Opening Tank Valve

[TMHeimer]

A DM -- or anyone else for that matter -- touching my kit would result in me throwing them over the side! Thankfully we don't have DMs on our boats, just independent divers who are responsible for their own kit and welfare. (Maybe that sounds a tad strong, but a boatload of essentially solo divers are like that -- touch their kit at your peril!)

I think it was against the end-stop and in the warmer ambient temp topsides, the valve contracted in the colder water and jammed. When reaching back over one's shoulder, you don't have much purchase to turn the valve, certainly not as much as when you're standing in front of the valve.

For me it's just logic that you open a valve fully, then back it off a smidge, say 1/8th or 1/4 turn. When doing a shutdown, you've drilled it into your muscle memory which way to turn the valve for off and on.

I'm not keen on the other advice which is to only partially open the valve. There be a load of potential problems, like not opening it enough to breathe when you're deep.


I always, like OCD always, test my breathing gas three times. Firstly when standing in front of the kit when I'll turn the gas on both sides, then breathe from each regulator and check the pressure. Secondly when I've got into the kit and put the necklace on -- breathe from it -- and routed the longhose around -- then breathe from it at least three breaths. Finally as I get up to waddle over to the jump I'll always breathe from it for another three breaths (and ensure there's gas in the wing).

The same applies with a rebreather, but in this case it's the pre-breathe to ensure the scrubber's woken up. Even so, before I waddle over to the jump it's check the PPO2 (oxygen levels) and some gas in the wing.

Critical to know that you can breathe when you jump, and that you will float on the surface. Was on a boat once where somebody died because they didn't ensure that.

Good points. As far as turning off the valve when your done I just know that if it turns only a little and stops I went the wrong way....
I've read on SB that is fairly common over here for boat DMs to check that the dumb divers have their air on and the odd time they wind up turning the air off. Big discussions on SB over that.

 

[PfcAJ]

Hmphh.. That's a different take on why quarter turn back may be useful. Is it possible there is a defect with your RH valve? I'm not familiar with twins, but can't see that a quarter turn back (or less than that as I do) can really hurt anything. Now maybe if you consider boat DMs fooling with your valve to make sure your air is on and mistakenly turning it off. Not a concern if you take 3-4 breaths before jumping in or are diving solo from shore.

Baloney. There is no mechanism to lock a valve like that.
Take it apart and look at em. They don’t work like that.

 

[TrimixToo]

Need to disagree with you on this.

Turning on oxygen rich/pure oxygen mixes can create an oxygen fire with the hammer effect. Easier to turn all cylinders on slowly.

Quarter turn back means that changes in ambient pressure or temperature won't jam the valve open. Yes, the damn twinset RH valve was jammed for me where I couldn't reach behind to turn it off (during an underwater test); needed a proper turn off on the surface.

But do carry on.

As most valve seats are not made of the world's most O2-compatible materials when last I looked, I'm actually not sure slow opening is better. (I've been opening them slowly, as taught, but now I wonder.) Can one open the valve quickly enough to make a really significant pressure wave occur in the short gas path to the HP seat? Other than the valve seat what's flammable in that gas path (should be "nothing" if it was properly cleaned, right)? But slow opening increases the velocity of the O2 across the seat, which seems as though it might increase the chance of toxic gas being created in the valve.

I'd be fascinated to learn how changes in ambient temperature could cause a valve to jam. The entire assembly is brass, with all parts carrying the same coefficient of thermal expansion. And, with the valve open, there's tank pressure inside, which is "usually" a bit higher than ambient.

I'm willing to learn. What am I missing? Probably something...

 

[Wibble]

Alas I don’t know why it jammed, just that it was too tight to move when reaching over my shoulder. The isolator valve was fine so had there been a break, I could save half the gas to get out of Dodge.

BTW my rebreather valves are all slow opening, which makes opening much easier.

 

[TrimixToo]

As most valve seats are not made of the world's most O2-compatible materials when last I looked, I'm actually not sure slow opening is better. (I've been opening them slowly, as taught, but now I wonder.) Can one open the valve quickly enough to make a really significant pressure wave occur in the short gas path to the HP seat? Other than the valve seat what's flammable in that gas path (should be "nothing" if it was properly cleaned, right)? But slow opening increases the velocity of the O2 across the seat, which seems as though it might increase the chance of toxic gas being created in the valve.

I'd be fascinated to learn how changes in ambient temperature could cause a valve to jam. The entire assembly is brass, with all parts carrying the same coefficient of thermal expansion. And, with the valve open, there's tank pressure inside, which is "usually" a bit higher than ambient.

I'm willing to learn. What am I missing? Probably something...

That should say, "...slow opening increases how long high-velocity O2 flows across the seat...."

 

[FinnMom]

I still open valves this the reg slightly purging just because occasionally air is going to start hissing from somewhere. Usually hissing air is because the o-ring of the 1st stage is missing or damaged, or the 1st stage simply isn't on correctly, and just days ago it happened because one reg hose wasn't properly secured to the 1st stage. I've also had it happen because the o-ring of a reg hose was at the end of its usable life. I just like they slow & easy lets make sure this works as I expect it to approach.

