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

5-32 API RECOMMENDED PRACTICE 581
tube
The adjusted characteristic life, and adjusted POF, P ,of the bundle is calculated using
f , adj
mod
from
Equation (5.63) using Equation (5.65).
⎡
tube
⎛ t ⎞
Pf , adj
= 1−exp
⎢−⎜ ⎟
⎢ η
⎣ ⎝ mod ⎠
5.5.3.3.3 Effects of Bundle Life Extension Efforts
β
⎤
⎥
⎥⎦
η
(5.65)
Minor repairs and cleaning operations performed on bundles prior to inspection do not affect the life of the
bundle. However, there are life extension methods that are often implemented during shutdowns that return
the bundle to service in an improved condition. An adjustment is made to the inspection interval based on
Table 4.2 for life extension methods and by determining a life extension factor, LEF . The adjusted service
duration, tadj , is calculated with the LEF using Equation (5.66).
dur
dur
( 1 )
tadj = + LEF ⋅t
dur
(5.66)
Note the actual service duration, t dur
, is the time period in years between the bundle installation date and
the inspection date that the life extension method was performed, as shown in Equation (5.67).
tdur
= Inspect Date − Install Date
(5.67)
The Effective Installation Date, Bundle Installation Date , is calculated using tadj
adj
dur
, as shown in
Equation (5.68).
Bundle Installation Date = Inspect Date − tadj
(5.68)
adj
Consequence of Failure
Bundle failure is defined as a tube leak. Financial consequence of failure (COF) is determined based on the
bundle criticality which includes costs associated with lost opportunity due to production downtime,
environmental impact costs, and costs associated with maintenance and replacement of the bundle. The
consequence of an unplanned shutdown due to a bundle tube leak is determined using Equation (5.69).
tube
Ratered
C ⎛
⎞
= ⎜Unit ⋅ ⋅D ⎟⋅ Outage + Cost + ( Cost ⋅ matcost)
+ Cost
⎝ 100 ⎠
f prod sd mult env bundle maint
dur
(5.69)
Where
Dsd
is the time in days for a planned or unplanned shutdown and matcost factor is from Table 4.3.
Risk Analysis
Risk over time is calculated to determine what inspection is required to manage risk. Uncertainty exists when
relevant, credible data is lacking. More relevant data reduces the amount of uncertainty in the risk
calculation. Information from inspection is often needed to improve confidence in the damage states and
damage rates associated with bundles. Risk for bundles is a function of time is the product of the POF and
the COF in financial terms, as shown in Equation (5.70).
Risk = P ⋅ C
(5.70)
tube tube tube
f f f
Risk Matrix
A risk matrix is a valuable visual tool for identifying high risk bundles. The risk of each bundle is
characterized by the POF and COF categories, shown in Part 1, Section 4.3.2.2 and enables each bundle to
be plotted on the risk matrix as shown in Part 1, Figure 4.2 and Figure 4.3.