When liberal stress allowance is turned on, the thermal expansion stress range is 93% of the B31.3 Code allowable. The maximum sustained code stress in the model is 8% of the sustained allowable. There is thus "a lot" of "stress" available to "go over" and help out on the allowable stress range. When the liberal stress allowable is turned off (in the configuration file), my thermal expansion stress range is 144%. I can't rationalize why this is OK. Is it OK?

Using the "liberal stress allowable" allows you to take credit for the actual calculated sustained stress loads as allowed in B31.3. Back in the day before computers were used as frequently, to be conservative, it was assumed that SL = SH just as a worst case.

Unless your client/project does not allow you to take credit for the actual sustained stresses calculated in the system, you should be fine.

Some clients specifically indicate in their stress specification not to use the allowable liberal stress. I have an experience with one of our clients.

This will increase the system's reliability, especially if the line is operating at a creep range.

Just make sure to check the client specification if it's written there. If not, the code I believe allows using this, since there is enough margin of safety for allowable stress, considering other factors such as SIF's.

Cheers!

I guess that you refer to the fact that you expect to have 92% available from sustained stress. Yes, but you compare with different allowable so that amount of available stress (92%) is less in term of percentage respect to the expansion allowable.