sticky valves

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maxdeeep

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We are diving Luxfer AL80s w/convertible Thermovalves in a hot salty environment where the tanks dry out thoroughly between dives. After a full fill, when the tanks are immersed, we have on occasion observed a tiny stream of bubbles coming from between the tank and the valve. Upon closer observation and removal of the valve we are seeing substantial salt deposits on the threads and 0-ring and even salt inside the cylinders. Worse, we sometimes are unable to remove the valve without very considerable force and in the process have sometimes destroyed both valve and cylinder. This is a pity because AL80s are pretty costly where we have to buy them. While the preventive action is to frequently remove and clean the valve/tank and replace the valve o-ring, the various solutions suggested to aid in removal of the sticky valve non-destructively have not always worked to our satisfaction.

Something we haven't tried but which I thought up on my own(!?) was to deliberately introduce about 100ML of fresh water into the cylinder by draining it, turn the cylinder upside down and re-pressurize the the cylinder and see if the water dissolves the salt deposits in the threads from the inside out.

Any notions on whether this might actually work? Also, should we slightly acidify the water with a few (how many?) drops of acetic acid, and what pressure should we use?

Curt
 
While the preventive action is to frequently remove and clean the valve/tank and replace the valve o-ring, the various solutions suggested to aid in removal of the sticky valve non-destructively have not always worked to our satisfaction.

I see no reason to do that "frequently". What you do need to do is check for any leaks in that neck valve o-ring - the stream of fizzing bubbles. Then remove the valve, inspect the tank and change the -214 o-ring. Also, make sure you are not introducing water into the tank during filling which I suspect is your real problem. After all, SW will not enter a tank through a leaking neck o-ring as long as the tank is maintained under pressure.

And yes, once you have contaminated a tank with SW, you do need to do a proper service - more thorough than you are suggesting.
 
...After a full fill, when the tanks are immersed, we have on occasion observed a tiny stream of bubbles coming from between the tank and the valve. Upon closer observation and removal of the valve we are seeing substantial salt deposits on the threads and 0-ring...

Sounds like the valves are not getting torqued down to specs, and salt water is attacking the o-ring. See the cylinder manufacturer specs for proper torquing of the valve. Most manufacturers are recommending 50 foot-pounds.

...and even salt inside the cylinders....

There are only a limited number of ways that salt water can get into a cylinder: (1) wet fills, (2) breathing a cylinder to zero and hitting the regulator purge valve, or (3) water leaks through the neck o-ring. Which one applies to you?

It sounds like you are getting leaks through the o-ring and cylinder neck. Generally this is caused by improper installation of the valve (i.e., not torquing to specs) or re-using cylinder neck o-rings, or both.

...Worse, we sometimes are unable to remove the valve without very considerable force and in the process have sometimes destroyed both valve and cylinder...

This is a known problem of bimetal corrosion in a salt water environment, secondary to having salt water inside of the cylinder. It's called galvanic corrosion. The University of Rhode Island did a study into this problem back in 1975:

Corrosion of steel and aluminum scuba tanks / Francis C. Cichy, Hilbert Schenck, John J. McAniff, URI Marine Technical Report, 1975

Basically, they found that using a lubricant on the valve threads helps prevents the flow of electrons between the two dissimilar metals (aluminum cylinder and brass valve) and reduces the incidence of galvanic corrosion and sticky valves. The type of lubricant doesn't seem to matter, as long as it is a lubricant that is approved for the intended use (i.e., use an oxygen-compatible lubricant in Nitrox clyinders).

Of course, if you eliminate the electrolye solution (salt water), then you also eliminate galvanic corrosion altogether.

Thread lubricants may not required by the manufacturer, but lubricants do reduce the incidence and extent of galvanic corrosion.

...While the preventive action is to frequently remove and clean the valve/tank and replace the valve o-ring, the various solutions suggested to aid in removal of the sticky valve non-destructively have not always worked to our satisfaction...

That's because you are not servicing your cylinders frequently enough. Based on your statements, you should be servicing your cylinders (i.e., performing visual inspections) at least quarterly or even monthly. If you service your cylinders more frequently, you'll catch the problems before they've developed very far.

...the various solutions suggested to aid in removal of the sticky valve non-destructively have not always worked to our satisfaction...

Once the two metals have bonded together through galvanic currents, your only choice is to use brute force to shear the metals apart.

...Something we haven't tried but which I thought up on my own(!?) was to deliberately introduce about 100ML of fresh water into the cylinder by draining it, turn the cylinder upside down and re-pressurize the the cylinder and see if the water dissolves the salt deposits in the threads from the inside out...

Water isn't going to dissolve metal. Remember, the problem is not "salt deposits," the problem is that you have welded aluminum to brass through galvanic currents. No amount of soaking in fresh water is going to dissolve the weld.

In summary...

I recommend that you:

(a) torque your valves to the specifications recommended by the cylinder manufacturer. I use a long torque wrench and crow foot wrenches:

674323774.jpg


(b) avoid wet fills and breathing cylinder to zero, and

(c) inspect/service your cylinders at least monthly until you get this problem under control, then consider doing inspections on a quarterly basis.

Valve installation torque values for Luxfer aluminum alloy cylinders and liners - Luxfer: Setting The Standard Worldwide
 
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I would add be sure the O rings being used are the correct size and material.
 
Doc Harry covered most of the things you should check. You did say that you change the valves?? I have seen before that ALI valves are replaced with M25 thread (European) valves and not the imperial 3/4 valves. I am not saying this is the case but please check it.

a M25 valve will fit a 3/4 tread with disastrous results as more than 80% of the tread contact surface are not in touch. I have seen cylinders with this config blow of valves during filling. This could be the cause of the air leaks and explains the wear on both cylinder and valves.

Safe Diving
 
I would add be sure the O rings being used are the correct size and material.


....and correct o-ring hardness which is Duro 90.

Until you have the problem sorted, avoid using a fill tank by taking the time to do a slow dry fill. A wet fill can be of benefit if the operator knows what he's doing, but it sounds like you have someone filling tanks that could use a refresher.
 
I recommend that you:

(a) torque your valves to the specifications recommended by the cylinder manufacturer. I use a long torque wrench and crow foot wrenches:

674323774.jpg

I've always wondered, wouldn't using a crow foot make the t/wrench inaccurate? The moment arm is off-centre... or is the distance negligible?

Any insights / actual experience appreciated since I already own a t/wrench and am thinking of getting a crow foot for torquing the valves on my new tanks : )
 
Perhaps it is negligible, but if you're going through the trouble to use a proper torque wrench and crow's foot you may as well be as precise as possible. Here is a handy calculator to make it easy. Torque Wrench Adapter Calculator - CNCexpo.com


....and remember to pull rather than push on the handle.
 
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That's a cool converter... thanks!
 
sierradelta, when using an offset such as a crow's foot, the torque will be the same if you offset the foot 90° to the tq wrench. The tq wrench is calibrated to the center of rotation, and setting the CF perpendicular to the wrench ensures the torque will be as set on the wrench, and requires no calculations.

If you must use it at any other angle than 90°, then the calculator will come in handy. But in all my years, I have always been able to find a way to keep it perpendicular.
 
https://www.shearwater.com/products/teric/

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