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If you shoot and pierce a vessel that is pressurised, especially if it’s near its pressure limit anyway, it will explode. Even if it’s just filled with an inert gas.

Actually even just making a hole by any means will significantly increase the chances of an explosion. This because the smallest crack can start a chain reaction where the material at the leading edge of the crack is bearing too much stress and the bonds break which then passes the stress on to the material straight after that. This all happens very quickly and can even accelerate as the hole grows bigger and gas starts moving.

It’s very lucky that the crack that formed didn’t propagate like that. It could be for many reasons though, like the crack front could have run into thicker material or a weld line or something like that and if luckily stopped the crack.

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Cracks in pressure vessels can propagate faster then the speed of sound. Which means your pressure vessel goes to flinders before any pressure is relieved.
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Not a direct answer, but there is a standard code for what actions emergency services should take, and the code[1] for this stuff is 3YE, which means "Use foam or dry agent, substance reacts violently/is explosive, BA [breathing apparatus] use is essential, evacuate vicinity, contain spill." So there must be some reason not to let the stuff out.

[1] This is the hazchem code. I think the US uses a different system. A list is here: https://www.ricardo.com/media/radn55jg/dangerous-goods-emerg...

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The US uses the Emergency Response Guide or ERG, which similarly lists more or less all hazardous materials and suggests appropriate responses.

Full document is here: https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/2024-04/...

Relevant guide is 129, which is on PDF page 189.

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SPILL OR LEAK

• ELIMINATE all ignition sources (no smoking, flares, sparks or flames) from immediate area.

• All equipment used when handling the product must be grounded.

• Do not touch or walk through spilled material. • Stop leak if you can do it without risk. • Prevent entry into waterways, sewers, basements or confined areas.

• A vapor-suppressing foam may be used to reduce vapors.

• Absorb or cover with dry earth, sand or other non-combustible material and transfer to containers.

• Use clean, non-sparking tools to collect absorbed material.

Large Spill

• Dike far ahead of liquid spill for later disposal.

• Water spray may reduce vapor, but may not prevent ignition in closed spaces.

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The spark could have caused an explosion.
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I read that was the primary concern and wondered about drilling/piercing techniques that could avoid a spark. Spraying water sounds like the dumbest one but some kind of mud applied or something. CNC machines don’t seem to be light shows but i know very little.
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My "I have no clue how this works" proposal to minimize the chance of sparking would be to re-purpose a waterjet cutter...
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Okay now hear me out. What about we introduce more chemicals and eat through the tank with acid?
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Wait... how could that have helped? It's a toxic material under pressure so why would we want to vent it further?
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I guess that begs the next point though - the high pressure system inside would want to violently vent out to the low pressure system outside through the relatively tiny hole.

Delta P... when it's got you... it's got you.

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