## Fitness-for-Service Analysis Procedure – Determine Acceptability of Flaw

1. When the flaw is not subjected to significant pressure or temperature cycles (greater than 20% of design pressure or 50°F), compare the stress intensity at the flaw (KI per C above) with the critical stress intensity for fracture initiation (KIC per D above) at ambient and operating temperatures.

a. The flaw is acceptable for operation if KI is less than KIC at the operating temperature. (It is recommended that the flaw size determined in this manner should be reduced by ½ to provide a safety factor of 2 for acceptable crack size.)

The flaw is unacceptable for continued operation if KI exceeds KIC at the operating temperature. The flaw must be repaired, or the operating pressure downrated until KI is less than KIC.

b. The flaw is acceptable for startup and shutdown at full operating pressure if KI is less than KIC at ambient temperature (i.e., atmospheric temperature during startup or shutdown).

The flaw is unacceptable for startup and shutdown at full operating pressure if KI exceeds KIC at ambient temperature. Pressure during startup and shutdown will have to be limited such that KI is lower than KIC, or the temperature will have to be increased before full operating pressure is attained.

2. If the flaw is subjected to significant pressure or temperature cycles, determine the extent of crack growth by fatigue until the next scheduled shutdown.

a. Estimate the number of pressure or temperature cycles until the next scheduled shutdown. (It is recommended to double the number of anticipated cycles to obtain a safety factor of 2.)

b. Determine the extent of crack growth for the number of anticipated cycles for the stress intensity at the flaw (KI per C) using the fatigue crack growth properties (per D). (It is recommended to double the crack growth/cycle to obtain a safety factor of 2 for fatigue crack growth rate.)

c. The flaw is acceptable for continued operation if it will not grow to a size such that KI exceeds KIC prior to the next scheduled shutdown. (It is not anticipated that the crack will grow to a size that will require repair at the next scheduled shutdown because of the dual safety factors recommended for anticipated cycles and crack growth rate. Therefore, this should be considered an indefinite postponement of repair.)

The flaw is unacceptable for continued operation if it will grow to a size such that KI exceeds KIC prior to the next scheduled shutdown. The crack should be repaired, or the operating pressure downrated such that the crack will not grow by fatigue to a size where KI exceeds KIC.

Categories: In-Service Inspection | Leave a comment