Draining a tank too fast

[PaulS_1]

I bought my first tank this summer, a Worthington HP130. I am relatively new to diving, so I called up where I bought it from, and asked what I should do with my tank for 8 months of storage. The guy told me I should drain my tank so I’ll have fresh air the next time I use it, and the lower pressure is easier on the tank. What he didn’t tell me was how fast to drain it. Now I didn’t open it up very much, but enough that condensation formed on the valve and tank. I freaked out and stopped it. I started with around 2200PSI and am now at 1800PSI. I came on this site, and I see it’s mentioned a few times not to drain the tank too fast for fear of condensation. Now I feel stupid for doing it, and I’m pissed off at the shop for not telling me. Do I need to worry about condensation inside the tank? What should I do?

Thanks in advance.

 

[MSilvia]

I wouldn't be too worried about it.

 

[cayman diver]

the air should be dry so no problems however more for protocal people empty tanks slowly, very noisy as well so in the dive shop i work in we empty them slowly.

 

[ClayJar]

As the air expanded through the valve, its temperature lowered. This cooled the valve below the dew point of the air outside the tank, so condensation formed. The air *inside* the tank, however, should have *significantly* less moisture, and it's quite unlikely that any condensation could have formed inside (and if any did, when the temperature returned to ambient, it would evaporate anyway).

Additionally, the condensation that formed on the outside of the valve certainly could not enter the tank while there was still significant pressure on the inside forcing air out. If, on the other hand, you'd cranked it all the way open, getting it nice and soaked with condensation (but not quite frozen), then perhaps once there was no pressure difference between the inside and outside, you may manage to get some water inside.

You didn't do anything wrong, but if you go more slowly, you'll condense less (i.e. you go slowly enough that the metal of the tank and valve conducting heat maintains the valve at a temperature greater than the dewpoint of the surrounding air). Don't go all the way to zero, which is easy enough to do if you stop when it sounds slower and check the pressure.

 

[mike_s]

I bought my first tank this summer, a Worthington HP130. I am relatively new to diving, so I called up where I bought it from, and asked what I should do with my tank for 8 months of storage. The guy told me I should drain my tank so I’ll have fresh air the next time I use it, and the lower pressure is easier on the tank. What he didn’t tell me was how fast to drain it. Now I didn’t open it up very much, but enough that condensation formed on the valve and tank. I freaked out and stopped it. I started with around 2200PSI and am now at 1800PSI. I came on this site, and I see it’s mentioned a few times not to drain the tank too fast for fear of condensation. Now I feel stupid for doing it, and I’m pissed off at the shop for not telling me. Do I need to worry about condensation inside the tank? What should I do?

Thanks in advance.

Emptying it too quickly is stressfull on the tanks and also promotes temperature change (PV=nRT) which promotes condensation. A little conensation can be expected depending on the humidity, but basically just empty it very slowly and you'll be fine. It might take 20-30 minutes, but just crack it open and check it every now and then. As the pressure goes down, you might need to open the valve a little more as time progresses..

Don't empty it all the way. Keep at least 500psi in it. If you empty it all the way it will most likely have a moisture issue inside the tank which could promote corrosion.

yes is is less stressful on the tank to store it at a lower PSI, but when they told you that "you'd have fresher air" next time you'd use it (after a new fill), whoever told you that is a quack. The only reason that the air would loose freshness is that it wasn't clean air to start with. (meaning it had excess moisture or compressor oil in the air mix or the air intake was near something that could suck in fumes, such as vehicle or motor exhaust fumes. This is more common with older surplus compressors or shops that don't maintenance their compressor or filter system properly.

If you got a good quality air fill to start with from a reputable source, then your air should be just as good the next year.


BTW.... I think you'll find that a majority of the people just store their tanks with whateve PSI is in them at the current moment. I don't think that most folks empty their tanks for "winter storage" or anything. I doubt you'ff find any supportive documentation that a tank failed because it was stores at rated pressure over the winter.

 

[lamont]

if you take an old crappy first stage reg, hook up an old LP hose to it, ziptie a weight on the end of it and submerge it in water you can turn the screaming noize into bubbling...

 

[JeffG]

if you take an old crappy first stage reg, hook up an old LP hose to it, ziptie a weight on the end of it and submerge it in water you can turn the screaming noize into bubbling...

ba...throw a towel over it and crank the valve.

 

[MSilvia]

ba...throw a towel over it and crank the valve.

That's pretty much what I do. I figure if my valve can take being submerged in the ocean, it can take a little external condensation.

 

[cayman diver]

Emptying it too quickly is stressfull on the tanks and also promotes temperature change (PV=nRT) which promotes condensation.

But filling a tank is more stressfull the heat buildup is far worse than the cooling effect.

 

[Dive-aholic]

ba...throw a towel over it and crank the valve.

Exactly! Condensation can't form in a pressurized environment. You see it on the outside due to lack of pressure, but not on the inside. And if you don't care about the noise, don't even worry about the towel.

That being said, don't even drain it. I keep my tanks stored at the last pressure I left them at...which is usually full. The higher pressure will not hurt the tank, especially if it's a steel tank. The stress on tanks comes from filling them, not keeping them stored.

BTW, 8 months!!!!!

What is wrong with you????? I can barely go 8 days without diving!!! :shakehead