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

3.A-2 API RECOMMENDED PRACTICE 581
It is important to note that the flammable consequence results are not highly sensitive to the exact material
selected, provided the molecular weights are similar, because air dispersion properties and heats of
combustion are similar for all hydrocarbons with similar molecular weights. This is particularly true for straight
chain alkanes, but becomes less true as the materials become less saturated or aromatic. Therefore, caution
should be used when applying the Level 1 consequence analysis table lookups to materials (such as
aromatics, chlorinated hydrocarbons, etc.) not explicitly defined in the representative fluid groups of Table
3.A.3.1. In such cases, it is recommended that a Level 2 consequence analysis be performed using a fluid
property solver to determine the consequences of release.
3.A.3.1.2.2
Example
As an example of determining the various properties of mixtures, by applying Equation (3.A.1), a material
containing 10 mol% C3, 20 mol% C4, 30 mol% C5, 30 mol% C6, and 10 mol% C7 would have the following
average key properties:
a) MW = 74.8;
b) AIT = 322.1 °C (629.8 °F) ;
c) NBP = 39.2 °C (102.6 °F);
d) density = 621.5 kg/m 3 (38.8 lb/ft 3 ).
The best selection from the materials in the representative fluids list of Table 3.A.3.1 would be C5, since the
property of first importance is the NBP, and C5 has a NBP of 36 °C (97 °F), which is lower than the calculated
NBP of our example mixture. It is non-conservative to select a representative fluid with a higher NBP than
the fluid being considered, e.g. C6-C8 when modeling a fluid with a weighted NBP of 99 °C (210 °F).
3.A.3.1.2.3
Example with a Mixture
If a mixture contains inert materials such as CO2 or water, the choice of representative fluid should be based
on the flammable/toxic materials of concern, excluding these materials. This is a conservative assumption
that will result in higher COF results, but it is sufficient for risk prioritization. For example, if the material is
93 mol% water and 7 mol% C20, using C20 and the corresponding inventory of the hydrocarbon provides a
conservative COF. A Level 2 consequence methodology may be used to more accurately model the release.
3.A.3.1.2.4
Toxic Mixture
If the mixture contains toxic components and a toxic consequence analysis is required, a flammable
representative fluid is still required, even when the toxic component is a small fraction of the mixture. In this
situation, the representative fluid is selected, as described in Section 3.A.3.1.2.1 and Equation (3.A.1).
3.A.3.1.3
Fluid Properties
Representative fluid properties for the Level 1 consequence analysis are provided in Table 3.A.3.1. The
properties of fluids (or individual components of mixtures) can be found in standard chemical reference
books.
3.A.3.2 Release Hole Size Selection
3.A.3.2.1
Overview
Part 2 of this document defines release hole sizes that represent small, medium, large, and rupture cases for
various components or equipment types. This predefined set of release hole sizes are based on failure size
distributions observed in piping and pressure vessels. The range of release hole sizes were chosen to
address potential on-site and off-site consequences. For on-site effects, small and medium hole size cases
usually dominate the risk due to a higher likelihood and potential for on-site consequences.