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

3-20 API RECOMMENDED PRACTICE 581
a) For Continuous Releases—A blending factor is calculated to smooth the results for releases near the
continuous to instantaneous transition [4,536 kg (10,000 lb) released in less than 3 minutes, or a release
rate of 25.2 kg/s (55.6 lb/s)] using Equation (3.19).
fact
IC
n
⎡⎧rate
⎫ ⎤
n
= min ⎢⎨
⎬, 1.0⎥
⎢⎣⎩
C5
⎭ ⎥⎦
(3.19)
When instantaneous equation constants that are not provided in Table 4.8 and Table 4.9 for the reference
fluid, the blending factor is defined in Equation (3.20).
fact = 0.0
(3.20)
IC
n
b) For Instantaneous Releases—Blending is not required for instantaneous releases [4,536 kg (10,000 lb)
released in less than 3 minutes, or a release rate of 25.2 kg/s (55.6 lb/s)]. The blending factor, fact ,
for an instantaneous release is defined in Equation (3.21).
fact = 1.0
(3.21)
IC
n
The blended release area is calculated using Equation (3.22). Note that this area is proportionate to the
proximity of the actual release rate, rate , is to the continuous/instantaneous transition rate of 25.2 kg/s
(55.6 lb/s).
n
( 1 )
CA = CA ⋅ fact + CA − fact
(3.22)
IC−blend INST IC CONT IC
n n n n n
4.8.6 Blending of Results Based on AIT
Consequence area calculations yield significantly different results depending on whether the autoignition not
likely consequence equations are used or the autoignition likely consequence area equations are used. The
consequence areas are blended using Equation (3.23).
( 1 )
AIT −blend AIL AIT AINL AIT
CA = CA fact + CA − fact
(3.23)
The AIT blending factor,
AIT
fact
, is determined using the following equations.
IC
n
AIT
fact = 0
for T + C ≤ AIT
(3.24)
s
6
( T − AIT + C )
AIT s
6
fact = for Ts
+ C6 > AIT > Ts
−C6
2⋅C6
(3.25)
AIT
fact = 1
for T −C ≥ AIT
(3.26)
s
6
4.8.7 Determination of Final Flammable Consequence Areas
The final flammable consequence areas are determined as a probability weighted average of the individual
(blended) flammable 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.