Overfilling LP Steel Tanks -- How bad is it?

[WD8CDH]

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!

Most yoke modern and many older regulators are rated to be used up to 3500psi. Some are even rated to 4000psi or higher. Most HP tanks have convertible DIN/K valves that can be used with yoke regulators. An even better yoke only valve is the Sherwood 5000 series valve with the smaller o-ring that works quite well at high pressures with yoke regulators.

Cousteau even used yoke regulators at 5000psi quite extensively.

I would suggest going with a modern HP tank. Their stronger steel allows higher pressure without thicker and heavier walls so the buoyancy is similar to LP tanks of the same physical size.

Some of the older HP tanks were made with the same strength steel as LP tanks just with thicker walls. They tend to be very heavy and negative buoyancy.

 

[WD8CDH]

"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.

Not true, The Faber high pressure tanks (and all other HP tanks) are made with thicker walls and/or stronger material than their lower pressure versions. They are absolutely different tanks not just re-labeled tanks.

 

[Cave Bum]

Most HP tanks have convertible DIN/K valves that can be used with yoke regulators.

Some do, but the valves that are convertable are the 200 bar valves and so are rated lower then service pressure of the tank (14.72 psi * 200 = 2944 psi). The 300 bar valves are too thick to fit a yoke regulator over, at least all the ones I have. In North Florida 200 bar valves are not a problem but I've heard of fill stations in other parts of the country and the world refusing to do high pressure fills with the 200 bar valves.

Cousteau even used yoke regulators at 5000psi quite extensively.

Legendary cave explorer Sheck Exley, I've been told, used all yoke valves and he was filling to 4200 psi and above. I was told this was because of the cost of switching the valves on his 50 plus tanks was prohibative. So for all those record setting dives that he made to 800 feet and below, he had yoke valves.

One consideration that I caution prospective buyers about is where they are going to use the tanks. If they're going to use them exclusively in North Florida LP tanks are no problem. If they are going to use them elsewhere, I'd suggest HP tanks with 300 bar valves and manifolds or they might find that their LDS won't fill them to the higher pressure. I've even heard of LDS's refusing to fill even HP tanks above 3000 psi because they have turned down their pressure regulators to prevent an inattentive employee from accidently leaving the tank hooked up and overfilling it.

 

[WD8CDH]

Some do, but the valves that are convertable are the 200 bar valves and so are rated lower then service pressure of the tank (14.72 psi * 200 = 2944 psi). The 300 bar valves are too thick to fit a yoke regulator over, at least all the ones I have. In North Florida 200 bar valves are not a problem but I've heard of fill stations in other parts of the country and the world refusing to do high pressure fills with the 200 bar valves.



Legendary cave explorer Sheck Exley, I've been told, used all yoke valves and he was filling to 4200 psi and above. I was told this was because of the cost of switching the valves on his 50 plus tanks was prohibative. So for all those record setting dives that he made to 800 feet and below, he had yoke valves.

One consideration that I caution prospective buyers about is where they are going to use the tanks. If they're going to use them exclusively in North Florida LP tanks are no problem. If they are going to use them elsewhere, I'd suggest HP tanks with 300 bar valves and manifolds or they might find that their LDS won't fill them to the higher pressure. I've even heard of LDS's refusing to fill even HP tanks above 3000 psi because they have turned down their pressure regulators to prevent an inattentive employee from accidently leaving the tank hooked up and overfilling it.

Long answer:
200 bar is not the service pressure of the valve. 200 bar DIN is just one of the slang terms for the DIN 477-13 standard connection and 300 bar DIN is the common slang term for the slightly longer DIN 477-56 standard connection. Some other common names for the DIN 477-13 connection are 230 bar DIN, 232 bar DIN, 240 bar DIN, 250 bar DIN and 5 thread DIN. (the 300 bar DIN is sometimes called 7 thread DIN.

232 bar DIN is probably the most common and most accurate name for the normal usage of the 5 thread DIN connection but still is not the rated service pressure of the valves. (The slight differance between a direct conversion of 232 bar to psi and 3442psi is due to the differance in standard temperature of the metric system and the Imperial system of measurement) Both versions of the DIN valve usually have an actual higher service pressure, for example, OMS rates all of their DIN valves for service pressures up to 4500psi. OMS SCUBA Valves & Manifolds http://www.OMSdive.com
The actual service pressure will depend on the burst disk. The Poseidon Cyklon 300 used the so called "200 bar DIN" connection but was officially rated for 300 bar service.

