Announcement

Collapse
No announcement yet.

Pipe union question: oil, dope, or...

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Pipe union question: oil, dope, or...

    A likely simple question that I can't find a definitive answer to with the 'internet search beings.' When assembling black iron pipe unions, what is used in assembling the two flanges? I am not talking about the NPT threads which connect the existing two pipes/nipples. I mean the flange connection that defines the union itself.

    I get the impression all that is used is a light coat of oil---the ones I bought have such on the flanges already. Am I right that dope is in fact not recommended for this connection? Thanks for the help and clarification.

  • #2
    The faces on both halves of the union are machined to fit (seal) without the addition of any sealant.

    Posts on an HVAC chatroom generally confirm this thinkin. http://www.diychatroom.com/f17/black...wet-dry-95849/
    Last edited by Rosco-P; 06-06-2012, 03:02 PM.

    Comment


    • #3
      Thank you very much, Rosco-P (nice moniker too!). That is basically what I thought; good to get confirmation of the fact. Next step---finally a real supply of compressed air in the shop. Can't wait!!

      Comment


      • #4
        Keep in mind that unions are to connect pipes that are in line. Not to correct miss alinement. That is were the leaks come from.

        Bob

        Comment


        • #5
          There's also absolutely nothing wrong with a little oil or even a thin layer of dope,

          unions do handle very slight mis-alignments and at the very least will straighten out and seat better if allowed to set into place (with lube) whilst tightening...

          Comment


          • #6
            I actually just ran compressed air lines in my shop as well. I'll post pics later when I have some time. I did not use any sealant on the union.
            Stuart de Haro

            Comment


            • #7
              I generally use pipe dope on the threads, and usually just a little bit on the faces of the union halves. The stuff on the threads allows a tighter connection and the stuff on the faces helps in some cases against corrosion. One has to be careful if the line needs to be clean, or if foreign matter can clog something down the line, but for an air line in a shop unless you go overboard on the application you will most likely not cause any trouble. Oh, and it also makes for easier disassembly down the trail if needs be.

              rollin'

              Comment


              • #8
                Being in the gas industry we use more of an anti-seize compound on the threads of the union. Pipe dope will make it near impossible to get apart once it is set. The threads on the union are not tapered and wont release as easy.And the better unions will have a machined brass ring in the female side to aid in sealing.

                Comment


                • #9
                  The brass bit is interesting to hear. Mine have that. They're nothing special---just the big box "Made in China" variety:

                  It makes sense to me to use a lubricant or anti-seize compound on the collar threads. They are not part of the seal. They are there only to create the seal by compressing the two flanges. Thanks again, everyone, for a thorough reply to my question.

                  Comment


                  • #10
                    +1 for the anti-seize compound. In a previous job I installed and serviced large ammonia refrigeration systems. We used schedule 80 black iron pipe. In some places we used unions. Our procedure was to use xpando on the pipe threads and nickel anti-seize on the union threads. With this combo I never had a leak with ammonia proof tested to 240 psi.
                    Robin

                    Happily working on my second million Gave up on the first

                    Comment


                    • #11
                      A light coat of coppercoat works well, used to use lead sealant on gas unions but might be hard to come by with the green movement........
                      Opportunity knocks once, temptation leans on the doorbell.....

                      Comment


                      • #12
                        Originally posted by rdfeil
                        +1 for the anti-seize compound. In a previous job I installed and serviced large ammonia refrigeration systems. We used schedule 80 black iron pipe. In some places we used unions. Our procedure was to use xpando on the pipe threads and nickel anti-seize on the union threads. With this combo I never had a leak with ammonia proof tested to 240 psi.
                        +1 on the XPando. I've installed several black iron pipe air systems for myself and friends using it on all the joints and have never had a leak. Not exactly easy to break the joints apart in the future however (a propane torch helps).

                        Here's a link to the stuff:

                        http://www.xpando.com/pjc.html

                        You can get it from most plumbing supply houses or even McMaster Carr. A 14 oz can is a lifetime supply for a non-pro plumber.

                        Brian
                        Taxachusetts

                        Comment


                        • #13
                          Not a plumber but there's just some common sense guidelines when dealing with unions,
                          Like I stated a thin layer of lube or dope on the sealing surface itself doesn't hurt and will help seat things more uni-form,

                          What's also good practice is a mild cinch-up and then grab the knuckle and oscillate it so that it settles both pipes into the closest position - this ensures a straighter line of the coupling also and eliminates a higher unit pressure favoring one side (you can see this effect on some unions in the copper sealing area after disassembly.)

                          The proof is also in the pudding when you go back to re-tighten and all tension is lost -- if this is the case then repeat the above procedure before finally pouring the coals to it and tightening it up for good...

                          Comment

                          Working...
                          X