Question Should you overfill lp tanks without a + stamp?

[macado]

Running a tank dry will allow water in via the neck o-ring. Some people around here do this religiously and the tanks get so rusted no amount of tumbling will save them.

Still probably an example of grossly mishandling the tank.

Are you just repeating old fallacies and information that your dive shop gave you? I'm not going to say it never happens but getting water inside the tank mostly occurs because of a fill station operator error.

Please explain to me how an o-ring and valve that holds back 3000psi of gas suddenly decides to "let" water in at atmospheric pressure?

The primary way this happens is that there is water on/inside the DIN fill whip or present on the valve on the cylinder that gets forced into the tank at the time of filling from a fill station operator that doesnt purge the fill whip or SCUBA valve first before connecting thereby forcing water INTO the cylinder.

The other scenario is the tank is drained at some point on the dive, the dive continues and the tank is then taken deeper. This happens a lot on inflation bottles...ask me how I know this. I suppose this sometimes could happen with stage bottles/pony bottles.

For example - 1 ATA (Sea Level/0ft/0m) is 14.7psi if it's truly empty. You then take that tank to 30m/99ft which 4ATA or 58.8psi. The outside pressure now exceeds the internal pressure. The first stage connection is loose (due to lack of pressure) and water is now let in the tank. I'm happy to be corrected here if this is not the case but this is my understanding of how water gets inside the tank.

Even draining your tank to say 150psi and then going down to 60m (200ft). The outside pressure is still only 102.9psi at 7ATA. Water is not going to get inside the tank.

EDIT: I was beaten by @Tracy 's post

 

[sharknutz]

Are you just repeating old fallacies and information that your dive shop gave you? I'm not going to say it never happens but getting water inside the tank mostly occurs because of a fill station operator error.

Please explain to me how an o-ring and valve that holds back 3000psi of gas suddenly decides to "let" water in at atmospheric pressure?

The primary way this happens is that there is water on/inside the DIN fill whip or present on the valve on the cylinder that gets forced into the tank at the time of filling from a fill station operator that doesnt purge the fill whip or SCUBA valve first before connecting thereby forcing water INTO the cylinder.

The other scenario is the tank is drained at some point on the dive, the dive continues and the tank is then taken deeper. This happens a lot on inflation bottles...ask me how I know this. I suppose this sometimes could happen with stage bottles/pony bottles.

For example - 1 ATA (Sea Level/0ft/0m). You then take that tank to 30m/99ft which 4 ATAs or 58.8psi. The outside pressure now exceeds the internal pressure. The first stage connection loose (due to lack of pressure) and water is now let in the tank. I'm happy to be corrected here if this is not the case but this is my understanding of how water gets inside the tank.

Even draining your tank to say 150psi and then going down to 60m (200ft). The outside pressure is still only 102.9psi at 7ATA. Water is not going to get inside the tank.

EDIT: I was beaten by @Tracy 's post

Dang have you been writing this for an hour?

 

[macado]

Dang have you been writing this for an hour?

On a phone with poor Internet but thanks for your concern.

 

[rob.mwpropane]

This is how I store all my tanks. @rob.mwpropane, I've got some tanks I need you to fill for me!

You sound like the kind of guy that would like to buy some pristine AL6351 tanks from me.. I give you good deal my friend! Like new, circa 1985, lol

 

[grf88]

I believe the tank I mentioned become partially filled with seawater as a result of the practice of swimming the empty tanks back to the beach as the fill station was at a different location from where the dive boat was kept. My theory was that the valve was open and the tank was hot before being dropped into the water resulting in a lot of water being sucked into the tank on its contact with the colder water. The tank was then filled without noticing the water content and left for several months before it was needed again. I found it when I tested for CO and did provide a controversial post about it several years ago.

 

[L13]

Can you cite any incident anywhere that a tank passed hydro but had issues after the fact? If a tank passes hydro I'm just not sure what could possibly happen within 5 years that would cause it to become "dangerous". Unless maybe grossly mishandled.

VIP's are nowhere near as important as they once were. The LDS can charge the tax all day long, but it's not for your safety, lol.

Rust.

Every cylinder that fails hydro transitioned from a condition that could pass a hydro to a condition that fails a hydro at some point between hydros. For steel tanks this usually occurs due to rust, often inside.

If (as you seem to be suggesting) a steel cylinder never experiences damage between hydros, why hydro it again? Once would be enough (in that fantasy case).

 

[CT-Rich]

All joking aside be aware that a burst disk blowing in close proximity can necessitate a change of underwear .

My parents wouldn't let me store my tank at their house after a Burst disk failed. Scared the **** out of them both. Of course they are both dead now (for unrelated reasons).

If tanks aren't popping off like firecrackers in the hot Florida sun, or blowing up under the gentle management of central American tank jockies, they might be more durable than you give them credit for. I pray for an occasional cave fill, my tanks are HP.

 

[Cio]

@rob.mwpropane permanent expansion for regular hydro vs REE for + rating. failing the REE means the tank is expanding too much at higher pressure. Repeated overfills will then cause the tank to start thinning the walls until it eventually fails the permanent expansion for hydro.

Although technically correct, the walls thin, this has nothing to do with what's being discussed. Cyclic stress work hardens the material, and the material loses its elasticity. Hydro fails are for a loss of elasticity, not permanent expansion. The test method does not measure how much the tank has expanded during its life, it measures the delta from test load to rest which is a measurement of elasticity.

 

[runsongas]

no, hydro fails on 3AA tanks are for permanent expansion. having too high of a REE (the criteria for the +) is an indication of deviation from the desired linear region on the stress/curve and an indication the yield strength of the steel has started to decrease. repeated stressing at higher than normal pressures from cave filling will start to compound this until eventually the yield strength is low enough it will be lower than the hydro test pressure at which the tank will undergo strain hardening and fail for the permanent expansion criteria.

 

[Tracy]

no, hydro fails on 3AA tanks are for permanent expansion. having too high of a REE (the criteria for the +) is an indication of deviation from the desired linear region on the stress/curve and an indication the yield strength of the steel has started to decrease. repeated stressing at higher than normal pressures from cave filling will start to compound this until eventually the yield strength is low enough it will be lower than the hydro test pressure at which the tank will undergo strain hardening and fail for the permanent expansion criteria.

The only 3AA scuba tanks being harmed by cave filling would be 2250 steel 72s and older. Those do not like 3600 psi and will fail hydro within a few test cycles if repeatedly filled to that pressure.
3600 psi is still well in the elastic range of modern 2400psi scuba tanks. A cave fill will not be the death of modern low pressure steels. I see almost no permanent expansion at 4000 psi hydro pressure. I have destroyed several tanks over the years playing with them seeing where that limit is. 3600 psi isn't it.