Broken Faber 120 Steel KNowb

[gshine]

Managed to drop a tank after diving for 25 years - First one and the knob broke off - I have a new complete valve assembly and would like to find a way to bleed off the tank so I can pop the broken valve. Do not want a repair as I would always be concerned over stem damage deeper in the valve. How can I bleed off 3500 psi safely so I can pop the valve - I have read that the burst valve can be unscrewed but would like confirmation or other ideas please. I assume the burst disk is the nut on the opposite end of the valve than the on/off air-flow knob.

Thanks in advance
Gary

 

[Scared Silly]

I would want to see a picture because breaking the valve knob off does not necessarily mean the valve stem is broken off to the point that it can not be turned. For that matter depending on the damage the valve stem can be removed and replaced. Which would then could potentially allow one to continue to use the valve. Note the valve stem moves the valve seat, the valve seat is what actually creates the seal in the valve. They are separate parts.

Yes the burst disk assembly is the nut on the "opposite" side of the valve face (or opposite the valve knob). And worst case could be removed with some projectile potential.

Here is a diagram of a valve assembly:
https://scubaengineer.com/pictures/valves/thermo/thermo_exploded_dwg.pdf

 

[JahJahwarrior]

Take the knob of your new valve with a flathead screwdriver.
The big nut under the knob, loosen that with a big wrench and remove the stick (valve stem) that you snapped on your old valve.
Take the same big nut off the old valve and remove the broken in half stick.
Insert the new stick. The stick will have an oring, and there is a white washer that might stay on the stick or might stay stuck in the nut, either way, you want it assembled with one washer and one oring.
Put the nut back on, put on the knob and loosen it.
No need to replace the entire valve, but since you have it, you might as well.

 

[broncobowsher]

Since you already have a new valve, don't take it apart to cannibalize parts for the old valve.

Two ways to do it. Since the stem is broken you have easy access to the gland nut. Take that off. The remainder of the stem will look like a big screwdriver and it turns a slot on the actual valve. Just take a regular screwdriver and open the valve a little. Walk away. Come back later and open a little more, walk away. Repeat until empty.

The other way. If the stem broke so bad you can't get it off. The burst disk. Not really remove it, just crack it loose. For this you really should be in an open place in case something decides to go flying. It is a bolt looking thing, because it is a hollow bolt with an engineered failure point in it. A socket that fits correctly. A ratchet set to loosen (that is important as you will want to be on the other side of the tank from where you are working and make sure you don't get flipped on that lefty loosey thing). Loosen slightly, when it starts leaking, walk away. come back later and loosen slightly more. Keep repeating until the bolt is completely out. The whole time aimed away from people and keep your hands clear. If that bolt doesn't start leaking until it is all the way out would be bad. 3500 PSI pushing a slug of metal. Thus keep it facing something safe and not at you or people. And if it shoots out, the tank is a rocket as well. Strapped to a pole is a good idea.

I much prefer just using the valve to bleed the tank dry. Burst disk is a last resort.

 

[2airishuman]

How can I bleed off 3500 psi safely so I can pop the valve

HP air is dangerous, get in-person advice and supervision. Seriously. Especially if there's anything other than air in the cylinder. Wear gloves, hearing protection, and goggles, and make sure the tank is thoroughly secured, most shops would put it in a tank vise before fiddling with it.

Every SCUBA tank valve I've seen has a floating stem that is not actually attached to the valve seat. It is ordinarily possible to carefully remove the packing nut (aka glad nut or bonnet nut) that holds the stem in place, then remove the damaged stem with pliers, without releasing any air. If the valve stem is stuck to the packing nut then it may turn when you start removing the packing nut, which will solve your problem too.

Once that is done, the damaged parts can be replaced with new ones and the contents of the tank used on the next dive. Alternatively, the valve seat can be carefully opened with pliers to bleed off the gas, so that the entire valve can be removed for inspection or replacement.

If the valve body itself is bent it is possible that the gland not may not be able to be removed or that the valve seat can't be loosened but that's rare. It takes a fair amount of force to get the packing nut loose, especially on Thermo valves and other designs that use a copper crush washer rather than an O-ring. Experience and judgment is required to know how much force is really excessive.

Removing the burst disc on a full tank is a last resort because the disk may blow rather than leak.