Accident report/ question

[Joshua Smith]

There's an accident report up on Rebreather World:

http://www.rebreatherworld.com/rebreath ... -cave.html

It happened in an old mine, and the divers were on CCRs, but my question isn't specific to CCRs or caves, or anything like that, it's more general:

IMO, aside from a laundry list of mistakes and oversites, this guys biggest problem was the fact that he was unable to shut off the flow of gas into his loop from his diluent tank, even after closing the valve, as in a "boom" drill.
This makes no sense to me. I'm no expert, but how can a valve on a scuba tank suddenly loose the ability to stay closed? I can see one wearing out, and developing a leak over a lot of use and a long period of time, but it would become really, really obvious pretty quickly, wouldn't it? As in: "That tank I filled last night is empty this morning. Hmmm. That's odd."

Am I missing something? Or can valves fail quite suddenly like that?

[boydski]

Am I missing something? Or can valves fail quite suddenly like that?

Any debris that is caught between the valve seat and the valve gate (plug) will prevent it from closing completely. In the case of the accident, it appears that the valve seat was erroded, allowing gas to pass the valve even though it was completely closed.

I have seen several tank valves that have failed. Usually in the closed position (so they can't be opened under pressure), but it is possible to fail in the open position, or to get something caught in the valve the prevents it from closing all the way.

[Joshua Smith]

Am I missing something? Or can valves fail quite suddenly like that?

Any debris that is caught between the valve seat and the valve gate (plug) will prevent it from closing completely. In the case of the accident, it appears that the valve seat was erroded, allowing gas to pass the valve even though it was completely closed.

I have seen several tank valves that have failed. Usually in the closed position (so they can't be opened under pressure), but it is possible to fail in the open position, or to get something caught in the valve the prevents it from closing all the way.

Sure, makes sense, but......ok, forget this was on a rebreather for a second. Let's just say it was a single al 80 or something. You fill the tank. You shut the valve off. And then you go to set it up to dive. There's some period of time before you hook your first stage up to the tank, right? If the valve was leaking catastrophically, as seems to have been the case here, wouldn't the average person notice that? Or did the leak somehow occur after he hooked the first stage up and charged it, which is the only way I could see this happening? The valve seat was erroded to the point that it held on until the unit was charged, and just let go after this poor guy's add valve also failed? It seems pretty crazy to me that someone could ignore that much corrosion, and that the valve could just.....stop.....working.....all at once, and in conjunction with an entirely different failure at the same time.

If he hadn't been in a mine, I guess he would have just rocketed to the surface....

[boydski]

Let's just say it was a single al 80 or something. You fill the tank. You shut the valve off. And then you go to set it up to dive. There's some period of time before you hook your first stage up to the tank, right? If the valve was leaking catastrophically, as seems to have been the case here, wouldn't the average person notice that? Or did the leak somehow occur after he hooked the first stage up and charged it, which is the only way I could see this happening? The valve seat was erroded to the point that it held on until the unit was charged, and just let go after this poor guy's add valve also failed? It seems pretty crazy to me that someone could ignore that much corrosion, and that the valve could just.....stop.....working.....all at once, and in conjunction with an entirely different failure at the same time.

I think it is unlikely that a catastrophic leak would be ignored at the surface, but slow leaks are common. It is also possible that some debris was blown into the tank when filled that nicked the seat when the regulator was pressurized. If the dip tube was missing or loose, any sand, grit, rust, etc. that is in the tank would be very abrasive to the valve seat on a typical rebreather tank (they are generally valve down rather then OC's typcial valve up). I don't think that it is a very common failure, but I've taken the valves off of several tanks that "appeared" to be empty and weren't. (makes for a quick trip to change one's underwear).

[Joshua Smith]

Let's just say it was a single al 80 or something. You fill the tank. You shut the valve off. And then you go to set it up to dive. There's some period of time before you hook your first stage up to the tank, right? If the valve was leaking catastrophically, as seems to have been the case here, wouldn't the average person notice that? Or did the leak somehow occur after he hooked the first stage up and charged it, which is the only way I could see this happening? The valve seat was erroded to the point that it held on until the unit was charged, and just let go after this poor guy's add valve also failed? It seems pretty crazy to me that someone could ignore that much corrosion, and that the valve could just.....stop.....working.....all at once, and in conjunction with an entirely different failure at the same time.

I think it is unlikely that a catastrophic leak would be ignored at the surface, but slow leaks are common. It is also possible that some debris was blown into the tank when filled that nicked the seat when the regulator was pressurized. If the dip tube was missing or loose, any sand, grit, rust, etc. that is in the tank would be very abrasive to the valve seat on a typical rebreather tank (they are generally valve down rather then OC's typcial valve up). I don't think that it is a very common failure, but I've taken the valves off of several tanks that "appeared" to be empty and weren't. (makes for a quick trip to change one's underwear).

Lol- yeah, I bet it would! OK, that's starting to make more sense. Yes, most CCRs have the tanks in the valve down position, and the victim's Inspiration definitely did. I'm sure there's more to the story than he's reporting, and I'm also pretty sure he's someone I wouldn't want to dive with. But, I guess it's theoretically possible, the way he described it.

[CaptnJack]

Say the valve seat itself (the nylon part) came unglued from the brass plug holder bit. The valve could close and completely seal if it remained in alignment when the knob was turned. Now when you open the valve, the nylon part falls out and lodges sideways in the space. Can't close the valve anymore.

Just a hypothetical but having that nylon bit loose and rattling in there could setup the scenario.