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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-13
The fundamental soil property required in the analysis is the soil hydraulic conductivity, k
h
. The hydraulic
conductivity as a function of soil type is provided in Table 4.7 based on water. The hydraulic conductivity for
other fluids can be estimated based on the hydraulic conductivity, density, and dynamic viscosity of water,
denoted as k , ρ
h,
water w
, and µ
w
, respectively, and the density and dynamic viscosity of the actual fluid
using Equation (5.33).
l w
h, prod
= kh,
water ⎜ ⎟⎜ ⎟
ρ
w
µ
l
k
⎛ ρ ⎞⎛ µ ⎞
⎝ ⎠⎝ ⎠
Fluid Seepage Velocity for Storage Tank Bottom
(5.33)
The seepage velocity of the fluid in the storage tank bottom or product through the soil is given by Equation
(5.34) where k
h
is the soil hydraulic conductivity and p
s
is the soil porosity.
vel
s,
prod
k
h,
prod
= (5.34)
ps
Calculation of Fluid Seepage Velocity for Storage Tank Bottom
1) STEP 7.1 – Determine properties including density, ρ
l
, and dynamic viscosity, µ
l
, of the stored fluid. If a
Level 1 analysis is being performed, select the representative fluid properties from Table 4.5.
2) STEP 7.2 – Calculate the hydraulic conductivity for water by averaging the upper and lower bound
hydraulic conductivities provided in Table 4.7 for the soil type selected using Equation (5.35).
k
h, water 31
( kh, water−lb
+ kh,
water−ub
)
= C
(5.35)
2
3) STEP 7.3 – Calculate the fluid hydraulic conductivity, k , for the fluid stored in the storage tank using
h,
prod
Equation (5.33) based on the density,
conductivity for water, k , from STEP 7.2.
h,
water
ρ
l
, and dynamic viscosity,
µ
l
, from STEP 7.1 and the hydraulic
4) STEP 7.4 – Calculate the product seepage velocity, vel , for the fluid stored in the storage tank
s,
prod
using Equation (5.34) based on fluid hydraulic conductivity, k , from STEP 7.3 and the soil porosity
h,
prod
provided in Table 4.7.
Release Hole Size Selection
A discrete set of release events or release hole sizes are used for consequence analysis as outlined in Table
4.8.
Calculation of Release Hole Sizes
The following procedure may be used to determine the release hole size and the associated generic failure
frequencies.
a) STEP 8.1 – Determine the release hole size, d
n
, from Table 4.8 for storage tank bottoms.
b) STEP 8.2 – Determine the generic failure frequency, gff
n
, for the d
n
release hole size and the total
generic failure frequency from Part 2, Table 3.1 or from Equation (5.36).
gff
tot
4
= ∑ gff
(5.36)
n=
1
n