Hi fellow pipe stressers,

I was wondering if I can pick you brain for the below DN150 STD CS piping system which has a portion buried 1.5m underground as shown in the attached pic.
Delta temperature for buried piping is 45 deg C. The lengths of the buried portions are marked up in red. From CII buried module, the VAL calculated is about 15 m.
I modelled the whole system as shown below and had thermal overstress issues on every bends which are expected. However the overstress issue still exists (about 140%) even with large dia bend up to 20D on most bends.
My question is, have I done the correct approach on modelling the whole buried piping as shown below? Is the thermal overstress of over 300% using LR elbows to be expected in such buried piping arrangement? What's the best way to model this buried piping / Or is it acceptable to just model the straight length of 30m (2 x VAL) from AG/UG points? Thanks for any input you may have.

Daprin

In an ideal world, you would forgo analysis in CAESAR, not because CAESAR doesn't give you the answer you need, but because you know CAESAR will predict what you already know.

I would expect that the line temperature remaining hot could be a point you would like to revisit.

I have earned my own repudiations for establishing underground loops to address temperatures the end user refused to budge on. Maybe you can have better luck there.

Similarly, you can try calculating heat losses to the earth and recommend your own temperature.

If you have FEA Tools you can try to use it. Although it's not always a guarantee that it will reduce stress.

If still overstressed, try increasing section modulus by increasing bend thickness, but noting the welding requirements for dissimilar thickness from ASME (there is a limitation up to how much you can increase).

If you are sure of the number of cycles you can use 3125 cycles to use the 1.2 increase in expansion stress allowable, but note that this has limitations also and I think not applicable for high strength steel due to difference in fatigue properties (inferior I think in low cycle regime). Just make sure, you read the Code requirements before using this and check first if you can use it.

I have an idea before of exposing the portion with failure (not buried) to gain some flexibility (unrestrained). But check with your company standards and project requirements.

Warm Regards,

It is not very clear if you have in the SOW also the burried line.
If you do not, you can specify that is included only for boundary condition and to delete the allowable.
Other thing: do you have an anchor block underground? you can stop there.

Many times also the advices are wrong due to incomplete input
Regards,

You could also put the elbows in an insulation box to allow them to move and install flexible / crushable insulation along a portion of the connecting pipe to allow the pipe to move.

Hi Tom, thanks for your suggestion. Are you able to share pictures how you do this arrangement?

Hi Michael, you have made your point there regarding the heat loss. At the moment I can only assume a number. The max design of 110 deg C is mainly causing this. Would it be acceptable to assume the max temp for buried piping is the operating temperature?