Chemical Engineering July 2022 - 22

Cover Story
Atmospheric Pressure
Releases: Which Require
Dispersion Modeling?
Dispersion modeling of atmospheric releases from pressure-relief valves and other sources
can be time-consuming. Follow this screening process to categorize which installations require
detailed modeling and focus effort where it is most consequential
John Burgess,
Dustin Smith and
Thuc Ngo
Smith & Burgess
Process Safety
Consulting
IN BRIEF
DESIGNED
ATMOSPHERIC
RELEASES
NON-FLAMMABLE, NONTOXIC
FLUID
FLAMMABLE/
COMBUSTIBLE FLUID
QUANTITATIVE ANALYSIS
TOXIC FLUIDS
CONCLUDING REMARKS
T
he safe design of atmospheric relief/venting
systems is imperative in
chemical and petrochemical facilities
when pressure-relief systems are designed
to release flammable or toxic materials
directly to the atmosphere (as compared
to a release to a flare or incinerator). Many
facilities have the potential for atmospheric
releases, either from existing installations or
new designs that require simple discharge
systems. Recent incidents and regulatory
pressure have many facility designers asking
the following questions:
* How many relief valves in the facility vent to
the atmosphere?
* Are these relief valves safe?
* Can we ensure that the relief installations
meet the
requirements for
RAGAGEP (recognized and generally
accepted good engineering practices)?
To
ensure
that
atmospheric
discharges
terminate at a " safe location, "
as required by good engineering practices
(for example, the American Society
of Mechanical Engineers Boiler and
Pressure Vessel Codes, section VIII UG135(f)),
engineers and designers should
follow a systematic decision-making
process. In this article, a safe location is
an installation that meets the criteria in
the American Petroleum Institute Standard
521 (Pressure-Relieving and Depressurizing
Systems) [1] but may not
meet more stringent requirements or environmental
rules.
To determine the safety of atmospheric
releases from pressure-relief valves, facilities
can use industry-available modeling
tools (such as DNV Phast, BakerRisk
SafeSite3G and so on) to perform
22
FIGURE 1. Designed releases of materials to the atmosphere can
occur from pressure-relief valves, such as the one shown here, and
other equipment
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2022
dispersion
modeling
of
the
releases
for
consequence analysis. However, modeling
every atmospheric release can become
overwhelming and produce a large volume of
data without adding value. Therefore, a more
practical solution may be to apply a simple
screening process that will categorize safe
installations based on RAGAGEP practices.
Following the screening process allows time
and effort to be focused on those installations
that fail simple screening and that require
detailed modeling.
This article
consolidates the
simplified
methods in API 521, and provides a decision-making
process such that typical plant
engineers with everyday tools can screen
Smith & Burgess
http://WWW.CHEMENGONLINE.COM
Chemical Engineering July 2022

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