Blasting
Content
---
PROCEDURE HANDBOOK SURFACE PREPARATION AND PAINTING OF TANKS AND CLOSED AREAS
SEPTEMBER 1981
Prepared by: COMPLETE ABRASIVE BLASTING SYSTEMS, INC. IN COOPERATION WITH . AVONDALE SHIPYARDS, INC.
Report Documentation Page
Form Approved OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
1. REPORT DATE
2. REPORT TYPE
3. DATES COVERED
SEP 1981
4. TITLE AND SUBTITLE
N/A
5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER
Procedures Handbook Surface Preparation and Painting of Tanks and Closed Areas
6. AUTHOR(S)
5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Naval Surface Warfare Center CD Code 2230 - Design Integration Tools Building 192 Room 128 9500 MacArthur Bldg Bethesda, MD 20817-5700
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
8. PERFORMING ORGANIZATION REPORT NUMBER
10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release, distribution unlimited
13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF:
a. REPORT b. ABSTRACT c. THIS PAGE
17. LIMITATION OF ABSTRACT
18. NUMBER OF PAGES
19a. NAME OF RESPONSIBLE PERSON
unclassified
unclassified
unclassified
SAR
123
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
FOREWORD
This research project was performed under the National Shipbuilding Research Program. The project, as part of this program, is a imperative cost shared effort between the Maritime Administration and Avondale shipyards, Inc. The development work was accomplished by Complete Abrasive Blasting systems, Inc. under subcontract to Avondale Shipyards. The overall objective of the program is improved productivity and, therefore, reduced shipbuilding costs to meet the lower Construction Differential Subsidy rate goals of the Merchant Marine Act of 1970. The studies have been undertaken with this goal in mind, and have followed closely the project Outline approved by the Society of Naval Architects and Marine Engineers’ (SNAME) Ship Production Comittee. Mr. James A. Giese, of Camplete Abrasive Blasting Systems, served as Project Manager and Ms.polly Medlicott as technical writer. On behalf of Avondale Shipyards, Inc., Mr. John Peart was the R & D Program Manager responsible for technical direction, and publication of the final report. Mr. Ben Fultz of Offshore Power Systems performed editorial services. Program definition and guidance was provided by the members of the 023-1 Surface Preparation Coatings Cammittee of SNAME, Mr. C. J. Starkenburg, Avondale Shipyards, Inc., Chairman. Also we wish to acknowledge the suppxt of Mr. Jack Garvey and Mr. Robert Schaffran, of the Maritime Administration. Special thanks are given to the numerous suppliers listed below for their valuable contribution of information (see Annex A for complete address ad telephone numbers). Aeroduct-Porter Company Aerovent, Inc. Air pollution Systems, Inc. American Air Filter Ccmpanyr Inc. American Coolair Corporation Anaconda Metal Hose Bry-Air Cargocaire Engineering Company Carter-Day
Central Engineerirq, Inc. (Vac/All) Clemco Industries Cleveland Metal Abrasives Cincinnati Fan and Ventilator Company, Inc. Complete Abrasive Blasting Systems, Inc. (CAB) Coppus Enqineering D.P. Way (Ultra Vac) Dryomatic Enpire Abrasive Equipment Corporation Flexaust Flint Abrasives General Air Division Zurn Industries H.B. Reed and Company, Inc. Hartzell Propeller Fan Company IRS/International, Inc. Kathabar - Medland Ross Key-Houston, Inc. Pauli and Griffin Pure-Aire, Inc. Strobic Air Corporation Super Products (Supersucker)Torit Division, Donaldson Company, Inc. Unimin Coloration United McGill Corporation Vacublast Corporation Van Air Systems, Inc. W.W. Sly Manufacturing Company Wedron Silica Company Wheelabrator-Frye, Inc. Whitehead Brothers Company
ii
Executive Summary A desperate need exists in shipyards for the proper planning enclosed areas and
execution of surface preparation ad coating operations in tanks and other
q
Abrasive blasters and painters are exposed to high
Other shipyard concentrations of dust and hazardous organic vapors. personnel are exposed to the potential dangers of explosion and fire. Another aspect of the need for better planning concerns the inefficient utilization of capital, manpower and material assets. AS an example, many extra manhours of labor are consumed in tank surface preparation operations because the abrasive blaster, when operating in tanks, just cannot see what he is blasting due to dust accumulation. Also, many square feet of painted surface are lost due to solvent entrapment during cure resulting in catastrophic peramature paint failure. Until the publishing of this report no single document existed With could be used by shipyard planners to effectively, efficiently and safely plan painting operations in confined areas. The information contained within this handbook includes:
q
Identification of the requirements and related problems associated with surface preparation and painting of tanks an enclosed areas. Identification of personnel exposure limits Identification of monitoring equipment for measurement of fume and dust concentrations and ventilation rates. Identification of maximum allowable concentrations and ventilation requirements for abrasive blasting and coatings application Identification of suitable ventilation and abrasive blast equipment for shipyard operations. In addition to the abve pints, a practical model for upgrading the
q
•
q
q
blast-paint department is offered. Throughout the course of this study, emphasis was placed on increasing productivity and improving enviromnental conditions. These pints can be achieved through a management sponsored systematic program of planned improvements based on” recomendations within this report.
iii
TABLE OF CONTENTS Page -i iv v vi 1. Conclusions 1.1 The Role of Managaement 1.2 Recommendations 1.3 Cost Savings 1.4 Summary 2. Use of the Handbook 3. Ventilation 3.1 Introduction 3.2 Technical Discussion 3.2.1 Ventilation During Abrasive Blasting 3.2.2 Ventilation During Painting 3.2.2.1 Lower Explosive Limit 3.2.2.2 Explosive Vapor Detection 3.2.2.3 Threshold Limit 3.3 Equipment Selection 3.3.1 Fans 3.3.2 Ducting 4. Dust Collection 4.1 Introduction 4.2 Technical Discussion 4.3 Equipment Selection 5. Dehumidification 5.1 Introduction 5.2 Technical Discussion 5.2.1 Principles of Condensation 5.2.2 Determining Dehumidification Requirements 5.3 Selection of Dehmidification Equipment 6. Abrasive Blasting 6.1 Introduction 6.2 Abrasive Blasting Equipment 6.3 Compressed-Air Drying Equipment 6.4 Abrasive Delivery and Storage 6.5 Abrasive Recovery Equipment 6.5.1 Selection Criteria 6.5.1.1 Portable Unit with Single-Chamber Collecticm Tank 6.5.1.2 Mobile Unit with Single-chamber Collecti= Tank 6.5.1.3 Portable Unit with Double-Chamber Automatic Discharge Tank 7. Model High Production Abrasive Blasting and Coating Pier 1.1 1.1 1.2 1.3 1.5 2.1 3.1 3.1 3.1 3.1 3.3 3.5 3.8 3.10 3.12 3.12 3.18 4.1 4.1 4.1 4.2 5.1 5.1 5.1 5.2 5.10 5.14 6.1 6.1 6.1 6.7 6.12 6.13 6.13 6.15 6.18 6.19 7.1
Foreword
Annex A - Suppliers List Annex B - Selection of Abrasives Annex C - Abrasive Cost Comparison iv
LIST OF FIGURES Page 3.1 3.2 3.3 3.4 3.5 3.6 3.7 4.1 4.2 4.3 4.4 4.5 5.1 5.2 5.3 5.4 5.5 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 7.1 7.2 7.3 7.4 7.5 7.6 Explosive Vapor Detector Schmatic of Centrifugal and Axial Fans Schematic of Axial Fan Centrifugal Fan Duct-Axial. Fan Branch Entry and Elbow Radius Design for Dinting Layout Ventilation Diagram of Large Enclosed Spaces, Small Tanks, and Multiple Tanks Schematic of Venturi Wet Scrubber’ Venturi Wet Scrubber Dust Collection -Unit Reverse jet Continuous Duty Dry Fabric Collector Unit Mobile Dust Collection/Dehurnidification System Mobile Dry Cartridge Dust Collection System Battery-Operated Psychron Magnetic Surface Thermameter Sample Psychrometric Report Form Schmatic Dry HoneyCcmbe Dehumidification Principle Model HC 9000 SEA Special Dry HoneyCombe Dehumidification Unit Portable Single-Chamble Multiple Outlet Blast Machine Single-Chamber Multiple Outlet Blast Machine Double-chamber Automatic Filling Principle Double-chanber Automatic Filling Multiple Outlet Blast Machine Mobile Steel Grit Blasting and Recovery System Schematic of Mobile Grit Blasting and Recovery System in Operation Compressed-Air Dryer and After-Cooler Model for Compressed-Air Drying System Pneumatic Delivery Truck Delivery and In-plant Distribution System for Abrasives Two Portable Vacuum Units with Single-Chamber Collection Tanks Mounted on Stand Portable Vacuum Units with Automatic Discharge Tank Mobile Vacuum Recovery Truck with Single-Chamber Collection Tank Panoramic View of Model Blast and Coat Pier 1000 Ton Abrasive Storage Hopper View of Ship Deck with Properly Installed Equipment Schematic of Unit Coating Container Cross-section Drawing of Ship Cargo Tank with Blast-Coat Equipment Installed Drawing of Tank Blast-Coat Operation 3.93.14 3.14 3.15 3.15 3.20 3.24 4.3 4.4 4.5 4.6 4.6 5.5 5.5 5.6 5.15 5.18 6.3 6.3 6.4 6.5 6.7 6.8 6.9 6.11 6.12 6.14 6.16 6.17 6.18 7.3 7.4 7.5 7.8 7.9 7.10
v
LIST OF TABLES Page Table I Table Table Table Table Table Table Table Table II III IV V VI VII VIII IX Ventilation Volumes Recomended to Maintain Solvent Vapor Concentrations below 10% of the lower Explosive Properties of Common Solvents Paint Vapor Concentration versus Ventilation Volume Friction Loss Per 100 feet of Ducting Area and Circumference of Circles Quick Dewpoint Reference Table Wet Air Factor Dehumidifier Moisture Remove Rate Comparison of Wet and Dry Desiccant Dehumidifier Units Comparison of Typical DH Units
3.4 3.6 3.11 3.22 3.26 5.3 5.12 5.13 5.16 5.17
Table X
vi
1.
Conclusions
1.1 Background The advent of huge, complex ocean-going vessels represents millions of dollars in capital investment. Corrosion prevention through blast-cleaning and painting is essential for protecting the value of these ships as capital assets and for prolonging the productive life of the vessels. Yet, for the most part, few guidelines exist for planning critical protective coatings (painting) operations during new construction, especially in high performance areas, such as ballast tanks and enclosed areas. Without exaggeration the blast-paint operation at sane shipyards can
be characterized as the dirtiest, most disorganize, wasteful and even dangerous area in the yard. These conditions many times result from a lack of guidance concerning basic principles and apparent lack of knowledge concerning available technology and equipment. The net result is a staggering wastage of manpower, materials, and time. An attempt to dispense with in-house painting operations by subcontracting blast-paint operations only provides a Short term solution, since responsibility for coating failures or production delays ultimately rests with shipyard management. The only possible long-term solution to these problems is to approach the surface preparation and coatings operation as a unified system. An experienced professional manager, using a systems approach to planning and Coordination of the total program, can:
q Modernize q Reduce q Improve
equipment
dependency on other services environmental renditions
1.1
The task of converting the blast-paint section into a profitable, productive and clean department must become a high priority for managers of U.S. shipyards. Economical modernization of this operation can be accomplished by otherwise successful companies. clearly, management plays a critical role in the development of a professional, efficient, surfacs preparation and coatings department. 1.2 Project Results This project achieved the defined objective of creating a procedural Handbook detailing ventilation rates surface preparation and painting and procedures required for the of tanks and enclosed areas. This accom-
plishment is a step toward solving the problem areas discussed above. The handbook on Surface Preparation and Painting of Tanks and Enclosed Areas provides a tool which can be used by shipyard personnel to:
q Reduce
labor hours for both blast-paint operation and for support
services and equipment.
q Write
procurement specifications for capital equipment procurement
q
Reduce worker exposure to hazardous conditions Reduce facility and equipment losses more competitive painting catastrophic paint operations
q
q Plan
q Reduce
failures
q Reduce
interference between crafts during construction
The net result will be a savings in dollars expended to produce ships.
1.2
1.3 Recommendations 1.3.1 Blast-Paint Department Management should commit high-caliber, technically capable personnel at all stages
to the program to insure competency, efficiency and quality of the operation. These personnel should include:
qA
surface preparations expert trained in quality control to coordinate between the shipyard and the ship owner. This individual
would also be responsible for the inspection of cleaned surfaces and for monitoring dust-collection and dehumidification systems.
qA
coatings specialist (ideally a chemsit) to review coating specifications, oversee application, sample coatings both at delivery and on finished surfaces, and maintain Ongoing data records of the coatings performance under actual shipyard condiitions. This individual would aid in the selection of appropriate coatings and preclude legal applications arising from coatings failure.
q An
instructor for an in-house program to train
employees in the use
of blasting, ventilating, dehumidifying, painting and compressedair drying machinery. General, components of the blast-paint operation which fully considered by management are:
q Development
should be care-
of an overall organizational plan of a program check list to include all equipment of procedures and inspection techniques
q Develooment
q Standardization
q Establishment
of a comprehensiver equipment maintenance program
1.3
q Coordination
of transportation, delivery and storage of materials, to include support logistics
In the drydock area, modifications might include such things an endramp access so that equipment could be moved in and out without a crane, an and deck, increased elevator or other personnel lifting system between dock system.
electrical services and installation of a high-volume compressed air piping Such improvements would result in a marked reduction of down-time during the blasting and coating operation. Finally, a carefully designed permanent installation is (see Section 7) practically a must for the efficient completion of major jobs. The essential elements of a Properly designed facility are: Large, enclosed space providing protection from the weather Equipment to control ambient air renditions Adequate utility hook-ups for electrical, water, compressed air and other services Permanent, properly designed ventilation system State-of-the-Art abrasive blasting and handling machinery permanently installed for maximum output Railroad track locatd next to the shelter for materials and equipment transport. Section 7 discusses one way of establishing a well-organized operations base for large blasting and painting jobs.
1.4
1.3.2 Naval Architects and Marine Engineers Naval architects and marine engineers must be the aware of problems faced by the shipbuilding/ship repair industry and encouraged to incorporate design changes which facilitate construction activities. Some suggestions are:
q Constructing
permanent scaffolding supports in tanks
q Placing
permanent openings in bulkheads larger, more conveniently located hatch or cargo covers
q Providing
on deck. These changes vaild greatly improve materials and personnel access for future maintenance activities. 1.4 Cost Savings By using the handbook published as a result of this study and by systmatizing the blast-paint operations as recommended, shipyards should save 30% to 50% of blast-paint operational rests. Generally, cost-savings will result in the following areas:
q Reduction
of support services required. By utilizing the proper
equipment and by making recommended modifications to existing facilities, dependence on support services would be significantly reduced. LOst prduction time waiting air hookup, water, etc. ) would
q improvement
on required services (cranes,
be eliminated. Many costly problems and
of environmental conditions.
delays are used by the messy, dirty conditions associated with the blast-paint operation. These include contaminated air, high worker turn-over, non-compliance with governmental health and safety regulation, disposal of wastes, and constant housekeeping.
1.5
Recovery and reuse of abrasive. Specialized equipment can enable the department to utilize inexpensive abrasives for some jobs in addition to recovering and recycling more expensive abrasive-materials for other jobs. Reducing expenditure of rapidly consumed abrasives can add up to surprisingly large savings. (See Annex c).
Improvement of quality. Catastrophic coatings failures can obviously result in enormous costs for shipyards. A systematic approach to the total blast-paint operation, using proper equipment, correct procedures and careful record-keeping will assist in avoiding premature 1.5 Summary Preparing surfaces of enclosed tanks for coatings, including necessary ventilation and air treatment operations, is but one part of the construction and repair of a ship. However, it must be recognized, that these operations are just as essential as those performed by fabrication, mechanical or other shipyard manufacturing departments. The blast-paint department depends on many support services and a variety of specialized equipment to complete projects. Technology is available which can correct both the environmental and worker safety problems associated with abrasive blasting in shipyards. This technology can be expensive, but ignoring the problems will be more expensive. It is recognized that there are many possible ways to solve existing problems or meet defined objectives. This report provides one proposed process by describing equipment and by outlining procedures which are now available to the modern shipyard. paint failures.
1.6
2.
Use of the Handbook
An attempt has been made to organize this handbook in such a manner as to the reading easy and data presentation logical. The discussion proceeds from ventilation through dust collection and dehumidification to abrasive blasting. Section 7 discusses a model abrasive blasting and Painting pier which utilizes the principals presented. The sciences of ventilation, dust collection, dehumidification and abrasive blasting and painting are each extremely sophisticated engineering fields. This handbook will not qualify the reader as an expert in any of these disciplines, but it does present certain basic principles, that, then followed, will help assure a well planned operation. The reader should follow the presentation as written. If dust collection and/or dehumidification are not deemed to be required, then these sections can be scanned. However, be forewarned that a simple statement that these operations are luxuries and not necessary without verificaticn through actual measurement will lead to many disastrous experiences. Each section of the handbook maintains a technical discussion followed by equipment selection. The technical discussion includes examples and sample calculations. In many cases, a simple substitution of different numbers, depending on job size, is all that is necessary to obtain required planning factors. characteristics. The equipment selection discussion describes equipment Knowing the calculated planning factors and equipment
characteristics a lead to the proper equipment selection for a given blast-paint operation.
2.1
3. Ventilation 3.1 Introduction There are two primary proposes for ventilating tanks and enclosed areas :
q Operator
health and safety
•
Operator visibility
These purposes are accomplished by removal of contamination air from the space and replacement of fresh air to the space. a total air treatment system. 3.2 Technical Discussion The following sections present general guidelines for determining ventilation requirements. Later sections discuss the design of the air-handling system to meet specific ventilation objectives. Additional detailed design information is contained within Industrial. Ventilation - A Manual of Recommnended Practice. That manual, which is published by the American Conference of Governmental Industrial Hygienists and endorsed by the Sheet Metal and Air Conditioning Contractors National Association, can be obtained from the Committee on Industrial Ventilation P. O. Box 453, Lansing Michigan 48902. 3.2.1 Ventilation During Abrasive Blasting The amount of ventilation required during blasting depends on the following four variables. Percentage figures indicate the relative importance in calculating requirements:
q q q
Where dehumidification
and/or dust collection is indicated, ventilation is the basic component of
Size of tank (cubic feet ) Number of blast operators Amountof corrosion on tank surface
60% 15% 15%
3.1
q Dusting
or breakdown characteristics of abrasive
10%
(see Annex B for discussion of Abrasives)
Ventilation is measured in terms of the volume of air movement over time, expressed as cubic feet per minute (CFM) . A general guideline to. providing an adequate environment in closed tanks would be one (1) complete air change every three minutes during the blasting operation. For example, a centerline or “Jumbo” tank with a 100,000 cubic foot capacity would require. approximately 33,000 CFM of ventilation. Generally speaking, the greater the number of complete air changes, within reason, the better the resulting visibility in the tank. Any one of. the listed variables can significantly affect renditions inside the tank. For example, if the amount of dust beinq generated increases due to an excessively corroded tank surface and/or high abrasive breakdown, the supervisor can compensate for these coditions by changing one or more of the other variables. He may choose to decrease the number of blast operators, stop blasting and mechanically descale the tank to improve surface conditions or increase the amount of ventilation in the tank. Unlike ventilation for paint or welding fumes, dry airborne dust created by abrasive blasting consists of relatively large particles. Since the particles can be sea, it is easy to monitor the success of the A more detailed discussion of the ventilation system in removing dust.
ranges of abrasive breakdown characteristics, tank surface conditions and cleaned surface, standards will be described in Section 6, Annex B and Annex c. The balancing of in-g and outgoing air is an important aspect of a ventilation system. If clean air is blown into the tank while muchless dirty air is being extracted, the result is air turbulence. The dirty air will subsequently be blown out any crack or opening in the tank. Similarly, the extraction of too much air relative to treated incoming air will result in inproper dehumidification for condensation control. Air circulation balance is achieved then the total amount of incoming air, treated or untreated, equals the total amount of air being exhausted. Conditions within
3.2
the tank, i.e., visibility, temperature or humidity, are thus maintained within a predictable, controlled range of efficiency and in accordance with safety requirements. 3.2.2 Ventilation During Painting During painting operations in confined spaces, the air in these areas becomes laden with paint overspray and solvent vapor. The health and safety hazards presented by these conditions dictate that ventilation requirements be carefully calculated and subsequently monitored throughout the painting. operation. To better understand the calculation of ventilation requirements, the following two definitions are necessary: LOWER EXPLOSIVE LIMIT (LEL) : Ihe lower limit of flammability or
explosibility of a gas or vapor at ordinary ambient temperature expressed in percent of the gas or vapor in air by volume. THRESHOLD LIMIT (TL) : The values for airborne toxic materials which are to be used as guides in the control of health hazards and represent time weighted concentration to which nearly all workers may be exposed 8 hours per day over extended periods of time without adverse effects. Whereas regulatory requirements dictate that the ventilation volumes be sufficient to dilute solvent vapor to at least 25 percent of the lower explosive limit of the specific solvent being sprayed, 10 Percent is a more commonly used design factor which insures explosion and fire prevention under varying conditions. Table I contains ventilation volumes recommended to maintain solvent vapor concentratons below 10 percent of the LEL for representative tank volumes.
