United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/034i Dec. 1987
SEPA Project Summary
Prevention Reference Manual:
Chemical Specific, Volume 9:
Control of Accidental Releases
of Chlorine
D. S. Davis, G. B. DeWolf, J. D. Quass, and K. P. Wert
Recent headlines of accidental re-
leases of toxic chemicals at Bhopal and
Chernobyl have created the current
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 planned series of companion
manuals addressing accidental releases
of toxic chemicals.
Chlorine has an IDLH (immediately
Dangerous to Life and Health) con-
centration of 25 ppm, which makes it a
substantial acute toxic hazard.
Reducing the risk associated with an
accidental release of chlorine involves
identifying some of the potential causes
of accidental releases that apply to the
processes that use chlorine. In this
manual examples of potential causes
are identified, as are measures that may
be taken to reduce the accidental release
risk. Such measures include recom-
mendations on plant design practices;
prevention, protection, and mitigation
technologies; and operation and main-
tenance practices. Conceptual cost
estimates of possible prevention, pro-
tection, and mitigration 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 report that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
The accidental release of a toxic chemi-
cal, methyl isocyanate, in Bhopal, India,
in 1984 was a milestone in creating an
increased public awareness of toxic re-
lease problems. There have been other
less dramatic incidents of toxic chemical
releases in the past, and the chemical
industry was aware of this problem long
before this event. Safety and loss preven-
tion have long been standard parts of
industrial activity, and over the years
industry has made many advances in this
area. There is renewed interest, however,
in reviewing technology and procedures
for preventing, protecting against, and
mitigating accidental releases.
As an aid to regulators and industry
personnel charged with reducing the
probability and consequences of accidental
toxic chemical releases, technical manuals
have been prepared that address preven-
tion, protection, and mitigation measures
for releases. This chemical specific
manual on chlorine is part of that series.
Chlorine is a major commodity chemical
in industry. The major industrial uses of
chlorine are: numerous organic and in-
organic chemical syntheses, bleach
manufacture, cooling tower water treat-
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ment, drinking water treatment, waste-
water disinfection, and repackaging for
resale.
Potential Causes of Releases
Potential chlorine releases may be
either liquid or vapor. Liquid spills can
occur when chlorine is released at or
below its boiling point of -34°C (-29.3°F),
or when a sudden release of chlorine at
temperatures above -34°C results in
vapor flashing. Direct releases of vapor
gas can also occur.
Chlorine is frequently stored in 68 kg
(150 Ib) cylinders and 1 ton (907 kg)
containers. These containers are equipped
with fusible plugs as a form of pressure
relief. Although fire is not the most
frequent hazard, it may be the most
serious, since fire can melt the fusible
plug of a container at 70°C (158°F),
allowing most of the chlorine in the con-
tainer to escape. Defective fusible plugs
have also failed to melt, allowing fire to
rupture the container. Corrosion or poor
bonding between the lead alloy plug and
the plug retainer allows moisture to ac-
cumulate, causing corrosion at the con-
nection, leading to the chlorine leak. One
frequent cause of chlorine emissions is
failure of the copper tubes commonly
used to connect cylinders and containers
to process equipment.
Process causes of a chlorine release
include: (1) excessively high chlorine feed
rate to a bleach reactor leading to exces-
sive exothermic reaction, combined with
failure of the cooling system; (2) backflow
of chlorination water to a chlorine cylinder;
(3) loss of agitation in batch reactor sys-
tems; (4) excess chlorine feed leading to
overfilling or overpressuring equipment;
(5) photo-lamp failure in photochemical
reactor; and (6) overpressure of a chlorine
storage vessel caused by overheating from
reactions.
Equipment causes of accidental re-
leases result from hardware failure such
as excessive stress caused by improper
construction or installation, mechanical
fatigue and shock, thermal fatigue and
shock in bleach reactors, brittle fracture
(especially in carbon steel equipment),
creep failure in equipment subjected to
extreme operational upsets, and corrosion.
Operational causes of accidental
chlorine releases involve incorrect oper-
ating or maintenance procedures, or
operator error. Examples are overfilled
storage tanks, errors in loading and un-
loading procedures, inadequate main-
tenance, and lack of inspection and non-
destructive testing of vessels and piping
to detect corrosion weakening.
Hazard Prevention and Control
The prevention of accidental releases
relies on a combination of technological,
administrative, and operational practices
applied to the design, construction, and
operation of facilities where chlorine is
stored and used.
The most important process design
considerations are aimed at preventing
overheating and overpressuring systems
containing chlorine. Temperature moni-
toring is important, not only because of
potential overpressure or equipment
weakening caused by overheating, but
also because chlorine can react with
many metals above a certain activation
temperature. Chlorine can also cool itself
while off-gasing and potentially reach
temperatures below the safe operating
range of some metals.
