Silicone & Nitrox - OK?

[redacted]

Just how bad is silicone around recreational nitrox mixes? I understand it is not OK for high concentrations of O2 (more than 40%) but how about those mixes that we treat like "air"? I have a shop that still uses silicone on tank valve threads if it is not an O2 cleaned tank. They do PP blending and subscribe to the CGA standard of anything with O2 greater than 23% must be O2 cleaned. But these "air" tanks may be filled elsewhere, from a banked source, exposing the silicone to higher concentrations of O2. Is that a problem?

 

[skyking]

To quote the Oxygen Hacker's Companion: "It's one of those things you may get away with ten or a hundred times, but if on the thousanth time it blows up in your face you're going to feel pretty stupid in those last micro-seconds".

 

[redacted]

To quote the Oxygen Hacker's Companion: "It's one of those things you may get away with ten or a hundred times, but if on the thousanth time it blows up in your face you're going to feel pretty stupid in those last micro-seconds".

Was he talking about PP blending or banked fills?

 

[DCBC]

Just how bad is silicone around recreational nitrox mixes? I understand it is not OK for high concentrations of O2 (more than 40%) but how about those mixes that we treat like "air"? I have a shop that still uses silicone on tank valve threads if it is not an O2 cleaned tank. They do PP blending and subscribe to the CGA standard of anything with O2 greater than 23% must be O2 cleaned. But these "air" tanks may be filled elsewhere, from a banked source, exposing the silicone to higher concentrations of O2. Is that a problem?

They use silicone around 100% O2 in healthcare, so I suspect it's safe.

 

[Doc Harry]

Just how bad is silicone around recreational nitrox mixes? I understand it is not OK for high concentrations of O2 (more than 40%) but how about those mixes that we treat like "air"? I have a shop that still uses silicone on tank valve threads if it is not an O2 cleaned tank. They do PP blending and subscribe to the CGA standard of anything with O2 greater than 23% must be O2 cleaned. But these "air" tanks may be filled elsewhere, from a banked source, exposing the silicone to higher concentrations of O2. Is that a problem?

Your shop sounds like they know what they're doing. It's best to keep Nitrox cylinders O2-clean. Even if you're not doing PP blending, "dirty" cylinders can contaminate the fill station.

Only two instances that I can think of where you might not need an O2-clean cylinder for Nitrox. (But I subscribe to the notion that anything above 22% needs to be O2 clean.)

(1) If you are filling your own cylinders from your own compressor, then you're the one playing Russian Roulette and you can do whatever you want.

(2) If you always go to one and only one dive shop for fills, and have an agreement with that dive shop regarding your cylinders, then that's between you and the dive shop.

Remember, if you are taking your cylinders somewhere for fills, the people at risk for explosions are the fill station operators and any innocent customers who may be standing near the fill station.

 

[DA Aquamaster]

A few thoughts...

1. Nitrile/buna N o-rings have been used in aviaton O2 and medical applications for decades along with silicone based lubricants.

2. The valves in those applications are generally designed for O2 service, come up to pressure slowly and have well designed flow paths with the sharp edges, sharp bends and dead ends basically engineered out of them to avoid heating that could provide a potential ignition source.

3. Not much of number 2 applies to scuba valves - they are in most cases far less than ideal for O2 service.

4. O2 clean is relative. In terms of nitrile versus EPDM versus viton o-rings, the difference is not that some are fire proof, as all of them will burn, the difference is just that the ignition temperature is higher by a couple hundred degrees in basically that order. In a 100% O2 environment and with a high enough temperature to start the fire, anything will burn - including the steel or aluminum tank walls. Even if perfetly clean, fuel still exists and will soon build up as "clean" gasses will still carry some traces of hydrocarbon contaminants. So you can clean and asemble it in a clean room, with suitable materials and non-hydrocarbon based lubes, but what is "clean" today probably won't be after 6 months of regular fills. By then you are just hoping for clean enough.

5. Regardless of how clean it is, you still want to strive to prevent potential ignition sources. Open the valve slowly with the purge slightly depressed.

6. On one side of the argument you have an operational history where rubber o-rings and silicone lubes have done fine with mixes under 40%. On the other side, you have most agencies except NOAA that require O2 clean proceduires for anything in contact with mixes over anythwere from 23-25% O2 - and an army of lawyers who will argue that it is prudent to do so. So if the science doesn't make people nervous, ambulance chasing lawyers will.

Personally, I clean my own tanks and valves - and I pay very careful attention to the valve, as the potential fuel in the valve is going to be more problematic in the tank as there are far fewer sources of ignition in the tank.

But if I also do my own O2 cleaning and might look at it differently if I had to pay a shop to clean 4 sets of doubles, 4 deco bottles and 4 stage bottles every year.