Changing Scuba Tank Valve

[hqduong]

Another newb question.

I want to replace the tank valve on my tank because I want to go all DIN. Can I remove the valve myself and just twist on a new one? Anything else I need to do?

 

[Randy43068]

It's a DIY project for sure.

Obviously, drain the tank.
If the valve is tight, tap it with a rubber mallet on the valve knob. Not too hard. Experts don't agree with this, but it's routinely done. If you're not comfortable doing it, get a big wrench.

Unscrew the old valve, screw the new valve on and bump it tight with the heal of your hand. Making sure to use a brand new neck O-ring.

Ensure on the new tank valve that the dip tube is tight. Most of the time on new valves, it's only finger tight. Use a pair of pliers to give it a little snug if it's loose. Try not to mar the dip tube surface. If you don't it's not really a problem.

There you have it, nothing to it! Any questions PM and I'll help all I can.

 

[tstormdiver]

Yes,...as long as the threads are the same size as the original size/ Just make sure you empty the cylinder first. You may also want to makesure the neck O-ring is lubricated.

 

[Randy43068]

Tammy is correct. However, the old 7/8 thread is pretty rare these days. Besides, it ain't gonna fit if the threads are not the same. :d

 

[oxyhacker]

The general feeling (though it is far from unaminous) is that the neck O-ring should NOT be lubed, since it is static and doesn't need lubing, and anyhow is not really acting as an O-ring at all but rather as a crush seal. Lube will just encourage it to extrude.

The threads, though, should be lightly lubed, with an appropriate lube, silicone or oxy-compatible depending on what the tank will see, as a dielectric barrier to prevent dissimilar metal corrosion between the valve threads and the tank threads. This is especially important with aluminum tanks.

You may also want to makesure the neck O-ring is lubricated.

 

[tstormdiver]

I meant lubed enough to prevent dry rotting.

 

[Doc Harry]

Ensure that the valve actually works and has truly drained the cylinder before you try to remove the valve. You could have a really bad day if you try to remove the valve while the cylinder is still under pressure.

Tapping on the valve with a mallet is bad form and a good way to bend something. Commonly you can bend the valve orifice itself (especially DIN valves), knocking it out of round and preventing the fill whip or regulator from seating properly. You can also bend components of the valve stem.

A better technique is to put a large wrench onto the valve, using the flatforms that are meant for a wrench, then twist off the valve until it is loose enough to be unscrewed by hand. You might need a padded chain clamp to hold the cylinder. Generally I tighten the clamp only just enough to barely keep the cylinder from slipping as I torque the valve with the wrench.

A small amount of an appopriate lubricant on the valve threads helps prevent bimetal galvanic corrosion.

Each cylinder manufacturer requires a certain minimum number of intact threads must be present. Assuming that your latest valid visual inspection has checked this, and assuming that you haven't damaged any threads in the process, then you should be good to go to screw the valve back in place with an appropriate new, unlubricated cylinder neck o-ring.

Each cylinder manufacturer requires that valve be torqued to specs. Almost all require 50 foot-pounds, but it varies from manufacturer to manufacturer. In my experience, 50 ft-lbs is just beyond my ability to tighten by hand. In other words, if I tighten a cylinder valve by hand as tight as possible, I still need a little bit of torque with a torque wrench to get it to 50 ft-lbs. But that's just me, and I know this because I have checked it with a torque wrench.

I use a long-handled torque wrench and crow feet for torquing all of my cylinders. Of course, it should be obvious that the purpose of torquing a valve to specs is to prevent the valve from loosening and turning into a high-power projectile. If you are using DIN valves with twin cylinders and isolation manifold, then it is obviously unlikely that the valve would loosen and spin off under pressure.

All of this is also assuming that this is a scuba cylinder for plain air. If you are doing this for a NITROX cylinder, then you are opening a whole other can of worms in terms of oxygen cleaning the valve, appropriate o-rings and lubricants, etc.

 

[Randy43068]

not a problem because it's replaced every year.

 

[Randy43068]

Doc.. you're textbook correct, but mostly BS.

If the tank has pressure in it, that valve isn't coming off, at least not easily. Also, as far at torquing it on, it's BS too. Text book correct but 500-to-3K of pressure will hold it in place.

A rubber mallet isn't going to do any harm as long as it moves with gentle tapping. If not, use a wrench.

 

[elan]

The service guy that works in my LDS once told me that they had abnormal o ring extrusion rates on worthingtons when they were screwing in the valves hand tight on worthingtons. Worthington told them to RTFM and it was in the spec that they require some minimal torque. Extrusions stopped after they started following the spec.