Half-Turn to close the tank - why???

[mrobinson]

another reason is; if the valve is full open & brutus the body building deck hand thinks the tank is shut tight, and tries to crank it open, valve damage can happen. Although it sounds far fetched, i've seen a tank cranked past the point of open, we had to use a vise grips to close it.

I agree with this one. That's why I'm liberal when people ask 1/4 or half turn? It doesn't really matter because both serve the purpose...

Very interesting to learn more about the older tanks though! Thanks!

 

[String]

The vale on one of my tanks is "old" and indeed does stick when full open. A recent freeflow which neither myself nor buddy could fix underwater was enough to make me back it off 1/2 a turn.

My newer tanks dont have an issue though so i leave those full on all the time.

 

[mrobinson]

While under pressure, valves do strange and unpredictable things, like jamming open. The small turn back may not be necessary, but "an ounce of prevention," well, you know the story.

I think this quote is also a great reason too. Thanks to the tech divers for the tip of leaving it open for doing valve drills!

 

[Charlie99]

My impression was that the "1/4 turn" practice was just a carryover from some industrial plants where this practice helps in checking valve status.

If a valve is closed, then you don't want to open it up when checking it's condition. But if the valve is open, testing its position by closing it just a partial turn isn't going to affect anything. So you test valve positions by trying to close them. This works, unless you have open valves that are stuck on the backseat and resist closing, and therefore appear to be closed when they are fully open. OTOH, if you back off a quarter turn after opening, then it's obvious that it's open when you come along and test it by trying to close it.

Just my $0.03 (inflation).

 

[Kim]

I agree with the comments about the valve seats and sticking. If you turn a valve full open then it will create more friction between the valve threads when it is stopped from opening more. Sometimes this results in having to use slightly more pressure to turn it off - ie stick. For me this could be a problem if you have to reach back over your shoulder and shut down the tank U/W as with my arm in that position I can't use my hands or fingers with the same strength as usual. When I first practiced valve shutdowns I actually had that situation once - I could reach the valve, but not apply enough pressure to break the 'stick'.

 

[friscuba]

I've had one or two engineers over the years tell me that backing off a quarter turn is standard procedure with all valving, not just scuba. We should probably be asking engineers this question, not divers.

FWIW, for years we've been hearing "never point the pressure guage at anyone" during class.... but how many people have seen one actually explode? - I finally saw it happen last fall. Inside of the guage wasn't corroded at all, just went.

later,

Steve

hey everyone,

sorry for such a basic question, but i was having a discussion with a friend about the procedure for turning on air before a dive. i always opened the tank fully, followed by a half-turn in the direction to close the tank. i wanted to verfiy:

1. is this procedure always performed?

2. why is it performed?

my friend says that you only do the half-turn back for boat diving. his reason being, that this helps the divemaster/boat captain who is assisting divers into the water verify that the tank is open. sounds confusing, buthis reasoning is it's easy to see if the tank is open if you only have to turn it a little bit.

okay, my girlfriend/buddy and i think his reasoning is incorrect, and we have both always done the half-turn back for every dive we've ever done. however, we cannot think of a reason why we've been doing this!!! the books are packed as we're moving, so we can't look it up.

someone, please settle this for us! thanks for any insight.

victor

 

[String]

When I first practiced valve shutdowns I actually had that situation once - I could reach the valve, but not apply enough pressure to break the 'stick'.

I had that issue with a freeflow, couldnt provide the force needed reaching back to kill it. Neither could buddy. In fact it required a spanner and substantial leverage to move the thing once on the surface.

 

[mwn]

Glad you asked, I always wondered about that. I figured it was one of those things that everybody did because it has always been done.

 

[WarmWaterDiver]

OK, I'll answer as an engineer - and this is simply what I do, NOT an instruction for anyone else. Everyone has to make their own choices.

Backing off 'a bit' from a full mechanical limit stop open position for all except quarter turn style valves has been a standard practice through my industrial career, and I also do this with my scuba tanks. I didn't get this practice in school, but from my father who was a shift working refinery operator from the time he returned from WWII to when he retired in 1981. If you've ever had to apply a pipe wrench of 24" or greater size, much less such a tool with a 'cheater bar' extension, to try to get a valve that normally runs open to even start to close, you begin to appreciate backing off just a tad from the hard mechanical stop at the end of valve travel. Then, think of doing this with the valve and its associated container filled at pressure with something other than air - say hydrogen, or natural gas, etc. Nice dry gases make very poor lubricants - in fact, they have a tendency to provide a 'stripping' action. But even compressed air is dangerous enough from a contained energy standpoint that I would rather not have to take tools to try to get a valve designed to operate by hand to operate.

The mechanical engineers call this 'breakaway torque' if I remember correctly (I'm not a mechanical engineer). It takes more force to start a valve to travel off its seat than to stroke the valve once off its seat. Just like it takes more force to start a car rolling, than to roll along - friction. If someone has well and truly 'jammed' a valve against its mechanical stop, or forced it so hard it started to damage the mechanical stop, the 'breakaway torque' could be significantly different, as expressed by several folks here.

Matt, looks like you & I follow similar practice.

A 'half-turn' rule sounds like overkill to me, especially if the valve is of the quarter-turn design (90 degrees of rotation = full closed to full open) . . . like ball valves as an example, or plug valves as another example. I just back off the slightest bit so the valve isn't forced 100% against the end of its mechanical travel, whether on scuba tanks or other applications - even the water faucet for the hose to water the grass at my house, for example.

I don't know the history of the evolution of the scuba tank valve, but I'd like the quarter-turn type feature myself - you can tell at a glance from the stem position whether the valve is open or shut, and one can easily employ purpose-built plastic or metal 'locks' to keep in one position (safety shower and eyewash station potable water supply lines are one example where this is handy).

Think of the "J" part of the "J" valve design, without the 'reserve' function - just a 90 degree rotation for open or closed, period. Why wouldn't a 90 degree rotation scuba tank valve be functional? Might be OK at differential pressure of 500 PSI or so, but maybe too much torque when working with 3000 PSI or so differential pressure without having an overly long valve handle to get leverage. Probably why we have the style valves in common use today on scuba tanks.

I have seen a few 'vintage' tank valves on Ebay that employ a lever that is pushed across the valve body, but those seem extremely non-ergonomic, especially when working against a 3000 PSI neighborhood of differential pressure, so the short life of that design is understandable.

On pressure gauge failures, I've never personally witnessed one on scuba gear, but have experience with the phenomenon in the industrial world. Pointing away when first pressurizing makes LOADS of sense to me. I don't know of a good way to predict when my SPG might fail - but I'm open to any tips anyone else might have.

I don't get overly upset if someone other than my dive buddy (wife - also engineer) 'touches' my tank valve, but I deliberately stop and put it where I had it set before proceeding further when this happens.

Just my choice.

 

[WarmWaterDiver]

I've had one or two engineers over the years tell me that backing off a quarter turn is standard procedure with all valving, not just scuba. We should probably be asking engineers this question, not divers. . . .


Steve

BTW, Engineers and Divers are not required to be mutually exclusive groups. I am one example, my wife another, and the founder of ANDI still yet another.