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PROCEDURE HANDBOOK

SURFACE PREPARATION AND PAINTING 0 F TANKS

AND CLOSED AREAS

SEPTEMBER 1981

Prepared by:

COMPLETE ABRASIVE BLASTING SYSTEMS, INC.
IN COOPERATION WITH .
AVONDALE SHIPYARDS, INC.

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Procedures Handbook Surface Preparation and Painting of Tanks and

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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 ship¬
yards, 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 Ccmpany r 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 Engineering
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

Executive Summary

A desperate need exists in shipyards for the proper planning and
execution of surface preparation ad coating operations in tanks and other
enclosed areas . Abrasive blasters and painters are exposed to high
concentrations of dust and hazardous organic vapors. Other shipyard
personnel are exposed to the potential dangers of explosion and fire.

Another aspect of the need for better planning concerns the ineffi¬
cient 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 sguare
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:

• 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
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

Fo reword i

Executive Surinary iii

Table of Contents iv

List of Figures v

List of Tables vi

1. Conclusions 1.1

1.1 The Role of Managaement 1.1

1.2 Recommendations 1.2

1.3 Cost Savings 1.3

1.4 Summary 1.5

2. Use of the Handbook 2.1

3. Ventilation 3.1

3.1 Introduction 3.1

3.2 Technical Discussion 3.1

3.2.1 Ventilation During Abrasive Blasting 3.1

3.2.2 Ventilation During Painting 3.3

3.2.2.1 Lower Explosive Limit 3.5

3.2.2.2 Explosive Vapor Detection 3.8

3.2.2.3 Threshold Limit 3.10

3.3 Equipment Selection 3.12

3.3.1 Fans 3.12

3.3.2 Ducting 3.18

4 . Dust Collection 4.1

4.1 Introduction 4.1

4.2 Technical Discussion 4.1

4.3 Equipment Selection 4.2

5. Dehumidification 5.1

5.1 Introduction 5.1

5.2 Technical Discussion 5.1

5.2.1 Principles of Condensation 5.2

5.2.2 Determining Dehumidification Requirements 5.10

5.3 Selection of Dehmidification Equipment 5.14

6 . Abrasive Blasting 6.1

6.1 Introduction 6.1

6.2 Abrasive Blasting Equipment 6.1

6.3 Compressed-Air Drying Equipment 6.7

6.4 Abrasive Delivery and Storage 6.12

6.5 Abrasive Recovery Equipment 6.13

6.5.1 Selection Criteria 6.13

6 .5.1.1 Portable Unit with Single-Chamber

Collection Tank 6.15

6 .5.1.2 Mobile Unit with Single-chamber

Collecti= Tank 6.18

6 .5.1.3 Portable Unit with Double-Chamber

Automatic Discharge Tank 6.19

7. Model High Production Abrasive Blasting and Coating Pier 7.1

Annex A - Suppliers List

Annex B - Selection of Abrasives

Annex C - Abrasive Cost Comparison

LIST OF FIGURES

Page

3.1 Explosive Vapor Detector 3.9-

3.2 Schmatic of Centrifugal and Axial Fans 3.14

3.3 Schematic of Axial Fan 3.14

3.4 Centrifugal Fan 3.15

3.5 Duct-Axial. Fan 3.15

3.6 Branch Entry and Elbow Radius Design for Dinting Layout 3.20

3.7 Ventilation Diagram of Large Enclosed Spaces, Small Tanks,

and Multiple Tanks 3.24

4.1 Schematic of Venturi Wet Scrubber' 4.3

4.2 Venturi Wet Scrubber Dust Collection -Unit 4.4

4.3 Reverse jet Continuous Duty Dry Fabric Collector Unit 4.5

4.4 Mobile Dust Collection/Dehurnidification System 4.6

4.5 Mobile Dry Cartridge Dust Collection System 4.6

5.1 Battery-Operated Psychron 5.5

5.2 Magnetic Surface Thermameter 5.5

5.3 Sample Psychrometric Report Form 5.6

5.4 Schmatic Dry HoneyCcmbe Dehumidification Principle 5.15

5.5 Model HC 9000 SEA Special Dry HoneyCombe

Dehumidification Unit 5.18

6.1 Portable Single-Chamble Multiple Outlet Blast Machine 6.3

6.2 Single-Chamber Multiple Outlet Blast Machine 6.3

6.3 Double-chamber Automatic Filling Principle 6.4

6.4 Double-chanber Automatic Filling Multiple Outlet

Blast Machine 6.5

6.5 Mobile Steel Grit Blasting and Recovery System 6.7

6.6 Schematic of Mobile Grit Blasting and Recovery System

in Operation 6.8

6.7 Compressed-Air Dryer and After-Cooler 6.9

6.8 Model for Compressed-Air Drying System 6.11

6.9 Pneumatic Delivery Truck 6.12

6.10 Delivery and In-plant Distribution System for Abrasives 6.14

6.11 Two Portable Vacuum Units with Single-Chamber Collection

Tanks Mounted on Stand 6.16

6.12 Portable Vacuum Units with Automatic Discharge Tank 6.17

6.13 Mobile Vacuum Recovery Truck with Single-Chamber

Collection Tank 6.18

7.1 Panoramic View of Model Blast and Coat Pier 7.3

7.2 1000 Ton Abrasive Storage Hopper 7.4

7.3 View of Ship Deck with Properly Installed Equipment 7.5

7.4 Schematic of Unit Coating Container 7.8

7.5 Cross-section Drawing of Ship Cargo Tank with

Blast-Coat Equipment Installed 7.9

7.6 Drawing of Tank Blast-Coat Operation 7.10

LIST OF TABLES

Page

Table I Ventilation Volumes Recomended to Maintain

Solvent Vapor Concentrations below 10% of the
lower Explosive 3.4

Table II Properties of Common Solvents 3.6

Table III Paint Vapor Concentration versus Ventilation Volume 3.11

Table IV Friction Loss Per 100 feet of Ducting 3.22

Table V Area and Circumference of Circles 3.26

Table VI Quick Dewpoint Reference Table 5.3

Table VII Wet Air Factor 5.12

Table VIII Dehumidifier Moisture Remove Rate 5.13

Table IX Comparison of Wet and Dry Desiccant

Dehumidifier Units 5.16

Table X Comparison of Typical DH Units 5.17

SECTION 1
Conclusions

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 stagger¬
ing wastage of manpower, materials, and time.

An attempt to dispense with in-house painting operations by sub¬
contracting 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:

•Modernize equipment

•Reduce dependency on other services

•Improve 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 accom¬
plished 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 and procedures required for the
surface preparation and painting 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:

•Reduce labor hours for both blast-paint operation and for support
services and equipment.

•Write procurement specifications for capital equipment procurement

• Reduce worker exposure to hazardous conditions

• Reduce facility and equipment losses

•Plan more competitive painting operations

•Reduce catastrophic paint failures

•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
to the program to insure competency, efficiency and quality at all stages
of the operation. These personnel should include:

•A 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.

•A 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 condi-
itions. This individual would aid in the selection of appropriate
coatings and preclude legal applications arising from coatings
failure.

•An instructor for an in-house program to train employees in the use
of blasting, ventilating, dehumidifying, painting and compressed-
air drying machinery.

