In-service inspection refers to inspection after new equipment is placed in service, although these inspections are usually made during scheduled shutdowns. An inservice inspection program will significantly improve reliability and safety of operation, and increase productivity and profitability by preventing unscheduled shutdowns for emergency repairs.
Pressure vessels can deteriorate during normal operation and periods of process upset due to:
1. Internal and external corrosion.
2. Thermal aging of material.
3. Mechanical and thermal fatigue.
4. Stress corrosion (environmental) cracking.
5. Internal erosion.
6. Hydrogen attack.
7. Hydrogen blistering.
8. Creep and stress-rupture.
Materials for the construction of pressure vessels are selected to minimize deterioration during service (see Section 500), but their actual performance is not always exactly as predicted. Consequently, a well planned in-service inspection program is necessary to assure reliable and safe operation.
All of the above forms of deterioration will not necessarily occur with each vessel. The inspection program for a vessel should be established to detect only those forms of deterioration that may occur during normal operation or process upsets. The specific forms of deterioration that the in-service inspection program covers should be determined by experienced pressure vessel and materials engineers, with regard to the process environment and service conditions.
Nondestructive examination (NDE) techniques can detect most forms of deterioration, and these techniques can be carried out at an early stage to permit continued operation until repair or replacement can be scheduled. Therefore, in-service inspection will prevent most equipment failures that cause unscheduled shutdowns for repairs and interrupt production.
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