Welding a through pipe into a pierced blind
Welding a through pipe into a pierced blind
(OP)
For many years, I've specified letting through-pipe into a blind flange for various functions on pipeline drips (e.g., a level float, a stinger for a drain, etc.). I've never really thought about the welding technique beyond being sure that I had an approved procedure.
This week a colleague asked me if we would be welding the pipe on both the top of the blind and the bottom (see attached drawing), or just on the top. I said "just the top" and he asked about crevice corrosion potential (the system is wet natural gas at 300 psig with 6% CO2) in the unsealed hole. I don't think I've ever had a failure below the weld (I think I would know if a drain line stopped draining, but I'm not certain). He checked with a couple of welding shops and one said "you have to weld both the top and bottom", the other said "we just weld the top".
My concern is that welding both top and bottom will trap some air that could get to pretty high pressure during the welding process, but I don't have an answer to the crevice corrosion question. Does anyone have a feel for a recommended method of sealing around piercings in a blind?
The blind and the through-pipe are both manufactured to API 5L, the pipe is Grade B.
This week a colleague asked me if we would be welding the pipe on both the top of the blind and the bottom (see attached drawing), or just on the top. I said "just the top" and he asked about crevice corrosion potential (the system is wet natural gas at 300 psig with 6% CO2) in the unsealed hole. I don't think I've ever had a failure below the weld (I think I would know if a drain line stopped draining, but I'm not certain). He checked with a couple of welding shops and one said "you have to weld both the top and bottom", the other said "we just weld the top".
My concern is that welding both top and bottom will trap some air that could get to pretty high pressure during the welding process, but I don't have an answer to the crevice corrosion question. Does anyone have a feel for a recommended method of sealing around piercings in a blind?
The blind and the through-pipe are both manufactured to API 5L, the pipe is Grade B.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Welding a through pipe into a pierced blind
I would call for both welds.
RE: Welding a through pipe into a pierced blind
For aggressive acid and alkali lines, we always preferred inside welding, so no experience about crevice corrosion.
I think air trapping may not be a big issue (unlike pad plates, where you have some space for the air to get displaced and pressure built up) as the welding process is not instantaneous and air may be expelled outside as the weld progresses. Bevel the inside edge of flange if still in doubt.
RE: Welding a through pipe into a pierced blind
RE: Welding a through pipe into a pierced blind
You can drill the hole as europipe said (usually not in the weld itself, though), to vent the cavity- some do that with slip-ons too. We don't, and don't have a problem. Any air which needs to escape will do so during the completion of outer weld. If you need subsequent passes, the pressure can't really get all that high in there due to heating of the trapped gas, if that's your concern. The ratio of Kelvin temperatures between room temp and the melting point isn't really all that high.
RE: Welding a through pipe into a pierced blind
By that time, the pipe and blind flange are pretty hot, so the air gap will be hot too - say about 150 to 125F at least. So, when you close the last little gap in the weld, you actually will end up with a "hot" air space compared to ambient, and so on cooling of the whole blind flange, will get a a very slight vacuum between the two welds.
RE: Welding a through pipe into a pierced blind
David