Information Required for Fitness-for-Service Analysis
It is essential to have the following information to perform properly a fitness-forservice analysis:
1. Accurate sizing data for cracks (or other flaws).
2. Toughness and fatigue data for the material of construction at ambient and operating temperatures.
3. Applied stress developed by the operating pressure at the location of the crack or other flaw.
Cracks or other flaws can be accurately sized by ultrasonic testing (UT), but the proper procedures must be used by experienced technicians. Shear wave UT procedures are generally used for sizing, and time-based techniques usually produce more accurate results than amplitude-based techniques. It may be necessary to verify the accuracy of the procedures and the performance of the technicians by constructing a “mock-up” with implanted defects that simulates the size and geometry of the pressure vessel component containing the crack. Ultrasonic imaging techniques are available that can provide a permanent visual record of the size and orientation of the crack, and they should be used to support the fitness-for-service analysis of critical equipment.
The toughness of the material of construction can be estimated from CV-impact data, but more accurate toughness data obtained directly from KIC, COD, or J-integral tests should be used whenever they are available. The fatigue data should show the crack growth rate as a function of the cyclic stress intensity at the tip of the growing crack. The toughness and fatigue data should also reflect the behavior of the material in the process environment. Especially important are environments where hydrogen charging can promote crack growth (including stress-corrosion cracking), and high operating temperature where creep can occur.
The applied stress developed by the operating pressure at the location of a crack or flaw must be accurately determined. The primary membrane and bending stresses used for design are usually sufficient for pressure vessel components with simple geometries. However, it may be necessary to perform a finite-element stress analysis to determine discontinuity and secondary, i.e., thermal, stresses for locations that have a complex geometry.
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