United States
Environmental Protection
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/039a Dec. 1987
Project Summary

Prevention  Reference  Manual:
Control Technologies.
Volume  1:  Prevention  and
Protection Technologies for
Controlling Accidental
Releases of Air  Toxics
D. S. Davis, G. B. DeWolf, and J. D. Quass
  The probability that  an accidental
release will occur depends on the
extent to which deviations (in magni-
tude and duration) in the process can
be tolerated before a loss of chemical
containment occurs. Development of a
satisfactory control system and equip-
ment capable of withstanding devia-
tions requires adherence to sound
process and physical  plant design
principles and to appropirate proce-
dures and management practices.
  Control of a process is achieved by
manipulating the variables of  flow,
temperature, pressure,  composition,
and quantity. A control system can be
improved by improving the quality,
specifications, and maintenance of
physical components, and by duplica-
ting components where warranted.
Process changes that enhance control
of the system will reduce the proba-
bility of a chemical release.
  The probability that equipment will
fail and cause a release can be reduced
by considering various aspects of
physical plant design. Whereas process
design involves process operating
conditions, physical plant design
involves hardware, equipment compo-
nents, and their placement within a unit
or plant. A third way to reduce the
probability of a release is to maintain
proper management practices and
personnel training procedures. Finally,
protection system technologies such as
flares, scrubbers, and enclosures offer
a last line of defense against the
occurrence of accidental toxic chem-
ical releases.
  Each of these areas is discussed in
terms of its relation  to categories of
hazards, its effectiveness in reducing
the probablity of  a  release, and its
economics of application.
  This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory. Research
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).

  Accidental toxic  chemical releases,
fire, and explosion  are the three types
of major accidents in chemical process
plants.Toxic chemical releases can occur
as a result of fire or explosion, but  may
also occur in their absence. The purpose
of preventing toxic chemical releases is
to prevent harm to human health and to
the environment. An  accidental toxic
chemical release is the final event in a
sequence of events  leading to  the
release. This  chain of events can be
broken by preventing the initial event or
any intermediate, enabling, event from
occurring. Preventing a particular acci-

dental toxic release depends on correctly
identifying  individual events  and event
chains, on  knowing the relative proba-
bility of the events, and on the skill and
knowledge  of  the individuals charged
with the problem.
  Preventing accidental releases means
preventing the loss of primary chemical
containment. The general classes of
equipment  that contain chemicals are
vessels, piping, and process machinery.
To ensure containment, a process must
operate in proper sequence under accep-
table conditions  of temperature,  pres-
sure, and composition. Equipment must
be operated within design limits defined
by specifications for pressure, tempera-
ture, chemical compatibility, mechanical
stresses, and the physical limits imposed
by the process system.
  A toxic chemical release  can occur
when there has been a physical  break-
down of equipment or a loss of process
control that leads to a breach in primary
containment.  The problem requires
consideration of process design, physical
plant design, operation, and supervision
and management.

Process Design
  Modifications of fundamental process
chemistry,  operations,  and  equipment
can  reduce the  severity  of  operating
conditions, the quantities of toxic mate-
rials, and  process  complexity.  Such
modifications can  increase process
control, thereby reducing the probability
of an accidental chemical release. Pro-
cess changes  involve the principles  of
addition,  substitution, deletion, and
duplication (or redundancy). Substitu-
tion,  for example, might involve the
replacement of a toxic chemical with one
less toxic. Each process chemical, oper-
ation, or piece of equipment should be
evaluated in terms of how changes based
on these principles might reduce the
probability of an accidental chemical
release.  Process  changes involve such
factors as: (1) control characteristics of
the process, (2) process thermodynamics
and chemistry, (3) flow measurement
and control, (4) pressure measurement
and control, (5) temperature measure-
ment and control, (6) quantity measure-
ment and control, (7) mixing systems, and
(8)  composition measurement and
   Adequate control of a process means
 maintaining the prescribed conditions for
all controlled variables within the pro-
cess. These variables are in turn main-
tained through control  system action on
the manipulated variables of flow,
temperature, pressure, composition, and
quantity. These variables are controlled
by manipulating the flow of some process
stream. The specified value of  the
controlled variable in the process system
is  referred to as its  set  point.  Since
deviations  in variable set points  can
cause deviations from acceptable  pro-
cess conditions, they are the fundamen-
tal source of process hazards. An assess-
ment of the probability of a toxic chemical
release depends on the extent to which
deviations in magnitude and duration can
be tolerated before a  loss of chemical
containment occurs. Once these limits
of control are defined,  appropriate
modifications that enhance control can
be planned.
  Major control  system  components
include: (1) sensing and measurement,
(2) controllers, (3) final control elements,
(4) switches and alarms, (5) emergency
shutdown and interlock systems, and (6)
computer control. The  effectiveness of a
process control system lies in its ability
to minimize the magnitude and duration
of deviations in the process variables of
chemistry, flow, pressure,  temperature,
quantity, and composition.