General, components of the blast-paint operation which should be care¬
fully considered by management are:

•Development of an overall organizational plan

•Develooment of a program check list to include all equipment

• Standardization of procedures and inspection techniques

• Establishment of a comprehensiver equipment maintenance program

1.3

•Coordination of transportation, delivery and storage of materials,
to include support logistics

In the drydock area, modifications might include such things an end-
ramp access so that eguipment could be moved in and out without a crane, an
elevator or other personnel lifting system between dock and deck, increased
electrical services and installation of a high-volume compressed air piping
system. 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

• Eguipment to control ambient air renditions

• Adeguate 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 perma¬
nently 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 incor¬
porate design changes which facilitate construction activities. Some
suggestions are:

•Constructing permanent scaffolding supports in tanks

•Placing permanent openings in bulkheads

•Providing larger, more conveniently located hatch or cargo covers
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:

•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 on required services (cranes,
air hookup, water, etc. ) would be eliminated.

•improvement of environmental conditions. Many costly problems and
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 obvi¬
ously 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 paint failures.

1.5 Summary

Preparing surfaces of enclosed tanks for coatings, including necessary
ventilation and air treatment operations, is but one part of the construc¬
tion 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 avail¬
able 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.

1.6

SECTION 2
Use of the Handbook

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
verification 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. The equipment selection discussion describes equipment
characteristics. Knowing the calculated planning factors and equipment
characteristics a lead to the proper equipment selection for a given
blast-paint operation.

2.1

SECTION 3
Ventilation

3. Ventilation

3.1 Introduction

There are two primary proposes for ventilating tanks and enclosed
areas :

•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. Where dehumidification
and/or dust collection is indicated, ventilation is the basic component of
a total air treatment system.

3.2 Technical Discussion

The following sections present general guidelines for determining
ventilation reguirements. 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. 0. Box 453,
Lansing Michigan 48902.

3.2.1 Ventilation During Abrasive Bla sting

The amount of ventilation required during blasting depends on the
following four variables. Percentage figures indicate the relative im¬
portance in calculating requirements:

• Size of tank (cubic feet ) 60%

• Number of blast operators 15%

• Amountof corrosion on tank surface 15%

3.1

10 %

•Dusting or breakdown characteristics of abrasive
(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 adeguate 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
ventilation system in removing dust. A more detailed discussion of the
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 un¬
treated, 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 require¬
ments, 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. ) Ventilation Volume (CFM)

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

1,000

1,200

1.500
2,000

2.500
3,000
4, 000
5,000
6,000

10,000

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 is necessary to maintain visibil¬
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 require¬
ments 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
solvents. Since these limits are subject to change, the latest Federal
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 ex¬
tracted 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.

3.5

TABLE II

PROPERTIES OF COtMON SOLVENTS

Lower

explosive

Threshold

Cubic feet

limit (T.'FT.)

limit (TT.)

of vapor

in percent

per gallon

by volune

of liquid

of air

Solvent

at 70°F

Acetone .....

44.0 '

2.6

0.1

Amyl Acetate (iso) .

21.6

1.0

0.01

Amyl Alcohol (n) .

29.6

1.2

raiiy jl nxuj\- lidw; •••••••••»•••

Z37 • U

^ «->

JL. Z

Benzene .

36.8

1.4

0.001

2ir*o+* ra-f-ci f r-A

OA Q

i n

-i. • /

n m c

Butyl Alcohol (n) .

35.2

1.4

0.01

Butyl Cellosolve .

24.8

1.1

Cellosolve .

33.6

1.8

Cellosolve Acetate .

23.2

1.7

Cyclohexanone .

31.2

1.1

0.005

1,1 Dichloroethylene .

42.4

5.6

0.01

1,2 Dichloroethylene .

42.4

9.7

0.02

Ethyl Acetate .

32.8

2.5

0.04

Ethyl Alcohol .

55.2

4.3

0.1

Ethyl Lactate .

28.0

1.5

Methyl Acetate .

40.0

3.1

0.02

Methyl Alcohol .

80.8

7.3

0.02

A r\ n

• o

o r

Z*J

Methyl Ethyl Ketone .

36.0

1.8

0.02

Mo-Khirl rj—Drvorri r~\ Tfo4-rw»o

■sn a

i t;

n no

Naphtha (VM&P) (76^ Naphtha) ...

22.4

0.9

0.01

Naphtha (100° flash) Safety

Solvent - Stoddard Solvent .. .

23.2

1.1

0.01

Propyl Acetate (n) .

27.2

2.0

0.02

Propyl Acetate (iso) .

28.0

1.8

Propyl Alcohol (n) .

44.8

2.1

0.02

Propyl Alcohol (iso) .

44.0

2.0

Toluene .

30.4

1.4

0.02

Turpentine .

20.8

0.8

0.01

Xylene (o) .

26.4

1.0

0.01

3.6

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

follows:

Dilution Volume = 4(100-1 ^4)30.4

= 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 solv. evap.
gal ., of solvent 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 1 gpm paint

0.4 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 ventila¬
tion volume to prevent explosions are:

o» The 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 m-
easure, 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 reguiring 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 0 f 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 reduce-
the 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:

n ■ -i , ■ T7 , (100—TL) v

Dilution Volume = — 1 ——-

TL s

Where TL is expressed in percent by volume of air and vs is cubic feet
of vapor per gallon of solvent.

The dilution volume for the threshold limit of toluene solvent can be
calculated as follows:

Dilution Volume = 30.4 cu. ft.

= 151,970 cu. ft.

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 as calculated below:

Ventilation Volume

.1 51,970 cu. ft.
gal . of Solv .

x 1 gpm paint x 0-44 gal. — SOIV.

gal. paint

- 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

Paint Vapor Concentration - % by Vol. of Air

Table III - Paint Vapor Concentration versus Vent Volume

Paint Application Rate = 1 GPM

39.4 Ft^ Vapor Per

Gal. of Solvent

0.4 Gal. of Paint Solvent

Ventilation Volume - CFM

Maintaining the paint vapor 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 solvent-
based paints in tanks. Another alternative is to limit the paint applica¬
tion 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 movement 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

• 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. These fans are available in ranges of
10,000 to 50,000 CEM capacity. Due to their low static pressure ratings,
they require minimum horsepower (3-10HP). 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 and
spark-resistant construction. See Figure 3.4.

Centrifugal fans are capable of moving large volumes of air at high
static pressure, and therefore, are used in conjunction with dust Collec¬
tion and dehumidification systems. These fans can operate efficiently when
Connected to ,long runs of duct work. The increased static pressure capabil¬
ity 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.

A I R
Flow

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./ft
centerline 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 horse¬
power 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:

•Sizing, including factors of CFM, static pressure, velocity
requirements, and fan specifications.

•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 established design
objectives.

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:

• Air volume in CFM

•Distance air is to be moved

•Static pressure limitation of available fans

•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
reduced and there is danger of the duct collapsing. 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.

3.19

BRANCH ENTRY

Branches should enter at gradual expansions and at an angle of 30° or less (preferred)
to 45° if necessary.

ELBOW RADIUS

doesno^permif 6 2 ° r 21/2 diameters centerline radius except where space
Figure 3.6: Branch Entry and Elbow Radius Design for Ducting Layout

3.20

Same examples of static pressure loss for various types and sizes of
are as follows (assume 9000 CFM ventilation requirement):

• 18" smooth ducting will generate 1.7" of static pressure drop
100' of duct, and will provide a velocity of 5,000 FPM. (See Table
Iv)

•18" flexible ducting has a static pressure drop of 2.8" per 100'.
Adding one 90 degree bend along 103' will increase static pressure
‘drop to 3.2”. Two balds along -200‘ increases to 8.4" and three
bends along 3(X) increases to 12.6". the snaller cross-sectional
'area inside the flexible ducting, due to surface irregularities,
increases the velocity to 5, 500 FPM as compared to the smooth
ducting (see figure 3.6 for proper branch entry and elbow radius/
designs).

