I don't have much to add, aside from a few observations and clarifications.
From the video, it appears to be a mix of steel cylinders. Although the footage is fairly grainy, it seems to me that all the remaining cylinders are steel. If I had to hazard a guess, the shape of those cylinders would make me point to ECS or Vítkovice. Both are highly reputable manufacturers that produce excellent cylinders. Given the Eastern European location, Vítkovice would be the more likely source. But again, this is only speculation.
It was stated somewhere that SCUBA cylinders have a wall thickness of around 1/8" (3.2 mm), which is not quite correct. Wall thickness depends on the alloy used, the test pressure, and the outer diameter. Historically, three different formulas have been applied:
- Bach-Clavarino formula (US): Often predicts failures at higher pressures than seen in practice.
- Mean Diameter Formula (EU): Can be unsafe for thin walls and overly conservative for thick walls.
- Lamé von Mises formula: Currently regarded as the most accurate.
The EU historically applied the Mean Diameter Formula, while the US used Bach–Clavarino. In 1981, an ISO workgroup compared all three and concluded that Lamé–von Mises was the most accurate. This was subsequently adopted in ISO 9809. The US is still stuck on the Bach-Clavarino formula.
Steel SCUBA cylinders generally have a wall thickness between 4.0 mm and 5.0 mm, while very small steel oxygen bottles might be as thin as 2.5 mm. Modern chrome-molybdenum steels have a yield strength of around 755–930 MPa and a tensile strength of 1030–1100 MPa. I would argue that the 1/8" is a tad too low for what is seen in most steel cylinders.
Aluminium fares worse. The modern 6061-T6 alloy has a yield strength of 280–295 MPa and a tensile strength of 330–345 MPa. This lower strength requires a much thicker wall: a typical S80 cylinder will have walls of 12 mm or more.
In general, the larger the diameter and the higher the test pressure, the thicker the wall must be.
Both steel and aluminium cylinders today are designed to burst without fragmentation (clause 10.4.3.3 of ISO 9809 and ISO 7866). They are intended to split along the side wall, with the tear not normally propagating into the base or neck. Mishandling or neglect can, however, still lead to fragmentation, though this is rare with modern alloys.
Burst discs are designed for one specific scenario: over-pressurisation. European countries do not require them on SCUBA cylinders, although some gases are supplied in cylinders with pressure relief devices (PRDs). It is highly unlikely that over-pressurisation was a factor here, as compressors are fitted with reliable safety valves. While these can be tampered with, I doubt that is what happened in this case.
I was also surprised to see some comments suggesting that rapid corrosion can occur within 100 days, even with distilled water. A so-called
"100-day rule" was mentioned, which I had never heard of. There is no question that water ingress is harmful to any cylinder, steel or aluminium, but the idea that failure could occur in just 100 days is, in my experience, greatly exaggerated.
Most cylinders that suffer water ingress do so from the compressor itself, which by design, will have water with almost no minerals or salts. While not ideal, I have seen enough wet cylinders from compressors to know that it usually takes far longer than 100 days for corrosion to reach a critical point. In fact, even a couple of years of such "
compressor-wetness" may not necessarily cause failure, though obviously this is not something anyone should test deliberately.
The 100-day reference originates from Francis C. Cichy’s report, which specifically examined saltwater ingress. That is a completely different scenario, and extremely rare. I have only personally observed it twice. Saltwater does indeed cause extraordinarily rapid deterioration, much faster than I ever expected. In just a few weeks, it can fuse brass valves to aluminium cylinders, for example. The attached photographs are from one such case.
View attachment 915909
This is why I find annual visual inspection so valuable in the SCUBA industry. Almost all cylinders will survive a full year of
"compressor-wetness”. If inspections are carried out annually, the chance of a dangerous cylinder slipping through is extremely low (excluding gross negligence such as deliberate overfilling). Annual inspection weeds out problem cylinders before they become hazardous.
Without more details, speculation is all we have. Often in such a case, the cause of events is fairly straightforward:
- The compressor was poorly maintained and delivered partially wet air (a common issue). Cost-cutting may have been at play.
- Visual inspections were skipped for an extended period. Again, cost-cutting could be a likely issue.
- A cylinder deteriorated to the point of bursting during filling. No over-pressurisation necessary.
As for how a bystander so far away was badly injured, either the blast produced shrapnel (which modern cylinders are not designed to do), or more likely, it propelled a nearby object with enough velocity to cause serious harm.
For perspective, an S80 charged to 207 bar will explode with about 0.45 MJ if charged with a diatomic gas such as air. That is roughly equivalent to 100 grams of TNT. A US-made Mk II hand grenade in World War II carried 52 grams of TNT, which means the energy released by a bursting S80 is comparable to two of those grenades detonating. I'm not an expert by any means on explosions, but the math does not leave me surprised of such an outcome. Quite the contrary, I would have expected even more bodily harm to more bystanders.
Either way, it serves as another reminder of the importance of adhering to annual inspection schedules.
