Fitness-for-Service Analysis of Cracks or Other Flaws
The concepts of fracture mechanics can be applied to determine if a pressure vessel containing a crack larger than permitted by the ASME Code for new construction is “fit-for-service” at its intended operating conditions. A critical crack size, required for failure of the vessel, can be determined for the material of construction at the vessel’s operating pressure (i.e., applied stress). Cracks smaller than this critical size will not cause a pressure vessel to fail when it is returned to service. However, the crack could grow by fatigue during operation if the vessel is subject to pressure or temperature fluctuations. Under these circumstances, the fatigue crack growth rate for the stress intensity at the crack tip must be calculated, to determine if the crack will grow to a critical size to cause failure of the vessel prior to the next scheduled shutdown when another inspection can be made.
The toughness of carbon and low-alloy pressure vessel steel depends on the temperature. Therefore, separate analyses may have to be made for the ambient temperature startup and elevated temperature operating conditions. Crack sizes that are acceptable for operation of a pressure vessel at elevated temperatures may present a risk of failure during startup at ambient temperature. In this situation, it may be necessary to restrict the pressure during startup until the vessel is warmed up, or increase the minimum pressurizing temperature (MPT). The restriction on pressure would also apply during a shutdown, in that the pressure may have to be reduced before the vessel is allowed to cool to ambient temperature.
Hydrotest of a pressure vessel could present a much more serious risk of failure than startup at ambient temperature. Although the toughness of the material at ambient temperature is the same for both hydrotest and startup, the applied stress associated with the hydrotest pressure is appreciably higher than that developed at the operating pressure. Therefore, a crack size that is entirely acceptable for both startup and operation could cause a pressure vessel to fail during hydrotest. In other words, a hydrotest is not proof that a fitness-for-service analysis has been performed satisfactorily and should not be conducted with a pressure vessel that contains a crack, unless a separate analysis has been made for the hydrotest conditions.
Neither API 510 nor the National Board Inspection Code provides for the fitnessfor-service analysis of a pressure vessel containing a crack, but they do not explicitly prohibit making such an analysis. ASME Code, Section XI, “Inservice Inspection of Nuclear Power Plant Components,” does provide for the fitness-for-service analysis of ASME Code, Section III, pressure vessels, and has been accepted by the Nuclear Regulatory Commission (NRC). The methodology given in Section XI for making an analysis has been adapted for ASME Code, Section VIII, pressure vessels in this section. Therefore, the acceptance of Section XI by NRC can be cited for precedence if it is necessary to obtain the approval of an authorized inspector.
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