In large bore thin cryo piping in B31.3, no pressure sttiffening gives conservative results compared to default in B31.3 which takes pressure stiffening in bends.

This pressure stiffening in bends reduces bend SIF & k both as per B31.3 App D point 6.

It is possible to have lower pressure than design pressure in piping.

Just like taking bourdon effect makes Cryo piping design non conservative, this pressure stiffening in bends does it, too!

How do our forum members tackle this situation -

in B31.3 Cryo pressure stiffening in bends - Yes or No ?

reg,
sam

Could you simply not run it at a range of pressures and plot the stresses? If not sufficiently below allowable stress, narrow the increments to isolate worst case scenario?

Especially now with the advent of B31J becoming mandatory in the 2020 B31.3 code, the COINCIDENT pressure and temperature pairs should be run in CII. Recently the trend has been to use DESIGN pressure for all runs to save time with the logic being that the NOMINAL SUS stress should be conservative or higher than the operating case. Also a good practice that we have using for a long time is to include a WNC and/or WNC+H to see what is happening in the INSTALLED case prior to startup; Structural also uses these types of loads to check OCC overturning.

In thinner wall piping systems the SUS deflections will be affected by the internal pressure, but even if the effect is insignificant in many cases the latest good practice is to attempt to get the REAL ANSWERS using reasonable time frames and resources.

With FEAtools, PCL-Gold, and reality, the BEND sifs and flex VARY with pressure and ALSO with layout of close fittings (ELLs, flanges, etc).

For future work an attempt will be made to model actual loading scenarios along with the "conservative" DESIGN checks if justified.

Just take a bit more care to describe the cases and care with the input so the poor checkers life will just be a formality.

I could then see the need to have a discussion with the end user or process engineer whether rapid depressurization of the system is a valid design point.

Unfortunately, because pressure both adds to and relieves stresses, it's hard to say where that maximum point could lie in any given system.

Thanks for the lively discussion on this topic.
reg,
sam