Close Call at the LDS and a High Pressure Reminder...

[kelemvor]

Some cave divers seek to abrogate the safety factors of an Overpressure relief valve in the mistaken (in my mind) belief that there is a real possibility of this valve releasing underwater inside a cave.

Third person report, of course. However I've had a cave instructor tell me he has had it happen to him. The story was big fill on a cold morning with say 40 degree air. Jump into the water with an overfilled tank and the temperature instantly increases by about 30 degrees (72 degree spring water) and pop. Obviously, in FL that's a winter only scenario.

I don't think I've ever heard him (or anyone else for that matter) advocate eliminating burst discs. Increasing the capacity of disks, yes. For example some guys put discs for 3000psi tanks into lp 2400psi tanks.

 

[KDAD]

[QUOTE="kelemvor, post: 7733626, member:]
Certainly shakes my confidence in burst disks. I was under the impression they allowed "safe" release of gas. I wouldn't call a tank flying around the shop "safe".
[/QUOTE]

A tank flying around a shop is not safe but it is a lot better than one exploding. The burst disk relieves the tank from over pressure although that pressure does have a small hole through which to escape so if the tank is not strapped sown it will move around. A valve without a burst disk will allow the tank pressure to keep increasing until something gives and that may be a very unpleasant experience

 

[John C. Ratliff]

Third person report, of course. However I've had a cave instructor tell me he has had it happen to him. The story was big fill on a cold morning with say 40 degree air. Jump into the water with an overfilled tank and the temperature instantly increases by about 30 degrees (72 degree spring water) and pop. Obviously, in FL that's a winter only scenario.

I don't think I've ever heard him (or anyone else for that matter) advocate eliminating burst discs. Increasing the capacity of disks, yes. For example some guys put discs for 3000psi tanks into lp 2400psi tanks.

Okay, these guys don't understand physics. Burst discs are designed to burst at about 140% of rated pressure. 140% of 2400 psig is 3360 psig. I don't have time right now to go through the Universal Gas Law equation, but they caused this themseleves. More later...

Repeated overfilling of tanks causes damage. In steel, it is in weakened walls. In aluminum, especially the earlier AL 80 tanks, it lead to sustained load cracks (SLC) in the threads. Overfilling is not a good idea. Get a larger capacity tank, or sling more (although I don't like the streamlining of this configuration).

SeaRat

 

[kelemvor]

Okay, these guys don't understand physics. Burst discs are designed to burst at about 140% of rated pressure. 140% of 2400 psig is 3360 psig. I don't have time right now to go through the Universal Gas Law equation, but they caused this themseleves. More later

No question there, they definitely caused it themselves. Still, it does happen.

 

[Aqua-Andy]

I keep reading in this thread "tank flying around the shop" I doubt that it actually flew around the shop and would bet that it tipped over and the valve knob hit something to cause it to bend. The "blown out ceiling tile" I'm sure was just the air escaping and the air pushed the light weight tile out of the way. I have had a disk blow on one of my tanks, yes it was loud and I'm sure some of the customers may have soiled their pants. The air did blow a picture off the wall and the glass broke when it hit the floor. But no tank flying around the shop.

 

[KDAD]

^^^^
Yeah what he said. Take a full tank and open the valve without a regulator on it. What happens. The air rushes out and maybe the tank rocks around or falls over. If it falls over it may then spin around but I don't think that it will become airborne.

 

[CuzzA]

I keep reading in this thread "tank flying around the shop" I doubt that it actually flew around the shop and would bet that it tipped over and the valve knob hit something to cause it to bend. The "blown out ceiling tile" I'm sure was just the air escaping and the air pushed the light weight tile out of the way. I have had a disk blow on one of my tanks, yes it was loud and I'm sure some of the customers may have soiled their pants. The air did blow a picture off the wall and the glass broke when it hit the floor. But no tank flying around the shop.

Perhaps you and @KDAD are reading a different thread or you both need to work on your reading comprehension skills as I never once said the tank was flying around the shop. The tank DID NOT just tip over. Had it tipped over it would have been impossible to damage the knob and valve stem. There was nothing else near it that it hit. The tank was off the ground sideways and landed on the knob. A failed flip is the best way I can describe it.

Apparently you two think compressed gas is no big deal, but I wonder if you have small children or are willing to put YOUR face next to a burst disc letting go. I recall a recent thread on here where either a valve o ring extruded or burst disc let go at night and the op was in his garage trying to find the source among tanks and consequently put his hand in front of the gas escaping and it basically tattooed air into his hand and under his skin.

 

[CuzzA]

^^^^
Yeah what he said. Take a full tank and open the valve without a regulator on it. What happens. The air rushes out and maybe the tank rocks around or falls over. If it falls over it may then spin around but I don't think that it will become airborne.

The physics of opening a "VALVE" which only slowly releases gas at the rate of which someone turns the knob is far different than an abrupt release of gas under pressure.

 

[KDAD]

Compressed gas is a big deal. Hence the safety measures such as a burst disk. What you are describing is not much different from what I described. I could definately see a tank do a little flip upon an abrupt release of HP gas, even scurry around on the floor. Kind of like what jumping jack fireworks do.

As far as small children go, I don't even like to bring my 8 yo to a big box store like home depot for the potential dangers. If I do she is kept on a very short leash - she is my future dive buddy after all.

 

[CuzzA]

A couple more pics of the bent valve stem, busted vindicator and cracked knob. I will look for the other thread I referenced and will post here.