Burst disks????
Being fairly new to cave diving and fill station practices, but after a few fills at Amigo's I realized that I need to do something about my burst disks (like plugging them). What do most people do with thieir burst disks? Do you typically put in larger rated disks in or double them, or just plug em. I cringe when I put 3800 in my HP tanks, what do most people fill their tanks too?
I have 5000psi burst disks in my doubles
All of my doubles (2400 steels) have 3500 burst disks in them. I don't play with doubling, plugging, etc.
Whoever said money can't buy love never bought a puppy.
Stl 72's get 3500'sOriginally Posted by Slüdge
LP 95's, 85's, 125's get 5k's
HP 100's get 5k's
Bwahatake the tanks off your back and I can show you more clear stuff.
Safety Assemblies (burst disc) are designed to fail when they reach a certain rated pressure with a 5% tolerance on either side. The specified burst disc for a cylinder is based on what it's hydrostatic test pressure is. Most cylinders test pressure is 5/3rds working pressure some cylinders are 3 halfs working pressure.
Some standards:
3000 psi aluminum cylinder will have a 5000 psi safety
2400 psi steel cylinder will have a 4000 psi safety
3180 psi steel will have a 5300 psi safety
3442 psi steel will have a 5160 (5250) psi safety
3500 psi steel will have a 5250 psi safety
4350 hoop wrap aluminum will have a 7250 psi safety
It's strongly recommended that the safety be changed at least at every hydro test date. Cylinders that get used a LOT should have the safety changed at least annually.
Remember that the cylinder rated pressure is at 70f. It's not uncommon for a cylinder pressure to be higher or lower depending on the final temperature. Professional cylinder filling technicians know how to take these temperature changes into consideration so that your cylinder is properly filled. I would not cringe at all when your 3500 psi cylinder shows on a non-precision pressure gauge 3800 psi. The standard safety assemblies for those cylinders are just fine.
Cheers
JDS
Legally a new burst disc has to fail withina range of 90% to 100% of the test preswsure - not within 5% plus or minus of a given pressure.
3AA steel and 3AL aluminum tnaks have test pressure of 5/3rd the service pressure. In the case of 3AA tanks that can have a plus rating for a 10% overfill, the service pressure not the plus rated overfill pressure is still used to detemrine the test pressure.
So for a 2250 steel tank the burst disc pressure should be between 3375 and 3750 psi, for a 2400 psi steel tank the burst disc burst disc should rupture at between 3600 and 4000 psi and for a 3000 psi tank the burst disc should rupture between 4500 and 5000 psi.
Special permit tanks such as the PST, Worthington and Faber 3442 psi tanks, the test pressure is 3/2 the test pressure, or in this case 5163 psi. The burst disc in turn should reputure between 4647 psi and 5163 psi.
Faber also made a 3AA steel tank with a 3180 psi service pressure tank that when overfilled by 10% had a pressure of 3498 psi. As a 3AA tank, the test pressure was 5300 psi and the burst disc pressure would be 4770 to 5300psi.
Obviously, there is some overlap in the 10% range for 3000 psi, 3442 psi and 3180 psi tanks and a burst disc designed to fail at 5000 psi would keep them all happy.
Or said differently, there is very little difference between a burst disc intended for 3000 psi service pressures and one intended for use with 3442 psi tanks due to the differences in how the test pressures are calculated. The difference in maximum burst disc pressure is only 163 to 300 psi for 3000 psi, 3442 psi and 3180 psi tanks.
Practically speaking, any of the above burst discs will work for a 2400 psi tank overfilled to 3600 psi, while the discs intended for 2250 or 2400 psi service won't cut as there is little or no margin between fill pressure and the minimum pressure where the burst disc could fail.
Another option are burst discs intended for 4350 psi service tanks where the minimum burst disc pressure will be 5872 psi for tanks tested at 3/2s or 6525 psi for tanks tested at 5/3 rds the service pressure.
A final option is to just use a stainless steel disc that is not designed to fail like a copper disc.
What ever the approach, it is a good idea to replace the burst disc asembly when ever the tank is hydro tested or every 2-3 years if the tank either sees a lot of cycles or is a 2400 psi tank cave filled to 3600 psi.
Doubling discs is done frequently, but it is not a great idea. The burst disc assembly is not intended to have 2 discs in it and in some valves it can actually lead to a burst disc failure.
Doubling is in my opinion just misguided paranoia as if the burst disc is replaced on a suitable schedule it will be reliable. I have had burst discs fail shortly after installation, but if the burst disc itself is faulty it will fail soon after the initial fill - within a few hours.
Over the long term, corrosion and numbers of cycles will cause the disc to corrode or fatigue and they will eventually fail at the service pressure, but not in the time frames mentioned above.
That's very technical and informative but is it really necessary to plug or double discs?
Has there been many burst disc failures? Or is this unfounded paranoia?
If you have a properly serviced safety assembly in your cylinder and it releases then there is probably a good reason for it to release.
In the USA it's required to have a properly rated safety assembly in the cylinder valve. Rendering it inoperable is foolish. We can get into the whole legal and moral discussion about it but that would go on for 30 posts or more. We can address why in the USA it is required but Europe does not require it, and that will run for 30 posts or more as well.
I just serviced some personal valves the other day. I serviced them a year ago as well. Upon inspection of the safety assembly it was indented far enough, and there was some minor corrosion on it that I opted to change it. I use the Thermo type assembly that is the one piece assembly with the pressure rating stamped and the disc held in place with a copper ring. I tossed out the old one and installed the new one to 120 inch pounds torque. I have zero fear that the assembly will blow out under normal use. Next year If it needs replacement I will replace again. It's the cost of diving.
As a DOT facility (we do hydro tests ) it's hard for me to suggest that you overfill cylinders or put improper safety assemblies in cylinders. It's best that you just not do it.
As to the "cave fill" you probably never heard of a Wahoo fill. It may have been 1991 and Dick Long was up on Long Island diving with us. He set up his tanks (twin 80 alum splits) , turned on the gauge and it read zero. He said to me .. Joel this tank is empty. I said not it's not. He said yes it is, I looked at his gauge. It was an older Poseidon SPG that only had readings to 3000 psi and the needle was at Zero .... We put another gauge on it and it read 4,000. He said "you cant fill tanks to that pressure" I asked him if he wanted me to drain it down to 3000... he said no i will keep that extra gas!
FYI when those cylinders got hydro tested a few years later they all failed.
In more than 20 years handling hundreds of thousands of cylinders (yes it adds up) I've had 4 safety release. 2 were on a set of 3442 tanks we were transporting cross country - the guy was using a 300 bar dive rite manifold that had safeties in them for LP tanks (4000 psi release) the cylinders were in a hot trailer box and let go (as they should have) the other was on some old 72s that a customer brought in for filling and it was their time. The valves had not been serviced in 20 years.
..... gotta go fill some tanks
Cheers
JDS
Phillip,
You don't have to worry about it in the DR since we never get anything over rated tank pressure for fills. Don't want to over fill the alum tanks anyway.
I only had one burst, but that was enough for me!
I see a lot of answers here, and many are correct, but the main thing to consider is what you actually fill to, not the "rated" pressure.
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