API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

3-96 API RECOMMENDED PRACTICE 581
5.8.6 Flash Fires
5.8.6.1 General
Flash fires, like VCEs, can occur as a result of a delayed ignition of a vapor cloud. The source of the vapor
cloud could either be from a vapor or two-phase jet release or evaporation off the surface of an un-ignited liquid
flammable pool. Dispersion modeling of the cloud is required to evaluate the extent of a vapor cloud since the
amount of flammable material and the area covered by the flammable portion in the cloud is needed. See the
general discussion on cloud modeling presented in Annex 3.A.
5.8.6.2 Flash Fire Consequence Area
A flash fire is a deflagration (not detonation); however, unlike VCEs, the flame speed is relatively slow and
overpressures (blast waves) do not occur. Flash fires are much more common than VCEs and last for no more
than a few tenths of a second. Unlike pool or jet fires (immediate ignition), flash fires need not consider radiation
effects away from the fire boundary, since the combustion process is of short duration and relatively low
intensity. The consequence area for personnel from a flash fire, flash
CA , is merely the flammable cloud
inj , n
boundary and no further.
As with VCEs, a suitable cloud dispersion model that can handle plumes (continuous release with steady state
analysis) as well as puffs (instantaneous releases that required a transient model) should be used. The cloud
dispersion model is used to determine the boundary area of the vapor cloud that contains flammable material
that is at or above the LFL of the mixture in the cloud. The resultant boundary area will equal the consequence
area for serious injury to personnel. As a general rule of the thumb, the consequence area associated with
damage to an equipment component from flash fires, flash
CA , is limited to 25 % of the area for serious injury
cmd , n
to personnel.
CA
= 0.25⋅ CA
(3.175)
flash
flash
f , cmd , n
inj,
n
5.8.7 Determination of Flammable Consequence for Each Release Case (Hole Size)
For each hole size or release case selected, the flammable consequence area is calculated as a probability
weighted consequence area of all of the potential event outcomes on the event tree as shown in Equation
(3.176) and Equation (3.177). For component damage, use Equation (3.176); for personnel injury, use
Equation (3.177).
CA
⎛ ppool ⋅ CA + pjet ⋅ CA + pfball ⋅ CA + ⎞
= ⎜ ⎟
⎝
⎠
pool jet fball
flam
n f , cmd , n n f , cmd , n n f , cmd
f , cmd , n
⎜
vce flash
pvcen ⋅ CAf , cmd , n
+ pflashn ⋅CAf , cmd , n
(3.176)
CA
⎛ ppool ⋅ CA + pjet ⋅ CA + pfball ⋅ CA + ⎞
= ⎜ ⎟
⎝
⎠
pool jet fball
flam
n f , inj, n n f , inj, n n f , inj
f , inj,
n
⎜
vce flash
pvcen ⋅ CAf , inj, n
+ pflashn ⋅CAf , inj,
n
(3.177)
5.8.8 Determination of Final Flammable Consequence Areas
The final flammable consequence areas are determined as a probability weighted average of the individual
flammable consequence areas calculated for each release hole size. This is performed for both the component
damage and the personnel injury consequence areas. The probability weighting utilizes the generic
frequencies of the release hole sizes selected per Section 4.2.
The equation for probability weighting of the component damage consequence areas is given by Equation
(3.178).