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

5.A-4 API RECOMMENDED PRACTICE 581
η
η
1
N
⎛1
β
β ⎞
mod
= ⎜ ∑ti,
dur
r ⎟
i=
1
mod
⎝
⎠
1
2.568 2.568 2.568 2.568
( 22) + ( 18) + ( 16)
⎡
⎤
= ⎢
⎥ = 22.16 years
⎢⎣
2
⎥⎦
(5.A.4)
The characteristic life for the specific bundle experience is higher than the 20.45-year characteristic life
calculated using the matching bundles from the reliability library (Figure 5.A.1).
It should be noted that this method assumes that the operating conditions for the bundle have not changed
for the time period being evaluated and has not been redesigned. Changes in metallurgy, process
conditions, or bundle design should be considered to determine if all the past bundle history is representative
of the current bundle under evaluation.
5.A.7
Nomenclature
The following lists the nomenclature used in Section 5.A.1.
β
η
is the Weibull shape parameter that represents the slope of the line on a POF vs. time plot
is the Weibull characteristic life parameter that represents the time at which 62.3 % of the
bundles are expected to fail, years
tube
P
f
is the probability of the bundle failure, failures/year
pve % is a goodness of fit test method for the data
r
t
2
is a goodness of fit test method for the data
is time, years