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

3-82 API RECOMMENDED PRACTICE 581
in Annex 3.A, Tables 3.A.3.3 through 3.A.3.6. These curves take into consideration the release rate and flash
point. In general, the lower the flash point of the released fluid, the higher the probability of ignition. Using
these curves eliminates the need to blend results between the continuous and instantaneous results as
required in the Level 1 consequence analysis.
The mass fraction of flammable fluid in the release fluid mixture,
release rate of flammable material:
rate rate mfrac
flam
n
flam
mfrac , must be known to calculate the
flam
=
n⋅ (3.106)
The liquid and vapor portions of the flammable release rate are determined using Equation (3.107) and
Equation (3.108).
( )
rate = rate ⋅ − frac
(3.107)
flam flam
l, n n
1
fsh
rate = rate ⋅ frac
(3.108)
flam flam
v,
n n fsh
As an alternative to using Figure 5.5 and Figure 5.6, the probability of ignition at ambient conditions of a
flammable liquid or vapor release may be calculated from Equation (3.109) or Equation (3.110), respectively.
Since these are a function of release rate, the probabilities of ignition are calculated for each of the release
hole sizes selected. Note that when the flammable liquid or vapor release rate exceeds a rate that would
indicate an instantaneous release [4,535.9 kg (10,000 lb) release in 3 minutes or less], a maximum value of
25.22 kg/s (55.6 lb/s) should be used for flam
flam
rate or
ln
rate in Equation (3.109) and Equation (3.110).
,
vn ,
( 12 fp ) ( 12 fp ) ( 12 fp )
2 3
⎛⎡a+
b⋅ C ⋅ T + c⋅ C ⋅ T + d⋅ C ⋅ T ⎤+
⎞
amb ⎜⎢⎣
⎥⎦
⎟
poiln
,
= exp⎜ 2 3
⎟
flam
⎜ ⎡e+ f ⋅( C12 ⋅ Tfp ) + g⋅( C12 ⋅ Tfp ) + h⋅( C12 ⋅Tfp )
⎤⋅ln
( C4 B
⋅ratel , n ) ⎟
⎝ ⎢⎣
⎥⎦
⎠
where,
a = -1.368924E-01
b = -7.598764E-03
c = 8.282163E-06
d = -6.124231E-09
e = 6. 876128E-02
f = 1.193736E-04
g = 2.081034E-07
h = -4.057289E-11
(3.109)