×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Welding a through pipe into a pierced blind

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.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

RE: Welding a through pipe into a pierced blind

As far as I can see this is very much the same as a Slip-On flange weld except the "inside" weld is turned inside-out.
I would call for both welds.

RE: Welding a through pipe into a pierced blind

I did observe corrosion when inside welding is not done for cooling water and chilled water pipes (with slipon flanges) but that was not serious enough to damage the piping.

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

When your concern is trapped air (pressure) you can drill a little hole in the outside weld and fill it with kit before painting.

RE: Welding a through pipe into a pierced blind

Both welds, David- just like a slip-on.  Inner, then outer.  The only time we rely on a single fillet is for 1/2"-2" socket welds.

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

Realistically, you've been welding around the pipe for a few minutes before you get to a point where the "air volume" could be closed off and trapped.

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

(OP)
I talked to a couple of more welders here and got exactly the same answer racookpe.  Thanks.

David

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources