press purge button when opening tank valve?

[Hank49]

It's not the 200 bar that is the problem, its that first 9 bar (IP). "Opening slowly" applies to the initial cracking of the tank valve so the change of pressure in the 1st stage from ambient to 9 bar occurs slowly. Once your system has reached IP, the 1st stage should be locked up and the rate of the change of pressure as you go from 9 to 200 bar shouldn't matter much so crank away. My own tank valves still work pretty easily so it is not hard to open it just a little bit. Rental tanks are often another story so I just developed the habit of pressing a purge button when I go to turn on a tank valve. I then watch the SPG and when it starts to register I release the purge button and crank the tank valve the rest of the way open.

I assume 9 bar is about 135 psi. I have not fully made this metric conversion but I'm inching my way towards it.

1 bar is 14 psi or there abouts. When you open the tank valve, the air that comes out expands instantly by 190 times the volumn it was in the tank. So even if you can sneak out 1 cc (about the size of 1 1/2 pencil erasers) of air, it instantly becomes 190 CCs, probably enough to fill the cylinder head of the reg and all the lp hoses. I'm just saying that not purging and thinking you can open slowly and achieve the objective of gently setting the piston stem down on the seat....well, you may be able to but you have to have the touch and concentration of a brain surgeon. and a cooperative tank valve. Not all are smooth. The only solution is, as you said, purge while you open.

 

[carbon]

...Anyway, the point is that all of this could be avoided with the simple practice of opening valves slowly while purging. Oh yeah... And keep your hand wrapped AROUND the knob, instead of over the end of it - just in case. Count the missing fingers on older divers' hands if you don't believe me.

Hi SeaJay,

Can you explain some more about the missing fingers, and why wrapping your hand around the knob helps?

Thanks,
Carbon

 

[SeaJay]

Yeah -

When a valve explodes, it usually becomes three pieces - the valve body (which stays put on the end of the tank), the burst disk or plug (which usualy fragments and flys off like it's shot out of a gun), and the valve handle - which does the same thing as the burst plug. If you look at the end of the valve knob, you'll see that the whole valve is really just a long bolt that enters the side of the valve - it's this bolt that shoots off and can do damage.

...Which is why you don't want your hand over the end of it if you can help it. When opening a valve, place your fingers AROUND the knob and turn, rather than over the end of it and turn. That way, if something explodes, you're less likely to have fingers taken off.

Sharp people will also stand clear of the burst disk or plug, face their pressure gauge away from their face (some hold it down with their foot) and also make sure to open SLOWLY, while purging.

...And no, it doesn't take "the precision of a brain surgeon" to open a valve slowly, unless you've got a seriously hosed valve, which should be replaced. Last I looked, they cost about $40 for a new one - and you'll get a free VIP out of it.

Just take your time opening your valve and make sure that fingers, hands, toes, eyes, etc. are all clear and/or protected from places where something could go wrong.

Burst disks/plugs and the end of the valve, along with your pressure gauge, the O-ring on the face of the valve, etc. - these are places to simply be aware of.

 

[carbon]

Thanks SeaJay,

I hadn't thought of the flying bolt part. I'll wrap the fingers from now on!

Carbon

 

[mikedarmody]

I've never seen anyone do this. I guess it depends on how many times you do it and whether it impacts the mfr's maintenance/inspection cycle. RTFM.

 

[chickdiver]

What typically happens during combustion in this case is that the valve itself can explode - both the pressure relief (bolt or disk) and the valve knob will pop off, becoming high speed shrapnel. If there's a high (>40%) O2 percentage in the tank as well, the whole thing could easily become a bomb. You don't want this.

...Anyway, the point is that all of this could be avoided with the simple practice of opening valves slowly while purging. Oh yeah... And keep your hand wrapped AROUND the knob, instead of over the end of it - just in case. Count the missing fingers on older divers' hands if you don't believe me.

Seajay-

Respectfully- on these particular points you are (based on my experiences) wrong. The valve will explode (I had one do this- in my hand- 2.5 years ago. The valve knob itself ahttered, essentially turining the jagged plastic into an electric knife. The valve seat essentially disintegrated (flashed) and there is a hole in the remains large enough to put a #2 pencil through. That created a huge amount of brass shrapnel that ended up in my hand (It took over 4 hours to debride the wound in the ER, and I still had brass slivers in it months later). Incidentally, the burst disk remained intact.

As for the positioning of the hand you recommend- well, my hand was wrapped around the valve. The major damage to my hand occured in the area of the palm just adjacent to the index and middle fingers, and between them. There was significant bruising and hairline fractures of the phalanges throughout the palm of my hand. I have very small hands (about 3.5" at the widest point of the palm)- so you can imagine what this would have done to a man's hand. Frankly I don't beleive there is any hand position that produces any more safetythan any other. I was extremely lucky in that not only did I NOT loose my hand, I also had no permanent damage other than scarring.

 

[Viscya]

Ok folks, this seems pretty simple.

Looking at the ratings on my reg, the first stage is rated at twice the flow rate of the second stage. Check your IP with the purge pushed. Does it drop to 0? No it drops by 3-5%.

Therefore, if I open the valve with the purge pushed, I allow some gas to pass through the second stage. The first stage goes from full open to mostly, closed. From here releasing the purge allows the valve to close the rest of the way. At this point the IP has gone from 0 to 95%. By this time enough gas has escaped to allow the first stage to cool. Given the human reaction time, the second stage valve will not close instantly when I release the purge, and the first stage valve can gently close.

This as opposed to cranking the tank valve open and allowing the inertia of the piston to slam the seat from a full open position. BDs have less inertia, but would also slam shut.

Should you open the valve slowly? Sure, every little bit helps, but push the purge also.

Also, I believe Curt’s figures were based on a LP hose failure. In his test, at 1 ATA, a free flowing second stage took 255 seconds to drain a 80cf tank.

 

[rubbachicken]

i was taught to do that from day one
i've seen very few others out there do the same, when ever i get buddied up with someone less experienced than myself i try to get them to do the same, i tried with more experienced divers aswell, but they didn't seem want to know
at the end of they day if it might help make my gear last a little longer, i'm all for it