Question Valve for 100% O2 Deco Bottle

[InWay2Deep]

I use regular valves and put Green vindicator knobs on them, feel its $$ better spent.

 

[broncobowsher]

One of those rebreather fires, I remember questioning the details. Back when there were good websites for that stuff.

They were having issues with the OPV leaking. Instead of checking the IP and finding the 1st stage had an IP creep issue and the OPV was actually working correctly, they removed the OPV. This let the IP creep with no escape. The plastic threads tore out and the friction of the threads tearing out in a pure O2 system lit it up. Yes it happened when the O2 was turned on. but it wasn't the valve itself that did it. Had to look at the details to find what really happened and not just "opened the O2 valve and the rebreather caught fire" of the headline.

 

[Agro]

Never ever heard chrome is a problem for oxygen. Why should it be? Poseidon Xstreams are certified for 100% oxygen and are chrome plated all over. I don't believe the story "chrome is bad".

The ox valve is not lubricated? I can hardly believe that. Any O2 valve I know, any O2 regulator I know is lubricated. A non lubricated o-ring will not last very long in a 3'0000 psi valve, you add a failure point to your equipment.

Slowly opening, fine thread: A traditional valve will always open rather fast, no matter what kind of thread you have. You can try(!) to open the valve slowly, sometimes it works, sometimes it does not. If you want a slowly opening valve in every(!) situatiation you need a needle valve. No manufacturer produces needle valves for O2 diving cylinders. So this seems to be unneccessary.

 

[EricSm]

So, in your opinion, take every reasonable measure that could plausibly improve O2 safety? Don't read this as me doubting what you said--my diving philosophy is indeed to take every reasonable measure to stack the safety odds in my favor. If switching my O2 bottles' valves out for DGX O2 Pros is in line with my philosophy, I won't hesitate. I just hadn't seen them mentioned before, that's all.

By the way, now that you mention the nylon valve seat, I am wondering why DGX did not include a less-combustible valve seat among the features of the O2 Pro? I know little about this stuff, but my understanding is that we don't see such a valve seat in regulators for scuba O2 use because the standard material (nylon?) is reasonably non-combustible, and an even less combustible material would be an expensive aerospace type, of which someone would then have to custom-manufacture valve seats. So I guess with the O2 Pro DGX decided on a compromise of an essentially standard tank valve with a couple of fairly straightforward but plausibly combustion-resistant features at a reasonable price point.

I imagine he misspoke about the valve seat being nylon. A normal ~40% O2 regulator would use a viton seat. A regulator in ~100% O2 service would be fitted with a PTFE (Teflon) seat, which is white and looks like Nylon. A PTFE seat has an autoignition temperature of at least 450C while nylon has a melting point of 250C.

 

[EricSm]

Never ever heard chrome is a problem for oxygen. Why should it be? Poseidon Xstreams are certified for 100% oxygen and are chrome plated all over. I don't believe the story "chrome is bad".

The ox valve is not lubricated? I can hardly believe that. Any O2 valve I know, any O2 regulator I know is lubricated. A non lubricated o-ring will not last very long in a 3'0000 psi valve, you add a failure point to your equipment.

Slowly opening, fine thread: A traditional valve will always open rather fast, no matter what kind of thread you have. You can try(!) to open the valve slowly, sometimes it works, sometimes it does not. If you want a slowly opening valve in every(!) situatiation you need a needle valve. No manufacturer produces needle valves for O2 diving cylinders. So this seems to be unneccessary.

The disadvantage with chrome is that one of the (many) ignition mechanisms for oxygen service is particle impact. Particle impact is is the most common method for metal ignition in oxygen systems. Heating from adiabatic compression is the most common source of fires overall e.g. quickly open a 1st stage and pressurize a volume of gas. The temperature rise is astounding.

Chrome tends to flake off and thus introduce metal particles into a flowstream. This could plug the particularly small MAV orifice for non-electronic rebreathers. Brass has excellent oxygen compatibility to 10,000 psi and is sort of self lubricating given its relatively soft constituents.

 

[EricSm]

One of those rebreather fires, I remember questioning the details. Back when there were good websites for that stuff.

They were having issues with the OPV leaking. Instead of checking the IP and finding the 1st stage had an IP creep issue and the OPV was actually working correctly, they removed the OPV. This let the IP creep with no escape. The plastic threads tore out and the friction of the threads tearing out in a pure O2 system lit it up. Yes it happened when the O2 was turned on. but it wasn't the valve itself that did it. Had to look at the details to find what really happened and not just "opened the O2 valve and the rebreather caught fire" of the headline.

I'm not familiar with that incident, but from your description of threads tearing out after the O2 valve being opened, it sounds like the the equivalent of opening a ball valve in an oxygen system, a sudden pressurization causing heating via adiabatic compression.

I read through the 4 pages of posts and figure I'll mention it here. This is not directed at broncobowsher. This is supposed to be a technical section where people provide answers based on science, but there is a lot of 'yeah this is probably good' or 'I bought a compressor and an O2 intensifier and haven't blown up yet' or no training on handling O2 service is best - just go with common sense. If you don't know you don't no.

