Re: Pipe dope on union mating surfaces?
Posted by dick on April 20, 19100 at 09:53:51:
In response to Re: Pipe dope on union mating surfaces?

I am not familiar with the variety of pipe dope available on the retail market. however i work with and design various seals and use commercial (oilfield) pipe dopes and sealants.

For drill pipe threads that seal on a metal to metal shoulder, somewhat like a pipe union, do not use a compound with large particles of teflon or other stuff. by large, i mean if you rub it between your fingers and it feels gritty then those are "large" particles. pipe dope used on these connections is primarily a lubricant and an anti-galling agent. It causes more of the torque to be translated into axial force pushing the two sealing surfaces together. the seal is formed when one or both of the sealing surfaces is deformed enough to fit perfectly against the other surface. The grease or dope helps the seal a little but not much.

Tapered tubing threads, similar to standard pipe threads , seal by forming a very long spiral leak path when screwed together. As more torque is applied the the threads deform and the spiral leak path gets skinnier. Pipe dope for these threads must be a lubricant, an anti galling agent , and plug up the spiral leak path. The most common dope used in the oilfield contains fairly large particles of teflon. for extreme pressure tests we usualy wrap the pin thread with 3-4 layers of teflon tape, then apply grease containing moly and graphite. Some pipe dope is just a thick paste with no chunks of anything in it . this type "seals" by filling the spiral leak path with a very viscous liquid (paste)that is just too thick to be pumped out by the available pressure. This type is generaly not as good for higher temps and pressures.

This is probably what my daughter calls "information overload" I don't need to know all that crap just answer my question!!!!

No matter what type of dope you use, Torque the hell out of everything.

: I (a civilian) got a lecture from an experienced plumber the other day that the proper procedure for using copper unions was to apply pipe dope to the WHOLE fiting - threads, friction surface (where nut turns) AND on the mating surfaces.

: Being a civilian, I had never used pipe dope on a union, and in particular, felt that the the nature of soft metal would have the union seal better with no dope on the mating surfaces.

: So, what is good practice with unions? Is there an definitive reference on this in some plumbing standards manual?

: Just curious.

: David

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