Cave Fills and Burst Disc Rating?

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A recent thread on VIPs indicates that there is absolutely nothing standard about them. If the inspector is trained and certified by an agency like PSI, then the inspector SHOULD follow that agency's standards, but there is no law about this. There is no law about VIPs at all, so anyone can print VIP stickers and slap them on tanks after doing whatever he or she thinks is appropriate, which could in fact be nothing at all.
 
Ill have to look . there is something that says the inspection has to include the valve since it its a safety device and it prevents the tank from being used for other than it was made for.
 
Ill have to look . there is something that says the inspection has to include the valve since it its a safety device and it prevents the tank from being used for other than it was made for.
Something in what?
 
Just to throw a red herring in, I had a burst disc fail in an -AL 80 about 18 years ago. The tank was hydro tested every year. The tank had been filed a couple of weeks prior to the disc failure, and been stored inside, not in a hot car. The disc failed at about 2 in the morning on a cool night. It was like a scene from the exorcist, curtains flying every bit of paper in the room airborne.
dive shop replaced the disc, never happened agin
 
Something in what?

I would have to dig into my course materials if i can find them. Which i dont intend to spend the time to look . Whether it be n the fed regs or an organizational item i don't know. I don't do any tanks for anyone but for myself. Its a non issue for me I just looked at my vip stickers adn it does not specifically say valve. I know it is in the process somewhere from the course i took whether it be in the hydro or vip process.

The PSI inspection form calls for valve inspection if present and whether the disk was replaced or not. Every item on the PSI form is there as result of a fed reg or other controlling agency such as CGA ect.
 
Let's clear up some of the information drift in a few areas.

Test pressures

3AA and 3AL tanks are hydro tested to a pressure equal to 5/3rds the service pressure.

So for an AL 80 with a 3000 psi service pressure, the test pressure will be 5000 psi.

Some other common 3AL and 3AA service and test pressures:
3,300 psi (super 80, AL 100, MP 72, etc) service pressure = 5,500 psi test pressure
1,800 psi (some older steel 45s) service pressure = 3,000 psi test pressure.
2,015 psi ( other older 45s, plus the Catalina AL 14) service pressure = 3,358 psi test pressure
2.250psi (steel 72) service pressure = 3,750 psi test pressure
2,400 psi (low pressure 85, 95, 104, etc) service pressure = 4,000 psi test pressure
3,180 psi (PST 65, 80, 100 and 120) service pressure = 5,300 psi test pressure

Exempt or Specials Permit tanks are made to a different design standard and they are tested to 3/2 the service pressure (1.5x the service pressure).

3,442 psi (most E7 and E8 series tanks) service pressure = 5,163 psi test pressure
3,500 psi (Genesis HP tanks) service pressure = 5,833 psi test pressure

The test pressure for these Exempt and Special Permit task is thus slightly less relative to the service pressure - 10/6 of the service pressure for 3AA and 3AL tanks compared to 9/6 of the service pressure for E and SP tanks.

Burst disc pressures

Burst discs are required by law to fail at between 90% and 100% of the tank's test pressure, regardless of whether it is a steel, aluminum, or composite tank.

That however is the burst pressure when it is new. Burst discs flex and they fatigue with fill cycles. General speaking you need to replace the burst disc every time you send the tank in for a hydro test, which is every 5 years for steel and aluminum tanks and every 3 years for composite tanks. That ensures the burst disc will get retired before it fails.

Generally speaking, if a burst disc fails during the course of a normal fill, or shortly after a fill (the two times they are most likely to fail) it's because the burst disc is way past its prime.

That 90% to 100% window is handy as it means some burst disc assemblies can accommodate different tanks.

For example the 5000 psi Test Pressure burst disc assembly used for an AL 80 is one of the most versatile assemblies around as it must rupture between 4500 and 5000 psi. Thus if a burst disc ruptures at 5000 psi, it will work for these service pressures:

3,000;
3,180;
3,300; and
3,442 psi.

The 5,250 Test Pressure burst disc is also hand to have as the minimum burst pressure is 4,725 psi and it will work in:
3,180;
3,300;
3,500; and it is a close match for
3,442 psi tanks.

Both the 5000 and 5250 burst disc assemblies work fine for a 2400 psi tank cave filled to 3600 to 4000 psi.

Fatigue limits matter

Aluminum fatigues with every fill. The good news is that Luxfer tests its aluminum tank designs to the full test pressure for 10,000 cycles. That pretty much ensures that the fatigue life at the service pressure is not going to be exceeded during the useful life of the tank - and despite what some shops claim, that useful life is far longer than 20 years.

But...overfilling a steel tank is frowned upon as it does in fact have a fatigue life.

Steel has:
- an elastic limit - the point to which it can be deformed and return to it's original shape;
- a fatigue limit - the point at which it will return to its original shape, but start to fatigue; and
- a plastic limit - the point at which it will suffer permanent deformation.

For a steel tank, the three limits occur in that same order, although the separation between the elastic limit and the fatigue limit varies based on the steel used and the tank's design. That's important when comparing 3AA and E or SP series tanks.

3AA steel tanks are over engineered and they have the advantage that they effectively have no fatigue limit as the forces they experience at the test pressure are still below the fatigue limit. That's why there's a 20 plus year history of 2400 psi tanks being filled to 3600 psi in cave country with no issues with tanks failing or even failing hydro tests every five years.

