Cave Fills and Burst Disc Rating?

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CuzzA

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I'll preface this by saying I do not want to start a debate on the practice of cave fills and instead I have a specific question that is a result of an incident where the burst disc on my tank plug let go. The specifics can be found here Close Call at the LDS and a High Pressure Reminder...

I'll also point out that I do not dive caves and have no desire to dive outside my training; however, I think this is the best place to ask this question so I posted it here.

I purchased two used Faber low pressure 112's when I first started diving. The tanks came with 4000 psi rated burst disc plugs. I appreciate the fills I get from my tanks and shop and that is the reason I bought the tanks. However, I'm curious to know from those of you who have experience if I have the correct plug/disc for the typical fills I receive. Normally my shop fills them to around 3500-3600 and they cool to 3100-3200. On occasion I'll get a fill that holds at 3500.

Given the fills I typically want and receive should I change to a higher rated disc/plug for my tanks?
 
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.
 
PRD Safety Assembly (Burst Disk Kit) | Dive Gear Express®

look at the description there. 4000psi discs are for LP steels. Personally I put 5000psi discs in everything just to keep one disc on hand. These are for the 3000psi aluminum tanks, but are only 250psi lower than the rating for the HP steels. I do still leave 4000psi discs in the valves for my LP72's though.

Notice carefully that these all conveniently coincide with the hydro pressure of these cylinders *the 3442 burst discs are rated at 5250 which is hydro pressure of the old 3500psi skinny neck tanks*.

While 4000psi is the hydro rating of the LP steels, it is cutting it way too close to the fill pressures of most compressors, so when cave filling to 3800psi to cool to 3600psi, that's just way too close for me. I wish there was a 4500psi burst disc, but there isn't, so I follow Europes rules where burst discs don't actually exist and it doesn't bother me
 
Thanks for the replies. I think I'm going to have them swapped to 5250.

@kelemvor I think modern valves have the disc preassembled on the plug and will state the burst rating right on the outside. Older valves allowed for changing of just the disc. I'd be curious to know if your shop actually did change them since they specialize in cave training.
 
Thanks for the replies. I think I'm going to have them swapped to 5250.

I'm not absolutely sure about this, but I think that the cylinder wall steel will yield before the pressure reaches 5250. If so the safety of the setup would be comparable to using a blanking plug that doesn't blow at all.

I myself would not do that. I have HP120s I use when I want lots of gas.

I think modern valves have the disc preassembled on the plug and will state the burst rating right on the outside. Older valves allowed for changing of just the disc. I'd be curious to know if your shop actually did change them since they specialize in cave training.

The reality of what's out there is a little more sketchy.

The idea behind the newer one-piece plugs is that they are, as you point out, stamped with the rated burst pressure on the outside of the plug, making it possible to determine, by inspection, whether the correct (or desired, I guess) disc is installed.

I have however encountered three-piece plugs where the burst pressure is similarly marked out the outside of the plug. Since it would be possible to mismatch the disc and the threaded portion of the plug, whether deliberately or accidentally, it is not possible to rely upon the marked pressure as being accurate.
 
Notice carefully that these all conveniently coincide with the hydro pressure of these cylinders *the 3442 burst discs are rated at 5250 which is hydro pressure of the old 3500psi skinny neck tanks*.

My newer HP120s are marked for a service pressure of 3442 PSI and a hydro test pressure of 5250 PSI.
 
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.
 
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.
 
https://www.shearwater.com/products/swift/
http://cavediveflorida.com/Rum_House.htm

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