Questions about LP72 tanks

[guruboy]

How many would you like? I sell them for 75 each and I can ship. I have about twenty for sale at the moment.

---------- Post added August 26th, 2015 at 06:55 PM ----------

Since there seems to be a lot of confusion about good old steel 72s, let us try and set the record straight.
Liner or no liner, doesn't matter much. The liners tumble out overnight. I have never had one I couldn't remove. You will not cut into the wall enough to harm the tank in the time it takes to remove a plastic liner.
Whether or not your LDS is capable, that is a whole different boat.
The only thing that kills a 3AA tank is rust, inside, outside, doesn't matter where, but if you don't see rust, assume you have a good tank.
I have hydroed at least 100 72s in the last three years and I have had ONE fail hydro, several have failed visual inspection due to rust, but just one on the hydro end. (That tank still befuddles me, it is beautiful,it must have been overpressured to the moon sometime in the past.)
As to the plus hydro, they should not be getting one. Period. End of story.
The Manufacturers of steel 72s are no longer in the business, there is no way to confirm an REE number.
The PST document that is often alluded to has a misprint on the tank pressure line for the steel 72 tank. There is no other version that I have seen. With the misprint and no way to confirm it with PST, that document is invalid.
Any retester doing this for a living is not going to risk his license to plus stamp a 40 year old scuba tank for somebody.
If they are willing, that is on them. The problem lies in that they have no paperwork should they ever be called to point on their decision.

When you hydro 3AA stamped steel tanks, do you use the round-out procedure on them?

 

[Luis H]

By definition, all Scuba cylinders are high pressure cylinders. In the US that definition comes from the CFR and the Compress Gas Association.

A low pressure cylinder is the kind you buy in a hardware store or an auto-part to hold shop air (from 90 psi to maybe as much as 150 psi). Propane cylinders are also low pressure cylinders. I can’t remember the pressure threshold were they are classified as high pressure cylinders, but it is very low.

The scuba industry is a very small portion of the compress gas business and it decided to make its own naming convention. I find it very annoying and very inconsistent with the bigger compress gas industry and the rest of the world.

We have a common, technical language. Why does the new kid in town has to start changing the established names?

There are very good technical reasons to differentiate true LP cylinders from actual HP cylinders.

LP cylinders do not need to be hydro tested. They just get visual inspections.

All HP cylinders are hydro tested and are re-qualified in a similar manner. There is no technical difference between an 1800 psi cylinder and a 3500 psi cylinder.

There have been 3AA cylinders rated for 4500 psi. The actual wall stress design on those cylinders is exactly the same as for the 1800 psi 3AA cylinder.

The newer exemption (or special permit)steel cylinders are made of higher strength material, but they are design using the same methodology and the same type of calculations as the 3AA cylinders.


Added: I will admit that I have occasionally used, incorrectly the terms HP and LP cylinders, when talking to other divers, but that is not correct and it still annoys me.

 

[Luis H]

I went back to the hydro shop and asked them about the steel tank testing. They are aware of the round-out procedure like the one mentioned in D100E but they only use it on steel tanks stamped with the exemption (according to them there is a separate section for that). On 3AA stamped steel tanks, they only do the standard hydro test. They will not perform the round-out procedure on a 3AA stamped tank.

I recommend that you do not do business with them anymore. They are making their own rules and not following manufacturers recommendations.


If the coating of your WK cylinder is a factory coating, it is hot-dipped galvanized. That was the only type of factory galvanizing done back in those days. I am positive that WK used it.

The newer types of zinc coatings used today (like from Faber) have a more environmentally friendly process, but that was not as much of a concern back then.

 

[guruboy]

Great. Thanks a lot, Luis. I wanted to make sure I wasn't wrongly applying the round-out procedure on a tank that wasn't hot dipped galvanized.

I have a printout of D100E taped to the tank to take to the next hydro shop.

 

[Eric Sedletzky]

By definition, all Scuba cylinders are high pressure cylinders. In the US that definition comes from the CFR and the Compress Gas Association.

A low pressure cylinder is the kind you buy in a hardware store or an auto-part to hold shop air (from 90 psi to maybe as much as 150 psi). Propane cylinders are also low pressure cylinders. I can’t remember the pressure threshold were they are classified as high pressure cylinders, but it is very low.

The scuba industry is a very small portion of the compress gas business and it decided to make its own naming convention. I find it very annoying and very inconsistent with the bigger compress gas industry and the rest of the world.

We have a common, technical language. Why does the new kid in town has to start changing the established names?