• 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 repre¬

sents 30, 000 CFM. As can be seen from the table, duct sizes
from 20" to 80" in diameter diameter will carry the required air

volume.

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

CU FT OF AIR PER MINUTE

.01

100 ooo

90 000

80 000

70 000

60 000

50 000

40 000

30 000

20 000

10 000
9 000
8 000

.02 .03 .04 .06 .08 .1

FRICTION LOSS IN INCHES OF WATER PER 100 FT

° n S ' andard Air °' °- 075 ,b p " CU “ denS '“ y n ° Wi " 9 lhroV ^ h ° rer °^' '°™d. galvanized me,al ducts having approximately 40 joints per 100 ft.)

Friction of Air in Straight Ducts for Volumes of 1000 to 100,000 Cfm

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 = . 1 " 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 appro¬
priate 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 see n, 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. To allow for maximum portability and ease of
set-up and 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

CLEAN AIR.

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 ppg pick-up point were
positioned higher in the tank. The duct - openings should be separated as
much as possible. See Figure 3.7.

PROPERLY VENTILATING
TANK.

35.000 CFM DIRTY AIR OUT FAN.

CLEAN AIR 35.000 CFM FAN.

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 particu¬
larly 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
for a too little space.

Metal ducting should be used for all straight runs. Flexible fabric-
reinforced ducting, which is more expensive and is subject to high wear and
tear, Should be used for inking connections to machinery and to small,
inaccessible tanks. 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 portability 0

After the system has been installed, periodic inspection of the
ductwork should be made to insure air-tightness. In addition, every new
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
square feet or 5,632 CFM. See Table V for area and circumferences of
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

TABLE 'v - Area and Circumference of Circles

Siam.

AREA

CIRCUMFERENCE !

AREA

CIRCUMFERENRE

Inches

S qua r e
Inches

Sq.ua re
Feet

Inches

Feet |

Inches

Squar'd

Feet

Inches

Feet

.7854

.0054

3-1416

.2618

706.8

4.909

94.25

7.854

1.767

.0123

4.712

.3927

754.7

5.241

97-39

8.116

3.1^

.0218

6.28

.5236

804.2

5-585

100.5

8.376

4.910

.0341

7.854

.6544

855-3

5-940

103.7

8.639

.0491

9*42

.7054

907.9

6.303

j.uo.8

0.901

9.620

.0668

11.00

• 9164

35 ■

962.1

6.611

109.9

9.163

12.57

.0873

12.57

1.047

.1017.8

7.069

113.1

9.426

15.90

.1105

14.14

1.178

37---

1075.2

7.467

116.2

9.686

19.63

.1364

15.71

1.309

1134.1

7-876

119.4

9.948

5a-

Zj.fb

• iopU

I?.26

1.439

x££ • 5

xu. 21

28.27

.1964

18.85

1.571

1256.6

8.727

125.7

10.47

33.18

.2305

20.42

1.702

1320.2

9.168

128.8

10.73

oQ JiQ

o£oo

Ol nn

-* Oon

ViOf l»

n 601

1 <V 1 A

44.18

.3068

23.56

1.964

1452.2

10.08

135.1

11.26

50.27

.3491

25.13

2.094

1520.5

IO.56

138.2

11.52

56.75

.3940

26.70

2.225

1590.4

11.04

141.4

11.78

63.62

.4418

28.27

2.356

1661.9

11.54

144.5

12.04

clL

On RA

(VI ww

JLftOO

on o?r

0 ;.o->

ITSIl Q

fl£

*•« *

Hi n 4.

TO on

10'

78.54

31.42

2.618

I809.5

12.57

150.8

12.57

95.03

34.56

2.880

1885.7

13.10

153.9

12.83

1151

7^54

00 nr\

3 -14?

ioai.»:

13-64

157.1

to no

132.7

.9218

40.84

3.403

2042

14.19

160.2

13.35

153.9

1.069

43.98

3.665

2124

14.75

163.4

13.61

J-5

17b. 7

1.227

47.12

3.927

2206

15.32

166.5

13.88

201.0

1.396

50.26

4.189

2290

15.90

169.6 .

14.14

226.P

1.576

63_4i

4.451

246?

17.10

175.9

14 .ft*

254.4

1.767

56-55

4.712

2642

18.35

182.2

15.18

283-5

1.969

59.69

4.974

2827

19.63

188.5

15.71

314.1

2.182

62.83

5.236

3019

20.97

194.8

16.33

346.3

2.405

65.97

5-498

3217

22.34

201.1

16.76

380.1

2.640

69.11

5-760

3421

23.76

207.3

17-28

^15.4

2.685

72.26

fc>.021

3632

25.22

2.1.3.6

17.80

452.3

3.142

75-40

6.283

3848

26.73

219.9

18.33

490.8

3.409

78.54

6.545

4072

28.27

226.2

18.86

530.9

3-687

81.68

6.807

29.87

232.5

19-37

572.5

3-976

84.82

7-069

4536

31.50

238.8

19.90

<-U

( 0

87.96

?Oiu

**yyo

J_J. -w

2G.*+2

660.5

4.587

91.11

7.592

5027

34.91

251.3

20.94

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 ventilation e Examples for the most part have been f or
ventilation of ambient, untreated air. The next section will identify the
components of the dust collection system, which cleans the dust-laden a i r
exhausted by ventilation.

3.27

SECTION 4
Dust Collection

4. Dust Collection

4.1 Introduction

The utilization of dust collection equipment to clean contaminated
exhaust air resulting from manufacturing activities is an existing tech¬
nology 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
ard will substantially reduce job-site housekeeping.

Current or pending federal legislation may seen force every shipyard
and contractor to clean the contaminated air generated by abrasive blasting

an< 3 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.

•wet scrubber
•Dry Fabric
•DrY 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.

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 cleaning 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 assembled 0 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 equip¬
ment. The moisture laden air increases the load on dehumidification equip¬
ment. 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 - FAN MOTOR

11 - PUMP DISCHARGE

12 - PUMP

13 - PUMP INLET

14 - SLUDGE CONVEYOR

15 - CUSTOMER'S SLUDGE CONTAINER

16 - SLUDGE CONVEYOR DISCHARGE

17 - 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 eguipment, 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 25,000-
CFM unit. Frequent handling of this type of unit will result in increased
maintenance and repair costs. See Figure 4.3.

A A

FIGURE 4 . 3 : Reverse-jet Continuous Duty Dry Fabric Collector Unit
- 70,000 CFM

The dry fabric collector is most efficiently utilitized in semi¬
permanent, pierside, barge-mounted, or railcar-mounted arrangements.This
system is also appropriate for large capacity permanent installations.

For individual tank jobs reguiring 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 cleg 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 all-
weather, 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 equipment selection should be based on the per¬
formance requirements of the intended application. As a general selection
guide1ine:

• 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 .

• For stationary applications requiring a single unit of over 35,000
CFM, the dry fabric collector is best.

• 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

SECTION 5
Dehumidification

5. DEHUMIDIFICATION

5.1 Introduction

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.
The resulting surface contamination promotes poor adhesion of the protec¬
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 require¬
ments. 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.

5.2.1 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 carrying-
capability 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 psychrom¬
eter, 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 condi-
itions change.

Once these general principles are understocd, several points must be
remembered in connection with the dehumidification of air in shipboard
tanks .

5.2

TABLE VI. QUICK DEWPOINT REFERENCE TABLE

CRY BULB TEMPERATURE °F

95 90 85 80 75 70 65 60 55 50 45 40 35 30

90 89 -

W 85 82 83 - - - -.

