EPA/600/8-87 / 03 4o
January 1989
PREVENTION REFERENCE MANUAL:
CHEMICAL SPECIFIC
VOLUME 15: CONTROL OF ACCIDENTAL
RELEASES OF SULFJR TRIOXIDE
by:
D. S. Davis
G. B, DeWolf
K. E. Hummel
J. D. Quass
Radian Corporation
Austin, Texas 78766
EPA Contract No. 68-02-3994
Work Assignment 94
and
EPA Contract No. 68-02-4286
Work Assignment 41
EPA Project Officer
T. Kelly Janes
Air and Energy Engineering Research Laboratory
Research Triangle Park, North Carolina 27711
AIR AND ENERGY ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
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TECHNICAL REPORT DATA t
(Pleese read Instructions on the reverse before completh PB89 - 1 55055
1. REPORT MO, 2,
EPA/600/8-87/034o
** mini minium in: E
4. TITLE AND SUBTITLE _
Prevention Reference Manual: Chemical Specific,
Volume 15: Control of Accidental Releases of Sulfur
Trioxide
5. REPORT DATE
January 1989
6, PERFORMING ORGANIZATION CODE
7. AUTHQR(S)
D, S, Davis, G. B. DeWolf, K. E. Hummel, and
J. D, Quass
B, PERFORMING ORGANIZATION REPORT NO.
DCN 88-239-004-41-07
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Radian Corporation
P. O. Box 9948
Austin, Texas 78766
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO,
68-02-3994, Task 94, and
68-02-4286, Task 41
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air and Energy Engineering Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 6/87 - 9/88
14. SPONSORING AGENCY CODE
EPA/600/13
is,supplementary notes AEERL project officer is T. Kelly Janes, Mail Drop 62b, 919/541-
2852.
i6. abstract The report, discussing sulfur trioxide (S03), is one of a series addressing
the prevention of accidental releases of toxic chemicals. S03, a clear oily liquid or
solid at typical ambient conditions, has an Immediately Dangerous to Life and Health
(IDLH) concentration of 20 ppm, which makes it an acutely toxic hazard. Reducing
the risk associated with an accidental release of S03 involves identifying some of
the potential causes of accidental releases that apply to process facilities than manu-
facture or use the chemical. The manual identifies examples of potential causes and
measures that maybe taken to reduce the accidental release risk. Such measures
include recommendations on plant design practices, prevention, protection, and
mitigation technologies, and operation and maintenance practices. Conceptual cost
estimates of example prevention, protection, and mitigation measures are provided.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.lOBNTIFIEfiS/OP.EN ENDED TERMS
c, COSATI Field/Group
Pollution Design
Sulfur Trioxide Maintenance
Toxicity
Emission
Accidents
Cost Estimates
Pollution Control
Stationary Sources
Accidental Releases
13B
07B 15E
06T
14G
13L
15 A, 14 A
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
116
20. SECURITY CLASS fThtspagt)
Unclassified
22, PRICE
EPA Form 2220-1 <9.73}
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ABSTRACT
The accidental release of a toxic chemical at Bhopal, India, in 1984 was
a milestone in creating an increased public awareness of toxic release
problems. As a result of other, perhaps less dramatic incidents in the past,
portions of the chemical industry were aware of this problem long before these
events. These same portions of the industry have made advances in this area.
Interest in reducing the probability and consequences of accidental toxic
chemical releases that might harm workers within a process facility and people
in the surrounding community prompted the preparation of this manual and a
series of companion manuals addressing accidental releases of toxic chemicals.
Sulfur trioxide, a clear oily liquid or solid at typical ambient
conditions, has an IDLH (Immediately Dangerous to Life and Health)
concentration of 20 ppm, which makes it an acute toxic hazard. Reducing the
risk associated with an accidental release of sulfur trioxide involves
identifying some of the potential causes of accidental releases that apply to
the process facilities that manufacture or use sulfur trioxide. In this
manual, examples of potential causes are identified as are specific measures
thay may be taken to reduce the accidental release risk. Such measures
include recommendations on plant design practices, prevention, protection and
mitigation technologies, and operation and maintenance practices. Conceptual
cost estimates of example prevention, protection, and mitigation measures are
provided.
Preceding page blank
iii
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ACKNOWLEDGEMENTS
This manual was prepared under the overall guidance and direction of T,
Kelly Janes, Project Officer, with the active participation of Robert P.
Hangebrauck, William J. Rhodes, and Jane M. Bare, all of U.S. EPA. In
addition, other EPA personnel served as reviewers. Radian Corporation
principal contributors involved in preparing the manual were Graham E. Harris
(Program Manager), Glenn B. DeWolf (Project Director), Daniel S. Davis,
Jeffrey D, Quass, Kirk E, Hummel, and Sharon L. Wevill. Contributions were
also provided by other staff members. Secretarial support was provided by
Roberta J, Brouwer .and others. A special thanks is given to many other
people, both in government and industry, who served on the Technical Advisory
Group and as peer reviewers.
iv
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TABLE OF CONTENTS
Section Page
ABSTRACT . ii±
ACKNOWLEDGEMENTS iv
FIGURES vi
TABLES yii
1 INTRODUCTION 1
1.1 Background 1
1.2 Purpose of This Manual 1
1.3 Uses of Sulfur Trxoxide .................. 2
1.4 Organization of The Manual 3
2 CHEMICAL CHARACTERISTICS 4
2.1 Physical Properties ... 4
2.2 Chemical Properties and Reac^t lvx i. y ............. 7
2.3 Toxicological and Health Effects 9
3 PROCESS FACILITY DESCRIPTIONS ........ 11
3.1 Sulfur Trioxide Manufacture 11
3.2 Sulfur Trioxide Consumption ] 5
3.3 Storage and Transfer 20
4 PROCESS HAZARDS 25
4.1 Potential Causes of Releases . 25
4.1.1 Process Causes ..... 26
4.1.2 Equipment Causes 26
4.1.3 Operational Causes 28
5 HAZARD PREVENTION AND CONTROL 29
5.1 Background 29
5.2 Process Design 30
5.3 Physical Plant Design 32
5.3.1 Equipment , 32
5.3.2 Plant Siting and Layout 43
5.3.3 Transfer and Transport Facilities .... 46
5.4 Protection Technologies 46
5.4.1 Enclosures 47
5.4.2 Scrubbers 48
5.5 Mitigation Technologies 49
5.5.1 Secondary Containment Systems 50
5.5.2 Flotation Devices and Foams 55
5.6 Operation and Maintenance Practices 56
5.6.1 Management Policy 57
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TABLE OF CONTENTS (Continued)
Section Page
5.6.2 Operator Training ....... 58
5.6.3 Maintenance and Modification Practices . 62
5.7 Control Effectiveness 65
5.8 Illustrative Cost Estimates for Controls 68
5.8.1 Prevention and Protection Measures .... 68
5.8.2 Levels of Control 68
5.8.3 Cost Summaries 71
5.8.4 Equipment Specifications and Detailed Costs 71
5.8.5 Methodology 92
6 REFERENCES 101
APPENDIX A 104
APPENDIX B 10?
FIGURES
Number
3-1 Conceptual diagram of typical sulfur trioxide manufacturing
process 12
3-2 Conceptual process flow diagram of typical continuous
sulfur trioxide film sulfonation process .... 17
3-3 Conceptual diagram of typical liquid sulfur trioxide storage
system 21
3-4 Conceptual diagram of typical liquid sulfur trioxide transfer
systems 22
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TABIDS
Number Page
2-1 Physical Properties of Sulfur Trioxide 5
2-2 Exposure Limits for Sulfur Trioxide/Sulfuric Acid Mist 10
2-3 Predicted Hunan Health Effects of Exposure to Various
Concentrations of Sulfuric Acid Aerosols 10
3-1 Typical Sulfonated/Sulfated Products Manufactured From Sulfur
Trioxide and Organic Feedstocks ..... 16
5-1 Some Process Design Considerations for Processes Involving Sulfur
Trioxide 32
5-2 Materials of Construction for Sulfur Trioxide Service 34
5-3 Aspects of Training Programs for Routine Process Operations ... 60
5-4 Examples of Major Prevention and Protection Measures for Sulfur
Trioxide Releases 66
5-5 Estimated Typical Costs of Major Prevention and Protection
Measures for Sulfur Trioxide Release 69
5-6 Summary Cost Estimates for Potential Levels of Controls for
Sulfur Trioxide Storage Tank and Sulfonation System 72
5-7 Example of Levels of Control for Sulfur Trioxide Storage Tank . . 73
5-8 Example of Levels of Control for Sulfur Trioxide Sulfonation
Reactor 75
5-9 Estimate of Typical Capital and Annual Costs Associated With
Baseline Sulfur Trioxide Storage System 76
5-10 Estimated Typical Capital and Annual Costs Associated with
Level 1 Sulfur Trioxide Storage System 77
5-11 Estimated Typical Capital and Annual Costs Associated with Level
2 Sulfur Trioxide Storage System ........ 78
5-12 Estimated Typical Capital and Annual Costs Associated with Baseline
Sulfur Trioxide Sulfonation System . . 80
vii
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TABLES (Continued)
Number Page
5-13 Estimated Typical Capital and Annual Costs Associated With Level
1 Sulfur Trioxide Sulfonation System 81
5-14 Equipment Specifications Associated with Sulfur Trioxide Storage
System ...... 82
5-15 Details of Material and Labor Costs Associated with Baseline Sulfur
Trioxide Storage System ... 85
5-16 Details of Material and Labor Costs Associated with Level 1 Sulfur
Trioxide Storage System . 86
5-17 Details of Material and Labor Costs Associated with Level 2
Sulfur Trioxide Storage System . 87
5-18 Equipment Specifications Associated with Sulfur Trioxide
Sulfonation ..... 88
5-19 Details of Material and Labor Costs Associated with Baseline
Sulfur Trioxide Sulfonation System 90
5-20 Details of Material and Labor Costs Associated with Level 1
Sulfur Trioxide Sulfonation System . . 91
5-21 Format for Total Fixed Capital Cost 93
5-22 Format for Total Annual Cost 95
5-23 Format for Installation Costs 100
viil
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SECTION 1
INTRODUCTION
1.1 BACKGROUND
Increasing concern about the potentially disastrous consequences of acci-
dental releases of toxic chemicals resulted from the Bhopal, India, accident
of December 3, 1984, which killed approximately 2,000 people and injured thou-
sands more. A toxic cloud of methyl isocyanate was released. Concern about
the safety of process facilities handling hazardous materials increased fur-
ther after the accident at the Chernobyl nuclear power plant in the Soviet
Union in April of 1986.
While headlines of these incidents have created awareness of toxic re-
lease problems, there have been other, perhaps less dramatic, incidents in the
past. Interest in reducing the probability and consequences of accidental
toxic chemical releases that might harm workers within a process facility arid
people in the surrounding community prompted the preparation of this manual
and a series of companion manuals addressing accidental releases of toxic
chemicals.
Historically, major incidents in the United States involving sulfur tri-
oxide do not appear to have been common, although a release of sulfur trioxide
in Baltimore, Maryland, in January 1978, caused respiratory injuries to 35
people (1).
1.2 PURPOSE OF THIS MANUAL
The purpose of this manual is to provide technical information about
sulfur trioxide and specifically about prevention of accidental releases of
1
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sulfur trioxide. The manual addresses technological and procedural issues,
related to release prevention, associated with the storage, handling, and
process operations involving sulfur trioxide as it is used in the United
States, This manual does not address uses of sulfur trioxide not encountered
in the Unxted States .
This manual is intended as a summary for persons charged with reviewing
and evaluating the potential for releases at facilities that use, store, haa-
cile| or rnanutacture sulfur trroxrde, It is not mtended as a specxfrcation.
manual, and in fact refers the reader to additional technical manuals and
other information sources for more complete information on the topics dis-
cussed. Other information sources include manufacturers and distributors of
sulfur trioxide, and technical literature on design, operation, and loss pre-
vention in facilities handling toxic chemicals.
1.3 USES OF SULFUR TRIOXIDE
Sulfur trioxide (SO^) is a commodity chemical, produced by the catalytic
oxidation of sulfur dioxide (SO^). In this work, data on the production of
sulfur trioxide were not found, but based on the relative production of
surfactants, a use of 300 million pounds per year of sulfur trioxide is
estimated for recent years. Sulfur trioxide is primarily used as a
sulfonating/sulfating agent to produce anionic surfactants. These include:
• Linear alkylbenzene sulfonates;
• Alcohol sulfates; and
• Alcohol ether sulfates.
Storage systems for liquid sulfur trioxide include 55-gallon drums and
bulk storage tanks.
2
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Irt addition to anhydrous sulfur trioxi.de, oleum (fuming sulfuric acid
composed of sulfuric acid and sulfur trioxide) is also used. This manual
focuses primarily on anhydrous sulfur trioxide, but some considerations also
apply to oleum.
1.4 ORGANIZATION OF THE MANUAL
Following this introductory section, the remainder of this manual pre-
sents technical information on specific hazards and categories of hazards for
sulfur trioxide and their control. These are examples only and are
representative of only some of the hazards that may be related to accidental
releases.
Section 2 discusses physical, chemical and toxicological properties of
sulfur trioxide. Section 3 describes the types of facilities -which manufac-
ture and use sulfur trioxide in the United States. Section 4 discusses pro-
cess hazards associated with these facilities. Hazard prevention and control
are discussed in Section 5. Costs of example storage and process facilities
reflecting different levels of control through alternative systems are also
presented in Section 5. The examples are for illustration only and do not
necessarily represent a satisfactory alternative control option in all cases.
Section 6 presents a reference list. Appendix A is a glossary of key
technical terms that might not be familiar to all users of the manual, and
Appendix B presents selected conversion factors between metric (SI) and
English measurement units.
3
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SECTION 2
CHEMICAL CHARACTERISTICS
This section of the report describes the physical, chemical, and toxico-
logical properties of sulfur trioxide as they relate to accidental release
hazards.
2.1 PHYSICAL PROPERTIES
Anhydrous sulfur trioxide is a clear, colorless, oily liquid with a
strong, acrid odor. Liquid sulfur trioxide freezes at temperatures around
90°F, Sulfur trioxide is hygroscopic and fumes upon exposure to moist air.
Sulfur trioxide fumes combine with moisture in the air to form submicron
sulfuric acid mist particles that are visible and form clouds of dense white
fumes. Selected physical properties of sulfur trioxide are listed in Table
2-1.
Traces of water or sulfuric acid can catalyze the polymerization of liq-
uid sulfur trioxide to solid forms that are difficult to renielt. Polymeriza-
tion can be inhibited by adding various patented stabilizers to the liquid,
such as 0.3% dimethylsulfate with 0.005% boric oxide (2). Sulfur trioxide can
be absorbed in solutions of concentrated sulfuric acid; the resulting product
is known In the U.S. as oleum. Because of the hygroscopic nature and high
reactivity of sulfur trioxide, spills and leaks of liquid can result in
hazardous releases of sulfuric acid mist to the atmosphere. In addition,
since the vapor density of sulfur trioxide and sulfuric acid mist are greater
than that of air, releases will remain close to the ground and could create a
potentially dangerous situation for workers and surrounding communities.
Sulfuric acid mist forms an opaque, white, dense cloud that resists dispersal
and has been used as a smoke screen (3). This opaque "smoke" obscures vision
4
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TABLE 2-1. PHYSICAL PROPERTIES OF SULFUR TRIOX.DE
Reference
CAS Registry Number
Chemical Formula
Molecular Weight
Normal Boiling Point
Melting Point (*/-phase)
Liquid Specific Gravity (l^O-l)
Vapor Specific Gravity (air-1)
Vapor Pressure
Vapor Pressure Equation
7446-11-9
S03
80.06
112.6aF § 14.7 psia
62.2°F
1.84 @ 100°F
2.8 @ 68°F
5.41 psia @ 77°F
4
4
7
7
7
4
log P,
A -
T+C
where: P_^ = vapor pressure, mm Hg
I" - temperature °F (50 < T < 160)
A - 7.8663, a constant
B = 2086, a constant
C = 306.4, a constant
Liquid Viscosity
Solubility in Water
Specific Heat at Constant
Pressure (vapor)
Specific Heat at Constant
Pressure (liquid)
Latent Heat of Vaporization
Heat of Dilution
1.30 centipoise (3 100 °F
Complete*
0.19 Btu/(lb-cF) <3 212°F
0.77 Btu/(lb-°F) Q 100°F
235.3 Btu/lb (3 112.6 °F
907 Btu/lb
* Reacts violently with water.
(Continued)
5
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TABLE 2-1. (Continued)
Reference
Coefficient of Thermal
Expansion
Additional Properties Useful in
Determining Other Properties
from Physical Property Correla-
tions ;
Critical Temperature
Critical Pressure
Critical Density
0.00111 per ° F § 64DF 2
424.9 °F 4
1232 psia 4
39.49 lb/ft3 4
6
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and makes visual identification of the source of a leak of sulfur trioxide or
the volume involved, difficult; this can hamper efforts to control a leak or
spill.
Liquid sulfur trioxide has a large coefficient of thermal expansion. As
a result, liquid-full equipment is a special hazard, A liquid-full vessel is
a vessel that is not vented and is filled with liquid sulfur trioxide with
little or no vapor space present above the liquid. A liquid-full line is a
section of pipe that is sealed off at both ends and is full of liquid sulfur
trioxide with little or no vapor space. In these situations, there is no room
for thermal expansion of the liquid, and temperature increases can result in
containment failure.
2.2 CHEMICAL PROPERTIES AND REACTIVITY
Sulfur trioxide is a highly reactive chemical. The most significant
chemical properties contributing to the potential for releases are as follows:
• Anhydrous sulfur trioxide combines with moisture in the air to
form sulfuric acid mist. Sulfuric acid mist consists of visi-
ble submicron particles that form a dense white cloud. Sul-
furic acid mist is corrosive to most metals such as cast iron,
steel, copper, copper alloys, and aluminum. The mist is also
an extreme inhalation hazard.
• Sulfur trioxide can solidify at temperatures at or below 90°F.
