Question Should you overfill lp tanks without a + stamp?

[rjack321]

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.

you realize this takes 2 or 3 decades? at the earliest

 

[runsongas]

@Tracy @rjack321 just to be clear, you both are claiming it will take 20 to 30 years of cavefills every time (at how many fills a year?) for a 3AA tank that already fails the REE criteria for the plus (not regular hydro permanent expansion) to reach permanent expansion failure?

 

[Tracy]

@Tracy @rjack321 just to be clear, you both are claiming it will take 20 to 30 years of cavefills every time (at how many fills a year?) for a 3AA tank that already fails the REE criteria for the plus (not regular hydro permanent expansion) to reach permanent expansion failure?

I didn't say that. I have never actually seen a scuba tank fail for a plus that didn't also fail overall. Not saying it doesn't happen, I'm sure it does. Just never seen it.
To take it even further, I have only ever had two steel scuba tanks fail hydro ever. Out of probably close to 1000 tanks.
Usually when I have a steel tank fail, it is on the visual inspection. Those are the ones I typically play with to find the elastic limits of the steel.
Aluminums fail, not often, but it isn't rare. If I have a steel tank fail, you can guarantee I am double checking everything and testing it again.

And no, I'm not claiming 20-30 years of cave fills will fail anything. Since they fall within the elastic limit, I would consider them to still have an unlimited service life.
I personally know of one set of late 70s PST 104s that were filled to 3600 daily for the last 40 years. They have been retired for about 5 years now as the company installed a plumbed in air system for testing regulators but the tanks are still good and would still pass. You have seen the tanks, they were featured in many dive advertisements over the years.

 

[100days-a-year]

I've seen 1960s 72s filled to 3000 still in use hundred of times a year. Last hydro date on some were in the 60s.
I was conveniently behind a bulkhead every time I saw them being filled, in case.

 

[goldfishtornado]

20-30 years of cave fills

There's been some discussion about the theoretically expected endurance in the "It says right on my tanks..." and "Why not overfill Aluminum cylinders?" threads. Not an endorsement of overfilling tanks, but my estimation is that for 3AA tanks the old "10,000 hydrostatic tests at 4000 psi," is very roughly correct. From the S-N curve on the latter post, the 46 ksi point is about 10,000 cycles and about 4,000 psi. IMO, conservatively you should count every diurnal cycle as one "cycle" with respect to work hardening, not fills. That puts you at just under 30 years.

Barring any other defects, 46 ksi is at the point where tanks should be considered decade scale consumable, 30 ksi endurance if effectively indefinite. In between is probably in the range of longer than human lifespan time scales. I think this lines up with the empirical observations by @Tracy.

All of this assumes pristine tanks though, so it's not a statement on a reasonable safety margins in actual use.

 

[Tracy]

I've seen 1960s 72s filled to 3000 still in use hundred of times a year. Last hydro date on some were in the 60s.
I was conveniently behind a bulkhead every time I saw them being filled, in case.

72s at 3000 psi is within the elastic limit, it isn't hurting them. 3600 is damaging the tank and they will die.

 

[rjack321]

@Tracy @rjack321 just to be clear, you both are claiming it will take 20 to 30 years of cavefills every time (at how many fills a year?) for a 3AA tank that already fails the REE criteria for the plus (not regular hydro permanent expansion) to reach permanent expansion failure?

I'm saying that well over 20 years of cave filling is totally normal and not leading to permanent expansion failures at hydro time. Which is born out by others in this thread and the actual experience of both hydro facilities and shops which are comfortable with cave fills in 3AA tanks.

The number of tanks which pass at 5/3s pressure but failed the REE for a plus is basically non-existent. The only 3AA tanks not "earning" a plus are those where the REE isn't stamped on the crown.

 

[ofg-1]

One thing I did not see discussed is the hydro station doing the tests. Some steel tanks (PST) require a pre stress before taking the expansion readings. If you are taking tanks to a fire equipment company, they may have no clue what the plus mark is for. The person that holds the DOT certificate may not be the person actually doing the test. If I remember correctly the test gauge has to be calibrated annually.

My point, if you are just dropping your tanks at a dive shop, and have no idea who they are taking them to, don't be surprised if you get less than optimal results.

Final comment. I know a guy complaining about bad air odor in his al 80. The tank was just back from a shop that did the hydro and vip, reassembled it and filled it. We took the tank apart and there was a dead mouse in it. Sometime between the vip and the valve being installed, ole Mickey was roaming around the shop and found a good place to hide.

 

[Manatee Diver]

My point, if you are just dropping your tanks at a dive shop, and have no idea who they are taking them to, don't be surprised if you get less than optimal results.

Honestly this is why I like my local dive shop, they do the hydros in house.

I know the dude that does the hydros for a lot of the shops in the High Springs area, he is a diver as well.

ole Mickey was roaming around the shop and found a good place to hide.

Walt would grieve for it. Bob wouldn't care.

 

[rob.mwpropane]

Honestly this is why I like my local dive shop, they do the hydros in house.

I know the dude that does the hydros for a lot of the shops in the High Springs area, he is a diver as well.



Walt would grieve for it. Bob wouldn't care.

LOL.... may he rest in peace.

I've mentioned it on here before, but after having a tank fail hydro I did some investigating. Turns out all the local shops use the same hydro place and they don't know wtf they're doing when it comes to galvanized tanks. I called the LDS and asked him how often do steel tanks fail.. he said "here and there". That's a far cry from @Tracy and his 2 in 1000. Makes you wonder...