E 80 75 77 78 - - - - - - - - _ _ _

T 75 66 69 71 73 - - - - - - - - - _

1 0 57 60" 63 65 68 ---------

B 65 45 49 53 56 60 62 - -- -- -- -

U 60 26 35 40 46 50 54 57 -

L 55_9 22 30 37 42 47 51 - - - - - -

B 50_
45
T 40_
E 35_
M 30_
P 25_
20
F 15
10

0 17 27 34 41 46 -----

_ -4 15 26 33 40 - - - -

_ -4 15 27 34 - - -

_ 4 20 28 - -

_ -12 11 24 -

_ -4 15

• 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 310F, 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 proce¬
dure will provide detailed records of job conditions for future use.

The battery-operated Psychron Model 566, available from the Environ¬
mental Service Division of Bendix, provides wet- and dry-bulb tempera¬
ture 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 Instru¬
ments, 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, anc [ j_ s 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

DB-DRY BULB TEMPERATURE WB-WET BULB TEMPERATURE DP-DEW FOINT TEMPERATURE

DATE

TIME

FOREMAN

WEATHER*

OUTSIDE

DB WB DP

INSIDE

DB WB DP

TANK

SURFACE

TEMP

DIFFERENT

SURFACE TEMP

INSIDE DP

1/4/80

0800

WHITING

CLEAR

45/40/34

45/40/34 45 ABOVE

W.L.

+ 10F

40 BELOW

W.L.

+ 5F

1200

WHITING

CLEAR

60/50/41

60/50/41 60

ABOVE

W.L.

+25F

40 BELOW

W.L.

+ 5 F

1600

WHITING

CLOUD

50/48/45

50/48/45 50 ABOVE

W.L.

+ SF

CHANGING

40 BELOW

W.L.

- 5F

2000

BIBBO

CLEAR

40/38/35

40/38/35 40

ABOVE

W.L.

+ 5F

40 BELOW

W.L.

+ 5F

2400

CROTTY

CLEAR

40/38/35

40/38/35 33

ABOVE 1

W.L.

- 2F

CHANGING

40 BELOW

W.L.

+ 5F

0400

GIESE

RAIN

50/48/45

50/448/45 33

ABOVE

W.L.

-12F

40 BELOW

W.L.

- 5 F

•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 40°F

Ambient air temperature: dry-bulb 45°F

Ambient air temperature: wet-bulb 40°F

Dewpoint temperature (see Table VI) 34°F

Surface temperature: above water line 45°F

Surface temperature: below water line 40°F

(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.

12:00 NOON

Water temperature 40°F

Ambient air temperature: dry-bulb 60°F

Ambient air temperature: wet-bulb 50°F

Dewpoint temperature (see Table VI) 41°F

Surface temperature: above water line, 60°F

Surface temperature: below water line 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

°F

Ambient

air temperature:

dry-bulb

Ambient

air temperature:

ret-bulb

8‘

Dewpoint

temperature

(see

Table VI)

°F

Surface

temperature:

above

water line

°F

Surface

temperature:

below

water line

°F

(relative humidity 85%)

A storm enters the area, bringing additional moisture With in turn
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 50° F , is still 50 F above
the dewpoint temperature, so condensation will not occur on those surfaces.
DH required below water line.

8:00 P.M.

Water temperature 40°F
Ambient air temperature: dry-bulb - 40°F
Ambient air temperature: wet-bulb 38°F
Dewpoint temperature (See Table VI) 35°F
Surface temperature: above water line 40°F
Surface temperature: below water line 40°F

The storm has passed and ambient air is dryer. Surf ace temperatures,
both above and below the water line, are again 5°F higher than the dew-
pint. No DH required.

5.8

12:00 MIDNIGHT'

Water temperature

Ctj

Ambient

air temperature:

dry-bulb

Ambient

air temperature:

wet-bulb

Dewpoint

temperature (See

Table VI)

35 °F

Surface

temperature: above water line

33%

Surface

temperature: below water line

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 surface temperature above water is only 33°F. DH
reguired above waterline.

4:00 A.M.

Water temperature

40%

Ambient

air temperature:

dry-bulb

50 °F

Ambient

air temperature:

wet-bulb

48 °F

Dewppoint temperature

(see

Table VI)

45°F

Surface

temperature:

above

water line

33%F

Surface

temperature:

below

water line

40°F

(relative humidity 85%)

Surfaces both above and below the water line have cooled during the
night to temperatures below the dewpoint. Thus condensation will occur on
all ship's surfaces exposed to ambient air. DH reguired.

During this 24-hour period, three different coditions were experi¬
enced.

c a

• 4:00- P.M. the storm passed through and raised ambient air moisture
levels. The dewpint rose above the temperature of the surface
belcw the water line caused condensation below the water line.

• 12:00 Midnight surfaces above the water line mold through heat
radiation to a temperature lower than the dewpoint and mndensa-
tion 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
outlines the methodology of determining dehumidification requirements.

5.2.2 Determining Dehumidification Requirements_

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.

TABLE VII

WET AIR FACTOR
TANK SURFACE TEMPERATURE

at; an qk an *71;

0 .014 .025

0 .011
0

.035 .044 .051
.021 .030 .037
.010 .019- .026
0 .009 .016

_ 0 .007

.057 .062 .066
.042 .048 .051
.032 .037 .040
.022 .027 .031
.013 .018 .022
.007 .011 .015
0 .005 .009

0 .004

.069 -

♦055 -

.044 .048
.035 .038 .040
.026 .029 .032
.019 .022 .024
.013 .016 .019
.008 .011 .014
.004 .007 .016
0 .003 .006

0 .003

To determine the amount of moisture in the air, follow dewpoint
temperature across, surface temperature down (measure surface temperature
of tank with a magnetic thermometer. Figure 5.2). The intersection is the
wet air factor, in pounds per hour per CEM. Example: dewpoint temperature
50°F, surface temperature 45°F = .011 lbs.

TABLE VIII

DEHUMIDIFIER MOISTURE REMOVAL RATE - LB/HR
DRY BUIB TEMP F

*3n

284

320

356

392 428

—

70“

249

"277"

304

332

359

386

—

„D

220

244

269

294

318

356

393

—

-60

179

202

226

249

272

310

347

384

—

162

182

202

223

243

260 278 295 312

—

T*\

±3^

156

174

191

208

221

^34

24/

260

266

—

•45

152

160

179

196

214

217

220

223

226

—

■i. U

i cn

1 CLA
xvrr

i *7n
u_ / z*

~i on

J.UU

i m
1Q1

103

lOt

*i /-in
103

—

127

136

145

148

150

152

153

154

156

—

116

119

122

121

Ti
* *-»

n a

1 -X

*7C
/ ^

This graph shows the moisture-removal rate of one (1) Cargocaire Mxlel
HC-9000 SEA Special 9000 CEM Dehumidification Unit under varying condi¬
tions . (Refer to manufacturer for performance data cn other systems).
Follow dewpoint temperature across, dry bulb temperature down. Intersection
is moisture removal rate in Ibs/hr. for model specified. Example: dewpoint
temp. 50°F, dry bulb temp. 75°F = 208 Ibs/hr.

5.3 Selection of Dehumidification Equipment

Three types of air treatment systems - be used to control dewpoint
temperature.

• Air-conditioning

•Wet desiccant dehumidifiers

•Dry desiccant dehumidifiers.