While the convertible valves may be rated to 4500 psi, the common usage is up to 3442psi with the yoke insert, well below their maximum rating but well above 200 bar. In almost 40 years of commercial and sport diving, I have never seen a 5 thread DIN valve that wasn't rated to at least 3500psi.

I have never found a LSD that would refuse to fill a 3442psi rated tank to 3442psi just because it had the shorter 5 thread DIN valve. There is no legal or technical reason for them to do so as long as the proper burst disk was installed.

Short answer:
"200 bar DIN" is fine at 3442 psi.

 

[rjack321]

The main reason you don't hear about it is that steel tanks fail in a different mode then aluminum tanks do. Aluminum tanks, which are in the T6 heat treat condition, are brittle and fracture, sending out small pieces of shrapnel. Steel tanks split, simply releasing the pressure with a loud noise but usually without hurting anyone. The old news media adage "if it bleeds, it leads" means that steel tank failures rarely make the news.

Incorrect. AL tanks do not rupture like a hand grenade. The DOT specs definitively prohibit this failure mode. They rupture by splitting into one or two pieces most often starting at the crown area.

 

[rjack321]

I am a really big fan of the LP 95, but frankly if you take away the option of N FL cave fills, the HP130/E8-130/X8-130 all make a lot more sense and the LP tank is rapidly approaching dinosaur-hood.

3AA tanks have been around ALOT longer than any exemption series tank. Its a shame DOT hasn't rolled the 9791 etc exemptions into a new approved tank category. Until they do I would consider the exemption tanks young whippersnappers with a spotty track record of keeping their legal paperwork in order.

 

[DA Aquamaster]

Why doesn't anyone ever ask about overfilling HP cylinders? Conventional wisdom would suggest that if a LP tank is over engineered enough to withstand a 3600psi fill in a 2400psi rated tank then a 4700psi fill in a 3500psi rated tank would be reasonable as well. Not saying I advocate such fills but LP steels aren't the only cylinders you can physically put more air into.

The 3AA low pressure tanks have test pressures that are 5/3rds the service pressure. In contrast, the test pressure for most special permit steel tanks is 3/2 the service pressure. So assuming each type of tank had the same 3000 psi service pressure, the 3AA tank would have a 5000 psi test pressure and the special permit tank would have a 4500 psi test pressure.

So one reason is that the safety margin is less for special permit tanks.

Another reason is that they do not have the same lenght of time in service.

A third reason is the steel alloys are different and, unlike 3AA steel tanks, heat treatment of the alloy is usually involved.

So different materials, different engineering/safety standards and less service history all add to to be the reasons why no one significantly overfills HP steel tanks. Now, I do get 3700 psi ot so fill with my 3442 psi tanks when I take them to N FL, so there is a bit of an overfill, but nothing major - but then again an HP tank is already pretty efficient from a weight/volume/bouyancy perspective.

 

[siikik]

I'm curious... given the technical specifications provided by Cave Bum, why is there a concern about the cylinder exploding on an overfill? With the exception of weak material, which should mean that it didn't pass it's hydro, wouldn't the burst disc rupture first?


Ken

 

[Cave Bum]

Incorrect. AL tanks do not rupture like a hand grenade. The DOT specs definitively prohibit this failure mode. They rupture by splitting into one or two pieces most often starting at the crown area.

Incorrect. The shop where I worked, prior to when I started, had an AL40 Deco tank explode while it was hooked up to the O2 whip but not yet being filled. Sadly, the fill station attendant was killed. It was dropped from waist level and landed on the valve. The explosion, in addition to killing the attendant, completely destroyed the front of the shop. The valve was found about a year later out in the woods about 1000 feet from the shop. The pictures I've seen indicate the tank fragmented. There was a hole in it with missing material.

 

[Rick Inman]

Incorrect. The shop where I worked, prior to when I started, had an AL40 Deco tank explode while it was hooked up to the O2 whip but not yet being filled. Sadly, the fill station attendant was killed. It was dropped from waist level and landed on the valve. The explosion, in addition to killing the attendant, completely destroyed the front of the shop. The valve was found about a year later out in the woods about 1000 feet from the shop. The pictures I've seen indicate the tank fragmented. There was a hole in it with missing material.

Wow. This is horrible. Sorry to hear this.

Question, if the tank was not yet being filled, how much PSI was still in the tank? Was it o2, and was there any fire related to the event?