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In a laboratory in a university building, a gas tap is accidentally opened at the end of the working day. The gas tap is connected to a bank of large acetylene cylinders. The gas tap is ejecting 0.15 cubic meters of gas per minute. The gas tap remains open for about 8 hours before the acetylene spreading through the building finds an ignition source and is ignited, meaning that 7.2 cubic meters of instrument-grade acetylene spreads through the large 1940's era naturally ventilated multi-story building before ignition.

When the acetylene ignites, there is about 250 cubic meters of acetylene/air mixture that is above the lower explosive limit of 2.5% Acetylene in a 1940's style naturally ventilated multistory concrete slab/brick veneer building with large glass windows.

My question is how large an explosion will occur? At what range is the explosion likely to break or crack windows?

Please do not dwell on the lack of safety implicit in this event being able to occur... I am well aware of it, and it is part of my story . Please focus solely on the damage that this event might inflict upon a university campus, including gardens, trees and other buildings, as well as the surrounding suburbs.

If these conditions will not cause an explosion, then please assume conditions in which an explosion will occur with this amount of acetylene, and provide a damage estimate from that starting point.

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    $\begingroup$ When you give measurements in cubic meters, you mean volume at sea level and room temperature right? Just nitpicking here... $\endgroup$ – Guran Feb 5 at 8:36
  • $\begingroup$ @Guran Standard Temperature and Pressure, yes. $\endgroup$ – Monty Wild♦ Feb 5 at 9:31
  • $\begingroup$ Many university labs are equipped with fume hoods, which vent 100 cfm or more to the outside air, and because there is a chance something unpleasant was left/spilled inside one, they don't shut them off at the end of the day. I don't know if a lab in the 1940s would turn them off (you should clarify whether you mean the building's construction or the date of the events), but they should have at least have had them. $\endgroup$ – Mike Serfas Feb 5 at 15:06
  • $\begingroup$ @MikeSerfas This isn't Earth, and this lab was constructed so that stuff wouldn't escape. No fume hoods... but also no fumes expected. $\endgroup$ – Monty Wild♦ Feb 5 at 22:22
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From a purely energetics viewpoint:

7.2 cubic meters of acetylene is 7.2 * 1.1 = 7.92kg of acetylene
Complete combustion in air, releases 50200 kJ/kg
Thus total energy release is 3.98e8 Joules
Which is the same as 95.1kg of TNT explosive

HOWEVER!!

At atmospheric pressure, Acetylene in air does not detonate, it merely deflagrates very,very quickly.
You need to increase the pressure to 1.4-1.7 bar for detonation to occur.

Your building will undergo a huge fire, but with very little concussive force and no visible shockwave.

It sounds as if you want a detonation for your story, not a fireball? Add some oxygen!
It you can increase the atmospheric oxygen inside that building, you can achieve detonation.
Alternately: if the building is well sealed. Sufficiently so that the initial deflagration increases pressure to the required 1.4bar (not easy!!), then the remainder of the Acetylene will detonate. Realistically, this sort of thing would only occur in a sealed tunnel, or more usually in a pipe or similar small confined space.
Idea: Instead of leaking acetylene, just have a premix Oxy-Acetylene torch (not ignited) do the leaking. This will provide the suitable mix to have an explosive mixture, rather than a merely flammable one.

Deflagration vs. Detonation: the very basics.

Deflagration is burning of a material. The flame-front propagates subsonically. The resultant is a flamey fire, quite possibly in a fireball form. It releases a lot of energy in the form of heat, but the release rate occurs over a number of seconds, and there is virtually no shockwave just a whoosh.
Example: a trail of gunpower burning.
ALSO example: a keg of gunpowder exploding. Yes, that is not a detonation it is a very energetic deflagration.

Detonation is burning, but with the flame-front propagating at supersonic speed. Combustion thus occurs in a tiny fraction of a second. While total energy release is similar to a deflagration of the same components, the energy is concentrated over a much smaller timespan, typically milliseconds or less. A detonation releases less of its energy as simple heat, and (much) more as a shockwave. Detonations are immensely damaging to matter around them, but do not cause much fire.
Example: a Stick of Dynamite exploding. Another example: The 2020 Beirut explosion.

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  • $\begingroup$ If the lab into which the acetylene was released was sealed (or mostly sealed), and had an internal volume of about 14 cubic meters, the acetylene would detonate then? $\endgroup$ – Monty Wild♦ Feb 5 at 9:28
  • $\begingroup$ @MontyWild as a rough guide, acetelyne and air does not detonate, but acetylene and oxygen does. In your room, it would only detonate if the overpressure fom deflagration reached 1.4 or more atmospheres of pressure, it is highly unlikely your walls will resist that. Your building will fall part in a great gout of fire, but would only break windows around it by flying debris, not shockwave. Thus very limited range. $\endgroup$ – PcMan Feb 5 at 9:36
  • $\begingroup$ It is not unreasonable for the lab in question to be able to be sealed airtight and underground, for story reasons, if that would help. The seals might not leak until a significant overpressure was achieved. $\endgroup$ – Monty Wild♦ Feb 5 at 9:41
  • $\begingroup$ @MontyWild that could work. Containing the initial explosion would also serve to ensure complete combustion. Basement lab would make a beautiful crater, but would contain the shockwave somewhat. Will make a great mess either way. If you DO achieve detonation, you can treat it as a 95kg stack of TNT (68kg C4) being exploded. Lots of explosion modelers for high explosives out there $\endgroup$ – PcMan Feb 5 at 10:02
  • $\begingroup$ Could you please suggest one or two of these explosion modellers? $\endgroup$ – Monty Wild♦ Feb 5 at 10:35
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This is what 7 cubic meters does:

enter image description here

enter image description here

It killed 33 people and wounded 69 others.

As explored by PcMan, "Explosion" is a technical term. I'm not going to get into a lexical argument here - It will "explode" as per the common vernacular and that's all I'll say on it.

Acetylene sinks in air, and will pool at ground level. It will burn when mixed with oxygen, but a multistory building the gas mix will be imperfect - The presence of a basement below the leaky doors / windows also influences what happens - there was a basement capturing the gas in this explosion. In any case you'll get a gradient of fuel / oxygen, with more fuel at the bottom and more oxygen at the top. This gradient reduces the power of the "explosion" - it doesn't all go up at once.

One difference between this explosion and your hypothetical explosion you're describing is this explosion was caused by the similar Propane (46MJ/Kg) vs Acetylene (48MJ/Kg) - yours could theoretically be slightly more energetic. Both sink in air, allowing them to pool in the same manner.

This was a 1996 explosion in Puerto Rico. A documentary explored this disaster and is on youtube.

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  • $\begingroup$ This was a PROPANE explosion. With suitable mixture with air, propane does detonate, not just deflagrate. Detonation does big damage to structures. Deflagration does not, it just sets everything in sight on fire. $\endgroup$ – PcMan Feb 5 at 15:07
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    $\begingroup$ Acetylene is H-C≡C-H, which has the same molecular weight as N≡N, so it doesn't sink in air in the long term. However, it is true that coming out of a cylinder we would expect it to be very cold starting out. $\endgroup$ – Mike Serfas Feb 5 at 15:11
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To integrate the two existing answers, which are already pretty good, don't forget that things will get worse, damage-wise, if the building is made with a wooden frame instead of a concrete frame.

Wood, as opposite to concrete, is flammable. And a university campus contains a lot of paper, very easily flammable. A post-explosion fire is almost granted, starting from paper, wooden splinters and then beams, and once it gets to the frame the damage would be even worse than what you can see in the images posted by Ash.

enter image description here (image source)

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