102.2.4 Ratings: Allowance for Variation From Normal Operation. The maximum internal pressure and temperature allowed shall include considerations for occasional loads and transients of pressure and temperature.
It is recognized that variations in pressure and temperature inevitably occur, and therefore the piping system, except as limited by component standards referred to in para. 102.2.1 or by manufacturers of components referred to in para. 102.2.2, shall be considered safe for
occasional short operating periods at higher than design pressure or temperature. For such variations, either pressure or temperature, or both, may exceed the design values if the computed circumferential pressure stress does not exceed the maximum allowable stress fromAppendix A for the coincident temperature by
(A) 15% if the event duration occurs for no more than 8 hr at any one time and not more than 800 hr/year, or
(B) 20% if the event duration occurs for not more than 1 hr at any one time and not more than 80 hr/year
302.2.4 Allowances for Pressure and Temperature Variations. Occasional variations of pressure and/or temperature may occur in a piping system. Such variations shall be considered in selecting design pressure (para. 301.2) and design temperature (para. 301.3). The most severe coincident pressure and temperature shall determine the design conditions unless all of the following criteria are met:
(a) The piping system shall have no pressure containing components of cast iron or other nonductile metal.
(b) Nominal pressure stresses shall not exceed the yield strength at temperature (see para. 302.3 of this Code and S y data in BPV Code, Section II, Part D, Table Y-1).
(c) Combined longitudinal stresses shall not exceed the limits established in para. 302.3.6.
(d) The total number of pressure–temperature variations above the design conditions shall not exceed 1 000 during the life of the piping system.
(e) In no case shall the increased pressure exceed the test pressure used under para. 345 for the piping system.
(f) Occasional variations above design conditions shall remain within one of the following limits for pressure design.
(1) Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than (a) 33% for no more than 10 h at any one time and no more than 100 h/y, or (b) 20% for no more than 50 h at any one time and no more than 500 h/y
The effects of such variations shall be determined by the designer to be safe over the service life of the piping system by methods acceptable to the owner. (See Appendix V.)
(2) When the variation is self-limiting (e.g., due to a pressure relieving event), and lasts no more than 50 h at any one time and not more than 500 h/y, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increasedcondition by not more than 20%.
(g) The combined effects of the sustained and cyclic variations on the serviceability of all components in the system shall have been evaluated.
(h) Temperature variations below the minimum temperature shown in Appendix A are not permitted unless the requirements of para. 323.2.2 are met for the lowest temperature during the variation.
(i) The application of pressures exceeding pressure temperature ratings of valves may under certain conditions cause loss of seat tightness or difficulty of operation. The differential pressure on the valve closure element should not exceed the maximum differential pressure rating established by the valve manufacturer. Such applications are the owner’s responsibility.