Physical Plant Design
  Physical  plant design considerations
address the specific hazards caused by
hardware failure, the proper design and
construction  of equipment  to reduce
those hazards,  and the  siting and  con-
figuration  of the equipment  within the
process plant. A plant's safety is often
determined more by the quality of the
basic design  than by the addition of
special  safety  features, though  such
features can make an important contri-
bution to the safety of the plant. Specific
hardware-related prevention measures
can be identified for reducing the prob-
ability of accidental chemical releases.
  Plant  equipment must be designed to
avoid fire, explosion,  or accidental
chemical  release.  Numerous  industry
codes and standards  of practice  have
been developed by various organizations
in response to  knowledge gained from
actual accidents in anticipation of oper-
ating and  safety  problems based on
technical analysis. These standards and
codes should be the  starting  point for
plant design,  although many specific
situations may require  more  stringent
   Siting (the location of the process plant
within a  community) and layout  (the
positioning of  equipment within the
process facility) are important aspects of
release  prevention and  control. Certain
siting  conditions can  influence the
probability of a release, such as those
affected by natural disasters and climate,
and also by circumstances at neighboring
facilities  or transportation  accidents. A
facility in a flood-prone area, for example,
may require different types of founda-
tions for storage tanks or process equip-
ment. Because an accidnetal release may
affect the surrounding area, location of
population  centers,  passing  traffic,
waterways, and nearby businesses must
be taken into consideration. Prevailing
wind speed  and direction and  climate
must also be considered when designing
a facility.
  A  properly  designed  plant layout
reduces the potential for and consequen-
ces of an accidental release by making
it easier to  operate  the  plant  and by
segregating hazardous areas within the
facility.  Increased distances  between
process  units,  for instance,  tend  to
reduce  the  potential for and/or the
effects of an accidental release.  Opti-
mum spacing  arrangements provide
straight, unobstructed access ways that
continue from one end of  a unit to the
other and  that connect with  roads
surrounding the unit. Piping associated
with various utilities  and process  units
should be segregated, and more than one
escape route should be available for any
location within the plant. A safe control
room, located in an area of minimum risk
of exposure to fires, explosions,  or toxic
releases, should be designed to minimize
the potential for injury to  employees if
an  accident occurs.  Storage facilities
should be located on  no more than two
sides of a processing area. Storage tanks
should be located as far  from nearby
population centers as possible, and large
inventories of hazardous chemicals
should not be allowed to accumulate.
  Vessels, piping,  machinery, and pro-
cess instrumentation must be  con-
structed of materials  appropriate to the
process  characteristics.  Process  fluid
characteristics such as alkalinity, acidity,
abrasiveness,  and reactivity  must  be
considered in combination with pressure
and temperature extremes.

Procedures and Practices
  Quality hardware, contained mechan-
ical equipment, and protective devices all
increase plant  safety; however,  they
must be supported by the safety policies
of management and  by constraints on
their operation  and maintenance.  Man-
agement policies  and procedures  must
be designed to match the level of risk
in the facilities where they will be  used

