United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/028 Nov 1987
Project Summary
Prevention Reference Manual:
User's Guide, Overview for
Controlling Accidental Releases of
Air Toxics
D. S. Davis, G. B. DeWolf, and J. D. Quass
Concern about the potentially dis-
astrous consequences of accidental
releases of toxic chemicals has led to
increased interest in reducing the proba-
bility and effects of such releases. This
User's Guide, the first in a series of
manuals, presents an overview of avail-
able methods for identifying, evaluating,
and controlling hazards in facilities that
use, manufacture, or store acutely toxic
chemicals that could be released into
the environment. Hazardous chemicals
and their key characteristics are dis-
cussed, followed by a discussion of the
potential hazards in process and physical
plant design and in operational proce-
dures. Formal methods of hazard iden-
tification and evaluation are discussed
and their major features compared, and
an overview of control principles for
prevention, protection, and mitigation
is presented. Examples of control tech-
nologies are listed, and an example
guide for inspecting facilities is pre-
sented. Important references on the
topic of accidental toxic chemical re-
lease prevention are cited.
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
The User's Guide introduces govern-
ment agency personnel, industry man-
agers, technical people, and others con-
cerned with reducing the risk of accidental
toxic chemical releases to the broad area
of accidental chemical release prevention,
protection, and mitigation. The manual
serves as a guide to the more detailed
information in the companion set of
manuals and to the general technical
literature.
Other manuals in the series will cover
chemical specific information, prevention
and protection control technologies, and
mitigation control technologies. These
manuals compile technical information
so that approaches can be developed for
preventing and controlling accidental toxic
chemical releases. They cover various
aspects of control, including identification
of causes, methods of hazard identification
and evaluation, and prevention, protec-
tion, and mitigation measures.
The User's Guide identifies industrial
chemicals of primary concern and sum-
marizes some of the fundamental causes
of releases in process operations.
Methods used in hazard identification
and evaluation are discussed, and an
overview of the general principles of
hazard control is presented. The manual
also presents a guide for inspecting facili-
ties and defines some typical costs of
accidental release prevention.
Chemical and Process
Operations Hazards
Chemical hazards within a process
facility must first be identified, then
ranked according to the danger posed.
The primary basis for selecting chemicals
that represent a potential release hazard
-------
is their acute toxicity, though other
properties are also important. The main
methods of expressing toxicity are im-
mediate danger to life and health (IDLH),
Threshold Limit Value (TLV), low lethal
concentration (LCLO), and 50% lethal
concentration (LC50). Although the relative
acute toxic hazard of chemicals may be
ranked according to any of these criteria,
the EPA suggests using the IDLH as the
primary criterion for estimating con-
sequences of accidental releases.
Significant physical and chemical pro-
perties include boiling point, vapor pres-
sure, heat of vaporization, density,
viscosity, reactivity, flammability, explo-
sivity, exothermicity, and corrosiveness.
Vapor pressure, vapor density, and the
IDLH are the minimum data needed to
determine if a specific chemical is an
acute toxic air release hazard.
The basic causes of accidental releases
include: process or operational hazards
causing pressure or temperature to ex-
ceed process equipment limits; equip-
ment containment failures; operational
or maintenance errors; and external
damaging factors such as fire, explosion,
or mechanical stress.
Each of these general causes can have
many contributing causes, forming a chain
leading to the final event which results in
the release.
Process design considerations impor-
tant to preventing accidental toxic
chemical releases include: process char-
acteristics and chemistry, overall process
control, flow control, pressure control,
temperature control, quantity control,
mixing effects, and composition control.
Failures in any of the above systems
could cause the system state to exceed
the design limits of the equipment or the
ability of an operator to respond quickly
or accurately enough to maintain control
of changes occurring in the process.
