Chemical Engineering July 2022 - 26

Public relations. Another consideration
is the effect the release may
have on the public. The designer
needs to consider if a system will
routinely release. Releases should
be reviewed for the potential to form
" clouds, " loud noises or odors. Any
of these factors may interfere with
operations or cause concern with
the public.
Flammable/combustible fluids
These releases require more engineering
review than non-flammable/
non-toxic releases. The discharge of
potentially flammable material to atmosphere
is generally not preferred.
However, the risks associated with
atmospheric releases can be minimized
by designing the system carefully.
An extensive review of incident
reports from various databases, including
the Major Accident Reporting
System (MARS), Major Hazard
Incident Data Service (MHIDAS), and
the National Fire Information Reporting
System (NFIRS), concludes that
incidents associated with atmospheric
releases directly from relief
devices, which are not part of a collection
header or blowdown drum,
are very rare. Therefore, with proper
design that follows good engineering
practices, facilities can ensure relief
devices discharging flammable fluid
to atmosphere are discharging to a
" safe location. "
For systems with flammable or
combustible fluids, or both, system
designers should consider the following
analysis (in addition to the criteria
for the non-flammable/combustible
and non-toxic fluids):
Fluid phases. For vapor releases, a
release of flammable material begins
at the process concentration. Then,
as the fluid in the piping exits and
begins to mix with the atmosphere,
the concentration drops through the
upper flammable limit (UFL), and at
some point is diluted to below the
lower flammable limit (LFL). After
the concentration drops below the
LFL, the effluent ceases to be a fire
or explosion risk. All releases of potentially
flammable material must be
reviewed to ensure that the fluid has
safely passed through the LFL before
the " cloud " reaches a point of interest
(such as an ignition source, grade,
26
platform, equipment and so on).
Scenarios that could release flammable
liquids or solids to atmosphere
are generally not acceptable and require
mitigation. The release case
needs to be mitigated through one of
the following mechanisms:
1. Dedicated safety instrumentation
(compliant with ISA S-84 or other
applicable standard) to the point
that the potential for release is no
longer credible
2. The relief system design must
ensure that the liquid and solids are
contained and are not vented (for example,
a blowdown drum or tank) to
the atmosphere.
At the time of writing this article,
not all commercially available dispersion-modeling
software accurately
predicts the dispersion of liquids and
solids. Unless engineers and plant
designers can validate the model
findings, the validity of the predicted
flammable concentrations for liquid
and solid releases may not be useful.
Installation. Relief-device exit piping
should be pointed vertically
upward. With proper design of the
device, discharge configuration has
historically been a safe means of
disposing flammable vapors.
Semi-quantitative analysis. The
primary concern when analyzing
flammable atmospheric releases is
the potential to form a flammable
cloud near ignition sources or workers.
API STD 521 7th edition, section
5.8 provides a series of guidelines to
determine if a relief device discharging
flammable vapor to atmosphere
is acceptable based on the " jet " momentum
and velocity mixing effects
of the discharge. The greater the exit
velocity, the more turbulently the material
mixes with air. This results in the
concentration of the released material
quickly decreasing to below the LFL.
Even in releases where there are high
winds present, higher velocities from
vertically oriented vents are directed
more vertically than horizontally.
The following is a semi-quantitative
method (in accordance with
API STD 521 7th edition in §5.8) to
determine if a release is to a " safe
location. " To be considered safe, an
affirmative response to all of the following
is required:
* The molecular weight (MW) of the
fluid is less than 80 g/mol for a hydrocarbon
release (additional information
below in Note 1)
* The exit velocity is greater than 100
ft/s (additional information below in
Note 2)
* The ratio of the exit velocity to wind
speed is greater than 10
* There is no equipment or work
areas at or above the release point
horizontally for 50 ft
* The relief/jet temperatures are
close to or above the expected atmospheric
temperatures
* The qualitative considerations
from the non-flammable/combustible
and non-toxic fluids section
have been reviewed
Note 1. For hydrocarbon releases
with a MW greater than approximately
100 g/mol, there is an increasing
chance for condensation.
Relief streams continuing with other
fluids may condense as well. Depending
on the velocities and concentration
of the fluid at the point
which the temperature drops, an explosive
atmosphere may be formed.
(More information on this topic can
be found in Refs. 2 and 3.
Note 2. Relief devices are not expected
to flow at rated capacity all
of the time. The guidance in Section
5.8 of API STD 521 7th edition says
to consider that relief devices close
at approximately 25% of the rated
capacity. Thus, a designer may need
to consider a range of flowrates for
an installation based on the results of
the relief-systems analysis.
If the installation does not meet all
the listed criteria above, consider the
quantitative analysis procedure in the
following section.
Quantitative analysis
This section presents the simplified
analysis presented in API STD 521
7th edition Figures 8 and 9. The figures
are condensed into a simple
equation to calculate the horizontal
downwind distance that the flammable
vapor may travel prior to dilution
below LFL concentration. For
an atmospheric relief device to be
considered to discharge to a " safe
location, " no potential ignition source
should be located within that radius
at or above the release elevation .
Figure 2 depicts an atmospheric
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