Trace amounts of water or sulfuric acid can catalyze the
formation of solid sulfur trioxide. Solid sulfur trioxide can
exist in three trimorphic phases: gamma (7, mp — 62.2SF),
beta (jS, mp - 90.5®F), and alpha (Or, mp = 144°F) . The a-phase
is the most stable, and both or- and fi- forms melt to give
liquid f-sulfur trioxide. Once solid polymeric (frozen)
sulfur trioxide has formed, higher temperatures (122-167°F)
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are required, to fully convert the polymers back to liquid
monomer. Since the liquid "Y"P'nase has a normal boiling point
of 112"F, it can rapidly vaporize at temperatures required to
melt the o-sulfur trioxide. Due to the sudden increase in
vapor pressure, there is a risk of overpressure and rupture of
containers and vessels*
• Considerable heat is evolved when sulfur trioxide or oleum is
diluted with water. Violent reactions can result from the
inappropriate addition of water or caustic solutions to these
materials,
• Anhydrous sulfur trioxide attacks cast iron, copper and copper
alloys, silver, tantalum, titanium, zirconium, neoprene, poly-
propylene, and fluoroelastoraers, With the addition of water,
it can behave like oleum, and in the presence of sufficient
water, like sulfuric acid, with attendant corrosive effects on
cast iron, mild steel, stainless steels, most copper and nic-
kel alloys, and aluminum. No elastomer has been found that is
non-reactive with sulfur trioxide (5). Both fluorinated
synthetic rubbers and polytetrafluoroethylene are reportedly
resistant to sulfur trioxide (5).
• Anhydrous sulfur trioxide reacts exothermically with organic
materials, and may cause ignition when in contact with com-
bustible materials such as sawdust or oily rags.
• Anhydrous sulfur trioxide reacts with cyanides and sulfides to
produce toxic hydrogen cyanide and hydrogen sulfide, respec-
tively. In addition to the toxicity hazard of these gases,
this can result in potentially explosive mixtures in confined
areas, since both hydrogen cyanide and hydrogen sulfide are
flammable.
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2.3 TOXICQLOGICAL AND HEALTH EFFECTS
Sulfur trioxide is highly toxic, and is a highly corrosive arid severe
irritant to the skin, eyes, and respiratory system. It rapidly dehydrates
body tissues arid causes severe burns» The effects of exposure to sulfur
trioxide in the liquid or vapor form are not well documented. The inhalation
of dry sulfur trioxide fumes in a confined area will cause immediate
destruction of the lungs and upper respiratory tract. If swallowed, liquid
sulfur trioxide will cause immediate destruction of the tissues of the mouth
and esophagus. Contact with the eyes may result in a total loss of vision.
Table 2-2 reports the lowest published concentration for toxic effects (TC^)
of sulfur trioxide by inhalation.
Exposure limits for sulfur trioxide have not been established by OSHA or
ACGIH. However, since sulfur trioxide reacts rapidly with moisture in the air
to form sulfuric acid mist, exposure limits for sulfuric acid mist are inter-
preted as including sulfur trioxide. The Permissible Exposure Limit (PEL) for
sulfuric acid mist (and hence, sulfur trioxide) is reported in Table 2-2.
Inhalation of the mist, while much less hazardous than pure sulfur trioxide,
is strongly irritating and may cause permanent lung damage.
Initial effects of human overexposure to sulfuric acid mist include: eye
corrosion with corneal or conjunctival ulcerations; skin burns or ulceration;
irritation of the upper respiratory passages; temporary lung irritation
effects with cough, discomfort, difficult breathing, or shortness of breath;
or damage to tooth enamel. Exposure to higher levels may cause severe lung
damage; the lung injury may be delayed. There are no reports of human sensi-
tization. Individuals with preexisting diseases of the lungs may have in-
creased susceptibility to the toxicity of excessive exposures. Table 2-3
presents a summary of predicted human health effects of exposure to various
concentrations of sulfuric acid aerosols.
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TABLE 2-2. EXPOSURE LIMITS FOR SULFUR TRIOXIDK/SULFURIC AGID MIST
Exposure Concentration
Limit (ppm) Description Reference
IDLH 2Qa The concentration defined as
posing an immediate danger to
life and "health (i.e. causes
toxic effects for a 30-minute
exposure).
PEL 0.3a A time-weighted 8-hour exposure
to this concentration, as set
by the Occupational Safety and
Health Administration (OSHA),
should result in no adverse
effects for the average worker.
TG^q 9^ This concentration is the lowest
published concentration causing
toxic effects (irritation) for
a 1-minute exposure.
& Exposure limit for sulfuric acid mist,
for sulfur trioxide.
TABLE 2-3. PREDICTED HUMAN HEALTH EFFECTS OF EXPOSURE TO VARIOUS
CONCENTRATIONS OF SULFURIC ACID AEROSOLS
mg HLSO,/mJ
I V
Predicted Effect
0.5 - 2.0 barely noticeable irritant
3.0 - 4.0 coughing, easily noticeable
6.0 - 8.0 decidely unpleasant, marked alterations in
respiration
Source: Reference 2,
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SECTION 3
PROCESS FACILITY DESCRIPTIONS
This section briefly describes the manufacture and uses of sulfur
trioxide in the United States. Major hazards of these processes associated
with accidental releases are discussed in Section 4, Preventive measures
associated with these hazards are discussed in Section S.
3.1 SULFUR TRIOXIDE MANUFACTURE
Sulfur trioxide is manufactured by the catalytic conversion of sulfur
dioxide gas. The sulfur trioxide can either be used directly after conversion
(in sulfonation plants using a sulfur burner with a converter), or it can be
absorbed into weak sulfuric acid or oleum for manufacture of concentrated
sulfuric acid, oleum, or liquid sulfur trioxide (using the "contact process").
Liquid sulfur trioxide is obtained by the distillation of strong oleums.
Figure 3-1 illustrates the contact process for a double-absorption
sulfur-burning plant producing sulfuric acid, oleum, and liquid sulfur tri-
oxide. The double-absorption plant uses an intermediate absorption step to
improve overall sulfur dioxide conversions to 99,5-99.8%.
There are several sources of sulfur-containing raw materials. These
include spent/waste sulfuric acid, hydrogen sulfide C^S), and most commonly,
liquid elemental sulfur. Regardless of the source of sulfur, the first step
in the manufacture of sulfur trioxide is the production of a continuous,
contaminant-free gas stream containing appreciable amounts of sulfur dioxide
and some oxygen. The sulfur dioxide rich gas stream is then passed over a
vanadium oxide catalyst to form sulfur trioxide gas. The sulfur trioxide gas
is subsequently cooled before use in sulfonation plants, or before entering a
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PRODUCT
ACID
Figure 3-1. Conceptual diagram of typical sulfur trioxide
Source: Adapted from Reference 2.
manufacturing process.
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series of absorption cowers which are part of the contact process. A dry gas
stream entering the catalytic converters is desirable; the air used for
burning elemental sulfur is generally pre-dried, while other processes dry the
sulfur dioxide stream after it lea-res the combustion chamber. The gas leaving
the converter typically contains about 10 volume percent sulfur trioxide, with
the balance being dry air.
Plants producing oleum or liquid sulfur trioxide are different from those
producing only sulfuric acid since they have one or two additional packed
towers irrigated with oleum ahead of the normal sulfur trioxide absorption
towers. The absorption towers are typically carbon steel vessels lined with
acid-proof brick and mortar and packed with ceramic rings or saddles. Partial
absorption of sulfur trioxide occurs in these towers and sulfuric acid is
added to maintain the desired oleum concentration. Because of the high sulfur
trioxide vapor pressure of high-concentration oleums, only 30-60% of the
sulfur trioxide present in the product gas is absorbed in the oleum tower.
The remaining sulfur trioxide is absorbed by concentrated (98-99%) sulfuric
acid.
Liquid sulfur trioxide is obtained by heating oleum in a steel boiler to
generate sulfur trioxide vapor, which is then condensed. Oleums with greater
than 40% free sulfur trioxide are made by mixing sulfur trioxide with low-
concentration oleums.
High hazard areas in the manufacturing process, excluding bulk storage
and transfer (discussed in Section 3.4) include the following;
• Feed treatment to remove water;
• Converter;
• Sulfur trioxide cooler;
• Absorbers;
• Oleum boiler;
• Sulfur trioxide condenser; and
• Oleum mixing.
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The feed treatment process to remove water is a critical area of the
process because water and sulfur trioxide combine to form sulfuric acid which
rapidly corrodes many materials of construction. A properly designed system
should use materials of construction which take this corrosion potential into
account. Deficiencies or failures in the drying tower could lead to a
protracted corrosion problem resulting eventually in equipment failure.
Because of the exothermic nature of the oxidation of sulfur dioxide to
sulfur trioxide, there is the potential for overheating resulting in
overpressure in the catalytic converter. This could lead to subsequent
equipment failure and loss of containment.
The sulfur trioxide cooler on the converter outlet stream contains a
dilute gaseous stream of sulfur trioxide. However, failure of the tubing in
the heat exchanger could result in a leak into the cooling water system,
leading to sulfuric acid formation and a'potential corrosion problem.
A loss of liquid flow to any of the absorption towers would result in a
buildup of sulfur trioxide gas leading to overpressure and a potential
release. Insufficient cooling of the liquid absorber feed could also lead to
overtemperature and overpressure since the absorption of sulfur trioxide in an
exothermic process.
The heater for the oleum boiler may be a steam-heated jacket, tube bun-
dle, or coil. Failure of the jacket or tubing could result in a violent reac-
tion between the steam and oleum in the reboiler. This could lead to rupture
of the boiler and a direct release of oleum and/or sulfur trioxide. Also,
failure of the temperature control system could result in overheating which
would cause an overpressure of the boiler and a possible release.
The water-cooled condenser for condensing liquid sulfur trioxide presents
the possibility of a leak into the cooling water system, leading to sulfuric
acid formation and a potential corrosion problem.
14
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The oleum mixing units are used to strengthen sulfuric acid or oleum.
They require a supply of liquid or vaporized sulfur trioxide and nrust be
equipped for flow control. Failure of the flow control system could result in
excessive sulfur trioxide feed, leading to overpressure and a possible direct
release of oleum or sulfur trioxide,
'3.2 SULFUR TRIOXIDE CONSUMPTION
The primary use of sulfur trioxide in the United States is for the
sulfonation or sulfation of organic compounds. Table 3-1 presents a listing
of some of the chemicals produced and their end uses. This subsection
summarizes the major technical features, related to release hazards, of
typical processing facilities found in the United States.
Although the terms sulfonation and sulfation are often used interchange-
ably, they differ chemically. Sulfonation involves the addition of an SO^
group into an organic molecule to form a sulfonate; either a sulfuric acid
(-SO^H), a salt (-SO.^Na), or a sulfonyl halide (-SO^X). Sulfation, on the
other hand, involves the introduction of an SO^ group into an organic molecule
to form a sulfate with the characteristic -OSO^- configuration.
Probably the largest use of liquid sulfur trioxide is for the sulfonation
of dodecylbenzene to produce dodecy1bensene sulfonate. This material is wide-
ly used in industrial detergents and is highly biodegradable under anaerobic
conditions. Also, alkyl sulfates are produced by sulfation of long chain
primary alcohols using sulfur trioxide. These products are useful as deter-
gent powders in dishwashing formulations, and as shampoo ingredients.
A block diagram of a typical sulfur trioxide sulfonation/sulfation pro-
cess is shown in Figure 3-2, This is one of several possible configurations
(9,10,11). The processes differ primarily in the configuration of the reactor
unit. These reactors employ continuous falling-film sulfonation. The gaseous
sulfur trioxide is diluted with dry air or nitrogen to a concentration of 4 to
15
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TABLE 3-1. TYPICAL SULFONATED/SULFATED PRODUCTS MANUFACTURED
FROM SULFUR IRIOXIDE AND ORGANIC FEEDSTOCKS
Reactant (example)
End Product (example)
Uses
Alkylated benzene from
straight chain normal
paraffins (dodecylbenzene)
Alkyl aryl sulfonates
(dodecylbenzene
sulfonate)
Industrial, detergents
Long chain primary alcohols
(lauryl alcohol)
Alkyl sulfates (sodium
lauryl sulfate)
Shampoo, dishwashing
powder
Fats and oils (lard; castor,
soybean, or peanut oil)
Sulfonated fats and
oils
Wetting agents,
detergents,
emulsifiers
Alpha olefins
Alpha-olefin sulfonates
Personal care/
household
products
Ethoxylated alcohols
Ethoxylated alcohol
sulfates
Detergents,
emulsifiers
Petroleum products (topped
crude oil)
Petroleum sulfonates
Tertiary oil recovery
Linear polystyrene
Linear water soluble
sulfonated polystyrenes
Ion exchange resins
Substituted benzenes
Sulfonated alkyl
benzenes
Intermediates for
dyes, drugs, and
insecticides
Long-chain alkylated
benzenes
Oil soluble sulfonates
Lubricant additives,
emulsifiers, rust
preventives
Source: References 2 and 5,
16
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Figure 3-2. Conceptual process flow diagram of typical continuous sulfur trioxide film
sulfonation process.
Source: Adapted from Reference 2.
-------
10 vol% sulfur trioxide. The gas feed rate is matched to the liquid (organic)
feed for essentially complete reaction. The reaction occurs at atmospheric
pressure and between 85 and 144°F. The gaseous residence time is very short
(order-of-magnitude <0.2 sec), and the gas-liquid contact is turbulent for
complete mixing. The reactor is cooled with cooling water to remove the heat
of reaction.
The source of sulfur trioxide gas can either be a sulfur
burner/converter, or liquid sulfur trioxide vaporised before dilution with
air. According to one sulfonation equipment vendor (12), nearly all new
sulfonation plants employ the sulfur burner/converter as a source of sulfur
trioxide. However, many older existing plants may still use liquid sulfur
trioxide. Liquid sulfur trioxide is delivered to the vaporizer by a propor-
tioning pump to maintain the proper ratio of sulfur trioxide to organic feed.
Upon leaving the reactor, liquid sulfonic acid product is separated from
the effluent or spent gas in a liquid separator or cyclone. The liquid
sulfonic acid is then subjected to further downstream processing (for example,
aging/digestion, hydrolysis, or neutralization). The effluent gas, consisting
of particulate-laden air (entrained liquid sulfonic acid droplets) and small
amounts of unreacted sulfur trioxide, is sent through demisters or scrubbers
for cleanup before venting.
From a sulfur trioxide release perspective, a fundamental characteristic
of the sulfonation process is the use of sulfur trioxide as a reactant. Be-
cause of the high reactivity of pure sulfur trioxide, it is diluted with dry
air or nitrogen,
High hazard areas in the sulfonation process, excluding bulk storage and
transfer (discussed in 3.4), include the following:
• Liquid sulfur trioxide proportioning pump;
• Vaporizer to convert liquid sulfur trioxide to vapor; and
• Reactor,
18
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The above mentioned hazard areas apply to sulfonation processes using
liquid sulfur trioxide. Sulfonation processes using a sulfur-burner and
converter would have the same hazards as noted earlier in Section 3.1 for
production of gaseous sulfur trioxide (converter and cooler), in addition to
the reactor. However, neither the liquid sulfur trioxide proportioning pump
nor the vaporizer would be included.
Failure of the flow control on the liquid sulfur trioxide proportioning
pump could result in excess sulfur trioxide being fed to the reactor. This
may cause overpressurization of the reactor or unreaeted sulfur trioxide could
cause a downstream process upset. This could result in either overpressuriza-
tion or emergency venting; if the scrubber oil the reactor vent line is either
not properly sized or not operating, a direct release of sulfur trioxide could
result.
The liquid sulfur trioxide vaporizer must be equipped with temperature
controls to prevent unintentional cooling and resultant solidification of the
liquid sulfur trioxide. Remelting of solidified sulfur trioxide can result in -
excessive pressure increases which may cause a rupture in the vessel. Also,
failure of the steam jacket or tubing could result in a violent reaction be-
tween the steam and liquid sulfur trioxide in the vaporizer. This could lead
to rupture of the vaporizer and a direct release of sulfur trioxide.
The reactor must be equipped with controls and monitors for temperature
and pressure. Temperature must be regulated to prevent cooling and solidifi-
cation of sulfur trioxide.
Pressure must be monitored to avoid emergency venting. A loss of cooling
could result in increased temperature and potential overpressure. Also, a
loss of air flow to the reactor could result in a concentrated sulfur trioxide
feed stream. Since the reaction occurs immediately and exothermically, there
is the potential for a runaway reaction if the cooling capacity cannot handle
the excess load at the entrance section of the reactor.
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3.3 STORAGE AND TRANSFER
Sulfur trioxide is difficult to store because of its high, freezing point.
It is necessary to maintain the sulfur trioxide at 95 to 105°F and to exclude
even trace quantities of water, sulfuric acid or other impurities to avoid
polymerization to solid, high-melting forms (see Subsection 2,1). Liquid
sulfur trioxide is sold in both stabilized and unstabilized forms. The
stabilized form is more resistant to polymerization.
Sulfur trioxide is shipped in drums, in tank trucks, and tank cars (13).
Figures 3-3 and 3-4 show typical storage and transfer systems. These figures
are only conceptual representations of typical storage and transfer systems.
Actual systems will vary in the design and method of storage and transfer.
Storage tanks may be located indoors within a temperature-controlled hot
room. Alternatively, storage tanks may be located outdoors if kept heated and
well insulated. Under no circumstances should storage tanks have internal
steam coils due to the risk of tube failure and' potential violent reaction or
explosion (4).
All sulfur trioxide lines should either be heat traced and insulated, or
located within the hot room. This is also true for valves, pumps, metering
devices, vaporizers, and other associated equipment. If heat tracing is used,
there should be a 1/4-inch thick insulation layer between the heat trace and
the sulfur trioxide line to prevent localized overheating ("hot spots") (13).
Since liquid sulfur trioxide freezes at 9C°F, it is crucial that all transfer
lines be kept heated to prevent solidification of sulfur trioxide. If a
transfer line were to become blocked due to solidification, stagnation could
result in overheating or over-pressurization of pumps, or render shut-off
valves inoperative.
20
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MOTOR
Figure 3-3. Conceptual diagram of typical liquid sulfur trioxide
storage system.
Source: Adapted from Reference 4.
