United States
Environmental Protection
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).
  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
  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

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-
  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
     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
                        Center for Environmental Research
                        Cincinnati OH 45268
VA t-.-IMl.TY
                                                                                  !   DiCiO'8/  j -;l,yr
                                                                                 A	/r Tl =  o .2
Official Business
Penalty for Private Use $300

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