Tank blowout - airloss rate

[Santa]

Just recently I had the pleasure of having a tank blowout at 26 metres. The old O-ring gave up the ghost. As one may imagine it happened pretty early in the dive, like 8-10 minutes in so I had plenty of air and surfaced after a regular safety stop without trouble aided only for the last minute by my buddys octo. I did not suck my tank completely dry because I wanted to at least try to avoid flooding my first stage.

But it started me thinking about rates of air loss. For example I could tell that my air wasn't going to be gone in like seconds so I went for a diver I knew was more experienced (he was officially my buddy anyway, but happened to not be the closest diver at the time. If pressed for time I would have gone with anyone or if that was not possible surfaced when my tank was dry preferring possible DCS to drowning any day of the week).

Based on my comp readouts and a little adding and subtracting I've calculated a loss of around 20 bar/minute erring to the conservative side (it was a standard 12 litre aluminum tank). This includes my breathing from the tank during ascent and safety stop so there's an individual element. Depth is of course a major factor as well breathingwise so maybe a sober question might be

"At what rate (bar/min)?" does air leave an open-minus-half-a-turn 12 litre alu tank filled to 200 bars?".

Then we can always screw around with adding our own individual breathing averages and dive-profile considerations I suppose.

(Personally I would just try to construct a standard worst-case scenario and consider everything less added safety)

 

[spankey]

very interesting. I have no idea but I am also keen to find this out.

glad to here u handled it well.

 

[SmileMon]

Just out of curiosity, was it DIN or Yoke?

In rescue class they told me that if a LP hose explodes, it takes 6 min to drain and if its a HP hose, it takes 20 min to drain, about tank o-rings, well, it depends how big the gap is.

one of the biggest problems in my opinion about LP hose/orings is 1st stage freezeup.

If its a yoke valve and a rental, just ask for an o-ring when you rent the tank, if its your tank, just make a regular check before a dive day and look if the oring is bad, if its a DIN o-ring, just replace it once every few months but look at it and make sure it doesn't have cracks.

Nothing can replace a GOOD buddy or a pony bottle, from my short experience, a pony is preffered.

 

[Hoyden]

Just recently I had the pleasure of having a tank blowout at 26 metres. The old O-ring gave up the ghost. As one may imagine it happened pretty early in the dive, like 8-10 minutes in so I had plenty of air and surfaced after a regular safety stop without trouble aided only for the last minute by my buddys octo. I did not suck my tank completely dry because I wanted to at least try to avoid flooding my first stage.

But it started me thinking about rates of air loss. For example I could tell that my air wasn't going to be gone in like seconds so I went for a diver I knew was more experienced (he was officially my buddy anyway, but happened to not be the closest diver at the time. If pressed for time I would have gone with anyone or if that was not possible surfaced when my tank was dry preferring possible DCS to drowning any day of the week).

Based on my comp readouts and a little adding and subtracting I've calculated a loss of around 20 bar/minute erring to the conservative side (it was a standard 12 litre aluminum tank). This includes my breathing from the tank during ascent and safety stop so there's an individual element. Depth is of course a major factor as well breathingwise so maybe a sober question might be

"At what rate (bar/min)?" does air leave an open-minus-half-a-turn 12 litre alu tank filled to 200 bars?".

Then we can always screw around with adding our own individual breathing averages and dive-profile considerations I suppose.

(Personally I would just try to construct a standard worst-case scenario and consider everything less added safety)

See http://thedecostop.com/forums/showthread.php?t=5296&highlight=equipment+failure+test for some answers to your questions.

 

[www.scubamazing.com]

Interesting and complicated. Congrats on handling this dive emergency with grace.

I can see how it coudl be calculated, but then there are so many variables. Obviously you'll lose more at deeper depths. And different problems will lose it a different rates. Hose vs. o-ring etc...

I agree, the best you could probably hope for is a worst case scenario. Guess that's why it's best to dive by thirds (1/3 out, 1/3 in, 1/3 for emergency).

I'm going to roll these ideas over in my mind for a while, let use know what you find out.

 

[Ed Palma]

In rescue class they told me that if a LP hose explodes, it takes 6 min to drain and if its a HP hose, it takes 20 min to drain, about tank o-rings, well, it depends how big the gap is.

Hey Mon, I'm just curious about those values: 6 min for LP and 20 min for HP?

Just checkin'...

 

[SmileMon]

Yep, check the link Hoyden posted, it seems 6 minutes is too long, that guy did a test on a high performance reg if I read correctly.

You can try it yourself, just open the hose connection to the 2nd stage while having the whole thing pressurized, I would get good ear protection before doing it!!!!

I didn't test mine yet, maybe its a good time to open a thread with regulator models and their performance on a freeflow/blowup...

 

[Ed Palma]

So it would actually be better to have the HP line blow out 'coz it lasts longer?

I'm confused... (and I don't have access to the page ) Will try again later.

While mulling it over, I figure the worst case scenario would be a failure of the tank neck o-ring. Whether at the surface, or underwater, the pressure of the air in the tank would push through the ruptured ring at around 3000 PSI (because it bypasses the regulator). So I think that the rate of emptying on the surface would more or less approximate the same time at depth.

That's my 0.02.

(I'm off to check out that thread!)

 

[TheRedHead]

How did your buddy and the other divers around you respond? It seems to me that this would make a huge cloud of bubbles and attract attention. You wrote that you went for your buddy (the closest diver). Did he notice it before you swam towards him?

 

[Vicente]

The HP has a much smaller orifice, unscrew it from the 1st stage and look at the end of the hose. So even at the higher pressure it takes much more time to pass an equivalent amount of gas. All you hook up on this port is an SPG so it doesn't need to be any larger since there is not constant gas movement.

The LP hose however has a large opening since you are constantly draining from it. I've seen a LP hose rupture in a pool session, and it made a bang that momentarily stuns and surprises you. In the pool of course the student just stood up and someone turned the valve off.

Losing the valve O-ring is another matter of course than a hose. Be interesting to see what the worst-case is for that. A good reason for close-by buddy or pony bottle. It's not clear from original post, if this was the O-ring at the join of 1-stage to valve, or the one at the junction of the valve and neck of tank.