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Tips April - June 2001
Allowable Displacement Range (SA) versus Material Yield Strength
Question: "...it was noted that some values of SA exceeded the yield strength of the material, for example: For A335P5, the allowable stress SA according to CAEPIPE is 39500 psi at 400 deg.C, while the min. yield strength of this material acc. to ANSI B31.3 code is 23800 psi at 400 deg. C and 30000 psi at standard condition. Can you please explain?"
Reply: S(A) is the allowable displacement stress range and, using B31.3 Eqn. (1b), will approach 1.6 times the yield stress, if S(L) values are negligible. This is correct and is based on the assumption of "shakedown" or "self-springing" of the piping which is assumed to be ductile. (See B31.3 para. 319.2.3).
Note: We strive to provide accurate data for the material properties. Sometimes, typographical errors do creep in. It is recommended that you verify the properties before using them.
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.
See technical article on a quicker method for evaluating piping vibration by Dr. R. T. Hartlen (under Tech Articles).