3.3
Table I Ventilation Volumes Reccommended To Maintain Solvent Vapor concentrations Below 10% of the Lower Explosive Limit.
Tank Volume (Cu. Ft. ) 670 1,340 2,000 2,800 5,600 8,400 11,200 14,000 28,000 56, 000 84,000 112,000 168,000
Ventilation Volume (CFM)
1,000
1,200 1,500 2,000 2,500 3,000 4,000 5,000 6,000 10,OOO 15,000 20,000 30,000
3.4
In addition to safety factors, paint overspray can accumulate in enclosed tanks a blind workers wiht a dense particle fog. As in blasting, a relatively large Volume of ventilation ity and insure production efficiency. It is important to note that the ventilation objective for abrasive blasting recommended in Section 3.2.1, (approximately one air-change every three minutes) will, in most oases, maintain solvent vapor concentrations below the required percentage of the lower explosive limit, as well as maintain good visibility. By using the guidelines contained within this handbook and by requiring workers to USe respirators for painting, the sane ventilation system can, in most oases, be utilized for both blasting and painting operations. It must also be remembered that ventilation requirements extend through the paint curing process. The next two sections contain information on how to calculate LEL and TL. Table II contains current information on the LEL and TL for some common Since these limits are subject to change, the latest Federal solvents. Regulation should be used to calculate actual requirements. 3.2.2.1 Lower Explosive Limit Most paints used in marine applications contain solvents which rapidly evaporate during spraying. As stated above, sufficient air must be extracted from the tank during painting to limit the concentration of the flammable solvents to no more than 25% of their lower explosive limit (LEL) . The following example is used as a guide in demonstrating the principles involved in calculating required. ventilation volumes for specific solvents. Toluene is selected as the representative solvent. is necessary to maintain visibil-
3.5
Step One - Calculate Dilution Volume The minimum amount of air required (dilution volume per gallon of solvent, in cubic feet) is obtained from the following equation, where. vs is the cubic feet of vapor per gallon of solvent: 4 (100-LEL)VS Dilution Volume (cu. ft.) = By selecting the appropriate values for LEL and vs from Table 11, the dilution” volume required per gallon of toluene solvent is calculation as
Dilution Volume = 4(100-1 .4)30.4 1.4 = 8,564 cu. ft. of air per gallon of toluene Step Two - Calculate Ventilation Volume The required Ventilation Volume, in CFM, is found by multiplying the dilution vol- per gallon of solvent by the number of gallons of solvent evaporated per minute. Ventilation Volume ( CFM) = Dilution Volume (cu. ft. ) x gal ., of solvent gal. of solv. evap. nun.
In our example, several workers are painting in an enclosed tank. They are applying toluene thinned paint at a combined rate of one gallon per minute (gpm). The paint is 40% solvent. The ventilation volume required to maintain the solvent vapor concentration in the tank safely below the LEL is calculated as follows: Ventilation Volume = 8,564 cu. ft. gal . of Solv . x l gpm paint x O.4 gal solvent 1 gal. paint
3.7
Ventilation Volume =
3,426 CFM (for toluene)
This ventilation volume is the minimum amount required to prevent the hazardous accumulation of flammable paint vapor. The important factors to remember in determining the minimum ventilation volume to prevent explosions are: o % T h e rate at which the paint is being applied (gallons per minute), o The amount of flammable solvent in the paint. Tank size is not the controlling parameter. However, in larger tanks a
greater amount of paint vapor would probably be generated due to the increased number of workers. Water-based painting requires almost no dilution volume to prevent explosion since these paints contain only 1% to 2% flammable solvents. 3.2.2.2 Explosive Vapor Detection Two basic types of devices are used for explosive vapor detection. The type primarily used in the petrochemical industry is equipped with a heated catalytic element which is a possible source of ignition. As a safety measure, the element is protected by a fine mesh “Davy” screen that prevents flame propagation. Temperature of the heated element increases during exposure to a flammable atmosphere resulting in degradation of the sensing element. ThiS characteristic necessitates frequent recalibration. When located in an area where paint can deposit on the sensor, an additional problem is created. The fine screen is readily clogged by paint which requires frequent removal for cleaning. The detection principle reccomended for shipboard tank applications uses a “cold sensor” whihc does not degrade with time or exposure to flammable Vapors. No protective screen is used. The sensing element housing protects the instrument from physical damage. Sensitivity to paint solvents
3.8
is god, and the electronic alarm circuitry is simple and rugged. Since the detection element is not heated, power consumption is much lower than with heated element types. Portable battery-operated units can operate units several days before requiring recharging. See Figure 3.1. Simple construction tion and operation make this instrument suitable for fixed installation such as hood exhausts or duckwork which are not accessible for service and maintenance. The use of these instruments and the determination of hazardous conditions should be restricted to individuals trained certified as 'Competent Personnel'.
Figure 3.1: Explosive Vapor Detector
3.9
3.2.2.3 Threshold Limit Limiting the flammable paint vapor concentration to 25% of the LEL is sufficient to prevent explosion hazard, but this concentration is too high -for workers to breathe. Additional ventilation must be provided to reducethe paint solvent vapor concentration below the maximum levels allowed for workers on a routine basis. This concentration, called the threshold limit (TL), varies with the individual solvents used. A listing of the values for various solvents is contained in Table II. The dilution volume per gallon, of solvent required to maintain a concentratiocn below the threshold limit is given by: (1OO-TL) v TL s Where TL is expressed in percent by volume of air and vs is cubic feet Dilution Volume = of vapor per gallon of solvent. The dilution volume for the threshold limit of toluene solvent can be calculated as follows: (l00.0.02) 30.4 cu. ft. 0.02 = 151,970 cu. ft.
Dilution Volume =
Referring back to the previous example in paragraph 3.2.2.1 the ventilation volume rate required to maintain the vapor concentration below the TL requires 60,790 CFM Ventilation as calculated below:
Volume 0.44 gal. SOIV. gal. paint
=
151,970 cu. ft. gal . of Solv .
x 1 gpm paint x
= 60,790 CFM (for Toulene) This ventilation volume is the minimum required to maintain tank at an acceptable TL value. Table III shows graphically the resultant paint Vapor conncentration for various ventilation volumes.
3.10
concentration below the threshold limit requires extremely large volumes of fresh air, generally more than required for LEL maintenance or blasting generations. These volumess are difficult to provide due to air-handling equipment space limitations and cost, especial, ly when dehumidification of the incoming air is necessary. An alternative solution is to require workers to use respirators when applying solventbased paints in tanks. Another alternative is to limit the paint application rate to coincide with the required blasting ventilation volume. The same ventilation equipment can then do an effective job for both operations. As stated earlier, water-based paints require only a small fraction (about 5%) of the ventilation volume required for solvent-based paints. This can be easily provided by the blasting ventilation volume. 3.3 Equipment Selection Proper ventilation consists of equipment for moving air, equipment for directing or channeling tile air and the efficient setup of this equipment. The following paragraphs discuss the principles of air movcment and the proper selection of equipment necessary to effect efficient operations. 3.3.1 Fans Fans are used to ventilate tanks by exhausting dirty air and/or by blowing in fresh air. Fans can be selected from a wide variety of sizes and types for different applications. The most important factors involved in determining the fan requirements are: Type of ventilation system required Amount of ventilation required
Maintaining the paint vapor
Static pressure required Available space
3.12
Generally speaking, the objective is to choose a fan which provides required air volumes at proper static pressures with minimum horsepower and space utilization. The two preferred types of fans for marine ventilation are duct-axial and centrifugal. See Figures 3.2 and 3.3. Compressed air driven fans are also commonly used by shipyards for general ventilation. Hbwever, air driven fans have low efficiency rating relative to power requirements and are therefore not suitable for moving the large volumes of air. If the fan is to be used simply to ventilate the tank with ambient, untreated air, the duct-axial fan is the best @ice. This fan is ideal for portable applications where large volumes of air are blown or exhausted through only 50 to 100 feet of ducting at low static pressure. Having a simple heavy-duty design, the duct-axial fan can be successfully operated in abrasive and dirty renditions. 10,000 to 50,000 CEM capacity. These fans are available in ranges of Due to their loW static pressure ratings,
they require minimum horsepower (3-1OHP). In addition, duct-axial fans can be mounted either vertically or horizontally. Fans used for blast-paint operations should always be ordered with explosion-proof electric motor spark-resistant construction. See Figure 3.4. and
Centrifugal fans are capable of moving large volumes of air at high static pressure, and therefore, are used in conjunction with dust Collection and dehumidification systems. These fans can operate efficiently when Connected to ,long runs of duct work. The increased static pressure capability of centrifugal fans result in increased horsepower ratings (25-250+HP). See Figure 3.5.
3.13
Figure 3.2: Centrifugal Fans - Air enters the center of the impellers in an axial direction and is discharged by the impellers radially through the fan outlet. It is generally used When high static pressures are required, above 10-15 inches water column.
AIR FL O W
Figure 3.3: Axial Fans - Air enters and discharges in a straight line, parallel to - fan housing. It is generally used when a high volume of air is required, with the fan occupying the least amount of space.
3.14
The required fan capacity can be calculated based on the size of the tank and the frequency of air changes necessary for adequate visibility. For example, an air damage every three minutes in a typical 50,000 cu. ft. wing tank would require a fan capacity of 16,500 CEM. A 100,000 CU./ftcenterline tank would require a fan capacity of 33,000 CFM for the same air change frequency. Fan capacity specifications are based on standard cubic feet per minute (SCEM) ratings. A SCEM represents one cubic foot of air at 70°F moving at a rate of one foot per minute. Air cooler than 70°F, and there fore denser, moves slower through a fan than warmer air. Also more horsepower is required to move a given Volume at a given rate of cold air than of warm air. Fans are designed for varying maximum static pressure potentials. Fan static pressure is required to overcome the resistance or friction of air moving through ducting. Figures 3.4 and 3.5 Static pressure requirements are calculated based on the size, length, and number of bends of the ductwork. Size is the cross-sectional dimension of the duct. To demonstrate the effect of bends and elbows on static pressure loss, one foot of 18” duct with a 90 deg. elbow has the equivalent resistance of approximately 28 feet of straight duct. Static pressure requirements are also increased by air passing through air treatment equipment. The static pressure requirement for a fan should be determined after the ducting and equipment layout for the ventilation system has been designed. As an example, assume a fan must blow 9,000 CFM of air through a dust collection unit and 200 feet of 18” flexible ducting. The dust collector and the size and length of ducting each result in a 5” loss of static pressure for a total pressure loss of 10”. Therefore the fan must have at least 10” of static pressure potential in order to maintain the 9,000 CFM required. See Table IV, for friction loss per 100 feet of various sized duct
In many cases, the rated fan static pressure may be sufficient to pull or push the air in the volume required. Generally, duct-axial fans used in single-purpose ventilation systems should have at least 1“ static pressure capability, and preferably 2“. Centrifugal fans used with dust collection equipment should be ordered with a minimum 12” static pressure rating. In a well-designed, permanently installed air handling system, fans can be located at practically any distance from the tank and still operate efficiently. However, on jobs of short duration where portability and ease of installation are desired, the fan should be placed as close to the tank as possible in order to reduce the amount of ductwork required. Duct-axial fans can be ordered with special adapters enabling then to be mounted directly into ‘Butterworht’ openings and cargo hatches. Ideally the exhaust fan(s) should be placed over the ‘Butterworth’ opening(s) and ducted to two sides of the tank bottom. Fresh air should be blown into the tank through the cargo hatch. This arrangement will distribute the clean air uniformly through the tank and through the same passage operators use to enter and
exit.If deck space is severely limited fans may be platform-mounted. If
possible, fans should be isolated from commiunication areas because of high noise levels. Installed fans Should be checked periodically with a manometer. This device measures air flow. Measured reductions in air flow of an installed system can be indicative of wornm parts such as impellers or destructed ducts . In In summary, the most flexible type of fan for ventilating tanks with ambient air is the duct-axial type with a rated capacity of 30,00040,000 CFM, 2" of static pressure and a 4248” spark-proof case. Centrifugal fans with greater capacity for static pressures are primarily designed for use in air-treatment systems. Exact specifications will depend on the layout of the ductwork and/or treatment systems.
3.17
3.3.2 Ducting Well-designed and Properly laid-out ductwork is essential to an. efficient air handling system. Ducting design requires a thorough knowledge_ 1 of requirements , accurate data on equipment performance specifications, accessibility, duct length and weight and volume of material to be moved i.e., abrasive dirt, solvent fumes, etc. The two main areas of design criteria for ducting are: including factors of CFM, static pressure, velocity
q Sizing,
requirements, and fan specifications.
q Layout,
including type of job, ducting material,placement, and
monitoring of the system. The general objective for the ductwork design is a system of the smallest dimensions which combines the lowest practical static pressure requirements with sufficient velocity to transport the airborne materials. 3.3.2.1 Sizing Sizing is the most critical consideration in selecting ducting because it determines, thethw the actual CEM, static pressure, and velocity of the air-f low in the finished ventilation system meets objectives. established design
1
For detailed information pertaninig to duct design, consult Industrial Ventilation: A Manual of Recommended Practices
3.18
Four factors must be considered when selecting duct size:
q Air volume in CFM
q Distance q Static
air is to be moved
pressure limitation of available fans q Air velocity requirements With these four pieces of information, Table IV can be used to select the proper duct size. As discussed earlier, air volume requirements are based on the size of the confined area and the characteristics of the material requiring venting. The distance the air is to be moved is simply the length of the ducting. Normallya fan which best meets the air volume requirements is selected from the existing capital inventory. The static pressure rating of the selected fan then becomes a design parameter Which must be considered in the final ducting size selection. Velocity calculations are based on the characteristics of each type of materiel to be vented. If the duct is too large, resulting in a decrease in critical particle velocity along the length of the ducting the suspended material will fall out of the air-stream and build up in the bottom of the duct. As the duct fills, the ventilation capacity of the system is severely As a rule, airborne dust resulting from abrasive blasting requires a critical particle velocity of 3,500 FPM. Static pressure loss along the length of the ducting is dthectly related to the size (internal cross-sectional area) of the duct. If the duct is too small,the static pressure required to offset frictional losses may overload the fan capacity, resulting in a reduction of air volume moving through the system. It must be remembered that as static pressure requirements increase, more energy (HP) is required to operate the system. Excessive energy requirements not only increase cost but may also restrict ventilation equipment usage at same locations within the yard. reduced and there is danger of the duct collapsing.
3.19
Same examples of static pressure loss for various types and sizes of are as follows (assume 9000 CFM ventilation requirement):
q 18”
smooth ducting will generate 1.7” of static pressure d r o p
100' of duct, and will provide a velocity of 5,000 FPM. (See Table Iv)
q 18"
flexible ducting has a static pressure drop of 2.8” per 100’. Adding one 90 degree bend along 103’ will increase static pressure increases to 8.4” and three cross-sectional irregularities,
bends along 3(X)
‘ d r o p t o 3 . 2 ” . Two balds along -200‘
increases to 12.6". the
‘area inside the flexible ducting, due to
increases the velocity to 5,500 FPM as to the smooth ducting (see figure 3.6 for proper branch entry and elbow radius/ designs).
q 24”
smooth ducting has a static pressure drop of 6” per 100’.
However, velocity at the velocity CFM
is 3,200 FPM, which would be
marginal to transport abrasive dust. The air volume would have to be increased in order to move grit dust through this duct size. (Accurate static pressure figures for various CFM and duct sizes can be obtained from manufacturer’s specifications) . Example: A ship tank is scheduled for abrasive blasting. The size and
configuration of - tank is such flat 30,000 CFW of air and 300' of duct are required for proper ventilation. The available fan is a 30,000 CFM duct-axial. rated at 2" static pressure. Step One: Look at Table
IV.
Select the line on the y axis which repreAs can be seen from the table, duct sizes diameter will carry the required air
sents 30,000 CFM.
volume.
from 20" to 80" in diameter
Step Two:
Calculate the maximum allowable static pressure drop for each 100‘ of duct based on fan rating. This allows use of Table IV
3.21
which is expressed in frictional loss in inches of water (static pressure) per 100’ of duct lenght.
300' ÷ 100' = 3 lengths of 100'
2.0" static pressure + 3 length = .7” per 100’ allowed
Step Three:
Again leek at Table IV. Follow the x axis to the paint which corresponds to a frictional loss of .7. Trace up this line to the intersection of the line which corresponds to 30,000 CFM. The diagonal line which intersects with x and y axis and represents 'in. duct diam. ' reads 34. Therefore, the appropriate size duct appears to be 34”.
Step Four:
Verify that the duct size selected will maintain the proper velocity to keep abrasive dust suspended-3,500 FPM) . The FPM velocity line in alSO a diagonal line. As can be seen, the velocity of the air ranges from 4500 to 5000 FPM which is in excess of the minimum velocity required to transport abrasive dust (3500 FPM) .
Solution:
In this ample the 34” ducting
would
be the correct choice.
In conclusion, ducting which is not carefully and properly sized will greatly reduce the efficiency of the total ventilation system, and will result in problems related to equipment, visibility and worker safety. 3.3.2.2 Layout When blasting marine tanks, the operator is faced with many different types of applications and tank configurations around which the ducting layout must be designed. set-up and To allow for maximum portability and ease of breakdown, the yard should stock ducting components in a variety
of sizes and quantities. However, the shipyard should have some standard systems which are designed for the most frequent types of jobs.
3.23
In many cases, ventilation air is not distributed uniformly through the tank. As a result, only parts of the tank are properly ventilated, while other areas remain contaminated . Clean air must be ducted into the tank in such a manner that the ductwork extends down no more than 6“ below the tank top. Since the heavier airborne dust particles tend to settle to - bottom of the tank, the dirty air removal duct should be positioned in such manner that the pick-up opening is near the tank bottom. This arrangement permits the dust particles to naturally fall toward the bottom of the tank and be exhausted much faster than if the pick-up point were positioned higher in the tank. The duct openings should be separated as much as possible. See Figure 3.7.
Figure 3.7:
Ventilation Diagrams of Enclosed Spaces, Snail Tanks and Multiple Tanks
3.24
Some tank configurations and/or production requirements necessitate ventilation between tanks. This can be accomplished by cutting access holes through common bulkheads or through decks. These access holes are particularly advantageous when setting up a complete tanker job. The resulting cross-ventilation saves considerable time through standardization of duct sections. Blanks can be used to close off outlets or inlets when not in service. This practice also provides additional access entrances to each tank and avoids the constant problem of personnel ard materials competing fora too little space. Metal ducting should be used for all straight runs. Flexible fabricwear and
reinforced ducting, which is more expensive and is subject to high inaccessible tanks.
tear, Should be used for inking connections to machinery and to small, Round duct is usually the best choice because it maintains a uniform air velocity and withstands higher static pressure. All duct work for tankblasting ventilation should be durable yet light for optimum portabilityo periodic inspection of the ductwork should be made to insure air-tightness. In addition, every new After the system has been installed,
system Should include access for measuring devices tO monitoring
velocity, CFM and static pressure at various points along the ducting. The Pitot tube is the standard air velocity meter. By multiplying the velocity reading in FPM by the cross-sectional area of the duct in square feet, the actual CEM at that point can be calculated. For example, at a point on a straight run of 18” ducking, the air velocity is measured to be 3,200 FPM. The 18” round duct has a cross-sectional area of 254.4 square inches, or 1.76 square feet. The CFM at that Point would be 3,200 X 1.76 See Table V for area and circumferences of square feet or 5,632 CFM. circles. A manometer is used to measure static pressure. If measurements of CFM, static pressure, and velocity reveal that
ventilation objectives are not being met, modification or repair of the ductwork and/or the fan may be necessary. A common problem with fans used
3.25
for blasting ventilation is worn impellers caused by abrasive dust. If the fan does not have sufficient capacity, ducting must be straightened or shortened. The problem may also be caused by improperly sized ducting, constrictions or air leaks. Ducting that has been used for ventilation during painting should be
inspected for paint build-up on the interior surfaces before it is used for blasting ventilation. Friction created by the abrasive dust combined with flammable paint solvent particles can create a fire hazard. In addition, excessive paint build-up will receive the efficiency of the ventilation System. In this section, basic procedures and guidelines have been given for general marine tank ventilatione Examples for the most part have been for ventilation of ambient, untreated air. The next section will identify the components of the dust collection system, which cleans the dust-laden air exhausted by ventilation.
