Many of the forms of deterioration that occur in pressure vessels are characterized by the formation of cracks, at least in their terminal stages before failure actually occurs. Analytical methods are available to evaluate the effect of these cracks on the integrity and reliability of the vessel if it is returned to service. These methods are commonly referred to as fitness-for-service analyses, because they can predict the effect that flaws will have on the performance of a vessel under actual service conditions. Employing fitness-for-service analyses can have significant benefit by:
1. Avoiding the expense of unnecessary repairs.
2. Allowing the vessel to be returned to service until repairs or replacement can be scheduled without interrupting production.
3. Planning in-service inspection programs that will prevent the failure of vessels during operation resulting in unscheduled shutdowns.
Guidelines for performing a fitness-for-service analysis of a pressure vessel are given in this section.
The inspection of pressure vessels during a shutdown occasionally reveals the presence of flaws that exceed the acceptance limits of ASME Code Section VIII and the Company specification that may have been used for the original design and construction. The flaws most commonly detected are corrosion and indications of cracks. Detection of flaws (including cracks) that exceed the ASME Code acceptance limits does not necessarily indicate that a pressure vessel is unfit for continued operation within its original design conditions. Fitness-for-service analyses can determine if pressure vessels with flaws developed during operation can be returned to service without significant risk of failure.
An experienced pressure vessel engineer should be consulted for making a fitnessfor-service analysis. This engineer should have the following qualifications:
1. Appropriate experience concerning the design, operation, inspection, and repair of pressure vessels.
2. Familiarity with the effects that the process environment can have on the behavior of the materials of construction.
3. Knowledge of fracture mechanics.
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