Field-Fabricated Spheres, Materials of Construction (MOC), SA-516-60

[dorbsdee]

Hi there,
Reference is made from Pressure Vessel Design Manual, 4th Ed. by Dennis R. Moss & Michael Basic, page 594, Procedure 9-7: Field-Fabricated Spheres. It's about Materials of Construction (MOC), it mentioned that "SA-516-60 may be
used to eliminate the need for PWHT in wet H2S service". Does anyone know the reasons for this?
Typically most of the client (operators) requires PWHT for pressure vessel under severe service (Wet H2S, Sour).
Although, the reference design manual is a guideline only, and it's not part of the contract with client, I just wanted to know.
Thanks in advance for your help.

DorbsDee.

 

[marty007]

In my experience, the specification for PWHT and/or calling out of SA-516-60 steel revolves around the issue of hardness. By going with SA-516-60 rather than SA-516-70, you are typically (but not always) going to get a softer steel, that is more easily able to meet the required hardness limits. That said, there are some overlaps in material requirements, and you could very well get dual certified material.

The area of concern however would be the welding. You would have to ensure in some way that the production welding is also producing material with suitable properties for sour service.

 

[metengr]

The issue concerns susceptibility to stress corrosion cracking (SCC) in the base metal heat affected zone because of hardness. Typically, 22 HRC scale is the threshold hardness for SCC, and that is the reason PWHT may be employed even with SA 516 Grade 60 or 70. The key word is may.

 

[Christine74]

I don't agree with the recommendation because in most instances, plate that is certified per SA-516 Gr. 60 is dual certified per SA-516 Gr. 70. So specifying SA-516 Gr. 60 is no guarantee of reduced hardness.


-Christine