3.27
4. Dust
Collection
4.1 Introduction
The utilization of dust collection equipment to clean contaminated exhaust air resulting from manufacturing activities is an existing technology with widespread use throughout the world. The possible exception is shipyard adaptation of dust collection for blast-paint operations. If properly used by shipyards, dust collection will eliminate many of the problems associated with Contaminated air from abrasive blasting and Coating; operations, will insure compliance with EPA and OSHA regulations will substantially reduce job-site housekeeping. Current or pending federal legislation may seen force every shipyard contractor to clean the contaminated air generated by abrasive blasting painting. ClearlY, management would be well-advised to begin assessing requirements and identifying dust collection equipment which will must efficiently meet existing and potential regulations.
4.2 Technical Discussion
There are three types of dust collection equipment which are adaptable to shipyard blast-paint operations.
q wet q Dry q DrY
scrubber Fabric Cartridge
Wet scrubbers impinge the dust-laden air with moisture, wetting the dust and causing it to settle due to increasd weight. The resulting sludge is drained of moisture and discharged by conveyor from the machine in a semi-dry condition. One reccomended type of wet collector is of the venturi design. This design combines high constant efficiency and portability with low operating costs and low operating noise levels.
4 1
Dry fabric (baghouse) collectors use a series of fabric bags which filter dirty air drawn across or through the banks of filter elements (bags) . The retained dust is then removed at regular intervals by blowing compressed X through the fabric bags, by shaker or by vibratirg systems . The dislodged dry dust then falls into hoppers for disposal. Reverse- jet continuous duty dry fabric dust collectors are reccommended for shipyard applications because of the high humidity conditions. This design provides increased air flow and, therefore more complete cl caning of the filter media. However, this system has a higher initial cost and requires more maintenance. Dry cartridge systems collect and discharge dust in the same manner as dry fabric or baghouse systems but have capacities of only 5-10,000 CFM. Because the cartridge is rigid in the collector, the filter media does not require removal. for transport. Cartridges are replaced as necessary. 4.3 Equipment Selection The most important selection criteria for dust collection equipment in the shipyard are as follows: o Portability o CFM and static pressure requirments o Type of particles handled o Efficiency and consists 4.3.1 Portability Portability is a crucial consideration in selecting dust collection equipment, and includes factors such as machine size, transportability, set-up time and ease of placement. If the shipyard frequently handles individual tank blasting jobs and/or multi-tank projects, a wet venturi system would be a good choice. This 25,000 CFM unit is compact (13.75’ high X 8’ wide X 18’ long), with a dry weight of approximately 12,000 lbs. The wet scrubber can be transported
completely assembledo Because the fan is mounted on top of the machine, extra ducting is not required between the fan and scrubber. The unit can be disassembled or reassembled in about 8 man-hours. The removal of the fan and transition piece make it a legal load for transporting outside-the yard. Due to a low center of gravity, the unit can be located ship without problems. See Figures 4.1 and 4.2. The primary limitation of the wet venturi system is that it cannot be used with dehumidification equipment. The moisture laden air increases the load on dehumidification equipment. When projects dictate dehumidified air, the dry fabric or cartridge collector is the recommended choice of equipment.
1 – CUSTOMER’S INLET DUCT 2– INCOMING DIRTY AIR 3 – WATER DISTRIBUTOR 4 – ADJUSTABLE RECTANGULAR VENTURI 5 – WASH SECTION 6 – SOLUTION TANK 7 – MOISTURE ELIMINATORS 8 – FAN 9 – OUTGOING CLEAN AIR
10 11 12 13 14 15 16 17
– FAN MOTOR – PUMP DISCHARGE – PUMP – PUMP INLET – SLUDGE CONVEYOR – CUSTOMER’S SLUDGE CONTAINER – SLUDGE CONVEYOR DISCHARGE – SLUDGE CONVEYOR DRIVE
FIGURE 4.1 - Schematic of Venturi Wet Scrubber
FIGURE 4.2:
Venturi Wet Scrubber Dust Collection Unit - 25,000 CFM
The standard design of the dry fabric collector is less suitable for portability than the wet venturi or dry cartridge equipment, i.e., a 25,000 CFM unit is 27’ high x 12’ wide x 25’ long and weighs about 13,000 pounds. It has a much higher center of gravity making it unstable when placed on the ship deck. If a dry unit is to be remved or transported, the bags usually should be removed to avoid tearing. Bag removal is a dirty and unpleasant task. In most designs, the fan and rotor are not mounted on the unit . These must be disconnected and transported separately for moving. Approximately 150 manhours are required to set-up or disassemble a 25,000CFM unit. Frequent handling of this type of unit will result in increased maintenance and repair costs. See Figure 4.3.
4 4
FIGURE 4.3:
Reverse-jet Continuous Duty Dry Fabric Collector Unit - 70,000 CFM
The dry fabric collector is most efficiently utilitized in semipermanent, pierside, barge-mounted, or railcar-mounted arrangements.This system is also appropriate for large capacity permanent installations. For individual tank jobs requiring dehumidification, a combination of dry cartridge and dehumidification units (10,000 CFM each unit) represents a high-performance, totally portable system. (See Figure 4.4). The dry cartridge dust collector system is also ideal for trailer-mounting because of its compact design. A system of up to 40,000 CFM (consisting of four 10,000 CFM units) can be mounted complete with fan and motor on a single 40’ trailer. Since the cartridge unit can be moved without disassembly, this system can be transported on roads as well as within the shipyard.
4.5
FIGURE 4.4:
Mobile Dust Collection/Dehumidification System - 10,000 CFM Each Unit
4.3.2 CFM and Static Pressure Each type of dust collection system can be assembled with high static pressure fans to accommodate long runs of ducting. However, the Wet Venturi Scrubber is restricted to a 50,000 CFM volume capacity as the largest practical single unit. .Because of their modular design, single units of the dry fabric system can be designed with a Capacity in excess of 100,000 CFM.
FIGURE 4.5:
Mobile Dry Cartridge Dust Collection System- 40,000 (2’FM
4.3.3
Type of Particles Handled Dust created by abrasive blasting institutes a moderate load of fine
to medium sized particles. Both dry and wet systems are well-suited to handle these particles. However, the dry fabric collector cannot efficient ly handle wet particles as they tend to clog the filter media. This problem limits the use of dry fabric collectors during matings applications because the overspray is wet. If air ventilated durirg painting is to be cleaned by a dry fabric colector, an exdpendable paint arrestor filter should be used to filter the air before it is exhausted to the collector. Wet paint will quickly clcg and "blind" the bags. The wet collector can handle both dry and wet particles. The slightly
damp sludge resulting from the wet scrubber system is easier and cleaner to handle than the discharge from the dry system. The dry dust discharge can create a secondary air pollution problem during disposal. 4.3.4 Efficiency and Costs In terms of efficiency, operating most, and maintenance, the Wet scrubber offers several. advantages. It runs at a constant efficiency, has heavy-duty instruction with few moving parts, requires less maintenance and has lower replacement rests. The unit is also easily accessible for repairs and external inspection. The wet unit can be installed for allweather, year-round operation. The efficiency of the wet scrubber is not affected by air moisture in humid areas, although the use of water may introduce corrosive conditions within the collector. When ordering scrubbers, a corrosion-resistant mating such as a coal tar epoxy should be specified for all internal metal surfaces. The scrubber requires both electrical and wter service hook-ups, although water used by the unit is recirculated. In comparison, the dry system will operate efficiently only when air
conditions are dry enough to prevent condensation or moisture deposits on the fabric. Under humid renditions, dust will cake on the bags, resulting in loW efficiency and possible damage to the filter media. All openings and
4.7
fittngs on the suction side of the ductwork should sealed against moisture. The unit has a large number of parts and assemblies with limited accessibility which results in increased maintenance rests. An additional hazard of the dry system is the possibility of a “bagtiuse” fire. The ferrous oxide contained in blast dust residue may under certain conditions spontaneously ignite. Use of the wet scrubber system for abrasive blast air-cleaning eliminates the possiblity of collector fires. 4.3.5 Summary In summary, actual guideline:
q
equipment selection should be based on the per-
formance requirements of the intended application. As a general selection
For jobs requiring multiple units and volume
requiremmts of 15,000
-35,000 CFM (especially when frequent jobs of this range are widely
distributed around the yard) the wet scrubber system should be used .
q
For stationary applications requiring a single unit of over 35,000 CFM, the dry fabric collector is best.
q
For small, portable, short-term applications and/or here multiple units are required for recirculation of dehumidified or heated air dry cartridge type collector of 5,000-10,000 CFM are best
If dust collection equipment is properly installed and utilized, the environment in and around the blasting operation will be as desirable a place to work as any other area within the shipyard.
4.8
5.
5.1 Introduction
DEHUMIDIFICATION
Dehumidification (DH) is the process of removing moisture from ambient air. The removal of moisture from the ventilation air is an important process in preventing condensation (” sweat”) on internal tank surfaces during blasting and painting. Condensation occurring on surfaces which have just been blast-cleaned may cause rust bloom formation within a short time. adhesion of the protecThe resulting surface contamination promotes poor
tive coating and premature failure due to underfilm Corrosion. Blistering is another common type of paint failure usually causal by applying paint to a surface containing moisture. Blisters may also occur when the surface was originally dry during application but moisture entered the mating as it cured. Paint curing is a function of temperature, time and humidity. Since
curing requirements vary widely between water+born, epoxies, inorganic zincs, and other types of coatings, paint manufacturer’s specifications should be consulted for recommended atmospheric conditions. The use of dehumidification equipment througout the process of blasting, painting, and curing will prevent many coatings failures. 5.2 Technical Discussion The purpose of this section is to provide simple, clear explanations of condensation principles and the calculation of dehumidification requirements. In addition, information will be presented on the comparison, selection and utilization of DH equipment. A series of easy-t-understand tables for calculating DH requirements have been developed to avoid the complex psychrometric interpretations that have hitherto been necessary.
5 1
5.2.1 Principles of Condensation Condensation ocurs when warm, moisture-laden air contacts a cooler surface. As the air next to the surface is coaled, the moisture carryingcapability is reduced, and some of the water vapor is deposited as droplets on the cooler surface. This occurs naturally in the early morning when air warmed by the sun contacts cooler blades of grass or car windshields. The temperature at which the ambient air becomes saturated with water vapor is called the dewpoint temperature. Any reduction in the air temperature below the dewpoint (for example...when warm air contacts a cooler surface), causes moisture condensation. Reducing moisture in the air will lower the dewpoint temperature of that air. Dewpint temperature is determined by the difference in the wet- and dry-bulb temperatures. This difference can be measured by a psychrometer. See Table VI, Quick Dewpoint Reference Table, for examples of dewpoint.
To determine dewpoint
(air temperature at which moisture will condense
on surfaces), follow wet bulb temprature across and dry bulb temperature down. (These temperatures can be measured by a battery-operated psychrometer, Figure 5.1). The intersection is the dewpoint temperature. Example; wet bulb 60°F, dry bulb 75°F = dewpoint temperature 50°F. It is commonly believed that high air temperatures combined with high humidity create the greatest possibility of condensation. In shipyard operations, lower humidity combined with large day-to-night temperature swings and low sea water temperatures can present a greater potential condensation problem. During day-night temperature transition periods, surface temperatures will often be lower than dewpoint. Condensation in a ship tank can cccur during these periods, or anytime that weather condiitions change. Once these general principles are understocd, several points must be remembered in connection with the dehumidification of air in shipboard tanks .
5.2
Condensation will never occur if the dewpoint temperature of the air is kept lower than the surface temperature of the tank. Therefore, the general rule for condensation prevention is to maintain the air dewpoint temperature at least 5°F below the surface temperature. Heating the air in an enclosed tank does not remove moisture or change the dewpoint temperature. For example, air at 400F with 70% relative humidity has a dewpoint temperature of 31OF, file 80°F air with 17% relative humidity has an identical dewpoint of 31°F. . Dewpoint control can be easier to maintain when a ship is in the water than when in drydock. This is because the ship surface temperarures below the water line will remain relatively constant due to the heat sink of the surrounding water. When the ship is in drydock, the entire surface is exposed to air temperature shifts. Heat is also lost to the ambient air at night. Psychrometric readings should be measured and recorded every four hours during the entire blasting and painting operation. This procedure will provide detailed records of job conditions for future use. The battery-operated Psychron Model 566, available from the Environmental Service Division of Bendix, provides wet- and dry-bulb temperature readings as well as a scale to determine dewpoint. A surface lihermaneter with built-in clamping magnets can be easily attached to metal surf aces anyWhere on the ship for surface temperature readings. The Model 315F, available from Zorelco Measuring & Testing Instruments, 8520 Garfield, Blvd., Clevelan3, Ohio, is suitable for this purpose. See Figures 5.1 and 5.2.
5.4
FIGURE 501:
Battery Operated Psychron . Bendix
FIGURE 5.2:
Magnetic Surface Thermometer Zorelco
As an illustration of the practical application of dehumidification principles, the following example is offered. Readings were Compiled over a 24 hour period and entered on a Psychrometric Report (See Figure 5.3). This proposed report is one way of recording required data. Note that relative humidity is not a required reading, and is only given as a comparison between air temperature and moisture-holding capacity.
5.5
(SAMPLE) PSYCHROMETRIC REPORT 30B LOCATION NEW YORK HARBOR TANK WING TANK - SHIP WATERBORNE DP-DEW FOINT TEMPERATURE
DB-DRY BULB TEMPERATURE
WB-WET BULB TEMPERATURE
DATE
TIME FOREMAN WEATHER* OUTSIDE INSIDE DB WB DP DB WB DP
TANK SURFACE TEMP
DIFFERENT SURFACE TEMP INSIDE DP +1OF + 5F +25F +5F + SF - 5F + 5F + 5F - 2F +5F
-12F
1/4/80 0800
WHITING CLEAR
45/40/34 45/40/34 45 ABOVE W.L. 40 BELOW W.L.
60/50/41 60/50/41 60 ABOVE W.L.
1200 WHITING CLEAR 1600 WHITING CLOUD CHANGING 2000 BIBBO CLEAR
40 BELOW W.L. 50/48/45 50/48/45 50 ABOVE W.L. 40 BELOW W.L.
40/38/35 40/38/35 40 ABOVE W.L. 40 BELOW W.L. 40/38/35 40/38/35 33 ABOVE
2400 CROTTY CLEAR CHANGING 0400 GIESE
W.L. 40 BELOW W.L. W.L. 40 BELOW W.L.
50/48/45 50/448/45 33 ABOVE
-5F
*INDICATE:
SUNNY, CLOUDY, RAIN, SNOW, FOG, CLEAR, (CHANGING.
FIGURE 5.3: SAMPLE PSYCHOMETRIC REPORT FORM
5.6
The fOllowing renditions are based on a 50,000 cubic feet wing tank ventilated with 17,000 CFM of air. It should be noted that a sealed tank with no ventilation would present very different characteristics, as the stagnant, idle air on the inside would not be subject to radical tempera-’ture swings. 8:00 A.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 80%) Conditions at this time are condensation free, as surfaces both above and below Water line have temperatures above dewpoint. No DH required. 40°F 45°F 40°F 34°F 45°F 40°F
12:00 NOON Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line, Surface temperature: below water line 40°F 60°F 50°F 41°F 60°F 40°F
Conditions are the same as at 8:00 A. M., as only the ambient air temperature has increasd. No DH requirement.
5.7
4:00 P.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: ret-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 85%) A storm enters the area, bringing additional moisture With in turn o raises the dewpoint 5 F above the existing temperatures of surfaces below the water line. condensation will therefore occur on tank surfaces below the water line. The area above the water line, at 50oF, is still 50F above the dewpoint temperature, so condensation will not occur on those surfaces. DH required below water line. 40°F o 5 0 F 4 8
o F
45°F 50°F 40°F
8 : 0 0 P.M.
Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (See Table VI) Surface temperature: above water line Surface temperature: below water line
The storm has passed and ambient air is dryer.
40°F - 40°F 38°F 35°F 40°F 40°F
Surf ace temperatures, both above and below the water line, are again 5°F higher than the dewpint. No DH required.
5.8
12:00 MIDNIGHI’ Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (See Table VI) Surface temperature: above water line Surface temperature: below water line 40°F 40°F 38°F 35°F 33% 40°F
During the clear night, the surfaces of the Ship above the water line
are radiation heat into space, so the surface temperature above water line drops to 33°F. This temperature will not drop any further because heat is also being transferred from the warmer surfaces below the Water line. In this case, condensation is occurring on surfaces above waterline, since the dewpoint is 35°F and the required above waterline. surface temperature above water is only 33°F. DH
-
4:00 A.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewppoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 85%) Surfaces both above and below
40%
50°F 48°F 45°F 33%F 40°F
--
the water line have cooled during the Thus condensation will occur on DH required. coditions were experi-
night to temperatures below the dewpoint. all ship’s surfaces exposed to ambient air.
During this 24-hour period, three different
enced.
5 9
q
4:00- P.M. the storm passed levels.
through and raised ambient air moisture
The dewpint rose above the temperature of the surface
_ _ _ belcw the water line caused condensation below the water line.
q
12:00 Midnight surfaces above the water line mold through heat radiation to a temperature lower than the dewpoint and mndensation occurred.
4:00 A.M. condensation occurred on surfaces that had cooled during
the night . ___ These examples demonstrate the types of conditions which are commonly experienced by shipyards. These conditions require dehunidification of the air to prevent condensation on tank surfaces. The following section requirements. outlines the methodology of determining dehumidification
5.2.2 Determining Dehumidification Requirement s
Dehumidification requirements are determined by calculation the volume of conditioned air needed to control condensation inside a tank and then calculating the requisite number of DH units which will meet the defined objectives. These calculations can be easily accomplished by using data entered on the Psychrometric Report (Figure 5.3) in conjunction with Tables VI, VII, VIII. The instructions accomoanying each table gives specific examples of required calculations. Table VI gives the dewpoint temperature based on existing ambient dry and wet bulb temperature readings. Using the dewpoint and the existing tank surface temperatures, the amount of moisture in the ventilated air, expressed in pounds per hour per CFM, can be determined from Table VII. Determinations should be made for surfaces with above and below the water line. Table VIII contains the moisture removal capacity of a Cargocaire Model HC-9000 SEA Special 9000 CEM unit dehumidifier unit in pounds per hour This model and size dehumidifier was chosen for the example because it is a standard readily available piece of equipment. A table similar to Table VIII can be compiled using performance curve data for any other
existing DH system. The total amount of ventilation required for visibility and safety (see Section 3: Ventilation) is then multiplied by the wet air factor (Table VII ) to obtain the total amount of required moisture removal Table VIII is then used to determine the number of DH units needed to meet the dehumidification requirement for the specified volume of air. This, in turn, is the required amount of conditioned air as a ratio to the amount of untreated ambient air required for ventilation. Example: A ship tank is scheduled for. abrasive blasting. The size and
configuration of the tank is such that 30,000 CFM of ventilation is required. The dewpoint temperature of the ambient air is 50°F and the surface temperature of tank is 45°F. Dry Bulb Temperature of air is 75°F. Step One: Determine the wet air factor from Table VII . At a dewpoint temperature of 50° and relative humidify of 45%, the Wet Air Factor is .011. Step Two: Multiply ventilation requirement by Wet Air Factor 30,000 CFM X .011 lbS/hr/~ = 330 lbs/hr This is the quantity of moisture to be removed. Step Three: Fran Table VIII determine the moisture removal rate at a dewpoint temperature of 50°F and a dry bulb temperature of 75°F. In the example the water removal rate is 208
Ibs/hr .
