Failed Hydrostatic Tests

[diverbeans]

I have three vintage 71.2 steel tanks that have failed their hydro test in recent years. I often take a mixed lot of aluminum 80 3000psi and steel tanks to be filled at the same time. Many times I have come home with ALL of the tanks containing 3000psi. I would always bleed them down to 2250. I know the tanks are old. The oldest is from 1961. There are many older tanks still in service. I think the overfills damaged the tanks and caused them to fail the test. Has anyone had a similar experience?

[duckbill]

Of course, routine overfilling of cylinders should be avoided; but whether the 3000 psi causes permanent expansion or not probably depends on several other variables. Considering that hydostatic test pressures typically run 5/3 of working pressure, I doubt that an occasional 3000 psi overfill would shorten the cylinder lifespan by a whole lot. However, you did say "many times". (5/3 of 2250 psi is 3750 psi hydrostatic test pressure for permanent expansion calculations)

BTW, it is my understanding (and I could be wrong) that once the cylinder is overfilled there is no point to bleed the air back down unless there is a chance of further pressure build-up due to exposure to heat. I always bleed mine back down anyway, just in case I am wrong about that! It just makes me feel better (even though I know that once any damage is done, it is irreversible).

I don't know if this would help you in your situation, but I always mark and circle the working pressure of my 72s in black permanent marker on my yellow tanks, or write the pressure in bright yellow paint pen on my galvanized tanks, along with my last name. I also circle any "plus" ratings on the last hydro date stamp so the filler will take note.

[Broxton Carol]

That is a most interesting point. What if you had a 1800 psi tank in the lot? That may mean real danger to the careless shop employee. You have illustrated some can be a pain in the a--! Some wont fill vintage tanks at all. For years I have used a compressor of my own, so it saves any chance of this kind of mistake. a couple weeks ago on a trip to florida I filled over 24 cylinders, where and when I wanted!

[YankDownUnder]

Diverbeans,

The test station where my tanks are serviced says it is rare for a steel 72 to fail a test. They mark the tanks which have a low working pressure and you might want to consider doing that too. They just use a paint pen. You could even high-light the working pressure in that way. It might even be considered a 'status symbol' to have a pressure stenciled on the tank, a bit like the early & pre-Broxton imports. They had USE COMPRESSED AIR ONLY on them, as diving air was uncommon in the early days and a commercial industrial gas suppliers might fill the tanks with oxygen by mistake. Even early film scripts have the actors talking about diving with oxygen when they mean compressed air.

When I have a 72 filled, I stand there and watch. I have stopped several fills that would have been far too high. My local dive shop is a 'tech diver' shop and they over fill even HP tanks and let them cool back. They do a lot of deep wreck dives (250 ft) and need the gas. They certainly don't think 'vintage'.

I had a set of triple Broxtons blow a disc in my car. It was an old tube shaped blow plug and the escaping gas flipped the tanks over. I could have lost the glass from my car, but I had a window down. Having a disc blow while you are driving at freeway speeds could be dangerously distracting.

I don't know if the over-fills caused the damage to your tanks, but working pressures are marked that way for a reason. When your tanks are failed, the test station is doing you a favor. They don't want to fail tanks. Customers and shops are very unhappy, and no one wants that. A ruptured tank releases gas about as fast as detonated TNT, and is nothing to fool around with.

I would definetly mark the tanks, as discs bursting can be a problem and even regulators can be harmed. The HP seat on a US Divers Mistral may be harmed from too high a pressure and some pressure gauges are not rated high enough for some over-fills. My British Heinke Mk III regulator has a pressure gauge calibrated to 140 atmospheres (2058 psi) and I watch that one closely. I hope that helps.

[hardin]

Hey,,, I am an old FSDA & PADI instructor from the 70's. I worked at a dive shop on the east coast with hydro station. I care NOT to remember how many i have done including pallets of them from sub & destryoer tenders from all over the world. I remember only 2 tanks failing "hydro. One a new 80 sold on a Fri night & involved in a fire the same night. The disc blew so the tank did not explode. It was yellow with a browning of the paint in one area 8 inches X 4 inches. It made it to around 4200 psi when it failed. The temper had been taken out of the tank & the Alum mbulged in the brown area. The secont was a stell 72 that passed hydro. It was dirty & rusted inside so it was rolled with carbide bits. When the girl at the shop put it in the cooling tank to fill she walked off. She returned with the tank at about 1500 psi. She hollered & boogied out of the shop at a run. I went to her & ALL SHE COULD SAY WAS THE TANK IS LEAKING!!!!!! There was a pin hole leaking air out the side of the tank. Evidently when the tank was rolled it caused what ever rust was plugging the hole to get knocked loose opening the hole. Well that is the only two i can remember!
Later on!
Old Diver!