21
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A. PUMPED WITH SELF-CONTAINED PUMP
OExieie joints
G AIR DISPLACEMENT USING DRY AIR
Figure 3-4. Conceptual diagram of typical liquid sulfur trioxice
transfer systems.
Source; Adapted from Reference 4,
22
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Transfer of the sulfur trioxide to storage is accomplished using either
pumps or compressed gas pressure. Typically, compressed air is dried to a
minimum dewpoint of -40°F, and cleaned to remove oil and foreign matter before
being used to pressure the sulfur trioxide source vessel (5), A vent line
from the storage tank is commonly connected to a scrubber or mist eliminator.
The scrubber is designed to prevent the release of sulfuric acid mist when the
transfer is occurring. When pump transfer is performed, a closed loop between
the storage tank and the tank car is commonly used. The closed loop allows
for collection of the displaced sulfur trioxide vapors and prevents release to
the atmosphere.
The fume scrubber system consists of a packed absorption tower or mist
eliminator and an acid or water tank. It is used to control sulfur trioxide
vapors from the vent of storage tanks.
It should be mentioned that if solid (frozen) sulfur trioxide is formed,
it is possible to remelt the solid by careful heating. The remelting of solid
sulfur trioxide is a hazardous procedure. The formation of solid sulfur tri-
oxide is inhibited or minimized when using stabilized liquid sulfur trioxide.
However, with pure, unstabilized sulfur trioxide, a solid can form that is
difficult to remelt at normal storage temperatures (i.e., between 95 and
105°F).
Sometimes shipments of sulfur trioxide are received in drums, but the
contents are frozen. The best way to recover the liquid sulfur trioxide is to
place the sealed, frozen drum(s) in a hot room at 95 to 105°F for several
days. If solid, material is found in drums maintained at recommended storage
temperatures due to the presence of the hxgher meltrng 01 — phase soli-d sulfxir
trioxide, the temperatures can be raised to 109°F. With pure, unstabilized
sulfur trioxide, higher temperatures may be required to melt the solid. How-
ever, since the 7'Phase sulfur trioxide boils at 112.6°F, when the solid even-
tually does melt, it is to the liquid phase above its normal boiling point.
23
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Thus, the superheated liquid is vaporized, resulting in a large pressure in-
crease (the so-called "alpha explosion"). The pressure increase has enough
force and is sudden enough to shatter glass containers or rupture drums,
causing a direct release of sulfur trioxide.
All storage tanks should be equipped with level controls to prevent
overfilling and spillage. ^ closed loop recirculation line is often used to
provide a means for mixing the tank contents.
24
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SECTION 4
PROCESS HAZARDS
Some of the specific process hazards associated with the manufacture and
uses of sulfur trioxide were cited in the previous sections of this manual.
This section presents some examples of more general causes of releases which
may be common to any sulfur trioxide process,
Sulfur trioxide can be used safely in appropriate processing and storage
equipment. However, when exposed to the atmosphere, sulfur trioxide combines
with moisture in the air to form sulfuric acid mist. This acid mist can be
detected in air by its white fumes, pungent odor and irritant property.
Sulfur trioxide releases can originate from many sources including leaks
or ruptures in vessels, piping, valves, instrumentation connections, and pro-
cess machinery such as pumps and compressors. In addition, losses may occur
through leaks at joints and connections such as flanges, valves, and fittings
where failure of gaskets or packing might occur.
Potential sulfur trioxide releases may be liquid or vapor. Liquid spills
can occur when sulfur trioxide is at or below its boiling point of 112°F or
when a sudden release of sulfur trioxide above this temperature results in
evaporative cooling of the remaining liquid by vapor flashing. Direct
releases of vapor also can occur from the vapor spaces of containers or lines
containing the vapor. Liquid spills of oleum can release some sulfur trioxide
as well. However, the vapor pressure of sulfur trioxide in oleum is
considerably less than the vapor pressure of pure sulfur trioxide.
Nonetheless, the threat from oleum should not be discounted, especially if
spills occur on hot surfaces where evaporation rates are high.
25
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4.1 POTENTIAL CAUSES OF RELEASES
Failures leading to accidental releases may be broadly classified as due
to process, equipment, or operational causes. This classification is for
convenience only. Causes discussed below are examples only and do not
necessarily include all possibilities. A more detailed discussion of possible
causes of accidental releases is presented in a companion manual on control
technologies in the prevention reference manual series of which this present
manual is a part (14).
4.1.1 Process Causes
Process causes are related to the fundamentals of process chemistry,
control, and general operation. Examples of process causes of a sulfur
trioxide release include:
• Excess sulfur trioxide or organic feed to a sulfonation/sulfation
reactor leading to excessive exothermic reaction, combined with
failure of the cooling system;
• Backflow of process reactants to a sulfur trioxide feed tank;
• Inadequate water removal from organic feeds to the
sulfonafcion/sulfation process or feeds to the sulfur trioxide
converter over a long period of time leading to progressive
corrosion;
• Excess feeds in any part of the system leading Co overfilling or
overpressuring equipment;
• Loss of liquid flow to sulfur trioxide absorbers leading to
overpressure and overtemperature;
26
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• Loss of temperature control in cooling units (e.g., reactor or
condenser) or heating units (e.g., vaporizer); and
• Overpressure in sulfur trioxide storage vessels due to overheating
or overfilling. These situations may be caused by exothermic
reactions from contamination, fire exposure, or unrelieved
ovfi rf il 1 ing.
4.1.2 Equipment Causes
Equipment causes of accidental releases result from hardware failures.
Some possible causes include:
• Failure of vessels at normal operating conditions due to weakening
of equipment from excessive stress, or corrosion; excessive stress
can be caused by improper fabrication, construction, installation,
or external loadings;
• Overheating, especially for sulfonate on reactors ox sulfur trioxide
vaporizers;
• Mechanical fatigue and shock in any equipment. Mechanical fatigue
could result from age, vibration, or stress cycling, caused by
pressure cycling, for example. Shock could occur from collisions
with moving equipment such as cranes, or other equipment in process
or storage areas;
• Thermal fatigue and shock in sulfonation reactors or heat
exchangers;
27
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• Brittle fracture in any equipment, but especially in carbon steel
equipment subjected to extensive corrosion where hydrogen
embrittlement from hydrogen release by sulfuric acid attack may have
occurred. Equipment constructed of high alloys, especially high
strength alloys selected to reduce the weight of major process
equipment, might be especially sensitive where some corrosion has
occurred, or severe operating conditions are encountered;
• Creep failure in equipment subj ected to extreme operational upsets,
especially excess temperatures. This can occur in equipment
subjected to a fire, for example; and
• All forms of corrosion. For example, external corrosion from
fugitive emissions of sulfuric acid mist could lead to equipment
weakening,
4.1.3 Operational Causes
Operational causes of accidental releases' are a result of incorrect oper-
ating and maintenance procedures or human errors (i.e., not following correct
procedures). These causes include:
• Overfilled storage vessels due to lack of attention, failure to
shutoff flow, or failure to note a partially full vessel before
adding more material;
• Improper process system operation such as failure to make proper
connections and failure to close a valve when disconnecting a hose
line;
• Errors in loading and unloading procedures;
28
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Inadequate maintenance in general, but especially on water removal
unit operations, and pressure relief systems and other preventive
and protective systems;
Lack of inspection and non-destructive testing of vessels and piping
to detect corrosion weakening; and
Incomplete knowledge of the properties of a specific chemical,
chemical system, or process leading to unexpected corrosion or
chemical reactions that could cause excessive heat or pressure.
29
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SECTION 5
HAZARD PREVENTION AND CONTROL
5.1 BACKGROUND
Prevention of accidental releases relies on a combination of technologi-
cal, administrative, and operational practices. These practices apply to the
design, construction, and operation of facilities where sulfur trioxice is
stored and used. Considerations in these areas can be grouped as follows:
• Process design;
• Physical plant design;
• Operating and maintenance practices; and
• Protective systems.
In each of these areas, attention must be given to specific factors that
could lead to a process upset or failure which could directly cause a release
of sulfur trioxide to the environment, or result in an equipment failure which
would then cause the release. At a minimum, equipment and procedures should
be in accordance with applicable codes, standards, and regulations. In
addition, stricter equipment and procedural specifications should be in place
if extra protection against a release is considered appropriate (i.e., as
determined by a hazard assessment).
The following subsections discuss some specific aspects of the above
areas as they relate to release prevention. In addition, illustrative cost
estimates for different levels of control applied to storage and process
facilities are also included. More detailed discussions can be found in the
manual on control technologies, part of this prevention reference manual
series (14).
30
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5.2 PROCESS DESIGN
Process design involves the fundamental characteristics of processes
which use sulfur trioxide. This includes an evaluation of how deviations from
intended process design features might initiate a series of events that could
result in an accidental release. The primary focus is on how the process is
controlled in terms of the basic process chemistry and the variables of flow,
pressure, temperature, composition, and quantity. Additional considerations
may include mixing systems, fire protection, and process control
instrumentation. Modifications to enhance process integrity may result from a
review of these factors and could involve changes in quantities of materials,
process pressures and temperatures, the unit operations, the sequence of
operations, the process control strategies, and the instrumentation.
Table 5-1 shows the relationship between some process design
considerations and individual processes described in Section 3 of this manual.
This does not mean that other factors should be ignored, nor does it mean that
proper attention to just the considerations in the table ensures a safe
system. However, the items listed, and perhaps others, must be properly
addressed if a system is to be safe.
The most significant considerations are aimed at preventing overheating
and overpressuring systems containing sulfur trioxide. If sulfur trioxide is
fed from a storage vessel under its own vapor pressure, the primary means of
overpressure would be from overheating. Where sulfur trioxide is fed by
nitrogen padding of a storage vessel, or through pumps, overpressuring could
occur without overheating. Equipment failure without overpressure is possible
if corrosion has weakened process equipment. Temperature monitoring is
important, not only because of a potential overpressure or equipment weakening
due to overheating, but also because sulfur trioxide's corrosiveness increases
with temperature and solidifies to a difficult melting form at relatively high
temperatures.
31
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TABLE 5-1. EXAMPLES OF PROCESS DESIGN CONSIDERATIONS FOR PROCESSES
INVOLVING SULFUR TRIOXIDE
Process Design Consideration
Contamination (with water
especially)
Flow control of sulfur trloxide
feed
Temperature sensing and cooling
medium flow control
Adequate pressure relief
Corrosion monitoring
Temperature monitorin|
Level sensing and control
Process or Unit Operation
All
All
Sulfonation/sulfation reactors
Storage tanks, reactors,
heat exchangers
All, but especially recycle
circuits
All reactors, storage tanks,
vaporizers
Storage tanks
32
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5.3 PHYSICAL PLANT DESIGN
Physical plant design considerations include equipment, siting and
layout, and transfer/transport facilities. Vessels, piping and valves,
process machinery, instrumentation, and factors such as location of systems
and equipment must all be considered. The following subsections cover various
aspects of physical plant design beginning with a discussion of materials of .
construction. This section is not intended to provide detailed specifications
for the design of a facility handling sulfur trioxide. The discussion is
intended to be illustrative examples of some of the kinds of considerations
that are required in the design of the physical facilities to minimize the
chance of a sulfur trioxide release.
5.3.1, Equipment
Materials of Construction--
The proper selection of materials of construction for sulfur trioxide
service is dictated by conditions which directly and indirectly affect corros-
ion. Temperature, pressure, moisture content, flow velocity, aeration, and
impurities such as water or sulfuric acid are important considerations in
determining the appropriate materials. Table 5-2 presents a list of possible
materials of construction for sulfur trioxide service.
Mild steel is generally satisfactory for storage and handling of sulfur
trioxide and oleum (4, 13, 15, 16). In the absence of moisture, the rate of
corrosion is very slow (0.25 mil/yr at 86°F) (13), Sulfur trioxide spilled on
the exterior surface of the tank can become extremely corrosive as acid
becomes diluted with atmospheric moisture or rain water. For improved corro-
sion resistance, 304 stainless steel can be used (4,13).
Schedule 80 carbon steel pipe with welded flanged connections is commonly
used for sulfur trioxide service (13). However, 304 stainless tubing with
welded joints is desirable for use in the hot room environment, where Its lack
of maintenance requirements is a benefit (4,13).
33
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TABLE 5-2. MATERIALS OF CONSTRUCTION FOR SULFUR TRIOXIDE SERVICE
Satisfactory
Metals
Carbon steel*
Stainless Alloy 20**
302/304/321/347 Stainless
Steels**
316/317 Stainless Steels**
Hastelloy €
Monel
Ni-resist Iron
Gold
Hon Metals
Fluorocarbons (polytetrafluoroethylene)
Fluorinated synthetic rubbers
Polychlorotrifluoroethylena
Unsatisfactory
Metals
Cast Iron
Brass, Bronze, and Copper
Silver
Tantalum
Titanium
Zirconium
Hon Metals
Neoprene
Styrene-butadiene rubbers
Polypropylene
Pclyvinylidenefluoride
* Acceptable as long as sulfuric acid content is less than 0.1%; recommended
for piping and storage tanks.
** Recommended for pumps, valves, and areas of turbulence.
Source: References 4, 13. 15, and 16.
34
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For pumps, valves, and other areas of turbulence, stainless Alloy 20, 304
SS, or 316 SS are commonly used (4). Gaskets or packing should be made of
polytetrafluoroethylene, polychlorotrifluoroethylene, or other fluorocarbon
(4), Gaskets should be the envelope type, either encapsulated or restrained
with mesh (4).
Vessels--
A variety of storage and process vessels are used in sulfur trioxide
service. Examples include small storage cylinders, chemical reactors, absorp-
tion columns, heat exchangers, and large storage tanks. Each type of vessel
has certain specifications under various codes and standards which are sup-
posed to be adhered to in design and fabrication.
Sulfur trioxide storage vessels range in size from- 3 gallon drums for
sample quantities up to several thousand gallon storage tanks used by pro-
ducers. The shell of a tank inside a hot room does not need to be covered,
and thus the exterior surface can be easily inspected. This is one reason
most users of sulfur trioxide prefer to use hot room storage in place of
outdoor storage. The hazard associated with enclosed sulfur trioxide storage
is felt to be more than offset by the ability to regularly inspect the tank
and maintain the proper temperature. As a result of the relatively large
inventories contained in sulfur trioxide storage vessels, they represent one
of the most hazardous areas of a sulfur trioxide process facility.
In general, sulfur trioxide storage tanks should be designed and built in
accordance with the ASME Code for Unfired Pressure Vessels. Special consider-
ations may be as discussed in the code for lethal materials, or even stricter
standards may be appropriate, k pressure vessel capable of handling 50 psig
has been recommended by one vendor since this rating makes it possible to melt
the high melting a-phase with the resulting vapor pressure of 23 psig (4).
As stated earlier, the usual material for sulfur trioxide storage vessels
is mild steel. Because of the potential release of hydrogen during corrosive
attack by sulfuric acid mist, welding processes are usually carefully
35
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controlled to avoid heat-affected zone hardness which can lead to hydrogen
stress cracking. Shielded arc welding with, double butt-welded longitudinal
seams and single butt-welded girth seams with seal welds on the inside is
commonly used for vessels, including those in sulfur trioxide service.
Vessel nozzles are constructed of mild steel with ANSI Class 300 or
greater forged steel weld neck flanges. Bottom outlet nozzles are not often
used since there is a greater risk of losing the entire tank contents as the
result of valve or line failure. However, an ASME-coded tank might be
equipped with a bottom outlet (3). A sump and clean-out valve are typically
mounted at the bottom. Nozzles are usually double valved as a precautionary
measure. The following nozzles are usually specified for mounting on top of
the tank (13):
• One 22-inch minimum diameter manhole with cover;
• One manhole for submerged pump;
• One 4- to 6-inch nozzle for vent line;
« two 2-inch nozzles for inlet line and relief valve; and
• One 1-inch nozzle for level measuring.
Specific release prevention considerations for vessels include: over-
pressure protection, temperature control, and corrosion prevention. Relief
devices are not usually provided for 55-gallon drums. These drums must be
kept from fire exposure, contamination, and mechanical damage. Larger vessels
are usually equipped with pressure relief valves as are tank trucks and rail
cars. Process vessels are usually protected by pressure relief valves and/or
rupture discs. Since sulfur trioxide tends to corrode pressure relief valves,
they are frequently separated from the sulfur trioxide by a rupture disc. The
pressure relief valve is set to relieve slightly above the design working
pressure of the vessel, but well below the maximum allowable working pressure.
36
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Pressure relief valves arid rupture discs are designed to prevent explos-
ion by allowing a controlled release of overpressurized contents. These
relief systems are usually sized for flashing liquid caused by:
• Fire exposure (NFPA 30);
• Thermal expansion;
• Internal reaction/decomposition; and
• Excess supply rates.
Relief piping must be sized for adequate flow. To avoid direct discharge to
the atmosphere, an overflow tank might be provided for overpressured liquid.
If there is a possibility that overpressuring may occur due to causes other
than liquid thermal expansion, or where there is no overflow receiver, the
vessels should be relieved to either a point in the process which can handle
the discharge flow, or to a gas absorption system.
The foundations and supports for vessels are important design considera-
tions, especially for large storage vessels and tall equipment such as distil-
lation columns. Supports for storage tanks containing sulfur trioxide are
usually concrete saddles. Tubular support legs are usually avoided. The
supports must be protected from possible sulfuric acid mist contact since
rapid corrosion can result from dilution of the acid with moisture in the air.
Fugitive emissions of sulfuric acid mist can lead to significant external
corrosion of supports and bolting. These supports should be protected by
fugitive emissions control and regular maintenance of structural members.
Surface coatings of mastics or polymers may also help retard external corro-
sion.
Drum storage of sulfur trioxide is coannon where small to moderate quan-
tities are required. Drum storage temperatures should not exceed 112°F and
drums should not be allowed to come in direct contact with hot surfaces. Air
pressure is not used to transfer sulfur trioxide from drums; instead drums are
37
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emptied either by gravity flow or by pumping. A drying train is fitted to the
vent opening to prevent raolst air from being drawn into the drum while empty-
ing.
