United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/039b Aug. 1989
Project Summary
Prevention Reference Manual:
Control Technologies,
Volume 2. Post-Release
Mitigation Measures for
Controlling Accidental
Releases of Air Toxics
D. S. Davis, G. B. DeWolf, K. A. Ferland, D. L. Harper, R. C. Keeney, and
J. D. Quass
Reducing the possibility of acci-
dental toxic chemical releases re-
duces the possibility of harm to hu-
man health and to the environment
When such a release does occur,
however, its consequences must be
reduced. This can be accomplished
by means of a variety of mitigation
measures that can contain, capture,
destroy, divert, or disperse the
released chemical.
Mitigation measures begin with the
initial siting and layout of a facility to
decrease the area that would be
affected by a release. The extent of
the area potentially affected, the con-
centrations of toxic chemicals reach-
ing those areas, and the duration of
exposure can be estimated by vapor
or gas dispersion modeling. The ex-
tent and magnitude of an actual re-
lease can be determined using mete-
orological instruments. These sys-
tems, along with emergency planning
and training, are the first steps in the
mitigation process. Other measures
involve the use of mitigation tech-
niques such as leak plugging, con-
tainment systems, and spray or foam
systems. The general application
costs of these methods are discus-
sed.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
Post-release mitigation measures are
measures that can reduce the conse-
quences of an accidental toxic chemical
release after it has occurred. Mitigation
measures decrease the quantity of a
chemical that can reach the environment
and human or other receptors. These
measures also limit the area exposed to
the chemical and/or the duration of ex-
posure.
The release of a toxic chemical is the
final event in a sequence of events lead-
ing to the release. If the measures that a
facility uses to prevent or protect against
an accidental release fail to contain the
chemical, mitigation measures to reduce
the adverse effects of the release must
be invoked. The mitigation measures ad-
dressed in this manual, Volume 2 of the
Prevention Reference Manual series,
include (1) emergency planning, (2) siting
and layout, (3) dispersion modeling, (4)
detection and warning systems, (5)
meteorological instrumentation, and 6)
technical measures that can effectively
control a release.
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Emergency Planning and
Training
Emergency planning and training en-
sure the rapid and appropriate response
of the people charged with applying
mitigation measures and define the
mitigation measures to be employed.
The details of emergency planning and
training vary from facility to facility. In
general, however, any program should
address certain basic elements: program
initiation; hazard evaluation; countermea-
sures identification, evaluation, selection,
and implementation; resource require-
ments and availability; organization; and
mobilization and demobilization.
The Community Awareness and Emer-
gency Response (CAER) program de-
veloped under the auspices of the Chem-
ical Manufacturers Association discusses
the implementation of community emer-
gency response plans. The goal of this
program is to improve community aware-
ness and to integrate industrial emer-
gency response plans with those of the
community. Major steps m a CAER
program are (1) community status review
and program coordination, (2) facility
status review, (3) implementation, (4)
community involvement, (5) emergency
exercises.
The community emergency response
plan will determine the basic training
needs. The general objectives of training
are to increase the awareness, know-
ledge, and skills of management, and of
operating, maintenance, and special
emergency response personnel. Emer-
gency exercise activities involve integrat-
ing the training of plant personnel with
that of community emergency personnel.
Facility Siting and Layout
Plant siting and layout concerns the
placement of hazardous facilities relative
to sensitive receptors in the surrounding
community and within plant boundaries.
Important things to consider in this area,
apart from distance, include taking ad-
vantage of terrain features such as hills
that might act as natural barriers and
avoiding the funneling effects of valleys.
One study of the contribution of dif-
ferent hazard factors to accidental re-
leases has found that poor facility siting
played a role in 5.8% of the cases and
that poor layout of equipment within the
facility was a factor in 3.9% of the cases.
Although siting is usually carefully
examined only for new facilities, the ex-
pansion or modification of an existing fa-
cility may require a reevaluation, espe-
cially if the expansion involves chemicals
or processes that pose more hazard than
presented by the original facility. Poor
utility service, poor emergency response
and fire protection, off-site traffic conges-
tion that hinders the response of emer-
gency vehicles, and poor drainage are
possible effects of poor siting.
Proper layout concerns the placement
and spacing of the components and
equipment of a process facility to mini-
mize the consequences of an accidental
release. In a well-designed facility, pro-
cess operability will be made as smooth
as possible and hazardous areas will be
segregated.
Both the Chemical Manufacturers
Association (CMA) and the National Fire
Protection Association (NFPA) have is-
sued standards and guidelines for facility
layout. Some key features to be consid-
ered are facility boundaries, work bound-
aries, railway lines that pass through the
area, ignition sources, control rooms,
waste disposal areas, storage and pro-
duction units, and loading and unloading
areas.
Detection and Warning Systems
Detection and warning systems give
advance notice that a release is incipient
or has occurred; they also define the
magnitude and location of the release so
that other mitigation measures can be
taken. Detection and warning systems
built into the process control system are
widely used in the chemical process
industry. Such systems monitor process
operating conditions such as tempera-
ture, pressure, and flow rate, and trigger
audible and visual alarms when these
process variables exceed design limits.
Other detection systems identify hazards
after a release has occurred. Post-release
detection systems are important because
the more quickly an airborne release of a
hazardous material is detected, the
greater is the opportunity to control the
effect on the community.
