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FFP: Fitness For Purpose
"Repair or replace?" A short time ago only these were the only two possibilities available for a defect pressurized component. 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 quantifying 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 defect component can be predicted quantitatively.
KEMA has developed its 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 size defects with an accuracy of 1 mm. In addition, KEMA has experience in finite element analysis and assessment of creep crack growth by using C*-integral. Following the commonly accepted standards, such as R5 report and PD 6539:1994, KEMA has delivered numbers of services on FFP assessment of defective components at high temperature quantitatively.
For more information, please contact us.
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