Chemical Engineering July 2022 - 24

most atmospheric releases. The article
can be used as the basis for
screening
the
designed
releases
from atmospheric relief devices at
new and existing facilities. In addition,
this article provides qualitative,
quantitative and semi-quantitative
methods to screen the atmospheric
release based on the hazardous
properties of the discharged fluids,
including
tible/toxic,
non-flammable/combusflammable/combustible,
and
toxic materials.
Detailed dispersion modeling was
performed to validate the results of
the simplified equations presented
in this article. While there are always
rare circumstances that can occur,
under the most common conditions
for processing hydrocarbon streams,
the methods in this article are conservative
and can be used to screen
atmospheric
relief
device
installations.
However, each installation is
different and engineering judgement
must be used to ensure that the specific
installation meets all government
and company requirements. There is
no attempt here to create a methodology
that will be applicable to all
possible installations or discharge
compositions. The user takes responsibility
for how the methodologies
proposed here are executed and
interpreted. Environmental impacts
and permit compliance is outside the
scope of this article. However, these
methods may help achieve compliance
with the Risk Management Plan
(RMP; U.S. Environmental Protection
Agency) requirements in the U.S.
Designed atmospheric releases
Generally, in the chemical process
industries (CPI), there are two major
risks
associated with
atmospheric
discharges: (1) the generation of a
flammable or explosive mixture, and
(2) the accumulation of toxic materials
at a concentration level that may
cause harm. Therefore, this article
consolidates the simplified methods
in API STD 521 to screen the atmospheric
release based on the hazardous
properties of the discharged fluids
including flammability, combustibility,
physical effects from non-flammable/
non-toxic materials, and toxicity.
Methods and designs necessary
to comply with environmental regu24
lations
are not discussed
and must be
independently verified.
The focus here is
on designed releases
from atmospheric
pressure relief devices,
rather
than
" accidental "
releases
from loss of containment.
Consider the
following two cases:
1. An amine regenerator
overpressures and
the relief device works
as designed. This system
is in a gas plant
and the relief device's
final
disposition
is
Smith & Burgess
from an atmospheric
collection system. The
resulting vapor cloud
results in H2S area monitors activating
and the evacuation of all non-essential
personnel from the site. The
unit also shuts down.
FIGURE 2. After the atmospheric release of a vapor-phase material, the
downwind distance that the material will travel at concentrations between
its UFL and LFL must be determined
2. A forklift driver delivers equipment
near the amine regenerator
and backs into piping, resulting in
a leak from a control valve station.
This leak creates a vapor cloud that
activates H2S monitors. Consequently,
all non-essential personnel
from the site are evacuated and the
unit shuts down.
Which case is worse? Arguably
both cases could have been prevented
with better engineering design,
but in the first scenario the
overpressure system functioned as
designed and yet the facility was
evacuated. This article will help engineers
design installations such that
the first scenario should not occur.
The article focuses on designed releases
(such as relief-device tail piping),
as compared to facility
siting
concerns associated with loss of
containment scenarios (such as vessel
ruptures).
Atmospheric venting is designed
into many petrochemical and refining
facilities. Generally, designed or
planned atmospheric releases come
from several sources:
Control systems. Backpressure
regulators, compressor stability control
schemes and emergency shutdown
devices can discharge to the
atmosphere.
Pressure-relief devices. Pressure
relief valves, rupture disks, and conservation
vents can all have installations
that discharge the effluent directly
to the atmosphere.
Atmospheric collections systems.
These systems can range from the
collection of one or two relief devices
into a single vent stack to systems as
complex as flare systems terminating
with a vent instead of a flare tip. Special
considerations are required for
the analysis of the vent, especially in
global relief scenarios, as there is no
designed means to burn the effluent.
Flare systems. Flare systems are
designed to capture the effluent from
the facility, remove entrained liquids,
and burn the remaining vapor.
In addition to the listed vents,
many other vent sources may occur
in a facility. For example, in addition
to venting from the discharge location,
relief devices may vent from a
broken bellows or pilot-operation
system. Open pipe vents may occur
(typically steam or other inert systems),
but often there are process
vents from normally closed isolation
valves or through misaligned isolation
valves. Low-pressure tank vents
(pressure-vacuum vents, gooseneck
vents, or otherwise) typically vent directly
to atmosphere.
Often, these systems can be designed
safely with proper consideration
for the hazards of the fluid and
for the design of the vent or flare.
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