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Author: Subject: pipe thread sealing
Magpie
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[*] posted on 13-8-2010 at 13:14


Quote: Originally posted by merrlin  

At this point I would be inclined to epoxy a stainless 1/4NPT coupling to the male stub at the tee. Before epoxying the coupling, I would plug the coupling and attach it to the hose with PTFE tape and pressure test it. That way you can verify the coupling before epoxying it to your existing plumbing.


I'm thinking along these lines myself. Only I would use silver solder instead of epoxy as I don't want the maximum service temperature to be limited anymore than necessary. Although using the solder would be a lot more work and expense than would be the epoxy.

Another advantage to the coupling will be the female threads. These are needed for the pressure relief device that will form the "plug" in service.
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Magpie
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[*] posted on 18-8-2010 at 12:26


Strongly suspecting bad threads at the tee/FNPT-JIC connection I carried out the test proposed by merrlin and myself. That is, the female JIC on the hose end was connected to a nipple (NPT/JIC), NPT coupling, and plug as shown below. This system was then brought up to 1500 psig. All 9 threaded connections held.

Pressure would slowly drop off as various compression fittings were leaking slightly at these higher pressures. They were tightened and the leaks stopped. Finally a threaded connection at a brass pressure gage did start slowly leaking. I terminated the test at this point.

I now feel that if the NPT coupling is epoxied or silver soldered to the 1/4" MNPT at the tee on the autoclave (see picture) I will be able to proceed with hydrostatic testing of the autoclave itself to 1500 psig.

Thanks to all those contributing suggestions to help me seal this system.

coupling test.jpg - 114kB

[Edited on 18-8-2010 by Magpie]

[Edited on 18-8-2010 by Magpie]
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Magpie
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[*] posted on 18-9-2010 at 10:39


Quote: Originally posted by Magpie  

I now feel that if the NPT coupling is epoxied or silver soldered to the 1/4" MNPT at the tee on the autoclave (see picture) I will be able to proceed with hydrostatic testing of the autoclave itself to 1500 psig.


This is a followup to the leak that I could not stop before on the autoclave tree.

I did have a coupling silver-soldered to what I suspected were bad male threads on the tee. Today I pressure tested the system again. It held 1500 psig for 15 minutes with no leaks. So I now consider the autoclave to have successfully passed its hydrostatic pressure test.

As always comments, suggestions, and questions are welcomed.



autoclave tree.JPG - 40kBautoclave hydrostatic test pressure.JPG - 57kB




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Eclectic
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[*] posted on 18-9-2010 at 12:56


I'd go with silver solder every time...

Awww...come on...I built an autoclave designed for 50,000 psi

5 inch diameter 316+ cold rolled, 2" bore...


17-4 ph alloy is widely used in Autoclave Engineers high pressure fittings.

[Edited on 9-18-2010 by Eclectic]
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[*] posted on 18-9-2010 at 13:06


Let's see it....and your pressure test. :D

What is 17-4 ph alloy? And how does it compare in cost and availability to SS316 fittings?

[Edited on 18-9-2010 by Magpie]




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[*] posted on 18-9-2010 at 13:57


Quote: Originally posted by Magpie  
What is 17-4 ph alloy?
http://www.specialtysteelsupply.com/17-4ph-stainless-steel.php
http://www.sandmeyersteel.com/17-4PH.html
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[*] posted on 18-9-2010 at 17:24


Have you tried using flax fibers w/ PTFE? it is very strong. like this and this.
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[*] posted on 18-9-2010 at 17:45


http://www.autoclaveengineers.com/products/pressure_vessels/...

AE Style Closure. I can take photo of closure, alas, not body.
Pressure test would have required a 100 ton hydraulic press and special 1/2 sq inch piston modified plug

Patent US2424449
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Magpie
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[*] posted on 18-9-2010 at 17:49


Flax? That's an interesting suggestion. But my joints are now holding with just the Ace PTE joint sealing compound.

If your threads aren't right I'm now convinced the joint will leak no matter what sealant you use. And if your threads are really great you may not even need one. When you tighten to deformation you are really getting a compression seal. But the root-to-peak spiral still must be sealed, I suppose.


@Eclectic
OK, I believe you. Nice equipment, but likely nearly as expensive as Parr.

[Edited on 19-9-2010 by Magpie]

[Edited on 19-9-2010 by Magpie]




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[*] posted on 18-9-2010 at 17:51


Your pressure range would likely be happier using hydraulic line 10,000 psi designed fittings, but you seem to have it working ok using much cheaper standard plumbing.
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[*] posted on 18-9-2010 at 18:00


Quote: Originally posted by Eclectic  
Your pressure range would likely be happier using hydraulic line 10,000 psi designed fittings, but you seem to have it working ok using much cheaper standard plumbing.


Those are hydraulic fittings. Some have JIC compression ends and some have NPT ends.




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[*] posted on 20-9-2010 at 21:38


Magpie: before PTFE appeared, it was widely used by plumbers (ex). even if your threads are great, you need some sealing, even if it is a thin layer.
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[*] posted on 21-9-2010 at 08:25


Quote: Originally posted by denatured  
Magpie: before PTFE appeared, it was widely used by plumbers (ex). even if your threads are great, you need some sealing, even if it is a thin layer.


Yes, I agree. With the NPT design some sealant is required as nicely explained by watson upthread.

Flax is an interesting approach. Perhaps shredded dental floss would also work in a like manner. For general household water plumbing I have always used either Rectorseal No. 7, Ace PTE, or Teflon tape. If the threads are good they all seem to work satisfactorily.

Rectorseal No. 7 has been the industrial favorite for decades in the US. Mine was given to me by a millwright in a paper mill where I once worked.




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[*] posted on 23-9-2010 at 03:11


I'm curious how ptfe will hold up under pressure and high temp. I thought it had a relatively low temperature threshold compared to the kind of things chemists (especially mad ones) like to do.

I also thought JIC & NPT aren't meant to go together. I've learned the hard way the subtle differences between the threading types - you can sometimes get it part way but anything else you damage the threads to accomplish because of the differences in pitch angles etc.

http://www.malonespecialtyinc.com/tech.htm

We got introduced to the fun of different types of plumbing fittings when we got a leybold vacuum pump with BSP fittings. I have a brazed plate heat exchanger I can't play with til I get ahold of 4 ISO 1" fittings. Ugh.

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[*] posted on 23-9-2010 at 07:29


Quote: Originally posted by SWilkin676  
I'm curious how ptfe will hold up under pressure and high temp. I thought it had a relatively low temperature threshold compared to the kind of things chemists (especially mad ones) like to do.

I also thought JIC & NPT aren't meant to go together. I've learned the hard way the subtle differences between the threading types - you can sometimes get it part way but anything else you damage the threads to accomplish because of the differences in pitch angles etc.


As you say organic materials are usually limited to about 300C if not less, and I will be limited to applications below those temperatures.

When I said that some ends are JIC and some are NPT I did not mean to imply that I was mating the two.




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