Chemical Engineering July 2022 - 30

tration of a toxic compound in an
effluent stream that meets all the
criteria in the list given in the " semiquantitative
analysis " section. If
there is the potential for personnel
in the immediate vicinity of the area
of release plume (while it is still a jet),
detailed dispersion modeling may
be required.
(6)
Where:
CLimit = the highest concentration
that meets the risk acceptance for
the toxic material, ppm (see Table 2)
CEffluent = the concentration of the
toxic material in the effluent stream,
ppm
Determining an acceptable lower
concentration (CLimit) can be difficult,
but there are a variety of sources that
offer guidance (see Table 2). The list
in Table 2 is only provided for guidance.
It is the responsibility of the
system designer to ensure that all
regulatory and site requirements for
releases are met.
In summary, if systems meet the
qualitative criteria listed and if the
concentration limits satisfy Equation
(6), the maximum toxic concentration
to which workers may be exposed
should be below the concentration
that meets the risk-acceptance criteria.
For example, if the relief-device
toxic vapor release has a H2S concentration
of 2,000 ppm (or less),
then the peak exposure will be below
the IDLH (immediate danger to life
and health) concentration of 100
ppm for locations where personnel
are present. Corporate or regulatory
requirements may be different than
this example for H2S.
This screening method has limited
applicability for streams that contain
high concentrations of toxic materials
or for facilities that handle extremely
toxic chemicals. Users are cautioned
if using this screening method for
materials that do not have characteristics
similar to light hydrocarbons,
because the jet-mixing effect of
those streams might be different. For
those systems, detailed dispersion
modeling might be needed.
30
Concluding remarks
The methods presented here are
simplified and can be readily used
by system designers to screen atmospheric
relief-device installations
to meet the " safe location " criteria as
presented in API STD 521. Following
the methods outlined in this article,
a system designer should be able
to document each of the following
questions:
* Do these relief valves discharge to
a " safe location " ?
* Can we show that the discharge
location meets good engineering
practices?
* Which relief-device systems require
additional analysis (dispersion
modeling)?
For each atmospheric discharge,
if the provided systematic decisionmaking
process is followed and documented,
atmospheric
discharges
can
be shown to vent to a " safe
location, " as required by good engineering
practice. Detailed dispersion
modeling was performed to validate
the results of the simplified equations
presented here. While there are
always rare circumstances that can
occur, under the most common conditions
that hydrocarbon streams are
processed, the methods discussed
here are conservative and can be
used to screen atmospheric reliefdevice
installations.
n
Edited by Scott Jenkins
Disclaimers
All information presented in this article is meant to be
used by a qualified and experienced engineer or relief
system designer. The content of this article is general in
nature and based on the guidance provided in API STD
521. It is up to the end user to verify the safety and/or
legality of any installations. With any predictive model,
there may be variations between the results predicted
and the results that occur from any given incident. For
energetic or extremely toxic substances, or those where
the concentrations predicted are close to the acceptable
limits and risks, additional analysis should be performed.
The user is cautioned to design a system that
has a safety margin that allows for the inherent errors
in this type of estimation. As with other aspects of these
evaluations, good engineering judgment and knowledge
of the specific installations must be considered.
This article is offered for general informational purposes
only. Readers should consult with a qualified profession
regarding their specific projects or work. The authors
disclaim any and all liability arising from or concerning
the analysis contained in this article.
References
1. API Standard 521, " Pressure-relieving and Depressuring
Systems, " American Petroleum Institute, 7th ed.,
2020.
John Burgess is a co-founder
and principal consultant Smith &
Burgess (same address as above).
He has extensive experience providing
assistance to facilities that
process highly hazardous chemicals
in maintaining compliance
with the OSHA process safety
management (PSM) standard.
Burgess has over 25 years of experience
in relief and flare system designs and PSM
compliance. Burgess received a B.S.Ch.E. degree from
Texas Tech University and a M.S. from University of Missouri.
Burgess is a licensed engineer in the state of
Texas.
Thuc Ngo is a lead consultant
at Smith & Burgess (same address
as above). She has over 10 years
of experience working on over 100
projects in
designs. She received
and M.S.Ch.E. degrees
from the
University of Houston. Ngo is a licensed
engineer in the state of
Texas.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2022
2. Gant, S. and others. Generation of Flammable Mists
from High Flashpoint Fluids: Literature Review, Health
and Safety Laboratory (HSE) Symposium Series, number
158, 2013.
3. Burgoyne, J.H., The Flammability of Mists and Sprays,
IChemE Second Symposium on Process Hazards, 1963.
3. " Documentation for Immediately Dangerous To Life
or Health Concentrations (IDLHs), " Centers for Disease
Control and Prevention. Centers for, May 1994. 12 November
2013, www.cdc.gov.
4. Frank, Michael V. " Choosing Safety: A Guide to Using
Probabilistic Risk Assessment and Decision Analysis in
Complex, High-consequence Systems. " Washington,
D.C., Resources for the Future, 2008.
5. Loudon, D. E. " Requirements for Safe Discharge of
Hydrocarbons to Atmosphere, " 28th Mid-year Meeting
of the American Petroleum Institute's Division of Refining.
Philadelphia, Pa. 15 May 1963. Speech.
6. Centers for Disease Control and Prevention (CDC),
www.cdc.gov, 04 October 2013.
7. Turner, D. Bruce. Workbook of Atmospheric Dispersion
Estimates, U.S. Department of Health, Education,
and Welfare, 1970.
8. Weber, A. H. " Atmospheric Dispersion Parameters in
Gaussian Plume Modeling, " U.S. Environmental Protection
Agency, 1976.
9. DeVaull, George E., John A. King, Ronald J. Lantzy,
and David J. Fontaine. " Understanding Atmospheric
Dispersion of Accidental Releases, " New York: American
Institute of Chemical Engineers, 1995.
Authors
Dustin Smith is a co-founder
and principal consultant at Smith
& Burgess (7600 W. Tidwell Rd.,
suite 600, Houston, TX 77040;
Email:
info@smithburgess.com;
Phone: 1-713-802-2647; www.
smithburgess.com) a process
safety consulting firm headquartered
in Houston. He has extensive
experience helping facilities that
process highly hazardous chemicals maintain compliance
with the OSHA process safety management (PSM)
standard. Smith has over 25 years of experience in relief
and flare system designs and PSM compliance. Smith
received a B.S.Ch.E. from Texas A&M University. Smith
is a licensed engineer in the state of Texas.
relief & flare system
B.S.Ch.E.
http://www.cdc.gov http://www.cdc.gov http://www.smithburgess.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2022

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