Although the Code does not use the term "maximum allowable working pressure," the concept is useful in a discussion of the allowances for variations. Pressure design of piping systems is based on the design conditions. However, because piping systems are assemblies of standardized parts, there is quite often significant pressure capacity in the piping beyond the design conditions of the system. The allowances for variations are relative to the maximum permissible design pressure for the system. The allowances for variations are not used in sustained (longitudinal), occasional (wind, earthquake), or displacement (thermal expansion) stress evaluations. They are only used in pressure design.
Increases in pressure and temperature above the design conditions are permitted for short-term events, as long as several conditions are satisfied, one of which is that this maximum allowable working pressure is not exceeded by more than some percentage. Thus, the variation can be much higher than the design conditions, yet remain permissible.
As an alternative to the prior two paragraphs, if the variation is self-limiting, such as by accumulation during a pressure-relieving event, and lasts no more than 50 hours at any one time nor more than 500 hours per year, an allowable variation of 20% is permitted without the owner's approval and without requiring that the designer determine that the piping system is safe with the variations.
The following conditions are requirements for use of the variations:
The piping system shall not have pressure containing components of cast iron or other non-ductile metal.
The nominal pressure stress (hoop stress for straight pipe or, for rated components, the pressure divided by the allowable pressure times two-thirds the yield strength) must be less than the yield strength of the material.
The longitudinal stresses must be within the normally permitted limits.
The total number of pressure-temperature variations above the design conditions must be less than 1000 over the life of the system (note that this is the number anticipated in the design of the system, not some count taken during operation of the system; the ASME B31.3 Code is for design of new piping systems).
The maximum pressure must be less than the test pressure; this can be a limitation if pneumatic or alternative leak testing was used.
If the above conditions are satisfied, and if the owner approves, the pressure rating or allowable stress (essentially the maximum allowable working pressure) may be exceeded by 33% for events that are not more than 10 hours at any one time nor more than 100 hours per year, and by 20% for events that are not more than 50 hours at any one time nor more than 500 hours per year. It is clear how a variation in pressure is handled. There is sometimes confusion relative to variations in temperature. The variation in temperature decreases the allowable stress or pressure rating. Thus, the stress or pressure may exceed the allowable value during a variation in temperature, without a change in pressure.
If the above variations are used, the designer must determine that the piping system, including the effects of the variations, is safe over the service life of the piping, using methods that are acceptable to the owner. Note that the pressure test provides such assurance for piping operating below the creep regime. For piping at elevated temperatures, within the creep regime for the material of construction, the pressure test does not ensure long-term pressure integrity. Therefore, Appendix V of ASME B31.3 was provided to evaluate the effect of short-term variations at elevated temperature on the life of the piping system. #Little_PEng