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.
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