Overfilling LP Steel Tanks -- How bad is it?

[Rick Inman]

LP steel tanks are regulary overfilled (THOUSANDS every year), which is why we always hear about them blowing up.

Wait a miniute. Make that, never...

As to rules, blame that on Andy and his friends.

 

[Cave Bum]

Wouldn't know . . . I'm not an engineer educated in the disciplines of metalurgy, metal casting and structural engineering.

I am an engineer and do work in metalurgy. I've also worked in fill stations in cave country.

The basic calculations state that for design purposes you want to limit stress to .6 the yield strength. Steel tanks are made from AISI 4130 Chrome-Moly steel with a minimum yield strength of 63,100 psi.

The pressure vessel design standard called out in ASME Section VIII is more conserative but the above gives us a starting point.

Based on a 43 lbs empty weight on 95's and the size of the tank the average wall thickness would be about 1/4". In actuality the base is thicker then the wall, though so say 3/16" min wall thickness.

So we have about 7100 lbs per sq inch that the 3/16" material is capable of withstanding and it's seeing 3500-4000 psi. That's about a 2 to 1 safety factor.

Additionally, the bust discs in these tanks are 4500 psi burst discs. You do increase risk when you double disc tanks or replace burst disks with stainless steel discs as is common in cave country. This is completely unnessary since the 4500 psi burst disc won't pop in normal use. These excessive burst discs don't put the diver at risk. They do, however, put the fill station operator at risk and they also put firefighters at risk if the tank is exposed to a fire.

Okay, so based on the above, the fill pressures in cave country won't stretch the material of the tank beyond it's elastic limit, meaning that it won't permanently damage it.

THIS IN NO WAY SAYS THAT I'M SAYING IT'S OKAY TO OVERFILL STEEL TANKS.

The rating is actually a DOT (Dept of Transportation) standard and I believe DOT is the only agency that can enforce that standard and they can only do so on public highways. Note I said 'I believe'. I'm not sure of this fact, take it with a grain of salt.

 

[rjack321]

The DOT burst disk for a 2400psi (+ or not) lp tank is 4000psi (hydro pressure). You'll want to upsize this to 5000psi (3,000psi service pressure) or 5250psi (3442/3500psi service pressures) if you plan on overfilling a 2400psi tank. It's vastly more likely to blow at 3500 than at 2640 (2400+10%).

 

[Cave Bum]

LP steel tanks are regulary overfilled (THOUSANDS every year), which is why we always hear about them blowing up.

Wait a miniute. Make that, never...

This does not apply to steel 72's. They are being blown up from overfilling on a regular basis. Those are much thinner walled and those should never be overfilled (2250 plus 10% equals 2475 psi).

The tanks this thread is talking about is the 85, 95, 104, 108, 112, 121 series of tanks.

I'm sure most all the folks posting here know that but it occurred to me none of us had specified that for the casual reader. I thought I'd make that clarification...

 

[halocline]

This does not apply to steel 72's. They are being blown up from overfilling on a regular basis. Those are much thinner walled and those should never be overfilled (2250 plus 10% equals 2475 psi).

Really? Funny, I've never heard of a single incidence of a steel 72 blowing up from being overfilled. Surely this would be news worthy as fill stations would be destroyed and people would be dying. Do you have any examples, any documented case of this happening? I'd be interested in that. I'm not denying your statement, I just never heard this before.

I have always assumed that steel 72s were over-engineered for their service pressure. This is why it's almost unheard of for them to fail hydro, despite the fact that every one still in service is decades old. I spoke to a hydro guy once who told me he has never seen one fail.

I think that in part, the service pressure of those tanks was influenced by the fact that lower pressure for compressed air was the norm in the 1950s and 60s, and also by the fact that many steel 72s were reputedly used as fire extinguishers, which operated at much lower pressures.

I would be very interested to know the wall thickness and other specific structural information on LP72s as opposed to current 3AA 2400 tanks.

 

[Mark Michaud SELAUSAR]

"Overfill" can be subjective. Faber tanks are made in Italy. Their fill pressure rating is 4000+ across the Atlantic. I have read that it is 2400 here because DOT decided that was a safe pressure due to the US using yoke as opposed to DIN. It is not about the tank but aqbout the whole package. An extruded oring would be catastrophic and it cannot hold that pressure safely.

A captured oring (Din / most of Europe) protects that. Europe also does not use burst disks. I am sure this can all be argued, but it is common knowledge that most cavers fill to 3600 psi and I personally have never heard of a tank blowing. It may have but I have not heard of one. My point is this.......I don't think it is unsafe to overfill to the pressures used (3600) if the valve is Din, the bottles are clean and inspected regularly (yearly), and properly maintained.

I don't know about worthington's pst, etc. but they are also filled the same way and the results seem to be the same. The main things is to educate yourself and make your decision based on that. You will be diving that bottle. Are you ready to accept the risk? Or, is there really a risk at all. Hmmmmmm.

 

[DA Aquamaster]

Steel 72's are designed to the same 3AA engineering requirements as other 3AA steel tanks like the LP 95, 108, etc. So the walls are thinner but they are thinner due to the lower service pressures they were designed for.

There is also more to it than just wall thickness. Larger diameter and longer lenght also has effect on wall stress, and consequently those factors also drive the wall thickness.

None the less, a steel 72 with a 2250 psi service pressure overfilled to 3375 psi would be over filled to 150% of the service pressure. Similarly, a 2400 psi tank over filled to 3600 psi is also overfilled to 150% of the service pressure. Given the same engineering standard, a 3375 psi fill in a steel 72 won't be stressing the tank any more than an LP 95 filled to 3600 psi.

But as cave bum points out, there is a great deal of predjudice against overfilling a 3AA steel 72 by the same people who over fill other 3AA steel tanks without giving it a second thought.

And as Matt points out, exploding steel 72's are as rare as exploding steel 95's.

-----

Faber steel 95's are lighter than PST and Worthington LP 95's and all of them have similar external dimensions. That would suggest that the walls are a bit thicker on the PST and worthington tanks.

-----

With a properly designed yoke valve, there is face to face contact between the face of the valve and the inlet fitting of the regulator, so the o-ring is also fully captured just like it is on a DIN valve. The difference is that the yoke is more prone to stretching than the threads in a DIN valve. This was a much bigger issue in the past with lighter yokes designed for 2250 psi service pressures that could stretch enough to extrude an o-ring when attached to a 3000 psi tank but is not an issue today with heavier yokes with 7/16" yoke screws.

 

[Web Monkey]

So I live in Florida & I've noticed my LDS routinely overfills tanks...I wanna buy something like an LP80/85 to be overfilled. My question is..how bad/dangerous is it really to do this?

Why overfill? If you're buying tanks, get tanks that hold what you want.

Terry

 

[H2Andy]

oh dude

you must come and see the dark side of the force

 

[AceszHigh]

Why overfill? If you're buying tanks, get tanks that hold what you want.

Terry

Valid point..
For me, the motivation to overfill was because I have a Yoke regulator so I don't want an HP tank. And the LP100's seem much bulkier than the 80 or 85.

Honestly I just now started researching tanks, so I don't know too much. But I read somewhere that one brand (Faber maybe?) LP100 was -14 lbs when full. That wouldn't work for me..as I only wear 12 lbs of weight and would prefer to have some of that be ditchable!