Air conditioning cools air through the use of refrigeration coils to
condense out moisture from the 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. While this method can theoretically control
condensation, and can be effective for small tanks, it is extremely
inefficient, uses excessive amounts of energy, and does not to remove
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 humidi¬
fiers 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

The dry system operates on the same fcasic principle as the wet unit,
except that the desiccant is dry. The desiccant is usually attached k to a-
rotating core within the unit, ihe dry desiccant unit is simpler in design
and requires less auxiliary equipment, infrequent desiccant replacement,
and lower initial and maintenance costs. In addition, it presents ro
corrosion problems and can be essentially self-cleaning. Because of its
simplicity, portability, and ability to operate unattended (requires no
adjustments), the design of this system is particularly well-suited for
marine ooating applications. For these reasons, this system has been
selected by most shipyards and marine painting contractors in the U.S. (See
Figure 5.4).

REACTIVATION

REACTIVATION A1R 0UTLET

FIGURE 5.4: Schematic of Dry BoneyCcmbe Dehumidifcation Principle

EH units can be designed for CEM capacities frcm 50,000 to 100,000
CEM. For coating applications, where portability is an important factor,
the most flexible size is the 9000 CEM unit (see Figure 5.5). When con¬
ditioning large volumes of air, the individual units are combined to
produce the desired capacity. DH equipment can be located on deck, skid-
or trailer-mounted pier side, or barge-mounted. If dirty air frcm the tank
is to be cleaned, dehumidified, and recirculated to the blasting areas, the
EH units must be located downstream frcm the dust collector.

5.15

TABLE IX

COMPARISON OF WE7T AND DRY DESICCANT HU MID IF IER S

Basis of Comparison

Wet System

Dry System (HoneyConibe)

Auxiliary equipment
required

Filters, reac. discharge
duct and steam valves
and traps, water valves

React, discharge duct
steam valves and traps

Minimum delivery
dewpoint

10°F

-60°F

Installation

Utilities

Ducts .

Solution piping

Regenerator (separate
from conditioner)

Utilities

Ducts

Cleanability

Replace solution
(monthly analysis required)

Vacuum surfaces

Regeneration/

Process Air Separation

Separate units

Positive seals 0 leakage
@ 12" 10-12 years life

Conditioner Housing

Life Expectancy

12-15 years

Corrosion

Unlimited, no corrosion
(unit #1 still in
operation after 15 years)

Dust Filtration

Scrubber effect
(Contaminates and/or
reacts with solution)

Self-cleaning. Particles
up to 1000 microns will
pass through wheel.

35% prefilter‘typical.

Humidity Control

Solution concentration

Slew response

Bypass damper system
Immediate response +1%RH

Suitable for ,
outdoor installation

yes

Shutdown precautions

Periodic running required ■'

None required

Desiccant carry-over

Common (extremely
corrosive) desiccant
replacement required.

None

Desiccant

crystallization

May occur - requires
shutdown and cleaning

N/A

Energy modulation

Optional in response to
moisture load

Desiccant replacement

Check monthly, $800 per

55 gallon drum

• Permanent except for
ambient sulphur. Typical

field reimpregnation
every 3/5 years.

Cost $200.

TABLE X

TYPICAL COMPARISON

The following table is a comparison of dehumidification equipment
based cn current catalogg. It is for a 6000 CEM dehunidifier operatic at
an inlet condition of 95 F, 120 grains per pound, using 100 psig saturated
steam far reactivation energy.

Supplier

Cargocaire (dry HoneyCcmbe)

Kathabar (wet)

Model

HC-6000

1250 - 14C

Delivered air volume
(scfm)

6000

Utilities

460/3/60 electrics

100 psig steam

460/3/60 electrics
100 psig steam

85°F water

Weight (lbs.)

2150

9820

Floor space (sq. ft.)

Steam required
(lb./hr.)

491

629

Electricity required
(HP)

8 - 1/4

SECTION 6

Abrasive Blasting

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
lbs. 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
resulting operating time of about 30 minutes. 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

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 mutliple outlets
are now available. The main design features of these machines are:

•Large abrasive capacity which allows extended Periods of uninter¬
rupted operation.

•Bulk pneumatic refilling from delivery trailers, completely sealed
system which provide weather protection for abrasives.

•Multiple-nozzle outlets and fast equipment set-up.

•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:

• 6-ton

• 22-ton

• 4 0-ton

Each machine is designed for a specific application. See Figures 6.1
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 opera¬
tors working in a central area. This machine is particularly well-suited
for spotblasting contaminated areas on new fabrications. Forty-ton units

Figure 5,1s

Figure

Portable Single-chamber Multiple Outlet Blast Machine
- 6 Ton Capacity

.2: Single-chamber Multiple Outlet Blast Machine

- 40 Ten Capacity (Photos oourtesty of CAB Inc.)

6 .'

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 f or 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.
maintaining the bottom chamber constant pressure, the top chamber can
be depressurized and filled with abrasive. The abrasive will be automatic¬
ally 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.

MSA-6 FILL CYCLE

LEGEND

1 ABRASIVE CHARGING VALVE

2 THREE WAY VALVE

3 AIR CYLINDER

4 CA 8 DYNATROlPROBES

5 OVERHEAD STORAGE HOPPER

6 AIR SUPPLY VALVE

7 MANIFOLD DRAIN VALVE

8 AIR FLOW VALVE

9 GRIT FLOW VALVE

10 ABRASIVE METERING VALVE

11 ABRASIVE SHUT OFF VALVE

12 MOISTURE SEPARATOR

13 CONTROL BOX

14 AIR RESERVE TANK

Full

Fillin

Figure 6.3: Schematic of Double-Chamber Automatic Filling Principle

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 essen¬
tial 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 auto¬
matic 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:

• 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) .

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:

•Existing compressed-air volume and pressure capabilities.

• Abrasive storage capacity

•Crane capacity

•Abrasive delivery schedule

• Number, frequency and location of blasting jobs.

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:

• Deliquescent

• Refrigerated

• Regenerate

For abrasive blasting operations, the deliquescent system provides the
most trouble-free solution to cleaning and drying the air. In addition, it
has the lowest initial cost and is least expensive to maintain. High-
volume units are available Which are constructed with liftirg eyes to
permit easy relocation. See Figure 6.7.

DELIQUESCENT
COMPRESSED AIR DRYERS.
Provide dependable dry, clean,
compressed air with zero energy
required to operate. Lower initial
and operating costthan powered
type dryers. Deliver a safe,
compensating dew point about
20°F less than the entering air
temperature, so no moisture can
condense in your air system.

AIR-COOLED AFTERCOOLERS
Economical Turbo-Cool After- f
copiers use atmospheric air ™
(it s free!) to cool 350° F air
from the compressor to within
5°, 10°, 15°, or 20°F to the
ambient air, condense up to
70% of the moisture from the
raw compressed air. Horizontal
and vertical models for indoor or
outdoor installation, capacities from
30 to 3500 SCFM.

FIGURE 6.7

Refrigerated units represent a complex systems which requires qual-
ifid 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.

r c\

The regenerative system also requires qualified service personnel for
maintenance. These units do not remove oil without the addition of pre- and
after-filters, 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 cool in the air
lines before entering the dryer. In addition the dryer will catch any
contaminants which might have entered into the system.

Figure 6.8: Model for Catpressed-Air

Drying System

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 deliq¬
uescent 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 pro¬
tection and coatings application. It also eliminates plugging or clogging
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.

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 eguipment 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 guantity 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 avail¬

able :

• 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 blast¬
cleaning, the following criteria Should be considered:

•Equipment operation with reduced labor

•Support services requirments

•Equipment size in relation to available space in the work area

•Hose size needed to operate at maximum efficiency

•Initial 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
in setting up the vacuum system. 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.

