How Long to Drain a 80 Cubic Foot Cylinder - 4 Failures

[Bob DBF]

Yeah, maybe, if you don't panic. For a burst disk (is that an O-ring?) it said you have 72 secs from a full 80 cf -- that's not a lot of time to get to the surface from 60 feet. And then no air to inflate your BCD when you get there. You better have a pony or a buddy!

So we're all practicing oral inflation, right?

- Bill

Burst disc is the over-pressure relief, set below hydro pressure, so the tank will not rupture if subjected to overheating or a crazed fill monkey.
It is a copper disc under a small nut, with holes in it, on the tank valve. (Disc set below)

I haven't run across one that burst underwater. Yet.

For those of us that were trained at 60' / min to the surface with no safety stop, it is plenty of time, even if solo. Gets to be more of an issue as you go deeper.

With an empty tank I float on the surface so I have plenty of time to inflate, but yes. One of the many reasons for proper weighting.

Most of these issues can be avoided by inspecting and maintaining your gear regularly.


Bob
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"No matter what happens, somebody will find a way to take it too seriously" -Dave Barry

 

[grf88]

The fastest I have seen a tank drain was when my son's first stage diaphragm failed at 80 feet on a high current wreck in the St. Lawrence.

 

[Nemrod]

A second stage, no matter over balanced or whatever you may, cannot flow more air than an (cut) open LP hose. Sorry, if the one data set shows different it is wrong. Plain and simple wrong. No air flows out of a second stage that did not first flow through the LP hose. If the hose can sustain a particular maximum flow for a free flowing second then it can flow even more when unrestricted by the second stage volcano orifice and seat interference. A second stage could never deliver more air than the hose it is connected to and the port that is connected to can deliver.

Regarding an HP leak, from a HP hose being damaged, most modern first stages have a very tiny restricting orifice and most modern HP hoses also have a restricting orifice. The potential flow rate through a burst HP hose is very small.

By the way, there is no IP when a LP hose is cut, IP exists when there is a regulation mechanism in place by the first and second stages, thus the term regulators. The pressure on an open flowing hose without restriction is ambient until there is a restriction.



N

---------- Post added March 15th, 2014 at 04:40 PM ----------

The fastest I have seen a tank drain was when my son's first stage diaphragm failed at 80 feet on a high current wreck in the St. Lawrence.

I have had a diaphragm fail also, now that I am thinking on it, it unseated on a MR12 first stage. The flow rate was substantial.

N

 

[CT-Rich]

I had a burst disk go on an AL 80 once when I was away at college. Scared the crap out of my parents and they refused to let me store scuba tanks in their house any more....

My guess as to why the the drain times on at depth are so similar is because the changes in relative pressure are so small. A tank at the surface has a pressure of 3000 psi above ambient (~15 lb) at 99 fsw the pressure from the water is ~60 psi. At depth the pressure would be 2940 psi, not a noticeable difference.

The LP going will make a much greater difference because the 1st stage can handle a much larger volume and feeds air at a pressure of fixed relative to ambient. I imagine when you have a catastrophic failure of a burst disk underwater it is very exciting...

(apologies if I am restating what was already posted, I haven't read the whole thread)

---------- Post added March 15th, 2014 at 07:38 PM ----------

Yeah, maybe, if you don't panic. For a burst disk (is that an O-ring?) it said you have 72 secs from a full 80 cf -- that's not a lot of time to get to the surface from 60 feet. And then no air to inflate your BCD when you get there. You better have a pony or a buddy!

So we're all practicing oral inflation, right?

- Bill

This would be one of those rare moments when ditching weight would be appropriate (at the surface), unless you are capable of doing the dreaded "oral inflation" of legend...
[video=youtube;icbscq_Z6vI]http://www.youtube.com/watch?v=icbscq_Z6vI[/video]

 

[Tiberius Tenebris]

I had a burst disk go on an AL 80 once when I was away at college. Scared the crap out of my parents and they refused to let me store scuba tanks in their house any more....

