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Temperature Limitations in Piping Component Materials
ASME B31 .3, Para. 323.2 Temperature Limitations describes low temperature requirements to assure adequate material toughness in piping components. In B31.3, if adequate toughness is assured, the basic design premise of -shakedown- or -self-springing is maintained and the allowable stresses or strains (see Note 2) are appropriate. The situation addressed by the B31.3 toughness requirements is if the piping system is cold enough and the stress high enough and the piping component material thick enough a failure, a brittle fracture, can occur. Two potential interrelated conditions can lead to that failure: (I) when it's cold enough and if a significant portion (a thick enough part or a sufficient volume) of the piping component exceeds the yield stress throughout then a brittle fracture will initiate, however, (II) if the piping component exceeds the yield stress at the other end of a loading cycle, e.g., from thermal expansion, the piping component will experience -shakedown- or -self-springing- and stresses will incrementally diminish in the -hotter- condition(s) and incrementally increase in the cold condition(s). Thus, while a brittle fracture may not occur in an initial loading condition, if a stress range between any of the piping component's normal, abnormal, or environmental conditions results in yielding of the piping component, -shakedown- or -self-springing- can result in a stress increase in the cold condition and a brittle fracture may occur.
The simplest way of assuring this is to keep the combined stress due to all the coincident loadings in the cold condition(s) below the material yield stress. This is described in potential failure condition (I) above and is the intent of General Note (a)(3) of B31.3, Fig. 323.2.2B. Potential failure condition (II) is not described in the General Notes of Fig. 323.2.2B but nonetheless should be considered. (In a future revision to the Code, B31.3 is intending to add words to this effect).
Local stresses perhaps should also be considered. Currently, however, B31.3 says they don't need to be considered by the statement in General Note (a)(3) of Fig. 323 .2.2B that -stress intensification factors are not included in this calculation-. Because of the complexity of the problem and lack of research in this area, it may be some time before the effect of local stresses on brittle fracture can be cogently introduced into B31.3 or other B31 Codes.
Note 1: See B31.3, Para. 319.2.3(a) and (b). B31.1, Para. 119.2 describes the same phenomena and may be clearer. (Note also, B31.1 is considering the addition of toughness rules similar to those in B31.3.)
Note 2: The B3 1.1 and B31.3 references in footnote 1 suggest that a stress analysis approach, rather than a strain analysis approach, is used in the B31 Codes because it is easier and more "convenient."
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.