Burst discs: Yes they burst when they shouldn't

[KevinNM]

In reality?
I use 3000psi burst discs on my LP72's and all AL tanks since they get filled to 3300 and cool off to about 3k ish.

I use 5250s on everything, because that is what the valves I get come with.

 

[tbone1004]

I use 5250s on everything, because that is what the valves I get come with.

I do actually replace mine at hydro time, so when they get replaced they get either the right ones or blanking plugs. I have a bunch of blanking plugs about to go in

 

[Akimbo]

... I have a bunch of blanking plugs about to go in

Where can you get blanking plugs in the US (sincere question, nothing implied)? Are they solid plugs or disks with a really high failure pressures?

The way I understand it is it wouldn't be illegal for me as an individual to use them... true???

 

[tbone1004]

Where can you get blanking plugs in the US (sincere question, nothing implied)? Are they solid plugs or disks with a really high failure pressures?

The way I understand it is it wouldn't be illegal for me as an individual to use them... true???

northeast scuba supply, and they are discs but I don't believe that they could fail under any pressure that the valve seat would be able to withstand. They "look" like normal burst discs from the outside. I don't know the full legality of using them in the US for personal use

S/S Blanking Plug for Burst Discs

 

[2airishuman]

The way I understand it is it wouldn't be illegal for me as an individual to use them... true???

It's a grey area and varies from one state to the next.

USDOT PHMSA's jurisdiction begins when hazardous materials used in commerce are loaded into or onto a vehicle (or vessel or aircraft) and ends when the materials are unloaded. It is the compressed gas, not the cylinder or valve, that is considered a hazardous material. Anything over approximately 40 PSI is considered hazardous.

So, if you have a bunch of cascade cylinders, and you never transport them full, USDOT PHMSA's rules don't apply. Or if you have a cylinder that is never transported in commerce, the rules don't apply. If the transportation is on a commercial vessel (dive boat), or incidental to commerce (teaching), then you have to play by the rules. There's a "materials of trade" exemption to some of the rules, but not the ones that govern packaging of gasses (i.e. cylinders and valves).

Even then, be careful, because many states and localities require pressure vessels to be inspected and certified under state and local rules, and have an exception for DOT cylinders. That's true in Minnesota, and the inspection and insurance requirements are more onerous than USDOT. There's a separate exception for SCBA cylinders, but not for SCUBA cylinders. Whether the SCBA provision applies to SCUBA is the sort of question lawyers make their money on. Whether a DOT cylinder has to be in full compliance with DOT rules to be exempt from state law is another question of the same sort.

Of course, no one will known unless there is an accident related to the blanking plug, and it won't affect you unless you survive it...

 

[JamesBon92007]

easier way to think about the burst disc pressure, and I think the actual ruling is that they should be no less than 90% of the test pressure, and no more than the test pressure of the cylinder
3aa-2250psi=3375psi-3750psi
3aa-2400psi=3600-4000psi
3al-3000psi=4500-5000psi
3al-3300psi=4950-5500
3442psi=4645-5162psi
3500psi=4725-5250psi

In reality?
I use 3000psi burst discs on my LP72's and all AL tanks since they get filled to 3300 and cool off to about 3k ish.
I use 3442 or 3500psi burst discs on all steel bottles since they normally get filled to close to 4000psi in cave country. As burst discs need to be replaced, they are getting blanking plugs from NESS installed. Obviously not kosher, but I VIP my own tanks, and it's less bad than doubling up burst discs imo.
.

I recently got a bunch of "new" steel 72s hydroed (new to me) so now I am going to replace the burst discs. Some of them are the old three-piece type. I believe they are all 3/8-24 but one listing I read says "UNF." Does this sound right?

Also, is there any reason why I can't put a one-piece unit in place of a three-piece unit?

I normally fill my tanks to 2700 psi and they cool down to about 2650. Is the 3AA-2400 psi = 3600-4000 psi the correct choice? The hydro guy said they were tested to 3750 psi so 4000 seems a bit high however 3375 (the first choice on the list) seems a bit low. Unfortunately the best deal I can find right now is for 5000 burst pressure which seems way too high for my steel 72s.

In nearly 50 years of diving I've only had one burst disc failure and that was a slow leak and happened early on in my diving career. My dive shop didn't even charge me to replace it. Heck, they usually didn't even charge me for air. And no, he wasn't dating my sister or anything

 

[tbone1004]

@JamesBon92007 me personally? I either plug them all or put the burst discs for the HP steel tanks in because it's easier to keep them all the same. I'm not worried about the tank failing at all so I go for convenience on them.

Switching to the new one piece depends on the valve, but I think you can swap them over in most valves

 

[Storker]

I would rather see safety systems on compressors to prevent overfilling and burst disks on valves eliminated. Unfortunately it would require something like RFID tags built into cylinders and smart pressure switches on compressors.

Depends on how many pressure ratings you have. Here on the right side of the pond, we have three: 200 bar, 232 bar and 300 bar. It's physically impossible to mount a properly designed 300 bar fill whip to a 200 or 232 bar tank valve. Our club's compressor has two 300 bar fill whips and is equipped with an auto-shutoff at 330 bar. It also has a 330 bar pressure relief between the compressor and the filters. So if it's physically impossible to mount a 300 bar fill whip on a 200/232 bar tank valve, how can we fill 200 and 232 bar tanks, I hear you ask? We use a 300 bar -> 200/232 bar adaptor, which is equipped with an overpressure relief that blows if the pressure exceeds ~235 bar.

It's possible to overfill a 200 bar tank by some 30 bar, but that's well within test pressure (50% above working pressure, IIRC). You have to work pretty hard to be able to significantly overfill a 232 bar tank or a 300 bar tank.

 

[tbone1004]

@Storker our whips here are the same for DIN. You'll only have a 300bar din whip that will mount to 200bar with no problem *literally looks just like the regulator fitting*, and then they will have a spin on yoke adapter *again identical to the regulator ones, the Poseidon's seem to be common in cave country*. Others will have yoke only fittings and we will use screw in fill adapters. Very different filling procedures.
Fill pressure may be controlled by an adjustable regulator, but often is just controlled by the tank monkeys looking at the fill gauges to stop them appropriately

I'd just put the lp steel burst discs in them and call it a day. It's really close enough.

@JamesBon92007
Burst Discs
You can replace the three piece to a one piece, or you can just replace the disc and/or the washer itself on the three piece. Your call, but they're cheap and easy to replace DIY.

 

[Storker]

You'll only have a 300bar din whip that will mount to 200bar with no problem *literally looks just like the regulator fitting*, and then they will have a spin on yoke adapter *again identical to the regulator ones

Interesting. IIRC our yoke adapters can only be fitted to a 232 bar fill whip (or adapter).

There's also a difference in the number of threads. A 300 bar DIN fitting has 7 threads, a 200/232 bar has only 5. Not an issue for regs, since they're designed to fit both 200/232 bar valves and 300 bar valves, but the fill whips are designed to make it physically impossible to screw a 300 bar fill whip into a 200/232 bar tank valve.