B31 Thermal / Displacement Stress Range 20 ksi allowable cap / 0.72 SMYS origins

[BBQ Engineer]

Hello Everyone,
I was wondering if anyone was aware of the origins of the 20 ksi cap found in B31.3's displacement stress range calculation for hot and cold allowables. I know the stress range formula was based off of Markl's work in the 50's, however I never came across the 20 ksi limitation. Perhaps it's related to the pipe properties used in his experiment, and the weak correlation fatigue has with yield strength found in typical material with SMYS < 80 ksi? At least that was the best thing I could come up with...

Also, in the older versions of code, the expansion stress limit was originally limited to 0.72 SMYS. Is anyone aware of where this came from, and if there's an actual fatigue based technical reasoning behind his?

Thank you so much for your help!

 

[LittleInch]

Well the pipeline B 31.4 and B 31.8 0.72 Design factor is reported to come from a safety factor of 1.25 based on the fact that pipes were tested to 90% of yield in the pipe mill.

1.25 was a commonly used structural safety factor at the time ( 19 20's)

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[BBQ Engineer]

@ LittleInch

Thanks for the response! Yeah... I figured it was some variation of 0.8 x 0.9, but was this just based off of typical design and test parameters / factors? So would it be fair to say the 0.72 SMYS rule was not based off of any sort of fatigue testing / rule or sorts?

 

[LittleInch]

Not that I'm aware of. as said 0.72 DF for pipelines was there in the first version of the codes back in the 1920's. I don't believe they did much in the way of testing in those days.

It has stood the test of time though and even though 0.8 is available for gas lines, very few seem to be designed to it. For various reasons I've looked for the reasoning and asked around and have never found a definitive reference or reason other than what I said above.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.