Step Four:
Divide the quantity of moisture to be removed by the moisture removal rate of the dehumidifier. This will provide the number of units required. 330 lbs/hr + 208 lb/hr/unit = 1.59 units
Solution:
1.59 or 2 units of 9,000 CFM capacity each are required. This means that approximately half of the 30,000 (2FM of ventilating air must be dehumidified.
5 11
5.3
Selection of Dehumidification Equipment Three types of air treatment systems - be used to control dewpoint
temperature.
q Air-conditioning q Wet q Dry
desiccant dehumidifiers desiccant dehumidifiers. conditioning cools air through the use of refrigeration coils to
Air
An air conditioning system may be adequate to control condensation in year-round warm climates; however, as the temperature approaches 45°F, moisture from the air will freeze on the coils making the system ineffective for dewpoint control. Furthermore, air conditioning units are not designed for rugged, dirty condition or portability. These units also require specialized maintenance. Therefore, air conditioning units have not proven to be reliable for typical marine coating applications. Heaters are sometimes used to raise surface temperatures inside the
tank above the dewpoint.
condense out moisture from the air.
condensation, inefficient,
While this method can theoretically control and can be effective for small tanks, it is extremely and does not to remove
uses excessive amounts of energy, moisture from the air.
Until the late 70’s, wet desiccant systems were the most frequently used in U. S. shipyards. This system operates by pumping a desiccant solution through a spray header tube in the contactor. When the air to be dried is drawn past the contactor, moisture in the air is absorbed by the desiccant. The moisture-laden desiccant is then cycled through an exhaust air stream which removes the collected moisture. The wet desiccant system requires piping and regular replacement of the desiccant solution, plus auxiliary support equipment. (See Tables IX and X). Wet desiccant humidifiers are not reccomended for shipyards because of the high initial and maintenance costs, the requirment for a full time operator, and the large unit size and weight.
5.14
6.
Abrasive Blasting
6.1 Introduction Abrasive blasting is the process by which steel surfaces are cleaned of contanination through the use of abrasives striking the surface at relatively high velocity. This process requires a wall-coordinated program of carefully selected equipment and materials, experienced operators, and organized services in order to guarantee success. 6.2 Abrasivee Blasting Equipment The selection and placement of the abrasive blasting equipment is critical to the success of any tank blasting project. This equipment requires the greatest amount of consumable materials (i. e., abrasives) used in the shipyard. A single abrasive blaster will use approximately 1,500 Ibs. of abrasives per hour. An average cost for delivered mineral slag abrasive today is approximately $40.00 per ton. Therefore, one blaster will consume .$30 .00 worm of materials per hmr. In addition, abrasive blasting requires a wide range of costly support services, including compressed air, crane service, dust collection, dehumidification, staging, and clean-up crews. Proper selection of equipment and materials can increase labor productivity and significantly reduce the amount of materials and services required. In the past, small capacity blast machines were used. These units usually held between 600 and 1,000 lbs. of abrasives with a maximum Shall abrasive storage hoppers of 3 to 5 ton capacity were placed overhead by crane or forklift. These timers require replacement at least once a day, and often more frequently. If a crane or forklift was not available to lift the hoppers, the machines ran out of abrasives, and the result was wasted manpower and lost prduction. On jobs which required large amounts of blasting, the use of these machines resulted in very loW productivity and high abrasive resulting operating time of about 30 minutes.
6.1
consumption (spillage).
This size equipment should only be used for
light-duty jobs requiring minimum blasting. Larger capacity, bulk abrasive blasting machines with are now available.
q Large
mutliple outlets
The main design features of these machines are:
abrasive capacity which allows extended Periods of uninter-
rupted operation.
q Bulk
pneumatic refilling from delivery trailers. completely sealed
system which provide weather protection for abrasives.
q Multiple-nozzle
outlets and fast equipment set-up.
q Unattended
machine operation.
These design features permit less dependence on crane service, less abrasive consunption, less labor, faster set-up and cleaner operation with little spillage. The basic machine has a single chamber With operates 2 to 8 outlets. Both portable or stationary models are available with capacities from 6 to 40 tons. These machines are commonly manufactured in three sizes:
q q q
6-ton 22-ton 40-ton See Figures 6.1
Each machine is designed for a specific application. and 6.2.
The 6 to 8 ton unit can be mounted on wheels or skid. A 22-ton machine is supported by legs and is basically portable. The 22-ton units are primarily used for larger spot-blasting jobs which require several operators working in a central area. This machine is particularly well-suited for spotblasting contaminated areas on new fabrications. Forty-ton units
6 2
are usually used for stationary blasting
projects in Which the work pieces
are transported to the blast area. These wits provide sufficient storage capacity for several operators and are often used when high production rates are required. For large tank blasting jobs, or for external hull work, the 40-ton units can be mounted at the head of a drydock or aboard . ship In addition to single-chamber blasting machines,
another type of system has been developed for large abrasive blasting requirements such as cleaning multiple tanks or huge repair jobs. This unit is a double-chamber system Which fills automatically from overhead storage tippers. While maintaining the bottom chamber constant pressure, the top chamber can be depressurized and filled with abrasive. The abrasive will be automatically transferred to the lower chamber when the top chamber is closed. The blast operator, is never stopped because of a lack of abrasives. This unit is especially recommended for the Shipyard which is pursuing internal tank blasting contracts. See Figures 6.3 and 6.4.
Full Figure 6.3:
Filling
Schematic of Double-Chamber Automatic Filling Principle 6 4
In nest cases, the blast machine should be located as close to the work . area as possible to avoid air pressure drop through the blast hose. It is important to note flat the properly sized blast hose and nozzle is essential to the operation.
Figure 6.4: Double-Chamber Automatic Filling Multiple Outlet Blast Machine (Photo courtesy ofCAB Inc.,)
6.5
One method of locating the blast equipment close to the job site is to use a mobile, self-contained blast and recovery,trailer mounted system. Figure 6.5 is a picture of an existing mobile unit. This system is designed to recirculate steel abrasives. Refer also to Figure 6.6 which is a schematic which demonstrates one possible use for such a machine.In this system grit is cleaned (A) by means of a pneumatic separator. clean material falls into storage hopper (B) Dirty airborne dust is exhausted from can system. From the storage hopper, abrasive is directed into an automatic filling two-chamber blast machine (C) The abrasive is then transfered under pressure through the blast hose to the work area (H) Spent abrasive is manually vacuumed utilizing the vacuum (E) mounted on the trailer. Abrasive is deposited into a automatic dumping machine (1)) which directly transfers collected abrasive back into the pneumatic separator (A). For doing external work a special staging is required which will collect all the abrasive rebounding from the work surface. The blaster (B) stamps on a grated floor which permits the abrasive to fall through into a collection hopper. There, it is automatically collected by vacuum hose and returned to the trailer. Clean air is directed into the enclosure through a vent there dirty air is removed by an exhaust fan (F). when selecting and utilizing blasting equipment, the following items should be noted:
q
A single operator should be able to blast 100-250 sq. ft. per hour (depending on the condition of the steel surface) . Each 1/2” nozzle in operation will require approximately 1,500 pounds of abrasive (sand or slag) per hour. Each nozzle will require approximately 300 CFM at 110 pounds per square inch (psi) .
q
q
6.6
Figure 6.5:
Mobile Steel Grit Blasting and Recovery System (Photo courtesy of CAB Inc. )
Thus selection of new eqquipment must be based on:
q Existing
compressed-air volume and pressure capabilities. storage capacity
q Abrasive
q Crane
capacity delivery schedule frequency and location of blasting jobs.
q Abrasive
q Number,
6.3
Compressed-Air Drying Equipment Compressed-air drying equipment is required to remove impurities from
the compressed air system. Contaminants which
normally
enter the system
include moisture and dirt from the ambient air, and oil from the compressor itself. As the air is compressed, these impurities combine to form an extremely dirty and corrosive mixture. The resultant contaminated air
6.7
drastically reduces the efficiency of the blast operation by clogging nozzles and depositing moisture and impurities on the tank surface. This cotiition will also contaminate abrasives and ruin steel grit abrasives. There are three types of compressed air drying systems:
q Deliquescent q Refrigerated q
Regenerate
For abrasive blasting operations, the deliquescent system provides the most trouble-free solution to cleaning and drying the air. In addition, it Highhas the lowest initial cost and is least expensive to maintain. volume units are available Which are constructed with liftirg eyes to permit easy relocation. See Figure 6.7.
FIGURE 6.7
Refrigerated units represent a complex systems which requires qualifid service personnel to assure dependable year-round operation. This unit is not well-suited for portable applications, or for use in areas where dirty, dusty air will contaminate the filter and condenser fins. For these reasons, refrigerated systems should only be installed in permanent indoor locations.
6 9
The regenerative system also requires qualified service personnel for maintenance. These units do not remove oil without the addition of pre- and and are not designed for portable application without modifications for protection during handling. Selection of appropriately sized equipment is based on the total equipment CFM requirement. A practical guide is to assume a service factor of 300 CEM delivered at 110 psi per blaster. If a central compressor is used to distribute air throughout the yard, the CEM delivered to any given point will not exceed the amount that is Passed through the orifices in the blast nozzles. The CFM per nozzle can thus be used to estimate the total CFM of required compressed air. Optimun utilization of the deliquescent dryer requires large volumes of air to be processed at high pressures. Therefore, it is imprtant to measure air pressure available at the points Were the dryers might be installed prior to ordering a system. As an example, a unit which is capable of processing 2,300 CEM at 125 psi may only process 1,550 CFM at 80 psi. The location of the dryer depends on the compressed air distributing System, (i.e., portable or stationary). Since all air will be used outdoors, the dryer must also be located outdoors. The unit should be placed in the coolest area possible to avoid radical changes of temperature between the drying point and use point to prevent condensation in pipes downstream of the dryer. In warm climates, When the temperature of the compressed air exceeds 100°F, an air- or water-coaled after-cooler is required to reduce the temperature of the compressed air. The chemical in the deliquescent dryer will be subject to deterioration in direct proportion to the rise in temperature over 100°F. For portable applications, it is best to locate the dryer within 50’ of the blast machine. This gives the air an opportunity to Coo1 in the air lines before entering the dryer. In addition the dryer will catch any contaminants which might have entered into the system.
6 10
after-filters,
Since abrasive blasting requires large volumes of air, often in surges, an air reserve tack is recomended. This tank should be at least equal in size to the CFM usage and be installed downstream of the deliquescent unit. Normally, a receiver is placed ahead of the dryer to allow additional cooling exposure before entering the dryer. If an after-cooler is used on a portable basis, it should be placed in a location that will insure maximum ambient air flow around the unit. Often times a surplus heat exchanger may be used as an after-cooler. Air is circulated through the tubes while the unit is immersed in the water (Figure 6.8 ). Compressed air is passed through a submerged heat exchanger to cool it
and to remove the moisture from the air stream. The air is then passed into a chemical deliquescent dryer for final removal of contaminants. Compressed air drying equipment offers the final assurance for proper surface protection and coatings application. It also eliminates plugging of abrasives in the blast machine caused by moisture. 6.4 Abrasive Delivery and Storage In the past, abrasives were delivered to the shipyard and distributed to the blast machine hoppers in a variety of ways from hundred pound bags to railcars. Today, the most efficient method for distributing abrasives to blasting locations within the shipyard is by pneumatic delivery trailers. These units are operated at low pressure, and transfer the abrasives pneumatically through a discharge hose to the blast machine or storage hopper. See Figure 6.9. or clogging
FIGURE 6.9:
Pneumatic Delivery Truck 6.12
The main advantage of this trailer unit is mobility and the direct transfer of the abrasives for use in blasting. If a local supply of abrasives is available, and if the blasting equipment is accessible to the trailer, materials can be picked up and transferred directly to blast machines. If the supply source is distant, installation of a large-capacity bulk storage hopper should be considerd. The storage hopper can be loaded directly from a railcar or truck. Depending on the quantity of materials used and the time needed for replacement materials to arrive, this storage hopper should have a capacity of from 500 to 1000 tons. See Figure 6.10. Large sealed portable hoppers should also be made available for
installation in areas Were frequent blasting takes place, or where the pneumatic trailer cannot reach blast units. These units should be sized as large as possible without exceeding the lifting capacity of available cranes. The hoppers can be filled by the pneumatic trailer and then lifted by crane and positioned over the blast units. By decreasing the number of lifts required, these large hoppers can reduce the number of times cranes are required. Distributing and storing abrasives in completely enclosed pneumatic delivery trailers and storage tippers also reduces the possibilities of spillage and moisture contaminations. If delivery and storage in large bulk is feasible, purchase of a yard-owned pneumatic trailer will reduce the overall cost of the operation. Materials can be bought in bulk and the laker required for handling and distribution will be reduced. 6.5 Abrasive Recovery Equipment 6.5.1 Selection Criteria After a ship tank has has been blast cleaned, spent abrasives, abrasive dust, and paint chips must be removed in order to ready the tank for painting. Usually this is accomplished by vacuum machines which suck the particles and other debris out of the tank through a flexible hose.
6.13
There are three different types of vacuum recovery machines available:
q q q
Portable unit with single-chamber collection tank Portable unit with automated discharge tank Mobile truck unit with single-chamber collection tank
In selection a vacuum recovery system for shipboard tank blastcleaning, the following criteria Should be considered:
q Equipment q Support
operation with reduced labor
services requirments
size in relation to available space in the work area q Hose size needed to operate at maximum efficiency
q Initial
q Equipment
and maintenance rests
6.5.1.1
Portable Unit with Single-Chamber Collection Tank
The portable single-chamber vacuum recovery tank is designed to operate unattended and can be located close to the worksite. This unit is equipped with an easy to handle, flexible 4“ hose. The unit does not use fabric dust filtration media, and the vacuum producer can be separated from the collection tank. These equipment characteristics allow for flexibility If the unit is to be positioned on deck, a crane is usually required for placement. Suction is created by a high-performance liquid ring-type vacuum purpose. The average abrasive removal rate of the unit is ten tons of abrasive debris per hour.In addition, this type vaccum pump can handle large amounts of dust particles which carry over from the secondary dust cyclone tank (larger particles settle out) . The unit is powered by a 50 to 70HP motor and requires water and electrical service hook-up. Both equipment maintenance and initial crest are low ($25,000). See Figure 6.11. in setting up the vacuum system.
6.15
Figure 6.11:
Two Portable Vacuum Units with single-chamber Collection Tanks hunted on Stands
6.16
Portable single-chamber units can be hooked up to a pneumatic discharge device if the material is to be disposed directly from the area without using a tank . The collection tank of the portable single-chamber unit can be placed a an elevated platform so that a dump-truck can pick up the abrasives for disposal or recovery. If abrasive recycling is desired, the collection tank insures that the recovered abrasives are protected for reuse. 6.5.1.2- Mobile Unit with SingleChamber Collection Tank This unit is permanently truck-mounted for mobility. The unit has an
average production rate of ten tons per hour. Performance is increased by moving the unit closer to the job site which is sometimes difficult in shipyard operations. Some units are capable of removing water and other fluids. This system is designed to operate with a 6“ to 8“ hose and is equipped with a positive displacement vacuum pump. Being mobile, this unit requires an attendant. The power take-off unit which is driven by the truck engine requires increased maintenance, especially men used on a continuous basis. The truck system can only be operated for short periods at a time before being shut down and driven away for disposal. The initial cost of the mobile unit is high (about $125,000) See Figure 6.13.
Figure 6.13:
Mobile Vacuum Recovery Truck with Single+Chamber Collection Tank
6.18
Although the mobile unit is suited for some for most internal tank-cleaning jobs.
shipyard applications,
maintenance requirments and short-cycle performance make it impractical .-
6.5.1.3 Portable Unit with Double Chamber, Automatic Discharge Tank This unit can be moved to an area where needed for abrasive recovery. No attendant is necessary and each component can be separated. It is designed to operate with a 4“ to 6” hose and has an average obtainable production rate of ten tons per hour. Like the portable single-chamber units, the initial cost is relatively-low (35,000 - $40,000); however, the dust collection filter must be periodically replaced. Suction is produced by a positive displacement, rotary Vacuum pump. These pumps are subject to damage should any dust carry-over from the air filter system. The pump can be troublespome if not prperly maintained. The large number of moving parts are an additional disadvantage resulting in increased maintenance rests See Figure 6.12. The portable unit with double-chanber automatic discharge tank is not suited for must tank vacuuming. Debris removed along with the ebrasive cannot pass through the discharge valves and may lodge between the valve and seat, resulting in a vacuum leak.
-
6.19
6.
Dehunidification units are skid mounted for placement aboard ship (also see figure 5.5). .-
7.
The facility should also contain a high pressure water main for wash-down (not shown) .
8.
A duct storage cradle should be used to move large amounts of duct without damage (not shown)
9.
Dehumidification duct should be prefabricatd in sections to accomodate long runs with minimum set-up time.=
10. 11.
Dust collection duct
is also prefabricate.
A 5000 gallon #2 diesel fuel storage tank is located adjacent to compressor station.
12•
The air compressor station must have sufficient capacity to provide compressed air to 16 blasters plus additional gear. A 6000 CEll compressor station is shown. This station is equipped with a water cooled aftercooler to insure that the temperature of the compressed air entering the dryer is less than 1000F. This compressor is diesel powerd to facilitate portable application.
13. 14. 15•
Pier management and quality control are located in an office on site. A portable crane is necessary to provide lifting services. A shelter can be used to provide protection for inclement weather the sun. and
16.
A staging platform cradle can be used to enable the crane to lift larger amounts of scaffolding (not shown) .
Figure 7.3 is a photograph which shows a ship docked at the abrasive blasting and coating pier. The portable dehumification units are located
7.2
midship with prefabricated ducts extending forward and aft. Flexible ducts are connected between the prefabricated side. There are thee difference ducts. ducts and the cargo access ports (hatches). The dust collecting duct can be seen midship on the port (water) Each duct haS two 90° turns which
positions the discharge end OVer the side of the ship and into barge mounted dust collection equipment. Four each vacuum recovery units are mountedon the dock on the starboard (land side) side of the ship. Figure 7.4 contains a schematic of a unit coating container. These containers are designed such that all painting equipment and materials necessary for a specific area are kitted prior to starting the job. The container is then positioned as close to the actual operation as possible. The unit matings container provides a clean, sheltered work center. Figure 7.5 is a cross-sectional drawirg of a large ship cargo tank The cargo tank covers have been removed for personnel and equipment access. In some cases equipment is placed directly over the openings. Dirty air (detail A) is extracted from the tank internal by duct work In most all cases efficiency of the from the intake side of a fan. ventilating process is greatly improved by running the duct work the shortest possible distance. Even when a penetration through the side shell is required, the procedure is generally less expensive than running ductwork through the ships interior. A dust collector should be used to clean the dust-laden air before being exhausted. Clean air can be routed into the work space from a penetration in bulkhead of an adjacent tank or be directed into the tank parallel to the dirty air exhaust duct. If a dust collector can be conveniently located pierside, substantial saving in set-up time
H
be achieved with a mobile unit.
On projects or climates which require dehumidification (detail B) either a mobile or portable unit may be used. If the volume of air required is too large for a single III unit the the DH air may be recoverd and recirculated by cleaning the dirty exhausted air by a dust-collector. This process of recirculation enables the dew point to be continually controlled in the work space utilizing a minmum of DH.
7.6
For an area requiring a large air volume (detail C), fresh ambient or treated air may be introduced into the tank through the car~ hatch cover. Dirty air should be extracted near the bottom of the tank and conveyed by duct through fans and collected on the ship’s deck. Sometimes large access holes are required in the deck. In these cases, special openings should made for duct access as well as for personnel lifting devices. Figure 7.6 is a close-up drawing of a ship tank area. Limited access into the area can be improved with penetrations thru bulkheads, side shell, and/or decks. Fresh ambient or conditioned air should be introduced at the highest access point. The eXhaust intake should be at the lowest point
.-
within the tank and on the opposite side from the fresh air inlet. Blind spots of stagnant” air inside tank can be prevented with proper placement of duct . Consideration should always be given to recycle treated air as dotted line indicates. When performing blasting and painting operations in closed tanks, several safety points must be remembered. All abrasive blast equipment, operators, nozzles and the object that is being blast cleaned must be grounded. Procedures must be established to control entry into tanks and enclosed areas until such time as the area is declared “gas free” by a trained and certified competamt person. For tanks that have been in service, this should be accomplished prior to initial entry, after Work breaks and prior to the start of a new shift. All lighting must be explosion proof. At least one person should be positioned outside the tank, adjacent to the access in case of energency.