If you have an O2 cylinder on a summer day (doesn't matter in the scheme of things) at 45C and that cylinder is full at 3300 psi. You connected it to your rebreather valve block and whack open the bottle valve, the temperature at the (hopefully) PTFE valve seat is given by the following formula (everything is in Kelvin and Pa for pressure). Gamma or y is 1.4

T2 = T1 * (P2/P1)^((y-1)/y)

T2 = 1495K or 1222 C or 2231 F

The autoignition temperature of Viton is 250C and the autoignition temperature of PTFE is somewhere above 450C. We are talking many times the autoignition temperature of any plastics, seals, elastomers. So, every time you open a 100% O2 cylinder it should ignite the valve seat and then the valve itself though a kindling reaction, the only reasons it doesn't are:
1.) The valve was opened slow enough to dissipate the heat into the brass valve body through use of the correct slow opening valve
2.) The finger oils or hydrocarbons weren't present to contribute to the kindling reaction, but you'll probably find soot in the mouth of your rebreather O2 bottle o-ring seat if you look...
3.) The volume of gas that is contained between the bottle valve and the regulator block is quite small so there is relatively little thermal mass (thermal energy to transfer).
4.) If you were using an SPG or swift transmitter on a whip from your valve block, you have an orifice spool at the hose end nearest the valve block so as not to quickly compress all the gas contained in the SPG whip (as that has a lot more energy or thermal mass).

 

[rjack321]

I imagine he misspoke about the valve seat being nylon. A normal ~40% O2 regulator would use a viton seat. A regulator in ~100% O2 service would be fitted with a PTFE (Teflon) seat, which is white and looks like Nylon. A PTFE seat has an autoignition temperature of at least 450C while nylon has a melting point of 250C.

There is no Teflon in any scuba reg seat that's for sure. Teflon is also a completely impossible as a scuba valve seat as well - it's far too soft and will extrude around the sealing cone under pressure. They look and act like nylon because they are nylon seats.

I'm not familiar with that incident, but from your description of threads tearing out after the O2 valve being opened, it sounds like the the equivalent of opening a ball valve in an oxygen system, a sudden pressurization causing heating via adiabatic compression.

I read through the 4 pages of posts and figure I'll mention it here. This is not directed at broncobowsher. This is supposed to be a technical section where people provide answers based on science, but there is a lot of 'yeah this is probably good' or 'I bought a compressor and an O2 intensifier and haven't blown up yet' or no training on handling O2 service is best - just go with common sense. If you don't know you don't no.

If you have an O2 cylinder on a summer day (doesn't matter in the scheme of things) at 45C and that cylinder is full at 3300 psi. You connected it to your rebreather valve block and whack open the bottle valve, the temperature at the (hopefully) PTFE valve seat is given by the following formula (everything is in Kelvin and Pa for pressure). Gamma or y is 1.4

T2 = T1 * (P2/P1)^((y-1)/y)

T2 = 1495K or 1222 C or 2231 F

The autoignition temperature of Viton is 250C and the autoignition temperature of PTFE is somewhere above 450C. We are talking many times the autoignition temperature of any plastics, seals, elastomers. So, every time you open a 100% O2 cylinder it should ignite the valve seat and then the valve itself though a kindling reaction, the only reasons it doesn't are:
1.) The valve was opened slow enough to dissipate the heat into the brass valve body through use of the correct slow opening valve
2.) The finger oils or hydrocarbons weren't present to contribute to the kindling reaction, but you'll probably find soot in the mouth of your rebreather O2 bottle o-ring seat if you look...
3.) The volume of gas that is contained between the bottle valve and the regulator block is quite small so there is relatively little thermal mass (thermal energy to transfer).
4.) If you were using an SPG or swift transmitter on a whip from your valve block, you have an orifice spool at the hose end nearest the valve block so as not to quickly compress all the gas contained in the SPG whip (as that has a lot more energy or thermal mass).

Well the gas hitting the valve seat is *colder* than your 45C starting gas temperature due to adiabatic expansion so your efforts to provide predictability to oxygen (which are a bit optimistic) are kinda fundamentally flawed.

 

[EricSm]

Well the gas hitting the valve seat is *colder* than your 45C starting gas temperature due to adiabatic expansion so your efforts to provide predictability to oxygen (which are a bit optimistic) are kinda fundamentally flawed.

No wonder diesel engines never worked in practice. The temperature drop from throttling through the intake valves made it impossible in practice. I joke.

If the initial temperature were -10 C (14 F), the T2 temperature would be 963 C (1236 F) so it still doesn't matter in the scheme of things. The net action is a volume of gas is going from 14.7 psi to 3300 psi creating a significant temperature rise. A diesel engine goes from 14.7 psi to ~ 235 psi (16:1) and that is sufficient to run our interstate transportation infrastructure.