A release may also occur if the condi-
tional state of the process exceeds the
physical limits of the equipment or if
these limits deteriorate below those re-
quired for containment. The plant design
must be such that equipment and compo-
nents can withstand normal operating
conditions for the anticipated life of the
facility and can tolerate abnormal condi-
tions within certain bounds. The com-
plexity and operability of the equipment
may influence how well a process is
controlled and how easily equipment and
components are maintained. Some
causes of mechanical failure are: exces-
sive stress, external loading, overpres-
sure, overheating, mechanical fatigue and
shock, thermal fatigue and shock, brittle
fracture, creep, and chemical attack. Pre-
vention of these failure modes can be
incorporated into initial facility design
and construction. Though some of these
conditions relate to operating conditions
that can change over time, many failures
can be prevented by the proper selection
and use of construction materials.
Another hazard area in process opera-
tions involves human error in: decision
making; physical actions controlling a
process; and the planning and supervision
of the design, construction, and operation
of facilities at all stages of a plant's life
cycle. Examples of this kind of hazard are
a lax management policy that does not
enforce its own safety standard or an
operator who takes a wrong action at a
control panel. Management policy must
address special safety procedures for toxic
chemicals through all parts of the chemi-
cal process life cycle, and the skill and
knowledge of the operators must match
the needs of the process. Communica-
tions procedures in high-hazard facilities
should receive high priority, and main-
tenance practices must ensure that the
original specifications are adhered to and
that all special preventive and protective
systems are functional.
Methods for Hazard
Identification and Evaluation
Various formal and systematic methods
for hazard identification and evaluation
are used for facilities that manufacture,
use, or store toxic chemicals. Hazard
identification procedures can be divided
into four main classes: experience, aug-
mented experience, analytical methods,
and creative methods.
The experience method relies on com-
paring a new situation to a known past
situation. In the augmented experience
method, each step of a process is re-
viewed to determine what would happen
following equipment failures, process
upsets, or operating errors. The analytical
approach uses logic diagrams (such as
fault trees, event trees, and cause-
consequence charts) or checklists. These
methods can be combined with quantita-
tive data on probabilities to identify and
evaluate the hazard potential of a facility.
Other hazard identification procedures
include safety reviews: Dow and Mond
Hazard Indices; Hazard and Operability
(HAZOP) Studies; Fault Tree and Event
Tree Analyses; and Failure Modes, Effects,
and Criticality Analyses (FMECAs).
The evaluation step attempts to rank
hazards qualitatively or quantitatively or
both, seeks to identify measures that '
reduce the probability that the hazard will
be realized, and examines the potential
consequences of the hazard if it is
realized.
Overview of Principles of Control
The control of accidental chemical
releases consists of three basic levels of
control: prevention, protection, and miti-
gation. Prevention measures consider
operational and hardware aspects of a
chemical process system. When preven-
tive measures fail, a second level of con-
trol deals with protection from releases,
or the containment, capture, neutraliza-
tion, or destruction of a toxic chemical
after its release from primary contain-
ment but before its release into the en-
vironment.
If a protection system is deficient and
allows a toxic vapor or gas to escape to
the environment, the consequences may
be reduced by effective mitigation mea-
sures. Mitigation refers to equipment and
procedures that can be used to reduce
the concentration of a chemical below
levels that would otherwise occur.
Technical measures include the use of
water sprays or steam curtains, barriers
for dispersion and diversion, and proce-
dures such as closing doors and windows
and evacuation.
-------
D. Davis. G. DeWolf, and J. Quass are with Radian Corporation, Austin, TX
78766.
T, Kelly Janes is the EPA Project Officer (see below).
The complete report, entitled "Prevention Reference Manual: User's Guide
Overview for Controlling Accidental Releases of Air Toxics." (Order No. PB
87-232112/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
VA t-.-IMl.TY
! DiCiO'8/ j -;l,yr
A /r Tl = o .2
Official Business
Penalty for Private Use $300
fcHA/600/S8-87/028
0000329 PS
U
S EHVIR PROTECTIOH HG6NCY
5 1I8R*RT
230 S DEARBORN STREET
CHICAGO It
------- |