The effectiveness of any safety program
is determined by a company's commit-
ment to it,  as  shown by  the  safety
performance  measures established, the
incentives provided for attaining safety
goals, and the resources committed  to
safety and hazard control.
  Because  human error is  a  common
cause of accidental chemical releases,
personnel selection and training, and the
maintenance of a qualified, experienced
work force is a significant part of release
prevention.  Signal  detection,  signal
filtering, probability estimation, manual
control, and fault diagnosis are important
operator skills. The  qualifications and
abilities of  personnel in high-hazard
facilities should be greater than for those
in other process facilities.  Management
is responsible for ensuring that adequate
descriptions of standard policy and safety
procedures are  available  and also  for
supporting these procedures by  means
of staff training programs, and by estab-
lishing operating and  maintenance
practices for: handling upset and emer-
gency  conditions, using safety  equip-
ment, and performing plant audits.
  Emergency training  includes such
topics as: (1) recognition of alarm signals,
(2) performance  of specific safety func-
tions, (3) use of specific equipment, (4)
evacuation  instructions, (5) firefighting,
and  (6)  rehearsal  of  emergency
  Proper plant maintenance and  modi-
fication procedures  are necessary  to
ensure the integrity of chemical process-
ing  equipment and more effective pro-
duction. Since these activities are also
a primary source of accidental releases,
proper  maintenance and modification
practices are an important part of acci-
dental   release  prevention.  Accidents
frequently happen when workers incor-
rectly identify the equipment needing
maintenance. It  is essential  to have
positive isolation of both process mate-
rials and moving parts during mainte-
nance activities. Permit systems and up-
to-date  maintenance procedures reduce
the potential for  accidents during main-
tenance operations  by  specifying the
work to be  done, defining individual
responsibilities, eliminating or protecting
against  hazards, and  ensuring that
appropriate inspection and testing proce-
dures are followed.
 The potential for an accidental chem-
ical release  may also be  reduced  by
repairing or  replacing  equipment that
seems  headed for failure. Some of the
most common tests for examining equip-
ment conditions  are (1) metal thickness
and integrity testing, (2) vibration testing
and  monitoring,  and  (3)  relief  valve

Protection Technologies
  The technology of protection involves
equipment and systems used to capture
or destroy a  toxic chemical that has
escaped from primary containment. Such
technologies  include flares, scrubbers,
enclosures,  and  incinerators,  each  of
which represents an addition to the basic
process system it protects. The appropri-
ate application and proper design  of
these systems for toxic chemicals must
be evaluated  on a case-by-case basis.
  Flares are routinely used in the chem-
ical  process  industries  to  dispose  of
intermittent or emergency emissions of
flammable waste gases because they can
handle  larger flow variations than can
process  combustion  devices such  as
boilers. Many units include venting steps
to the flare system during  processing.
Though flares can be a useful protection
against  accidental releases  of toxic
chemicals, because of potential second-
ary hazards their use requires a thorough
analysis of each specific application.
  Scrubbers,  a traditional  method  of
absorbing toxic  gases  from  process
streams, can be used for controlling toxic
gas releases from vents and pressure
relief discharges, from  process  equip-
ment,  or  from secondary containment
enclosures.  Absorbers  are useful for
protecting against accidental releases of
toxic chemicals, but there may be circum-
stances where their use is difficult. As
with flare systems, it is important  to
determine that use of an absorber does
not exacerbate  the  original problem.
Absorbers differ from flares in that they
are often dedicated  to  specific  units
because of the diverse  nature of gas
contaminants and the  poor turndown
ratio of absorbers. The types of absorbers
most applicable to accidental chemical
releases are spray towers, packed tow-
ers, and Venturis.
  Enclosures are containment structures
that can capture toxic chemicals spilled
or vented from  storage  or  process
equipment,  thereby  preventing  their
immediate discharge to the environment.
Enclosures contain the spilled  liquid  or
gas until it can be transferred to other
containment, discharged at  a controlled
rate, or transferred at a controlled rate
to scrubbers  for  neutralization.  Enclo-
sures can be constructed around individ-
ual equipment items, process units, and
entire plants,  depending  on the nature
of the hazard being controlled. The most
common procedure involves  directed
ventilation; e.g., fume hoods, exhaust
blowers, fans, and  specialized heating
and  air-conditioning  systems.  Under-
ground storage tanks  can also be used
for certain applications.

    D. S. Davis, G. B. DeWolf, andJ. D. 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:  Control
      Technologies,  Volume 1: Prevention and  Protection Technologies  for
      Controlling Accidental Releases of Air Toxics," (Order No. PB 87-228 656/
      AS; Cost: $24.95, 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

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