Yet another LP 72 question

[John C. Ratliff]

Remember that these are very old tanks. That article states that overfilling will shorten their life span, and that's a 2005 article. When their history is unknown, it is another reason not to overfill these tanks. Even a remote possibility of an explosion is not acceptable to me.

SeaRat

 

[lowviz]

Remember that these are very old tanks. That article states that overfilling will shorten their life span, and that's a 2005 article.

Overfilling past the elastic limit will indeed shorten their lives or even end them. Overfilling below the elastic limit is fine. But one has to go about this correctly. https://en.wikipedia.org/wiki/Yield_(engineering) Scroll down to elastic limit.

Aluminum has no elastic limit. They will all fail over time. Ask any aircraft engineer. Steel will act like a balloon if properly treated. NEVER exceed the elastic limit. Steel 72's are overly conservatively rated at 2250 psi. IMHO, years and numbers support the claim that 2800 psi is within the elastic limit. NO cumulative damage.

...//... When their history is unknown, it is another reason not to overfill these tanks. ...

I would agree, but passing hydro and a really good VIP is an excellent indicator that nothing serious has been done to them.

 

[DA Aquamaster]

The article is heavily slanted and it's using a single source with a known bias toward following the rules t the letter:

Undercurrent discussed overfilling tanks with Bill High, President of Professional Scuba Inspectors, Inc. (PSI) and the dive industry’s leading expert on tanks. He told us unequivocally that continuous over-pressurizing of either aluminum or steel scuba tanks has a “cumulative effect, so the danger of an explosive rupture increases over time.”

Bill is also leaving out some critical details in the bolded portion of the excerpt. If he were only talking about aluminum tanks his paraphrased statement would be correct, as aluminum tanks have a fatigue life.

Luxfer tests their 3AL tanks to 10,000 cycles, but overfilling the an aluminum tank will increase the stress and reduce the fatigue life, so over fills of aluminum tanks are very bad juju.

A hydrostatic test measures permanent expansion and it is in essence ensuring that the tank is not significantly exceeding it's elastic limit at test pressure, as evidenced by little or no measurable permanent expansion. That's true for both aluminum and steel tanks and if a tank has fatigued to the point that the elastic limit is exceeded during the hydro test, it will demonstrate excessive permanent expansion and it will fail the test.

Steel tanks however differ from aluminum tanks as steel tanks not have a fatigue life, but rather have a fatigue limit, and if the tank stresses remain under the fatigue limit, they can withstand an unlimited number of cycles.

There is also a difference in steel tanks. Tanks that divers refer to as "low pressure" steel tanks are steel tanks are designed to the "3AA" engineering standard and that standard has proven to be very conservative over the last 50 plus years.

There are also special permit or exempt permit tanks that are not designed to the 3AA standard, and their engineering standards are not as conservative.

This difference matters as carbon steel has a fatigue limit, an elastic limit, and a plastic limit. As noted above, unlike an aluminum tank, if a pressurized steel tank does not exceed it's fatigue limit it can be cycled to that pressure indefinitely. If the fatigue limit is exceeded but it is still inside the elastic limit, the tank will expand slightly but it will still return to it's original dimensions. However, each excursion above the fatigue limit is now causing fatigue in the metal, and it will have a fatigue life.

Exempt and special permit tanks, with their thinner walls and less conservative engineering standards cannot be over filled without going boldly forward into crash test dummy territory. So no one should be overfilling them.

2400 psi service pressure 3AA steel tanks on the other hand have a 20 year plus history of being cave filled to 3600 psi (a 150% over fill) and they are a) not blowing up, and b) they are still passing hydro tests with flying colors. That speaks to the conservative nature of the 3AA standard and the high probability that even 150% overfills are still under the fatigue limit for the tank. To put it in perspective a 2400 psi tank has a test pressure of 4000 psi, equal to 5/3rds the service pressure.

Steel 72s are a little different as they have a 2250 psi service pressure rather than a 2400 psi service pressure. That means the test pressure is 3,750 psi. A comparable 150% overfill is 3,375 psi. I don't know anyone who actually fills them to that pressure but 3000 psi is common in cave country and 2,800 psi is a little conservative by comparison.

Steel tanks due rupture, and the article suggests that overfilling is the cause, but fail steel tank is virtually always due to line corrosion in the tank (a line of rust pits in the tank) or a particularly deep it in a sidewall that creates a stress riser in the tank.

