United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/034I Feb. 1988
Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 12:
Control of Accidental Releases of
Sulfur Dioxide
D. S. Davis, G. B. DeWolf, J. D. Quass, and M. Stohs
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
has prompted preparation of this manual
and a series of companion manuals on
the control of accidental releases of
toxic chemicals. This manual on sulfur
dioxide (SO2) summarizes information
to help regulators and industry per-
sonnel identify and control release
hazards associated with SO2.
To reduce the risk associated with an
accidental release of SO2, the potential
causes of accidental releases in process
facilities that handle and store SO2 must
be identified. The SO2 manual provides
examples of such causes, as well as
measures that may be taken to reduce
the accidental release risk. Such mea-
sures include recommendations on:
plant design practices; prevention, pro-
tection, and mitigation technologies;
and operation and maintenance prac-
tices. Conceptual cost estimates of
example prevention, protection, and
mitigation measures are provided.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
Increasing concern about the potentially
disastrous consequences of accidental
releases of toxic chemicals has prompted
preparation of a series of manuals for
regulators and industry personnel on the
prevention of accidental releases of toxic
chemicals. The manual on S02 is one of
several chemical-specific manuals that
address issues associated with the stor-
age, handling, and process operations
involving toxic chemicals as they are used
in the U.S.
Historically, there appear to have been
no major releases of S02 in the U.S.
Major incidents elsewhere have not been
common.
The major industrial uses of SO2, which
is a major commodity chemical in industry,
are in: sulfuric acid production, food pro-
cessing, water and waste water treat-
ment, the sulfate pulping process for the
manufacture of paper, metallurgical ap-
plications, sulfation and sulfonation of
specialty chemicals, and repackaging.
Potential Causes of Releases
S02, a toxic, highly irritating gas that
can have immediate effects on the eyes,
throat, lungs, and skin, is extremely stable
to heat, even up to 2000°C. It does not
form flammable or explosive mixtures
with air; it will, however, react with water
or steam to produce toxic and corrosive
fumes.
Large-scale accidental releases of S02
can result from leaks or ruptures of large
storage vessels (including tank cars on
site) or failure of process machinery {e.g.,
pumps and compressors) that maintains
a large throughput of S02 gas or liquid.
Smaller releases may come from ruptured
lines, broken gage glasses, or leaking
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release include enclosures and scrubbers"
Enclosures contain the spilled liquid or
gas until it can be transferred to scrubbers
for neutralization or to other containment
and discharged at a controlled rate. Con-
crete block or concrete sheet buildings
and bunkers are suitable for S02.
Scrubbers such as spray towers, packed
bed scrubbers, and Venturis can be used
to control S02 releases from vents and
pressure relief discharges, from process
equipment, or from secondary contain-
ment enclosures. Alkalit e solutions such
as calcium hydroxide, sodium hydroxide,
or sodium carbonate are used to achieve
effective absorption.
If an accidental release occurs, mitiga-
tion technologies can reduce the con-
sequences. Such measures include
physical barriers such as dikes, impound-
ing walls, and basins, as well as water
spray and fogs and foams where applic-
able. The purpose of mitigation technique
is to divert, limit, or disperse the spilled or
released chemical to reduce the atmo-
spheric concentration and the area
affected.
Since accidental releases of toxic
materials may result from deficiencies in
operation and maintenance as well as from
deficiencies of design, the safe operation
of plants using SO2 is important. Em- (
ployees should be trained in the proper
procedures for handling S02, in potential
hazards, cleanup, and emergency proce-
dures. Well-defined procedures and
practices can decrease the possibility of a
hazardous release and can reduce the
magnitude of any release that occurs.
valves, fittings, flanges, valve packing, or
gaskets.
S02 can promote equipment failure
because of its high coefficient of thermal
expansion and the corrosiveness of sul-
furous acid, which is formed when dry
S02 contacts moisture. Accidental re-
leases may be caused by problems in the
process, with equipment, or in plant
operations.