Vapor Dispersion Modeling
Vapor dispersion modeling is used to
predict the extent, duration, and concen-
tration of the plume or cloud of released
toxic vapor or gas. Numerous dispersion
models of varying levels of sophistication
and accuracy, and with varying ability to
be verified by actual field data, are
available. The results predicted by these
models depend on a source term that
describes the characteristics of the initial
release, and a dispersion term that de-
scribes the characteristics of the cloud or
plume. These models can inform de-
cisions about plant siting and layout, the
placement of detection and warning
systems and meteorological instrument*
tion, and the selection of technici
mitigation measures.
Models for predicting the effects (
accidental releases must be able t
handle short-term releases at high or lo
concentrations and at variable releas
rates. They must be capable of modelin
a release/dispersion of heavier- an
lighter-than-air materials and materia
that have the same density as air. Sue
models should simulate a variety i
possible release forms, such as a releas
from a boiling pool of liquid, or th
release from a hole in a pressurize
vessel.
Vapor dispersion mathematical mode
ing may be used to assess hazards ar
plan the emergency response, and
give emergency response personnel i
formation during an actual accidenl
release.
Meteorological Instrumentatior
Meteorological data can be used to n
vapor/gas dispersion studies to plan f
an emergency response to an acciden
toxic chemical release. Also, real-tin
meteorological data are essential f
choosing the correct mitigation ai
emergency response actions during
actual release. Meteorological data c
also be used to analyze past events,
predict the consequences of vario
hypothetical accidental release scenarir.
and in facility design so that potent
toxic release points can be located
minimize the exposure to employees, t
surrounding community, and the envirc
ment in general.
Secondary Containment
If an accidental toxic chemical relea
occurs, containment systems are used
reduce the area exposed to the vapi
and to contain the liquid until measui
can be taken to recontain or destroy <
released material. While some cants
ment measures are successful w
gases, most apply to spilled volal
liquids. Stopping or reducing the flow c
chemical at its source, such as closinj
valve or plugging a leak, is also
containment measure. Remotely op
ated emergency isolation valves are
effective way of stopping the flow
material. Where a large hole in a ves
is the source of escaping chemical,
leak must be plugged. The three types
leak plugging are chemical patches i
plugs, physical patches and plugs, ;
methods for stopping the flow upstre
of the leak.
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Physical barrier containment systems
jsually consist of curbing, trenches,
excavated and natural basins, and earth,
steel, or concrete dikes. The inventory of
toxic material and its proximity to other
portions of the plant and the community
are primary considerations in the selec-
tion of a system. The secondary contain-
ment system should be able to contain
spills with minimum damage to the facil-
ity and its surroundings and with a min-
imum potential for escalation of the
event.
Spray, Dilution, and Dispersion
Systems
Spray systems, routinely used in the
chemical process industries for fire
protection, are also used to disperse,
dilute, and absorb released airborne
chemicals. Spray systems rely on fixed
or mobile equipment that applies a spray
of water, other materials, or a condensing
cloud of steam directly to the plume or
cloud of noxious chemical. Some spray
systems are similar to fire fighting
systems.
The primary purpose of sprays and
steam curtains is to dilute the toxic gas or
chemical with air or by absorption of the
gas in the liquid drops. Spray- or steam-
induced warming of cold vapor clouds
that form from liquefied gas releases can
also dilute the heavier-than-air cloud or
plume. Heating the cloud will decrease its
density, causing it to rise, thus
decreasing ground-level concentrations
of the toxic vapors downwind of the
release.
The two most common spray systems
are fixed and mobile water sprays. Some-
times a reactive water solution is used,
such as a mild aqueous alkaline spray
system. Steam curtains are fixed-pipe
systems designed so that the individual
jets combine to form a continuous curtain
of steam that entrains sufficient air to
dilute the gas or vapor concentration to
below its toxic and/or flammable limit.
Foam Systems
Foams, used to control and extinguish
certain types of hydrocarbon fires
involving spilled liquids, are used when
the fire might not be effectively controlled
by water sprays These systems are
based on special chemical materials that
generate foams whose characteristics are
tailored to the chemical characteristics of
the material to which they are applied.
Foams act as a physical barrier to pre-
vent or decrease evaporation from liquid
surfaces. The application of a foam
blanket to a liquid spill may prevent the
release of a flammable gas or vapor from
reaching an ignition source in concentra-
tions that could result in an explosion or
fire. Foams can also help prevent plant
personnel or public exposure to danger-
ous concentrations of a hazardous gas or
vapor being emitted from the surface of
the liquid. The properties of foam that
make it effective for fighting fires are:
• The ability to blanket the spilled
liquid surface with a material of lower
density than liquid, thereby cutting
off the source of combustion air;
• The suppression of flammable va-
pors so they will not be emitted to
the atmosphere to mix with air;
• The prevention of nearby flames
from heating the spilled liquid co-
vered by the foams; and
• The cooling of the spilled liquid with
water draining from the foam and
surrounding surfaces to help prevent
reignition.
The six types of foam used to control
vapors from chemical spills are (1)
regular protein foams, (2) fluoroprotein
foams, (3) surfactant foams, (4) aqueous
film-forming foams, (5) alcohol or polar
solvent foams, and (6) special foams
(such as Hazmat NF* foams).
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D. S. Daws, G. B. DeWolf, K. A. Ferland, D. L. Harper, R. C. Keeney, and J. D.
Quass are with Radian Corp., Austin, TX 78720-1088.
T. Kelly Janes is the EPA Project Officer (see below).
The complete report, entitled "Prevention Reference Manual: Control Technologies,
Volume 2. Post-Release Mitigation Measures for Controlling Accidental Releases
of Air Toxics," (Order No. PB 89-755 0631 AS; Cost: $28.95, 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 Project 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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