The reactors used in sulfur trioxide-related processes represent possible
sources of major releases since they contain a large portion of the sulfur
trioxide used in their respective processes. These reactors must, therefore,
be properly constructed of appropriate materials of construction.
The feed to the reactors must be dry since moisture accelerates the
corrosion rate of materials used in construction (e.g., carbon steel).
Provisions should be made for exclusion of moisture during any process shut-
downs, or purging of the reactors at shutdown and before startups.
Since the suifonation/sulfation reaction is exothermic, a jacketed
tubular reactor is commonly used to maintain the desired reaction temperature.
Water is the cooling medium. This equipment must be designed to prevent water
leakage into the reactor. Provisions must be made for corrective action'if
such leakage should occur. Common precautions include ensuring that the water
pressure is lower than the process fluid pressure, and monitoring the pH of
the cooling water. If leakage should then occur, the acid would enter the
water system and be detected by the pH monitoring.
A pressure relief system along with a means of gas purging is also
incorporated into the reactor designs since hydrogen gas may build up as a
result of corrosion. All vents should be routed to a gas scrubber.
Absorption towers and vaporizers present significant release hazards
since they contain large amounts of sulfur trioxide in relatively pure form.
The conditions under which this equipment operates (especially the vaporizer
boiler) are severe and as a result these areas are potential sites for a
release.
38
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Piping--
As with sulfur trioxide vessels, sulfur trioxide pipework design must
reflect the pressure, temperature, and corrosion concerns associated with use
of the chemical. Careful attention must be paid to pipework and associated
fittings since failures of this type of equipment are major contributors to
accidental releases of chemicals, Regardless of the physical state of the
sulfur trioxide, there are some general guidelines for sulfur trioxide piping
systems. The first is simplicity of design; the number of joints and connect-
ions should be minimized. In addition to being securely supported, pipes
should be sloped, with drainage at the low points. Piping should be con-
structed so as to allow room for thermal expansion of the pipe and should be
protected from exposure to fire and high temperatures. Placement of valves
should ensure isolation of leaking pipes and equipment. In addition, all
piping not enclosed in a heated structure should be heat traced and insulated.
The correct design and use of pipe supports is essential to reduce
overstress and vibration which could lead to piping failure. The supports
should be designed to handle the load associated with the pipe, operating and
testing medium, insulation, and other equipment. Factors which must be
considered include thermal expansion and contraction, vibrations caused by
pumping and fluid flow, bending moments as a result of overpressure in the
pipe, and external loads such as winds or ice accumulation.
At a minimum, piping networks are usually pressure tested to meet the
requirements specified by ANSI code B31.3 (17). If a gas is used, it must be
dry to prevent corrosion and resulting hydrogen gas formation. If water is
used, the piping must be drained and blown dry before use.
As a result of the possible buildup of hydrogen gas from corrosive
effects and the relatively low boiling point of sulfur trioxide, rupture due
to overpressure is possible especially in liquid-full lines. Protection
against this is commonly provided by installing a pressure relief valve or
rupture disk (IS).
39
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All piping should be situated away from fire and fire hazards since the
presence of hydrogen gas could trigger an explosion. If possible, piping
carrying sulfur trioxide should not be routed near other processes or piping
networks -which might present an external threat (e.g., piping carrying highly
corrosive materials, high pressure processes). Pipe flanges should be situ-
ated so as to minimize potential hazards from drips and small leaks since
these could cause rapid external corrosion. In addition, the piping network
should be protected from possible impact and other structural damage.
Several types of valves including gate, globe, ball, relief, excess flow,
and check configurations are used in sulfur trioxide-containing systems.
Valves for sulfur trioxide service should be made of 316 or 347 stainless
steel or stainless Alloy 20 (4). Valve stem packing of polytetrafluoroethyl-
ene is preferred. Whenever possible valves should be flange-mounted horizon-
tally, and positioned with the valve stem up (13). All valves should be heat
traced and insulated. Valves should be purchased with a stainless steel guard
to protect the operator against acid leaking through the packing (13).
Pressure relief devices should be installed on all sulfur trioxide
containing vessels where the chemical can be blocked in. Such devices are set
to relieve at a pressure approximately equal to the design pressure (which is
greater than the normal operating pressure) for each specific piece of equip-
ment .
Process Machinery--
Process machinery refers to rotating or reciprocating equipment that may
be used in the transfer or processing of sulfur trioxide. This includes pumps
which may be used to move liquid sulfur trioxide where gas pressure padding is
insufficient or inappropriate. No information is available on compressors
that might handle gaseous sulfur trioxide.
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Pumps --Many of the concerns and considerations for sulfur trioxide piping
and valves also apply to pumps. To assure that a given pump is suitable for a
sulfur trioxide service application, the system designer should obtain inform-
ation from the pump manufacturer certifying that the pump will perform prop-
erly in this application.
Funrps should be constructed with materials which are resistant to sulfur
trioxide at operating temperatures and pressures. They should be installed
dry and oil-free. It is especially important that their design not allow
sulfur trioxide or lubricating oil to enter seal chambers where they may
contact one other. Sulfur trioxide freezes at a fairly high temperature so
temperature considerations are important. The pump's supply tank should have
high and low level alarms; the pump should be interlocked to shut off at low
supply level or low discharge pressure. External pumps, should be situated
inside a diked area, and should be accessible in the event of a tank leak.
The type of pump selected depends on pumping requirements and operating
conditions. Centrifugal, rotary, positive displacement, and sealless pumps
are used to pump sulfur trioxide. The pumps used should be constructed of
suitable materials such as 316 on 347 stainless steel, or stainless Alloy 20
which are resistant to sulfur trioxide corrosion (4). Liquid sulfur trioxide
can be transferred using submerged pumps, canned pumps, or pumps with dry
packing or shaft seals.
Canned motor pumps are centrifugal units in which the motor housing is
interconnected with the pump casing. Here, the process liquid actually serves
as the bearing lubricant. These pumps must be operated with a flooded suction
and freezing must be avoided. An alternative concept is the vertical pump
often used on storage tanks. These pumps consist of a submerged impeller
housing connected by an extended drive shaft to the motor. The advantages of
this arrangement are that the shaft seal is above the maximum liquid level
(and is therefore not wetted by the pumped liquid) and the pump is self-
priming because the liquid level is above the impeller.
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Pumps using stuffing boxes and packing should be provided with double-
faced seal chambers designed to prevent sulfur trioxide from contacting any
reactive material. These chambers can be purged with an appropriate inert
fluid such as dry and oil-free nitrogen, or a suitable seal liquid. The seal
gas pressure should exceed the tank pressure by an appropriate margin. A seal
fluid back-up system should be considered (19).
Magnetically-coupled pumps are constructed with a rotating magnetic field
as the pump-motor coupling device. Diaphragm pumps are positive displacement
units in which a reciprocating flexible diaphragm drives the fluid. In both
of these pump types, exposure of packing and seals to the pumped liquid is
eliminated.
For metering service, diaphragm puntps are commonly used. However, a
major consideration in the application of such pumps is that at some point,
diaphragm failure will probably occur. Such a failure could lead to a release
of the liquid being pumped. These pumps may have a pressure relief valve on
the outlet, bypassing to the suction.
Improper operation of pumps as a result of cavitation, running dry, and
deadheading can cause damage and failure of pumps. If cavitation is allowed
to occur, pitting and eventual serious damage to the impeller can result.
Running a pump dry as a result of loss of head in a feed tank, for example,
can seriously damage a pump. Finally, pumping against a closed valve can have
serious ramifications, A pump bypass or kick-back is useful in avoiding such
an occurrence. Failure of a pump, for whatever reason, can eventually lead to
a hazardous release.
Centrifugal pumps often have a recycle loop back to the feed container
which prevents overheating if the pump is deadheaded. This is an important
consideration in sulfur trioxide systems since sulfur trioxide corrosiveness
increases with increasing temperature. Deadheading is also a concern with
positive displacement pumps. To prevent rupture, positive displacement pumps
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commonly have a pressure relief valve which bypasses to the pump suction.
Because of the probability of eventual diaphragm failure, the use of diaphragm
pumps should, be carefully considered in view of this hazard potential.
Pumps are not always necessary; in some circumstances, liquid sulfur
trioxide is moved by gravity and/or pressure padding. With sulfur trioxide
drums, the liquid may be displaced from the vessel by the force of gravity.
With other types of vessels, an inert gas such as dry nitrogen may be used to
force liquid from tank. Padding system designs must reflect the operating
conditions and limitations (e.g., required flow rate) and therefore must be
custom designed for a process.
Miscellaneous Equipment--
Pressure Relief Devices --Information on specific relief valve types for
sulfur trioxide service is not readily available. Some characteristics for
other hazardous chemical service seem to-apply for sulfur trioxide, however.
For vessels, an acceptable relief valve is of angle body construction with a
closed bonnet and a screwed cap over the adjusting screw. These valves are
normally used in combination with a rupture disc or a breaking assembly with a
pressure indicator to monitor the pressure of the space between disc and
valve. Typical valve construction materials include a cast carbon steel body:
nickel plated steel spring; and nickel-copper or nickel-chromium-molybdenum
alloy nozzle, disc adjusting ring, nozzle ring, and spindle guide. The inlet
flange should be ANSI Class 300 or greater and the outlet flange should be
ANSI Class 150 or greater. Valves of this construction which also have
fluorinated synthetic rubber "0" ring seat seals need not have a rupture disc
or breaking pin. Other types of pressure relief devices are acceptable as
long as they are constructed of materials suitable for sulfur trioxide service
and meet the general requirements of the ASME boiler and Pressure Vessel Code,
Section VIII, Division 1 (17).
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Rupture discs are constructed of appropriate sulfur trioxide-resistant
materials. Impervious graphite rupture discs fragment upon overpressure, and
therefore, should not be used in conjunction with relief valves. Connections
can be screwed, flanged or socket-welded for connections smaller than two
inches. However, connections two inches or larger should be flanged or
butt-welded. The flanges should be constructed of forged carbon steel and be
rated in accordance with the associated piping system. Because operating
pressures exceeding 70X of a disc's burst pressure may induce premature
failure, a considerable margin should be allowed when sizing rupture discs.
When it is possible to draw a vacuum on the disc, supports should be provided
to prevent damage to the disc (IS),
5.3.2 Plant Siting and Layout
The siting and layout of a particular sulfur trioxide facility is a
complex issue which requires careful consideration of numerous factors. These
include: other processes in the area, the proximity of population centers,
prevailing winds, local terrain, and potential'natural external effects such
as flooding. The rest of this subsection describes general considerations
which might apply to siting and layout of sulfur trioxide facilities.
Siting of facilities or individual equipment items should be done in a
manner that reduces personnel exposure, both plant and public, in the event of
a release. Since there are also other siting considerations, there may be
trade-offs between this requirement and others in a process, some directly
safety related. Siting should provide ready ingress or egress in the event of
an emergency and yet also take advantage of barriers, either man made or
natural, which could reduce the hazards of releases. Large distances between
large inventories and sensitive receptors (e.g., control room, hazardous
process units, population centers) are desirable.
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Layout refers to the placement and arrangement of equipment In the
process facility. All anhydrous sulfur trioxide, oleum, and sulfuric acid
storage and handling equipment should be located away from other potentially
hazardous storage and handling facilities. Such 'equipment is usually located
in concrete enclosures or pits surrounded by curbing or walls to prevent acid
spills from contaminating surrounding areas and also to serve as a boundary
for the restricted acid area. Concrete containment areas are often treated
with polymeric sealers to increase the containment efficiency. The drain
system for the containment area should be designed to cause storm water to be
routed to wastewater treating. The system should be capable of directing
spilled sulfur trioxide, oleum, or sulfuric acid to a high capacity neutral-
ization facility.
Various techniques are available for formally assessing a plant layout
and should be considered when planning high hazard facilities (18).
General layout considerations include:
• Large inventories of sulfur trioxide should be kept away from
potential sources of fire or explosion;
¦ Vehicular traffic should not be routed too near sulfur trioxide
process or storage areas if this can be avoided;
• Where such traffic is necessary, precautions should be taken to
reduce the chances for vehicular collisions with equipment,
especially pipe racks carrying sulfur trioxide across or next
to roadways;
• Sulfur trioxide, oleum or sulfuric acid piping preferably should not
be located adjacent to other piping which is under high pressure or
temperature, or which carries flammable materials; and
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• Storage facilities should be segregated from the main, process -unless
the hazards of pipe transport are felt to outweigh the hazard of the
storage tank for site-specific cases.
Because heat increases the corrosiveness of sulfur trioxide and causes
thermal expansion of liquid sulfur trioxide, measures should be taken to
situate piping, storage vessels, and other sulfur trioxide equipment so they
are less exposed to heat sources. Hot process piping, equipment, steam lines,
and other sources of direct or radiant heat should be avoided or systems
should be designed for heat induced corrosion and pressure increases. Storage
areas should also be situated away from control rooms, offices, utilities,
other hazardous storage, and laboratory areas by distances similar to those
specified for flammable materials (18). Special precautions should be taken
to keep sulfur trioxide storage vessels away from potential fire or explosion
sources.
In the event of an emergency, there should be multiple means of access to
the facility for emergency vehicles and crews. Storage vessel shut-off valves
should be readily accessible. Containment around liquid storage tanks can be
provided by diking. Dikes reduce evaporation while containing the liquid. It
is also possible to equip a diked area to allow drainage to an underground
containment sump. This sump should vented to a scrubber system for safe
discharge. A hot room can also serve as a full containment system if it is
designed to vent to a scrubber. This type of secondary containment could be
considered for large-volume liquid sulfur trioxide storage tanks.
5.3.3 Transfer and Transport Facilities
Transfer and transport facilities where both road and rail tankers are
loaded or unloaded are potential accident areas because of vehicle movement
and the intermittent nature of the operations. Therefore, special attention
should be given to the design of these facilities.
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As mentioned in the previous section, tank car and tank truck facilities
should be located away from sources of heat, fire, and explosion. Equipment
in these areas should also be protected from impact by vehicles and other
moving equipment. Tank vehicles should be securely moored during transfer
operations; an interlocked barrier system is commonly used. Sufficient space
should be available to avoid congestion of vehicles or personnel during -
loading and unloading operations. Vehicles, especially trucks, should be able
to move into and out of the area without reversing. High curbs around trans-
fer areas and barriers around equipment should be provided to protect equip-
ment from vehicle collisions.
When possible, the transfer of sulfur trioxide should be made using fixed
rigid piping. In situations which require flexible hoses or tubes, precau-
tions, must be taken to ensure sound connections. Avoiding cross-contamination
of chemical materials is also a key concern.
5.4 PROTECTION TECHNOLOGIES
This subsection describes two types of protection technologies for
containment and neutralization. These are:
• Enclosures; and
• Scrubbers.
A presentation of more detailed information on these systems is presented in
a companion manual on control technologies of the prevention reference manual
series.
5,4.1 Enclosures
Enclosures refer to containment structures which capture any sulfur
trioxide spilled or vented from storage or process equipment, thereby prevent-
ing immediate discharge of the chemical to the environment. The enclosures
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contain the spilled liquid or gas until it can be transferred to some other
type of containment, discharged at a controlled rate which would not be in-
jurious to people or the environment, or transferred at a controlled rate to
scrubbers for neutralization.
The use of specially designed enclosures for sulfur trioxide storage
appears to be fairly common. It is not known whether sulfur trioxide process
equipment is typically enclosed or not. If the process is enclosed, the
enclosure is a hot room which is primarily designed to maintain the liquid or
vaporized sulfur trioxide temperature at 95 to 105°F (20). Locating toxic
operations in the open air has been mentioned favorably in the literature,
along with the opposing idea that sometimes enclosures may be appropriate
(18). The desirability of enclosure depends partly on the frequency with
which, personnel must be involved with the equipment. A common design ration-
ale for not having an enclosure where toxic materials are used is to prevent
the accumulation of toxic concentrations•of a chemical within a work area.
However, if the issue is protecting the community from accidental releases,
then total enclosure may be appropriate.
Care must be taken when an enclosure is built around pressurized equip-
ment. It would not be practical to design an enclosure to withstand the
pressures associated with the sudden failure of a pressurized vessel. An
enclosure would probably fail as a result of the pressure created from such a
release and could create an additional hazard. In these situations, it may be
determined that an enclosure is not appropriate. If an enclosure is built
around pressurized equipment then it should be equipped with some type of
explosion protection, such as rupture plates that are designed to fail before
the entire structure fails.
The types of structures that appear to be suitable for sulfur trioxide
storage and process equipment are buildings or bunkers constructed of
concrete. If silicate concrete is used, these structures would be resistant
to attack by sulfur trioxide or sulfuric acid mist (15). The building should
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have a ventilation system designed to draw in air when the building is vented
to a scrubber. The bottom section of the building should be liquid tight to
retain any liquid sulfur trioxide that might be spilled. There should be a
minimum of two exits which have doors elevated and accessible by steps inside
and outside. The room should also include a valved draw-off line from below
floor level to permit removal of the flooded liquid sulfur trioxide to a
backup storage system. Enclosures should be equipped with continuous
monitoring equipment and alarms. Alarms should sound whenever detected
concentrations are approaching lethal levels.
5.4.2 Scrubbers
Scrubbers are a traditional method for absorbing toxic gases from process
streams. These devices can be used for the control of sulfur trioxide re-
leases from vents and pressure relief discharges, from process equipment, or
from secondary containment enclosures.
Sulfur trioxide discharges could be contacted with sulfuric acid {93% or
greater) in any of several types of scrubbing devices. Makeup water would be
added to maintain the desired acid strength. Types of scrubbers that might be
appropriate include spray towers, packed bed scrubbers, and Venturis. Other
types of special designs might be suitable, but complex internals subject to
corrosion do not seem to be advisable.
Whatever type of scrubber is selected, a complete system would include
the scrubber itself, a liquid feed system, makeup equipment, reservoir,
demister and a blower for some systems. If such a system is used as protec-
tion against emergency releases, consideration must be given to how it would
be activated in time to respond to an emergency load. One approach used in
some process facilities is to maintain a continuous circulation of scrubbing
acid through the system. For many facilities this would not be practical, and
the scrubber system might be tied into a trip system to turn it on when it is
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needed. However, with this system a quantity of sulfur trioxide would be
released prior to actuation of the scrubber (i.e., starting up a blower and
turning on the flow of liquid).