 

[AfterDark]

Don't ever remember hearing this one. I've been opening valves without purging the 2nd stage on the same regulators since 1969 and go 3-4 years between service of said regulators. I wonder why I've never had a problem with over 2000 dives on the same regulators? They are Scubapro regulators to boot!

I always back off the stop of any valve I open be it scuba, garden hose faucet, propane grill tank you name it if I open a valve I always take it back off the stop a wee bit.

 

[TMHeimer]

Baloney. There is no mechanism to lock a valve like that.
Take it apart and look at em. They don’t work like that.

I don't disagree. He did said his RH valve was "jammed". You figure it out, it's out of my league having never dived doubles. I only suggested there was something wrong with it since it jammed.

 

[Wibble]

I don't disagree. He did said his RH valve was "jammed". You figure it out, it's out of my league having never dived doubles. I only suggested there was something wrong with it since it jammed.

Maybe I could have phrased it as "tight" as in too tight to manipulate with my arm behind my head -- lets be honest, there's no purchase/leverage when your arm and hand is contorted into such an awkward position. It could have been that it was opened "too tightly".

Waiting for someone to mention "my team" to which I'll respond what team when diving solo / self reliant

 

[MaxTorque]

Can one open the valve quickly enough to make a really significant pressure wave occur in the short gas path to the HP seat?

It's not the dynamic pressure (ie the velocity) that is the issue, it is the time it takes for that pressure to change!

When you compress a gas it releases thermal energy, ie heat. That is un-avoidable. You tank has a certain pressure in it, your downstream system starts broadly at ambient pressure, so at some point, that gas in that part of the system WILL undergo the compression required to raise it up. What happens when you increase pressure slowly is there is time for that heat created to be conducted away, and so the average temperature (note important difference between heat and temperature) of that gas will not rise too much. In this case, peak gas temperatures are not high. Slam open the valve however and the compression happens quickly, and there is simply not enough time for the heat energy to escape from the gas, so the gas itself gets to a higher temperature. You first stage is a heavy lump of brass, and can suck in a large amount of heat before it gets warmed up, as long as their is time for that to occur. The opposite, a process where there is no time for heat to flow into our out of a gas stream is called an Adiabatic process btw.

Now, yes, dynamic effects caused by high gas velocity can add to this effect, but remember that to get to a high velocity energy must have been expending on the gas (to accelerate it) and so it'll be starting colder in the first place.

I'd be fascinated to learn how changes in ambient temperature could cause a valve to jam. The entire assembly is brass, with all parts carrying the same coefficient of thermal expansion. And, with the valve open, there's tank pressure inside, which is "usually" a bit higher than ambient.

I'm willing to learn. What am I missing? Probably something...

There is one mechanicsm that i can just about see, but it's pretty marginal.

A cylinder valve carries the pressure load reaction on the valve stem spindle, and not on the threaded shuttle seat. When the tank is opened, the stem spindle gets shunted sideways by the high cylinder pressure, away from the shuttle seat. Modern valves have nice PTFE discs or seals to ensure a very low friction co-efficient, meaning the valve stays easy to turn under these loads.

If you open the valve fully, it actually gets stopped from turning at the point the shuttle seat has backed far enough out of it's threaded chamber to come into hard physical "end stop" contact with the stem spindle, which for a full tank at high pressure is currently pushed fully away from the shuttle by the HP. Those two parts are normally not axially connected because the stem spindle and the shuttle seat use a blade & slot type arrangement:

Now as the tank decreases in pressure during the dive, the axial pressure load on the stem spindle falls, and that part possible is able to move towards the shuttle seat, pushed by the relaxing sealing o ring and PTFE disc.


That much is for sure. The next bit is my speculation:

Could that tiny movement and increase in axial load on the shuttle, which carries NO pressure load when open, cause it to jam or at the least become significantly harder to turn, especially with back gas that can be hard to manipulate at the best of times?

I'm going to say yes.

Dirty threads, perhaps slightly damaged or miss-shappen threads right at the end of the shuttle seat chamber, perhaps even a build up of old grease or lubricant. perhaps with some valve designs an axial load can cause the shuttle seat to skew a bit in its threads and bind a bit more?

Likely? No so much. Possible? certainly.


If you measure up inside a modern cylinder valve, it's pretty obvious that after approx 1.5 turns from closed, the opening curtain area of the shuttle seat is greater than that of the inlet and exit drillings / passages, so no flow benefit occurs from further opening. The only reason to open the valve fully as mentioned is make sure it can only be turned in one direction to avoid possible confusion.

So whilst i don't do the "turn back a bit" thing, mainly to ensure the only way the knob can meaningfully turn is the opposite way to which it currently sits. ie if a valve is OPEN, it can only turn in the CLOSE direction, i do ensure that i haven't opened the valve HARD against it's end stop.