Question about ASME B31.3 Section 345.4.2(c)

[Robin_L]

I noticed that in the 2014 edition (and onwards to the current 2018 edition) of ASME B31.3 Section 345.4.2, the following clause was added regarding S_T/S factors:

​

"c) in those cases where the piping system may not include pipe itself, any other component in the piping system, other than pipe-supporting elements and bolting, may be used to determine the S_T/S ratio based on the applicable allowable stresses obtained from Table A-1 or Table A-1M. In those cases where the piping system may be made up of equivalent lengths of more than one material, the S_T/S ratio shall be based on the minimum calculated ratio of the included materials."

For reference here is parts a and b of 345.4.2 Link

I'm wondering what the motivation for using the minimum calculated ratio is. To me this is saying, if I have a pipe that has an S_T/S ratio of say, 2, my test pressure must exceed 1.5(P)(2), however if I replace half of this section with some other material that happen to only a S_T/S ratio of say, 1.9 (it de-rates slower with temperature), by this clause I can lower the test pressure of the entire system to 1.5(P)(1.9), which is less than the minimum hydrotest pressure required for the part of the component that de-rates faster with temperature?

 

[XL83NL]

In certain circumstances, e.g. a system that changes from one material to another (by means of a DMW rather than flanges), one may find there are no other options left. Then it’s safety first, as you dont want to test over the Code specified yield. At least, the latter would be my reasoning of the code.