What this means for a 3AA steel tank is that provided it isn't rusted to the point of having pits or thinning of the metal, or damage to the valve threads, a 100 year old 3AA steel tank is just as sound today as it was the day it was made. I seen numerous WWI and pre-WWI era welding tanks meet the criteria for and receive a + rating despite being 100 year old tanks.

As an aside, the plus rating allowing the 10% overfill of 3AA steel tanks was a WWII invention to offset a shortage of welding tanks needed for war production. Thus most of the first tanks that were plus rated were al ready well past their first hydro test dates and there is no truth to the claim that a + rating can only be done on the first hydro test. It does however require an additional wall stress calculation and it requires documentation of the Rejection Elastic Expansion (REE) limit. Still, you'll find many RINs who refuse to plus rate tanks or claim it can't be done. In some cases it really can't if the RIN doesn't have factory documentation of the REE, but in most cases the RIN either doesn't know how to do it, or is too lazy to do it.

Now...with cave fills creeping up to 3800-4000 psi the last few years its possible that this extra 200-400 psi in current cave fill is starting to cause fatigue in these tanks, but it's not all that likely. Still....I don't want to see a fill pressure past the test pressure, even if it cools back to 4000 psi. I'm happy with a 3600-3800 psi fill when cool.

E and SP tanks are a different story as they are much less conservatively designed - which is why they are lighter for any given internal volume. Less metal = less weight, but also less strength and perhaps little or no margin between the elastic limit and fatigue limits. I don't know the specifics, but the safe assumption is that the test pressure on an E or SP tank will take a cycle off its fatigue life, so no one filled them much past the service pressure (3600 is pretty common for a 3442 psi tank in cave country and most shops everywhere else will round up to 3500 psi).

Cave fills and burst discs?

There are a couple schools of thought.

One school holds that a burst disc should be used and should reflect the fill pressure, and thus it should have a 5,000 or 5,250 psi test pressure burst disc installed.

The other school holds that a burst disc is a failure point and they advocate using a stainless steel disc that is not designed to rupture.

Then there are the mis guided souls in the middle who think using doubled burst discs is a good idea. It isn't. A burst disc plug in a valve is designed to very tight parameters and when you double the disc you're putting yourself squarely in crash test dummy territory as the two copper discs can interfere with each other and fail at a lower pressure than would a single disc.

The respective schools of thoughts are based on safety in a fire, where a burst disc (even a 5250 psi burst disc in a 2400 psi tank) can release the pressure in the tank before it ruptures, and safety in the water, where the theory is that a burst disc could fail and cause catastrophic gas loss.

In 30 plus years of diving and 20 plus years of technical or cave diving, I'd had exactly now burst disc leak in the water, and it was a very slow leak with a very small bubble every few seconds. I've also had exactly one burst disc fail, and it was a new disc that failed the day it was installed about 30 minutes after a cave fill. It was properly installed and torqued and I suspect it was just a defective disc.

Thus I'm pretty firmly in the "use a burst disc" camp as the in water risk is essentially non existent provided you change them out every 5 years or so.
 
I'll disagree on the tanks being lighter for the respective volume.
Blue Steel Scuba - Cylinder Specs

Check that chart as the water volume is accurate.
lp108's and FX133's are the same 17L tanks, but the 133's are 1.7lbs heavier with otherwise identical dimensions
 
no all compressed gas vessels the periodicity of the vips will be different but they all have requirements.

A visual inspection is a required element in the "re-qualification" of high pressure steel tank. People commonly call that the "hydro-test" but the hydro static test portion is only one part of the re-qualification process, with a VIP being the other essential part. And for 6351-T6 alloy aluminum tanks and eddy current inspection is also required as part of the re-qualification process.

3AA, 3AL and E and SP steel tanks generally have 5 year re-qualification intervals while composite tanks have 3 year intervals.

In contrast, and a supplement to the VIP done at re-qualification, the dive industry requires an annual VIP, and that's a very good idea for a couple reasons:

1. Scuba tanks are used in and filled around water, and any water inside the valve will get blown into the tank during the fill. That's why it's always a good practice to crack the valve on the tank for a second or two to blow out any water than might be inside the valve, and the same thing applies to the fill whip if there is any potential for it to have gotten wet as well.

2. RINs generally conduct the VIP prior to the hydro test as it's listed first in the regs, and because they feel it's a waste of time to hydro a tank that won't pass a VIP.

The problem with this is that I've seen 6351-T6 aluminum tanks pass a VIP and then come out of the hydro test with a visible crack in the neck. In essence the 5000 psi hydro test pressure as sufficient to cause the crack to propagate from undetectable to detectable limits. I've also heard a couple reports of 6351-T6 tanks leaking through a crack shortly after coming back from hydro, and I'll bet my next paycheck this is an artifact of doing the hydro test after the VIP.

Thus, I think it is important for the shop do a cursory VIP of the tank before sending it off for hydro (to ensure it has no obvious rust, corrosion, pits, or other defects that would fail the tank), and then do a thorough VIP once it comes back from hydro test, particularly with a 6351-T6 aluminum tank.
 
https://www.shearwater.com/products/perdix-ai/
http://cavediveflorida.com/Rum_House.htm

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