Examples of process causes of a SO2
release include:
• Excess S02 feed to a chlorine dioxide
reactor, leading to an excessive
exothermic reaction, combined with
failure of the cooling system;
• Backflow of process reactants to a
S02 feed tank;
• Loss of condenser cooling to distilla-
tion units;
• Inadequate water removal from
hydrocarbon feeds in the extraction
process, leading to progressive
corrosion;
• Excess feeds, leading to overfilling
or overpressuring equipment; and
• Overpressure in S02 storage vessels
resulting from overheating because
of fire, or from overfilling.
Equipment causes of accidental re-
leases resulting from hardware failure
include excessive stress because of im-
proper fabrication, construction or in-
stallation; mechanical fatigue and shock
resulting from age, vibration, or stress
cycling; thermal fatigue and shock in
reaction vessels, heat exchangers, and
distillation columns; brittle fracture,
especially in carbon steel equipment sub-
jected to extensive corrosion; creep failure
in high temperature equipment subjected
to extreme operational upsets; and all
forms of corrosion.
Incorrect procedures and human error
can result in an operation-caused ac-
cidental release. Examples are overfilled
storage vessels; improper process control
system operation; error in loading and
unloading operations; poor quality control;
inadequate maintenance, especially of
preventive and protective systems such
as pressure relief systems; and lack of
inspection and nondestructive testing of
vessels and piping to detect weakening
from corrosion.
Hazard Prevention and Control
To develop a thorough release preven-
tion plan, control must be maintained
over: process design, physical plant
design, protective systems, and operating
and maintenance practices.
Process design involves the basic
chemistry of a process and how this
chemistry is affected by the variables of
flow, pressure, temperature, and com-
position. The first concern in process
design is understanding how deviations
from expected conditions could result in
an accidental release. Any aspect of a
process may be modified to enhance the
integrity of the system. For example, the
quantities of materials used, the pressure
and temperature conditions, the type and
sequence of unit operations, control
strategies, and instrumentation may be
changed to increase safety and process
control.
Physical plant design concerns equip-
ment, siting and layout, and transfer/
transport facilities. Most common mate-
rials of construction are resistant to
commercial dry liquid S02, dry S02 gas,
and hot S02 gas containing water above
its dew point. These include cast iron,
carbon steel, copper, brass, and alumi-
num. Wet S02 gas, sulfurous acid, and
sulfite solutions, however, are all corro-
sive to many metals, including iron, steel,
nickel, copper-nickel alloys, and nickel-
chromium-iron alloys. Corrosion is a
serious hazard, so it is important to use
appropriate materials for applications that
may involve some exposure of the SO2 to
small amounts of moisture. Special care
should be taken to ensure that all replace-
ment parts or new equipment are made
of materials that are compatible with the
chemicals involved in the process.
The siting and layout of a S02 facility
must take into account other processes
in the area, the proximity of population
centers, prevailing winds, local terrain,
and potential natural external effects
such as flooding. Facilities and individual
equipment items should be situated so as
to reduce personnel and community ex-
posure in the event of release. Specially
designed buildings, barriers, and routes
for evacuation and access to emergency
equipment are important siting and layout
considerations. Storage rooms and other
large inventories of S02 should be kept
away from possible sources of fire or
explosion and should be located in cool,
dry, well-ventilated areas.
Transfer and transport facilities should
also be located away from sources of
heat, fire, and explosion. Congestion of
personnel and vehicles should be avoided,
and correct procedures should be followed
when unloading and handling even small
S02 storage vessels such as cylinders
and drums.
Protection technologies for the contain-
ment and neutralization of any S02
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D. Davis, G. DeWolf, J. Quass, and M. Stohs are with Radian Corporation,
Austin. TX 78766.
T. Kelly Janes is the EPA Project Officer (see below).
The complete report, entitled "Prevention Reference Manual: Chemical Specific,
Volume 12. Control of Accidental Releases of Sulfur Dioxide," (Order No.
PB 88-103 734/AS; Cost: $18.95, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
U.S. OFFICIALDOM
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