sealing NPT threads

[scubaguy]

I am building a blending station for mixing SCUBA breathing gases. The system will handle pure oxygen and helium at up to 2500 psi and Nitrox II (air with reduced nitrogen (@68%) and increased oxygen (@32%)) at pressures of up to 4000 psi. The manifolds are machined brass and the fittings going into the manifolds are either brass (line valves) or SS (316) check valves or #6 tube to #4 NPTM fittings. I have had large number of leaks, almost all of them between the manifold and SS fittings. I have tried using teflon tape with no success. I have tried using a thread sealant called LOX-8, with greater but not complete success. After reading other threads on this topic, I am considering trying both teflon tape and the LOX-8. The various components are all new and appear to be in good condition and manufactured to the stated specs.

My question is: "Are any other suggestions for things to try if this fails to stop the leaks?"

Thanks for your suggestions.

 

[europipe]

You can also try Locktite55.
It seals a little better than PTFE tape.

Greetings

 

[moltenmetal]

A breathing air and oxygen-compatible anaerobic pipethread sealant. Henkel Loctite would be the first place to look.

We've had good luck using heavy (>1.1 s.g.) teflon tape with a top-dressing of an anaerobic pipethread sealant.

 

[MikeHalloran]

You might try whirling the manifold threads instead of tapping them.



Mike Halloran
Pembroke Pines, FL, USA

 

[hacksaw]

leaks, high pressure helium, high pressure oxygen at 2500 to 4000 psi...try metal to metal seals

 

[scubaguy]

Thanks for your responses and the information. FWIW, I had spoken with Henkel Loctite and according to them, they produce no products which are both oxygen compatible and breathing gas compatible. They steered me to Fluoramics, the makers of LOX-8, which is oxygen and breathing gas compatible.
Fluoramics suggested that the tape might be unnecessary with the LOX-8, but no luck, hence my question here.

Hacksaw - can you give me a more specific suggestion for an appropriate metal to metal seal? Thanks.

 

[hydtools]

Get rid of the pipe threads. Try fittings like those by Swagelok.

http://www.swagelok.com/downloads/webcatalogs/EN/MS-01-34.PDF

Ted

 

[Tmoose]

I talked to a few plumbers lately. They all lament the inability of today's commerical NPT fittings to seal domestic water at less than 80 psi. I got decent water sealing by scrubbing male threads with a soft wire brush to deburr, and using 3 or more wraps of teflon tape.

NPT is only considered on "low pressure" hydraulic cylinders.
G straight pipe threads with o-ring face seals are used on high pressure.