API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-97CAflamf , cmd⎛4flam⎜∑gffn⋅CAf , cmd , n ⎟n=1= ⎜⎟⎜ gfftotal⎟⎜⎝⎞⎟⎠(3.178)The equation for probability weighting of the personnel injury consequence areas is given by Equation (3.179).CAflamf , inj⎛4flam⎜∑gffn⋅CAf , inj,n ⎟n=1= ⎜⎟⎜ gfftotal⎟⎜⎝⎞⎟⎠(3.179)In Equation (3.178) and Equation (3.179), the total GFF is as calculated in STEP 2.2.5.8.9 Calculation of Flammable Consequence Areasa) STEP 8.1—Determine the mass fraction of the release rate that contains a flammable component,flammfrac . This can be determined by adding the mass fractions of all flammable components in themixture.b) STEP 8.2—For each hole size, calculate the flammable release rate, rateflamn, using Equation (3.106). Alsoflamflamcalculate the liquid portion, rate , and the vapor portion,ln ,rate , of the flammable release rate usingvn ,Equation (3.107) and/or Equation (3.108), as applicable. Note that for two-phase releases both valuesshould be calculated.c) STEP 8.3—For each hole size, select the appropriate event tree using Figure 5.2 and Figure 5.3 and thephase of the fluid after flashing to atmosphere in STEP 1.3. For the leak cases (small, medium, and largehole sizes), use Figure 5.2. For the rupture case, use Figure 5.3.d) STEP 8.4—For each hole size, including the rupture case, calculate the probability of ignition of therelease.1) Determine the probability of ignition at ambient temperature for the liquid portion of the release,ambflampoiln,, using Equation (3.109) and the value of rate obtained in STEP 8.2. Note that for theln ,rupture case or some of the larger hole sizes a maximum value of 25.2 kg/s (55.6 lb/s) should be used.2) Determine the probability of ignition at ambient temperature for the vapor portion of the release,ambflampoivn,, using Equation (3.110) and the value of rate obtained in STEP 8.2. Note that for thevn ,rupture case and some of the larger hole sizes, a maximum value of 25.2 kg/s (55.6 lb/s) should beused.3) Determine the maximum probability of ignition for the liquid,using Equation (3.111) and Equation (3.112).aitpoi , and the vapor,laitpoi , at the AITv4) Calculate the probability of ignition for the liquid, poi , and the vapor,ln ,poi,temperatures using Equation (3.113) and Equation (3.114), respectively.vn, at normal storage5) For two-phase releases, calculate the probability of ignition, poi , at normal storage temperatures2,nusing Equation (3.115).
3-98 API RECOMMENDED PRACTICE 581e) STEP 8.5—For each hole size, determine the probability of immediate ignition given ignition.1) Obtain the probabilities of immediate ignition at ambient conditions for the liquid portion and the vaporambambpoii,portions of the release, lnpoii,and vn, from Table 5.3, based on whether the release is aninstantaneous or continuous liquid or vapor release.2) Calculate the probability of immediate ignition given ignition at storage conditions for the liquid portionpoii,of the release, lnpoii,, and the vapor portion of the release, vn, using Equation (3.128) andaitEquation (3.129). Use a value for the probability of immediate ignition at the AIT,poii=1.0.poii2,n3) For two-phase releases, calculate the probability of immediate ignition given ignition,normal storage temperatures using Equation (3.118) and the flash fraction,STEP 1.3.f) STEP 8.6—Determine the probability of VCE given a delayed ignition., atfracfsh, calculated in1) Determine the probability of VCE given delayed ignition, pvcedi , from Table 5.3 as a function of therelease type and phase of release. The probability of a VCE given delayed ignition for a liquid releaseis pvcedi ; for a vapor it isln ,pvcedi .vn ,2) For two-phase releases, calculate the probability of VCE, given delayed ignition, pvcedi2,nEquation (3.119) and the flash fraction,frac calculated in STEP 1.3.fshg) STEP 8.7—Determine the probability of flash fire given delayed ignition., using1) Determine the probability of flash fire given delayed ignition, pffdi , from Table 5.3 as a function ofthe