Overfill of Steel 72

[RonMurray]

I had a steel 72 filled last night and when I got it home I found that the operator had filled it to nearly 3000 psi. That's the normal pressure for an AL80 isn't it?

Anyway, the tank was hydro'd a month ago and originally certified for 2260 I believe. When I got it back from the the LDS that performed the hydro they had filled it to 2600 psi.

Is overfilling a pretty common practice? How much danger am I with this 3,000 psi tank in my garage? I'm sure these tanks are engineered with a large safety margin...

How far have you pushed the envelope on this?

 

[Scared Silly]

The hydro test is at 5/3 of the working pressure or for this cylinder 3750psi. You got about 4/3 of that. Many LP steels get jacked up above their working pressure. It is quite common to get what is known as a cave fill for cylinders that have a working pressure 2640 (2400 w/10% overfill allowed). Many have no problems putting 3000-3500psi in them.

I probably would not leave it stored that way for a long time. So my suggestion would be to go use the gas to lower the pressure rather than bleeding it off - a perfect excuse for a dive.

 

[Luis H]

If the pressure is 2600 psi, is no big deal. That tank was normally filled to 2475 with its 10% overfill rating. If it is filled to 3000 psi, well I suggest you take it diving before the weather starts getting hot.

The tank was tested at 3750psi and at that pressure the stress is supposed to be at the low end of the material yield strength (way below the ultimate strength). Therefore there is not much danger since the tank is in good condition, but the burst disk has been stresses a bit over its normal operation.

The minor overstressed on the burst disc is done (and you could have the disc replace on the next VIP) so I would not bother lowering the pressure before your next dive, unless you expect it to sit around in a hot car trunk or some other warm spot.

 

[DA Aquamaster]

As stated above the 5/3rds hydro test pressure is 3750 psi so I'd feel a lot better about a 3000 psi fill than I would a 3500 psi fill as it is still fairly far under the test pressure.

It is common in North Florida for shops to fill LP steel tanks with 2400 psi service pressures to 3500 psi. In that case the fill is only 500 psi below the hydro test pressure and they seem to do it for years with no particular issues other than perhpas a higher than average failure rate for hydro tests.

So filling a steel 72 to 3250 psi would be comparable and 3000 psi is under that figure so I woudl probably not be overly concerned.

The other side of the argument is that 3000 psi in a steel 72 is still only about 83.6 cu ft. - compared to the 71.2 cu ft you get at 2475 and the extra 12 cu ft of gas is probably not worth the extra strain on the tank. I tend to fill mine to 2600 (74.8 cu ft) and call it good.

The other issue was alsoi touched on above - the burst disc is designed to vent pressure at 90 to 100% of the test pressure (3375 to 3750 psi in this case) and that is when they are new with no cycles or corrosion. So it has been pushed close to it's limit and that may shorten it's life. Most often they ruputure during a fill when the pressur eis increasing but it is not unknown for them to rupture during a dive when the tank presure is falling and a blown burst disc will vent the entire tank over the course of several minutes so lots of bubbles means it is time to abort the dive and ascend. So if you are using the tank in an overhead environment, emptying the tank and replacing the disc now is a good idea, otherwise, just be aware of the potential for a failure and have a plan in mind so you are not caught by surprise if it bursts - and then have it replaced at the next VIP.

 

[Nemrod]

Just bleed the tank down to 2500 psi and don't worry any more. N

 

[Hoosier]

Is this also true that an old 72 tank has a thinner wall than any other LP tank's one?

 

[SparticleBrane]

LP72s have very thin walls.

 

[Luis H]

Is this also true that an old 72 tank has a thinner wall than any other LP tank's one?

The wall thickness on most steel tanks is in the same region. The table below has the minimum wall thickness for a number of tanks. See column 8 from the left.

The wall thickness is slightly lower, but it is proportional to the slightly lower working pressure and the smaller cylinder diameter. If you look at the calculated stress, it is actually lower than other steel tanks.

Most 3AA steel tanks are made from the same chrome-molly ASTM 4130 type of steel and the allowable design stress for all 3AA steel is the same 70,000 psi per CFR49 section 178.37

Oh if the pressure is 3000 psi or less, there is no need to lower the pressure to 2500 psi before the next dive. Just don’t let it get warm and go to a higher pressure. The time exposed to the 3000 psi is irrelevant. If there was any harm to the burst disks it is not time dependant.

 

[DA Aquamaster]

LP72s have very thin walls.

Read Luis's post a couple of times.

Steel 72's were designed and manufactured to the same 3AA criteria as the 2400 psi LP tanks that North Florida shops love to over fill even though the smaller diameter steel 72's are subject to slightly less stress. That bias tends to perpetuate the rumor that Steel 72's are some how inferior or conversely that the 2400 psi LP tanks are in some way superior in terms of strenght and safety when overfilled.

 

[RonMurray]

Thanks for all the great replies and information. I'll be using the tank this weekend and feel much better that I am not strapping a bomb onto my back.

Back when I got certified (1976) the book we used had a photograph that is burned into my mind of a white Impala with the trunk/rear-end blown out of it by a SCUBA cylinder. I was in my formative years and probably am scarred for life...