Physical plant design considerations
include equipment, siting and layout, and
transfer/transport facilities. Equipment
construction materials must be chosen to
prevent deterioration or product con-
tamination. Steel, cast and wrought iron,
copper and nickel alloys, some varieties
of stainless steel, and lead are common
construction materials in chlorine pro-
cesses. On vessels, relief devices provide
overpressure protection against cata-
strophic rupture or explosion by con-
trolling the release of the overpressured
contents. Vessels larger than cylinders or
1-ton containers are usually equipped
with pressure relief valves and rupture
disks. Even with these devices, however,
a catastrophic sudden release could occur.
Further protection can be gained if the
relief device is routed to a caustic
scrubber.
Overfilling can be prevented by using
level sensing devices, pressure relief
devices, and adequately trained per-
sonnel. Relief devices for chlorine over-
filling may be the same as or similar to
those used for gas pressure relief.
According to guidelines developed by
the Chlorine Institute, chlorine tanks,
usually constructed of normalized carbon
steel, should be designed to accept a tank
car dome assembly. In addition to vents,
containers should have valves that can
isolate the vessel from the process to
which the chlorine is being fed. As a
protection against corrosion, moisture
must be excluded from the tank, and it
should not be left in standing water or
exposed to moist air.
Another concern is the backflow of
material into a storage vessel. When
chlorine is being mixed with a liquid, the
liquid can be drawn back into the chlorine
container. Such backflow can be pre-
vented by a vacuum-breaking device, or a
barometric leg, check valves, and positive-
displacement pumps.
A chief concern in liquid chlorine pipe
domes and valves is overpressure caused
by thermal expansion of the chlorine, or
pressure pulses caused by shutting valves
rapidly. These pressures can rupture
pipes. An expansion chamber, consisting
of a rupture disk and a receiver chamber,
can be installed to prevent thermal ex-
pansion ruptures. Pressure pulses can be
avoided by selecting valves that do not
shut abruptly. Ball and plug valves should
be designed so that excess pressure in
the body cavity will relieve spontaneously
toward the high pressure side. Pipes and
valves and process machinery such as
pumps and compressors must be con-
structed of materials resistant to chlorine
at operating temperatures and pressures.
Facilities and equipment should be sited
to minimize personnel exposure in the
event of a release. Large inventories of
chlorine should be kept away from
sources of fire or explosion hazard. If
possible, chlorine piping should not be
located next to other piping under high
pressure or temperature. Storage facilities
should be segregated from the main pro-
cess and away from control rooms, offices,
utilities, and laboratories.
Protection technologies for facilities
that use or manufacture chlorine include
enclosures and scrubbers. While enclo-
sures for secondary containment of
chlorine spills or releases do not seem to
be widely used, they can be considered
for areas near especially sensitive re-
ceptors. Enclosures capture and contain
any chlorine spilled or vented from storage
or process equipment, preventing im-
mediate discharge of the chemical to the
environment. If enclosures are used, they
should be equipped with continuous
monitoring equipment and alarms. For
chlorine, concrete block or concrete sheet
buildings or bunkers are most suitable.
Scrubbers, which absorb toxic gases
from process streams, can be used to
control chlorine releases from vents and
pressure-relief discharges from storage
equipment, process equipment, or secon-
dary containment enclosures. Spray
towers, packed bed scrubbers, and ven-
turis are appropriate for chlorine dis-
charges. An alkaline solution is needed
to achieve effective absorption because
absorpotion rates with water alone would
require unreasonably high liquid-to-gas
ratios. In an emergency, however, water
scrubbing can be used in a makeshift
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scrubber if an alkaline solution is not
available.
If chlorine is released in spite of all
precautions, the consequences may be
reduced by mitigation measures such 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.
The choice of a mitigation technology for
a particular chemical depends on that
chemical's specific properties (flamma-
bility, toxicity, reactivity), as well as its
dispersion characteristics in the atmo-
sphere. Secondary containment systems,
such as impounding basins and dikes,
reduce the evaporation rate of a released
liquefied gas, as do flotation devices and
foams. However, even when measures
such as these are employed after a
chlorine release, a hazardous vapor cloud
will probably form. The primary means of
dispersing as well as removing chlorine
from the air is with water sprays or fogs.
The effectiveness of water sprays depends
on wind direction, on the distance of the
nozzles from the point of release, on the
fog pattern, and on nozzle capacity, pres-
sure, and rotation. If the right strategy is
followed, a "capture zone" can be created
downwind of the release into which the
chlorine vapor will drift and be partially
absorbed. In some cases, it may be pos-
sible to use fans and blowers to disperse
a vapor cloud.
Operation and maintenance practices
that can reduce the probability of a large
chlorine release include training em-
ployees in proper handling and storage
procedures. To prevent corrosion, chlorine
should be analyzed for water several
times a week, and pH readings of cooling
water and condensate can be taken
several times a day to detect internal
leaks.
D. S. Davis, G. B. DeWolf. J. D. Quass, and K. P. Wert are with Radian
Corporation, Austin, JX 78766.
T. Kelly Janes is the EPA Project Officer (see below).
The complete report, entitled "Prevention Reference Manual: Chemical Specific,
Volume 9. Control of Accidental Releases of Chlorine," (Order No. PB 87-
228 664/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
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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EPA/600/S8-87/034J
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