Repairing vessel shell - SA 516 Gr. 70 - Post Weld Heat Treatment

[jives]

Anyone have any guidance on the following:

We are going to cut out and then replace a section of shell from a pressure vessel. Shell material is SA 516 Gr. 70. Shell thickness is 3/4". MAWP of shell is 160 psig. Shell inside diameter is approximately 62". Fluid in shell is treated boiler water.

The vessel was heat-treated after it was manufactured.

After we weld the shell section back in, is it advisable to do post-weld heat treatment or stress relief at the repair welds ?

Jonathan Ives
Project Engineer
Transalta, Alberta Thermal Plants
Duffield, Alberta

 

[metengr]

I would presume you are performing a window, flush patch repair. The original code of construction is ASME Section VIII, Div 1. Based on the original code of construction PWHT requirements, PWHT is not required for this shell thickness.

So, I would not perform a PWHT. I would, however, increase then preheat to 250 deg F for the weld repair and use low hydrogen electrodes.

 

[metengr]

One other comment, which just dawned on me, is this pressure vessel a DC storage tank??

If so, and this would apply to DC storage tanks, I would perform a local PWHT of the repair welds. Despite the minimum PWHT requirements in the Code for not having to perform PWHT on nominal thickness less than 1.25”, I would perform a local PWHT pf the repair welds in this vessel. It has been shown that reducing the residual stresses form repair welds will reduce the susceptibility to either stress corrosion cracking or corrosion fatigue in vessel welds located below the water line.

 

[jives]

Thanks for the quick feedback.

The repairs will be made on the shell of a high-pressure feedwater heater. The shell will see boiler water at up to 160 psi, so I think we'll PWHT the repair welds to avoid stress-corrosion cracking.

Jonathan Ives
Project Engineer
Transalta, Alberta Thermal Plants
Duffield, Alberta

 

[unclesyd]

jives,
Could you post the reason for the repair, a nosey person would like to know ?

 

[SJones]

To follow up on your repair, check out Section 7 of API 510.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[weldtek]

I would also recommend following the guidelines in UW 40 for local PWHT.