Cold Forming after PWHT

[MikaelaD]

Hello,

We have a horizontal vessel that exceeded out-of-roundness tolerance of ASME VIII-1 Code after PWHT. The vessel became oval in shape along its length. Deformation is between the support saddles with largest deformation in the middle of the vessel and diminishes are you go out further from the middle towards the saddles. Vessel is 22'-0" ID x 100'-0" Tan/Tan made of 2.25" thick SA-516-70. Design pressure is 300 psig @ 150°F. Vessel fabricator's repair plan is to jack the vessel back to shape. The vessel shell will be plastically deformed and there are no plans for heat treatment. Fabricator claims the forming strain will be small and will not exceed the 5% extreme fiber elongation limit specified in the Code.

Vessel fabricator assumes the vessel shell, including the circumferential and longitudinal welds, will be plastically deformed. Welds are stronger than base metal so my concern is the welds will just undergo elastic deformation. The welds will try to spring back to original shape so there will be remaining stresses in the vessel, Without heat treatment, the vessel will retain the stresses throughout its design life. In this case, what are the risks of vessel failing? What are the damage mechanism at the welds and HAZ?

 

[0707]

You gave the answer to your question. What is the serviçe of the vessel? If it is out of tolerance acording to code after PWHT, you are the client!?

luis

 

[MikaelaD]

I am third party engineering contractor. I am not really involved in the engineering and procurement of vessel. I just performed cold eyes review to provide unbiased assessment. My assessment is to accept the vessel as is and perform Fitness-for Service evaluation, but AI does not want to sign the vessel and client wants the vessel repaired. I am okay with repair methodology but since vessel will undergo elastic-plastic deformations, heat treatment is necessary in my opinion. Concerns from fabricator and client is that vessel might deform again after the heat treatment.

Would appreciate if someone could refer me to a similar case or refer me to literatures about vessel or weld failures due to residual stresses in vessel. All I know about service of vessel is that it contains hydrocarbon.

 

[0707]

How was the PWHT done? In a heater? Properly acconditionated to avoid ovality?

luis

 

[MikaelaD]

The entire vessel was heat treated in a PWHT furnace. Heat source came from gas burners. Furnace design ensures not flame impingement. Root cause of ovality is improper support/bracing of vessel. The vessel had two permanent support saddles. Fabricator provided 1 temporary support in the middle and there were no internal spiders. It seems the vessel just deformed and became oval from its own weight.

 

[Tmoose]

How much out of tolerance is the ovality?
I'm assuming the vertical dimension got small, and the horizontal dimension expanded ( much like myself in recent years ).

I was going to say pop it back in the oven for a while on its side, until I saw it is 100 feet long.
Also your handle say you're from Texas, so it might be nice to run a furnace for a while, unless the gas is being saved for Grannie.

I'm surprised appropriate "repairs" options are not covered in detail in one of the ASME VIII codes.