Hi every one,

In my system pipe material in A312 TP304L.

the Design temprature in 210 C.

value of SC=115 N/mm2
Value of Sh=108 N/mm2

While calculating sustain case (W+P1+H) stress, caesar is using allowable value as 108 N/mm2 instead of 115 N/mm2.

Why it is so?

Code requirements.

For example, if you are using B31.3, read paragraph 302.3.5(c).

Sir as you told code requirements, may I ask u to please elaborate the the code requirements


As in 302.3.5 (c)

code is stating the weld joint reduction factor is 1.0 for longitudinal welded pipe.


As in 302.3.5 (e)

code is stating weld joint reduction factor is 1.0 upto 510 deg C and below. However at 815 deg C it should be taken as 0.5.

Now my question arises:

at design temperature 210 deg C

if the pipe is having longitudinal weld.

then for the same material

calculated sustain case stress should be equal to 115 N/mm2.
but the result it comes 108 N/mm2.

please justify your answer with proper data.

thanks in advance.

Your longitudinal stress due to sustained loads is used to confirm the system will not collapse. It should not collapse at any time - when the system is installed (oftentimes the cold state) and when the system is in operation (oftentimes the hot state). Since the basic allowable stress is lower in the hot condition (SH<SC), the "hot allowable stress" is the limit for SL.
W has nothing to do with this discussion.

Thank YOU SIR.

I just realized that in calculations with several operating temperatures, CAESAR II takes Sh, as allowable in the SUSTAINED cases, from the highest of the temperatures (lower allowable stress, then) .
Is this OK? Has the lowest Sh must be taken or should Sh1 be the one for SUS (with P1) and Sh2 with P2 and so on?

Thanks in advance

S. Gómez
Stress Engineer

Samuel,

This is as per code. Check ASME B31.3 302.3.5 (C).

Thanks Shahid, but I know that paragraph of the code. What I do not know is if in a system with different operating temperatures (more than one operating case) I have to take the Sh of the highest to compare with the stresses in all the sustained loads.
That is what I am asking. If I use different temperatures then the allowable is being drastically lowered, even if it is a case which does not happen often.

Caesar II is a computer program that will simply analyse the data you have entered in the input. If you have entered a certain temperature case then it will calculate the Sh value for it and use it in the sustained case analysis of B31.3 (if it is the lowest value of Sh).

As Dave Diehl said, the sustained case is designed to check the pipe does not fail under primary loadings at any time. It therefore has to consider the Sh of every temperature you enter, as you have implied that it could come under these conditions at some point in its life cycle.

CAESAR II, by default, uses the minimum S as the allowable stress for sustained loads. But you can choose other defined S's specified (by way of different temperatures)as your sustained stress limit. Find this entry on the Load Case Options tab of Static Analysis.
Sustained stresses may change because of load redistribution caused by pipe strain (e.g., lift off at resting supports). So since the calculated sustained stress is a function of temperature (because of pipe strain), you can associate a temperature for that state of "sustained" stress. And if you can associate a temperature with a sustained state, there is a related basic allowable stress at that temperature - Sh.
B31.3 302.3.5(c) states:
The sum of the longitudinal stresses due to sustained loads, SL, such as
pressure and weight in any component in a piping system (see para. 320), shall not exceed Sh, where Sh is taken from Table A-1 at the metal temperature of the operating condition being considered.