Fatigue Crack Repair

Mechanical and thermal fatigue cracks can be repaired by first removing by grinding, followed by weld build-up of the ground area to restore it to the minimum required thickness (see Section 800). The ground area should receive a magnetic particle examination (MT) or dye-penetrant examination (MT) to make certain that all of the fatigue crack has been removed. Furthermore, the remaining shell thickness in the ground area can be determined by ultrasonic examination (UT) from the opposite surface using a longitudinal wave procedure. Weld build-up of the ground area may not be necessary if, similar to local corrosion or pitting, the local primary membrane stress due to the reduced shell thickness meets the criteria in Appendix 4 of ASME Code, Section VIII, Division 2 (see Section 741).

Alternatively, the depth of the fatigue can be determined by ultrasonic examination (UT) using a shear wave procedure, and a fitness-for-service analysis can be made (see Section 750) to determine if the crack jeopardizes the integrity and reliability of the vessel. This may allow postponing repair until a shutdown can be scheduled or until replacement materials and parts can be obtained.

Whenever mechanical or thermal fatigue cracks are detected in a pressure vessel, the cause of the fatigue must be determined and eliminated by changes in design or operation. Unless this is done, it is likely that fatigue cracks will reappear in the same locations after the repairs have been made. Regardless of the actions taken to prevent recurrence of the fatigue cracks, all locations where they were detected should be re-examined during the next scheduled inspection. Postweld heat treatment can prevent the recurrence of corrosion fatigue cracks in deaerator vessels.

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