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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-15
Inventory Volume and Mass Available for Release
The amount of inventory in the storage tank available for release depends on the component being
evaluated. The available inventory is the entire contents of the storage tank for bottom components unless
the tank has an RPB.
Calculation of Storage Tank Bottom Inventory Mass
The amount of fluid available for release through storage tank bottoms is the fluid level up to the storage tank
design fill height or the operating fill height.
a) STEP 10.1 – Calculate liquid volume in the storage tank in m 3 (ft 3 ) using Equation (5.40).
Lvol
total
2
⎛π
D ⎞
tank
= ⎜ ⎟⋅
h
⎝ 4 ⎠
liq
b) STEP 10.2 – Calculate the total storage tank volume in barrels using Equation (5.41).
total
total
13
(5.40)
Bbl = Lvol ⋅ C
(5.41)
c) STEP 10.3 – Calculate the storage tank mass using Equation (5.42).
masstotal = Lvoltotal ⋅ ρl
(5.42)
Type of Release
The type of release for the storage tank bottom is assumed to be continuous.
Impact of Detection and Isolation Systems on Release Magnitude
Detection and isolation systems are not accounted for in the storage tank consequence analysis.
Release Rate and Volume for the Consequence of Failure Analysis
The release for the storage tank is assumed to be continuous, and the release rate is calculated from
Equation (5.43) where W is determined in STEP 9.4.
n
rate
n
= W
(5.43)
n
Storage Tank Bottom Release Volume
A step-by-step procedure for determining the release rate and volume is as follows:
a) STEP 11.1 – Determine the release rate, rate
n
, for each release hole size using Equation (5.43) where
the release rate, W
n
, is from STEP 9.4.
b) STEP 11.2 – Determine the leak detection time, t ld
, as follows:
1) t = 7 days for a storage tank on a concrete or asphalt foundation, or
ld
2) t = 30 days for a storage tank with an RPB, or
ld
3) t = 360 days for a storage tank without an RPB.
ld
c) STEP 11.3 – Calculate the leak duration, ld
n
, for each release hole size using Equation (5.44) based on
the release rate, rate
n
, from STEP 11.1, the leak detection time, t ld
, from STEP 11.2, and the total
volume, Bbl , from STEP 10.2
total
ld
n
⎡⎧Bbl
⎫
total
= min ⎢⎨ ⎬,
⎣⎩
raten
⎭
t
ld
⎤
⎥
⎦
(5.44)