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Tips January - March 2007
Low Temperature Material Use in B31.1 vs. B31.3
An inquirer comments that when one looks at B31.1, he finds that the min temp listed for B31.3, Curve B carbon steels is -20F (-29C) regardless of thickness. This is the way B31.3 used to look before the addition of Fig. 323.2.2A which defined 0.500" nominal wall and less to be -20F (-29C) with thicker cross sections having a "warmer" min design metal temp. Is B31.1 planning on moving to the B31.3 approach?
The inquirer further states that when developing B31.1 piping specifications, he would tend to use the minimum design metal temp derived from B31.3 in order to avoid the potential for brittle fracture of thick walled B31.1 piping (especially during hydrotest).
The inquirer suggests that B31.1 should be consistent with B31.3 in this respect. However knowing B31.1 to be developed on a more conservative basis, perhaps B31.1 could assign a thicker wall to the -20F (-29C) min design metal temp for B31.1.
In response to the inquirer, B31.1 is working on low temperature requirements similar to B31.3. But, while some might contend that B31.1 is the more conservative code, there are plenty of instances where B31.3 is the more conservative, e.g., severe cyclic service which suggests that the fatigue rules of B31.1 are not conservative enough. Another thing to remember is that B31.1 is for power piping which seldom, if ever, has cold pipe and thus, -20 degF operating conditions would virtually never occur. And, because B31.3 requires all support configurations be evaluated, including liftoff at ambient (see Interpretation 6-07), B31.3 infers that B31.1 is unconservative because in normal practice B31.1 does not normally consider the ambient condition an operating condition and stresses are seldom, if ever, evaluated. You might rightly conclude that developing low temperature material rules has not been high on B31.1's priority list of things to do. But since B31.1 is not sure what power applications might be seen in the future, they in the process of developing low temperature material rules into the code, in some measure to be consistent with B31.3.
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.