6.15

Figure 6.11: Two Portable Vacuum Units with single-chamber Collection
Tanks hunted on Stands

6.16

Figure 6.12: Portable Vacuum Units with Automatic Discharge Tank

6.17

Portable single-chamber units can be hooked U p to a pneumatic dis¬
charge 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 S o 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 shipyard applications,
maintenance requirments and short-cycle performance make it impractical
for most internal tank-cleaning jobs.

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

SECTION 7

Model High Production
Abrasive Blasting and
Coating Pier ,

7. Model High Production Abrasive Blasting and Coating Pier

Vi-irrH rtr/vlnnf n r\n ahracirro "Kl a <a4- *i rvrr anrl rv-\34- -? rorr nior eVinnl/^ Vv*

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established in an area which can be totally dedicated to painting and
painting related activities. The pier area should be large enough to
acccrrmodate the largest ship requiring painting. Ships can remain in the
water. Figure 7.1 shows a panoramic (photographic) view of such a layout.
In this photograph seme of the important aspects of the operation can be
seen. -These activities are listed below. The photograph is numerically
rsLGyscl to ths activity nurnbGir •

1. Bulk abrasive is delivered by railcar or pneumatic truck and is

transferred into one of two 1,000 ten capacity storage hoppers (also

ooo fi rrnra 7 PnnVof ol DTraf nrc i icorl nri1r-*=ar? 4-Vie* >-n-i 1 rv^^rr*

-A--A.-J W. ■— I ■ A. / * ___ >_* V A1. M WVU VAy WUAV\A'a J.U4. LV^Ui. J »

These operations require minimuti labor and the abrasive stays dry
regardless of weather conditions.

2. Mounted belcw each storage hopper is a double-chamber, autcmatic-
filling, multiple outlet blast machine (also see figure 7.2). These
machines are automatically filled without the necessity of an attend-
ant* Eight blasters can operate frcui each blast machine*

3. Two inch I.D. abrasive blasting pipes are routed through a trough
below grade level to the ship. The pipe size permits long runs with
minimum pressure drop. Grating is placed over the troughs to permit
crossing.

4. The ship is moored to the pier, (just off photograph.)

5. Portable vacuum units with single chamber collection tanks are mounted
on stands to permit discharge into a disposal truck (also see figure

C ft\ rrru * — ,'j - i i l xza j 3 — n 3 31 _ i _i_i •

1 . UiLLLb UcUi lllUWd dii-l piaUtU d-tj cHJtiUU tu tne pUJLXIC UI

use to reduce vacuum hose length. In this photograph the units are
stationed alongside the ship.

7.1

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 accomo¬
date long runs with minimum set-up time.=

10. Dust collection duct is also prefabricate.

11. 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 CE11
compressor station is shown. This station is eguipped 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. Pier management and quality control are located in an office on site.

14. A portable crane is necessary to provide lifting services.

15• A shelter can be used to provide protection for inclement weather and
the sun.

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

Figure 7.2: Close-up of 1000 ton capacity Abrasive Storage Hoppers

7.4

midship with prefabricated ducts extending forward and aft. Flexible ducts
are connected between the prefabricated ducts and the cargo access ports
(hatches) . The dust collecting duct can be seen midship on the port (water)
side. There are thee difference ducts. 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
from the intake side of a fan. In most all cases efficiency of the
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 explo¬
sion proof. At least one person should be positioned outside the tank,
adjacent to the access in case of energency.

7.7

Figure 7.4: Drawing of Unit Coatings Container

7.9

Figure 7.5: Cross Sectional Drawing of a Large Ship Cargo Tank

7.10

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Figure 7.6: Drawing of Ship Tank

ANNEX A
Suppliers List

ANNEX A

SUPPLIERS LIST

Abrasives Suppliers

/"»1 1 4 -r—_

^LCVCiCUiU I’ISLCU. ttLJXCUDXVt^D f Xiiu*

'' ‘305 Euclid Office Plaza, Cleveland, -Ohio
Flint .Abrasives

Rov T-1.d98_ .Toni in* M~, f^lPm fA17l CT5_')/M/I

-— — —X—“*•*** * f *• • / v V I-*- » /

H.B. Reed and Company, Inc. (Black Beauty)

8149 Kennedy Avenue, Highland, Indiana 46322, (219) 923-4200

Unimin Corporation

50 Locust Avenue, New Canaan, CT. 06840, (203) 966-8880
Wedron Silica Company

400 West Higgins Road, Park Ridge, Illinois 60063, (312) 692-3322
Whitehead Brothers Company

66 Hanover Road, Florham Pakr, NJ 07932, (201) 377-9100

Abrasive Blastina Eauirment SuddI iers

... . . ■■ , tk - it -- -- ±..r ~ —7

Clemco Industries

Jerrols at Upton Street, San Francisco, Ca 94124, (415) 282-7290

Complete Abrasive Blasting Systens, Inc. (CAB)

18250 68th Avenue, South, Kent, WA 98031, (206) 226-6012

A.l

Enpire Abrasive Equipment Corporation

2101 Cabot Blvd., West, Iangbome, PA 19047, (215 ) 752-8800

Pauli and Griffin

137 Utah Avenue at Wattis Way, South, San Francisco, CA 94080
(415) 873-4540

Vacublast Corporation

Post Office Drawer 885, Belmont, CA. 94002, (415) 592-2121
Wheelabrator-Frye, Inc.

451 Byrkit Street, Michavaka, Indiana 46544, (219) 255-2141

Ccmpressed-Air Dryers Suppliers

General Air Division Zum Industries

1335 West 12th Street, Erie, PA. 16501, (814) 453-3651

Pure-Aire, Inc.

Post Office Box 5584, Charlotte, NC 28225, (704) 377-4815
Van Air Systems, Inc.

349 Mechanic Street, lake City, PA. 16423, (814) 774-2631

Dust Collection Equipment Suppliers
Air Polution Systems, Inc.

18642 63th Avenue, -South, Kent, WA 98031, (206) 251-5330

American Air Filter Company, Inc.

200 Central Avenue, Louisville, KY., 40277 (502) 637-0011

506 73rd Avenue, NE, Minneapolis, MN. 55432, (612) 571-1000

Torit Division, Donaldson Company, Inc.

1135 Rankin Street, St. Paul, mm 55164, (612) 698-5330

W. W. Sly Manufacturing Company

21945 Drake Road, Strongville, Ohio 44136, (216) 238-2000

Dehumidification Equipment Suppliers _ '

Bry-Air

Post Office Box 269-T, Sunbury, Ohio 43074, (614) 965-2974

Cargocaire Engineering Company

79 Monroe Street, Anesbury, MA 01913

Dry ana tic

Airport Industrial Center, Gaithersburg, MD 27760, (301) 948-5000

Post Office Box 791, New Brunswick, NJ 08903, (201) 356-6000-

IRS/lntemational, Inc.

Post Office Box 94, Issaquah, WA 98027, (206) 392-3953

r \« A m — r *.,——3 4 ~

t JC o a

Aeroduct-Porter Canpany

315 Porter Building, Pittsburgh, PA 15219, (412) 391-100
Anaconda Metal Hose

700 South Main Street, Waterbury, CT 06723, (203) 574-8500

Flexaust

11 Chestnut Street, Amesbury, MA 01913, (617) 388-9700
United McGill Corporation

200 East Broadway, Westerville, Ohio 43081, (614) 882-7401

Fans Suppliers

American Cbolair Corporation

P-O. Box 2300, Jacksonville, EL 32203"

Aerovent, Inc.

Ash and Bauer Streets, Piqua, Ohio 45356, (513) 773-4611

Cincinnati Fan and Ventilator Company, Inc.

