Access hole for gamma inspection in headers

[huguette]

1 - On a P91 header (T=1063F P= 2090 psi)we need access for radiographic inspection. The proposed gamma plug, directly screwed in the thickness of the header, is a 1" plug acc to spec SA182 F22 class 3.
The header is 12" x 1.5"
Do you know problems about F22 screwed plug in P91 header?
2 - Are there any new regulations about high pressure gamma plug concept taking into account leakage experience or any other problems?

 

[metengr]

No. The threaded F22 RT plug should be acceptable in the P91 header despite the lower chromium content, and lower allowable stress values in comparison to the P91 material. Considering the function of the RT plug and loads in service, I don't believe that local creep deformation will be a problem with the threads.

I would recommend that you use the Pipe Fabrication Institute ES-16 for guidance in plug design, if you have not done so already.

http://www.pfi-institute.org/Product.htm

Make sure you radius (1/8" minimum) the corner on the RT plug hole at the header ID surface to avoid stress concentration. Machinists like to leave sharp corners unless told otherwise.

As far as leakage, threaded RT plugs, if installed correctly, last many years. However, at some point plugs could develop steam leaks. I have heard on occasion where RT plugs have been ejected because of creep damage in the header that started from the ID surface at a sharp corner. Typically, leaks will occur at the first sign of trouble.

I would not recommend using a stainless plug because of the dissimilar material concerns and potential for carbon migration in elevated temperature service.