The scrubber system must be designed so as not to present excessive
resistance to the flow of an emergency discharge. The pressure drop should be
only a small fraction of the total pressure drop through the emergency
discharge system. In general, at the liquid-to-gas ratios required for
effective scrubbing, spray towers have the lowest, and Venturis the highest
pressure drops. While packed beds may have intermediate pressure drops at
proper liquid-to-gas ratios, excessive ratios or plugging can increase the
pressure drop substantially. However, packed beds have higher removal
efficiencies than spray towers or Venturis.
In addition, the scrubber system must be designed to handle the "shock
wave" generated during the initial stages of the release. This is
particularly important for packed bed scrubbers since there is a maximum
pressure with which the gas can enter the packed section without damaging the
scrubber internals.
5.5 MITIGATION TECHNOLOGIES
If, in spite of all precautions, a large release of sulfur trioxide were
to occur, the first priority would be to rescue workers in the immediate
vicinity of the accident and evacuate persons from downwind areas. The source
of the release should be determined, and the leak should be stopped If pos-
sible. The next primary concern becomes reducing the consequences of the
released chemical to the plant and the surrounding community. Reducing the
consequences of an accidental release of a hazardous chemical is referred to
as mitigation. Mitigation technologies include such measures as physical
barriers, water sprays and fogs, and foams where applicable. The purpose of a
mitigation technique is to divert, limit, or disperse the chemical that has
been spilled or released to the atmosphere in order to reduce the atmospheric
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concentration and the area affected by the chemical. The mitigation tech-
nology chosen for a particular chemical depends on the specific properties of
the chemical including its flammability, toxicity, reactivity, and those
properties which determine its dispersion characteristics in the atmosphere.
If a release occurs from a liquid sulfur trioxide storage tank, the
exposed liquid will immediately combine with moisture in the air to form
sulfuric acid mist ("smoke"). The tremendous volumes of "smoke" can obscure
vision and make location of the source of the release more difficult. Prop-
erty damage and personal injury may also result from contact with sulfuric
acid mist. It is therefore desirable to minimize the liquid sulfur trioxide
surface area exposure which in turn will minimize the rate of mist formation.
Mitigation technologies which are used to reduce the surface area exposure
include secondary containment systems such as dikes, sumps, and enclosures.
A post-release mitigation effort requires that the source of the release
be accessible to trained plant personnel. Therefore, the availability of
adequate personnel protection is essential. Personnel protection will typi-
cally include such items as portable breathing air and chemically resistant
protective clothing,
5.5.1 Secondary Containment Systems
Specific types of secondary containment systems include excavated basins,
natural basins, dikes (earth, steel, or concrete) and high impounding walls.
The type of containment system best suited for a particular storage tank or
process unit will depend on the risk associated with an accidental release
from that location. The inventory of sulfur trioxide and its proximity to
other portions of the plant and to the community should be considered when
selecting a secondary containment system. The secondary containment system
should have the ability to contain spills with a minimum of damage to the
facility and its surroundings and with minimum potential for escalation of the
event.
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Secondary containment systems for sulfur trioxide storage facilities
commonly consist of one of the following:
• An adequate drainage system underlying the storage vessels which
terminates in an impounding basin having a capacity as large as the
largest tank served; and
• A diked area, with a capacity as large as the largest tank served.
These measures are designed to prevent the accidental discharge of liquid
sulfur trioxide from spreading to uncontrolled areas.
The most common type of containment system is a low wall dike surrounding
one or more storage tanks. Generally, no more than three tanks are enclosed
within one diked area to reduce risk. Dike heights usually range from three
to twelve feet depending on the area available to achieve the required volu-
metric capacity. The dike walls should be liquid tight and able to withstand
the hydrostatic pressure and temperature of a spill. Low wall dikes may be
constructed of steel, concrete or earth. If earthen dikes are used, care must
be taken to ensure that the sulfur trioxide cannot leak through the dike.
Piping should be routed over dike walls, and penetrations through the walls
should be avoided if possible. Vapor fences may be situated on top of the
dikes to provide additional vapor containment. If there is more than one tank
in the diked area, the tanks should be situated on berms above the maximum
liquid level attainable in the impoundment.
A low wall dike can effectively contain the liquid portion of an acci-
dental release and keep the liquid from entering uncontrolled areas. By
preventing the liquid from spreading, the low wall dike can reduce the surface
area of the spill. Reducing the surface area will reduce the rate of evapora-
tion. The low wall dike will partially protect the spill from wind; this can
reduce the rate of evaporation. A dike with a vapor fence will provide extra
protection from wind and will be even more effective at reducing the rate of
evaporation.
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A dike also creates the potential for sulfur trioxide and trapped water
to mix within the enclosed area. This may accelerate the rate of evaporation
and form large quantities of highly corrosive sulfuric acid mist. If mater-
ials that would react violently with sulfur trioxide are stored within the
same diked area then the dike will increase the potential for mixing the
materials in the event of a simultaneous leak. A dike also limits access to
the tank during a spill.
An impounding basin is well suited to storage systems where more than one
tank is served and a relatively large site is available for such a basin. The
spilled sulfur trioxide is directed to the basin by dikes and channels under
the storage tanks which are designed to minimize exposure of the other tanks
and surrounding facilities to the spilled liquid. Because of the high vapor
pressure of sulfur trioxide, the trenches that lead to the basin should be
covered to reduce the rate of evaporation. Additionally, the basin should be
located near the tank area to minimize the amount of sulfur trioxide that
evaporates as it travels to the basin.
This type of system has several advantages. The spilled liquid is
removed from the immediate tank area. This allows access to the tank during
the spill and reduces the probability that the spilled liquid will damage the
tank, piping, electrical equipment, pumps or other equipment. In addition,
the covered impoundment will reduce the rate of evaporation from the spill by
isolating the spill from wind or heating from sunlight.
A limitation of a remote impounding basin is that there is still the
potential for water or other incompatible materials to be trapped in the
impoundment and mix with the incoming sulfur trioxide. Remote impounding
basins do not reduce the impact of a gaseous sulfur trioxide release.
High wall impoundments may be a good secondary containment choice for
selected systems. Circumstances which may warrant their use include limited
storage site area, the need to minimize vapor generation rates, and/or the
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tank must be protected from external hazards. Maximum vapor generation rates
will generally be lower for "high wall impoundments than for low wall dikes or
for remote impoundments because of the reduced surface area. These rates can
be further reduced with the use of insulation on the wall and floor in the
annular space. High impounding walls may be constructed of low temperature
steel, reinforced concrete, or prestressed concrete. A weather shield may be
provided between the tank and wall with the annular space remaining open to
the atmosphere. The available area surrounding the storage tank will dictate
the minimum height of the wall. For high wall impoundments, the walls may be
designed with a volumetric capacity greater than that of the tank to provide
vapor containment. Increasing the height of the wall also raises the eleva-
tion of any released vapor.
One disadvantage of these dikes z s that the high walls around a tank may
hinder routine external observation. Furthermore, the closer the wall is to
the tank, the more difficult it becomes to access the tank for inspection and
maintenance. As with low wall dikes, piping should be routed over the wall if
possible. The closeness of the wall to the tank may necessitate placement of
the pump outside the wall, in which case the outlet (suction) line will have
to pass through the wall. In such a situation, a low dike encompassing the
pipe penetration and pump may be provided, or a low dike may be placed around
the entire wall.
One further type of secondary containment system is one which is struct-
urally integrated with the primary system and forms a vapor-tight enclosure
around the primary container. Many types of arrangements are possible. A
double-walled tank is an example of such an enclosure. These systems may be
considered where protection of the primary container and containment of vapor
(for events not involving foundation or wall penetration failure) are of
greatest concern. Drawbacks of an integrated system are the greater complex-
ity of the structure, the difficulty of access to certain components, and the
fact that complete vapor containment cannot be guaranteed for all potential
events.
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Provision should, be made for drainage of rainwater from diked areas.
This will involve the use of sumps and separate drainage pumps, since direct
drainage to stormwater sewers would presumably allow any spilled sulfur
trioxide to follow the same route. Alternatively, a sloped rain hood may be
used over the diked area which could also serve to direct the rising vapors to
a single release point (20), The ground within the enclosure should be graded
to cause the spilled liquid to accumulate at one side or in one corner. This
will help to minimize the area of ground to which the liquid is exposed and
from which it may gain heat. In areas where it is critical to minimize vapor
generation, surface insulation may be used in the diked area to further reduce
heat transfer from the environment to the spilled liquid.. The floor of an
impoundment should be sealed with a clay blanket to prevent the sulfur tri-
oxide from seeping into the ground; percolation into the ground causes the
ground to cool more quickly, increasing the vapor generation rate. Absorption
of the sulfur trioxide into water in the soil would also release additional
heat.
5.5.2 Flotation Devices and Foams
Other possible means of reducing the surface area of spilled hazardous
chemicals include placing impermeable devices on the surface, dilution with
water, and applying water-based foam. However, where sulfur trioxide releases
are concerned., none of these are completely satisfactory.
Placing a surface seal over a spilled chemical is a direct approach for
containing toxic vapors with nearly 100 percent efficiency. One such material
has been developed and tested to rapidly stop the generation of sulfuric acid
mist ("smoke") from a confined pool of liquid sulfur trioxide (21). The
surface seal is made of an inert halocarbon oil slurried with hollow glass
bubbles to make the oil float on the surface of the liquid sulfur trioxide.
The slurry can be mixed in advance for storage, but it must be remixed at
weekly intervals to prevent stratification and setting. The surface seal is
effective at capping a confined pool of liquid sulfur trioxide, such as
contained in a reservoir or sump.
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The use of foams in vapor hazard control has been demonstrated for a
broad range of volatile chemicals. However, since foams still contain water
they will react with sulfur trioxide to form sulfuric acid mist. Results of a
laboratory test program to develop a method for the containment and blanketing
of liquid sulfur trioxide spills showed that a mechanical foam is the fastest
method available to stop the "smoke" from a large or uncontrolled spill in an
unconfined area (3). A mechanical foam is a fluid aggregate of small bubbles
containing water dispersed in very thin films that make up the bubbles.
Mechanical foam should not be used in a confined area since the foam can react
violently with liquid sulfur trioxide to form hazardous sulfuric acid mist.
Finally, the dilution of a sulfur trioxide spill with water will result
in huge volumes of sulfuric acid mist or "smoke". Extreme caution should be
exercised using water of any kind, since violent spattering can occur. To
minimize spattering, a water fog or fine spray should be used (4). The
reaction of liquid sulfur trioxide with a water fog or spray is less violent
than the reaction with a full water stream. Water would be appropriate only
when the spill is too large to be controlled using absorbent material or a
surface seal.
One alternative for sulfur trioxide spills is to spread a non-reactive
absorbent material such as expanded clay or diatomaeeous earth. Once absorb-
ed, the sulfur trioxide residue can be removed from the area for dilution with
water, or neutralization with solutions of caustic materials.
5.6 OPERATION AND MAINTENANCE PRACTICES
Quality hardware, contained mechanical equipment, and protective devices
all increase plant safety; however, they must be supported by the safety
policies of management and by constraints on their operation and maintenance.
This section describes how management policy and training, operation, and
maintenance procedures relate to the prevention of accidental sulfur trioxide
releases. Within the sulfur trioxide industry, these procedures and practices
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vary widely because of differences in the size and nature of the processes and
because any determination of their adequacy is inherently subjective. For
this reason, the following subsections focus primarily on fundamental princi-
ples and do not attempt to define specific policies and procedures.
5.6.1 Management Policy
Management is a key factor in the control of industrial hazards and the
prevention of accidental releases. Management establishes the broad policies
and procedures which influence the implementation and execution of specific
hazard control measures. It is important that these management policies and
procedures be designed to match the level of risk in the facilities where they
will be used. Most organizations have a formal safety policy. Many make
policy statements to the effect that safety must rank equally with other
company functions such as production and sales. The effectiveness of any
safety program, however, is determined by a company's commitment to it, as
demonstrated throughout the management structure. Specific goals must be
derived from the safety policy and supported by all levels of management.
Safety and loss prevention should be an explicit management objective.
Ideally, management should establish the specific safety performance measures,
provide incentives for attaining safety goals, and commit company resources to
safety and hazard control. The advantages of an explicit policy are that It
sets the standard by which existing programs can be judged, and it provides
evidence that safety is viewed as a significant factor in company operations.
In the context of accident prevention, management is responsible for
(17,22):
• Ensuring worker competency;
• Developing and enforcing standard operating procedures;
• Adequate documentation of policy and procedures;
• Communicating and promoting feedback regarding safety issues;
• Identification, assessment, and control of hazards; and
• Regular plant audits and provisions for Independent checks,
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Additional discussion on the responsibilities of management can be found
in a manual on control technologies, part of this manual series.
5.6.2 Operator Training
The performance of operating personnel is also a key factor in the
prevention of accidental sulfur trioxide releases. Many case studies docu-
menting industrial incidents note the contribution of human error to acci-
dental releases (17), Release incidents may be caused by using improper
routine operating procedures, by insufficient knowledge of process variables
and equipment, by lack of knowledge about emergency or upset procedures, by
failure to recognize critical situations, and in some cases by direct physical
mistake (e.g., turning the wrong valve). A comprehensive operator training
program can decrease the potential for accidents resulting from such causes.
Operator training can include a wide range of activities and a broad
spectrum of information. Training, however, is distinguished from education
in that it is specific to particular tasks. While general education is
important and beneficial, it is not a substitute for specific training. The
content of a specific training program depends on the type of industry, the
nature of the processes used, the operational skills required, the character-
istics of the plant management system, and tradition.
Some general characteristics of quality industrial training programs
include:
• Establishment of good working relations between management and
personnel;
• Definition of trainer responsibilities and training program goals;
• Use of documentation, classroom instruction, and field training (in
some cases supplemented with simulator training);
58
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• Inclusion of procedures for normal startup and shutdown, routine
operations, and upsets, emergencies, and accidental releases; and
• Frequent supplemental training and the use of up-to-date training
materials.
In many instances training is carried out jointly by plant managers and a
training staff selected by management. In others, management is solely
responsible for maintaining training programs. In either case, responsibi-
lities should be explicitly designated to ensure that the quality and quantity
of training provided is adequate, Training requirements and practices can be
expected to differ between small and large companies, partly because of
resource needs and availability, and partly because of differences in employee
turnover,
A list of the aspects typically involved in the training of process
operators for routine process operations is presented in Table 5-3,
Emergency training includes topics such as:
• Recognition of alarm signals;
a Performance of specific functions (e.g., shutdown switches);
• Use of specific equipment;
• Actions to be taken on instruction to evacuate;
• Fire fighting; and
• Rehearsal of emergency situations.
Aspects specifically addressed in safety training include (17, 22):
• Hazard recognition and communication;
• Actions to be taken in particular situations;
59
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TABLE 5-3. ASPECTS OF TRAINING PROGRAMS FOR ROUTINE PROCESS OPERATIONS
Process goals, economics, constraints, and priorities
Process flow diagrams
Unit operations
Process reactions, thermal effects
Control systems
Process materials quality, yields
Process effluents and wastes
Plant equipment and instrumentation
Equipment identification
Equipment manipulation
Operating procedures
Equipment maintenance and cleaning
Use of tools
Permit systems
Equipment failure, services failure
•Fault administration
Alarm monitoring
Fault diagnosis
Malfunction detection
Communications, recordkeeping, reporting
Source: Reference 17.
60
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• Available safety equipment and locations;
m Use and familiarity with documentation such as;
plant design and operating manuals,
company safety rules and procedures,
procedures relevant to fire, explosion, accident, and
health hazards,
chemical property and handling information; and
• First aid and OPR.
Although emergency and safety programs typically focus on incidents such
as fires, explosions, and personnel safety, it is important that prevention of
chemical releases and release responses be addressed as part of these pro-
grams .
Much of the type of training discussed above is also important for
management personnel. Safety training gives management the perspective
necessary to formulate good policies and procedures, and to make changes that
will improve the quality of plant safety programs. Lees suggests that train-
ing programs applied to managers include or define (17);
• Overview of technical aspects of safety and loss prevention
approach;
• Company systems and procedures;
• Division of labor between safety personnel and managers with respect
to training; and
• Familiarity with documented materials used by workers.
61
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5,6,3 Maintenance and Modification Practices
Plant maintenance is necessary to ensure the structural integrity of
chemical processing equipment; modifications are often necessary to allow more
effective production. However, since these activities are also a primary
source of accidental release incidents, proper maintenance and modification
practices are an important part of accidental release prevention. Use of a
formal system of controls is perhaps the most effective way of ensuring that
maintenance and modification are conducted safely. In many cases, control
systems have had a marked effect on the level of failures experienced (17).
Permit systems and up-to-date maintenance procedures minimize the potent-
ial for accidents during maintenance operations. Permit-to-work systems
control maintenance activities by specifying the work to be done, defining
individual responsibilities, eliminating or protecting against hazards, and
ensuring that appropriate inspection and'testing procedures are followed.
Maintenance permits originate with the operating staff. Permits may be
issued in one or two stages. In one-stage systems, the operations supervisor
issues permits to the maintenance supervisor, who is then responsible for his
staff. Two-stage systems involves a second permit issued by the maintenance
supervisor to his workforce (17).
Another form of maintenance control is the maintenance information
system. Ideally, these systems should log the entire maintenance history of
equipment, including preventative maintenance, inspection and testing, routine
servicing, and breakdown or failure maintenance. This type of system is also
used to track incidents caused by factors such as human error, leaks, and
fires, including identification and quantification of failures responsible for
hazardous conditions, failures responsible for downtime, and failures respons-
ible for direct repair costs.
62
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Accidental releases are frequently the result of some aspect of plant
modification. Accidents result when equipment integrity and operation are not
properly assessed, following modification, or when modifications are made
without updating corresponding operation and maintenance instructions. In
these situations, it is important that careful assessment of the modification
results has a priority equal to that of getting the plant on-line.
For effective modification control, there must be established procedures
for authorization, work activities, inspection and assessment, complete
documentation of changes, including the updating of manuals, and additional
training to familiarize operators with new equipment and procedures (17, 22).
