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

3-14 API RECOMMENDED PRACTICE 581
mass
inv
N
= ∑ mass
(3.9)
i=
1
comp,
i
b) Component Mass—It is assumed that for large leaks, operator intervention will occur within 3 minutes,
thereby limiting the amount of released material (see Annex 3.A for additional background). Therefore,
the amount of available mass for the release is limited to the mass of the component plus an additional
mass, mass , that is calculated based on 3 minutes of leakage from the component’s inventory group.
add , n
This additional mass is calculated assuming the same flow rate from the leaking component but is limited
to a 203 mm (8 in.) release hole size. The additional mass is calculated for each release hole size
add , n
using Equation (3.10).
[ ]
mass = 180⋅ min W , W
(3.10)
add , n n max8
In Equation (3.10), the maximum flow rate, W
max8
, to be added to the release from the surrounding
components, W , [limited by a 203 mm (8 in.) diameter leak] is calculated using Equations (3.3), (3.6),
max8
or (3.7), as applicable, with the hole area,
A
n = 32,450 mm 2 (50.3 in. 2 ).
The maximum mass available, mass
avail,
n , for release is calculated using Equation (3.11).
{ }
mass = min ⎡
⎣
mass + mass , mass
avail, n comp add , n inv
⎤
⎦
(3.11)
Plant detection, isolation, and mitigation techniques, as described in Section 4.6, will limit the duration of the
release such that the actual mass released to atmosphere can be significantly less than the available mass as
determined above.
Further guidance on the basis of the above methodology for calculating the available mass and the inventory
grouping is provided in Annex 3.A.
4.4.3 Calculation of Inventory Mass
a) STEP 4.1—Group components and equipment items into inventory groups (see Annex 3.A).
b) STEP 4.2—Calculate the fluid mass, mass
comp
, in the component being evaluated.
c) STEP 4.3—Calculate the fluid mass in each of the other components that is included in the inventory
group, mass .
comp,
i
d) STEP 4.4—Calculate the fluid mass in the inventory group, mass
inv , using Equation (3.9).
e) STEP 4.5—Calculate the flow rate from a 203 mm (8 in.) diameter hole, W
max8 , using Equations (3.3),
(3.6), or (3.7), as applicable, with An
= A8 = 32,450 mm 2 (50.3 in. 2 ). This is the maximum flow rate that can
be added to the equipment fluid mass from the surrounding equipment in the inventory group.
f) STEP 4.6—For each release hole size, calculate the added fluid mass, mass , resulting from 3
add , n
minutes of flow from the inventory group using Equation (3.10), where W
n is the leakage rate for the
release hole size being evaluated and W
max8 is from STEP 4.5.