7.7
PROCEDURE HANDBOOK SURFACE PREPARATION AND PAINTING OF TANKS AND CLOSED AREAS
SEPTEMBER 1981
Prepared by: COMPLETE ABRASIVE BLASTING SYSTEMS, INC. IN COOPERATION WITH . AVONDALE SHIPYARDS, INC.
Report Documentation Page
Form Approved OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
1. REPORT DATE
2. REPORT TYPE
3. DATES COVERED
SEP 1981
4. TITLE AND SUBTITLE
N/A
5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER
Procedures Handbook Surface Preparation and Painting of Tanks and Closed Areas
6. AUTHOR(S)
5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Naval Surface Warfare Center CD Code 2230 - Design Integration Tools Building 192 Room 128 9500 MacArthur Bldg Bethesda, MD 20817-5700
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
8. PERFORMING ORGANIZATION REPORT NUMBER
10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release, distribution unlimited
13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF:
a. REPORT b. ABSTRACT c. THIS PAGE
17. LIMITATION OF ABSTRACT
18. NUMBER OF PAGES
19a. NAME OF RESPONSIBLE PERSON
unclassified
unclassified
unclassified
SAR
123
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
FOREWORD
This research project was performed under the National Shipbuilding Research Program. The project, as part of this program, is a imperative cost shared effort between the Maritime Administration and Avondale shipyards, Inc. The development work was accomplished by Complete Abrasive Blasting systems, Inc. under subcontract to Avondale Shipyards. The overall objective of the program is improved productivity and, therefore, reduced shipbuilding costs to meet the lower Construction Differential Subsidy rate goals of the Merchant Marine Act of 1970. The studies have been undertaken with this goal in mind, and have followed closely the project Outline approved by the Society of Naval Architects and Marine Engineers’ (SNAME) Ship Production Comittee. Mr. James A. Giese, of Camplete Abrasive Blasting Systems, served as Project Manager and Ms.polly Medlicott as technical writer. On behalf of Avondale Shipyards, Inc., Mr. John Peart was the R & D Program Manager responsible for technical direction, and publication of the final report. Mr. Ben Fultz of Offshore Power Systems performed editorial services. Program definition and guidance was provided by the members of the 023-1 Surface Preparation Coatings Cammittee of SNAME, Mr. C. J. Starkenburg, Avondale Shipyards, Inc., Chairman. Also we wish to acknowledge the suppxt of Mr. Jack Garvey and Mr. Robert Schaffran, of the Maritime Administration. Special thanks are given to the numerous suppliers listed below for their valuable contribution of information (see Annex A for complete address ad telephone numbers). Aeroduct-Porter Company Aerovent, Inc. Air pollution Systems, Inc. American Air Filter Ccmpanyr Inc. American Coolair Corporation Anaconda Metal Hose Bry-Air Cargocaire Engineering Company Carter-Day
Central Engineerirq, Inc. (Vac/All) Clemco Industries Cleveland Metal Abrasives Cincinnati Fan and Ventilator Company, Inc. Complete Abrasive Blasting Systems, Inc. (CAB) Coppus Enqineering D.P. Way (Ultra Vac) Dryomatic Enpire Abrasive Equipment Corporation Flexaust Flint Abrasives General Air Division Zurn Industries H.B. Reed and Company, Inc. Hartzell Propeller Fan Company IRS/International, Inc. Kathabar - Medland Ross Key-Houston, Inc. Pauli and Griffin Pure-Aire, Inc. Strobic Air Corporation Super Products (Supersucker)Torit Division, Donaldson Company, Inc. Unimin Coloration United McGill Corporation Vacublast Corporation Van Air Systems, Inc. W.W. Sly Manufacturing Company Wedron Silica Company Wheelabrator-Frye, Inc. Whitehead Brothers Company
ii
Executive Summary A desperate need exists in shipyards for the proper planning enclosed areas and
execution of surface preparation ad coating operations in tanks and other
q
Abrasive blasters and painters are exposed to high
Other shipyard concentrations of dust and hazardous organic vapors. personnel are exposed to the potential dangers of explosion and fire. Another aspect of the need for better planning concerns the inefficient utilization of capital, manpower and material assets. AS an example, many extra manhours of labor are consumed in tank surface preparation operations because the abrasive blaster, when operating in tanks, just cannot see what he is blasting due to dust accumulation. Also, many square feet of painted surface are lost due to solvent entrapment during cure resulting in catastrophic peramature paint failure. Until the publishing of this report no single document existed With could be used by shipyard planners to effectively, efficiently and safely plan painting operations in confined areas. The information contained within this handbook includes:
q
Identification of the requirements and related problems associated with surface preparation and painting of tanks an enclosed areas. Identification of personnel exposure limits Identification of monitoring equipment for measurement of fume and dust concentrations and ventilation rates. Identification of maximum allowable concentrations and ventilation requirements for abrasive blasting and coatings application Identification of suitable ventilation and abrasive blast equipment for shipyard operations. In addition to the abve pints, a practical model for upgrading the
q
•
q
q
blast-paint department is offered. Throughout the course of this study, emphasis was placed on increasing productivity and improving enviromnental conditions. These pints can be achieved through a management sponsored systematic program of planned improvements based on” recomendations within this report.
iii
TABLE OF CONTENTS Page -i iv v vi 1. Conclusions 1.1 The Role of Managaement 1.2 Recommendations 1.3 Cost Savings 1.4 Summary 2. Use of the Handbook 3. Ventilation 3.1 Introduction 3.2 Technical Discussion 3.2.1 Ventilation During Abrasive Blasting 3.2.2 Ventilation During Painting 3.2.2.1 Lower Explosive Limit 3.2.2.2 Explosive Vapor Detection 3.2.2.3 Threshold Limit 3.3 Equipment Selection 3.3.1 Fans 3.3.2 Ducting 4. Dust Collection 4.1 Introduction 4.2 Technical Discussion 4.3 Equipment Selection 5. Dehumidification 5.1 Introduction 5.2 Technical Discussion 5.2.1 Principles of Condensation 5.2.2 Determining Dehumidification Requirements 5.3 Selection of Dehmidification Equipment 6. Abrasive Blasting 6.1 Introduction 6.2 Abrasive Blasting Equipment 6.3 Compressed-Air Drying Equipment 6.4 Abrasive Delivery and Storage 6.5 Abrasive Recovery Equipment 6.5.1 Selection Criteria 6.5.1.1 Portable Unit with Single-Chamber Collecticm Tank 6.5.1.2 Mobile Unit with Single-chamber Collecti= Tank 6.5.1.3 Portable Unit with Double-Chamber Automatic Discharge Tank 7. Model High Production Abrasive Blasting and Coating Pier 1.1 1.1 1.2 1.3 1.5 2.1 3.1 3.1 3.1 3.1 3.3 3.5 3.8 3.10 3.12 3.12 3.18 4.1 4.1 4.1 4.2 5.1 5.1 5.1 5.2 5.10 5.14 6.1 6.1 6.1 6.7 6.12 6.13 6.13 6.15 6.18 6.19 7.1
Foreword
Annex A - Suppliers List Annex B - Selection of Abrasives Annex C - Abrasive Cost Comparison iv
LIST OF FIGURES Page 3.1 3.2 3.3 3.4 3.5 3.6 3.7 4.1 4.2 4.3 4.4 4.5 5.1 5.2 5.3 5.4 5.5 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 7.1 7.2 7.3 7.4 7.5 7.6 Explosive Vapor Detector Schmatic of Centrifugal and Axial Fans Schematic of Axial Fan Centrifugal Fan Duct-Axial. Fan Branch Entry and Elbow Radius Design for Dinting Layout Ventilation Diagram of Large Enclosed Spaces, Small Tanks, and Multiple Tanks Schematic of Venturi Wet Scrubber’ Venturi Wet Scrubber Dust Collection -Unit Reverse jet Continuous Duty Dry Fabric Collector Unit Mobile Dust Collection/Dehurnidification System Mobile Dry Cartridge Dust Collection System Battery-Operated Psychron Magnetic Surface Thermameter Sample Psychrometric Report Form Schmatic Dry HoneyCcmbe Dehumidification Principle Model HC 9000 SEA Special Dry HoneyCombe Dehumidification Unit Portable Single-Chamble Multiple Outlet Blast Machine Single-Chamber Multiple Outlet Blast Machine Double-chamber Automatic Filling Principle Double-chanber Automatic Filling Multiple Outlet Blast Machine Mobile Steel Grit Blasting and Recovery System Schematic of Mobile Grit Blasting and Recovery System in Operation Compressed-Air Dryer and After-Cooler Model for Compressed-Air Drying System Pneumatic Delivery Truck Delivery and In-plant Distribution System for Abrasives Two Portable Vacuum Units with Single-Chamber Collection Tanks Mounted on Stand Portable Vacuum Units with Automatic Discharge Tank Mobile Vacuum Recovery Truck with Single-Chamber Collection Tank Panoramic View of Model Blast and Coat Pier 1000 Ton Abrasive Storage Hopper View of Ship Deck with Properly Installed Equipment Schematic of Unit Coating Container Cross-section Drawing of Ship Cargo Tank with Blast-Coat Equipment Installed Drawing of Tank Blast-Coat Operation 3.93.14 3.14 3.15 3.15 3.20 3.24 4.3 4.4 4.5 4.6 4.6 5.5 5.5 5.6 5.15 5.18 6.3 6.3 6.4 6.5 6.7 6.8 6.9 6.11 6.12 6.14 6.16 6.17 6.18 7.3 7.4 7.5 7.8 7.9 7.10
v
LIST OF TABLES Page Table I Table Table Table Table Table Table Table Table II III IV V VI VII VIII IX Ventilation Volumes Recomended to Maintain Solvent Vapor Concentrations below 10% of the lower Explosive Properties of Common Solvents Paint Vapor Concentration versus Ventilation Volume Friction Loss Per 100 feet of Ducting Area and Circumference of Circles Quick Dewpoint Reference Table Wet Air Factor Dehumidifier Moisture Remove Rate Comparison of Wet and Dry Desiccant Dehumidifier Units Comparison of Typical DH Units
3.4 3.6 3.11 3.22 3.26 5.3 5.12 5.13 5.16 5.17
Table X
vi
1.
Conclusions
1.1 Background The advent of huge, complex ocean-going vessels represents millions of dollars in capital investment. Corrosion prevention through blast-cleaning and painting is essential for protecting the value of these ships as capital assets and for prolonging the productive life of the vessels. Yet, for the most part, few guidelines exist for planning critical protective coatings (painting) operations during new construction, especially in high performance areas, such as ballast tanks and enclosed areas. Without exaggeration the blast-paint operation at sane shipyards can
be characterized as the dirtiest, most disorganize, wasteful and even dangerous area in the yard. These conditions many times result from a lack of guidance concerning basic principles and apparent lack of knowledge concerning available technology and equipment. The net result is a staggering wastage of manpower, materials, and time. An attempt to dispense with in-house painting operations by subcontracting blast-paint operations only provides a Short term solution, since responsibility for coating failures or production delays ultimately rests with shipyard management. The only possible long-term solution to these problems is to approach the surface preparation and coatings operation as a unified system. An experienced professional manager, using a systems approach to planning and Coordination of the total program, can:
q Modernize q Reduce q Improve
equipment
dependency on other services environmental renditions
1.1
The task of converting the blast-paint section into a profitable, productive and clean department must become a high priority for managers of U.S. shipyards. Economical modernization of this operation can be accomplished by otherwise successful companies. clearly, management plays a critical role in the development of a professional, efficient, surfacs preparation and coatings department. 1.2 Project Results This project achieved the defined objective of creating a procedural Handbook detailing ventilation rates surface preparation and painting and procedures required for the of tanks and enclosed areas. This accom-
plishment is a step toward solving the problem areas discussed above. The handbook on Surface Preparation and Painting of Tanks and Enclosed Areas provides a tool which can be used by shipyard personnel to:
q Reduce
labor hours for both blast-paint operation and for support
services and equipment.
q Write
procurement specifications for capital equipment procurement
q
Reduce worker exposure to hazardous conditions Reduce facility and equipment losses more competitive painting catastrophic paint operations
q
q Plan
q Reduce
failures
q Reduce
interference between crafts during construction
The net result will be a savings in dollars expended to produce ships.
1.2
1.3 Recommendations 1.3.1 Blast-Paint Department Management should commit high-caliber, technically capable personnel at all stages
to the program to insure competency, efficiency and quality of the operation. These personnel should include:
qA
surface preparations expert trained in quality control to coordinate between the shipyard and the ship owner. This individual
would also be responsible for the inspection of cleaned surfaces and for monitoring dust-collection and dehumidification systems.
qA
coatings specialist (ideally a chemsit) to review coating specifications, oversee application, sample coatings both at delivery and on finished surfaces, and maintain Ongoing data records of the coatings performance under actual shipyard condiitions. This individual would aid in the selection of appropriate coatings and preclude legal applications arising from coatings failure.
q An
instructor for an in-house program to train
employees in the use
of blasting, ventilating, dehumidifying, painting and compressedair drying machinery. General, components of the blast-paint operation which fully considered by management are:
q Development
should be care-
of an overall organizational plan of a program check list to include all equipment of procedures and inspection techniques
q Develooment
q Standardization
q Establishment
of a comprehensiver equipment maintenance program
1.3
q Coordination
of transportation, delivery and storage of materials, to include support logistics
In the drydock area, modifications might include such things an endramp access so that equipment could be moved in and out without a crane, an and deck, increased elevator or other personnel lifting system between dock system.
electrical services and installation of a high-volume compressed air piping Such improvements would result in a marked reduction of down-time during the blasting and coating operation. Finally, a carefully designed permanent installation is (see Section 7) practically a must for the efficient completion of major jobs. The essential elements of a Properly designed facility are: Large, enclosed space providing protection from the weather Equipment to control ambient air renditions Adequate utility hook-ups for electrical, water, compressed air and other services Permanent, properly designed ventilation system State-of-the-Art abrasive blasting and handling machinery permanently installed for maximum output Railroad track locatd next to the shelter for materials and equipment transport. Section 7 discusses one way of establishing a well-organized operations base for large blasting and painting jobs.
1.4
1.3.2 Naval Architects and Marine Engineers Naval architects and marine engineers must be the aware of problems faced by the shipbuilding/ship repair industry and encouraged to incorporate design changes which facilitate construction activities. Some suggestions are:
q Constructing
permanent scaffolding supports in tanks
q Placing
permanent openings in bulkheads larger, more conveniently located hatch or cargo covers
q Providing
on deck. These changes vaild greatly improve materials and personnel access for future maintenance activities. 1.4 Cost Savings By using the handbook published as a result of this study and by systmatizing the blast-paint operations as recommended, shipyards should save 30% to 50% of blast-paint operational rests. Generally, cost-savings will result in the following areas:
q Reduction
of support services required. By utilizing the proper
equipment and by making recommended modifications to existing facilities, dependence on support services would be significantly reduced. LOst prduction time waiting air hookup, water, etc. ) would
q improvement
on required services (cranes,
be eliminated. Many costly problems and
of environmental conditions.
delays are used by the messy, dirty conditions associated with the blast-paint operation. These include contaminated air, high worker turn-over, non-compliance with governmental health and safety regulation, disposal of wastes, and constant housekeeping.
1.5
Recovery and reuse of abrasive. Specialized equipment can enable the department to utilize inexpensive abrasives for some jobs in addition to recovering and recycling more expensive abrasive-materials for other jobs. Reducing expenditure of rapidly consumed abrasives can add up to surprisingly large savings. (See Annex c).
Improvement of quality. Catastrophic coatings failures can obviously result in enormous costs for shipyards. A systematic approach to the total blast-paint operation, using proper equipment, correct procedures and careful record-keeping will assist in avoiding premature 1.5 Summary Preparing surfaces of enclosed tanks for coatings, including necessary ventilation and air treatment operations, is but one part of the construction and repair of a ship. However, it must be recognized, that these operations are just as essential as those performed by fabrication, mechanical or other shipyard manufacturing departments. The blast-paint department depends on many support services and a variety of specialized equipment to complete projects. Technology is available which can correct both the environmental and worker safety problems associated with abrasive blasting in shipyards. This technology can be expensive, but ignoring the problems will be more expensive. It is recognized that there are many possible ways to solve existing problems or meet defined objectives. This report provides one proposed process by describing equipment and by outlining procedures which are now available to the modern shipyard. paint failures.
1.6
2.
Use of the Handbook
An attempt has been made to organize this handbook in such a manner as to the reading easy and data presentation logical. The discussion proceeds from ventilation through dust collection and dehumidification to abrasive blasting. Section 7 discusses a model abrasive blasting and Painting pier which utilizes the principals presented. The sciences of ventilation, dust collection, dehumidification and abrasive blasting and painting are each extremely sophisticated engineering fields. This handbook will not qualify the reader as an expert in any of these disciplines, but it does present certain basic principles, that, then followed, will help assure a well planned operation. The reader should follow the presentation as written. If dust collection and/or dehumidification are not deemed to be required, then these sections can be scanned. However, be forewarned that a simple statement that these operations are luxuries and not necessary without verificaticn through actual measurement will lead to many disastrous experiences. Each section of the handbook maintains a technical discussion followed by equipment selection. The technical discussion includes examples and sample calculations. In many cases, a simple substitution of different numbers, depending on job size, is all that is necessary to obtain required planning factors. characteristics. The equipment selection discussion describes equipment Knowing the calculated planning factors and equipment
characteristics a lead to the proper equipment selection for a given blast-paint operation.
2.1
3. Ventilation 3.1 Introduction There are two primary proposes for ventilating tanks and enclosed areas :
q Operator
health and safety
•
Operator visibility
These purposes are accomplished by removal of contamination air from the space and replacement of fresh air to the space. a total air treatment system. 3.2 Technical Discussion The following sections present general guidelines for determining ventilation requirements. Later sections discuss the design of the air-handling system to meet specific ventilation objectives. Additional detailed design information is contained within Industrial. Ventilation - A Manual of Recommnended Practice. That manual, which is published by the American Conference of Governmental Industrial Hygienists and endorsed by the Sheet Metal and Air Conditioning Contractors National Association, can be obtained from the Committee on Industrial Ventilation P. O. Box 453, Lansing Michigan 48902. 3.2.1 Ventilation During Abrasive Blasting The amount of ventilation required during blasting depends on the following four variables. Percentage figures indicate the relative importance in calculating requirements:
q q q
Where dehumidification
and/or dust collection is indicated, ventilation is the basic component of
Size of tank (cubic feet ) Number of blast operators Amountof corrosion on tank surface
60% 15% 15%
3.1
q Dusting
or breakdown characteristics of abrasive
10%
(see Annex B for discussion of Abrasives)
Ventilation is measured in terms of the volume of air movement over time, expressed as cubic feet per minute (CFM) . A general guideline to. providing an adequate environment in closed tanks would be one (1) complete air change every three minutes during the blasting operation. For example, a centerline or “Jumbo” tank with a 100,000 cubic foot capacity would require. approximately 33,000 CFM of ventilation. Generally speaking, the greater the number of complete air changes, within reason, the better the resulting visibility in the tank. Any one of. the listed variables can significantly affect renditions inside the tank. For example, if the amount of dust beinq generated increases due to an excessively corroded tank surface and/or high abrasive breakdown, the supervisor can compensate for these coditions by changing one or more of the other variables. He may choose to decrease the number of blast operators, stop blasting and mechanically descale the tank to improve surface conditions or increase the amount of ventilation in the tank. Unlike ventilation for paint or welding fumes, dry airborne dust created by abrasive blasting consists of relatively large particles. Since the particles can be sea, it is easy to monitor the success of the A more detailed discussion of the ventilation system in removing dust.
ranges of abrasive breakdown characteristics, tank surface conditions and cleaned surface, standards will be described in Section 6, Annex B and Annex c. The balancing of in-g and outgoing air is an important aspect of a ventilation system. If clean air is blown into the tank while muchless dirty air is being extracted, the result is air turbulence. The dirty air will subsequently be blown out any crack or opening in the tank. Similarly, the extraction of too much air relative to treated incoming air will result in inproper dehumidification for condensation control. Air circulation balance is achieved then the total amount of incoming air, treated or untreated, equals the total amount of air being exhausted. Conditions within
3.2
the tank, i.e., visibility, temperature or humidity, are thus maintained within a predictable, controlled range of efficiency and in accordance with safety requirements. 3.2.2 Ventilation During Painting During painting operations in confined spaces, the air in these areas becomes laden with paint overspray and solvent vapor. The health and safety hazards presented by these conditions dictate that ventilation requirements be carefully calculated and subsequently monitored throughout the painting. operation. To better understand the calculation of ventilation requirements, the following two definitions are necessary: LOWER EXPLOSIVE LIMIT (LEL) : Ihe lower limit of flammability or
explosibility of a gas or vapor at ordinary ambient temperature expressed in percent of the gas or vapor in air by volume. THRESHOLD LIMIT (TL) : The values for airborne toxic materials which are to be used as guides in the control of health hazards and represent time weighted concentration to which nearly all workers may be exposed 8 hours per day over extended periods of time without adverse effects. Whereas regulatory requirements dictate that the ventilation volumes be sufficient to dilute solvent vapor to at least 25 percent of the lower explosive limit of the specific solvent being sprayed, 10 Percent is a more commonly used design factor which insures explosion and fire prevention under varying conditions. Table I contains ventilation volumes recommended to maintain solvent vapor concentratons below 10 percent of the LEL for representative tank volumes.