A comment was made in the article that the hydro test process does not test the bottom or shoulder of the tank, and that's only partially true. The limiting factor in a tank is the hoop stresses. Those are increased as a tank design is made longer or larger in diameter. A short, skinny tank can have much thinner walls than a long and fat tank as the hoop stresses are much smaller. The ends of the tank however are much thicker than the walls due to the process of making the tank, and those compound curved sections are also mechanically much stronger than the straight sides of the tank. So yes...a hydro test isn't really affected by the condition of the ends of the tank as they are no where near the limiting factor in the tank.

Bill High knows that as well, as he knows allowances for a pit in the base or shoulder of the tank are much larger than the allowance for a pit in the sidewalls of the tank. Ever wonder why the numbers on a tank are either on the shoulder (tank markings, test dates, etc), or on the base (lot numbers and control numbers for the manufacturing process)? It's because that's where the steel or aluminum is the thickest and it can be stamped without compromising the integrity of the tank.

In short, Bill is doing his job and protecting his business interests by insisting that everyone follow the rules and never exceed the service pressure stamped on any tank.

 

[John C. Ratliff]

DA Aquamaster, could you cite your sources for this information? That way, we could check for ourselves, and it gives credence to what you say.

SeaRat

 

[lowviz]

I purposely picked the most inflammatory support possible for not overfilling.

I am squarely with DA Aquamaster when it comes to my choice for my diving and my continued existence.

 

[DA Aquamaster]

DA Aquamaster, could you cite your sources for this information? That way, we could check for ourselves, and it gives credence to what you say.

SeaRat

I only took a couple years of aeronautical engineering before I figured out I liked flying them a whole lot more than I'd ever like designing them, but it covered the basics of metallurgy. Look up the terms "fatigue limit", "elastic limit" and "plastic limit" in relation to carbon steel and draw your own conclusions.

Then perhaps consider why the DOT will crawl all over a dive shop that overfills aluminum tanks, but has basically ignored every shop in north Florida that has been overfilling 3AA steel tanks for the last 20 plus years. I don't get the impression the DOT is all that concerned - probably because they've most likely considered a) the differences between 3AA and 3 AL tanks, b) the fatigue properties of steel and aluminum and c) the lack of catastrophic tank failures in cave filled 3AA steel tanks. The only tank failures I am aware of in cave country were related to dropped aluminum O2 tanks.

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Now to be fair, engineering standards and safety margins don't have a lot to do with a shop's willingness to over fill 3AA steel tanks.

Sadly, we have way too many lawyers in the US, and those that don;t go to work for the government end up looking around for people to sue. Unfortunately, in the US you can be sued for anything, whether the case has any merit or not as those same attorneys are fairly confident that they can still get a settlement even when there isn't a hope of winning the case in court as it's cheaper for an insurance company to sue than to cover the costs of successfully defending against the lawsuit.

Which is to say that most dives shops won't do cave fills as their insurance companies would probably drop them if anything did happen, leaving them totally exposed and probably out of business.

The challenge is different in cave country, as if a shop only filled tanks to their turn pressure, it would be out of business in short order. Thus, they provide cave fills to stay competitive and stay ion business and trust that nothing is going to happen and that they can deal with the lawsuit or limit their liability if something does happen.

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In that regard, I fill my own steel 72s to between 2800 and 3000 psi in my garage. I assume all the risk and I'm comfortable with that level of risk because I ensure they are properly re-qualified every 5 years, I do my own annual VIPs, and I maintain them in spotless condition.

I would not however be inclined to fill anyone else's steel 72s over the service pressure as I don't know the history of the tanks, I don't do the maintenance on those tanks and I don't feel like assuming the liability that would come with filling someone else's tanks.

 

[John C. Ratliff]

I find it interesting that cave divers are willing to overfill steel 72s, but will harp a lot about how a diver needs a more robust yolk on older regulators so that they can handle the increased pressure. And yet, in the U.S. Naval School for Underwater Swimmers, we routinely used DA Aquamaster regulators with the standard yolk on 3000 psig Navy Aluminum tanks (twin 90s) in 1967.

SeaRat

 

[BubbleGator]

The old steel 72 Nemrod I saved from scrap (made in 1979) looks real good inside, as I see no rust in the tank or on the surface (the hydro dudes will take off the boot - I did not look at that yet). Shouldn't I feel pretty confident about the integrity of that old soldier? By that I mean, that baby missed seven hydros at 5/3rds pressure. And it has rested in silence at minimum since five years after that when the hydro expired.

I was thinking if I get a new HP 117 I can use a tank-to-tank equalizer attachment to bump my steel 72 local fill from 2250 to about 2650. That would give me about the same cf as an aluminum 80, so I could hang with the group and not make my buddy come up early. And, doing that equalizer fill would reduce the HP 117 tank from 3442 (if I get that; I see on the board how common it is for HP tanks to be under-filled) to a much easier 3050 on my entry level regulator. My new Oceanic Alpha9/SP-5 is rated to 3500. How can it hurt to dive it at just over 3000?