Formal procedures and checks on maintenance and modification practices
must be established to ensure that such practices enhance rather than advers-
ely affect plant safety. As with other plant practices, procedure development
and complete documentation are necessary; However, training, attitude, and
the degree to which the procedures are followed also significantly influence
plant safety and release prevention.
The use and availability of clearly defined procedures collected in
maintenance and operating manuals is crucial for the prevention of accidental
releases. Well-written instructions should give enough information about the
process that the worker with hands-on responsibility for operating or main-
taining the process can do so safely, effectively, and economically. These
instructions not only document the path to the desired results, but also are
the basis for most industrial training programs (23, 24). In the chemical
industry, operating and maintenance manuals vary in content and detail, To
some extent, this variation is a function of process type and complexity;
however, in many cases it is a function of management policy. Because of
their importance to the safe operation of a chemical process, these manuals
must be as clear, straightforward, and complete as possible. In addition,
standard procedures should be developed and documented before plant startup,
and appropriate revisions should be made throughout plant operations.
63
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Operation and maintenance may be combined or documented separately,
Procedures should include startup, shutdown, hazard identification, upset
conditions, emergency situations, inspection and testing, and modifications
(17). Several authors think industrial plant operating manuals should include
(17, 22, 23, 24):
• Process descriptions;
• A comprehensive safety and occupational health section;
« Information regarding environmental controls;
• Detailed operating instructions, including startup and shutdown
procedures;
• Upset and emergency procedures;
• Sampling instructions;
• Operating documents (e.g., logs, standard calculations);
• Procedures related to hazard identification;
• Information regarding safety equipment;
• Descriptions of job responsibilities; and
• Reference materials.
Plant maintenance manuals typically contain procedures not only for
routine maintenance, but also for inspection and testing, preventive mainten-
ance, and plant or process modifications. These procedures include specific
items such as codes and supporting documentation for maintenance and modifica-
64
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tions (e.g., permits-to-work, clearance certificates), equipment identifica-
tion and location guides, inspection and lubrication schedules, information on
lubricants, gaskets, valve packings and seals, maintenance stock requirements,
standard repair times, equipment turnaround schedules, and specific inspection
codes (e.g., for vessels and pressure systems) (17). Full documentation of
the maintenance required for protective devices is a particularly important
aspect of formal maintenance systems.
The. preparation of operating and maintenance manuals, their availability,
and the familiarity of workers with their contents are all important to safe
plant operations. The objective, however, is to maintain this safe practice
throughout the life of the plant. Therefore, as processes and conditions are
modified, documented procedures must also be modified.
5.7 CONTROL EFFECTIVENESS
It is difficult to quantify the control effectiveness of preventive and
protective measures to reduce the probability and magnitude of accidental
releases. Preventive measures, which may involve numerous combinations of
process design, equipment design, and operational measures, are especially
difficult to quantify because they reduce a probability of a chemical release
rather than a physical quantity of chemical release. Protective measures are
more analogous to traditional pollution control technologies. Thus, they may
be easier to quantify in terms of their efficiency in reducing a quantity of
chemical that could be released.
Preventive measures reduce the probability of an accidental release by
increasing the reliability of both process systems operations and the equip-
ment. Control effectiveness can thus be expressed for both the qualitative
improvements and the quantitative improvements through probabilities. Table
5-4 summarizes what appear to be some of the major design, equipment, and
operational measures applicable to the primary hazards identified for the
sulfur crioxide applications in the United States, The items listed in Table
65
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TABLE 5-4, EXAMPLES OF MAJOR PREVENTION AND PROTECTION MEASURES
FOR SULFUR TRIOXIDE RELEASES
Hazard Type
Prevention/Protection
0 P ERATIONAL
Human error
Vehicular collisions
Overfilling
PROCESS
Water contamination
in organic feeds
to sulfonation/sulfation
Sulfur trioxide
flow control
Temperature sensing
and cooling medium
flow control
Temperature sensing
and heating medium
flow control
Reactor and vaporizer
temperatures
Overpressure
Atmosphere releases
from relief discharges
Increased training and supervision;
use of checklists; use of automatic
systems
Location; physical barriers; warning
signs; training
Redundant independent level sensing,
alarms, and interlocks; training of
operators
Continuous moisture monitoring;
Backflow prevention
Redundant flow control loops;
Minimal overdesign of feed systems
Redundant temperature sensors;
Interlock flow switch to shut off
SO^ feed on loss of cooling, with
relief venting to emergency scrubber
system
Redundant temperature sensors;
Interlock flow switch to shut off
S0_ feed on loss of heating, with
relief venting to emergency scrubber
system
Redundant temperature sensing and
alarms
Redundant pressure relief; adequate
size; discharge not restricted
Emergency vent scrubber system
(Continued)
66
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TABLE 5-4. (Continued)
Hazard Type
Prevention/Protection
EQUIPMENT
Vessel failure
Corrosion
Storage tank or line
rupture
Adequate pressure relief; inspection
and maintenance; corrosion monitor-
ing; siting away from fire and
mechanical damage; higher pressure
rating in the event of solid SO^
formation and required remelting
Increased monitoring with more
frequent inspections; use of pH
sensing on cooling water and steam
condensate loops; use of corrosion
coupons ; visual inspections;
non-destructive testing
Enclosure vented to emergency
scrubber system; diking; foams;
dilution; neutralization; water
sprays
A piece of metal of known composition
rates by allowing it to reside in the
the amount of corrosion as a function
which is used to monitor corrosion
corrosive environment and measuring
of time.
67
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5-4 are for illustration only and do not necessarily represent satisfactory
control options for all cases. These control options appear to reduce the
risk associated with an accidental release when viewed from a broad
perspective. However, there are undoubtedly specific cases where these
control options will not be appropriate. Each case must be evaluated
individually, A presentation of more information about reliability in terms
of probabilities is discussed in the manual on control technologies, part of
this prevention reference manual series,
5.8 ILLUSTRATIVE COST ESTIMATES FOR CONTROLS
This section presents cost estimates for different levels of control and
for specific release prevention and protection measures for sulfur trioxide
storage and process facilities that might be found in the "United States.
5.8.1 Prevention and Protection Measures
Preventive measures reduce the probability of an accidental release _from
a process or storage facility by increasing the reliability of both process
systems operations and equipment. Along with an increase in the reliability
of a system is an increase in the capital and annual costs associated with
incorporating prevention and protection measures into a system. Table 5-5
presents costs for some of the major design, equipment, and operational
measures applicable to the primary hazards identified in Table 5-4 for the
sulfur trioxide applications in the United States.
5.8.2 Levels of Control
Prevention of accidental releases relies on a combination of technologi-
cal, administrative, and operational practices as they apply to the design,
construction, and operation of facilities where hazardous chemicals are used
and stored. Costs are inherent in determining the degree to which these
practices are carried out. At a minimum, equipment and procedures should be
68
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TABLE 5-5. ESTIMATED TYPICAL COSTS OF MAJOR PREVENTION AND
PROTECTION MEASURES FOR SULFUR TRIOXIDE RELEASE®
Prevention/Protection Measure
Capital Cost
(1986 $}
Annual Cost
(IS86 $/yr)
Continuous moisture monitoring
7,500-10,000
900-1,300
Flow control loop
4,000-6,000
500-750
Temperature sensor
250-400
30-50
Pressure relief
- relief valve
- rupture disk
1,000-2,000
1,000-1,200
120-250
120-150
Interlock system for flow shut-off
1,500-2,000
175-250
pE monitoring of cooling water
7,500-10,000
900-1,300
Alarm system
25-0-500
30-75
Level sensor
— liquid level gauge
- load cell
1,500-2,000
10,000-15,000
175-250
1,300-1,900
Diking (based on a 10,000 gal. tank)
- 3 ft, high
— top of tank height, 10 ft.
1.200-1,500
7,000-7,500
150-175
850-900
Increased corrosion inspection^
200-400
a-Based on a 10,000 gallon fixed sulfur trioxide storage tank system and a 20
million pounds per year capacity sulfonate detergent process.
bBased on 10-20 lir. @ $20/hr.
69
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in accordance with applicable codes, standards, and regulations. However,
additional measures can be taken to provide extra protection against an
accidental release.
The levels-of-control concept provides a means of assigning costs to
increased levels of prevention and protection. The minimum level is referred
to as the "Baseline" system. This system consists of the elements required
for normal safe operation and basic prevention of an accidental release of
hazardous material.
The second level of control is "Level 1", "Level 1" includes the "Base-
line" system with added modifications such as improved materials of construc-
tion, additional controls, and generally more extensive release prevention
measures. The costs associated with this level^ are higher than the baseline
system costs.
The third level of control is "Level 2." This system incorporates both
the "Baseline" and "Level 1" systems with additional modifications designed
specifically for the prevention o£ an accidental release such as alarm and
interlock systems. The extra accidental release prevention measures incor-
porated into "Level 2" are reflected in its cost, which is much higher than
that of the baseline system.
When comparing the costs of the various levels of control, it is impor-
tant to realize that higher costs do not necessarily imply improved safety.
The measures applied must be applied correctly. Inappropriate modifications
or add-ons may not make a system safer. Each added control option increases
the complexity of a system. In some cases the hazards associated with the
increased complexity may outweigh the benefits derived from the particular
control option. Proper design and construction along with proper operational
practices are needed to assure safe operation.
70
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These estimates are for illustrative purposes only. It is doubtful that
any specific installation would find all of the control options listed in
these tables appropriate for their purposes. An actual system is likely to
incorporate some items from each of the levels of control and also some
control options not listed here. The purpose of these estimates is to illus-
trate the relationship between cost and control, and is not to provide an
equipment check list.
Two sets of cost estimates were prepared; one for a 10,000 gallon capac-
ity fixed liquid sulfur trioxide storage tank system and the other for a
liquid sulfur trioxide sulfonation reactor system for a 20 million pounds per
year capacity sulfonate detergent process. These systems are representative
of storage and process facilities that night be found in the United States.
5.8.3 Cost Summaries
Table 5-6 presents a summary of the total capital and annual costs for
each of the' levels of control for the liquid sulfur trioxide storage system
and the added level of control for the liquid sulfur trioxide sulfonation
reactor system. The costs presented correspond to the systems described in
Table 5-7 and Table 5-8. Costs associated with each of the levels include the
cost of the basic system plus any added controls. Specific costs and cost
information for each level of control for both the storage and process facil-
ities are presented in Tables 5-9 through 5-13.
5.8.4 Equipment Specifications and Detailed Costs
Equipment specifications and details of the capital cost estimates for
the liquid sulfur trioxide storage and the liquid sulfur trioxide sulfonation
reactor systems are presented in Tables 5-14 through 5-20.
71
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TABLE 5-6. SUMMARY COST ESTIMATES FOR POTENTIAL LEVELS OF CONTROLS
FOR SULFUR TRIOXIDE STORAGE TANK AND SULFONATION SYSTEM
System
Level of
Control
Total Capital
Cost
(1986 $)
Total Annual
(1986 $)
Fixed SO, storage
tank witn 10,000
gallon capacity
Baseline
Level No. 1
Level No,2
116,000
210,000
679,000
14,000
25,000
80,000
SO^ sulfonation
reactor system with
a 20 million pounds
per year capacity
Baseline
Level No. lc
2,174,000
2,404,000
261,000
289,000
<3»
Only one level of control in addition to the baseline was costed since the
baseline system inherently includes many of the controls discussed in this
manual. Level 1 simply provides redundant control measures.
72
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TABLE 5-7. EXAMPLE OF LEVELS OF CONTROL FOR SULFUR TRIOXIDE STORAGE TANKa
Process; 10,000 gal fixed liquid sulfur trioxide storage tank
Controls
Baseline
Level No. 1
Level No. 2
Flow;
Single check-
Add second check
Add a reduced-
valve on tank-
valve.
pressure device
process feed
with internal air
line.
gap and relief
vent to con-
taminant tank or
scrubber.
Temperature:
Local temperature
Add remote temp-
Add temperature
indicator. Temp
erature indica-
alarm. Tem-
sensor with con-
tor. Temp sensor
perature sensor
troller regulat-
v/ccntroiler reg-.
with controller
. ing immersion
ulating immersion
regulating hot-
heater and heat
heater and heat
room heater and
tracing.
tracing.
immersion heater.
Pressure:
Single pressure
Add second relief
Add rupture discs
relief valve,
• valve w/3-way
under relief
vent to
valve. Vent to
valves. Provide
atmosphere.
limited scrubber.
local pressure in-
Local pressure
dication on space
indicator.
between disc and
valve. Vent to
scrubber.
Quantity:
Local level
Independent re-
Add level alarm.
indicator.
mote level in-
Add high—low
dicator.
level interlock
shutoff for inlet
and outlet lines.
Location:
Away from traf-
Same
Same
fic, flammables,
and other hazard-
ous processes.
Materials of
Carbon steel.
Carbon steel with
3Q4SS.
Construction:
increased corro-
sion allowances.
1/8 inch.
(Continued)
73
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TABLE 5-7. (Continued)
Controls
Saseline
Level No, 1
Level No. 2
Vessel:
Tank pressure
specification 50
psig. Heat
traced and
insulated.
Tank pressure
specification 100
psig. Heat
traced and in-
sulated.
Tank pressure
specification 100
psig.
Piping;
Sch. 80 carbon
steel. Heat
traced and
insulated.
Sch. 80 304 SS.
Heat traced and
insulated.
Sch. 80 Monel
Process
Machinery;
Bottom discharge
from tank to
centrifugal
pump, Alloy 20
construction.
Submerged cen-
trifugal pump,
Alloy 20 con-
struction.
Magnetically-cou-
pled centrifugal
pump, Alloy 20
construction.
Enclosures:
None
None.
Concrete build-
ing.
Diking:
None
3 ft. high.
3 ft. high.
Scrubbers;
None
Small acid
scrubber.
Large acid
scrubber
w/hotroox vent
scrubber.
Mitigation:
None
Foam generator.
ETuorocarbon
surface seal.
aThe examples in this table are appropriate for many, but not all appli-
cations. This is only an exemplary system. Design must be suited to
fit the service.
74
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TABLE 5-8, EXAMPLE OF LEVELS OF CONTROL FOR SULFUR
TRIOXIDE SULFONATION REACTOR
Process: Sulfur Trioxide Sulfonation
Typical Operating Conditions: - Temperature 95-130"F
- Pressure: 30-40 psig
Controls
Baseline
Level No, 1
Temperature:
Provide local temperature
indicators on all process
streams and on the utility
fluid streams entering and
leaving the heat exchangers.
Remote temperature indicators
and controllers,
Add redundant tempera-
ture sensors and
alarms,
Pressure:
Provide local pressure indica-
tors on all fluid streams ¦
and at pump discharges,
Single relief valve on reactor.
Vent to scrubber.
Add remote pressure
indicators. Add second
relief valve.
Flow:
Provide local flow indicators
on all process streams. Pro-
vide local flow control on
reactor recycle line and S0^
vaporizer.
Add redundant flow
control loops.
Corrosion:
Visual inspection and periodic
pH monitoring of cooling water.
Add continuous pH
sensing of cooling
water. Add corrosion
coupons.
Material of
Construction:
316 Stainless Steel.
316 Stainless Steel,
Protective
Barrier:
Curbing around reactor/
settler.
3 ft. high retaining
wall.
Enclosures:
Scrubber:
None
Caustic venturi scrubber.
Steel building.
ESP with SOj scrubber.
75
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TABLE 5-9. ESTIMATE OP TYPICAL CAPITAL AND ANNUAL COSTS ASSOCIATED
WITH BASELINE SULFUR TRIOXIDE STORAGE SYSTEM
Capital Cost Annual Cost
(1986 $) (1986 $/yr)
VESSELS;
Storage Tank 63,000 7,300
Immersion Heater 4,800 570
Insulation 18,000 2,100
PIPING AMD VALVES:
Pipework 9,900 1,100
Check Valve 530 60
Globe Valves (7) 2,800 330
Relief Valve 1,600 190
Heat Tracing 710 80
Insulation 1,900 220
PROCESS MACHINERY:
Centrifugal Pump 4,000 460
INSTRUMENTATION:
Liquid Level Gauge 1,500 170
Local Temperature Indicator 1,900 220
Temperature Control
- Controller 1,900 220
- Sensor 190 25
Control Valve 2,800 330
no AnirmTDt? o tjt? a iTt n? cj •
Visual Tank Inspection (external) 15
Visual Tank Inspection (internal) 60
Relief Valve Inspection 15
Piping Inspection 300
Piping Maintenance 120
Valve Inspection 30
Valve Maintenance 350
TOTAL 00STS 116,000 14,000
76
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TABLE 5-10. ESTIMATED TYPICAL CAPITAL AND ANNUAL COSTS ASSOCIATED
WITH LEVEL 1 SULFUR TRIOXIDE STORAGE SYSTEM
Capital Cost Annual Cost
(1986 $) (1986 $/yr)
VESSELS:
Storage Tanks 114,000 13,000
Immersion Heater 4,800 570
Insulation 18,000 2,100
PIPING AND VALVES:
Pipework 16,000 1,800
Check Valves (2) 1,100 120
Globe Valves (7) 2,800 330
Relief Valves (2) 3,200 380
Heat Tracing 710 80
Insulation 1,900 220
PROCESS MACHINERY: -
Submerged Centrifugal Pump 6,500 750
INSTRUMENTATION:
Pressure Gauge 370 45
Liquid Level Gauge . 1,500 175
Remote Level Indicator 1,900" 220
Local Temperature Indicator 1,900 220
Remote Temperature Indicator 2,200 260
Temperature Control
- Controller 1,900 220
- Sensor 190 25
- Control Valve 2,800 330
SCRUBBER:
Brink® Mist Eliminator 27,000 3,100
DIKING:
3 ft High Concrete Diking 1,400 160
PROCEDURES AND PRACTICES:
Visual Tank Inspection (external) 15
Visual Tank Inspection (internal) 60
Relief Valve Inspection 30
Piping Inspection 300
Piping Maintenance 120
Valve Inspection 35
Valve Maintenance 400
TOTAL COSTS 210,000 25,000
77
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TABLE 5-11. ESTIMATED TYPICAL CAPITAL AND ANNUAL COSTS ASSOCIATED
WITH LEVEL 2 SULFUR TRIOXIDE STORAGE SYSTEM
Capital Cost
(1986 $)
Annual Cost
(1986 $/yr)
VESSELS;
Storage Tank
300,000
35,000
Immersion Heater -
4,800
570
PIPING AND VALVES:
Pipework
30,000
3,500
Reduced Pressure Device
1,500
170
Globe Valves (7)
2,800
200
Relief Valves (2)
3,200
470
Rupture Disks (2)
1,100
130
PROCESS MACHINERY:
Centrifugal Pump
19,000
2,200
INSTRUMENTATION:
Pressure Gauges (2)
740
90
Liquid Level Gauge
• 1,500
175
Remote Level Indicator
1,900
220
Level Alarm
380
45
High-Low Level Shutoff
1,900
220
Local Temperature Indicator
1,900
220
Remote Temperature Indicator
2,200
260
Temperature Alarm
380
45
Temperature Control
- Controller
1,900
220
- Sensor
190
25
- Control Valve
2,800
330
ENCLOSURE:
Concrete Building
19,000
2,300
SCRUBBER:
Acid Scrubber
280,000
33,000
DIKING:
3 ft. High Concrete Diking
1,400
160
(Continued)
78
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TABLE 5-11. (Continued)
Capital Cost
(1986 $)
Annual Cost
(1986 $/yr)
PROCEDURES AMD PRACTICES;
External Tank Inspection
15
Internal Tank Inspection
60
Relief Valve Inspection
300
Piping Inspection
120
Piping Maintenance
120
Valve Inspection
35
Valve Maintenance
400
TOTAL COSTS 679,000 80,000
79
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TABLE 5-12. ESTIMATED TYPICAL CAPITAL AND ANNUAL COSTS ASSOCIATED
WITH BASELINE SULFUR TRIOXIDE SULFONATION SYSTEM
Capital Cost Annual Cost
(1986 $) (1986 $/yr)
VAPORIZER3" 783,000 94,000
Cost includes vessels,
ntacbxnery, controls,
and instrumentation for
vaporizers
(See Table '5-18 for details),
SULFONATORa
Cost includes vessels, 1,150,000 138,000
machinery, piping, controls,
and instrumentation for
sulfonators
(See Table 5-18 for details).
