2airishuman
Contributor
The whole situation with burst discs is a mess.
Yes, they can fail, completely and without warning, well below their design pressure. As pointed out upthread I had this happen on a cylinder with an older valve and burst disc assembly with a single gas outlet drilling, unlike the current production ones that have two or three opposed drillings. The cylinder, an LP72 rated 3AA2250, with a + at hydro, had been properly filled to 2475 PSI, and was in the bed of a pickup truck in the sun on a warm day. The cylinder could not have been at more than about 120° F because I measured the surface temperature of a similar cylinder under similar conditions and it was below that. The increase in pressure due to heat would have been marginal, a few hundred PSI, well below the 3750 burst disc rating, well below any reasonable tolerance band. There was over $2000 of damage to the truck which was mostly covered by insurance. The damage was caused by the cylinder being propelled vertically and smashing the rear window in the cab and denting the rim of the bed.
The problem as I see it is that there are many compressor setups that are capable of delivering 4500 PSI to a yoke or DIN connector and where the only reason they do not is operator attention and vigilance. Some air plants are set up to allow 4500 PSI fills for SCBAs or the odd paintball customer. In theory the safe way to set up such a plant is to have it deliver 4500 PSI air to a CGA 347 whip and then be regulated down to lower pressures for DIN and yoke whips. Nobody, but nobody, does that.
So there is the risk, at least with LP steels (including modern ones, not just my old LP72s), that they will be pressurized sufficiently far beyond their test pressure to explode, if the compressor operator gets a phone call from his divorce lawyer at just the wrong time or has had too many martinis or is otherwise incapacitated. Boom, death, destruction, lifetime of guilt, liability, court cases, involuntary manslaughter, etc. for whoever put in doubled burst discs or a UNF threaded bolt or something rated far beyond what the cylinder walls could handle. Rare, yes. Preventable, yes.
Responsible diving is about minimizing risk and IMO that risk includes all aspects of the diving activity including preparation. It is also, to a degree, about following established safety practices.
I am not a technical diver and will leave it to those who are hold forth on the balance of risks for that specific activity.
For myself and the solo rec dives I go on a burst disc failure poses risks but not grave ones. While burst disc failures at depth can lead to an OOA emergency they nearly always occur near the beginning of a dive when nitrogen loading is not a major factor. From what I have been able to determine, they are a less common cause of OOA emergencies than either hose failures, plugged valves, or sudden catastrophic first stage failures (HP seat separation and diaphragm rupture both being reported from time to time).
In that light, I put properly rated burst discs in my valves.
I do make every effort to change them at hydro. The recent lack of availability of 3750 PSI burst discs complicates this, leading to a choice to use 4000 PSI burst discs or leave the old ones in place, neither of which is ideal.
Someone will point out that burst discs are not used in Europe. While air plants vary and Europe is a big place with widely varying regulatory realities, it is much more the case in Europe that 300 bar whips have 300 bar connectors and are regulated to 300 bar (or slightly over to allow for hot fills), and 200 bar whips have 200 bar connectors and are regulated to 200 bar. The safety problem of overfilled cylinders is solved a different way, in theory at least.
Yes, they can fail, completely and without warning, well below their design pressure. As pointed out upthread I had this happen on a cylinder with an older valve and burst disc assembly with a single gas outlet drilling, unlike the current production ones that have two or three opposed drillings. The cylinder, an LP72 rated 3AA2250, with a + at hydro, had been properly filled to 2475 PSI, and was in the bed of a pickup truck in the sun on a warm day. The cylinder could not have been at more than about 120° F because I measured the surface temperature of a similar cylinder under similar conditions and it was below that. The increase in pressure due to heat would have been marginal, a few hundred PSI, well below the 3750 burst disc rating, well below any reasonable tolerance band. There was over $2000 of damage to the truck which was mostly covered by insurance. The damage was caused by the cylinder being propelled vertically and smashing the rear window in the cab and denting the rim of the bed.
The problem as I see it is that there are many compressor setups that are capable of delivering 4500 PSI to a yoke or DIN connector and where the only reason they do not is operator attention and vigilance. Some air plants are set up to allow 4500 PSI fills for SCBAs or the odd paintball customer. In theory the safe way to set up such a plant is to have it deliver 4500 PSI air to a CGA 347 whip and then be regulated down to lower pressures for DIN and yoke whips. Nobody, but nobody, does that.
So there is the risk, at least with LP steels (including modern ones, not just my old LP72s), that they will be pressurized sufficiently far beyond their test pressure to explode, if the compressor operator gets a phone call from his divorce lawyer at just the wrong time or has had too many martinis or is otherwise incapacitated. Boom, death, destruction, lifetime of guilt, liability, court cases, involuntary manslaughter, etc. for whoever put in doubled burst discs or a UNF threaded bolt or something rated far beyond what the cylinder walls could handle. Rare, yes. Preventable, yes.
Responsible diving is about minimizing risk and IMO that risk includes all aspects of the diving activity including preparation. It is also, to a degree, about following established safety practices.
I am not a technical diver and will leave it to those who are hold forth on the balance of risks for that specific activity.
For myself and the solo rec dives I go on a burst disc failure poses risks but not grave ones. While burst disc failures at depth can lead to an OOA emergency they nearly always occur near the beginning of a dive when nitrogen loading is not a major factor. From what I have been able to determine, they are a less common cause of OOA emergencies than either hose failures, plugged valves, or sudden catastrophic first stage failures (HP seat separation and diaphragm rupture both being reported from time to time).
In that light, I put properly rated burst discs in my valves.
I do make every effort to change them at hydro. The recent lack of availability of 3750 PSI burst discs complicates this, leading to a choice to use 4000 PSI burst discs or leave the old ones in place, neither of which is ideal.
Someone will point out that burst discs are not used in Europe. While air plants vary and Europe is a big place with widely varying regulatory realities, it is much more the case in Europe that 300 bar whips have 300 bar connectors and are regulated to 300 bar (or slightly over to allow for hot fills), and 200 bar whips have 200 bar connectors and are regulated to 200 bar. The safety problem of overfilled cylinders is solved a different way, in theory at least.