5317 Creek Eoad, Cincinnati, Ohio 45242, (513) 984-0600

Coppus Engineering

341 Park Avenue, Vforchester, MA 01602

Hartzell Propeller Fan Canpany

910 South Downing Street, Piqua, Ohio 45356, (513) 773-7411

Strobic Air Corporaticn

207 Bunting Avenue, Trenton, NJ 08611, (609) 396-8216

Vacuum Equipment Suppliers

Central Engineering, Inc. (Vac/All)

4427 State Street, Milwaukee, WI 53209, (800) 558-6944

D.P. Way (Ultra Vac)

Post Office Box 09336, Milwaukee, WI 53209, (800) 558-6944

Key-Houstcn, Inc.

13911 Atlantic Blvd., Jacksonville, Florida 32225, (904) 241-0633

Super Products (Supersucker)

Post Office Box 27225, Milwaukee, VJI 53227

ANNEX B

Selection of Abrasives

ANNEX B

SELECTION CF ABRASIVES

Abrasives are hard, granulated materials which are propelled by
compressed air and impacted on the metal surface to remove contamination
such as corrosion. Because they are oonsumed in such large quantities,
abrasives represent one of the largest expendable materials costs in the
shipyard budget and therefore must be carefully selected and economically
utilized.

To obtain the highest cost savings, abrasives selection should be
determined by the specific job application. Factors include:

• Size and duration of job (i.e., number of tanks to be cleaned)

• Size and configuration of tank

• Condition of tank surface

• Availability of equipment

These considerations must be matched to the characteristics presented
by abrasive materials.

• Physical properties

• Availability

• Performance and safety

• Recyclability

The surface conditions of the tank will determine the coarseness
required of the abrasive. Heavily corroded surfaces require a coarser grit
material such as U.S. Sieve No. 8-20 (75,000 particles per pound), whereas
lighter corroded surfaces require a finer grit material such as U.S. Sieve
No. 25-40, (800,000 particles per pound). The following guidelines are

important in properly sizing abrasives.

B.l

Heavily corroded surface;

• First mechanically descaled are large rust deposits.

• Sand sweeping with coarse abrasive (U. S. Sieve No. 15-30).

• Final blast with medium-sized grit (U. S. Sieve No. 30-45).

Lightly corroded surface:

J -' » /rt

biz,oa yiit \u«

o. Dieve ino. ju-ou;

New and primed steel

• Blast with medium-io- fine sized grit (U. S. Sieve No. 40-70)

The abrasive grit size required for cleaning painted surfaces depends
on the type of coating to be removed. Generally, a medium to coarse mix
will provide best results.

Abrasive selection for tank surface conditions may be limited by the
availability of spscific abirasivas- Availability/ in tnm r dspcixls on tlic
location of the shipyard in relation to the source of supply and the market
situation.

Performance and safety, in terms of dust generation, blasting produc¬
tion rates, and visibility are directly related to the abrasive physical
characteristics, including chemical content, aggregate size, grain shape,

\-J 1 ■? 4*t r ( A a ^-4- 4-ft

i i'ooo cuia -i—u \ J- • V- • / LCOXOtciilbb 01iaL.UCJL •

The recyclability of the abrasive basically depends on friability
since this determines how much of the material can be effectively reused.

The following comparisons present the most efficient applications of
three types of abrasives, i.e., sand, mineral slag, and steel grit. Cost

CGTipciir i. sons aJtrs cx^ntcLinscl jlti Annsx C*

B.2

Sand

In the past, sand was the most ocnrnonly used material for abrasive
blasting, with the determination that silica sard presented a health hazard
to workers (silicosis), EPA and OSHA regulations have pressured shipyards
to use other types of abrasives. However, with the advent of air-fed
helmets for blast operators and dust collection equipment for dust control,

4*Tv^ nra T-vrv ^ I - A rrr. —'Uv--. — 4

i—r v^jl. tji.u *. * iib/uj.u a. c-v-ui 10 j.ucjl cu. oo t_i i_ C1J.CUU4.VC QUiaOXVC •

- • Sand is readily available to most shipyards. River and ocean sands are
not the best choices for use as abrasives. The ideal type of sand is
obtained frcm a quarry or sandpit. All sands should be properly graded and
cleaned prior to purchase. In addition to availability, the greatest
advantage of sand is low cost. Savings of frcm 30 to 100% can be realized
by using sand as opposed mineral slag for suitable applications.

Sand abrasives should cnly be used in enclosed tanks where dust
collection equipment and air-fed operator helmets are in operation.

Mineral Slag

As a result of federal legislation promoting the use of non-silica
abrasives, mineral slags have beccme the most ccrrmonly used material for
shipyard blasting. Because these abrasives are obtained as a by-product of
other industries several problems are created for shipyards. These problems
include such things as distance to supply sources, transportation, logis¬
tics, and unpredictable availability. Many times, the mineral slag supplied
to shipyards is not suitable due to non-uniform particle size and high dust
content. These conditions result in reduced productivity and increased

o Til 4*V»/"\n/"TT"» rmmesvol -svq *fv^vrrt h, v.<-n -!

• i w. WlUU^l A 11 LJ-A. A>—JL_ OU^J. UO V CJJ- ^ 1 l UJOLXj / 1|J QU V 011 l_Ciy C | I I

productivity over sand of similar quality is obtained.

Better grades of mineral slag do offer the possibility of recycling

duri na enclosed -hank 'hlas-Hner. The nonava i 1 ah>i 1 i -Hr o-F arlpnnafo eumlioe m=.r

- - ZJ- --- J. -»»Mjr

warrant investment in an automated recovery system. (Refer to Section 6.5)

B.3

Generally, slag abrasives are best-suited to external blasting

qvwO i Vknl 1 —>«4-i»n«4.iw« rrf 1 _J*

u*a liuxx/ vx'x^-.rv, cuxi oupbi.ou.uv^uuLC wt/JLJS.* j.Il£SSe dJJI, d5 _LVeS

should also be used for open blasting areas within the yard.

Steel Grit

Steel grit abrasive is more expensive than either sand or mineral
slag. This material is supplied in uniform particle size and is dust free.

TD«i r-L/r 4-Vio

■ -- ■ i. IVj f uu^

3rit is easily v^ntauiinated and ±. uiiitd. fey watt:!., and great

care must be taken that moisture does not enter through tank openings,
ventilation equipment or ocmpressed air system.

The principle advantage of steel grit is high resistance to shattering
and therefore suitability for recycling. Steel grit can be reused 100 times
if properly handled. If steel grit is used in conjunction with dehimidifi-
cation and ocmpressed air drying to felast snail or medium sized tanks, nigh
recyclability can yield substantial savings to the shipyard. See Figures

6.6 and 6.7.

B.4

• -- ANNEX C
Abrasive Cost Comparison

ttKIKTCV n

Ml < I l/\

Abrasive Cost Comparison

The following comparisons are based cn a blasting operation using l/2"
nozzles at 100 psi and 300 CEM. Under these conditions, each operator will
use approximately 1,500 pounds of sand or mineral slag per hour or 4,500
pounds of steel grit per hour. The average delivered cost for one ten each
of the following abrasives is.

Sand - $17.00
Slag - $40.00
Steel Grit - $350.00

Sand vs. Mineral Slag

With ten men blasting during an eight hour shift, abrasive consumption
would be:

10 men x 1,500 Ibs/man/hr = 15,000 lbs

1 ^ nnn The v ^ lire anfnal V\1 acf — OH 1 V\r—

VW .«■ *t. W tu.u . ■ ^ \J f wy ■ 1 /Q

90,000 lbs = 45 tons of abrasives used per shift
The cost of sand abrasive would be:

45 x $17.00/ton = $765.00 per diift for sand abrasives.