Too funny, I had a similar experience while living in my small university apartment. A disk burst in one of my AL80s at 3am (I was in a deep sleep at the time). I swear there is no noise like it, very difficult to describe, a bit like a jetliner might sound as it's about to enter ones living room. It was only when most of the pressure had been dumped that it sounded anything like compressed air coming from a cracked valve and it occurred to me what was happening. Hilarious in hindsight, at the time it sounded like the end of the world...

 

[NAM001]

The rate of bleed down is going to be a function of the hp to lp erduction of the regulator. not the lp to ambient. so a 50psi ambient change is minimal in regards to 3000 tp 140 psi differential of teh first stage reduction. or inother words the differece in 3000-140 and 3000 to 190 is not much change.

Ever since Curt wrote that article something about the results have seemed counter intuitive to me, namely that a blown burst disk or a blown LP hose will vent the SAME volume of air per second regardless of depth while a free flowing regulator vents more air the deeper you go.

Any theory crafting on why that would be the case?

R..

 

[Bob DBF]

This would be one of those rare moments when ditching weight would be appropriate (at the surface), unless you are capable of doing the dreaded "oral inflation" of legend...

If you were properly weighted, neutrally buoyant at 15' @ 500# in your tank with your BC empty, why would you need to add air in your BC if your tank was empty on the surface? Possibly because you really want to be on top of the surface, or may be you don't believe in snorkels.

Of course if you are overweighted...



Bob
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I may be old, but I'm not dead yet.

 

[jwelburn]

Fundamentals Of Constant Mass Flow Gas Systems - Rebreather World

This article goes into detail on why you won't see much change with depth on HP failures. (There are a number of other good articles that can help you understand the concept, this was just the first I ran across in a quick search for a decent link.) Gas will be held back by the narrowest point in the system. Flow of gas is generally limited to the speed of sound, so once the gas leaving the tank reaches that speed the flow rate will remain essentially constant. Gas will drain from the tank at a fixed rate until it crosses a a certain threshold. For example, if you fully open a tank valve: (numbers have been completely made up)
A tank at 00 feet will drain at 20 cubic feet per minute until the tank pressure reaches 100 psi, at which point the flow rate will begin to drop.
A tank at 99 feet will drain at 20 cubic feet per minute until the tank pressure reaches 110 psi, at which point the flow rate will begin to drop.
A tank at 198 feet will drain at 20 cubic feet per minute until the tank pressure reaches 120 psi, at which point the flow rate will begin to drop.
So if the flow rate only begins to change at around 100 psi, it's understandable that barely any difference will be seen between a tank draining at the surface and a tank draining at 300 feet.

I know this has been touched on already in this thread, but for the more curious among us I though you might like to better understand the physics behind it.

 

[CT-Rich]

If you were properly weighted, neutrally buoyant at 15' @ 500# in your tank with your BC empty, why would you need to add air in your BC if your tank was empty on the surface? Possibly because you really want to be on top of the surface, or may be you don't believe in snorkels.

Of course if you are overweighted...



Bob
------------------
I may be old, but I'm not dead yet.

my assumption is that if you can't figure out how to oral inflate a BCD on the surface, you probably would be over weighted. Most Newbies start out that way.

 

[Diving Dubai]

I remember my Ex GF's burst disc goin gas she was just about to make her giant stride into the water on her 2nd dive after the OW course that week - it gave me a fright and I was already in the water....

Untli I checked just now it never occurred to me that my Faber steels didn't have a burst disc - although it's obvious why not.

Interesting as this discussion is (and I'm with Diver0001 in that the HP vs LP flow rates are not intuitive, I can assure everyone that if I hear the noise of a freelow under water I won't be trying to establish whether its the burst disc, LP or HP then looking at my current contents, getting out a set of tables and calculating my time to empty... oh no! call me a coward but I'll be reaching for my buddy, looking at my contents and depth gauges and getting to the surface as fast and safely as possible. Once surrounded by breathable atmosphere then I'll figure out WFT just let go