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.
