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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-81
e) STEP 7.5—For each hole size selected, calculate the release rate of vapor (including entrained liquid
remaining in the jet, W ), using Equation (3.101).
jet
n
f) STEP 7.6—Calculate the mass fraction of entrained liquid, frac
entl
, within the jet portion of the release
using Equation (3.102).
g) STEP 7.7—Determine the vapor source rate and source area for the vapor cloud and flash fire dispersion
analysis.
1) For vapor releases, use the jet release rate, W , established in STEP 7.5.
jet
n
2) For liquid releases, determine whether the pool is boiling or non-boiling in accordance with Section
5.7.4. For boiling pools, calculate the evaporation rate, erate
n
, and the pool radius, r , using
pn ,
Equations (3.103) and (3.104). For non-boiling pools, calculate the evaporation rate, erate
n
, and the
pool radius, r , using Equation (3.105).
pn ,
5.8 Determine Flammable and Explosive Consequences
5.8.1 Event Tree Calculations
5.8.1.1 Overview
Event tree analysis determines the probabilities of various outcomes as a result of release of hazardous fluids
to the atmosphere. These probabilities are then used to weight the overall consequences of release.
The CCPS [14] defines an event tree as “a graphical logic model that identifies and quantifies possible outcomes
following an initiating event. The event tree provides systematic coverage of the time sequence of event
propagation, either through a series of protective system actions, normal plant functions and operator
interventions (a preincident application), or where loss of containment has occurred, through the range of
consequences possible (a postincident application).”
An overall event tree is presented in Figure 5.2. The COF portion fits within the overall methodology as shown
in Figure 5.2. POF (POL for leakage or POR for rupture) is a function of the GFFs for particular pieces of
equipment and the calculated damage state (DFs) of the piece of equipment or component being evaluated.
The determination of the POF is covered in Part 2 of this document.
The POF is then multiplied by the event probabilities as determined from the consequence analysis. Similar to
trees employed by the CCPS [14] to evaluate consequences of releases in process units, the event trees
presented in Figure 5.2 through Figure 5.5 display the potential outcomes that could occur from the initiating
event (a release). The event tree for the leakage cases, which corresponds to the small, medium, and large
release hole sizes as discussed in Section 4.2, is shown in Figure 5.3. The event tree for the rupture case is
shown in Figure 5.4.
5.8.1.2 Probability of Ignition Given a Release
For a release of a hazardous fluid, the two main factors that define the event outcome are the probability of
ignition and the timing of ignition, in other words, immediate vs delayed ignition.
A study by Cox, Lee, and Ang in 1990 [15] indicates that the probability that a flammable release will ignite is
proportional to the release rate of flammable material. Additional research on probabilities of ignition is provided
in Reference [16]. The curve fit for the Cox, Lee, and Ang work can be seen as the lowest curves in Figure
5.5, which applies to liquids, and Figure 5.6, which applies to vapors. The additional curves provided in these
figures are extrapolated to match the constant values assumed in the Level 1 consequence analysis provided