Cave Fills and Burst Disc Rating?

[CuzzA]

Good info. Thanks for posting.

Both of my tanks now have 5000 psi burst plugs and I will make sure to replace them at every hydro.

 

[2airishuman]

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.

Well, maybe I'm bad at math, but I get:

3500 * 3/2 = 5250

While it is true that 3442 * 3/2 = 5163, my six HP cylinders (two Worthington, two PST, two Faber) are stamped with a working pressure of 3442 and a test pressure of 5250. I conclude that this is typical.

A fact to ponder is that the ratio between the actual fill pressure of + rated 3AA cylinders ( 1.1 x rated ) and their test pressure (5/3 x rated) is 1.515151, which is approximately the same as the 3/2 ratio between the actual fill pressures of most E/SP cylinders and their test pressures.

I'm not an expert, but it has always been my understanding that the burst discs themselves have a 90%-100% manufacturing tolerance in the relief pressure, and that it is technically not permitted to install an under-rated disc.

 

[NAM001]

As far as using an under rated disk. You statement has merit. especially if the valve has a working pressure on it.
Example my Tank valves have stickers on them that say 3000. The stickers came with the valve. putting the 3000 sticker on the valve indicates the burst disc is the one for a 3000# tanks.
The valve for a 2400 use is the same valve but with a different disc in it. To put a 2000 psi valve on a 3000 system would potentially set up an incident where the disk would blow under normal system pressure.

If I ever get "CAVE FILLS again in my LP tanks I will have the disc changed to ones for a 3500 psi tank. I say this cause On one occasion I had 4k+ fill on a tank and it warmed up in the sun and the spg was reading 4400+. too close to min 4500 burst pressure for me. Especially with a costly trimix fill.

 

[Michael Guerrero]

plugs...

 

[Shotmaster]

The hydro tests elasticity of the metal to stretch and return within a percentage of the original dimensions and is a predictor of potential failure. The burst disk is to prevent catastrophic bursting of the vessel if the cylinder is exposed to heat (primarily from fires), and provides a controlled release of the pressure way before the integrity of the vessel would be challenged. The actual burst pressures are very, very high compared to service and test pressures. Our safety requirements are higher than most of the world. I cave fill mine and have no fears. Aluminum is a different story. It is very temperature sensitive, as low as 350 F can alter its structure as some guys found out from powder coating them.

 

[Rusty Shackleford]

I know of one LP steel failing hydro.

Instructor at my old shop was filling his to 4500 cold routinely. After many years and much abuse, his failed hydro.

He got greedy and got bit.

 

[Lukas Ircha]

I can't answer definitively. However, my tanks all have whatever discs XS Scuba ships in their valves that are labeled for "2400 PSI". My tanks have repeatedly been filled to 3600psi and occasionally to 3900psi (cool). They haven't given out yet, and unless the LDS changed the discs for free without telling me or charging me for the new ones, they're still the original discs.

I wouldn't use the 4000 psi burst discs if over filling LP tanks. It's too close to the rated burst pressure and I'd expect the disc to go sooner or later. My LP tanks have 5250 burst discs and I've filled to 4000psi several times (typically 3600) without issue.

I own a set of Faber LP112's. I regularly fill them to 3400 psi with a 4000 psi burst disc. I've had no problems at all.

 

[Lukas Ircha]

ok, so let's do some math.

What can cause a tank to exceed it's fill pressure to a real point.

Compressor failures, well most compressors can't pump beyond 5000psi anyway, and most are regulated to 4500psi if they can go up higher than that because the HP banks are 4500psi. Yes there are exceptions, but let's assume that worst case they can get filled to 4500psi which at 500psi over the hydro pressure for LP steels, it isn't going to do a bloody thing to them.

Heat. this is the biggest thing. Now, let's be super generous and say you got a fill to 3800psi at 0F because your fill station is in a freezer. Now let's say you drive to the desert during a 100* heat wave, in direct sun, and leave your tanks in your car where it is reported to get up to 160F inside the vehicle. The pressure is going to go up to about 5100psi. Still not going to cause a high pressure tanks burst disc to blow. Is this realistic? Heck no!

A more realistic scenario is a fill inside to 70F because you left them overnight and got them topped off to 3600 psi, then you let them bake in the Florida heat where it gets up to 160F in the car, and the pressure will get up to about 4200psi. I don't want 4000psi burst discs in my tanks because of that still less than realistic, but more probable scenario. I would like 4500psi burst discs if I had a choice, but the 5250's aren't going to hurt my feelings because I know that the pressure can only barely get above the hydro pressure of the LP tanks if they're filled way up.

Realistically the temps are not going to be 70/160, but more like 80/140, and with a good 3600psi fill that is only going to creep up to just shy of 4000psi. Remember also that this is assume the tank has fully cooled off to ambient temperatures which can take quite a long time and is not indicative of what the fills are going to be. In some shops the regulators are set to 3800psi to make sure that they cool off to around 3600psi, so you now have a 3800psi fill at roughly 110*, that isn't going to get that much higher in the car.

So TLDR is it is completely unrealistic for a tank to ever get well over about 4000psi due to pressure increase from the heat, it is highly unlikely to be connected to a compressor capable of wildly overfilling a tank to the point that you are worried about the walls bursting, and unless you are diving real old tanks with 2250 or lower fill pressure *i.e. LP72's*, I would just put the big 5000/5250psi burst discs in and not worry about it. I use 5000psi because of aluminum tanks and keeping everything consolidated.

 

[Lukas Ircha]

So what pressure would you cave fill those old steel 72s with 2250+psi rating? I got one from 1977 and inside is very clean and it would be nice to have 80 cf or gas in it. Can you go 2800psi and treat it as a 80 cubic foot tank? if yes can you let it sit for a week or more at those pressures? it is only 25% over fill so should be fine no? Is 3000psi crazy for these 40 year old tanks?

 

[tomfcrist]

So what pressure would you cave fill those old steel 72s with 2250+psi rating? I got one from 1977 and inside is very clean and it would be nice to have 80 cf or gas in it. Can you go 2800psi and treat it as a 80 cubic foot tank? if yes can you let it sit for a week or more at those pressures? it is only 25% over fill so should be fine no? Is 3000psi crazy for these 40 year old tanks?

This thread has been dead for only 2 years..so we are getting better.

Answer: It’s common to take 40 year old 72’s to 3k. There is no way however that I would pump them beyond that...I value my appendages too much.