OOppps I did a bad thing.

I am surprised the burst disc didn’t let go.

Normally I would say not to worry, just dive it, but 4000 is above the test pressure of a steel 72. The test pressure for a 2250 psi tank is 3750 psi (the burst disk should have let go at that pressure also).

I would lowered them to about 3000 and then finish them by diving.

I would also consider an early re-hydro. They are probably fine, but you have stressed them above the low yield point.


The hydro test pressure (the 3750 psi) takes the tank just below the yield point to check that the material is still elastic. That is why one of the important measurements is the residual expansion (or amount of deformation that did not return elastically). If the tank is good, the test should never stress the tank beyond its elastic limit.

By exceeding the test pressure you have caused a very small amount of permanent deformation. That in itself is not an issue, but any work hardening will decrease the elasticity of the metal and make it slightly more brittle (less elastic).

That why I recommend to have it re-hydroed. As I mentioned, it will probably be fine, but the only standard way to measure elasticity is with a the hydro test.

Luis,

I appreciate your expertise in this matter. The mechanics of the tanks, and the effects of pressure on the tanks, is important to be shared, and that is expertise many of us do not have.

I also am surprised that the burst discs did not let go. Is it possible that the wrong burst discs were installed, and they were for a 3000 psig tank? That would be worth finding out, in my opinion.

John

The burst disc does need to be replaced. As mentioned, it probably was the wrong burst anyways.

After I left for work this morning I was also thinking that you live in Alaska.

Cold temperatures will affect the ductility and fracture mechanics of carbon steels. The 3AA tanks are actually chrome-molybdenum steels which I don’t believe are as sensitive to cold temperatures, but I would not push it.

What I mean by that is to lower the tank pressure slowly so that it doesn’t cool down any further. Also avoid any thermal shock (sudden temperature changes), at least until you have lowered the pressure.


I think I am being a bit conservative since this particular alloy is probably fairly good in cold temperatures (under high stress), but I would not risk it, without doing more research on that particular alloy. The most common 3AA alloy is ASTM 4130. I don’t have any of my engineering references at home, but if I had time I bet that some reliable information is available on the internet from some of the steel suppliers.

At BIW (the shipyard that we supervise) they sharpie notch test a lot of structural steels used for heavy lifting due to the concern of fracture mechanics with cold temperatures, here in Maine.

As I mentioned earlier, you need to have the tank re-hydro test it.

Well, I checked all the tanks today and drained everything down to 3,000 psi. The highest tank pressure after cooling for a day and a half was 3500.

I'm thinking the gauge on the 'Ole windjammer is a bit off. I know Dave put a new pressure releif valve on it when he bought it so that he could fill HP tanks, but I'm thinking the 4,000 it showed on the compressor was optomistic... Old gauge is probably off by a few hundred psi.

Also, we have a hard time finding burst discs here. What normally happens is I order a few every year someone in the Seattle area and have them shipped up in the hopes it will last us awhile.

Between Dave Sulser and I(both Dave...) we have about 80 tanks. Most of the Al 80's are being retired and so we've been pulling valves and burst discs off the un-usable tanks as needed. One of the problems with diving in a small town that doesn't have a dive shop is that we have to get creative with maintainance sometimes. Although we've both refused to walk down the 3/8 bolt path, that just seems too sketchy.

Some of the burst dics are likely off of AL80's which is why they didn't rupture.

Since Dave and I both dive for a living(me part-time), we've used a few compressor over the years. The Mako Oceanus stays on the Sea Life, it's actually Kens compressor. The USD windjammer that we use is from the old dive shop.... Southeast Diving. Southeast diving was sold to a guy names Al in the late 90's the compressor was sold seperatley to Tom, who sold it to Dave last year. It's a nice piece of kit, runs really slow and cool compared to the Bauer.

Here is a source for a variety of burst disc:
vintagedoublehose.com/index.php/shop-online.html#ecwid:category=1964142&inview=category1964142&mode=category&offset=0&sort=normal


You should not reuse a burst disc. The moment a burst disc is re-installed, it will have lost its calibration. The burst pressure will not be what was predicted. It will likely let go at a lower pressure, but not always.

The disc and the screw work together as a designed pair. The screw is the cutter for the disc and that is why it needs to be replaced as a match pair.