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

3-84 API RECOMMENDED PRACTICE 581
5.8.1.3 Probability of Immediate vs Delayed Ignition Given Ignition
Given that ignition occurs, the probability of immediate vs delayed ignition depends on the type of release
(continuous or instantaneous), the phase of the release, and how close the released fluid’s temperature is
to its AIT. The probability of immediate ignition given ignition is designated in Figure 5.3 and Figure 5.4 as
poii . The probability of delayed ignition given ignition is( 1− poii)
.
As the event tree figures show, the determination that a specific event occurs is greatly dependent on whether
or not an ignition is either immediate or delayed. For example, an immediate ignition of a vapor release results
in a jet fire or a fireball. If this same release were to have a delayed ignition, the resulting event could be a
VCE or a flash fire. Likewise, a liquid release could either result in a flash fire, a VCE, or a pool fire depending
on whether or not it is an immediate or a delayed ignition.
The probability of immediate ignition given ignition of a flammable liquid release, poii , and a flammable vapor
ln ,
release, poii , can be estimated using Equation (3.116) and Equation (3.117). As an alternative, Cox, Lee,
vn ,
and Ang [15] provides a curve for the probability that an ignition will be an immediate vs a delayed ignition.
poii = poii
⎛
+
T − C ⎞
⋅ poii − poii
⎝ ⎠
( )
amb s 16
ait amb
ln , ln , ⎜ ⎟
ln ,
AIT − C16
poii = poii
⎛
+
T − C ⎞
⋅ poii − poii
⎝ ⎠
( )
amb s 16
ait amb
vn , vn , ⎜ ⎟
vn ,
AIT − C16
(3.116)
(3.117)
amb
amb
The probabilities of immediate ignition, given ignition at ambient conditions, poii and
ln ,
poii , are based
vn ,
on expert opinion and are provided in Table 5.3 for instantaneous and continuous releases of liquids and
vapors. At the AIT or higher, it is assumed that the probability of immediate ignition given ignition for all release
ait
phases, poii , is equal to 1.0. Equation (3.118) provides a linear interpolation for operating temperatures
between ambient and the AIT .
For two-phase releases, the probability of immediate ignition given ignition can be assumed to be the mass
weighted average of the probability calculated for liquid and the vapor as follows:
( 1 )
poii = frac ⋅ poii + − frac ⋅ poii
(3.118)
2, n fsh v, n fsh l,
n
5.8.1.4 Probability of VCE vs Flash Fire Given Delayed Ignition
A delayed ignition will result in the event outcome of either a VCE or a flash fire. The probability of VCE given
a delayed ignition, pvcedi , is dependent on what type of release occurs, instantaneous or continuous, and
whether the release is a liquid or a vapor. Currently, the assumptions for these probabilities are provided in
Table 5.3 and are in general agreement with the assumptions provided in Annex 3.A, Tables 3.A.3.3 through
3.A.3.6 for the Level 1 consequence analysis.
An improvement to these assumptions would be to prorate the probability of a VCE given a delayed ignition,
pvcedi , based on the NFPA reactivity number. A fluid with a higher NFPA reactivity will have a higher
probability of a VCE vs a flash fire. An even better method would be to use the flame speed for the particular
fluid of interest. Higher flame speeds will have a higher probability of a VCE vs a flash fire. The problem with
this method is that data for the flame speed of a particular fluid in a vapor cloud are not always available.