1) should I be worried that I know nothing about the 35 year old steel 72 left next to the trash?

2) would using an HP-117 tank and equalizer set up to jam that old steel tank from 65 to 77 cf be a sound plan? ( I'm old, but ignorant. Not a good combo lol

 

[DA Aquamaster]

I find it interesting that cave divers are willing to overfill steel 72s, but will harp a lot about how a diver needs a more robust yolk on older regulators so that they can handle the increased pressure. And yet, in the U.S. Naval School for Underwater Swimmers, we routinely used DA Aquamaster regulators with the standard yolk on 3000 psig Navy Aluminum tanks (twin 90s) in 1967.

SeaRat

Regardless of brand, yokes tended to get heavier over the years, and that was do to some instances of some of the thinner brass yokes stretching as 3000 psi tanks became the norm.

A modern heavy yoke with a 9/16" yoke screw will probably handle 4000 psi just fine, so stretching is no longer an issue.

However, the yoke knob creates one more line trap and it's a bit more exposed when scootering in a cave, etc.

Also cave divers routinely leave stage and deco bottles with the valve turned off in the cave. If the purge gets bump, the reg will be depressurized and in those circumstances a yoke reg is far more susceptible to having the yoke screw loosen when the diver picks up the tank, re-attaches it and completes a gas switch. Worse, if the yoke is loose, the o-ring can be extruded or lost, leaving the diver with a lost gas scenario, and I've had that happen on a wreck dive back in the dark ages before I switched to DIN about 20 years ago.

With a DIN connection, the connection is much less likely to loosen if the reg is depressurized, and if that happens the o-ring is much less likely to be displaced, and if it is displaced it is much less likely to be lost.

Foe cave and technical wreck diving it's just a much better system, and strength has basically nothing to do with it.

 

[DA Aquamaster]

The old steel 72 Nemrod I saved from scrap (made in 1979) looks real good inside, as I see no rust in the tank or on the surface (the hydro dudes will take off the boot - I did not look at that yet). Shouldn't I feel pretty confident about the integrity of that old soldier? By that I mean, that baby missed seven hydros at 5/3rds pressure. And it has rested in silence at minimum since five years after that when the hydro expired.

I was thinking if I get a new HP 117 I can use a tank-to-tank equalizer attachment to bump my steel 72 local fill from 2250 to about 2650. That would give me about the same cf as an aluminum 80, so I could hang with the group and not make my buddy come up early. And, doing that equalizer fill would reduce the HP 117 tank from 3442 (if I get that; I see on the board how common it is for HP tanks to be under-filled) to a much easier 3050 on my entry level regulator. My new Oceanic Alpha9/SP-5 is rated to 3500. How can it hurt to dive it at just over 3000?

1) should I be worried that I know nothing about the 35 year old steel 72 left next to the trash?

2) would using an HP-117 tank and equalizer set up to jam that old steel tank from 65 to 77 cf be a sound plan? ( I'm old, but ignorant. Not a good combo lol

I wouldn't even consider an overfill in a steel 72 unless it was freshly requalified and in excellent condition - i.e. no rust inside or out, no pits from past rust, and no liner that might be hiding rust or pits.

Then, as you're suggesting, I'd only stuff it with the gas needed for the dive, rather than pushing it to the limit.

One potential issue is the differences in buoyancy. A Faber 117 will be about 11 pounds negative in the water when full, while a steel 7s is only about 4 pounds negative when filled to that pressure. You'd need to ensure that you have the reserve buoyancy to float the larger tank, and I'm not sure how the larger tank will affect your trim in the water. Steel 72s are desirable in part because they trim out nicely, and are not so overly negative that they try to turn you upside down. The 117 is a 8" diameter tank that significantly heavier and more negative.

You'll need to double check the math, but if you equalized a FX117 at 3442 psi and a steel 72 at 2250 psi, you'd get an end pressure of just under 2900 psi, leaving 83.4 cu ft in the steel 72 and 98.6 cu ft in the 117. If you stop the transfill with 2,650 in the steel 72, you'd have 76 cu ft in the steel 72 and 106 cu ft in the FX117.

If you get a short fill in the 3300 psi range with the 117, the end pressure will be in the 2650 range, with 76 cu ft in the steel 72 and about 90 cu ft in the 117, You'll probably find your bottom times are about equal with both tanks given the extra effort and extra gas needed to dive the 117 and keep it's extra weight neutral during the dive.