Crush-Type Seal for plastic Tapered Pipe Threads?
Crush-Type Seal for plastic Tapered Pipe Threads?
(OP)
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.
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.






RE: Crush-Type Seal for plastic Tapered Pipe Threads?
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
RE: Crush-Type Seal for plastic Tapered Pipe Threads?
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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RE: Crush-Type Seal for plastic Tapered Pipe Threads?