3.3
Table I Ventilation Volumes Reccommended To Maintain Solvent Vapor concentrations Below 10% of the Lower Explosive Limit.
Tank Volume (Cu. Ft. ) 670 1,340 2,000 2,800 5,600 8,400 11,200 14,000 28,000 56, 000 84,000 112,000 168,000
Ventilation Volume (CFM)
1,000
1,200 1,500 2,000 2,500 3,000 4,000 5,000 6,000 10,OOO 15,000 20,000 30,000
3.4
In addition to safety factors, paint overspray can accumulate in enclosed tanks a blind workers wiht a dense particle fog. As in blasting, a relatively large Volume of ventilation ity and insure production efficiency. It is important to note that the ventilation objective for abrasive blasting recommended in Section 3.2.1, (approximately one air-change every three minutes) will, in most oases, maintain solvent vapor concentrations below the required percentage of the lower explosive limit, as well as maintain good visibility. By using the guidelines contained within this handbook and by requiring workers to USe respirators for painting, the sane ventilation system can, in most oases, be utilized for both blasting and painting operations. It must also be remembered that ventilation requirements extend through the paint curing process. The next two sections contain information on how to calculate LEL and TL. Table II contains current information on the LEL and TL for some common Since these limits are subject to change, the latest Federal solvents. Regulation should be used to calculate actual requirements. 3.2.2.1 Lower Explosive Limit Most paints used in marine applications contain solvents which rapidly evaporate during spraying. As stated above, sufficient air must be extracted from the tank during painting to limit the concentration of the flammable solvents to no more than 25% of their lower explosive limit (LEL) . The following example is used as a guide in demonstrating the principles involved in calculating required. ventilation volumes for specific solvents. Toluene is selected as the representative solvent. is necessary to maintain visibil-
3.5
Step One - Calculate Dilution Volume The minimum amount of air required (dilution volume per gallon of solvent, in cubic feet) is obtained from the following equation, where. vs is the cubic feet of vapor per gallon of solvent: 4 (100-LEL)VS Dilution Volume (cu. ft.) = By selecting the appropriate values for LEL and vs from Table 11, the dilution” volume required per gallon of toluene solvent is calculation as
Dilution Volume = 4(100-1 .4)30.4 1.4 = 8,564 cu. ft. of air per gallon of toluene Step Two - Calculate Ventilation Volume The required Ventilation Volume, in CFM, is found by multiplying the dilution vol- per gallon of solvent by the number of gallons of solvent evaporated per minute. Ventilation Volume ( CFM) = Dilution Volume (cu. ft. ) x gal ., of solvent gal. of solv. evap. nun.
In our example, several workers are painting in an enclosed tank. They are applying toluene thinned paint at a combined rate of one gallon per minute (gpm). The paint is 40% solvent. The ventilation volume required to maintain the solvent vapor concentration in the tank safely below the LEL is calculated as follows: Ventilation Volume = 8,564 cu. ft. gal . of Solv . x l gpm paint x O.4 gal solvent 1 gal. paint
3.7
Ventilation Volume =
3,426 CFM (for toluene)
This ventilation volume is the minimum amount required to prevent the hazardous accumulation of flammable paint vapor. The important factors to remember in determining the minimum ventilation volume to prevent explosions are: o % T h e rate at which the paint is being applied (gallons per minute), o The amount of flammable solvent in the paint. Tank size is not the controlling parameter. However, in larger tanks a
greater amount of paint vapor would probably be generated due to the increased number of workers. Water-based painting requires almost no dilution volume to prevent explosion since these paints contain only 1% to 2% flammable solvents. 3.2.2.2 Explosive Vapor Detection Two basic types of devices are used for explosive vapor detection. The type primarily used in the petrochemical industry is equipped with a heated catalytic element which is a possible source of ignition. As a safety measure, the element is protected by a fine mesh “Davy” screen that prevents flame propagation. Temperature of the heated element increases during exposure to a flammable atmosphere resulting in degradation of the sensing element. ThiS characteristic necessitates frequent recalibration. When located in an area where paint can deposit on the sensor, an additional problem is created. The fine screen is readily clogged by paint which requires frequent removal for cleaning. The detection principle reccomended for shipboard tank applications uses a “cold sensor” whihc does not degrade with time or exposure to flammable Vapors. No protective screen is used. The sensing element housing protects the instrument from physical damage. Sensitivity to paint solvents
3.8
is god, and the electronic alarm circuitry is simple and rugged. Since the detection element is not heated, power consumption is much lower than with heated element types. Portable battery-operated units can operate units several days before requiring recharging. See Figure 3.1. Simple construction tion and operation make this instrument suitable for fixed installation such as hood exhausts or duckwork which are not accessible for service and maintenance. The use of these instruments and the determination of hazardous conditions should be restricted to individuals trained certified as 'Competent Personnel'.
Figure 3.1: Explosive Vapor Detector
3.9
3.2.2.3 Threshold Limit Limiting the flammable paint vapor concentration to 25% of the LEL is sufficient to prevent explosion hazard, but this concentration is too high -for workers to breathe. Additional ventilation must be provided to reducethe paint solvent vapor concentration below the maximum levels allowed for workers on a routine basis. This concentration, called the threshold limit (TL), varies with the individual solvents used. A listing of the values for various solvents is contained in Table II. The dilution volume per gallon, of solvent required to maintain a concentratiocn below the threshold limit is given by: (1OO-TL) v TL s Where TL is expressed in percent by volume of air and vs is cubic feet Dilution Volume = of vapor per gallon of solvent. The dilution volume for the threshold limit of toluene solvent can be calculated as follows: (l00.0.02) 30.4 cu. ft. 0.02 = 151,970 cu. ft.
Dilution Volume =
Referring back to the previous example in paragraph 3.2.2.1 the ventilation volume rate required to maintain the vapor concentration below the TL requires 60,790 CFM Ventilation as calculated below:
Volume 0.44 gal. SOIV. gal. paint
=
151,970 cu. ft. gal . of Solv .
x 1 gpm paint x
= 60,790 CFM (for Toulene) This ventilation volume is the minimum required to maintain tank at an acceptable TL value. Table III shows graphically the resultant paint Vapor conncentration for various ventilation volumes.
3.10
concentration below the threshold limit requires extremely large volumes of fresh air, generally more than required for LEL maintenance or blasting generations. These volumess are difficult to provide due to air-handling equipment space limitations and cost, especial, ly when dehumidification of the incoming air is necessary. An alternative solution is to require workers to use respirators when applying solventbased paints in tanks. Another alternative is to limit the paint application rate to coincide with the required blasting ventilation volume. The same ventilation equipment can then do an effective job for both operations. As stated earlier, water-based paints require only a small fraction (about 5%) of the ventilation volume required for solvent-based paints. This can be easily provided by the blasting ventilation volume. 3.3 Equipment Selection Proper ventilation consists of equipment for moving air, equipment for directing or channeling tile air and the efficient setup of this equipment. The following paragraphs discuss the principles of air movcment and the proper selection of equipment necessary to effect efficient operations. 3.3.1 Fans Fans are used to ventilate tanks by exhausting dirty air and/or by blowing in fresh air. Fans can be selected from a wide variety of sizes and types for different applications. The most important factors involved in determining the fan requirements are: Type of ventilation system required Amount of ventilation required
Maintaining the paint vapor
Static pressure required Available space
3.12
Generally speaking, the objective is to choose a fan which provides required air volumes at proper static pressures with minimum horsepower and space utilization. The two preferred types of fans for marine ventilation are duct-axial and centrifugal. See Figures 3.2 and 3.3. Compressed air driven fans are also commonly used by shipyards for general ventilation. Hbwever, air driven fans have low efficiency rating relative to power requirements and are therefore not suitable for moving the large volumes of air. If the fan is to be used simply to ventilate the tank with ambient, untreated air, the duct-axial fan is the best @ice. This fan is ideal for portable applications where large volumes of air are blown or exhausted through only 50 to 100 feet of ducting at low static pressure. Having a simple heavy-duty design, the duct-axial fan can be successfully operated in abrasive and dirty renditions. 10,000 to 50,000 CEM capacity. These fans are available in ranges of Due to their loW static pressure ratings,
they require minimum horsepower (3-1OHP). In addition, duct-axial fans can be mounted either vertically or horizontally. Fans used for blast-paint operations should always be ordered with explosion-proof electric motor spark-resistant construction. See Figure 3.4. and
Centrifugal fans are capable of moving large volumes of air at high static pressure, and therefore, are used in conjunction with dust Collection and dehumidification systems. These fans can operate efficiently when Connected to ,long runs of duct work. The increased static pressure capability of centrifugal fans result in increased horsepower ratings (25-250+HP). See Figure 3.5.
3.13
Figure 3.2: Centrifugal Fans - Air enters the center of the impellers in an axial direction and is discharged by the impellers radially through the fan outlet. It is generally used When high static pressures are required, above 10-15 inches water column.
AIR FL O W
Figure 3.3: Axial Fans - Air enters and discharges in a straight line, parallel to - fan housing. It is generally used when a high volume of air is required, with the fan occupying the least amount of space.
3.14
The required fan capacity can be calculated based on the size of the tank and the frequency of air changes necessary for adequate visibility. For example, an air damage every three minutes in a typical 50,000 cu. ft. wing tank would require a fan capacity of 16,500 CEM. A 100,000 CU./ftcenterline tank would require a fan capacity of 33,000 CFM for the same air change frequency. Fan capacity specifications are based on standard cubic feet per minute (SCEM) ratings. A SCEM represents one cubic foot of air at 70°F moving at a rate of one foot per minute. Air cooler than 70°F, and there fore denser, moves slower through a fan than warmer air. Also more horsepower is required to move a given Volume at a given rate of cold air than of warm air. Fans are designed for varying maximum static pressure potentials. Fan static pressure is required to overcome the resistance or friction of air moving through ducting. Figures 3.4 and 3.5 Static pressure requirements are calculated based on the size, length, and number of bends of the ductwork. Size is the cross-sectional dimension of the duct. To demonstrate the effect of bends and elbows on static pressure loss, one foot of 18” duct with a 90 deg. elbow has the equivalent resistance of approximately 28 feet of straight duct. Static pressure requirements are also increased by air passing through air treatment equipment. The static pressure requirement for a fan should be determined after the ducting and equipment layout for the ventilation system has been designed. As an example, assume a fan must blow 9,000 CFM of air through a dust collection unit and 200 feet of 18” flexible ducting. The dust collector and the size and length of ducting each result in a 5” loss of static pressure for a total pressure loss of 10”. Therefore the fan must have at least 10” of static pressure potential in order to maintain the 9,000 CFM required. See Table IV, for friction loss per 100 feet of various sized duct
In many cases, the rated fan static pressure may be sufficient to pull or push the air in the volume required. Generally, duct-axial fans used in single-purpose ventilation systems should have at least 1“ static pressure capability, and preferably 2“. Centrifugal fans used with dust collection equipment should be ordered with a minimum 12” static pressure rating. In a well-designed, permanently installed air handling system, fans can be located at practically any distance from the tank and still operate efficiently. However, on jobs of short duration where portability and ease of installation are desired, the fan should be placed as close to the tank as possible in order to reduce the amount of ductwork required. Duct-axial fans can be ordered with special adapters enabling then to be mounted directly into ‘Butterworht’ openings and cargo hatches. Ideally the exhaust fan(s) should be placed over the ‘Butterworth’ opening(s) and ducted to two sides of the tank bottom. Fresh air should be blown into the tank through the cargo hatch. This arrangement will distribute the clean air uniformly through the tank and through the same passage operators use to enter and
exit.If deck space is severely limited fans may be platform-mounted. If
possible, fans should be isolated from commiunication areas because of high noise levels. Installed fans Should be checked periodically with a manometer. This device measures air flow. Measured reductions in air flow of an installed system can be indicative of wornm parts such as impellers or destructed ducts . In In summary, the most flexible type of fan for ventilating tanks with ambient air is the duct-axial type with a rated capacity of 30,00040,000 CFM, 2" of static pressure and a 4248” spark-proof case. Centrifugal fans with greater capacity for static pressures are primarily designed for use in air-treatment systems. Exact specifications will depend on the layout of the ductwork and/or treatment systems.
3.17
3.3.2 Ducting Well-designed and Properly laid-out ductwork is essential to an. efficient air handling system. Ducting design requires a thorough knowledge_ 1 of requirements , accurate data on equipment performance specifications, accessibility, duct length and weight and volume of material to be moved i.e., abrasive dirt, solvent fumes, etc. The two main areas of design criteria for ducting are: including factors of CFM, static pressure, velocity
q Sizing,
requirements, and fan specifications.
q Layout,
including type of job, ducting material,placement, and
monitoring of the system. The general objective for the ductwork design is a system of the smallest dimensions which combines the lowest practical static pressure requirements with sufficient velocity to transport the airborne materials. 3.3.2.1 Sizing Sizing is the most critical consideration in selecting ducting because it determines, thethw the actual CEM, static pressure, and velocity of the air-f low in the finished ventilation system meets objectives. established design
1
For detailed information pertaninig to duct design, consult Industrial Ventilation: A Manual of Recommended Practices
3.18
Four factors must be considered when selecting duct size:
q Air volume in CFM
q Distance q Static
air is to be moved
pressure limitation of available fans q Air velocity requirements With these four pieces of information, Table IV can be used to select the proper duct size. As discussed earlier, air volume requirements are based on the size of the confined area and the characteristics of the material requiring venting. The distance the air is to be moved is simply the length of the ducting. Normallya fan which best meets the air volume requirements is selected from the existing capital inventory. The static pressure rating of the selected fan then becomes a design parameter Which must be considered in the final ducting size selection. Velocity calculations are based on the characteristics of each type of materiel to be vented. If the duct is too large, resulting in a decrease in critical particle velocity along the length of the ducting the suspended material will fall out of the air-stream and build up in the bottom of the duct. As the duct fills, the ventilation capacity of the system is severely As a rule, airborne dust resulting from abrasive blasting requires a critical particle velocity of 3,500 FPM. Static pressure loss along the length of the ducting is dthectly related to the size (internal cross-sectional area) of the duct. If the duct is too small,the static pressure required to offset frictional losses may overload the fan capacity, resulting in a reduction of air volume moving through the system. It must be remembered that as static pressure requirements increase, more energy (HP) is required to operate the system. Excessive energy requirements not only increase cost but may also restrict ventilation equipment usage at same locations within the yard. reduced and there is danger of the duct collapsing.
3.19
Same examples of static pressure loss for various types and sizes of are as follows (assume 9000 CFM ventilation requirement):
q 18”
smooth ducting will generate 1.7” of static pressure d r o p
100' of duct, and will provide a velocity of 5,000 FPM. (See Table Iv)
q 18"
flexible ducting has a static pressure drop of 2.8” per 100’. Adding one 90 degree bend along 103’ will increase static pressure increases to 8.4” and three cross-sectional irregularities,
bends along 3(X)
‘ d r o p t o 3 . 2 ” . Two balds along -200‘
increases to 12.6". the
‘area inside the flexible ducting, due to
increases the velocity to 5,500 FPM as to the smooth ducting (see figure 3.6 for proper branch entry and elbow radius/ designs).
q 24”
smooth ducting has a static pressure drop of 6” per 100’.
However, velocity at the velocity CFM
is 3,200 FPM, which would be
marginal to transport abrasive dust. The air volume would have to be increased in order to move grit dust through this duct size. (Accurate static pressure figures for various CFM and duct sizes can be obtained from manufacturer’s specifications) . Example: A ship tank is scheduled for abrasive blasting. The size and
configuration of - tank is such flat 30,000 CFW of air and 300' of duct are required for proper ventilation. The available fan is a 30,000 CFM duct-axial. rated at 2" static pressure. Step One: Look at Table
IV.
Select the line on the y axis which repreAs can be seen from the table, duct sizes diameter will carry the required air
sents 30,000 CFM.
volume.
from 20" to 80" in diameter
Step Two:
Calculate the maximum allowable static pressure drop for each 100‘ of duct based on fan rating. This allows use of Table IV
3.21
which is expressed in frictional loss in inches of water (static pressure) per 100’ of duct lenght.
300' ÷ 100' = 3 lengths of 100'
2.0" static pressure + 3 length = .7” per 100’ allowed
Step Three:
Again leek at Table IV. Follow the x axis to the paint which corresponds to a frictional loss of .7. Trace up this line to the intersection of the line which corresponds to 30,000 CFM. The diagonal line which intersects with x and y axis and represents 'in. duct diam. ' reads 34. Therefore, the appropriate size duct appears to be 34”.
Step Four:
Verify that the duct size selected will maintain the proper velocity to keep abrasive dust suspended-3,500 FPM) . The FPM velocity line in alSO a diagonal line. As can be seen, the velocity of the air ranges from 4500 to 5000 FPM which is in excess of the minimum velocity required to transport abrasive dust (3500 FPM) .
Solution:
In this ample the 34” ducting
would
be the correct choice.
In conclusion, ducting which is not carefully and properly sized will greatly reduce the efficiency of the total ventilation system, and will result in problems related to equipment, visibility and worker safety. 3.3.2.2 Layout When blasting marine tanks, the operator is faced with many different types of applications and tank configurations around which the ducting layout must be designed. set-up and To allow for maximum portability and ease of breakdown, the yard should stock ducting components in a variety
of sizes and quantities. However, the shipyard should have some standard systems which are designed for the most frequent types of jobs.
3.23
In many cases, ventilation air is not distributed uniformly through the tank. As a result, only parts of the tank are properly ventilated, while other areas remain contaminated . Clean air must be ducted into the tank in such a manner that the ductwork extends down no more than 6“ below the tank top. Since the heavier airborne dust particles tend to settle to - bottom of the tank, the dirty air removal duct should be positioned in such manner that the pick-up opening is near the tank bottom. This arrangement permits the dust particles to naturally fall toward the bottom of the tank and be exhausted much faster than if the pick-up point were positioned higher in the tank. The duct openings should be separated as much as possible. See Figure 3.7.
Figure 3.7:
Ventilation Diagrams of Enclosed Spaces, Snail Tanks and Multiple Tanks
3.24
Some tank configurations and/or production requirements necessitate ventilation between tanks. This can be accomplished by cutting access holes through common bulkheads or through decks. These access holes are particularly advantageous when setting up a complete tanker job. The resulting cross-ventilation saves considerable time through standardization of duct sections. Blanks can be used to close off outlets or inlets when not in service. This practice also provides additional access entrances to each tank and avoids the constant problem of personnel ard materials competing fora too little space. Metal ducting should be used for all straight runs. Flexible fabricwear and
reinforced ducting, which is more expensive and is subject to high inaccessible tanks.
tear, Should be used for inking connections to machinery and to small, Round duct is usually the best choice because it maintains a uniform air velocity and withstands higher static pressure. All duct work for tankblasting ventilation should be durable yet light for optimum portabilityo periodic inspection of the ductwork should be made to insure air-tightness. In addition, every new After the system has been installed,
system Should include access for measuring devices tO monitoring
velocity, CFM and static pressure at various points along the ducting. The Pitot tube is the standard air velocity meter. By multiplying the velocity reading in FPM by the cross-sectional area of the duct in square feet, the actual CEM at that point can be calculated. For example, at a point on a straight run of 18” ducking, the air velocity is measured to be 3,200 FPM. The 18” round duct has a cross-sectional area of 254.4 square inches, or 1.76 square feet. The CFM at that Point would be 3,200 X 1.76 See Table V for area and circumferences of square feet or 5,632 CFM. circles. A manometer is used to measure static pressure. If measurements of CFM, static pressure, and velocity reveal that
ventilation objectives are not being met, modification or repair of the ductwork and/or the fan may be necessary. A common problem with fans used
3.25
for blasting ventilation is worn impellers caused by abrasive dust. If the fan does not have sufficient capacity, ducting must be straightened or shortened. The problem may also be caused by improperly sized ducting, constrictions or air leaks. Ducting that has been used for ventilation during painting should be
inspected for paint build-up on the interior surfaces before it is used for blasting ventilation. Friction created by the abrasive dust combined with flammable paint solvent particles can create a fire hazard. In addition, excessive paint build-up will receive the efficiency of the ventilation System. In this section, basic procedures and guidelines have been given for general marine tank ventilatione Examples for the most part have been for ventilation of ambient, untreated air. The next section will identify the components of the dust collection system, which cleans the dust-laden air exhausted by ventilation.
