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

5-14 API RECOMMENDED PRACTICE 581
Release Rate Calculation
Release rate calculations are provided for a leak in a storage tank bottom plate. The liquid head is assumed
to be constant in time, and the leak is into the ground that is modeled as a continuous porous media
approximated by soil properties typically used for storage tank foundations.
Storage Tank Bottom Release Rate
The product leakage flow rate through a small hole in the storage tank bottom is a function of the soil and
fluid properties as well as the liquid head (fill height) above the bottom. The flow rate equations can be found
in Rowe [3]. The flow rate through a storage tank bottom into a porous media is calculated using the
Bernoulli in Equation (5.37), Giroud in Equation (5.38), or Equation (5.39) based on the hydraulic
conductivity, k , and release hole size, d
h,
prod
n
.
W = C ⋅π
⋅d 2 ⋅g ⋅h ⋅ n for k > C ⋅ d
(5.37)
2 2
n 33 n liq rh, n h, prod 34 n
1
⎡
1.8 0.74
0.2 0.9 0.74
d ⎤
n
n
=
35
⋅
qo
⋅
n
⋅
liq
⋅
h, prod
⋅
rh, n h, prod
≤
37
⋅⎢ 0.4 ⎥
Cqo
⋅ hliq
W C C d h k n for k C
n
m
⎛C 39 + 2⋅log( dn
) −log( k , ) ⎞
h prod
2⋅ log( dn
) + 0.5⋅log( hliq
) −0.74⋅ ⎜ m
⎟
⎝
⎠
38
10
rh,
n
W = C ⋅ ⋅n for all other cases
Where m = C −0.4324⋅ log( d ) + 0.5405⋅log( h )
40
n
liq
⎢⎣
⎥⎦
(5.38)
(5.39)
In Equation (5.38), the parameter C is an adjustment factor for degree of contact with soil and ranges from
qo
C
qo
= 0.21 for good contact to C
qo
= 1.15 for poor contact. A value of C
qo
= 0.21 is recommended in the
consequence analysis.
If the storage tank bottom has a release prevention barrier (RBP), then the liquid height, h , to be used in
liq
the flow rate calculations is set to .0762 m (0.25 ft). If the storage tank does not have a release prevention
barrier, the liquid height, h , to be used in the flow rate calculations is the actual height of the stored
liq
product.
The number of release holes, n , for each release hole size is a function of the storage tank diameter and
rh,
n
is shown in Table 4.9.
Calculation for Storage Tank Bottom Release Hole Size
a) STEP 9.1 – For each release hole size, determine the number of release holes, n , from Table 4.4.
rh,
n
a. STEP 9.2 – Determine the hole area, A
n
, for each hole size from STEP 8.1 using Equation (5.6).
b) STEP 9.3 – Determine the hydraulic conductivity of the stored liquid, k , from STEP 1.4.
h,
prod
c) STEP 9.4 – For each release hole size, determine the flow rate, W
n
, using Equation (5.37), Equation
(5.38), or Equation (5.39), as applicable. The liquid height, h , to use in this calculation is determined
liq
as follows:
1) The storage tank has an RPB: h = 0.0762 m [0.25 ft]
2) The storage tank does not have an RPB: hliq
liq
= Actual Product Height