Changing Scuba Tank Valve

[Doc Harry]

Here is someone with valve problems that arise because they don't torque to specs.

http://www.scubaboard.com/forums/tanks-valves-bands/382261-sticky-valves.html

 

[ajduplessis]

In defence there is no confirmed feedback that a lack of torque on the valve resulted in the problems experienced in this thread.

I also believe that hand tight should be more than enough IMO. This does not make any manufactures recommendation null and void.

 

[delta-v]

Catalina's website recommends lightly lubing the threads with something called Dow Corning Compound 111 to prevent galvanic corrosion. Any idea if this compound is O2-safe? Are there any other 02-safe alternatives in case one can't find DCC111 in the stores?

Just checked online and the DCC111 is silicone-based... that means it can't be used for O2 fills right?

 

[Couv]

Catalina's website recommends lightly lubing the threads with something called Dow Corning Compound 111 to prevent galvanic corrosion. Any idea if this compound is O2-safe? Are there any other 02-safe alternatives in case one can't find DCC111 in the stores?

Just checked online and the DCC111 is silicone-based... that means it can't be used for O2 fills right?

You cannot go wrong with ChristoLube or Tribolube 71 available here: Tools - Lubricants - Northeast Scuba Supply
A few of us did a product review you can read here: http://www.scubaboard.com/forums/tribolube/263864-tribolube-have-you-tried-yet.html

 

[captain]

Once the valve is screwed into the cylinder to the point of metal to metal contact the full circumference of the neck ( you can't see light through a gap between the valve and tank, if light can't pass rubber sure in hell can't) then it is impossible for the o ring to extrude or leak provided the valve and cylinder sealing surfaces are not damaged. A slight further tightening to insure the valve isn't accidentally loosened in the process of handling the tank by the valve is all that is required. If it is tight enough that you can lift an empty tank by the valve and twist it back an forth without the valve loosing it is tight enough.

 

[Akimbo]

I am a lot more concerned about under than over tightening a valve. You need a pretty long cheater to over tighten a cylinder valve into a steel tank. There is also a vast torque range between:

Under Tight: Lose enough for the O-ring to extrude or unscrews with abusive handling or less.

Over Tight: Damages the threads, over stresses the valve body, or rounds the wrench flats. Some might add damage the chrome, but that can be done with a bad wrench or technique and still have an under-tight valve.

We are talking about a thread that is about 1" in diameter, there is no need to treat it like a small screw. You don’t have to go nuts either, but you have to be awfully strong to over tighten with wrench that is 12-14" long. The biggest problem most people have is finding a vise to hold the tank.

 

[delta-v]

Under Tight: Lose enough for the O-ring to extrude or unscrews with abusive handling or less.

I've no experience with o-ring extrusion so can't comment on that, but I wonder how easy it is for a valve to unscrew from a pressurised tank?

Uncoupling the 1st stage is well-nigh impossible without purging the system, so even on an 'empty' tank there would still be around 200-400 psi (15-30 bar) of pressure pushing the valve threads against the tank threads... or not? Just curious.

If so, then I wonder what would be the threshold pressure for keeping the valve tight. Would a dive that ended with 50-100 psi (3-7 bar) left in the tank, be at risk of the valve unscrewing during careless handling?

 

[Akimbo]

I've no experience with o-ring extrusion so can't comment on that, but I wonder how easy it is for a valve to unscrew from a pressurised tank?...

25 PSI, surprisingly not so hard. 3000 PSI, you will probably break something before getting it to back out enough for the O-ring to extrude and relieve the pressure. However, unscrewing under any conditions is not a bad definition of too loose.

…Would a dive that ended with 50-100 psi (3-7 bar) left in the tank, be at risk of the valve unscrewing during careless handling?

Perhaps a very low risk. I have not run tests, but my guess is that a valve loose enough to unscrew from careless handling at 100 PSI is more likely to have extruded the O-ring at 3000, If not, the impact would probably break its little brass neck (or valve stem) off. It all depends on how it is hit.

 

[delta-v]

Thanks Akimbo. Makes sense. Hopefully we'll all be spared the scenario of cylinder-turned-missile if the valve gets hit too hard. Having only seen them rocket-off in the movies, I wonder how fast and with how much force would a regular AL80 leave the boat or dock! Certainly fatal if it hit someone I suppose.

 

[akdeepdiver]

If I remember right, last year (or two years ago) in Europe, a tank was dropped on a dock. The valve sheared off and it shot across, and hit a diver in the chest. I believe it was a fatal accident. It is under the accidents and incidents forum if I recall.