There are very good technical reasons to differentiate true LP cylinders from actual HP cylinders.

LP cylinders do not need to be hydro tested. They just get visual inspections.

All HP cylinders are hydro tested and are re-qualified in a similar manner. There is no technical difference between an 1800 psi cylinder and a 3500 psi cylinder.

There have been 3AA cylinders rated for 4500 psi. The actual wall stress design on those cylinders is exactly the same as for the 1800 psi 3AA cylinder.

The newer exemption (or special permit)steel cylinders are made of higher strength material, but they are design using the same methodology and the same type of calculations as the 3AA cylinders.


Added: I will admit that I have occasionally used, incorrectly the terms HP and LP cylinders, when talking to other divers, but that is not correct and it still annoys me.

1800 psi is considered the start of high pressure in cylinders for gas storage - welding O2, dry nitrogen CO2, argon, etc. Acetalene is LP in the realm of fire extinguishers, propane, etc.
Scuba is in it's own world when it comes to terminology in LP and HP cylinders.
What a lot of people don't realize is hydro facilities don't really give a rats ass about scuba cylinders as much as divers think they care. Divers probably give them the hardest time out of all other customers combined, so I'm usually pretty respectful when I take stuff in. I know that if it becomes too much of a pain for them to deal with scuba tanks and the A holes who own them, they'll just tell us to take a hike, not worth it.
Part of the problem are dive shops that have placed an additional layer of inside regulation on scuba cylinders in the form of annual VIP's and the associated neurotic fanatasism on what they consider clean enough for breathing gas. I think it's funny because in the consumable O2 market for medical grade O2 there is no such craziness, and the craziness certainly doesn't exist in industrial applications.
The dive shops are the ones that are actually making hydro facilities not want to deal with scuba cylinders. One miniscual hint of flash rust, not even enough to make a bit of difference, and some shops will refuse to VIP them, which sends the tank and the pissed off customer back to the hydro shop.
Most hydro facilities don't VIP scuba tanks as dive shops require because it's just not something they do.
For this reason I clean my own tanks and very soon will be getting private fills.
So I'll finally be completely free of all the drama.

 

[guruboy]

Just wanted to follow-up on my tank. Took it to get re-tested using the 2-step process and came back with 1.4% permanent expansion (vs 14%). As a comparison, I had two other LP72 tanks which passed without the 2-step process and they were both in the 7-8% permanent expansion range. Makes a pretty big difference.

 

[guruboy]

Had yet another LP72 tank fail hydro. Supposedly failed even with the round-out procedure applied.

This time I had a matched pair that I found banded together with manifold. same born date (67), same last hydro (72), same interior/exterior visual condition.

One passed. Failure came back with 22.8% PE.

My failure rate on these tanks is about 30%. Not worth the trouble.

 

[Luis H]

Wow, that 22.8% is huge. I have never seen a cylinder fail with that much permanent expansion.

The only time I have seen numbers that high is because there was a leak in the system. Water from the high pressure side inside the cylinder, leaked into the jacket (hydro test container) and the burette showed the water that leaked as if it was a permanent expansion.

If they did do a proper round-out procedure just before the hydro test, it should have caught any system leaks. So this would have ruled out the possibility of a leak and it points to a legitimate cylinder failure, but I am still skeptical of such a high number. Specially if it has a twin cylinder that did OK. I assume that there is no chance that this cylinder was in a fire, but the other one wasn't.

 

[guruboy]

No burn marks or signs of trauma as far as I can see.

The other cylinder was 3% PE.
FWIW, they both had similar EE values.

 

[JamesBon92007]

Hi -


- Galvanized finish instead of painted seems to be the preference. What's the reason for this? Is this as simple as "if it looks galvanized, it is galvanized" or are there tanks which are galvanized but painted over? Any way to correctly identify them?
- Tanks with linings are bad (?). Any way to find this out from tank markings? My LDS does VIP before sending it off for hydro anyway, so they would confirm if this is a problem or not.
- Assuming the tank passes hydro+VIP, any reason using banked EAN32 would be unsafe, maybe due to any lining?

Thanks in advance,
elgoog

I bought two used 72s that were dipped in black epoxy or something but were galvanized. Enough of the "paint" on one was lifting so I could see the galvanized finish underneath. They were born in early 70s and have only been hydroed 2 or 3 times so sitting empty for decades didn't seem to hurt them any.

Given the choice I'd pick the plain galvanized finish. I have one that is painted (and not galvanized) and I need to repaint it every few years.