Why Sh is considerd as allowable limit for primay stress?
Why noy Sc?
Since both are allowable yield stresses at design and ambient temperature respectively.

The primary stresses exist regardless of the position or temperature of the piping system. Therefore (until recently) the Codes applied the minimum allowable (Sh) to the primary stress state.

The latest edition of B31.3 does permit the user to indicate which Sh should be applied to the various Sustained (primary) load cases. This will be supported in CAESAR II 2012.

Devendra patil,

Primary Stresses exist even the pipe is in hot conditions, so it shud be compared with SH (SH is the minimum code stress allowable) only...code is just comparing with the extreme maximum temperature conditions for safe design.....

Sri..