Reforming torispherical dish to ellipsoidal dish

[arksurat]

We have a peculiar problem. Our pressure vessel contractor has formed a 15% torispherical dish instead of 2:1 ellipsoidal. The material is 32 mm, 516 Grade 70, 600 mm Dia. The mistake has been realised after the dish has been welded to the shell. Now he wants to cut the dish and reform the shape to semi ellipsoidal. Is it recommended?
What precautions are to be taken during re forming and subsequent welding?

 

[SHG]

Hi arksurat
I think that it depend to the radius that you have and radius which you should recieve.
It is not practicable to recieve larger radius from smaler and also it depends to capabilities of your forming equipments.
In any way the table UG-37 of ASME Sec.viii-1 may be a guide to find the radius that required for an ellipsoidal head.
It shall be remembered that it is better that to heat treated the reformed head because of residual stresses due to excess forming.
I also remain to the final solution of your problem please inform us.

SHG

 

[prex]

It is possible, though for sure not recommended.
You could go over the amount of cold deformation allowed by code, due to the repeated forming, but a suitable heat treatment as suggested by SHG would correct this: however if the head has been already heat treated the first time, then you should really go for a new one.
Unless you have critical time constraints and can't find the material for a new head, I can't see a reason for not replacing the head, that would cost certainly less than reforming the old one.
Also, if you want to be correct, such an operation should be reported in the vessel book. prex

http://www.xcalcs.com

Online tools for structural design

 

[twjag]

arksurat
prex is correct re reporting. The error must be reported as a non-conformance. The removal, reforming, & other steps taken requires documenting on the non-conformance report, & be approved by the AI.
As to reforming for SA516 material (P-No. 1) the requirements of UCS-79(b) must be satisfied. If it were me, I would insist on PWHT regardless of vendors UCS-79(d) calculations.

prex
Re prior post forming heat treatment:
If head was cold formed, PWHT (stress relief) temperature per UCS-56 would be below upper critical temperature. Therefore, it is possible to perform another PWHT after cold reforming without damage to base material properties. One area that it might affect is vendor's WPS qualification.

For hot formed heads, the CMTR for the material is usually based upon a test specimen (from the same heat) that has undergone the same heat treatment as the head. Any hot reforming would likely require new materials CMTR, with same combination of all heat treatment regimes used for final reformed head (i.e., all heat treatments during forming & reforming).

I am interested in hearing other's points of view on this subject.