release type and phase of release. Alternatively, Equation (3.120) and Equation (3.121) can beused to obtain these values.2) For two-phase releases, calculate the probability of flash fire given delayed ignition, pffdi2,nEquation (3.122) and the flash fraction,frac calculated in STEP 1.3.fsh, usingh) STEP 8.8—Determine the probability of a fireball given an immediate release, pfbii , using Equation(3.123) or Equation (3.124).i) STEP 8.9—Select the appropriate event tree. For small, medium, and large hole sizes, select the eventtree from Figure 5.3 based on whether the release is a liquid, vapor, or two-phase release. For the rupturecase, select the event tree from Figure 5.4 based on whether the release is a liquid, vapor, or two-phaserelease.j) STEP 8.10—For each hole size, determine the probability of each of the possible event outcomes on theevent tree selected in STEP 8.9. As an example, the probability of each of the event outcomes for leakageof a vapor from a small, medium, or large hole size is shown below. All other event tree outcomes can becalculated in a similar manner.1) Probability of a pool fire given a release:ppoolvn ,= 0.0(3.180)
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3-98 API RECOMMENDED PRACTICE 581
e) STEP 8.5—For each hole size, determine the probability of immediate ignition given ignition.
1) Obtain the probabilities of immediate ignition at ambient conditions for the liquid portion and the vapor
amb
amb
poii
,
portions of the release, ln
poii
,
and vn
, from Table 5.3, based on whether the release is an
instantaneous or continuous liquid or vapor release.
2) Calculate the probability of immediate ignition given ignition at storage conditions for the liquid portion
poii
,
of the release, ln
poii
,
, and the vapor portion of the release, vn
, using Equation (3.128) and
ait
Equation (3.129). Use a value for the probability of immediate ignition at the AIT,
poii
=1.0.
poii2,n
3) For two-phase releases, calculate the probability of immediate ignition given ignition,
normal storage temperatures using Equation (3.118) and the flash fraction,
STEP 1.3.
f) STEP 8.6—Determine the probability of VCE given a delayed ignition.
, at
frac
fsh
, calculated in
1) Determine the probability of VCE given delayed ignition, pvcedi , from Table 5.3 as a function of the
release type and phase of release. The probability of a VCE given delayed ignition for a liquid release
is pvcedi ; for a vapor it is
ln ,
pvcedi .
vn ,
2) For two-phase releases, calculate the probability of VCE, given delayed ignition, pvcedi
2,n
Equation (3.119) and the flash fraction,
frac calculated in STEP 1.3.
fsh
g) STEP 8.7—Determine the probability of flash fire given delayed ignition.
, using
1) Determine the probability of flash fire given delayed ignition, pffdi , from Table 5.3 as a function of
the release type and phase of release. Alternatively, Equation (3.120) and Equation (3.121) can be
used to obtain these values.
2) For two-phase releases, calculate the probability of flash fire given delayed ignition, pffdi
2,n
Equation (3.122) and the flash fraction,
frac calculated in STEP 1.3.
fsh
, using
h) STEP 8.8—Determine the probability of a fireball given an immediate release, pfbii , using Equation
(3.123) or Equation (3.124).
i) STEP 8.9—Select the appropriate event tree. For small, medium, and large hole sizes, select the event
tree from Figure 5.3 based on whether the release is a liquid, vapor, or two-phase release. For the rupture
case, select the event tree from Figure 5.4 based on whether the release is a liquid, vapor, or two-phase
release.
j) STEP 8.10—For each hole size, determine the probability of each of the possible event outcomes on the
event tree selected in STEP 8.9. As an example, the probability of each of the event outcomes for leakage
of a vapor from a small, medium, or large hole size is shown below. All other event tree outcomes can be
calculated in a similar manner.
1) Probability of a pool fire given a release:
ppool
vn ,
= 0.0
(3.180)