That is so out there hypothetical that it doesn't border on fantasy vs reality it is miles into the fantasy realm.
While
this incident isn't exactly what the person described, what he refers to may not be as far-fetched as it seems. I do agree that this is an edge case and an unfortunate situation.
The usefulness of pressure relief devices (PRDs) such as burst discs is debatable. Their main advantage lies in extreme heat scenarios, like during a fire. In those situations, a fusible plug is far superior to a burst disc, as it vents the gas contents much earlier and can usually be inspected from the outside. Both PRDs have the drawback of expelling gas in typically no more than three directions. Escaping flammable gases from a PRD can easily create flames exceeding 10 meters - a violent 10-meter torch, if you will.
In SCUBA, most test pressures for cylinders, which a sound cylinder will withstand without issue, fall into the working pressures of higher-rated cylinders. For example, a 166bar (LP?) working pressure cylinder has a test pressure of 277bar, which is quite close to a 300bar cylinder. It would have to be an extraordinarily bad day for a cylinder to be filled to that pressure. The valve would have to be mismatched, and the compressor operator would need to have shown gross negligence. As a result, incidents from overfilling are extremely rare and are almost always due to other factors compromising cylinder integrity.
Fire and rescue services have specific protocols for dealing with cylinders in a fire, which usually boil down to:
"Stand as far back as you can and cool the cylinders. If you can’t safely cool them, just stay back." This is especially true for acetylene cylinders.
One major issue I have with burst discs, and a reason I’m glad they can’t be found in SCUBA gear in Europe, is that while their main advantage is in a fire (hopefully preventing the cylinder from exploding), there's no guaranteed way to know if they actually triggered. It is not guaranteed that the disc will trigger, it can fail to do its job, as as any other mechanical piece can fail. Maybe the pressure didn't reach critical levels as the cylinder wasn't fully charged, maybe the disc was faulty, maybe ...
Unless someone witnessed the burst disc rupture and the accompanying gas escape, it must be assumed that the cylinder is still fully charged. Handling a fully charged, fire-exposed cylinder is incredibly dangerous. For example, 6061 aluminum loses 50% of its tensile strength at just 200°C within half an hour. At 300°C, it loses 80% of its strength. Steel performs a bit better, but not by much. These temperatures are quickly reached in a fire.
Firefighters or anyone cleaning up after a fire must assume the cylinder is still fully charged, with the valve likely inoperable. Handling fire-exposed cylinders is often ill-advised, and transporting them for further examination or gas release could be disastrous. In some cases, the only solution is a controlled detonation, often achieved by shooting the cylinder from a distance.
A burst disc has a clear purpose: preventing explosions, particularly in a fire. However, I believe the downsides, especially for SCUBA, outweigh the benefits, particularly when its function is as concealed as it is in the event it actually triggers.
A pressurized cylinder in a fire sucks. In some cases, a burst disc might be advantageous, in others, it may make no difference. Worst of all, it can lead people to believe a cylinder is empty when, in reality, the burst disc failed to trigger.
