Equipment Exploding tank shrapnel injures boy - Ploče beach, Montenegro

[ZephyrNYC]

Corrected, thanks. I am not familiar with the region.

So, were there not other injuries?

" '... two other people sought medical attention due to minor injuries caused by the detonation,' the Police Department announced."

 

[iain/hsm]

Was it a neglected tank? No hydrostatic pressure tests done? No visual inspections?
I thought cylinders would fail in a controlled manner if they were only overfilled.
Was there an oxygen fire?

Not entirely cylinders can and do fail within the 5 year hydro test period especially the self filled scuba type cylinders. High tensile metal with a thin wall thickness for as light as possible weight for a self carry product not too heavy to avoid negative buoyancy hence narrow and long while shortening the cylinder height requires a larger outside diameter for the same water capacity and a greater positive buoyancy when approaching empty.

Similar within visual inspections 18 months / 2 year intervals again many cylinders have failed catastrophically during this period especially with the self fill brigade and the ignorant.

Cylinders in general won't fail or burst if over filled as the average scuba compressor both Bauer and Coltri cannot develop sufficient pressure due to the con rod upper needle bearing failing. or the motor/engine has insufficient power to compress

I cannot comment on the manufacturer of these bearings for reasons that previous comments about a certain words were deleted by our moderators. But its an interesting history.

Further most scuba compressors have multiple pressure relief valve interstage and final stage. While I have known of folk blanking off a leaking relief valve and you have those "Cave Fill" folk in your country
creating 620 bar (9000 psi) pressure to burst in a 207 bar (3000 psi) working pressure cylinder to achieve a pneumatic burst is not as easy as leaving the compressor running without attention.

Now the fact that no compressor operator or scuba diver was injured in the cylinder failure save this poor 14 year old boy some 50 meters (165 feet) away does indicate a possibility that this compressor if indeed it was running at the time was unattended.

Oxygen could have been a contributing factor granted and although I can show evidence of an industrial size oxygen cylinder on site. This would have more likely been a partial pressure method of filling and in the event of a ignition one would expect more damage to persons nearer the filling position again not 50 meters away.

 

[iain/hsm]

i thought that European tanks didn't have burst disks?

In general they don't fit burst discs to scuba cylinders in Europe for a number of reasons.
Our fire departments don't appear to have concerns regarding high pressure cylinders and fires by contrast with America the burst disc being a mandatory requirement.

We have also a climatic consideration and use a developed pressure calculation that the cylinder can withstand a ( heat ) developed pressure at 60 degrees C ( 140 F )
So a typical scuba cylinder in UK with a working presser of 232 BarG (3364 psiG) is capable of happily working up to 60 C and developing a pressure of 287 BarG (4161 psi)

This IMHO is where the ignorant amount us gets the feeling that cave fills are OK I guess

One other consideration in Europe is the cylinder pillar valve ejecting under load of which I think I emailed a post showing the spectacular results offshore off the platform stage. Again folk have died one woman in particular when the pillar valve ejected into her skull when lowering it into the boat.

Problem we have is in UK we used to use 3/4 BSPP threads while you lot use 3/4 NPSM and the Europeans in their wisdom decided to split the difference and create a European law requiring M25x 2 ISO metric thread be made mandatory. Hence these problems.

Now while I don't think this is the cause of this particular accident as the weight and solid mass of the valve would not have sufficient energy to cause severing someones hand a 50 meters and the additional multiple injuries to others in the vicinity suggests a metal cylinder shell fragmation where small sharp metal fragments would easily sever a hand at 50 meters and additional multiple fragments could easily find multiple targets.

In addition with the cylinder being unattended would also suggest a failure due to metal fatigue than while being filled and also explain why no divers were injured.

 

[rjack321]

Was it a neglected tank? No hydrostatic pressure tests done? No visual inspections?
I thought cylinders would fail in a controlled manner if they were only overfilled.
Was there an oxygen fire?

They are, in theory, designed and expected to fail without producing shrapnel. Staying one piece is the design goal but it isn't always attained

 

[iain/hsm]

This remains us with the logical conclusion of a cylinder explosion and subsequent fragmation of the cylinder wall into multiple small sharp edged projectiles eminently capable of severing flesh and body parts and with a light enough weight to project over a 50 meter 165 foot distance. Further the blast would be in effect "shaped" and directed over a narrow field especially if the cylinder was laid down on its side.
The detonation phenomenon would reflect back off the solid ground and in effect created a greater velocity of shrapnel in a smaller more confined blast area rather than a general 360 degree radial radius.
That poor kid 14 years of age on holiday with his parents.

 

[iain/hsm]

They are, in theory, designed and expected to fail without producing shrapnel. Staying one piece is the design goal but it isn't always attained

Not quite true Im afraid. You need to consider that cylinders or pressure vessels I should say are designed by engineers new. A combination of material tensile strength over diameter with pressure and wall thickness to tensile strength dictates weight.

