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

3-76 API RECOMMENDED PRACTICE 581
5.3 Release Rate Calculation
5.3.1 Source Term Modeling
Quantification of the consequence of a release event requires calculations of the release amount (or rate of
release), the duration of the release, and the state (e.g. gas, liquid or two-phase) of the material released. The
terminology used for determining these parameters is source term modeling. The source term is used as an
input to the various consequence models as well as the cloud dispersion analysis.
5.3.2 Determining the Release Phase
Estimation of the release amount or rate is covered for liquids and vapors (gases) in Section 4.3. For calculating
the release rate, the release phase must be determined. Note that the release phase is different than the phase
of the fluid at storage conditions or the phase of the fluid after flashing to atmosphere as described in Section
5.1.2 and Section 5.1.3. This is the phase immediately downstream of the release point and is used for
selecting the proper equation for calculating the release rate through the hole or crack opening.
To determine the release phase, the saturation pressure of the stored fluid at the storage temperature,
, must be determined.
if Psats ≥ Ps ≥ Patm
⇒ release phase is vapor
(3.95)
if Ps ≥ Psats > Patm
⇒ release phase is two-phase
(3.96)
if Ps ≥ Patm > Psats
⇒ release phase is liquid
(3.97)
5.3.3 Vapor Release Source
Psats
As shown in Equation (3.95), if the saturation pressure of the fluid at storage temperature, Psat
s
, is greater
than or equal to the storage pressure, P
s
, the fluid will be stored as a gas or vapor and released as a gas or
vapor. In this case, calculation of the theoretical release rate, W
n
, can be in accordance with Equation (3.6)
or Equation (3.7). Most gases will cool as they are depressured through an orifice, so in some cases,
condensation will occur and liquid rainout needs to be considered as presented in Section 5.7.2.
For supercritical fluids (stored above critical pressure or temperature), the release rate can be estimated using
Equation (3.6); however, in this case the specific heat ratio, k , should be evaluated at the NBP of the fluid
mixture or at standard conditions. This will result in a conservative release rate. More rigorous methods, such
as the HEM Omega [4] method, can be used to calculate the release rate of a supercritical fluid. In some cases,
supercritical fluids will condense upon release, and liquid rainout needs to be considered as presented in
Section 5.7.2.
5.3.4 Two-phase Release Source
As shown in Equation (3.96), if the saturation pressure of the fluid at the storage temperature, Psat
s
, is less
than or equal to the storage pressure, P
s
, but greater than atmospheric pressure, P
atm
, the fluid will be stored
as a liquid and will be released as a two-phase mixture. In this case, the release rate can be conservatively
estimated using the liquid Equation (3.3). Alternatively, a more accurate two-phase flow calculation may be
used. For this case, the effect of liquid entrainment in the released jet needs to be considered as well as
rainout. Methods for evaluating these effects are presented in Section 5.7.2.