SCRUBBER:
Caustic Venturi Scrubber 240,000 29,000
DIKING:
Curbing Around Reactor 1,200 150
TOTAL COSTS 2,174,000 261,000
£
System cost based on a 20 million pounds per year capacity system obtained
from Reference 12.
80
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TABLE 5-13. ESTIMATED TYPICAL CAPITAL AND ANNUAL COSTS ASSOCIATED WITH
LEVEL 1 SULFUR TRIOXIDE SULEONATION SYSTEM
VAPORIZER:
Baseline Cost (see Table 5—12)
Add to Baseline:
- Redundant Temperature
Sensors (4)
- Redundant Temperature
Alarm
- Redundant Pressure
Indicators (2)
- Redundant Flow Control
Loop
- pH Monitoring System
SULFONATOR:
Baseline Cost (see Table 5-12)
Add to Baseline
- Redundant Temperature
Sensors (12)
- Redundant Temperature
Alarm
- Redundant Pressure
Indicators (5)
- Redundant How Control
Loop
- Additional Relief Valves
DIKING:
3 ft. High Retaining Wall
Cj f'p TTR"R.TTP *
Wet Electrostatic Precipitator
with Packed Tower
ENCLOSURE:
Steel Building
Capital Cost Annual Cost
(1986 $) (1986 $/yr)
783,000
94,000
1,400
175
360
175
3,600
430
5,500
650
9,000
1,100
150,000
138,000
4,300
500
360
45
9,000
5,500
650
2,000
230
3,000
360
417,000
50,000
10,000
1,200
TOTAL COSTS 2,404,000 289,000
81
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TABLE 5-14. EQUIPMENT SPECIFICATIONS ASSOCIATED WITH SULFUR
TRIOXIDE STORAGE SYSTEM
Equipment Item Equipment Specification Reference
VESSELS:
Storage Tank
Immersion Heater
Insulation
PIPING AND VALVES:
Pipework
Check Valve
Globe Valves
Relief Valve
Heat Tracing
Insulation
Baseline: 10,000 gal. Carbon Steel
Storage Tank. 50 psig
Level #1: 10,000 gal. Carbon Steel
with 1/8 in. Corrosion Protection,
100 psig
Level #2: 10,000 gal. 304 Stainless
Steel 100 psig
75 kw Rating, Electric Bayonet
2 1/2 in. Thick Covering 10 ft.
Diameter "by 17 ft. Long Horizontal
Storage Tank
Baseline: 100 ft. of 2 in. Schedule
Carbon Steel
Level #1: 100 ft. of 2 in. Schedule 80
Type 304 Stainless Steel
Level #2: 100 ft. of 2 in. Schedule 80
Monel®.
2 in. Vertical Lift Check Valve,
Stainless Steel Construction
2 in. ANSI Class 300, .Type 316
Stainless Steel Construction
1 in. x 2 in., ANSI Class 300 inlet
and Outlet Flange, Angle Body,
Closed Bonnet with Screwed Cap.
Type 316 Stainless Steel Body and
Trim.
1/2 in. 0D Copper Trace Line, Heat
Transfer Cement
1 1/2 in. Thick CaSi including
Aluminum Waterproof Jacket
25, 26,
27, 28
25
26
29, 30
25, 26
25, 26
26
26
26
(Continued)
82
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TABLE 5-14. (Continued)
Equipment Item
Equipment Specification
Reference
Reduced Pressure
Device
Rupture Disk
PROCESS MACHINERY;
Centrifugal Pump
INSTRUMENTATION:
Pressure Gauge
Liquid Level Gauge
Level Indicator
Level Alarm
High-low Level
Shutoff
Temperature
Indicator/Con-
troller
Double Check Valve Type Device
With Internal Air Gap and Relief
Valve
1 in. Monel® Disk and Carbon Steel
Holder
Baseline: Single Stage, Alloy 20
Construction, Stuffing Box
Level #1: Single Stage Submerged,
Alloy 20 Construction, Double Mech.
Seal
Level #2; Magnetically-coupled,
Construction, Double Mech, Seal
Diaphragm Sealed., Hastelloy C
Diaphragm, 0-200 psi
Differential Pressure Type
Differential Pressure Type Indicator
Indicating and Audible Alarm
Solenoid Valve, Switch, and Relay
System
Thermocouple, Thermowell, Electronic
Indicator/Controller
Temperature Alarm Indicating and Audible Alarm
25
27, 31, 32
25, 26, 33
ENCLOSURES;
Building
Level #2: 10 in. Concrete Wails,
26-Gauge Steel Roof
25, 26, 34
25, 34
25, 34
25, 35, 36
25, 26, 34
25, 35, 34
25, 35, 36
35
(Continued)
83
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TABLE 5-14. (Continued)
Equipment Item Equipment Specification Reference
SCRUBBERS: Level #1: Brink® Mist Eliminator, 37
Carbon Steel Construction, Water
Spray, 500 ACFM Capacity
Level #2: Acid Scrubber with
Demister, Packed Tower with Ceramic
Saddles, Circulating Pump with
96-98% SO^. Spray Tower with
Brink® Mist Eliminator, Stainless
Steel Construction, Water Spray,
1000 ACFM Capcity
DIKING: Level #1 and #2; 6 in. Concrete 35
Walls, 3 ft. High
84
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TABLE 5-15. DETAILS OF MATERIAL AND LABOR COSTS ASSOCIATED WITH BASELINE
SULFUR TRIOXIDE STORAGE SYSTEM
Materials Labor Direct Indirect Capital
Coat Cost Costs Costs Cost
(1986 $}
VESSELS;
Storage Tank
29,150
13,100
42,250
14,800
63,000
Immersion Heater
2,650
660
3,310
830
4,800
Insulation
4,800
7,300
12,100
4,230
18,000
PIPING AMD VALVES:
Pipework
1,400
5,230
6,640
2,320 -
9,900
Check Valve
300
50
350
130
530
Globe Valves (7) .
1,750
180
¦ 1,930
680
2,800
Relief Valve
1,000
50
1,100
380
1,600
Heat Tracing
190
' 290
480
170
710
Insulation
500
750
1,250
440
1,900
PROCESS MACHINERY:
Centrifugal Pump
1,900
800
2,700
940
4,000
IN STRUMEMTATION:
Liquid Level Gauge
800
200
1,000
350
1,500
Local Temperature
Indicator
1,000
250
1,250
440
1,900
Temperature Control
— Controller
1,000
250
1,250
440
1,900
- Sensor
100
25
125
45
190
- Control Valve
1,500
380
1,880
660
2,800
TOTAL COSTS
48,000
30,000
78,000
27,000
116,000
85
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TABLE 5-16. DETAILS OF MATERIAL AND LABOR COSTS ASSOCIATED WITH LEVEL 1
SULfUR TRIOXIDE STORAGE SYSTEM
Materials Labor Direct Indirect Capital
Cost Cost Costs Costs Cost
(1986 $)
VESSELS;
Storage Tank
53,000
24,000
77,000
27,000
114,000
Immersion Heater
2,650
660
3,310
830
4,800
Insulation
4,800
7,300
12,100
4,230
18,000
PIPING AND VALUES;
Pipework
2,700
7,900
10,600
3,700
16,000
Check Valves (2)
600
100
700
130
1,100
Globe Valves (7)
1,750
180
1,930
680
2,800
Relief Valves (2)
2,000
200
2,200
700
3,200
Heat Tracing
190
290
480
170
710
Insulation
500
750
1,250
440
1,900
PROCESS MACHINERY:
Submerged Centrifugal
3,000
1,300
4,300
1,500
6,500
Pump
INSTRUMENTATION:
Pressure Gauge
200
50
250
90
370
Liquid Level Gauge
800
200
1,000
350
1,500
Remote Level Indicator
1,000
250
1,250
440
1,900
Local Temperature
Indicator
1,000
250
1,250
440
1,900
Remote Temperature
Indicator
1,200
300
1,500
530
2,200
Temperature Control
- Controller
1,000
250
1,250
440
1,900
- Sensor
100
25
125
45
190
— Control Valve
1,500
380
1,880
660
2,800
SCRUBBER:
Water Scrubber
12,000
6,000
18,000
6,300
27,000
DIKING:
3 ft High Con-
390
520
910
320
1,400
crete Diking
TOTAL COSTS
90,000
51,000
141,000
49,000
210,000
86
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TABLE 5-17. DETAILS OF MATERIAL AND LABOR COSTS ASSOCIATED WITH LEVEL 2
SULFUR TRIOXIDE STORAGE SYSTEM
Materials Labor Direct Indirect Capital
Cost Coat Costs Costs Cost
(1986 $)
VESSELS:
Storage Tank.
139.000
63,000
202,000
71,000
300,000
Immersion Heater
2,650
660
3,310
830
4,800
PIPING AND VALVES:
Pipework
11,000
9,000
20,000
7,000
30,0100
Reduced Pressure Device
800
200
1,000
350
1,500
Globe Valves (7)
1,750
180
1,930
680
2,800
Relief Valves (2)
2,000
200
2,200
700
3,200
Rupture Disks (2)
650
80
730
260
1,100
PROCESS MACHINERY;
Centrifugal Pump
9,000
3,900
. 12,900
4,500
19,000
INSTRUMENTATION:
Pressure Gauges (2)
400
100
500
180
740
Liquid Level Gauge
800
200
1,000
350
1,500
Remote Level Indicator
1,000
250
1,250
440
1,900
Level Alarm
200
50
250
90
380
High-Low Level Shutoff
1,000
250
1,250
440
1,900
Local Temperature
Indicator
1,000
250
1,250
440
1,900
Remote Temperature
Indicator
1,200
300
1,500
530
2,200
Temperature Alarm
200
50
250
90
380
Temperature Control
— Controller
1,000
250
1,250
440
1,900
- Sensor
100
25
125
45
190
- Control Valve
1,500
380
1,880
660
2,800
ENCLOSURE;
Concrete Building
6,100
6,600
13,000
4,500
19,000
SCRUBBER:
Acid Scrubber
130,000
59,000
189,000
66,000
280,000
DIKING:
3 ft High Concrete Dike
390
520
910
320
1,400
TOTAL COSTS
312,000
145,000
457,000
160,000
679,000
87
-------
TABLE 5-18. EQUIPMENT SPECIFICATIONS ASSOCIATED WITH SULFUR TRIOXIDK
SULFONATION
Eqtiipmen t It em
Equipment Specification
Reference
VAPORIZER:
System includes; 316 S.S. vaporizer,
316,S.S. piping, positive
displacement..metering pump,
interlocks, air compressor, air
chiller, drier, heat exchanger, local
and remote temperature
sensor/controller, local flow
indicator/controller, and local
pressure indicator.
12,25
SIXLFONATOR;
System includes; 316 S.S. falling
film reactor with 20 million Ib/yr
capacity, cyclone separator, heat
exchanger, interlocks recycle pump,
local and remote temperature
controller, local flow controller,
relief valve, and digester.
12,25
PIPING AND VALVES:
Relief Valve
2 in, x 3 in. Class 300 Inlet and
Outlet Flange, Angle Body, Closed
Bonnet With Screwed Cap, 316 S.S.
Body, Monel® Trim
26
Rupture Disk
2 in. Monel® Disk and Carbon Steel
Holder
27,31,32
INSTRUMENTATION:
Temperature Sensor
Thermocouple and Associated
Thermowell
25,26,36
Temperature Alarm
Temperature Switch
Indicating and Audible Alarm
Two-Stage Switch with Independently
Set Actuation
26,35,36
25,36
Remote Temperature
Indicator
Transmitter and Associated
Electronics Indicator
25,36
Remote Pressure
Indicator
Transducer, Transmitter and
Electronic Indicator
25,36
(Continued)
88
-------
TABLE 5—18. (Continued)
Equipment Item Equipment Specification Reference
Flow Control Loop 2 in. Globe Control Valve, Konel® 25,36
Trim, Flowmeter and PTD Controller
Flow Interlock System Solenoid Valve, Switch, and Relay 25,36
System
pH Monitoring System Electrode, Electrode Chamber, 25,36
Amplifier-Transducer and Indicator
DIKING: Baseline: 6 in. High Concrete 35
Curbing
Level #1: 3 ft, Eigh Concrete
Retaining
SCRUBBER: Baseline: ¥enturi Scrubber with 12,25
caustic spray
Level #1: Electrostatic Precipitator 19,25
and Packed Tower System
89
-------
TABLE 5-19. DETAILS OF MATERIAL AND LABOR COSTS ASSOCIATED WITH BASELINE
SULFUR TRIOXIDE SULFONATION SYSTEM
Materials Labor Direct Indirect Capital
Cost Cost Cost Cost Cost
mafi st
VAPORIZER:a 375,000 170,000 545,000 136,000 783,000
- Includes vessels,
machinery, piping,
controls, and
instrumentation for
vaporizer (see Table
5-18 for details),
SULFONATOR:3 550,000 250,000 800,000 200,000 1,150,000
- Includes vessels,
machinery, piping,
controls, and
instrumentation for
sulfonafcor (see Table
5-18 for details).
SCRUBBER:
Caustic Venturi Scrubber 115,000 52,000 167,000 42,000 240,000
DIKING:
Curbing Around Reactor 500 350 850 210 1,200
TOTAL COSTS 1,041,000 472,000 1,513,000 378,000 2,174,000
aSystem costs based on a 20 million pounds per year capacity system obtained
from Reference 12.
90
-------
TABLE 5-20. DETAILS OF MATERIAL AND LABOR COSTS ASSOCIATED WITH LEVEL I
SULFUR TRIOXIDE SULFONATION SYSTEM
Materials Labor Direct Indirect Capital
Cost Cost Cost Cost Cost
C1986 SH
VAPORIZER:
Baseline Cost 375,000 170,000 545,000 136,000 783,000
(See Table 5-19)
Add to Baseline:
Redundant Temperature
800
200
1,000
240
1,400
Sensors (4)
Redundant Temperature
200
50
250
60
360
Alarm
Redundant Pressure
2,000
500
2,500
620
3,600
Indicators (2)
Redundant Flow
3,000
750
3,750
950
5,500
Control Loop
pH Monitoring System
5,000
1,300
6,300 /
1,600
9,000
SULFGNATOR;
Baseline Cost 550,000 250,000 800,000 200,000 1,150,000
(See Table 5-19)
Add to Baseline
Redundant Temperature
2,400
600
3,000
720
4,300
Sensors (12)
Redundant Temperature
200
50
250
60
360
Alarm
Redundant Pressure
5,000
1,250
6,250
1,550
9,000
Indicators (5)
Redundant Flow
3,000
750
3,750
950
5,500
Control Loop
Second Relief Valve
1,300
50
1,350
340
2,000
DIKING:
3 ft. High Retaining 900 1,200 2,100 530 3,000
Wall
SCRUBBER; 200,000 90,000 290,000 73,000 417,000
Electrostatic
Precipitator with
Packed Tower
ENCLOSURE: 4,600 2,300 6,900 2,400 10,000
Steel Building
TOTAL COSTS 1,154,000 519,000 1,673,000 419,000 2,404,000
91
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5.8.5 Methodology
Tables are provided for control schemes associated with storage and
process facilities for liquid sulfur trioxide showing capital, operating, and
total annual costs. The tables are broken down into subsections comprising
vessels, piping and valves, process machinery, instrumentation, and procedures
and practice. The presentation of the costs in this manner allows for easy
comparison of costs for specific items, different levels, and different
systems.
Capital Cost--All capital costs presented in this report are shown as
total fixed capital costs. Table 5-21 defines the cost elements comprising
total fixed capital as it is used here.
The computation of total fixed capital as shown in Table 5-21 begins with
the total direct cost for the system under consideration. This total direct
cost is the total direct installed cost of all capital equipment comprising
the system. Depending on the specific equipment item involved, the direct
capital cost was available or was derived from uninst.illed equipment costs by
computing costs of installation separately. To obtain the total fixed capital
cost, other costs obtained by utilizing factors are added to the total direct
costs.
