Overfills on steel tanks

[divinginn]

I have been getting 3000 psi hot fills on my 72s for a few years now without problems,I personally wouldn't fill past 3k though,cool pressure about 2800.what makes me mad is the shops that fill them hot to 2200 or so and they cool down to under 2000 psi,had that happened last weekend,1900psi cool pressure,that is why I am in the market for my own compressor.

 

[DA Aquamaster]

A generic steel 72 holds 71.2 cu ft at 2475 psi (a 10% overfill), 86.3 cu ft at 3000 psi (a 133% overfill) and 92 cu ft at 3200 psi (a 142% overfill).

In comparison filling an LP 95 or LP 104 to 3400 psi is the same 142% overfill. That is I think significant as:

1. Both the steel 72 and the 2400 psi LP tanks are made to the same 3AA engineering specifications and have the same safety margins.
2. Steel tanks do not have a service life in a meaningful sense of the word and steel tanks made prior to WW1 (as in 1910 or so, and as in pushing 100 years old) will still pass hydro and even qualify for a plus rating if they are in good condition.
3. Within that spectrum, LP tansk have bene around long enough to be just as "old" as a steel 72 in similar condition, and in cave country the LP 95 has probably been much more abused in terms of overfills so discriminating against a steel 72 due to age is misguided.

So..if a shop chooses to exceed a tanks service pressure and overfill to the 140% range, it is just as safe (or unsafe depending on how you look at it) to fill a steel 72 to 3200 psi as it is to fill an LP 95 or LP 104 to 3400 psi.

That said, my experience is that most cave country fill monkeys will "trust" an LP 95 and fill it to 3400-3500 psi but will not "trust" a steel 72 and fill it to a comparable level of 3200 psi. They do that in the belief that the steel 72 is "older" and not as strong but that is more or less due to ignorance about certification standards and identical nature of the technology and metalurgy used in steel 72 and "newer" LP tank production.

Outside of cave country it is a moot point as it is often very hard to get a fill monkey to exceeed the service pressure at any point in the fill process. They often just do not understand that, for example, a 3000 psi tank tank hot filled to 3300 psi that drops to 3000 psi when the tank cools back down to room temp has in fact never been overfilled.

Personally I fill my steel 72's to 2600 psi on a regular basis (about 75 cu ft of gas and more or less equal to an AL80 at a dive shop average fill of 2900 psi) - but then I have them hydro tested and I do my own VIP's and tank tumbling so I know that they are in first rate condition, and I don't hesitate to turn one into a wind chime if I see anything I do not like whether it still passes a hydro test or not.

 

[Luis H]

I totally agree with DA Aqua Master post. All 3AA tanks are built to the exact same Code of Federal Regulations 49 paragraph 178.37 and for the most part they are made from the same alloy (ASTM 4130).

When you overfill any steel cylinder all you are doing is cutting into the safety factor. In an ideal world we don’t need a safety factor and the structural geometry of a simple cylinder is close to ideal. If it is a perfect cylinder, calculating stresses is very predictable and doesn’t even require a calculator.

From the experience with overfilling in Florida we can see that the real world of steel cylinders seems to follow the theoretical ideal world… but, they are still cutting into the safety factor. All you need to ask is “do you feel lucky today?” The odds are that you will be OK.

I also occasionally have filled my steel 72 to 2700 and 2800 psi. But again my 72’s are also in good shape.

About the safety disc…I would strongly recommend to replace it with a higher capacity one rather than totally disabling it. I have done some analysis of the performance of a steel 3AA cylinder in a fire and even a somewhat higher pressure safety plug will have much higher protection than no safety plug at all. I may be publishing some graphs in the near future.

 

[mpgunner]

I have heard that Faber steel tanks are of very high quality and they have no problem being over filled. Is this true? Can you trust a Faber LP 95 to be filled to 10 to 20% over?

 

[SparticleBrane]

^ Did you read anything that was just posted?

ALL tanks have to go through the same processes. One specific model of cylinder isn't necessarily going to be better than another, with regards to fill practices, if they are both conforming to the same standards.

A generic steel 72 holds 71.2 cu ft at 2475 psi (a 10% overfill), 86.3 cu ft at 3000 psi (a 133% overfill) and 92 cu ft at 3200 psi (a 142% overfill).

I was under the impression that 3000psi in a steel 72 is a 33% overfill and 3200psi is a 42% overfill. 133% and 142% overfills would be...a bit crazy. :11:

 

[nereas]

Ok maybe I need to back up and explain myself.

I dont need the over fills all the time, I have great air consumption and usually the al80's are overkill.

I can get tanks filled to anything I want since I am the one filling them most of the time as several friends have compressors.

I dont want to spend any more money on tanks right now, a friend is selling a set of 72's than have a single outlet manifold for faily cheap and I thought they may work for sidemount diving (I started a thread on that elsewhere).

Right now all I own are al80's and one can never pass up a good deal from a friend.

I dont have intentions of filling these things to 3k all the time but thought it would be more of a reason to get them if they could get over bumped once and a while should the need arise.

burst discs are of no worry to me, I am gonna start double discing all my tanks or just put in plugs (spare me the warnings please).

So really back to the question, what is it about 72's that sets them apart from other steel tanks which makes people shy away from overfills. Thanks

I don't have any problems with plugging burst disks. That is one less failure point, in fact.

I regularly have my steel 72 twins filled to 3000. No problems there either. I like nice round numbers on my SPG, and 3000 is nice and round. I would not go higher than that, myself.

Like DA said, steel is steel.

 

[deepstops]

Could it be that they don't sell Worthingtons?

No, it's probably because they're made in Canada.

In all seriousness, it's my understanding the tensile strength of the 3442 tanks differs from the 2400+ tanks but I have no idea why they'd single out not overfilling LP Worthingtons.

 

[DA Aquamaster]

^ Did you read anything that was just posted?

ALL tanks have to go through the same processes. One specific model of cylinder isn't necessarily going to be better than another, with regards to fill practices, if they are both conforming to the same standards.



I was under the impression that 3000psi in a steel 72 is a 33% overfill and 3200psi is a 42% overfill. 133% and 142% overfills would be...a bit crazy. :11:

It is what it is. At 3200 psi in a steel 72 you are overfilling the tank to 142% of its rated capacity. Call it 142% or just count the 42% over and above the already full 100% - either way it is the same capacity and saying it like it is is more honest.

Look at it another way...when NASA launches a space shuttle and operates its main engines at 106% of the original rated thrust, they call it "106%", not "6%".

That said, I do tend to fall back on old habits and convention when I refer to a 110% overfill as a 10% overfill as that term has been around since God was a PFC.

 

[Surface Tension]

It soulds to me there is no advantage to getting the steels if your 80's work for you. Those older steels aren't in the same class as some of the newer LP's out there and I wouldn't mess with them.

I fill my Faber LP 108's to 3500.

 

[WD8CDH]

FYI, the Special Permit that the Worthington Steel cylinders are made to allow for about 30% higher pressures than 3AA SCUBA cylinders but it also requires 30% stronger steel so there is the same safety factor.

Also, the 3AL (aluminum) cylinders require the same safety factor as steel tanks.