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

3-78 API RECOMMENDED PRACTICE 581
5.6 Estimate the Impact of Detection and Isolation Systems on Release Magnitude
5.6.1 General
The effects of detection and isolation systems are established in accordance with the Level 1 consequence
analysis. See Section 4.6.
5.6.2 Calculation for Detection and Isolation Systems
The step-by-step procedure for estimating the impact of detection and isolation systems is in accordance with
Section 4.6.6.
5.7 Determine the Release Rate and Mass for COF
5.7.1 General
The Level 2 consequence analysis models two-phase releases and distinguishes between the amount of the
theoretical release rate that releases to the atmosphere as vapor or as an aerosol (vapor with entrained liquid)
in the form of a jet and the amount of the release that drops to the ground as liquid to form a pool. Analysis
requires a fluid property package to isentropically flash (isenthalpic is acceptable) the stored fluid from its
normal operating conditions to atmospheric conditions. In addition, the effects of flashing on the fluid
temperature as well as the phase of the fluid at atmospheric conditions should be evaluated. Liquid entrainment
in the jet release as well as rainout effects could be evaluated to get a more representative evaluation of the
release consequences.
5.7.2 Aerosol and Rainout Modeling
When a release is two-phase, there is an amount of liquid entrained in the jet (vapor) portion of the release
(aerosol). The remaining liquid portion of the release, or rainout, can be estimated by the following correlation
recommended by Kletz [5] .
frac = 1−2⋅ frac for frac < 0.5
(3.98)
ro fsh fsh
frac = 0.0 for frac ≥ 0.5
(3.99)
ro
Other liquid rainout correlations are available from CCPS [6] , Davenport [7] , Prugh [8] , and Mudan [9] .
fsh
The fraction that flashes, frac , as it is released to the atmosphere can be determined using fluid property
fsh
software by isentropically (isenthalpically is acceptable) expanding the release fluid from the storage conditions
to the atmospheric conditions.
5.7.3 Calculation of Jet Release Rate and Pool Release Rate
Once the release rate is determined and the rainout fraction is estimated, the release rate for modeling pool
type consequences, W , and for modeling jet type consequences, W , can be determined as follows:
pool
n
jet
n
W = rate ⋅ frac
(3.100)
pool
n n ro
( 1 )
W = rate − frac
(3.101)
jet
n n ro