Chemical Engineering July 2022 - 28

release of a flammable vapor stream
as the concentration of effluent
starts above the UFL, is then within
the flammability
limits, and finally
drops below the LFL. The vertical
and horizontal distances to below
the LFL are based on API STD 521
7th edition Figure 8 and Figure 9,
respectively.
The data shown in API STD 521
7th edition Figure 9 were cubic feet
to develop an equation to calculate
horizontal downwind distance to
LFL. The maximum downwind horizontal
distance from jet exit to leanflammability
concentration limit for
petroleum gases can be expressed
as follows:
(1)
Where:
x is the horizontal distance downwind
to the LFL for a hydrocarbon
release, ft
dj is the inside diameter of the jet
release exit, in.
ρj is the density of the fluid just inside
the tip exit, lb/ft³
ρ∞ is the density of the ambient air,
lb/ft³
uj is the fluid jet exit velocity, ft/s
u∞ is the wind speed, ft/s
For an ideal gas,
(2)
Where:
MWj is the molecular weight of the
jet release fluid
MW∞ is the molecular weight of the
atmosphere, 28.8 g/mol
Pj is the pressure of the fluid just
inside the tip exit, which is typically
With the following conservative assumptions,
Equation (4) can be further
simplified:
u∞/uj ≈ 0.036, the maximum downwind
extents of the LFL
Pj/P∞ ≈ 1, the pressure at the maximum
vent velocity is near atmospheric
pressure
T∞/Tj ranges from 0.5 to 1, and
is assumed to be 1. For an atmospheric
temperature of 70 °F, the release
temperature would be limited
between 70°F and 600°F or less for
this assumption.
(5)
Engineers and designers can use
Equation
(5)
to
quickly
calculate
horizontal downwind distance to LFL
concentration.
Two hundred sixteen unique dispersion
models were performed in
DNV PHAST to validate Equation (5).
Each dispersion model was run
with
three
TABLE 1. FLUID CONDITIONS FOR THE DISPERSION MODELS RUN TO
COMPARE AGAINST EQUATION (5)
Fluid
Ethane
Ethane
n-Pentane
n-Pentane
n-Octane
n-Octane
28
Pressure, psig
50
250
50
250
50
250
Cold, ºF
50
200
325
375
525
Hot, ºF
100
100
400
400
600
600
different
weather
condi -
tions. The
conditions
for the different
leases
shown
Table
Each
atmospheric
pressure in the case of
a pipe exit, psia
P∞ is the pressure of the ambient air
/ atmosphere, psia
Tj is the fluid jet exit temperature,°R
T∞ is the temperature of the ambient
air / atmosphere, °R
The following is the result of substituting
Equation (2) into Equation (1)
and solving for the distance downwind
to the lower flammability limit:
(3)
the cases in Table 2 were run for
Crosby 1½G3, 4M6 and 6Q8 relief
devices. The discharge point has
the same pipe diameter as the reliefdevice
outlet. The wind speeds used
were 3.5 ft/s, 5 ft/s and 10 ft/s, all
with an atmospheric stability class
of D. The cases were run at the full
capacity of the relief device (as determined
by PHAST) and 25% of the
rated relief device capacity.
For each of the cases reviewed,
Equation (5) predicted
the
extent
(4)
reare
in
2.
of
of
the flammable zone to be 1.3 to
3.0 times farther than the horizontal
distance predicted by the PHAST
modeling. Also, for the smaller-diameter
vents with lower-MW fluids,
Equation (5) tended to overpredict
the horizontal distance by 1.3 times
more than the dispersion modeling.
For the larger vents with higher-MW
fluids, Equation (5) tended to overpredict
the horizontal distance by as
much as three times compared to
the dispersion modeling.
The dispersion modeling to test
Equation (5) showed that as much
as 1/6 to 1/3 of the cloud height may
be between the cloud centerline and
the lowest point where the LFL is
present. As such, detailed dispersion
modeling should be completed
if there is equipment above the discharge
location within the LFL horizontal
distance predicted by either
Equation (4) or Equation (5).
Note that this method does not
consider the effects of condensation
of the effluent stream, which must
be considered by the relief system
engineer/designer (see Note 2 in the
previous section).
Potential for ignition. API STD 521
requires the system designer to review
the potential effects of thermal
radiation on workers if the release
stream is ignited. This is the case
even when there is no potential ignition
source within the flammable
cloud. Ignition of the effluent stream
may still occur (for example, because
of static electricity, lighting, autoignition
and so on) and the impacts of
thermal radiation exposure should
be considered. Some facilities install
the ability to inject emergency steam
into the tail pipes of relief devices to
snuff out flames should the effluent
catch fire.
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