We usually flange at 2", and SW below (1 1/2" down) for sour service, and also glycol. Depending on pressure of course.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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I meant to say, depending on partial pressure of the H2S, to determine if it requires SW.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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Ashereng
Many thanks from your reply, for more information I have seen in Total spec that for below 2" it is possible to using SW in sour servic "H2S+water" in gas refinery except in 10000# rating but there is also some recommendation for using BW even in below 2" for toxic service.
I want to ask is the problem so serious that to be prohibited the using SW?

I don't have much experience with 10000#. Those would take you into API valves, well past ANSI.

For sour service, which for me is H2S, SW is fine. When you say toxic, what do you mean? I am not familiar with other toxic materials, as seen in chemical and other industries, so I could not say.

If your specs call for BW, then I guess I would go BW. If the specs allow for a choice, for H2S, we use SW.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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Hydrogen sulfide is toxic.  It is common to require all socket weld in acid gas service (one percent to 50 percent).  Flanges are normally permitted in normal sour gas both H2S and CO2.  Many piping specifications include socket weld below 2-inch.  The socket weld requirement often excludes control valves.

JL,

Yes, H2S is toxic. We typically call it sour. When amir mentioned toxic, I assumed it was something else.

SW, like I said, is typically what we use up to 1-1/2". 2" and up, we flange.

Control valves are also often SW, although most valves at 2" and up are flanged.

I don't see a lot of BW, which I think is what amir is wondering about. I guess for oil and gas, SW is good enough.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

Thanks everyone for the excellent feedback and comments. Your guidance on the welding type selection helped me nail down the problem.