For the longest time people diving in the overhead considered burst disks a failure point,no matter what service pressure they are. Think...something that's function by design is to fail,is something that is oxymoronic to over head diving, especially when manufacturing and thermal stressors can cause premature failure. Not saying I agree or disagree since this is a touchy area,but not existing has been a preferable choice.
Cave Fills and Burst Disc Rating?
- Thread starter CuzzA
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For the longest time people diving in the overhead considered burst disks a failure point,no matter what service pressure they are. Think...something that's function by design is to fail,is something that is oxymoronic to over head diving, especially when manufacturing and thermal stressors can cause premature failure. Not saying I agree or disagree since this is a touchy area,but not existing has been a preferable choice.
Burst disks as I remember have to hold 90% of test pressure. A lp tank (2400 psi) has a test pressure of 9/5 of 4000 ppsi so the disk has to hole to 3600 psi. A al80 has to hold 4500 psi before rupturing. You will see different markings related to the disks. IE a disk for a 2400 psi tank or the disk is stamped with the burst pressure on it of say 3650 and yo have to match it to the tank you are using.. If you look at a hp tank of 3500 you can see that the pressure of the disk can be quite high. whether the tesp pressure is 9/5 working pressure or it is stamped on the tank its 90%+ on the burst disk.
the rupture pressure of the tank is 4 times the working pressure. In the us it is called a safety factor, which is 4. In other countries it is 2 or 3. so with the same tank elsewhere you can fill it to higher pressures. an 1l80 ruptures at 12000 psi so with the safety factor of 4 the working pressure is 12000 devided by 4 or 3000psi.
I had a burst disk blow on a lp tank. I filled it to 3300 and then allowed tit to cool. Problem was the gage was 300 psi off and it got a fill to3600. it cooled to 3400. then it sat in the sun and it drove the psi to I guess around 3700 and the plug blew.
During the VIP the tank and valve is supposed to be inspected. that tank for the obvious but the valve is supposed to be verified to have the correct disk in it. when I bought my valves for my lp tanks I bought valves made for an al80. they are the same at 2400 psi valves except they have a higher burst disk in them. The valves themselves have 3000 stamped on them. That 3000 marking tells the vip guy it has a 4500 disk in it. I never allow my valves to be on the tank when it gets hydro'd. as they are required to do a vip before hydro and that means disk check and replace. If you look at a vip sticker, many say this tank and valve has been inspected...............................
During the VIP the tank and valve is supposed to be inspected. that tank for the obvious but the valve is supposed to be verified to have the correct disk in it. when I bought my valves for my lp tanks I bought valves made for an al80. they are the same at 2400 psi valves except they have a higher burst disk in them. The valves themselves have 3000 stamped on them. That 3000 marking tells the vip guy it has a 4500 disk in it. I never allow my valves to be on the tank when it gets hydro'd. as they are required to do a vip before hydro and that means disk check and replace. If you look at a vip sticker, many say this tank and valve has been inspected...............................
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KWS, I think you have some of your numbers wrong. You mention 9/5's for test when the hydro pressures are 5/3's, and then an AL80 rupture pressure at 4500psi, when it is 2.5x service pressure or 7500psi.
The burst discs due to have to fail at 90% of test pressure, but they are still rated at their nominal values, i.e. the hydro pressures of the tanks they are installed on.
VIP does not mean burst disc check and replace, especially not at hydro, but not in any VIP that I have ever had done. Burst discs are supposed to be replaced at hydro time as well as the valve being fully rebuilt, but I am unaware of any dive shops that do this is part of their basic vip service, you have to pay for the valve service, and during the valve service the disc is replaced, or at least supposed to be. It doesn't come in the valve service kits, so you have to make sure they do it
The burst discs due to have to fail at 90% of test pressure, but they are still rated at their nominal values, i.e. the hydro pressures of the tanks they are installed on.
VIP does not mean burst disc check and replace, especially not at hydro, but not in any VIP that I have ever had done. Burst discs are supposed to be replaced at hydro time as well as the valve being fully rebuilt, but I am unaware of any dive shops that do this is part of their basic vip service, you have to pay for the valve service, and during the valve service the disc is replaced, or at least supposed to be. It doesn't come in the valve service kits, so you have to make sure they do it
Kharon
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Sorry - didn't get to see the other posts before mine. As Emily Latella would say "Never mind".
Yes you are right it is 5/3. I brain farted 9/5 is temp conversion. thanks for the catch. The 4500 is the burst disk rupture pressure on a al80. 90% of test pressure. The burst disk check is seldom done but is is part of the vip I have never heard of 2.5 x working pressure in the US. The US is a supposed to be safety factor of 4. Perhaps that is 4 for steel tanks. This is one reason you cant fill foreign tanks in the US. They have lower safety factors and do not comply with DOT. You could probably check that theory with your lp steel and see if the rupture psi is 10-11k The plus rating is not part of the factor of 4.
My LP 108 doubles had 5250 burst disks in them. While having them filled for an open water dive at a local dive shop, the attendant noticed that one of the burst disks was hissing slightly. He told me he fixed it, and I didn't give it a thought. I assumed he had put a wrench on the nut and tightened it. No, he had replaced it with a standard disk for an LP tank. I didn't realize that until I had the tanks filled at Cave Adventurers. It did not go well. They probably remember it if you are in the area and want to ask.
The guidance I got was if they are nothing more than just a disk then yes if they are part of a plug then no. the inspection point is the pressure of the disk and being able to remove read the psi and reinstall them with out damaging them. so many times when you tighten them in place you can no longer read the disk pressure when removed. The off the record advice was if you don't over fill then there is little chance of the disks being weakened to the point of near blowing as you have in cave fills. many put in double disks to accommodate cave fills. The normal guy will never have a problem with the disc's. Most shops don't fool with the valves cause they cant compete with other shops charging 10-15 for vips by not doing the valve.
The thoroughness of eth vip varies with every shop doing one. many do not have "qualified" vip personell Ive seen shops with 14yo's doing vips. It takes about 5 min to do one is some shops. Those who are familiar with PSI knows it takes a lot longer than 5 min to do a roper vip.
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