Reading through all of these postings there seems to be a common view that you can’t check your occasional stresses at T2=25degC unless you can ABSOLUTELY guarantee that the occasional event won’t occur under the SUS+T2 situation.

However I don’t believe flexMen was trying to say this – rather he was trying to say that each case should be treated individually i.e.

Case 1 – calculate OCC stresses from OPE1+SEIS1-OPE1 using Sh for T1
Case 2 – calculate OCC stresses from OPE2+SEIS2-OPE2 using Sh for T2
Case 3 – calculate OCC stresses from OPE3+SEIS3-OPE3 using Sh for T3

What seems to have been overlooked in the ensuing discussion is that the stresses arising from the OCC loads in each of these cases can be different.

To illustrate this consider the following (hypothetical) case as per flexMan’s original example – a steam line to a turbine fitted with a bypass to a condenser.
Case 1 T1=500 degC (steamline to turbine hot)
Case 2 T2 = 25 degC (system on bypass, steamline at ambient)
Case 3 T3 = 560 deg C (steamline to turbine hot)

Assume the following situation:
- the pipeline runs along the x-axis
- there is a linestop installed on the pipeline with:
- gap back (-ve x direction) = 5mm
- gap forward (+ve x direction) = 100mm
- movements under thermal load at linestop are:
- Case 1 dx @ T1 = 80mm
- Case 2 dx @ T2 = 0mm
- Case 3 dx @ T3 = 90mm
- SEIS load acts in +ve x direction
- SEIS load has sufficient magnitude that from any operating condition it will close the forward gap to zero

So the displacements arising from the OCC case would be as follows:
- Case 1 = SEIS1 = 100mm gap – 80mm dx = 20mm
- Case 2 = SEIS2 = 100mm gap – 0mm dx = 100mm
- Case 3 = SEIS3 = 100mm gap – 90mm dx = 10mm

Assuming stresses are proportional to displacements in this case, then the OCC stresses for Case 2 will be much higher than the other two cases. It seems entirely reasonable that for Case 2 the code check should use Sh=25degC to take advantage of the higher allowable. This assumes that all OCC cases are combined with each OPE case.

Practically speaking it will often be a large task to generate all of these load combinations and I suspect that most users will prefer to use the current (conservative) CAESAR II methodology whereby the lowest value of Sh is used for checking all OCC cases.


John Luf
I note that your B31.3 RFI wording was slightly different to that posted on 12 Sept. The phrase:

“…should the occasional load stresses combined with the sustained stresses (ref: paragraph 302.3.6) be compared against the lowest value of Sh as determined from all of the operating temperatures be used as the basic allowable stress in paragraph 302.3.6.”

became:

“….should all coincident operating pressures and temperatures expected during operation be considered and shall the basic allowable stress used in para. 302.3.6 be based on the applicable temperature for each condition?”

So with the B31.3 committee response of “Yes”, my interpretation is that the methods proposed by flexMan are correct – provided that he checks all OCC cases against each OPE case.

This is where I become confused – this seems at odds with your statement “If the work you have performed as you proposed to do it was under the jurisdiction of B31.3 your questionable method does not meet the requirements of the B31.3 code.”

Can you explain how you came to this conclusion given the committee's response?

regards