ASME B31.3 2014 Hydrostatic Testing
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The second half of clause 345.4.2 (c) in ASME B31.3 2014 states as follows: In those cases where the piping system may be composed of equivalent lengths of more than one material, the ST/S ratio shall be based on the minimum calculated ratio of the included materials. How should this sentence be understood?
When the pipe system can consist of equivalent lengths of more than one material, it shall be calculated based on the minimum calculated proportion of the materials involved.
I still don’t understand; in what situations should this be used?
The ratio ST/S in the above formula must not be less than 1, nor may it exceed 6.5. If this ratio exceeds 6.5, 6.5 is used. If the test pressure determined from the above formula results in a nominal stress induced by pressure or axial stress that exceeds the yield point of the material at that test pressure, then the test pressure can be reduced to a value such that the maximum value of this stress is at the yield stress level.
The same pipeline system may consist of multiple sections made of different materials; for example, one section might be made of X70 while another is made of X60, and the ST/S value is calculated as the minimum value of those materials used