Crush-Type Seal for plastic Tapered Pipe Threads?

[SPociask]

Has anyone ever heard of using a crush seal, or some sort of o-ring seal on a tapered thread connection? I've always been of the opinion that a tapered pipe thread connection, by its very nature, precludes using a crush seal.

I am having a problem with plumbing leaks (liquid at 3 psi) between PTFE pipe connections and PVDF pipe connections (and PVDF pump head). The leaks start after several days, and reappear weekly. I believe it is because of the difference in durometer, between the two materials, and I may be experiencing some creep or cold flow in the softer teflon threads after tightening. Does this sound like a reasonable assumption? We are using teflon tape also. The crush seal was suggested as a solution, but I have my doubts that the concept can be effectively applied in a tapered pipe thread situation.

Under the circumstances, using identical plastics is not an option. Nor is using flared compression fittings. So I'm wondering if anyone has ever seen a successful application of o-rings or crush seals in a plastic taper thread?

I have included a pic of the leaking system, just for the heck of it. Nearly all the connections begin to leak after some days.

 

[MikeHalloran]

The only thing I've found that works in production on plastic/ plastic pipe threads is silicone rtv used as a thread sealant, just like loctite. That may not be available to you because of chemical compatibility limitations.

The materials are, after all, plastic, and by definition will move in response to continued stress.

One thing I can think of, in the instance where you have TFE female threads over a male fitting, e.g. on the pump discharge, is a metal sleeve over the TFE with an o-ring inside it, in an internal groove, or similarly trapped in a two=piece ring, so that the rubber stores some strain energy and keeps pressing on the TFE. That would be a custom part.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

You should not need Teflon tape and it probably does more harm than good.

Teflon or PTFE is very prone to creep or cold flow and is not really suitable for a thread except under very light load.

It is not because of the difference in durometer.

Regards
Pat
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[btrueblood]

Certainly divorcing the coupling aspect of the joint from the sealing aspect is not a bad idea for plastic materials, esp. ptfe. If you can re-tap the pump head (I'm thinking female NPT ports, not male nipples, as Mike assumed), you could try using an O-ring boss fitting. Examples are in the Parker O-ring handbook, amongst other places, or try getting a copy of MS33649.