3.27
4. Dust
Collection
4.1 Introduction
The utilization of dust collection equipment to clean contaminated exhaust air resulting from manufacturing activities is an existing technology with widespread use throughout the world. The possible exception is shipyard adaptation of dust collection for blast-paint operations. If properly used by shipyards, dust collection will eliminate many of the problems associated with Contaminated air from abrasive blasting and Coating; operations, will insure compliance with EPA and OSHA regulations will substantially reduce job-site housekeeping. Current or pending federal legislation may seen force every shipyard contractor to clean the contaminated air generated by abrasive blasting painting. ClearlY, management would be well-advised to begin assessing requirements and identifying dust collection equipment which will must efficiently meet existing and potential regulations.
4.2 Technical Discussion
There are three types of dust collection equipment which are adaptable to shipyard blast-paint operations.
q wet q Dry q DrY
scrubber Fabric Cartridge
Wet scrubbers impinge the dust-laden air with moisture, wetting the dust and causing it to settle due to increasd weight. The resulting sludge is drained of moisture and discharged by conveyor from the machine in a semi-dry condition. One reccomended type of wet collector is of the venturi design. This design combines high constant efficiency and portability with low operating costs and low operating noise levels.
4 1
Dry fabric (baghouse) collectors use a series of fabric bags which filter dirty air drawn across or through the banks of filter elements (bags) . The retained dust is then removed at regular intervals by blowing compressed X through the fabric bags, by shaker or by vibratirg systems . The dislodged dry dust then falls into hoppers for disposal. Reverse- jet continuous duty dry fabric dust collectors are reccommended for shipyard applications because of the high humidity conditions. This design provides increased air flow and, therefore more complete cl caning of the filter media. However, this system has a higher initial cost and requires more maintenance. Dry cartridge systems collect and discharge dust in the same manner as dry fabric or baghouse systems but have capacities of only 5-10,000 CFM. Because the cartridge is rigid in the collector, the filter media does not require removal. for transport. Cartridges are replaced as necessary. 4.3 Equipment Selection The most important selection criteria for dust collection equipment in the shipyard are as follows: o Portability o CFM and static pressure requirments o Type of particles handled o Efficiency and consists 4.3.1 Portability Portability is a crucial consideration in selecting dust collection equipment, and includes factors such as machine size, transportability, set-up time and ease of placement. If the shipyard frequently handles individual tank blasting jobs and/or multi-tank projects, a wet venturi system would be a good choice. This 25,000 CFM unit is compact (13.75’ high X 8’ wide X 18’ long), with a dry weight of approximately 12,000 lbs. The wet scrubber can be transported
completely assembledo Because the fan is mounted on top of the machine, extra ducting is not required between the fan and scrubber. The unit can be disassembled or reassembled in about 8 man-hours. The removal of the fan and transition piece make it a legal load for transporting outside-the yard. Due to a low center of gravity, the unit can be located ship without problems. See Figures 4.1 and 4.2. The primary limitation of the wet venturi system is that it cannot be used with dehumidification equipment. The moisture laden air increases the load on dehumidification equipment. When projects dictate dehumidified air, the dry fabric or cartridge collector is the recommended choice of equipment.
1 – CUSTOMER’S INLET DUCT 2– INCOMING DIRTY AIR 3 – WATER DISTRIBUTOR 4 – ADJUSTABLE RECTANGULAR VENTURI 5 – WASH SECTION 6 – SOLUTION TANK 7 – MOISTURE ELIMINATORS 8 – FAN 9 – OUTGOING CLEAN AIR
10 11 12 13 14 15 16 17
– FAN MOTOR – PUMP DISCHARGE – PUMP – PUMP INLET – SLUDGE CONVEYOR – CUSTOMER’S SLUDGE CONTAINER – SLUDGE CONVEYOR DISCHARGE – SLUDGE CONVEYOR DRIVE
FIGURE 4.1 - Schematic of Venturi Wet Scrubber
FIGURE 4.2:
Venturi Wet Scrubber Dust Collection Unit - 25,000 CFM
The standard design of the dry fabric collector is less suitable for portability than the wet venturi or dry cartridge equipment, i.e., a 25,000 CFM unit is 27’ high x 12’ wide x 25’ long and weighs about 13,000 pounds. It has a much higher center of gravity making it unstable when placed on the ship deck. If a dry unit is to be remved or transported, the bags usually should be removed to avoid tearing. Bag removal is a dirty and unpleasant task. In most designs, the fan and rotor are not mounted on the unit . These must be disconnected and transported separately for moving. Approximately 150 manhours are required to set-up or disassemble a 25,000CFM unit. Frequent handling of this type of unit will result in increased maintenance and repair costs. See Figure 4.3.
4 4
FIGURE 4.3:
Reverse-jet Continuous Duty Dry Fabric Collector Unit - 70,000 CFM
The dry fabric collector is most efficiently utilitized in semipermanent, pierside, barge-mounted, or railcar-mounted arrangements.This system is also appropriate for large capacity permanent installations. For individual tank jobs requiring dehumidification, a combination of dry cartridge and dehumidification units (10,000 CFM each unit) represents a high-performance, totally portable system. (See Figure 4.4). The dry cartridge dust collector system is also ideal for trailer-mounting because of its compact design. A system of up to 40,000 CFM (consisting of four 10,000 CFM units) can be mounted complete with fan and motor on a single 40’ trailer. Since the cartridge unit can be moved without disassembly, this system can be transported on roads as well as within the shipyard.
4.5
FIGURE 4.4:
Mobile Dust Collection/Dehumidification System - 10,000 CFM Each Unit
4.3.2 CFM and Static Pressure Each type of dust collection system can be assembled with high static pressure fans to accommodate long runs of ducting. However, the Wet Venturi Scrubber is restricted to a 50,000 CFM volume capacity as the largest practical single unit. .Because of their modular design, single units of the dry fabric system can be designed with a Capacity in excess of 100,000 CFM.
FIGURE 4.5:
Mobile Dry Cartridge Dust Collection System- 40,000 (2’FM
4.3.3
Type of Particles Handled Dust created by abrasive blasting institutes a moderate load of fine
to medium sized particles. Both dry and wet systems are well-suited to handle these particles. However, the dry fabric collector cannot efficient ly handle wet particles as they tend to clog the filter media. This problem limits the use of dry fabric collectors during matings applications because the overspray is wet. If air ventilated durirg painting is to be cleaned by a dry fabric colector, an exdpendable paint arrestor filter should be used to filter the air before it is exhausted to the collector. Wet paint will quickly clcg and "blind" the bags. The wet collector can handle both dry and wet particles. The slightly
damp sludge resulting from the wet scrubber system is easier and cleaner to handle than the discharge from the dry system. The dry dust discharge can create a secondary air pollution problem during disposal. 4.3.4 Efficiency and Costs In terms of efficiency, operating most, and maintenance, the Wet scrubber offers several. advantages. It runs at a constant efficiency, has heavy-duty instruction with few moving parts, requires less maintenance and has lower replacement rests. The unit is also easily accessible for repairs and external inspection. The wet unit can be installed for allweather, year-round operation. The efficiency of the wet scrubber is not affected by air moisture in humid areas, although the use of water may introduce corrosive conditions within the collector. When ordering scrubbers, a corrosion-resistant mating such as a coal tar epoxy should be specified for all internal metal surfaces. The scrubber requires both electrical and wter service hook-ups, although water used by the unit is recirculated. In comparison, the dry system will operate efficiently only when air
conditions are dry enough to prevent condensation or moisture deposits on the fabric. Under humid renditions, dust will cake on the bags, resulting in loW efficiency and possible damage to the filter media. All openings and
4.7
fittngs on the suction side of the ductwork should sealed against moisture. The unit has a large number of parts and assemblies with limited accessibility which results in increased maintenance rests. An additional hazard of the dry system is the possibility of a “bagtiuse” fire. The ferrous oxide contained in blast dust residue may under certain conditions spontaneously ignite. Use of the wet scrubber system for abrasive blast air-cleaning eliminates the possiblity of collector fires. 4.3.5 Summary In summary, actual guideline:
q
equipment selection should be based on the per-
formance requirements of the intended application. As a general selection
For jobs requiring multiple units and volume
requiremmts of 15,000
-35,000 CFM (especially when frequent jobs of this range are widely
distributed around the yard) the wet scrubber system should be used .
q
For stationary applications requiring a single unit of over 35,000 CFM, the dry fabric collector is best.
q
For small, portable, short-term applications and/or here multiple units are required for recirculation of dehumidified or heated air dry cartridge type collector of 5,000-10,000 CFM are best
If dust collection equipment is properly installed and utilized, the environment in and around the blasting operation will be as desirable a place to work as any other area within the shipyard.
4.8
5.
5.1 Introduction
DEHUMIDIFICATION
Dehumidification (DH) is the process of removing moisture from ambient air. The removal of moisture from the ventilation air is an important process in preventing condensation (” sweat”) on internal tank surfaces during blasting and painting. Condensation occurring on surfaces which have just been blast-cleaned may cause rust bloom formation within a short time. adhesion of the protecThe resulting surface contamination promotes poor
tive coating and premature failure due to underfilm Corrosion. Blistering is another common type of paint failure usually causal by applying paint to a surface containing moisture. Blisters may also occur when the surface was originally dry during application but moisture entered the mating as it cured. Paint curing is a function of temperature, time and humidity. Since
curing requirements vary widely between water+born, epoxies, inorganic zincs, and other types of coatings, paint manufacturer’s specifications should be consulted for recommended atmospheric conditions. The use of dehumidification equipment througout the process of blasting, painting, and curing will prevent many coatings failures. 5.2 Technical Discussion The purpose of this section is to provide simple, clear explanations of condensation principles and the calculation of dehumidification requirements. In addition, information will be presented on the comparison, selection and utilization of DH equipment. A series of easy-t-understand tables for calculating DH requirements have been developed to avoid the complex psychrometric interpretations that have hitherto been necessary.
5 1
5.2.1 Principles of Condensation Condensation ocurs when warm, moisture-laden air contacts a cooler surface. As the air next to the surface is coaled, the moisture carryingcapability is reduced, and some of the water vapor is deposited as droplets on the cooler surface. This occurs naturally in the early morning when air warmed by the sun contacts cooler blades of grass or car windshields. The temperature at which the ambient air becomes saturated with water vapor is called the dewpoint temperature. Any reduction in the air temperature below the dewpoint (for example...when warm air contacts a cooler surface), causes moisture condensation. Reducing moisture in the air will lower the dewpoint temperature of that air. Dewpint temperature is determined by the difference in the wet- and dry-bulb temperatures. This difference can be measured by a psychrometer. See Table VI, Quick Dewpoint Reference Table, for examples of dewpoint.
To determine dewpoint
(air temperature at which moisture will condense
on surfaces), follow wet bulb temprature across and dry bulb temperature down. (These temperatures can be measured by a battery-operated psychrometer, Figure 5.1). The intersection is the dewpoint temperature. Example; wet bulb 60°F, dry bulb 75°F = dewpoint temperature 50°F. It is commonly believed that high air temperatures combined with high humidity create the greatest possibility of condensation. In shipyard operations, lower humidity combined with large day-to-night temperature swings and low sea water temperatures can present a greater potential condensation problem. During day-night temperature transition periods, surface temperatures will often be lower than dewpoint. Condensation in a ship tank can cccur during these periods, or anytime that weather condiitions change. Once these general principles are understocd, several points must be remembered in connection with the dehumidification of air in shipboard tanks .
5.2
Condensation will never occur if the dewpoint temperature of the air is kept lower than the surface temperature of the tank. Therefore, the general rule for condensation prevention is to maintain the air dewpoint temperature at least 5°F below the surface temperature. Heating the air in an enclosed tank does not remove moisture or change the dewpoint temperature. For example, air at 400F with 70% relative humidity has a dewpoint temperature of 31OF, file 80°F air with 17% relative humidity has an identical dewpoint of 31°F. . Dewpoint control can be easier to maintain when a ship is in the water than when in drydock. This is because the ship surface temperarures below the water line will remain relatively constant due to the heat sink of the surrounding water. When the ship is in drydock, the entire surface is exposed to air temperature shifts. Heat is also lost to the ambient air at night. Psychrometric readings should be measured and recorded every four hours during the entire blasting and painting operation. This procedure will provide detailed records of job conditions for future use. The battery-operated Psychron Model 566, available from the Environmental Service Division of Bendix, provides wet- and dry-bulb temperature readings as well as a scale to determine dewpoint. A surface lihermaneter with built-in clamping magnets can be easily attached to metal surf aces anyWhere on the ship for surface temperature readings. The Model 315F, available from Zorelco Measuring & Testing Instruments, 8520 Garfield, Blvd., Clevelan3, Ohio, is suitable for this purpose. See Figures 5.1 and 5.2.
5.4
FIGURE 501:
Battery Operated Psychron . Bendix
FIGURE 5.2:
Magnetic Surface Thermometer Zorelco
As an illustration of the practical application of dehumidification principles, the following example is offered. Readings were Compiled over a 24 hour period and entered on a Psychrometric Report (See Figure 5.3). This proposed report is one way of recording required data. Note that relative humidity is not a required reading, and is only given as a comparison between air temperature and moisture-holding capacity.
5.5
(SAMPLE) PSYCHROMETRIC REPORT 30B LOCATION NEW YORK HARBOR TANK WING TANK - SHIP WATERBORNE DP-DEW FOINT TEMPERATURE
DB-DRY BULB TEMPERATURE
WB-WET BULB TEMPERATURE
DATE
TIME FOREMAN WEATHER* OUTSIDE INSIDE DB WB DP DB WB DP
TANK SURFACE TEMP
DIFFERENT SURFACE TEMP INSIDE DP +1OF + 5F +25F +5F + SF - 5F + 5F + 5F - 2F +5F
-12F
1/4/80 0800
WHITING CLEAR
45/40/34 45/40/34 45 ABOVE W.L. 40 BELOW W.L.
60/50/41 60/50/41 60 ABOVE W.L.
1200 WHITING CLEAR 1600 WHITING CLOUD CHANGING 2000 BIBBO CLEAR
40 BELOW W.L. 50/48/45 50/48/45 50 ABOVE W.L. 40 BELOW W.L.
40/38/35 40/38/35 40 ABOVE W.L. 40 BELOW W.L. 40/38/35 40/38/35 33 ABOVE
2400 CROTTY CLEAR CHANGING 0400 GIESE
W.L. 40 BELOW W.L. W.L. 40 BELOW W.L.
50/48/45 50/448/45 33 ABOVE
-5F
*INDICATE:
SUNNY, CLOUDY, RAIN, SNOW, FOG, CLEAR, (CHANGING.
FIGURE 5.3: SAMPLE PSYCHOMETRIC REPORT FORM
5.6
The fOllowing renditions are based on a 50,000 cubic feet wing tank ventilated with 17,000 CFM of air. It should be noted that a sealed tank with no ventilation would present very different characteristics, as the stagnant, idle air on the inside would not be subject to radical tempera-’ture swings. 8:00 A.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 80%) Conditions at this time are condensation free, as surfaces both above and below Water line have temperatures above dewpoint. No DH required. 40°F 45°F 40°F 34°F 45°F 40°F
12:00 NOON Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line, Surface temperature: below water line 40°F 60°F 50°F 41°F 60°F 40°F
Conditions are the same as at 8:00 A. M., as only the ambient air temperature has increasd. No DH requirement.
5.7
4:00 P.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: ret-bulb Dewpoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 85%) A storm enters the area, bringing additional moisture With in turn o raises the dewpoint 5 F above the existing temperatures of surfaces below the water line. condensation will therefore occur on tank surfaces below the water line. The area above the water line, at 50oF, is still 50F above the dewpoint temperature, so condensation will not occur on those surfaces. DH required below water line. 40°F o 5 0 F 4 8
o F
45°F 50°F 40°F
8 : 0 0 P.M.
Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (See Table VI) Surface temperature: above water line Surface temperature: below water line
The storm has passed and ambient air is dryer.
40°F - 40°F 38°F 35°F 40°F 40°F
Surf ace temperatures, both above and below the water line, are again 5°F higher than the dewpint. No DH required.
5.8
12:00 MIDNIGHI’ Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewpoint temperature (See Table VI) Surface temperature: above water line Surface temperature: below water line 40°F 40°F 38°F 35°F 33% 40°F
During the clear night, the surfaces of the Ship above the water line
are radiation heat into space, so the surface temperature above water line drops to 33°F. This temperature will not drop any further because heat is also being transferred from the warmer surfaces below the Water line. In this case, condensation is occurring on surfaces above waterline, since the dewpoint is 35°F and the required above waterline. surface temperature above water is only 33°F. DH
-
4:00 A.M. Water temperature Ambient air temperature: dry-bulb Ambient air temperature: wet-bulb Dewppoint temperature (see Table VI) Surface temperature: above water line Surface temperature: below water line (relative humidity 85%) Surfaces both above and below
40%
50°F 48°F 45°F 33%F 40°F
--
the water line have cooled during the Thus condensation will occur on DH required. coditions were experi-
night to temperatures below the dewpoint. all ship’s surfaces exposed to ambient air.
During this 24-hour period, three different
enced.
5 9
q
4:00- P.M. the storm passed levels.
through and raised ambient air moisture
The dewpint rose above the temperature of the surface
_ _ _ belcw the water line caused condensation below the water line.
q
12:00 Midnight surfaces above the water line mold through heat radiation to a temperature lower than the dewpoint and mndensation occurred.
4:00 A.M. condensation occurred on surfaces that had cooled during
the night . ___ These examples demonstrate the types of conditions which are commonly experienced by shipyards. These conditions require dehunidification of the air to prevent condensation on tank surfaces. The following section requirements. outlines the methodology of determining dehumidification
5.2.2 Determining Dehumidification Requirement s
Dehumidification requirements are determined by calculation the volume of conditioned air needed to control condensation inside a tank and then calculating the requisite number of DH units which will meet the defined objectives. These calculations can be easily accomplished by using data entered on the Psychrometric Report (Figure 5.3) in conjunction with Tables VI, VII, VIII. The instructions accomoanying each table gives specific examples of required calculations. Table VI gives the dewpoint temperature based on existing ambient dry and wet bulb temperature readings. Using the dewpoint and the existing tank surface temperatures, the amount of moisture in the ventilated air, expressed in pounds per hour per CFM, can be determined from Table VII. Determinations should be made for surfaces with above and below the water line. Table VIII contains the moisture removal capacity of a Cargocaire Model HC-9000 SEA Special 9000 CEM unit dehumidifier unit in pounds per hour This model and size dehumidifier was chosen for the example because it is a standard readily available piece of equipment. A table similar to Table VIII can be compiled using performance curve data for any other
existing DH system. The total amount of ventilation required for visibility and safety (see Section 3: Ventilation) is then multiplied by the wet air factor (Table VII ) to obtain the total amount of required moisture removal Table VIII is then used to determine the number of DH units needed to meet the dehumidification requirement for the specified volume of air. This, in turn, is the required amount of conditioned air as a ratio to the amount of untreated ambient air required for ventilation. Example: A ship tank is scheduled for. abrasive blasting. The size and
configuration of the tank is such that 30,000 CFM of ventilation is required. The dewpoint temperature of the ambient air is 50°F and the surface temperature of tank is 45°F. Dry Bulb Temperature of air is 75°F. Step One: Determine the wet air factor from Table VII . At a dewpoint temperature of 50° and relative humidify of 45%, the Wet Air Factor is .011. Step Two: Multiply ventilation requirement by Wet Air Factor 30,000 CFM X .011 lbS/hr/~ = 330 lbs/hr This is the quantity of moisture to be removed. Step Three: Fran Table VIII determine the moisture removal rate at a dewpoint temperature of 50°F and a dry bulb temperature of 75°F. In the example the water removal rate is 208
Ibs/hr .
