Hydrocarbon Processing - December 2021 - 52

Environment and Safety
The jet fire scenarios were modeled using proprietary softwarea,
based on the following general parameters:
* Release size (25 mm): The release size can range from
pinhole to fullbore rupture (TABLE 1). Different leak
sizes have different implications with relation to impact
potential. Guidance is available for selecting generic
hole sizes for performing the fire hazard analysis.3
The small and medium releases have a longer duration,
along with a greater potential for escalation and domino
effects. Large and rupture releases may have larger
hazard zones, but the shorter duration is less likely
to cause structural failure. For this case study, the
medium release category was selected, since the event is
still large enough, and the duration long enough,
to cause the greatest impact.8
* Release direction (horizontal): The accidental
release can occur in any direction (horizontal, vertical
or angled releases); however, the most hazardous ones
are horizontal releases because the resulting jet fire
will be closer to nearby equipment, with a high
probability of impairment. Per the jet fire correlations
implemented in the proprietary software, the flame
length and width will decrease with an increase in
the angle from the horizontal from 0° to ±90°.
* Release elevation (1 m from grade): The proprietary
software documentation recommends that the release
elevation should be set to greater than zero (e.g., 1 m)
for two reasons:
1. This release elevation is more realistic since most
releases are not at ground level.
TABLE 1. Jet fire release categories and hole sizes
Release category
Small
Medium
Large
Rupture
Identify the high risk areas
Identify and validate
the failure scenarios
Jet fire modeling
Release size range
0.1 in.-0.4 in. (3 mm-10 mm)
0.4 in.-2 in. (10 mm-50 mm)
2 in.-6 in. (50 mm-150 mm)
Full bore (equipment diameter)
Coarse QRA
HAZID
HAZOP
Process flow diagrams
Piping and instrument diagrams
Heat and material balance
Hazardous material scheudule
Safety philosophies
2. This elevation provides better modeling of droplet
behavior for continuous releases.
If a continuous release is at ground level, then the
program assumes that all liquid rains out immediately
to form a pool-whereas, if the release is above ground
level, the program models the evaporation and trajectory
of the droplets inside the cloud, and the proportion that
rains out will be lower and more realistic.
* Weather conditions (2F and 5D): The weather
condition is generally selected based on the wind rose
of the facility or data from the nearest meteorological
office weather station. The two most common wind/
weather conditions used in the UK-accepted by the
UK Health and Safety Executive agency for Control of
Major Accident Hazard (COMAH) reports-are:
° Wind speed at 2 m/sec and a Pasquill-Gifford
stability class F, representing very stable weather
during nighttime (2F)
° Wind speed at 5 m/sec and a Pasquill-Gifford
stability class D, representing neutral weather (5D).
The impact assessment was carried out by comparing the
jet fire modeling results with failure time criteria. As part of
a risk-based approach, dimensioning accidental loads (DALs)
provided a convenient metric to assess appropriate levels of
fire protection. Typical load measures include the duration of
flame impingement, exposure to radiation levels above a defined
value, heat transfer or temperature increases. Additionally,
exposure duration is the simplest and most widely reported
DAL, which was used in this methodology.9
The jet fire dimension
at time zero is generally used in the QRA to determine the
number of immediate fatalities; however, for asset protection,
longer fire durations are used. The failure of the unprotected
vessel or load-bearing member/support is assumed to occur
within 5 min in case of jet fire impingement.10
Protection measures were recommended based on the consequence
analysis results (i.e., jet fire modeling and the impact
assessment). The protection measures were applied by following
the hierarchy of control.
Results and discussion. In this case study, the PDH facility
utilizes a licensed technology designed to produce up to
750,000 tpy of propylene from propane feedstock. A PDH
flow schematic is shown in FIG. 2. During normal operation,
the PDH facility contains varying hydrocarbon compositions
at pressures as high as 22 barg, with an estimated 1,400 t of
total flammable liquid inventory.
A jet fire protection philosophy usually involves three eleImpact
assessment
Dimensioning accidental loads
Fire protection measures
FIG. 1. Proposed jet fire risk assessment methodology.
52 DECEMBER 2021 | HydrocarbonProcessing.com
ments: system de-inventory, passive fire protection and active
fire protection. However, due to operational constraints inherent
to the process, a system de-inventory process to minimize
the risk of sustained jet fire impingement on either process
equipment or structures, and to also mitigate subsequent escalation,
was not practically achievable nor economically viable.
Due to the licensed technology employed, the major
impediment to the successful emergency de-inventory of the
facility was that it could only be executed after a considerable
time delay upon initiation of emergency shutdown to bring the
facility to a safe state. Further, the inherently large flammable
inventory within the process necessitates the application of a
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