Which O-ring material? A Conundrum

[lexvil]

Way back in time, I worked on hydraulics on submarines, all o rings came individually packaged and had a shelf life, from cure date that was printed on the package, of 5 years and I’m pretty sure they were nitrile, if QA found an out of date package we got to do the job all over.

 

[Akimbo]

Way back in time, I worked on hydraulics on submarines, all o rings came individually packaged and had a shelf life, from cure date that was printed on the package, of 5 years and I’m pretty sure they were nitrile, if QA found an out of date package we got to do the job all over.

The Navy can get pretty anal. We would receive O-rings individually heat-sealed in a bag and tagged, open them in a class 100,000 clean room, run them through the vapor degreaser, bag & tag them as "Oxygen Clean", and use them on pure Helium service. Our tax dollars at work.

To be fair, this was the early 1970s not long after the Apollo 1 fire in 1967 and the NEDU fire in 1965. Oxygen fires were still sort of mystical. We had to wear Nomex clothing including boxers and use Nomex bedding, even when we were WAAY too deep to support a flame. We were only running 0.3 ATA so the atmosphere could only support combustion for about 2 days during decompression.

 

[-JD-]

@rsingler I had a few thoughts about potential failure modes as I was thinking about this, so more questions on that than material.

Assuming that you actually had not replaced the o-ring and the hose was installed 2 years ago new (I think I have that right) is there any indication of original production date? I'm wondering if it could simply be (possible significantly) old stock and therefore aged orings.

How compliant was the failed o-ring? (Did it feel brittle or was it more likely damaged)

Were you able to examine the groove edges of the hose for any indication of burring?


What type of angle-swivel was it? One reason I ask is that when I picked up my long hose from a local source I found that it wouldn't seal properly with my swivel (not necessarily the one I had the issue with and that hose was not in play for my failure) but works fine with my (HOG) reg. Thinking about it in this context:

• I should just proactively change the o-ring - it is probably old stock
• I'm wondering how much variation in the male and female barrel diameters may come into play in this situation.
  • Large female/small male diameters will leave a larger gap - I would think increasing the chance of extrusion/pinching during rotation/wiggling in use.

@Akimbo - just to ad a data point in my failures - those components had probably never seen salt and if they did (<6 with rinsing) there were 20+ FW dives since before starting the BB trip. Admittedly on the trip there was no rinsing and maybe 10-12 dives over 4 days before the failure.

 

[rsingler]

Well, I just tested twenty o-rings from multiple batches. The Nitrile and EPDM ALL had tensile strength double that of the Viton o-rings.
Then I tested Atomic Viton o-rings (of a higher durometer) and they were similarly low breaking strength.
I tested Scubapro 2-010 port plug o-rings, and they broke much higher (?EPDM?).

Re: the failed hose o-ring, the company that supplied the hose does a HUGE volume, so I doubt old stock.
The failed o-ring felt supple, and didn't look dried or cracked under the microscope. But as I suggested above, from the surface markings, it didn't look like one of my replacements. I wish I could tell what material from looking at it. The hose is advertised as "Nitrox ready" which may mean nothing vis a vis the o-ring, and just refer to the inner tubing. But maybe it means a Viton o-ring, too, which is beginning to fit my prejudice about that material

No causes that I could see in the bore of the second stage tube or 90-degree fitting. The 90-deg adapter is a DGX fitting, which I've been happy with, though I have disassembled each one I receive and put LocTite on the threads of the inner nipple to make sure it never disassembles.

If tensile strength bears any microscopic relation to tear resistance, then I conclude that a pinch on these Viton o-rings may be more likely to create a flaw that propagates to failure, the thinking being that the pinch is a microscopic pull on adjacent o-ring material, just like a tensile strength pull.

 

[Akimbo]

If tensile strength bears any microscopic relation to tear resistance, then I conclude that a pinch on these Viton o-rings may be more likely to create a flaw that propagates to failure,

Seems like a reasonable assumption but why not check directly with the O-ring gurus at Parker? They have always been helpful and informative when I emailed or phoned them.

 

[iain/hsm]

Where did you get a shelf life of 5 yrs for nitrile? All the searches I have done say 15 yrs, plenty long enough for scuba duty.

Easy. Five years from the manufacturer out east in China.
Add another five years for the major Importer distributor in USA on his shelf
And finally add the final five years of shelf life your talking about from the scuba brand supplier to the scuba shop you buy them from to the end user.

Buying them from a known supplier of a known compound, with a printed cure date, and batch number and in a sealed bag kind of solves a number of problems from the get go.

 

[iain/hsm]

Having decided it was cheap Chinese nitrile, I figured I'd replace it with a nice Viton o-ring

The amount of "Carbon Black" in your nitrile compound is in question here,
Its a cheap bulk filler. As the last question on shelf life unknown compounds result also in poor performance.

All my enthusiasm for using Viton o-rings for their 20+ year shelf life has just gotten called into question. What use is a "permanent" and less flammable oring material if its malleabllility is that poor?

1. Again you need to define the term "Viton" then specify the various compound blends of "Viton" available for you to consideration.

2. Consider also if it was indeed "Viton" and not FKM or FKM viton a similar compound but much cheaper out of China and the like. Sold to most end users as "Viton" but fake by performance criteria and standards.

Maybe there's a reason my JJ-CCR has nitrile o-rings, even if their shelf life is only 5 years...

Thoughts?

That's just ignorance on behalf of the JJ company and a lack of engineering know how
and application on the subject matter.

 

[iain/hsm]

What use is a "permanent" and less flammable oring material if its malleabllility is that poor?

It's a complex answer, based on the application.
By permanent I have assumed you mean a permanent shelf life and not operational life.

1. By example Viton (compound specific) is the preferred material for high pressure oxygen compressors in both static (seal) applications fittings tube ends (BOSS threads) etc and dynamic Piston rods etc. It is also workable up to a useful maximum temperature of around 350F say 200C

It fails safe mechanically with (not too rapid) loss of pressure and in areas of highest pressure and highest temperature the risk is further reduced by re design of the compound and the polymer volume.

Hence the "kindling effect" is reduced in these areas of special concern with additional safety introduced by using alternative metals than steel for the O-ring housings

And by example a 200 bar say 3000 psi 12 CFM pure oxygen compressor would be running 24/7 and have these dynamic O-rings replaced on an annual basis of up to 8000 hours running time. And the statics? replaced by and large when they feel like.

By comparison genuine 100% Viton specific compounds are not that expensive.
But for the example above the correct compound of genuine 100% Viton is that good in this specific application example.

 

[rsingler]

I respect your knowledge on this subject and concede that I'm dealing with some unknown qualities in my labeled "Viton" products.
That said, I can't either ignore nor explain why the unknown FKM samples in my shop AND the Atomic Viton HP seals both had tensile strength half of my nitrile and EPDM comparisons in the same size and similar duro.
Oxygen capabilities notwithstanding, if my Viton equivalents are more delicate to "pinch" damage on something as generally idiot-proof as a LP hose end service, then that makes Viton the poorer choice for Air/EAN40 service.
What is the specification I should be looking for on an ASTM D2000 spec that would give me equivalent tear-resistance?

 

[Endler's]

I vaguely remember a hose thread, in which tbone1004, said something about avoiding Viton O-rings on Second Stage hose swivel.

Perhaps he can recollect?