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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-31
Where trate 1,
trate2 and t
rate3
are the thinning states based on the measured corrosion rate from inspection
Bundle Bundle Bundle
D , D and D are the probabilities Part 2, Table 4.5.
and
1 2 3
The calculated bundle life,
PBL , is adjusted for inspection using Equation (5.60).
adj
PBL
adj
RWT
⋅ t
f orig
= (5.60)
t
rate,
adj
Where the failure point is defined as a fraction of remaining wall thickness,
3) Calculated Failure Data Based on Estimated Remaining Life
RWT .
The estimated remaining life, ERL , of the bundle is used to calculate bundle life if tube wall thickness data is
not available for calculation of a bundle tube thinning rate, or when the damage mechanism is not general
corrosion. The ERL is calculated using inspection data combined with accepted Fitness-For-Service
calculations based on the damage mechanism known or anticipated and the time in service, t
dur .
PBL = t + ERL
(5.61)
adj
dur
5.5.3.3.2 Adjustment to Failure Rate Based on Condition of Bundle
A bundle with a recommended two or more life cycles with inspection data is used to calculate a β
parameter for the matching bundle criteria (default to 3.0 if unknown) with a Weibayes analysis. The η
parameter is calculated using Equation (5.62).
1
β
N β
⎛ t ⎞
dur,
i
η = ⎜∑ (5.62)
i=
1 r ⎟
⎝ ⎠
Where N is the number of past bundles, t is the time in service for each bundle in years, r is the
dur,
i
number of failed bundles, and β is the Weibull slope parameter. This method assumes that the current
operating conditions for the bundle have not changed including changes in metallurgy, process conditions, or
bundle design. POF is calculated using Equation (5.55) for β and η .
η
A modified characteristic life,
mod
, for the bundle is calculated using Equation (5.63) if the bundle life is
calculated based on the last inspection using Equation (5.60) or Equation (5.63).
f
η
mod
1
⎛1 N β
β ⎞
= ⎜ ti,
dur
r ⎟
i=
1
⎝
∑ (5.63)
⎠
Where N is the number of past bundles, t
dur,
i is the time in service for each bundle in years, r is the
number of failed bundles, and β is the Weibull slope parameter. Note that if the bundle was replaced before
failure, a factor of 1.25 is applied to the service duration. This method assumes that the current operating
conditions for the bundle have not changed including changes in metallurgy, process conditions, or bundle
design. POF is calculated using Equation (5.55) for β andη .
The recommended inspection interval at the target POF for the bundle is calculated using Equation (5.64):
( ) 1
tube
ln 1
β
tinsp = ηmod ⋅ − ⎡
⎣ −P ⎤
f,tgt ⎦
(5.64)
mod