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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-17
4.7.1 Continuous Release Rate
For continuous releases, the release is modeled as a steady state plume; therefore, the release rate (units are
lb/s) is used as the input to the consequence analysis. The release rate that is used in the analysis is the
theoretical release as discussed in Section 4.3, adjusted for the presence of unit detection and isolations as
discussed in Section 4.6 [see Equation (3.12)].
( 1 )
rate = W − fact
(3.12)
n n di
4.7.2 Instantaneous Release Mass
For transient instantaneous puff releases, the release mass is required to perform the analysis. The available
release mass as determined in Section 4.4.2 for each release hole size, mass , is used to determine upper
avail,
n
bound release mass, mass
n , as shown in Equation (3.13).
⎡⎣{ } ,
⎤⎦ (3.13)
mass = min rate ⋅ld , mass
n n n avail n
In this equation, the leak duration, ld , cannot exceed the maximum duration
n
ld
max,n , established in Section
4.6.4 based on the detection and isolation systems present. Equation (3.14) can be used to calculate the actual
duration of the release,
ld .
n
ld
n
⎡⎧massavail,
n ⎫
= min ⎨ ⎬, 60⋅
⎢⎣⎩
raten
⎭
{ ldmax,
n}
⎤
⎥⎦
(3.14)
4.7.3 Calculation of Release Rate and Mass
a) STEP 7.1—For each release hole size, calculate the adjusted release rate, rate
n , using Equation (3.12),
where the theoretical release rate, W
n , is from STEP 3.2. Note that the release reduction factor, fact
di ,
determined in STEP 6.4 accounts for any detection and isolation systems that are present.
b) STEP 7.2—For each release hole size, calculate the leak duration, ld
n , of the release using Equation
rate ,
(3.14), based on the available mass, mass , from STEP 4.6 and the adjusted release rate,
avail,
n
n
from STEP 7.1. Note that the leak duration cannot exceed the maximum duration, ld , determined in
STEP 6.5.
c) STEP 7.3—For each release hole size, calculate the upper bound release mass, mass
n , using Equation
(3.13) based on the release rate, rate
n , from STEP 3.2, the leak duration, ld
n , from STEP 7.2, and the
available mass, mass , from STEP 4.6.
avail,
n
4.8 Determine Flammable and Explosive Consequence
4.8.1 Overview
Equations to calculate flammable and explosive consequence have been developed for the representative
fluids presented in Table 4.1. Consequence areas are estimated from a set of equations using release rate
(for continuous releases) or release mass (for instantaneous releases) as input. Technical background
information pertaining to the development of the empirical equations for the flammable consequence areas is
provided in Annex 3.A. An assumption is made that the probability of ignition for a continuous release is
constant and is a function of the material released and whether or not the fluid is at or above its AIT. The
probability does not increase as a function of release rate. For an instantaneous release, the probability of
max,
n