Blew a 4000psi burst disk at only 3100psi-WTF?

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AbyssalPlains

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Tucson, AZ
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Blew a brandnew burst disk while filling my LP95 to 3100. Got dive plans on the weekend and might not get a replacement in time. I have a 5000psi disk from another valve. Can I use that or is it dangerous to reuse a disk in a different valve? The valves are different brands. Does that matter?
 
Conventional wisdom is that once a burst disk comes out it goes into the metal recycling. So reusing the other would not be considered the best practice. But in a pinch I would consider doing it. Just make sure it is properly torqued.
 
The test pressure for an LP 95 is 4000 psi o the burst disc should fail at no less than 90% of that figure - or in other words 3600 psi.

A unitized burst disc assembly is easier to install. If you have a separate plug, disc and plastic washer, you need to stack the in the right order (plug -> disc -> plastic washer) and apply the appropriate torque firmly and quickly to prevent the plastic washer from deforming and extruding. if you stack them with the disc on the bottom, they will leak or fail at well below the stated pressure.

That said, burst discs can also fail if improperly installed or improperly torqued, and I have on rare occasions just had one that was bad and failed within an hour or two of the initial fill.

If you are overfilling a tank (not a recommendation, just an observation that it happens a lot some places) then, from a safety during the dive perspective (not safety in storage or transit), you probably want to use a burst disc that is matched to the actual fill pressure, not the stamped service pressure.

So a 3000-3100 psi fill presure would suggest a 3000 psi service pressure burst disc - one that will burst between 4500 and 5000 psi.

For a 3600 psi fill, you'd again probably want to use burst discs that are intended for 3442-3500 psi service pressures - 5163 to 5250 psi test pressure and burst pressures no less than 4650 to 4725 psi.

Just be aware that neither overfilling or using a burst disc for a higher than rated service pressure is neccesarily legal from a DOT perspective if the tanks are used in interstate commerce.
 
Thanks all for the info provided so far. However, at this point, I am totally confused.

I thought the fact that my burst plugs are stamped with "4000" means they burst at 4000 psi. Not at 3500, not at 4500, not at any other pressure. But from what I'm reading here it seems that a burst disk that is stamped with a certain number will (or should) burst at a number that has nothing to do with the stamped number? I just don't get it.

The way I looked at it was, My burst disks are stamped 4000, therefore I should be able to fill my tanks (in theory only) to 4000 psi without the disk blowing. Apparently it's more complicated in reality, since the stamped number and the actual bursting pressure of the disk are different.

TDL is shipping me a pair of replacement disks, most likely they will be stamped 4000. To what pressure can I safely fill before those disks blow?

Also, since I don't have a torque wrench, can someone give me a pointer as to how much I should tighten the safety plugs?
My valves are Thermo, so there is only one piece safety assembly, no washers or disks (at least from what I can tell).
 
Considering all the factors involved in manufacturing, materials and installation it is virtually impossible to have a burst disc burst at an exact pressure.
 
Considering all the factors involved in manufacturing, materials and installation it is virtually impossible to have a burst disc burst at an exact pressure.
Right, the most the DOT expects is for the burst disc to fail within a 10% range with an upper limit of not more than the test pressure so the bias will tend to be toward the lower end of that 10% range to reduce the number of burst discs that get rejected in any QA sample.

That is also under very carefully controlled test conditions with perfect installation, perfect valves, calibrated gauges, etc. In the real world the variance will be a bit larger and having a failure a 300-500 psi below spec would not be unheard of.

Thanks all for the info provided so far. However, at this point, I am totally confused.

I thought the fact that my burst plugs are stamped with "4000" means they burst at 4000 psi. Not at 3500, not at 4500, not at any other pressure. But from what I'm reading here it seems that a burst disk that is stamped with a certain number will (or should) burst at a number that has nothing to do with the stamped number? I just don't get it.

The way I looked at it was, My burst disks are stamped 4000, therefore I should be able to fill my tanks (in theory only) to 4000 psi without the disk blowing. Apparently it's more complicated in reality, since the stamped number and the actual bursting pressure of the disk are different.
As noted, 4000 psi is the correct test pressure for a 2400 psi tank (5/3rds the non plus rated service pressure stamped on the tank.

The DOT is mostly concerned that the disc fail at or below 4000 psi, so again it's in the burst disc manufacturer's interests to bias it toward the low side of the 10% range as having most of a sample in a test lot fail a few hundred psi low is fine, while having even one fail a 100 psi too high would be a "fail" for the lot. Under those ideal test conditions the burst disc could fail at only 3600 psi and still be within the required spec and everyone goes home happy.

The reality also is that the copper disc will flex with each cycle, fatigue, and corrode over time and all three will eventually lower the burst pressure to the point where most burst discs will eventually fail at or even below the rated fill pressure - usually during the fill cycle.

If you plan on doing 3000 psi fills on a regular basis, installing a 3000 psi service burst disc makes the most sense (stamped "5000 psi" with a theoretical burst pressure range of 4500-5000). That 4500 psi lower limit puts you a comfortable 1500 psi over the "normal" fill pressure.
 
Also, since I don't have a torque wrench, can someone give me a pointer as to how much I should tighten the safety plugs?
My valves are Thermo, so there is only one piece safety assembly, no washers or disks (at least from what I can tell).

You need a torque wrench. Harbor Freight has them for <$25. There's no way to communicate "how tight" without the proper wrench. And even if someone tried it would depend alot on the length of your lever (wrench)

If you merely got the same 4000 disks (really one peice plugs) keep your fills <3000psi. If you want >3000psi fill step up to a 5000 or 5250 disk.
 
I love discarded 70's art

Copper_Art_Poodle_web.jpg


and milk bottle screw caps?

With pre stressed, and pre compressed
the only torque wrench is in your hand
 
https://www.shearwater.com/products/teric/

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