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Crush-Type Seal for Tapered Pipe Threads?

05/29/2009 2:12 PM

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 teflon pipe connections and PVDF pipe connections (and PVDF pump head). I believe it is because of the difference in durometer, between the two materials, and I may be experiencing some creep in the softer teflon threads after tightening. 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 taper thread?

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#1

Re: Crush-Type Seal for Tapered Pipe Threads?

05/29/2009 9:56 PM

Your pressure is low. Did you try out the liquid sealants?

I don't know what is ths ealant for this application, never been in plastics, but the liquid sealanta have the property of filling up the gaps, and then again you don't have to put that much torque so as th crush the threads.

BTW: the torque to crush the threads are likely to seal better, since the softer material will flow and fill up the hap, unless your torque is washing them out.

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#2

Re: Crush-Type Seal for Tapered Pipe Threads?

05/29/2009 10:56 PM

I work with a fair amount of commercial and industrial equipment and I often see a very obvious and very effective solution.

Silicone. Pick your favorite color or brand! Works every time!

For high pressures two part epoxies do super too!

I have used JB weld on hydraulic fittings up to 5000 PSI peak pressure and never had a leak yet and some of those fitting are on constant use and have never leaked a drop in years. Plus if you do need to take a fitting apart it will let go and cleanup with a simple die grinder wire brush.

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#3

Re: Crush-Type Seal for Tapered Pipe Threads?

05/30/2009 1:05 AM

Yes I did once do so but only because of circumstance.

You will appreciate a change of fitting type and be wise. The push-pull fittings are capable for 3psi but maybe not for your application?

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#4

Re: Crush-Type Seal for Tapered Pipe Threads?

05/30/2009 1:44 AM

You are talking about pipe threads.

The first thing about those tapered threads is that they have to be within tolerance. Simple practice in a pipe fabrication shop is to adjust the threading dies so that a sample thread run into a fitting becomes too tight to continue by hand after no more then three turns.

If you are using teflon tape pipe sealant, or any of the brush on sealants, and after pulling the threaded pipe end and the fitting up tight you are still leaking it is because the threading dies are cutting too deep defeating the purpose of the taper because even when you run up to the end of the thread you still are not getting the appropriate squeeze which is the entire purpose of tapered thread cutting. Pipe dope, e.g., teflon tape or or brush on, are intended to seal irregularities in the surface of the metals but the main sealing effect is as a result of the taper squeeze.

Any other solution simply asks for more trouble.

Do it right. Go back and check the adjustment on the threading dies so that you cannot, by hand, run the threaded pipe into the fitting (Or coupling) by hand more than three turns before it is needful to utilize pipe wrenches.

j.

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Commentator

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#5

Re: Crush-Type Seal for Tapered Pipe Threads?

05/30/2009 10:37 AM

Good Answer Jack!! We also work on sites with mixed materials piping. Some of the standard pre-cuts, whether in steel or PVC have little or no quality control. We have found that "chasing" the threads with the appropriate die solves most issues and allows for a more uniform "plastic deformation" of the threads. Again, the key criterea is that you have only 3 (THREE!!) threads of engagement in hand tightening before tool use for the final seal.

A last resort trick that we sometimes use is Gas Line Teflon Tape. This is colored YELLOW (most likely for the ease of the inspectors) and is about 1 1/2 times as thick as the standard white. Sounds like a good match with your low pressure requirement.

Any chance there is a shock or vibration issue that is 'working' your connections loose??

Best on 'ya

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Join Date: May 2009
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#6

Re: Crush-Type Seal for Tapered Pipe Threads?

05/31/2009 2:11 PM

Hi , did you test with LOCTITE 55 ?

http://www.youtube.com/watch?v=3o4uMq6yQiQ

What is the fluid?

Also a 2 component epoxi can solve it

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