Yet another LP 72 question

[Eric Sedletzky]

What denominations do burst discs come it?
Are there specific ones for service pressures of 2250, 2400, 2600, 3000, and so on. Or are they more general like one for 2250 then a jump to 3000?
I'm asking because I put all 2250 discs in my 72's (like a good boy) but my dive shop fills them to 2800 to 3000 routinely and I'm not going to argue about more air, so if there is a specific 2400 or 2600 maybe that's better. One good hot day out in the sun with a 2250 disc and a 3000 fill and that sucker's goin' off.

 

[DA Aquamaster]

What denominations do burst discs come it?
Are there specific ones for service pressures of 2250, 2400, 2600, 3000, and so on. Or are they more general like one for 2250 then a jump to 3000?
I'm asking because I put all 2250 discs in my 72's (like a good boy) but my dive shop fills them to 2800 to 3000 routinely and I'm not going to argue about more air, so if there is a specific 2400 or 2600 maybe that's better. One good hot day out in the sun with a 2250 disc and a 3000 fill and that sucker's goin' off.

By law the burst disc has to rupture at between 90% and 100% of the test pressure. For a 3AA or 3AL tank the test pressure is 5/3rds the service pressure and for a special permit or exempt tank, the test pressure is 1.5x the service pressure.

For a 2250 psi 3AA steel tank the test pressure is 3,750 psi and the burst disc must rupture between 3375 and 3750 psi.

For a 2400 psi 3AA steel tank the test pressure is 4000, psi and the burst disc must rupture between 3600 and 4000 psi.

For a 3000 psi Aluminum tank, the test pressure is 5000 psi and the burst disc must rupture between 4500 and 5000 psi.

For a 3442 psi special permit tank, the test pressure is 5163 psi and the burst disc must rupture between 4647 psi and 5163 psi.

For a 3500 psi special permit tank those pressures are 5250 psi for the test pressure and the burst disc pressure must be between 4725 and 5250 psi.

Finally, for a 3AA tank with a 3498 psi service pressure the test pressure is 5830 psi and the burst disc pressure is between 5247 and 5830 psi.

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Burst discs and burst disc assemblies then are either packaged or marked with the service pressure of the tank it is intended for (2250, 2400, 3000, 3442, 3500), or are marked with the actual burst pressure (i.e. 4000, 5000, 5250), or both.

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Something to keep in mind however is that burst discs start to fatigue with the first fill and they get weaker with each successive cycle until they will eventually fail at the service pressure of the tank. They are in essence a consumable that needs to be replaced periodically. If you don't dive a lot, getting it replaced along with a valve rebuild every 5 years is adequate. If you dive a lot, that might not be soon enough to ensure you replace it before it fails, and if the tank is overfilled, it will fail much faster as it will experience much more stress with each cycle.

In that regard, I use a 3500 psi service pressure burst disc in my cave filled 2400 psi service pressure tanks. To me the risks and related consequences of having a burst disc fail during a cave dive are worse than the risks and potential consequences of a 5250 psi burst disc pressure.

Your options for a 2250 psi tank that you plane to over fill are a little scarier, as a 3000 psi burst disc won't rupture until between 4500 and 5000 psi, and the tank may fail before that pressure is reached in a fire. A 2400 psi service pressure burst disc that ruptures between 3600 and 4000 psi is a better choice by about a 1000 psi, but you'll want to replace it more often.

You can also make a 2250 psi service pressure burst disc work as well, but with a 3375 to 3750 psi burst pressure, the margin is potentially very thin if you're talking 3000 psi fills versus 2600-2650 psi fills.

 

[67macoshark]

Trident has 6 options for the standard 3/8" assemblies.
2015
2250
2400
3000
3300
3500

 

[Basking Ridge Diver]

Somewhat appropriate - on "How it is Made" TV episode tonight - they showed how they create/craft Aluminum tanks. For the final step they do a Hydro to check the tanks - they fill the tanks with water put them in a water bath and pressurize them - a 3000 psi tank looked like it went to 5200 psi to check the expansion and give the tank a pass rating. Very interesting show...
Now I could be wrong - I was expecting to see them push the psi to 4000 or so but it looked like they went well over that...

 

[BubbleGator]

Yeah.

#1) Forget the "+rating" thing.

Any 3AA2250 steel 72 in current hydro with a current VIP can be filled to 2800 psi repeatedly. Keep looking for a shop that will readily agree with this, it shouldn't take long to find a good one.

Now you have a steel 80.

After calling around again and getting nowhere with my local bigger name dive shops, I went to a more tech/industry fill place that was not your standard rec dive with lessons shop. The employee said they could not hydro the tank 'as it was older than 1990'. I left it and told the owner (who did the Nitrox fills) to call me. The owner called back and said come see him. I did, and he told me that my old steel 72 was a great tank; that he was fine filling it so long as it passed hydro and he had a chance to inspect it before the hydro. He would talk to his employee about aluminum versus steel tank characteristics.

When I asked about the plus + issue and can I get that 'great tank' bumped up a bit like they do in cave country, he just came out and asked what I wanted. I told him I was not comfortable with a 3500 fill, but wanted something that got me to around 80 cf to hang with the aluminum tank bottom times. He told me that 2800 would be 'no problem' (but wanted to keep it at or below 3K). He also said that he filled from a premixed tank of EAN32, and that him not doing partial pressure fills lessened his concerns about jamming it a bit.

Thanks to all who helped educate me on the issue. Now I need to get another one. .

 

[lowviz]

...//... He told me that 2800 would be 'no problem' (but wanted to keep it at or below 3K). ...

So glad you found what you were looking for.

Perfectly reasonable. 2800 anytime you wish, 3000 if you have a real reason. Be nice to your tanks, they aren't rented mules.

 

[DA Aquamaster]

Somewhat appropriate - on "How it is Made" TV episode tonight - they showed how they create/craft Aluminum tanks. For the final step they do a Hydro to check the tanks - they fill the tanks with water put them in a water bath and pressurize them - a 3000 psi tank looked like it went to 5200 psi to check the expansion and give the tank a pass rating. Very interesting show...
Now I could be wrong - I was expecting to see them push the psi to 4000 or so but it looked like they went well over that...

Aluminum tanks are made to the 3 AL standard. That requires hydro testing to 5/3rds the service pressure.

So for a 3000 psi service pressure aluminum tank, the test pressure is 5000 psi. Testing to 5200 psi would just be adding a bit more margin to the test to ensure a marginal tank is excluded.

The tanks are also required by law to be initially qualified (hydro test and VIP) by an independent third party tester. Consequently, the folks doing the testing in the video don't work for Luxfer, Catalina or Cyl-Tec (the companies that currently make aluminum tanks), they work for the independent tester that contracts with the tank manufacturer.

Tanks are also made in lots of 200 tanks and 1 out of every 200 tanks is randomly selected and tested to destruction to help ensure the entire lot is made to the proper standard. This includes pressure testing, a crush test, and tensile strength testing of metal cut from the tank wall in a specific shape and size "coupon".