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

RISK-BASED INSPECTION METHODOLOGY, PART 3—CONSEQUENCE OF FAILURE METHODOLOGY 3-75
For fluids that flash to two-phase, flash calculations at both the bubble point and the dew point of the flashed
mixture may be required depending on the composition of the fluid.
a) For pure fluids or binary mixtures (two components in mixture), additional calculations are not necessary
because in these cases the bubble point and dew point temperatures are the same and equal to the
isentropic flash temperature, i.e. Tb = Td = T .
f
b) For multi-component mixtures, both the bubble point and the dew point calculations are required.
5.1.4 Calculation of Fluid Properties
a) STEP 1.1—Obtain the stored fluid composition. For mixtures, concentrate on the major components within
the fluid mixture and attempt to get at least 90 % of the mixture identified and quantified. A more detailed
breakdown of the composition is not warranted, unless there are small quantities of toxic materials that
are in the mixture.
b) STEP 1.2—Using a fluid property solver, determine the fluid properties as specified in Section 5.1.2 a) for
the fluid at storage conditions. Research may be required to determine some of the fluid properties
required for the analysis, such as LFL, UFL, heat of combustion, and toxic limits. The analyst may need
to use MSDSs or other fluid databases, such as DIPPR [3] , to determine these properties. Mixing rules
(e.g. LaChatalier’s mixing principle for LFL and UFL) are available to determine properties of mixtures,
but in general a mole weighted method may be used as an estimate.
c) STEP 1.3—Using a fluid property solver, perform an isentropic flash (isenthalpic is acceptable) and
determine the flash temperature, T , the phase of the flashed fluid, and the fraction of fluid flashed,
f
frac .
fsh
d) STEP 1.4—Determine the bubble point or dew point temperature of the flashed fluid, as necessary.
1) For flashed liquid, determine the bubble point temperature, T
b
, at atmospheric pressure.
2) For flashed vapors, determine the dew point temperature, T
d
, at atmospheric pressure.
3) For fluids that flash to two-phase, the bubble point temperature, T
b
, at atmospheric pressure and the
dew point temperature, T d
, at atmospheric pressure should be determined. Note that for pure fluids
and binary mixtures, no calculation is required since the bubble point temperature and the dew point
temperature are equal to the flash temperature, T , as determined in STEP 1.3.
f
5.2 Release Hole Size Selection
5.2.1 General
As with the Level 1 approach, a discrete set of release events or release hole sizes are used, as shown in
Table 4.4.
5.2.2 Calculation of Release Hole Sizes
The step-by-step methodology for selecting the release hole sizes are in accordance with the Level 1
consequence analysis; see Section 4.2.2.