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

RISK-BASED INSPECTION METHODOLOGY, PART 5—SPECIAL EQUIPMENT 5-39
tube
Risk
f
is the risk of failure of the tube bundle, $/year
( )
tube
Risk
f
tr
n
is the risk of failure of the tube bundle at a planned bundle replacement frequency,
n
Rate
red
is the production rate reduction on a unit as a result of a bundle being out of service, %
Risk is the risk target, $/year
tgt
ROR
RWT
f
t
t
1
t
2
is the fractional rate of return or hurdle rate
is the failure point defined as a fraction of remaining wall thickness
is time, years
tr , $/year
is the service duration of the bundle at the upcoming turnaround (Turnaround Date1), years
is the service duration of the bundle at the subsequent turnaround (Turnaround Date2), years
t
dur
is the bundle duration or time in service, years
t β
dur,
i
is the time in service for the i th
bundle in a heat exchanger, years
t is the inspection interval, years
insp
t is the time from the bundle installation date to the plan date, years
plan
t
rate
is the thinning rate for the tube bundle, mm/year (inch/year)
t rate1
t rate2
t rate3
is the corrosion rate for damage state 1, mm/year (inch/year)
is the corrosion rate for damage state 2, mm/year (inch/year)
is the corrosion rate for damage state 3, mm/year (inch/year)
t rate , adj
t
s
is the probability adjusted corrosion rate
is the time step used in the optimization routine for bundle replacement frequency, days
tadj
dur
is the bundle duration or time in service adjusted for life extension activities, years
tr
n
is the bundle planned replacement frequency, year
t is the average measured tube wall thickness, mm (inch)
insp
t is the average furnished tube wall thickness, mm (inch)
orig
Unit is the daily production margin on the unit, $/day
prod