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FFP: Fitness For Purpose
"Repair or replace?" A short time ago only these two
possibilities for a defected pressurized component were available.
Actually, in many cases a small defect is allowable, if the
detected defect is smaller than the critical size determined by
fracture mechanics. The combination of modern NDT technology,
material science and fracture mechanics is the so-called
Fitness-for-Purpose (FFP) approach.
Failure by cracking may further be divided into three stages:
crack initiation, crack propagation and final failure when the
crack reaches a critical size. Life assessments are, therefore,
aimed at quantifing incipient damage prior to crack initiation, the
crack growth rate, and the critical crack size to failure. The last
two stages of life assessment are the main activities of the FFP
approach.
In power plants and process installations components are
subjected to high temperature, pressure, rotation, vibration and/or
corrosion conditions. Eventual failure can occur either by leakage
or rupture at the operating temperature or by rapid brittle
fracture at lower temperature during start-up/shutdown transients.
The main tasks of the FFP approach are to determine the crack
growth rate and the critical crack size under operating conditions,
so that residual lifetime of a defected component can be predicted
quantitatively.
KEMA has developed his own philosophy and flow chart of life
management. KEMA also developed modern NDT techniques, such as the
Time of Flight Diffraction (TOFD) system which can detect and
sizing defects with accuracy of 1 mm. In addition, KEMA has
experiences in finite element analysis and assessment of creep
crack growth by using C*-integral. Following the common accepted
standards, such as R5 report and PD 6539:1994, KEMA has delivered
numbers of services on FFP assessment of defected components at
high temperature quantitatively.
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