TTnr^T ci^nv* mrYl-ifinnc_ -f-*ho rv^erf* rvF mi noral

:1 art aKraoiTroc nrMil/1 •

w irwujL.<a •

45 tons x $40.00/ton = $1,800.00 per shift for mineral slag.
Cost difference for one eight hour shift would be:

$1,800 - $765 = $1,035

With a conservative assumption of 250 days a year of blasting:

250 x $1,035 = $258,750.00 cost difference per year.

This figure represents the possible yearly savings in abrasives costs
if sand were used in lieu of slag. The money saved could be used to
purchase all or most of the dust collection equipment required when using
sand abrasives.

Non-recycled Slag vs. Steel Grit

Calculating abrasive oosts per year and assuming six man-hours per day
and 250 blasting days per year,

6 hrs x 250 days = 1,500 nan "hours (M.H.) of blasting per year

per operator

At a consumption rate of 1,500 /lbs/M.H. for slag abrasives.

1,500 lbs x 1,500 M.H. = 2,250,000 lbs per operator

2,250,000

2,000

= 1,125 tons of slag per year for each operator.

At a consumption rate of 4,500 lbs/hr for steel grit.

4,500 Ibs/hr x 1,500 M.H. per year = 6,750,000 lbs per operator

6,750,000 lbs

2,000

= 3,375 tons of steel grit per year.

If properly utilized however, steel grit can be recycled up bo 100 times
(cycles), thus:

3,375 tons
100 cycles

= approx. 34 tons of steel grit per year per operator.

C.2

Cost per year far slag would be:

1,125 tons/yr x $40/'ton = $45,000.(JO per operator
Cost of steel grit for actual consumption would be: -

o a 1 _/__ / i__ _ <■»-» -t r\/^r\ _ _ _ _•_

uuiits/yj. a 9JJU/UJU = 9i_jL/^uu per qperax-or

Possible yearly savings in abrasives costs of using steel grit over
non-recycled slag, then are:

$45,000 - $11,900 = $33,000 yearly cost difference per operator

Calculating abrasive cost per square foot of cleaned surface, and assuming
200 square feet can be blasted by one operator in one hour:

200 sq. ft. x 1,500 man/hrs per year = 300,000 sq. ft. per man per

year

Cost per square foot for slag abrasives would be:

Ac rsr\r\ /- —

/UUu/ ys.

300,000 sq. ft.

= $0.15 per sq. ft. for slag.

Steel grit costs per square foot of blasted surface would be:

$ll,900/yr
3nn. nnn Sat f-t-.

- - -—a--

= approx $0.04 per sq. ft. for steel grit.

In conclusion, three 1/2" nozzles using steel grit will produce
900,000 sq. ft. of cleaned surface at a cost of $35,000 per year while an
equal area blasted with non-recycled slag will cost $135,000. Hie yearly
cost difference of $100,000 does not include costs of slag disposal or
those incurred in recycling the grit.

C.3

Recycled Slag

In this example an assumption was made that the abrasive will be recovered
frcm an enclosed tank and will be dry and easily collected. Under these,
conditions, slag oould be oollected into large vacuum recovery tanks. Since
the removal of the spent abrasive fran the tank internal is required in any
event, no cost has been assigned to removal for the purpose of evalua tin g
the economics of recycling. The same holds true for the disposal from the
collection tanks into a vehicle which would either transport the material
to a disposal area or to a recycling center. Therefore, the costs asso¬
ciated with recycling are basically those of the fixed plant and the
operator. '

If abrasive costs $40 per ton and one operator uses 1,500 pounds per hour,
the cost of slag will be:

1,500 lbs

2,000

$.75 tons

$40/ton x .75 = $30 per M.H.

With a good mineral slag abrasive, approximately 60% of the original
material may be used a second time and 30% of this twice-used abrasive may
be used still a third time. Thus, each ton of, new recyclable abrasive
(reused twice) is the equivalent of 1.78 tons.

(1.0 x 1) + (1.0 x 0.5) + (0.6 x 0.3) = 1.78.

Recycling can effectively reduce the abrasive costs frcm $40.00 per ten to:

= $22.47 per ton (three passes).

On the second pass, 60% of the original slag is reused:
1,500 Ibs/M.H. x .6 = 900 Ibs/M.H. which is recycled

C.4

This recycled slag equates to

you lbs
2,000

.45 tons/M.H.

On the third pass, 30% of the 2nd pass slag is reused.

r\AA *ii_/w tt _ n _ n_/\i tt

ruu a .o = z/u jujs/n.n*

= .135 tons/M.H.

These fig ur es may also Ids used i~o deteinnins -idie econcsnic valus of x'scycling
good-quality spent abrasives already in the shipyard. Each ten that is
recycled for a second pass is worth:

.6 x $40 = $24.00/ten,

and each ton vhen recycled for a third pass is additionally worth:

.6 x .3 x $35.00 = $7.20/tcn.

Therefore, the total value (or savings) available from recycling the
existing spent abrasive is:

$24 + $7.20 = $31.20/ton of cnce-used material

If a yard has ten men blasting tank internals for one shift, 250 days per
year, then the total amount of abrasive required would be:

r\r\r\ i * tt_—

15,000 M.H. x 1,500 Ibs/M.H. = 22,500,000 lbs.,

or 11,250 tons per year.

If we assume all new abrasive:

11,250 tor is/yx’ x $40/ton =

$450,000 per year for abrasives

C.5

If a recycling plant is added:

On the second pass, each ton of new abrasive is used.

1.0 + .6 a 1.6 times
This reduces the oost of abrasives to:

11,250 tons

1.6

— 7,032 tons x $40/ton = $281,250 per year.

Thus the abrasive savings frcm a first stage'of recycling is:

$450,000 - $281,250 = $168,750 yearly savings.

These savings would be doubled if two shifts were operating per day, and
would increase proportionately with the number of blasters.

With a third pass, each ton of new abrasive is used:

1.0 + .6 + (.3 x .6) = 1.78 times.

This further reduces the total abrasives costs to:

11,250 tons
1.78

= 6,320 tons x $40/ton = $252,809

per year.

The total abrasive savings frcm recycling becomes:

$450,000 - $252,809 = $197,191 yearly cost savings.

If the yard has an existing pile of spent (once-used) abrasives containing
10,000 tons of recyclable material, this slag may also be reused to further

C.6

reduce the new abrasives required in the first year of the recycling
plant's qperation. With recycling, this material represents:

.6 x (.3 x .6) = .78 tons

.78 x 10,000 = 7,800 tons of usable abrasive.

This additional savings amounts to:

-7,800 tons x $40/ton = $312,000 for .the first year.

If the yard has large quantities of spent material on hand which are

suitable for recycling, abrasive replenishment may rot be needed for a long
time.

The total possible savings for abrasive materials with the use of an
abrasive recycling system in the above example would be:

first year: $197,191 + $312,000 = $509,191

second and succeeding years: $197,191 per year.

With the initial oost of an abrasive recycling plant ranging frcm .$25,000
to $600,000, the plant aould pay for itself during the first year with
abrasive oost savings.

C.7

SHIPPRODUC
FACILITIES I
OUTFITTING AN!
INDUSTRIAL ENGINEE

SHIPBUILDI
DESIGN/PRODUI
COMPUTER AIDS
SURFACE PREPARi

ENVIRONM

TECHNOLI

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Full text of "DTIC ADA443671: Procedures Handbook Surface Preparation and Painting of Tanks and Closed Areas"

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