Step Four:
Divide the quantity of moisture to be removed by the moisture removal rate of the dehumidifier. This will provide the number of units required. 330 lbs/hr + 208 lb/hr/unit = 1.59 units
Solution:
1.59 or 2 units of 9,000 CFM capacity each are required. This means that approximately half of the 30,000 (2FM of ventilating air must be dehumidified.
5 11
5.3
Selection of Dehumidification Equipment Three types of air treatment systems - be used to control dewpoint
temperature.
q Air-conditioning q Wet q Dry
desiccant dehumidifiers desiccant dehumidifiers. conditioning cools air through the use of refrigeration coils to
Air
An air conditioning system may be adequate to control condensation in year-round warm climates; however, as the temperature approaches 45°F, moisture from the air will freeze on the coils making the system ineffective for dewpoint control. Furthermore, air conditioning units are not designed for rugged, dirty condition or portability. These units also require specialized maintenance. Therefore, air conditioning units have not proven to be reliable for typical marine coating applications. Heaters are sometimes used to raise surface temperatures inside the
tank above the dewpoint.
condense out moisture from the air.
condensation, inefficient,
While this method can theoretically control and can be effective for small tanks, it is extremely and does not to remove
uses excessive amounts of energy, moisture from the air.
Until the late 70’s, wet desiccant systems were the most frequently used in U. S. shipyards. This system operates by pumping a desiccant solution through a spray header tube in the contactor. When the air to be dried is drawn past the contactor, moisture in the air is absorbed by the desiccant. The moisture-laden desiccant is then cycled through an exhaust air stream which removes the collected moisture. The wet desiccant system requires piping and regular replacement of the desiccant solution, plus auxiliary support equipment. (See Tables IX and X). Wet desiccant humidifiers are not reccomended for shipyards because of the high initial and maintenance costs, the requirment for a full time operator, and the large unit size and weight.
5.14
6.
Abrasive Blasting
6.1 Introduction Abrasive blasting is the process by which steel surfaces are cleaned of contanination through the use of abrasives striking the surface at relatively high velocity. This process requires a wall-coordinated program of carefully selected equipment and materials, experienced operators, and organized services in order to guarantee success. 6.2 Abrasivee Blasting Equipment The selection and placement of the abrasive blasting equipment is critical to the success of any tank blasting project. This equipment requires the greatest amount of consumable materials (i. e., abrasives) used in the shipyard. A single abrasive blaster will use approximately 1,500 Ibs. of abrasives per hour. An average cost for delivered mineral slag abrasive today is approximately $40.00 per ton. Therefore, one blaster will consume .$30 .00 worm of materials per hmr. In addition, abrasive blasting requires a wide range of costly support services, including compressed air, crane service, dust collection, dehumidification, staging, and clean-up crews. Proper selection of equipment and materials can increase labor productivity and significantly reduce the amount of materials and services required. In the past, small capacity blast machines were used. These units usually held between 600 and 1,000 lbs. of abrasives with a maximum Shall abrasive storage hoppers of 3 to 5 ton capacity were placed overhead by crane or forklift. These timers require replacement at least once a day, and often more frequently. If a crane or forklift was not available to lift the hoppers, the machines ran out of abrasives, and the result was wasted manpower and lost prduction. On jobs which required large amounts of blasting, the use of these machines resulted in very loW productivity and high abrasive resulting operating time of about 30 minutes.
6.1
consumption (spillage).
This size equipment should only be used for
light-duty jobs requiring minimum blasting. Larger capacity, bulk abrasive blasting machines with are now available.
q Large
mutliple outlets
The main design features of these machines are:
abrasive capacity which allows extended Periods of uninter-
rupted operation.
q Bulk
pneumatic refilling from delivery trailers. completely sealed
system which provide weather protection for abrasives.
q Multiple-nozzle
outlets and fast equipment set-up.
q Unattended
machine operation.
These design features permit less dependence on crane service, less abrasive consunption, less labor, faster set-up and cleaner operation with little spillage. The basic machine has a single chamber With operates 2 to 8 outlets. Both portable or stationary models are available with capacities from 6 to 40 tons. These machines are commonly manufactured in three sizes:
q q q
6-ton 22-ton 40-ton See Figures 6.1
Each machine is designed for a specific application. and 6.2.
The 6 to 8 ton unit can be mounted on wheels or skid. A 22-ton machine is supported by legs and is basically portable. The 22-ton units are primarily used for larger spot-blasting jobs which require several operators working in a central area. This machine is particularly well-suited for spotblasting contaminated areas on new fabrications. Forty-ton units
6 2
are usually used for stationary blasting
projects in Which the work pieces
are transported to the blast area. These wits provide sufficient storage capacity for several operators and are often used when high production rates are required. For large tank blasting jobs, or for external hull work, the 40-ton units can be mounted at the head of a drydock or aboard . ship In addition to single-chamber blasting machines,
another type of system has been developed for large abrasive blasting requirements such as cleaning multiple tanks or huge repair jobs. This unit is a double-chamber system Which fills automatically from overhead storage tippers. While maintaining the bottom chamber constant pressure, the top chamber can be depressurized and filled with abrasive. The abrasive will be automatically transferred to the lower chamber when the top chamber is closed. The blast operator, is never stopped because of a lack of abrasives. This unit is especially recommended for the Shipyard which is pursuing internal tank blasting contracts. See Figures 6.3 and 6.4.
Full Figure 6.3:
Filling
Schematic of Double-Chamber Automatic Filling Principle 6 4
In nest cases, the blast machine should be located as close to the work . area as possible to avoid air pressure drop through the blast hose. It is important to note flat the properly sized blast hose and nozzle is essential to the operation.
Figure 6.4: Double-Chamber Automatic Filling Multiple Outlet Blast Machine (Photo courtesy ofCAB Inc.,)
6.5
One method of locating the blast equipment close to the job site is to use a mobile, self-contained blast and recovery,trailer mounted system. Figure 6.5 is a picture of an existing mobile unit. This system is designed to recirculate steel abrasives. Refer also to Figure 6.6 which is a schematic which demonstrates one possible use for such a machine.In this system grit is cleaned (A) by means of a pneumatic separator. clean material falls into storage hopper (B) Dirty airborne dust is exhausted from can system. From the storage hopper, abrasive is directed into an automatic filling two-chamber blast machine (C) The abrasive is then transfered under pressure through the blast hose to the work area (H) Spent abrasive is manually vacuumed utilizing the vacuum (E) mounted on the trailer. Abrasive is deposited into a automatic dumping machine (1)) which directly transfers collected abrasive back into the pneumatic separator (A). For doing external work a special staging is required which will collect all the abrasive rebounding from the work surface. The blaster (B) stamps on a grated floor which permits the abrasive to fall through into a collection hopper. There, it is automatically collected by vacuum hose and returned to the trailer. Clean air is directed into the enclosure through a vent there dirty air is removed by an exhaust fan (F). when selecting and utilizing blasting equipment, the following items should be noted:
q
A single operator should be able to blast 100-250 sq. ft. per hour (depending on the condition of the steel surface) . Each 1/2” nozzle in operation will require approximately 1,500 pounds of abrasive (sand or slag) per hour. Each nozzle will require approximately 300 CFM at 110 pounds per square inch (psi) .
q
q
6.6
Figure 6.5:
Mobile Steel Grit Blasting and Recovery System (Photo courtesy of CAB Inc. )
Thus selection of new eqquipment must be based on:
q Existing
compressed-air volume and pressure capabilities. storage capacity
q Abrasive
q Crane
capacity delivery schedule frequency and location of blasting jobs.
q Abrasive
q Number,
6.3
Compressed-Air Drying Equipment Compressed-air drying equipment is required to remove impurities from
the compressed air system. Contaminants which
normally
enter the system
include moisture and dirt from the ambient air, and oil from the compressor itself. As the air is compressed, these impurities combine to form an extremely dirty and corrosive mixture. The resultant contaminated air
6.7
drastically reduces the efficiency of the blast operation by clogging nozzles and depositing moisture and impurities on the tank surface. This cotiition will also contaminate abrasives and ruin steel grit abrasives. There are three types of compressed air drying systems:
q Deliquescent q Refrigerated q
Regenerate
For abrasive blasting operations, the deliquescent system provides the most trouble-free solution to cleaning and drying the air. In addition, it Highhas the lowest initial cost and is least expensive to maintain. volume units are available Which are constructed with liftirg eyes to permit easy relocation. See Figure 6.7.
FIGURE 6.7
Refrigerated units represent a complex systems which requires qualifid service personnel to assure dependable year-round operation. This unit is not well-suited for portable applications, or for use in areas where dirty, dusty air will contaminate the filter and condenser fins. For these reasons, refrigerated systems should only be installed in permanent indoor locations.
6 9
The regenerative system also requires qualified service personnel for maintenance. These units do not remove oil without the addition of pre- and and are not designed for portable application without modifications for protection during handling. Selection of appropriately sized equipment is based on the total equipment CFM requirement. A practical guide is to assume a service factor of 300 CEM delivered at 110 psi per blaster. If a central compressor is used to distribute air throughout the yard, the CEM delivered to any given point will not exceed the amount that is Passed through the orifices in the blast nozzles. The CFM per nozzle can thus be used to estimate the total CFM of required compressed air. Optimun utilization of the deliquescent dryer requires large volumes of air to be processed at high pressures. Therefore, it is imprtant to measure air pressure available at the points Were the dryers might be installed prior to ordering a system. As an example, a unit which is capable of processing 2,300 CEM at 125 psi may only process 1,550 CFM at 80 psi. The location of the dryer depends on the compressed air distributing System, (i.e., portable or stationary). Since all air will be used outdoors, the dryer must also be located outdoors. The unit should be placed in the coolest area possible to avoid radical changes of temperature between the drying point and use point to prevent condensation in pipes downstream of the dryer. In warm climates, When the temperature of the compressed air exceeds 100°F, an air- or water-coaled after-cooler is required to reduce the temperature of the compressed air. The chemical in the deliquescent dryer will be subject to deterioration in direct proportion to the rise in temperature over 100°F. For portable applications, it is best to locate the dryer within 50’ of the blast machine. This gives the air an opportunity to Coo1 in the air lines before entering the dryer. In addition the dryer will catch any contaminants which might have entered into the system.
6 10
after-filters,
Since abrasive blasting requires large volumes of air, often in surges, an air reserve tack is recomended. This tank should be at least equal in size to the CFM usage and be installed downstream of the deliquescent unit. Normally, a receiver is placed ahead of the dryer to allow additional cooling exposure before entering the dryer. If an after-cooler is used on a portable basis, it should be placed in a location that will insure maximum ambient air flow around the unit. Often times a surplus heat exchanger may be used as an after-cooler. Air is circulated through the tubes while the unit is immersed in the water (Figure 6.8 ). Compressed air is passed through a submerged heat exchanger to cool it
and to remove the moisture from the air stream. The air is then passed into a chemical deliquescent dryer for final removal of contaminants. Compressed air drying equipment offers the final assurance for proper surface protection and coatings application. It also eliminates plugging of abrasives in the blast machine caused by moisture. 6.4 Abrasive Delivery and Storage In the past, abrasives were delivered to the shipyard and distributed to the blast machine hoppers in a variety of ways from hundred pound bags to railcars. Today, the most efficient method for distributing abrasives to blasting locations within the shipyard is by pneumatic delivery trailers. These units are operated at low pressure, and transfer the abrasives pneumatically through a discharge hose to the blast machine or storage hopper. See Figure 6.9. or clogging
FIGURE 6.9:
Pneumatic Delivery Truck 6.12
The main advantage of this trailer unit is mobility and the direct transfer of the abrasives for use in blasting. If a local supply of abrasives is available, and if the blasting equipment is accessible to the trailer, materials can be picked up and transferred directly to blast machines. If the supply source is distant, installation of a large-capacity bulk storage hopper should be considerd. The storage hopper can be loaded directly from a railcar or truck. Depending on the quantity of materials used and the time needed for replacement materials to arrive, this storage hopper should have a capacity of from 500 to 1000 tons. See Figure 6.10. Large sealed portable hoppers should also be made available for
installation in areas Were frequent blasting takes place, or where the pneumatic trailer cannot reach blast units. These units should be sized as large as possible without exceeding the lifting capacity of available cranes. The hoppers can be filled by the pneumatic trailer and then lifted by crane and positioned over the blast units. By decreasing the number of lifts required, these large hoppers can reduce the number of times cranes are required. Distributing and storing abrasives in completely enclosed pneumatic delivery trailers and storage tippers also reduces the possibilities of spillage and moisture contaminations. If delivery and storage in large bulk is feasible, purchase of a yard-owned pneumatic trailer will reduce the overall cost of the operation. Materials can be bought in bulk and the laker required for handling and distribution will be reduced. 6.5 Abrasive Recovery Equipment 6.5.1 Selection Criteria After a ship tank has has been blast cleaned, spent abrasives, abrasive dust, and paint chips must be removed in order to ready the tank for painting. Usually this is accomplished by vacuum machines which suck the particles and other debris out of the tank through a flexible hose.
6.13
There are three different types of vacuum recovery machines available:
q q q
Portable unit with single-chamber collection tank Portable unit with automated discharge tank Mobile truck unit with single-chamber collection tank
In selection a vacuum recovery system for shipboard tank blastcleaning, the following criteria Should be considered:
q Equipment q Support
operation with reduced labor
services requirments
size in relation to available space in the work area q Hose size needed to operate at maximum efficiency
q Initial
q Equipment
and maintenance rests
6.5.1.1
Portable Unit with Single-Chamber Collection Tank
The portable single-chamber vacuum recovery tank is designed to operate unattended and can be located close to the worksite. This unit is equipped with an easy to handle, flexible 4“ hose. The unit does not use fabric dust filtration media, and the vacuum producer can be separated from the collection tank. These equipment characteristics allow for flexibility If the unit is to be positioned on deck, a crane is usually required for placement. Suction is created by a high-performance liquid ring-type vacuum purpose. The average abrasive removal rate of the unit is ten tons of abrasive debris per hour.In addition, this type vaccum pump can handle large amounts of dust particles which carry over from the secondary dust cyclone tank (larger particles settle out) . The unit is powered by a 50 to 70HP motor and requires water and electrical service hook-up. Both equipment maintenance and initial crest are low ($25,000). See Figure 6.11. in setting up the vacuum system.
6.15
Figure 6.11:
Two Portable Vacuum Units with single-chamber Collection Tanks hunted on Stands
6.16
Portable single-chamber units can be hooked up to a pneumatic discharge device if the material is to be disposed directly from the area without using a tank . The collection tank of the portable single-chamber unit can be placed a an elevated platform so that a dump-truck can pick up the abrasives for disposal or recovery. If abrasive recycling is desired, the collection tank insures that the recovered abrasives are protected for reuse. 6.5.1.2- Mobile Unit with SingleChamber Collection Tank This unit is permanently truck-mounted for mobility. The unit has an
average production rate of ten tons per hour. Performance is increased by moving the unit closer to the job site which is sometimes difficult in shipyard operations. Some units are capable of removing water and other fluids. This system is designed to operate with a 6“ to 8“ hose and is equipped with a positive displacement vacuum pump. Being mobile, this unit requires an attendant. The power take-off unit which is driven by the truck engine requires increased maintenance, especially men used on a continuous basis. The truck system can only be operated for short periods at a time before being shut down and driven away for disposal. The initial cost of the mobile unit is high (about $125,000) See Figure 6.13.
Figure 6.13:
Mobile Vacuum Recovery Truck with Single+Chamber Collection Tank
6.18
Although the mobile unit is suited for some for most internal tank-cleaning jobs.
shipyard applications,
maintenance requirments and short-cycle performance make it impractical .-
6.5.1.3 Portable Unit with Double Chamber, Automatic Discharge Tank This unit can be moved to an area where needed for abrasive recovery. No attendant is necessary and each component can be separated. It is designed to operate with a 4“ to 6” hose and has an average obtainable production rate of ten tons per hour. Like the portable single-chamber units, the initial cost is relatively-low (35,000 - $40,000); however, the dust collection filter must be periodically replaced. Suction is produced by a positive displacement, rotary Vacuum pump. These pumps are subject to damage should any dust carry-over from the air filter system. The pump can be troublespome if not prperly maintained. The large number of moving parts are an additional disadvantage resulting in increased maintenance rests See Figure 6.12. The portable unit with double-chanber automatic discharge tank is not suited for must tank vacuuming. Debris removed along with the ebrasive cannot pass through the discharge valves and may lodge between the valve and seat, resulting in a vacuum leak.
-
6.19
6.
Dehunidification units are skid mounted for placement aboard ship (also see figure 5.5). .-
7.
The facility should also contain a high pressure water main for wash-down (not shown) .
8.
A duct storage cradle should be used to move large amounts of duct without damage (not shown)
9.
Dehumidification duct should be prefabricatd in sections to accomodate long runs with minimum set-up time.=
10. 11.
Dust collection duct
is also prefabricate.
A 5000 gallon #2 diesel fuel storage tank is located adjacent to compressor station.
12•
The air compressor station must have sufficient capacity to provide compressed air to 16 blasters plus additional gear. A 6000 CEll compressor station is shown. This station is equipped with a water cooled aftercooler to insure that the temperature of the compressed air entering the dryer is less than 1000F. This compressor is diesel powerd to facilitate portable application.
13. 14. 15•
Pier management and quality control are located in an office on site. A portable crane is necessary to provide lifting services. A shelter can be used to provide protection for inclement weather the sun. and
16.
A staging platform cradle can be used to enable the crane to lift larger amounts of scaffolding (not shown) .
Figure 7.3 is a photograph which shows a ship docked at the abrasive blasting and coating pier. The portable dehumification units are located
7.2
midship with prefabricated ducts extending forward and aft. Flexible ducts are connected between the prefabricated side. There are thee difference ducts. ducts and the cargo access ports (hatches). The dust collecting duct can be seen midship on the port (water) Each duct haS two 90° turns which
positions the discharge end OVer the side of the ship and into barge mounted dust collection equipment. Four each vacuum recovery units are mountedon the dock on the starboard (land side) side of the ship. Figure 7.4 contains a schematic of a unit coating container. These containers are designed such that all painting equipment and materials necessary for a specific area are kitted prior to starting the job. The container is then positioned as close to the actual operation as possible. The unit matings container provides a clean, sheltered work center. Figure 7.5 is a cross-sectional drawirg of a large ship cargo tank The cargo tank covers have been removed for personnel and equipment access. In some cases equipment is placed directly over the openings. Dirty air (detail A) is extracted from the tank internal by duct work In most all cases efficiency of the from the intake side of a fan. ventilating process is greatly improved by running the duct work the shortest possible distance. Even when a penetration through the side shell is required, the procedure is generally less expensive than running ductwork through the ships interior. A dust collector should be used to clean the dust-laden air before being exhausted. Clean air can be routed into the work space from a penetration in bulkhead of an adjacent tank or be directed into the tank parallel to the dirty air exhaust duct. If a dust collector can be conveniently located pierside, substantial saving in set-up time
H
be achieved with a mobile unit.
On projects or climates which require dehumidification (detail B) either a mobile or portable unit may be used. If the volume of air required is too large for a single III unit the the DH air may be recoverd and recirculated by cleaning the dirty exhausted air by a dust-collector. This process of recirculation enables the dew point to be continually controlled in the work space utilizing a minmum of DH.
7.6
For an area requiring a large air volume (detail C), fresh ambient or treated air may be introduced into the tank through the car~ hatch cover. Dirty air should be extracted near the bottom of the tank and conveyed by duct through fans and collected on the ship’s deck. Sometimes large access holes are required in the deck. In these cases, special openings should made for duct access as well as for personnel lifting devices. Figure 7.6 is a close-up drawing of a ship tank area. Limited access into the area can be improved with penetrations thru bulkheads, side shell, and/or decks. Fresh ambient or conditioned air should be introduced at the highest access point. The eXhaust intake should be at the lowest point
.-
within the tank and on the opposite side from the fresh air inlet. Blind spots of stagnant” air inside tank can be prevented with proper placement of duct . Consideration should always be given to recycle treated air as dotted line indicates. When performing blasting and painting operations in closed tanks, several safety points must be remembered. All abrasive blast equipment, operators, nozzles and the object that is being blast cleaned must be grounded. Procedures must be established to control entry into tanks and enclosed areas until such time as the area is declared “gas free” by a trained and certified competamt person. For tanks that have been in service, this should be accomplished prior to initial entry, after Work breaks and prior to the start of a new shift. All lighting must be explosion proof. At least one person should be positioned outside the tank, adjacent to the access in case of energency.
7.7