[Captain]

Were the tanks failed as a result of visual inspection or hydro test. Were they done at a scuba shop or a commercial hydro test facility and finally did they follow PST procedure D-100. I will not allow a hydro test facility to test my tanks unless they are willing to follow D-100.

[duckbill]

Captain, is D-100 the "rounding out" procedure?

[Captain]

duckbill said:

Captain, is D-100 the "rounding out" procedure?

Yes it is. I ask my hydro service to follow this procedure, I gave them a copy of it. Although it is not mandated by DOT that it be done it is the same as the procedure that is allowed by DOT as an equipment calibration .

[scubadiverbob]

Diverbeans,

Many years ago, I used to hydro tanks at AquaLung Dive Center on Little Creek Rd. in Norfolk, VA. I also overhauled doublehose regs there. I was stationed at NSGD CincLantFlt back then. I'm still trying to find Matt (he taught me all this and was my dive buddy). If anyone remembers him, he had a Chowchow (was a brown dog) that used to attack the postal carrier when mail was delievered to the dive shop. His dog never went after anyone else.

I've never heard of an overfill causing a tank to fail hydro as DOT purposefully rates tanks at about half the pressure they can be filled to for safety reasons. Then there are blow disk in the valves to make sure they are not overpressured. The most common causes, I know of, for tanks failing hydro (excluding certain aluminium tanks; some Al tanks most shops will fail w/o a visual or hydro) is metal fatigue, excessive pitting, and too much rust (steel tanks filled at air stations where the air isn't filtered enough). Did you ever see a tank with a crack in the neck? That can cause it to fail vis/hydro. If you leave tanks setting in storage for long lengths of time it's a good idea to slowly bleed them down to 200 to 500 psi. This helps to prevent metal fatigue and if the tanks get warm in the summer they probably won't overpressurize.

Don't know why they even tested the Al tank you mentioned was in a fire. That's usually an automatic failure w/o testing.

Really, I don't think a 3000 psi fill on a 72 would cause it to fail if you bleed it down when you got home. I had a USD 72 from 1960 or eariler (first hydro on tank was 1960). It got overfilled a few times. I sold that tank to Jack (one of my dive buddies) and I think he is still using it.

Robert

[Captain]

A tank stored full will not cause metal fatigue. Metal fatigue is the result of being exposed to a force that is constantly cycled in a bending or stretching mode over a period of time. Bending a wire back and forth until it breaks is an example.
Filling and empting a tank ever so sightly stretches the metal, after a period of time the metal becomes fatigued and losses it's elasticy. Tanks are designed to be able to go through thousands of these cycles before the metal starts to fatigue, far more than any diver would put a tank through. Over filling by a gross amount will shorten the number of cycles to failure but instead of failure being 10,000 cycles it might be 9,000.
Steel tanks are best stored full. In case of fire the pressure rise will blow the burst disc before the strength of the steel is lessened. Because aluminum tanks are weakened at a much lower temperature the aluminum could be weakened to failure before the pressure rises to the point of blowing the burst disc so it is best to store them at a couple of hundred pounds if there is the possibility of fire .

[caveman]

I found a couple of 72's laying outside of the hydro facility that I use. I gave the guy that runs the place $20 for them so I could have the j valves that looked pretty good. When I took them home the valves had been left open for several weeks and each tank had water inside. When I cleaned them with detergent and dried them out they looked pretty good so I took them back to the same hydro facility and they both passed hydro.
I have quit a few 72's dating back to 1952 and none have ever failed testing. I may be just lucky, but I have a lot of respect for the ol' 72's.

[scubadiverbob]

I like steel 72's, also (what I dive with all the time). I do have an Al 63 tank. I use it to fill bicycle tires. Works great for that.

Got to go vote ...

Robert

[caveman]

Good idea. I have an aluminum 6. Use it to fill my dry suit, maby the same thing....

[william]

"Yes-sir-ree", the idea has come to me & to Cyndy too,ha, with the 2 small aluminum scuba tanks we still have in the back of the hall closet, the idea of having an aluminum tank failing during a dive, like you see in pictures,the Tops blown off of them) and blowing the back of one's head off is Scary-enough. So I will never strap one of those on again. Give us Any 1950s to now, Steel Scuba Tanks and I will be much more at ease in Vintage Diving,ha,ha

[scubadiverbob]

I used a conshelf VI first stage, a power inflator hose (the inflator end of it leaked, and an air compressor hose to put air in my bike tires. I cut the bad part off the power inflator hose and attached it to the compressor hose. Nice thing about the conshelf is I can adjust the air real easy for use with pnuematic tools (most are rated for 90 psi) so if no compressor is around to use, I just grab my tank. Works great!