Compression Valves/Fittings for Pressurized Oil System?

[George1985]

I have a static oil system which is pressurized max at around 450 psi. I want to replace the existing piping and valves with tubing with stainless steel compression fittings. Has anyone had problems using compression fitting such as Swagelok or Parker A-Lok in pressurized oil systems? I've done some research and it seems to me if they're installed properly there's no problem. If they're taken apart repeatedly will they lose their integrity and have to be replaced? Thanks in advance.

 

[Cockroach]

Both Swagelok and Parker-Hannifin are good products, I am particularly partial to the latter, Parker-Hannifin have the better quality product and customer support.

But regardless of manufacturer, compression fittings do loose their performance integrity given multiple break and make-up. Neither product is particulary better than the competitor. Remember that the threading are mechanical seals which get broken in, deformed and then backed out. You could get away with about three (3) make-ups if you follow the recommended torque.

Stainless steel is worse, probably two make-up at best. This is a response to material, lots of metal-to-metal transfer and galling. Threading compounds don't help much, you're dealing with metal interference and deformation to get a mechanical seal.

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[George1985]

Thanks for the reply,

What are your thoughts on using welded pipe versus compression fittings for oil systems? I would think that compression fittings are easier to install and maintenance but risk galling and incorrect installation contributing to leaks. I feel like using welded pipe in this application, although more difficult to install and maintain(cutting, re-welding), would be least succeptible to leakage and may come out cheaper in the long run.

 

[rmw]

I would think it depends on how much you anticipate having to take it apart. I have re-read your OP a couple of times and it doesn't seem like you are manufacturing this system, and if that is right, then productivity isn't an issue.

If you are rebuilding or refurbishing it and this is a one time thing, and you have the ability to clean it out after welding, then welded pipe is the way to go.

If you have some sort of need to take it apart in the future, or if you can't properly clean welded pipe, then tubing is the way to go.

Either brand would be acceptable to me.

rmw

 

[MikeHalloran]

Stainless compression fittings assemble better, and especially re-assemble better, if you apply a little Never-Seez to the threads before first assembly. ... that specifically means before even hand-assembling them 'for fun'.





Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

450 psig oil is no problem for compression fittings. They're routinely used for 3,000 psig hydrogen, which is a far more difficult sealing proposition.

We switch from compression fittings to pipe beyond 3/4" OD tubing, which has about the same bore as 1/2" pipe. Beyond 3/4" OD, there's questionable cost advantage to using tubing relative to pipe because the fittings and valves become so expensive.

All compression fittings have a limited number of make/break cycles. Anti-sieze paste does help but doesn't stop the joint degradation process.

Tell us more about your application and maybe we can help more. Why do you need to disassemble? How often?

 

[George1985]

Thanks for the replies everyone.

There are seven valves in this run of piping, a backpressure relief valve, check valve, and several needle valves for bypass, isolation, and pressure gauges. Sounds like compression fittings is the way to go since I want to design this system mainly for maintenance ease just in case they need to disassemble for a repair or something like that. The rest of the piping upstream and downstream of the collection of valves is welded.

I've never used compression fittings before and the only information I had been getting were from salesmen. Once again thanks for the information.