Posted by: charlot
Wall Thickness Calc for large diameter pipe - 01/25/21 02:50 PM
Hello Everyone
If I use a de-rated method (With design pressure/Temperature) to calculate the wall thickness of a large bore pipe (48"), do I need to consider wall thickness calc due to Hydrotest pressure separately(design pressure is almost ATM and Hydrotest increases the wall thickness)? If yes, do I have to consider C.A for that calculation and is it correct to consider 0.9 of the allowable as Hydrotest allowable?
Thanks
Posted by: Michael_Fletcher
Re: Wall Thickness Calc for large diameter pipe - 01/25/21 06:10 PM
You didn't specify piping code, but I'm going to assume ASME B31.3.
Do you need to consider wall thickness calc due to hydrotest pressure separately? Yes.
Firstly, 345.4.2 states minimum is 1.5 x "design pressure."
"Design pressure" is the pressure component of whatever pressure/temperature combinations result in the largest thickness.
E.G. if you're to design your pipe for 100 psi / 100 F and 20 psi / 1000 F, and 20 psi / 1000 F dictate the minimum thickness, then 20 psi is your "design pressure." Your hydrotest pressure would then be 20 x 1.5 x St/S. If 100 psi dictates the minimum thickness, then your hydrotest pressure is 100 x 1.5.
However, 345.2.1 states that if you exceed YS, then you should reduce the hydrotest pressure. I'm not sure which material this would apply to.
I never found a basis for 90% allowable stress for hydrotest. Anybody who's mentioned it to me hasn't been able to back up this value, either. Do please come back if you can find that reference.
I believe it's a "good idea" to include CA for hydrotest purposes, since if they modify the pipe later on they may fall out of compliance with not being able to properly safely hydrotest it to code requirements.
Either way, they'll want to inspect the pipe before they re-hydro. Whether they want to spend the money for the steel now or deal with consequences later will be up to them.
Including the CA will inevitably increase the "permissible hydrotest," should they wish to hydrotest to higher levels for other/future components that happen to have a higher pressure requirement.