Now a static pressure vessel using a high tensile steel with a wall thickness of 7.5mm would be capable of a working pressure of 350 bar (5000psi) over a 250mm diameter
However under your ASME VIII div 1 appendix 22 would require a wall thickness of 19mm (3/4")
Both as safe as each other with one using a much higher tensile steel material.

But non of these calculations are effective when the cylinder is heavily corroded with internal rust

It's known as the 100 day rule where a perfectly new steel cylinder will over 100 days with a few cc's of water internally will corroded sufficient for the shell to fragment.
The no burst option in the original design has in effect been eroded away.

 

[iain/hsm]

I should add that the excellent ASME requirements allow us as pressure vessel design engineers to use a much wider range of available steel materials but in addition they require a four times the allowable working pressure to burst. Similar our old trusted British Standards were also very similar.

While the EU European committee standards of vested interested parties allow a much narrow smaller range of steel materials but incorporate a much greater tensile strength while at the same time allow only a three times working pressure to burst. Its good for sales light weight and cheap to manufacture

The kicker here is that steel doesn't give a hoot about tensile strength and loves to convert itself from a high tensile strength into a thin wall rusty thin skin at the drop of a level of water vapour over -40 dew point so rust is created so quickly even a 2 minute hydro at you local dive shop creates a corrosion event.

Add to the mix your local scuba clown a lack of investment and a level of ignorance coupled with self packing any old junk zeolite off of EBay and China you have no idea how effective these chemical are and you have the recipe for these catastrophic events.

It's the June July August event. Cylinder new in June dive off your old junk Coltri compressor self pack with chemical you buy off a risky dink dive shop or EBay from China and the 100 days later time bomb comes up and bites you in the backside. Or in the case of this child he gets to loose his hand for someone's utter stupidity carelessness and utter contempt. Just saying.

 

[rjack321]

Not quite true Im afraid. You need to consider that cylinders or pressure vessels I should say are designed by engineers new. A combination of material tensile strength over diameter with pressure and wall thickness to tensile strength dictates weight.

Now a static pressure vessel using a high tensile steel with a wall thickness of 7.5mm would be capable of a working pressure of 350 bar (5000psi) over a 250mm diameter
However under your ASME VIII div 1 appendix 22 would require a wall thickness of 19mm (3/4")
Both as safe as each other with one using a much higher tensile steel material.

But non of these calculations are effective when the cylinder is heavily corroded with internal rust

It's known as the 100 day rule where a perfectly new steel cylinder will over 100 days with a few cc's of water internally will corroded sufficient for the shell to fragment.
The no burst option in the original design has in effect been eroded away.

The US DOT specifications for 3AL cylinders require:

"Three samples must be pressurized to destruction and failure may not occur at less than 2.5 times the marked cylinder service pressure. Each cylinder must remain in one piece."

The sample size is small though and obviously some cylinders can and do fracture ala a grenade.

 

[iain/hsm]

The US DOT specifications for 3AL cylinders require:

"Three samples must be pressurized to destruction and failure may not occur at less than 2.5 times the marked cylinder service pressure. Each cylinder must remain in one piece."

The sample size is small though and obviously some cylinders can and do fracture ala a grenade.

Ah I see what your driving at and yes I fully agree.

This is to prove the ductile nature of the material when new in manufacturing.
The elongation and deflection of the metal. Other rules require a small "coupon" of the side wall to be taken and further tested to prove the materials ductile nature. Also why elongation and a minimum proof stress are also a requirement.

But what you need to appreciate is that these requirement are for new cylinders under manufacture and critically internally the cylinder wall is smooth and bright shiny metal.

The same metal under a corrosion condition after say 100 days under various internal pressures even with compressed air let alone an elevated oxygen concentration are by contrast vastly more compromised. To the point where the stress over strain is approaching the point of fatigue.

Again the 100 day clause. You buy the cylinder new in the June and give it a wet fill (distilled water)
oxidising corrosion begins and during July corrosion eats into the wall reducing any safety margins, while by August some 100 days later the wall is so corroded the pressure integrity is weakened to the point of catastrophic failure and your original ductile retained wall calculation and sample test is no longer a valid condition of failure.

Add to this the drive to make the cylinders as light as possible the wall thickness in most scuba cylinders is around (just over) 1/8" not much margin for error or rust eating away at the wall.

 

[iain/hsm]

Here we go.​

I've been spitting feathers all the while looking for this document.​

​

Its the original University of Rhode Island $2.00 dollar report on 100 day test on scuba cylinder corrosion. I will try and Pdf a better copy when I find the original document.​

Probably do something in the compressor section also as they should be aware of these considerations. Printed in 1978 some 47 years ago back in the day when men were men and women knew the difference. I guess since then things have changed but the consequence of stupidity and ignorance remains.​