Connection issues of pressure lead lines on carbon steel pipes
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For example, if the process pipelines are made of carbon steel, the root valves for the instruments are generally also made of carbon steel (the root valve is selected based on the material of the pipeline). My approach is to use a flanged root valve, and for the instruments, I attach a stainless steel flange cover; holes are drilled in this flange cover, and stainless steel pressure transfer pipes are welded to it. This practice cannot be found in the design and installation manuals, which is why it is controversial. I would like to ask the experts here: how did you switch from carbon steel to stainless steel?
“Changing from carbon steel to stainless steel material involves dissimilar metal welding (which is generally not recommended), or connection via flanges. However, the root valves on instruments are usually welded valves to reduce the number of leakage points. In my opinion, it is not necessary to use stainless steel for the instrument pressure tapping pipes of carbon steel pipelines; if stainless steel must be used, your proposed solution can be adopted.
Why not drill NPT internal thread holes on the flange cover?
We usually drill a full 1/2\" NPTF hole in the flange cover, and then connect the instrument pressure tap using a ferrule joint.
You want to use stainless steel pressure transfer pipes, right? The method you mentioned is quite correct. For carbon steel pipelines, open the pipe, add a carbon steel flange + a root valve (the material doesn’t matter) + weld the stainless steel pressure transfer pipe to the stainless steel flange or to a blind plate with holes; or simply open the carbon steel pipeline directly, lead out the stainless steel pressure transfer pipe (200 mm is sufficient) + add a root valve (with internal threads at both ends) + stainless steel pressure transfer pipe
2 flanges, 1 carbon steel, 1 stainless steel.
We generally use socket-welded root valves, with a short sleeve welded to the side connected to the pipeline, and a stainless steel socket-welded terminal fitting welded to the side connected to the instruments.
Your approach is in line with the standards. However, I have a question: the short pipes on the pipeline side are likely made of carbon steel, and the socket weld valves are also probably made of carbon steel. This means there is an issue of welding carbon steel (socket weld valves) to stainless steel (connectors), right?