press purge button when opening tank valve?

[Spectre]

According to Curt Bowen's tests he did for a failed LP hose, an aluminum 80 drained in 83 seconsd. That's 36 psi/sec. For an AL 40 you could assume it would drain twice as fast; that's 72 psi/sec, or 144 psi over a 2 second period.

That's assuming it drains at a constant rate, where I would think the draining would slow as the pressure differential drops.

 

[SeaJay]

According to Curt Bowen's tests he did for a failed LP hose, an aluminum 80 drained in 83 seconsd.

I've never opened a valve fully on a full tank without a first stage attached, but I regularly bleed 1/2 a tank down so that I can do partial fills with low pressure O2. Even with no regulator attached at all, it takes many minutes to drain a tank - which can become freezing cold to the touch from the expanding gas. If you attached a regulator and open only a low pressure port, it'd take even longer to drain a tank.

...So 83 seconds, to me, sounds awfully fast, especially given that it's going through a 1st stage and the restriction of the LP port. I can't throw a BS flag without doing the experiment myself, but it sure does sound awfully fast.

The tank would certainly drop many degrees in temperature, too. I suspect that the outside of it might even frost over in humid environments if you were to drain that much gas in such a short period of time.

On the other hand, I've got a 100% O2 AL40 stage that I pressurize for every dive, but never use - it's my "emergency oxygen" stage, and is left either on the boat or on a 15' line overboard. Every time it's pressurized, I open the valve with the purge open - which takes more like 1/2 second, not two seconds. After 20 dives or so (maybe five or ten actual occurrences of pressurizing and depressurizing - I tend to leave my tanks on whenever I'm on the boat or at the dive site, and shut them off only when I leave the site or return to port), the tank's pressure has fallen from 3050 psi to about 2850. And that includes the occassional smartie that walks by and presses the purge to see if it's on, the occassional person who wonders if it feels any different to breathe 100% O2 (and therefore takes a drag), and the loss of gas from the repeated filling of the first stage and its attached hoses.

The reason why I press the purge when I open the valve (which I ALSO do very slowly) is twofold: Not only does it help prevent the valves in the regulator's second stages from marking their seating surfaces, but it also helps to keep the localized heat from an instant pressurization within the first stage down to a minimum - especially important when you're dealing with any breathing gas with more than 40% O2. Remember, it takes heat, fuel, and O2 for a fire. We already have O2 there, and there's fuel present even in the most "O2 clean" of tanks and regulators (which is why the concept of "O2 clean" is a dangerous one - "O2 clean" is great, but it does not guarantee the absence of fuel, especially after multiple tank fills). The only thing you can really control is heat - and so venting while purging helps to keep this at a minimum.

Venting while purging also helps with ventilation within the first stage - and the airflow helps to bleed off whatever heat is produced by the pressurization. This is why pressing the purge while opening the valve is a superior method to opening the valve super-slowly alone.

Me? I do both. Slow open AND press the purge. And no, it doesn't prematurely reduce tank pressure, even after repeated cycles. Well, nothing of any significance, anyway.

I do it even when I'm using a gas mix that's less than 50% - that way, I've formed a habit when opening valves (GUE calls it "muscle memory") and I'll never accidentally do it the wrong way and endanger myself or others around me by mixing high O2 percentages and localized heat within a first stage.

 

[Boogie711]

According to Curt Bowen's tests he did for a failed LP hose, an aluminum 80 drained in 83 seconsd. That's 36 psi/sec. For an AL 40 you could assume it would drain twice as fast; that's 72 psi/sec, or 144 psi over a 2 second period.

That's assuming it drains at a constant rate, where I would think the draining would slow as the pressure differential drops.

Much like Seajay's response would ALSO indicate:

As someone who routinely keeps the purge valve open as I open my valves, I can assure you, NWDiver, your brothers, the Fed Ex guy and anyone else:

there is no way to blow through 150 psi whilst holding the purge button and opening a valve.

Thank you.

 

[TCDiver1]

Curts' test seems reasonable and for the life of me, i can't think of a good reason he would fudge something like this.

I don't think you can compare it to draining a tank via only the valve since unless you have the tank strapped in a vise or something, the bleed pressure will likely be much less than the pressure allowed by the 1st stage.

So, unless i'm missing something, with everything but tank size remaining equal, the flow rate would be about the same at 36 psi/sec for any 3000 psi fill tank. A 40 would drain roughly twice as fast as an 80 only because the 40 is roughly 1/2 the volume of the 80.

Therefore, assuming a flow rate of 36psi/sec and holding the purge valve a full 2 seconds (which is longer than it sounds, i bet most people are closer to one full second) you have a purged volume of about 70 psi. A barely registerable amount on the best of analog gauges.

I think we may have just found a great use for one of those air integrated digital SPG's.

 

[jonnythan]

With the valve wide open, adding additional things past the valve opening won't help flow unless you attach some kind of high power vacuum. Cranking open the valve all the way should give the maximum possible flow rate out of the tank.

So, unless i'm missing something, with everything but tank size remaining equal, the flow rate would be about the same at 36 psi/sec for any 3000 psi fill tank. A 40 would drain roughly twice as fast as an 80 only because the 40 is roughly 1/2 the volume of the 80.

If it's 36 psi/sec for any 3000 psi tank, then any 3000 psi tank will empty in the same amount of time, whether it's an 80 or 6 cf tank. Obviously that's not the case. 36 psi in an Al 80 equals 72 psi in an Al40.

 

[TCDiver1]

If it's 36 psi/sec for any 3000 psi tank, then any 3000 psi tank will empty in the same amount of time, whether it's an 80 or 6 cf tank. Obviously that's not the case. 36 psi in an Al 80 equals 72 psi in an Al40.

True, i should have said Cf/sec not psi/sec. My point was, the flow rate would be the same if the test rig was the same therefore a true 40 Cf tank would drain twice as fast as a true 80 Cf tank.

 

[TCDiver1]

With the valve wide open, adding additional things past the valve opening won't help flow unless you attach some kind of high power vacuum. Cranking open the valve all the way should give the maximum possible flow rate out of the tank.

Have you ever cranked your tank valve wide open at 3000 psi to drain it?

 

[jonnythan]

Have you ever cranked your tank valve wide open at 3000 psi to drain it?

Not personally, but I've seen it done and seems like it usually takes a number of minutes.

I haven't sliced a low pressure hose either, but it couldn't possibly empty a tank faster than just cranking the valve open. All a regulator is is a passive series of restrictions to gas flow, it doesn't somehow amplify the gas delivery capability. If you want to get the same whipping effect, duct tape a garden hose to the valve and see how the air coming out of the end feels

 

[Boogie711]

Much like Seajay's response would ALSO indicate:

As someone who routinely keeps the purge valve open as I open my valves, I can assure you, NWDiver, your brothers, the Fed Ex guy and anyone else:

there is no way to blow through 150 psi whilst holding the purge button and opening a valve.

Thank you.

Just noticing - I guess for accuracy sake I should add "under normal circumstances."

I mean, if you were to crack open a valve while the purge was open and just stand there like a dummy for a while, then sure, you could one day get to 150 psi.

 

[Mako Mark]

Have you ever cranked your tank valve wide open at 3000 psi to drain it?

If you do it underwater, you can hold onto it and ride it around like a rocket powered scooter. (not very fast though)