The first group of other cost elements is indirect costs. These include
engineering and supervision, construction expenses, and various other expenses
such as administration expenses, for example. These costs are computed by
multiplying total direct costs by a factor shown in Table 5-21. The factor is
approximate, is obtained from the cost literature, and is based on previous
experience with capital projects of a similar nature. Factors can have a
range of values and vary according to technology area and for individual
technologies within an area. Appropriate factors were selected for use in
this report based on judgement and experience.
92
-------
TABLE 5-21. FORMAT FOR TOTAL FIXED CAPITAL COST
Item No, Item Cost
1 Total Material Cost -
2 Total Labor Cost -
3 Total Direct Cost Items 1+2
4 Indirect Cost Items (Engi-
neering & Construction
Expenses) 0,35 x Item 3a
5 Total Bare Module Cost Items (3 + 4)
6 _ Contingency (0.05 x Item 5)b
7 Contractor's Fee 0.05 x Item 5
8 Total Fixed Capital Cost Items (5+6+7)
£L * » ¦
For storage facilities, the indirect cost factor is 0.35. For process
facilities, the indirect cost factor is 0,25,
k For storage facilities, the contingency cost factor is 0.05. For process
facilities, the contingency cost factor is 0.10.
93
-------
When the indirect costs are added to the total direct costs, total bare
module cost is obtained. Some additional cost elements such as contractor's
fee and contingency are calculated by applying and adding appropriate factors
to the total bare module cost as shown in Table 5-21 to obtain the total fixed
capital cost.
Annual Cost--Annual costs are obtained for each of the equipment items by
applying a factor for both capital recovery and for maintenance expenses to
the direct cost of each equipment item. Table 5-22 defines the cost elements
and appropriate factors comprising these costs. Additional annual costs are
incurred for procedural items such as valve and vessel inspections, for
example. When all of these individual costs are added, the total annual cost
is obtained.
Sources of Information--
The costs presented in this report are derived from cost information in,
existing published sources and also from recent vendor information. It was
the objective of this effort to present cost levels for liquid sulfur trioxide
process and storage facilities using the best costs for available sources.
The primary sources of cost information are Peters and Timmerhaus (25),
Chemical Engineering (38), and Valle-Riestra (39) supplemented by other
sources and references where necessary. Adjustments were made to update all
costs to a June 1986 dollar basis. In addition, for some equipment items,
well-documented costs were not available and they had to be developed from
component costs.
Costs in this document reflect the "typical" or "average" representation
for specific equipment items. This restricts the use of data in this report
to:
• Preliminary estimates used for policy planning;
• Comparison of relative costs of different levels or system;
and
94
-------
TABLE 5-22. FORMAT FOR TOTAL ANNUAL COST
Item No. Item Cost
1 Total Direct Cost -
2 Capital Recovery on Equip-
ment Items G.163a x Item 1
3 Maintenance Expense on
Equipment Items 0.01 x Item 1
4 Total Procedural Items -
5 Total Annual Cost Items.(2+3+4)
aBased on a capital recovery factor at 10 percent cost of capital for 10
years,
95
-------
• Approximations of costs that might be incurred for a specific
application.
The costs in this report are considered to be "order of magnitude" with a
+50 percent margin. This is because the costs are based on preliminary
estimates and many are updated from literature sources. Large departures from
the design basis of a particular system presented in this manual or the advent
of a different technology might cause the system cost to vary by a greater
extent than this. If used as intended, however, this document will provide a
reasonable source of preliminary cost information for the facilities covered.
When comparing costs in this manual to costs from other references, the
user should be sure the design bases are comparable and that the capital and
annual costs as defined here are the same as the costs being compared.
Cost Updating--
All costs in this report are expressed in June 1986 dollars. Costs
reported in the literature were updated using cost indices for materials and
labor.
Costs expressed in base year dollars may be adjusted to dollars for
another year by applying cost indices as shown in the following equation:
. , . , new base year index
new base year cost = old base year cost x _
old base year index
The Chemical Engineering (CE) Plant Cost Index was used in updating cost for
this report. For June 1986, the index is 316,3.
Equipment Cost--
Most of the equipment costs presented in this manual were obtained
directly from literature sources of vendor information and correspond to a
specific design standard. Special cost estimating techniques, however, were
96
-------
used in determining the costs associated with vessels, piping systems, scrub-
bers, diking, and enclosures. The techniques used are presented in the
following subsections of this manual.
Vessels--The total purchased cost for a vessel, as dollars per pound of
weight of fabricated unit free on board (f.o.b.) with carbon steel as the
basis (January 1979 dollars) were determined using the following equation for
Peters and Timmerhaus (25):
Cost = 50 [Weight of Vessel in Pounds] ®
The vessel weight is determined using appropriate design equations as given by
Peters and Timmerhaus (25) which allow for wall thickness adjustments for
corrosion allowances, for example. The vessel weight is"increased by a factor
of 0.15 for horizontal vessels and 0.20 for vertical vessels to account for
the added weight due to nozzles, manholes, and skirts or saddles. Appropriate
factors are applied for different materials of construction as given in Peters
and Timmerhaus (25). The vessel costs are updated using cost factors.
Finally a shipping cost amounting Co 10 percent of the purchased cost is added
to obtain the delivered equipment cost.
Piping--Piping costs were obtained using cost information, and data
presented by Yamartino (29) and Barnett (30). A simplified approach is used
in which it is assumed that a certain length of piping containing a given
number of valves, flanges, and fittings is contained in the storage or process
facility. The data presented by Yamartino (29) and Barnet (30) permit cost
determinations for various lengths, sizes, and types of piping systems. Using
these factors, a representative estimate can be obtained for each of the
storage and process facilities.
Diking--Diking costs were estimated using Mean's Manual (35) for rein-
forced concrete walls. Several assumptions were made in determining the
costs. The dike must contain the entire contents of a tank in the event of a
97
-------
leak or release. Two dike sizes are considered: a three-foot high dike,
(six-inches thick) and a cop-of-tank height dike (ten inches thick), The
tanks are raised off the ground and are not volumetrically included in the
volume enclosed "by the diking. These assumptions facilitate cost determina-
tion for any size diking system.
Enclosures --Enclosure costs, were estimated using Mean's Manual (35) for
both reinforced concrete and steel-walled buildings. The "buildings are
assumed to enclose the same area and volume as the top-of-tank height dikes.
The concrete building is ten-inches thick with a 26-gauge steel roof and a
metal door. The steel building has 26-gauge roofing, siding, and metal door.
The cost of a ventilation system was determined using a typical 1,000 scfnt
unit and doubling the cost to account for ductwork and requirements for the
safe enclosure of hazardous chemicals.
Scrubbers--Scrubber costs were estimated using the following equation
from the Card (37) manual for scrubbers. The costs were based on the actual
cubic feet per minute of flow at a chamber velocity of 600 feet/minute.
Costs - 0.235 x (ACFM + 43,000)
3
A release rate of 10,000 ft /minute was assumed for the storage vessel systems
and an appropriate rate was determined for the process system based on the
quantity of hazardous chemicals present in the system at any one time. For
3
the sulfonacion reactor system, a rate of 1,000 ft /minute was assumed. In
addition to the tower, the costs also include pumps and a storage tank for the
scrubbing medium. The costs presented are updated to June 1986 dollars.
Installation Factors--
Installation costs were developed for all equipment items included in
both the process and storage systems. The costs include both the material and
labor costs for installation of a particular piece of equipment. The costs
were obtained directly from literature sources and vendor information or
98
-------
indirectly by assuming a certain percentage of the purchased, equipment cost
through the use of estimating factors obtained from Peters and Timmerhaus (25)
and Valle-Riestra (39). Table 5-23 lists the cost factors used or the refer-
ence from which the cost was obtained directly. Many of the costs obtained
from the literature were updated to June 1986 dollars using a 10 percent per
year rate of increase for labor and cost indices for materials associated with
installation.
99
-------
TABLE 5-23. FORMAT FOR INSTALLATION COSTS
Equipment Item Factor or Reference
Vessels:
Storage Tank 0.45
Expansion Tank 0.25
Piping and Valves;
Pipework Ref. 29 and 30
Expansion Loop Ref. 26
Reduced Pressure Device Ref. 26
Check Valves Ref. 26
Gate Valves Ref. 26
Ball Valves Ref. 26
Excess Flow Valves - Ref. 26
Angle Valves Ref. 35
Relief Valves Ref. 26
Rupture Disks Ref. 26
Process Machinery:
Centrifugal Pump 0.43
Instrumentation!
All Instrumentation Items 0.25
Enclosures; Ref. 35
Diking: Ref. 35
Scrubbers: 0.45
100
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SECTION 6
REFERENCES
1. Baltimore Plant Mishap Sends Toxic Gas Adrift. New York Times, January
5, 1978, p. 14.
2. Kirk-Othmer. Encyclopedia of Chemical Technology. Third Edition.
Volume 22. John Wiley and Sons, Incorporated, 1980.
3. Sumner, C.A. and J.R. Pfann. Sulfur Trioxide Spill Control. Stauffer
Chemical Company, Westport, CT, 1975.
4. Stauffer Chemical Company. Liquid Sulfur Trioxide Brochure, Westport,
CT, 1975.
5. E.I. duPont deNemours and Company (Inc.). Sulfur Trioxide Data Sheet,
Wilmington, DE, 1980.
6. Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals,
Tenth Edition. Merck & Co., Inc., Rahway, NJ, 1983.
7. E.I. duPont deNemours and Company (Inc.). Sulfur Trioxide Material
Safety Data Sheet. Wilmington, DE, 1980,
8. Registry of Toxic Effects of Chemical Substances, DHEW-NIOSH Publication
No. 79-100, 1983-84 Cumulative Supplement to the 1981-82 Edition.
Cincinnati, OH,
9. Vander Hey, J.E, (Allied Chemical Corporation). U.S. Patent #3,328,460,
June 27, 1967.
10. Knaggs, E.A, and M.L. Nussbaum (Stepan Chemical Company), U.S. Patent #
3,169,142, February 9, 1965.
11. Brooks, R.J. and B.J. Brooks (Chemithon Corporation). U.S. Patent #
3,259,645, July 5, 1966.
12. Telephone Conversation Between K.E, Hummel of Radian Corporation and a
Representative of Chemithon Corporation, Seattle, WA, May 1987.
13. E.I, duPont deNemours and Company (Inc.). Sulfur Trioxide and Oleum
Storage and Handling. Wilmington, DE, 1981.
14. Davis, Dan S., G.B. DeWolf, and J.D, Quass. Prevention Reference Manual:
Control Technologies, Vol. 1, Prevention and Protection Technologies for
Controlling Accidental Releases of Air Toxics. EPA-600/8-87-039a (NTIS
PB87-229656) , U.S. Environmental Protection Agency, Research Triangle
Park, NO, August 1987.
101
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15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Mellan, I. Corrosion Resistant Materials Handbook. Third Edition.
Noyes Data Corporation, Park Ridge, NJ, 1976.
Corrosion Data Survey (Metals Section). Fifth Edition. National
Association of Corrosion Engineers, Houston, TX, 1974.
ASME Boiler and Pressure Vessel Code, ANSI/A.SME. BPV-VIII-1. The
American Society of Mechanical Engineers, New York, NY, 1983.
Lees, F.P. "Hazard Identification, Assessment, and Control." Loss
Prevention in the Process Industries. Volumes 1 and 2. Butterworths,
London, England, 1980.
Perry R.H., and C.H. Chilton. Chemical Engineers' Handbook. Fifth
Edition. McGraw-Hill Book Company, New York, NY, 1973.
Hot Room Technical Bulletin. Stauffer Chemical Company, Westport, CT,
1977.
Aarts, J.J. and D.M. Morrison. Refrigerated Storage Tank Retainment
Walls. CEP Technical Manual, Volume 23. American Institute of Chemical
Engineers, New York, NY, 1981.
Liquid Sulfur Trioxide Surface Seal Technical Bulletin. Stauffer
Chemical Company, Westport, CT, 1978.
Chemical Manufacturers Association. Process Safety Management (Control
of Acute Hazards). Washington, DC, May 1985.
Stus, T.P. On Writing Operating Instructions. Chemical Engineering,
November 26, 1984.
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Manufacturers Association Process Safety Management Workshop, Arlington,
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Peters, M.S. and K.D. Timmerhaus. Plant Design and Economics for
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Pikulik, A. and H.E. Diaz. Cost Estimating for Major Process Equipment.
Chemical Engineering, October 10, 1977.
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102
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31. Barrett, O.H. Installed Cost of Corrosion-Resistant Piping, Chemical
Engineering, November 2, 1981.
32. Telephone Conversation Between J.D. Quass of Radian Corporation and a
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103
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APPENDIX A
GLOSSARY
This glossary defines selected terms used irt the text of this manual
which might be unfamiliar to some users or which might be used differently by
different authors.
Accidental release: The unintentional spilling, leaking, pumping, purging,
emitting, emptying, discharging, escaping, dumping, or disposing of a toxic
material into the environment in a manner that is not in compliance with a
plant's federal, state, or local environmental permits and/or that creates
toxic, concentrations in the air that are a potential health threat to the
surrounding community.
Cavitation: The formation and collapse of vapor bubbles in a flowing liquid.
Specifically, the formation and collapse of vapor cavities in a pump when
there is sufficient resistance to flow at the inlet side.
Chlorofluorocarbons: Organic compounds containing chlorine and/or fluorine
atoms within the molecule.
Creep failure: Failure of a piece of metal as a result of creep. Creep is
time-dependent deformation as a result of stress. Metals will deform when
exposed to stress. High levels of stress can result in rapid deformation and
rapid failure. Lower levels of stress can result in slow deformation and pro-
tracted failure.
Deadheading: Closing or nearly closing or blocking the discharge outlet or
piping of an operating pump or compressor.
Electromotive Series of Metals: A list of metals and alloys arranged accord-
ing to their standard electrode potential, which also reflects their relative
corrosion potential.
104
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Enthalpy: A thermodynamic property of a chemical related to its energy con-
tent at a given condition of temperature, pressure, and physical state.
Enthalpy is the internal energy added to the product of pressure times volume.
Numerical values of enthalpy for various chemicals are always based on the
change in enthalpy from an arbitrary reference pressure, temperature, and
physical state, since the absolute value cannot be measured.
Facility: A location at which a process or set of processes is used to pro-
duce, refine or repackage chemicals, or a location where a large enough inven-
tory of chemicals is stored so that a significant accidental release of a
toxic chemical is possible.
Hazard: A source of danger. The potential for death, injury or other forms
of damage to life and property.
Hygroscopic: Readily absorbing and retaining moisture, usually in reference
to readily absorbing moisture from the air.
Killed steel: Steel that is deoxidized with a strong deoxidizing agent such
as silicon or aluminum to reduce the oxygen content to such a level that no
reaction occurs between carbon and oxygen during solidification. Such a steel
will have a smaller grain size than a steel that is not killed. The grain
size affects the physical and chemical properties of the steel. Fully-killed
steel is fully deoxidized and has a smaller grain size than semi-killed steel
which is partially deoxidized and has a smaller grain size than steel that is
not killed.
Mild steel: Carbon steel containing a maximum of about 0,25% carbon. Mild
steel can be used where severe corrodants are not encountered or where pro-
tective coatings can be used to prevent or reduce corrosion rates to accept-
able levels.
Mitigation: Any measure taken to reduce the severity of the adverse effects
associated with the accidental release of a hazardous chemical.
105
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Olefinic hydrocarbons: A specific subgroup of aliphatic hydrocarbons sharing
the common characteristic of at least one unsaturated carbon-to-carbon atomic
bond in the hydrocarbon molecule. Aliphatic hydrocarbons are hydrocarbons
with a basic straight or branched chain structure, in contrast with ring
structured compounds.
Passivation film: A layer of oxide or other chemical compound of a metal on
its surface that acts as a protective barrier against corrosion or further
chemical reaction.
Plant: A location at which a process or set of processes is used to produce,
refine, or repackage chemicals.
Prevention: Design and operating measures applied to a- process to ensure that
the primary containment of toxic chemicals is maintained. Primary containment
means confinement of toxic chemicals within the equipment intended for normal
operating conditions.
Process: The sequence of physical and chemical states and operations during
the production, refining, or repackaging of chemicals.
Process machinery: Process equipment such as pumps, compressors, or agitators
that would not be categorized as piping or vessels.
Protection: Measures taken to capture or destroy a toxic chemical that has
breached primary containment but not yet entered the external environment.
Toxicity: A measure of the adverse health effects of exposure to a chemical.
106
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APPENDIX B
TABLE B-l. METRIC (SI) CONVERSION FACTORS
Quantity
To Convert From
To
Multiply By
Length:
in
cm
2,54
f|
5
cm.
0.3048
Area;
in.
6.4516
ft.
z
3
0.0929
Volume:
, J
cm.
16 . 39
ft3
J
0.0283
gal
¦j
m
0.0038
Mass (weight):
lb
0,4536
short ton (ton)
Mg
0.9072
short ton (ton)
metric ton (t)
0.9072
Pressure:
atm
kPa
] 01.3
mm Hg
kPa
0.133
psia
kPa
6.895
psig
kPa*
(psig+14,696)x(6.895)
Temperature:
°F
°c*
(5/9)x(°F-32)
«C
K*
° C—2 73.15
Caloric Value;
Btu/lb
kJ/bg
2.326
Enthalpy:
Btu/lbmol
kJ/kgmol
2.326
kcal/gmol
kJ/kgmol
4.184
Specific-Heat
Btu/lb-°P
kJ/kg-°C
4.1868
Capacity:
3
3
Density:
lb/ft
kg/m-
16.02
lb/gal
kg/m3
119.8
Concentration:
oz/gal
kg/m
quarts/gal
J! , J
cs| /m
m^/min
25,000
Flowrate:
gal/min
0.0038
gal/day
tn^/day
m /min
0.0038
ft /min
0.0283
Velocity:
ft/mln
m/mi-TL
0.3048
ft/sec
m/sec
0.3048
Viscosity:
eentipoise (CP)
kg/in - s
0.001
^Calculate as indicated.
107
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