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

3-10 API RECOMMENDED PRACTICE 581
4.1.4 Estimation of Ideal Gas Specific Heat Capacity Ratio
If the value of the ideal gas specific heat capacity ratio is unknown, an estimate can be made provided a value
of the constant pressure specific heat capacity, C , is available, using Equation (3.1).
p
C
p
k =
C − R
p
(3.1)
The constant specific heat capacity,
4.1.5 Flammable Fluid Types
C , may be evaluated using the equations provided in Table 4.2.
p
In the initial development of the Level 1 methodology, the results of the consequence analysis were correlated
with equations and presented in lookup tables. As the consequence area results were reviewed, it became
clear that adjustments had to be made to smooth out the results at the instantaneous to continuous release
transition and as the operating temperature approached the autoignition for each fluid (see Section 4.8.5 and
Section 4.8.6).
a) TYPE 0 Fluids—For the initial set of reference fluids, the consequence area equations were not smoothed
and there remained step changes in the equations. Instead, adjustment factors and blending factors were
applied to the consequence area equations. These initial fluids are designated here as TYPE 0 and can
be found in Table 4.1.
b) TYPE 1 Fluids—Instantaneous to continuous blending was performed during development of subsequent
reference fluids and the resulting consequence area equations accounted for the adjustments. As a result,
instantaneous to continuous blending factors need not be applied to TYPE 1 fluids.
4.1.6 Release Phase
The dispersion characteristics of a fluid and the probability of consequence outcomes (events) after release
are strongly dependent on the phase (i.e. gas, liquid, or two-phase) of the fluid after it is released into the
environment. Most releases from pressurized units are two-phase releases, especially if the fluid is viscous or
has a tendency to foam. Released fluids operating under pressure above their boiling points will flash and
produce a two-phase release. Guidelines for determining the phase of the released fluid when using the Level
1 consequence analysis are provided in Table 4.3, if more sophisticated methods are not available.
Consultation with process or operations personnel is appropriate in this determination. For steam, the release
phase is gas/vapor. For the representative fluid, Acid/Caustic, the release phase is always liquid (see Section
4.1.3).
Where more rigorous calculations are desired in order to include the effect of two-phase flashing releases as
described in Section 5.3.4, a Level 2 consequence analysis should be performed.
4.1.7 Calculation of Release Phase
a) STEP 1.1—Select a representative fluid group from Table 4.1.
b) STEP 1.2—Determine the stored fluid phase: liquid or vapor. If stored fluid is two-phase, use the
conservative assumption of liquid. Alternatively, a Level 2 consequence analysis can be performed.
c) STEP 1.3—Determine the stored fluid properties.
1) For a stored liquid:
— stored liquid density, ρ
l
[kg/m 3 (lb/ft 3 )], can be estimated from Table 4.2;