stress due to sustain case and expansion case

Reply from another forum:

1) Stress due to sustained forces are called sustained stresses :- weight etc (in simple language, forces whose magnitude dosent change , weight of pipe , dosent change in cold and hot condition.) So if these stresses are more
than yield stress , of material under consideration , it will lead to failure.

2)Expansion stresses , are generated by heating of pipe from cold to hot condition.( From installed condition the pipe goes to operating condition ).
Expansion stresses decrease with passage of time due to local yielding of pipe , ( only a small portion/area of pipe yields ),leading to re-distribution of strains generated. Only a small area of pipe is plastic , rest is elastic.
So local yeilding is allowed by ASME codes :- B 31.3 / B31.1 , in expansion cases..

3) Sustained stresses are checked with , allowable stress value at hot/operating temperature.( Sh).

4)Expansion stresses calculated have to be within allowable stress range as per code .

5) The method of mitigating sustained stresses and operating stresses is of opposite type. Additional supports on line will reduce sustained stresses , but will increase expansion stresses.

6) Depending upon layout , it is also important to check sustained stresses in hot condition ie :- hot sustained condition. Also the expansion stresses have to be calculated , for the entire temperature range.( eg :- from installation temperature to hot temp , from installation to low ambient ( winter temp).

By Jatin Prabhu