Gas explosions. (For the brain dead)

[harry]

For a gas explosion to occur, there needs to be three things.

1. A leak. This can be very very small.

1. An accumulation of gas. This can occur in any non-ventilated space where there is a leak. Over a period of weeks or months.

2. A source of ignition. Electric spark is ideal.

Typical scenario. Someone blocks up the pantry ventilation where there is a gas meter.

They go on holiday for a few weeks There is a very tiny leak on the gas meter. Normally this would be dissipated by the pantry ventilation. But there is no ventilation and the whole house fills with gas while they are away as no-one is opening the pantry door/other doors in the house.

When they get back, it's dark, someone opens the door an operates a light switch and boom, dead people house demolished. Or the have a fag in their gob.

So, only the brain dead block ventilation holes appertaining to gas installations.

[Jethro_uk]

It's actually quite hard to get a gas leak to "explode". You need a quite narrow ratio of gas/air. Which is one reason why a the Calor canister in a car "bomb" they tried in Glasgow (and NYC IIRC ?) had the engineers in the Ops department pissing themselves with laughter when I worked there.

Obviously gas leaks aren't a great thing, and can be very dangerous. But as a *reliable* explosive, gas (methane, propane, butane) leaves a lot to be desired.

A fracture in the high pressure (75 bar) gas grid can throw a flame 400

+metres.

[Martin Brown]

They also need to have no sense of smell not to notice any leak. (more common than you might think)

There was a famous one not far from me where a DIY gas CH installation on bottled gas went up spectacularly. The blackened shell of the house serving as a warning to others since the insurers declined to pay out.

I live within the danger zone for the high pressure ethylene pipeline at

1400psi (100bar). It is reckoned that any leak would roll down to the beck and flash back after reaching the sewage pumping station. Noise estimates for a serious breach at 30m are 140+dB

Planning permission for a fishing lake was turned down on the grounds of the lake being too close to the pipeline!!!

[Adrian]

So this airbrick in the pantry is the ONLY ventilation in the whole house? Hermetically sealed apart from that?

[John Rumm]

1. The ratio of gas to air must be in the range of ~5% to ~15%

There, fixed it for you.

[John Rumm]

Have you not seen harry's house?

[The Natural Philosopher]

+1.

In general the best* way to use gas to blow up soimeth8ing is to open a gas valve and leave a naked flame burning.

At some point the concentration gets to critical and BOOM

*GCHQ/NSA I am not advocating this as a desirable activity. Apologies for triggering your deep pizza inspection code.

[NY]

As used to great effect as a booby-trap in the film (originally TV mini-series) "Bellman and True", except that in that case they used a spark, rather than a naked flame, to ignite the gas, and a wrench operated by opening the door to start the release of gas so that it would not explode prematurely.

Conversely if you enter a house that is filled with gas, don't *just* open the windows without also turning off any sources of ignition, otherwise the gradually reducing gas concentration could reach critical from the opposite direction. I had this drummed into me on a safety course - turn off the gas and disable the ignition sources first and only then should you open the windows. For maximum safety, do all these things and then run like bloody hell in case it explodes as the concentration reduces to critical and you've forgotten about an ignition source.

LOL

[Adrian]

We know there's some way of a lot of hot air getting out of it...

[fred]

Calor gas is heavier than air so will accumulate from floor level up.( Hence the danger on boats where it will accumulate in the bilges. Proper safety measure is a spark proof extractor operated before starting engines.)

Mains gas is lighte than air so will build from the ceiling down.

Calor gas had more chance in escaping under doors etc. but mains gas will have filled the room before it gets down there.

ISTR seeing a tv programmes where they attempted to blow us a gas filled car with a spark. Very unspectacular. Same trick with the car filled with petrol fumes blew it up like a bomb

[Jethro_uk]

Mythbusters.

[newshound]

Bottled gas is of course much more dangerous than natural gas, especially if there is a cellar, because it "pools". If you have a sudden big leak then the chill factor may add to the problem.

Similarly for boats.

[harry]

Possibly well sealed enough to cause a problem.

[harry]

Not neccsarily. A burning front can run through a gas filled void/building drawing it's own combustion air in after it.

Just as the flame strikes back" in a bunsen burner if the air is progressively turned up.

[Jethro_uk]

We had deodorised gas at Calor - not sure why.

Apparently mercaprtans have a garlicky odour ... the French Gas Board had a flood of false alarms in the 60s when natural gas was introduced there.

(I was told that by a half-French colleague when I worked at British Gas in 1986. The starting point was my discovery in an old filing cupboard of loads of scratch'n'sniff cards from the late 60s which British Gas issued to customers used to town gas.)

[John Rumm]

Which is not a gas explosion...

[Jethro_uk]

Is it more correctly an inflagration ? Like gunpowder ...

[Blanco]

Trouble is that while usenet works fine for hot air, it doesn?t for gas with harry.

[harry]

This is the one that knocks the building down and leaves the inhabitants (relatively) unharmed.

[Brian Gaff]

Yes, try the cocoa tin with a hole trick.

I have in the past seen gas engineers looking for a leak in the street using a naked flame and watching for its colour to change. I said, are you trying to blow us up and the guy told me that in the open air the mixture is far too poor to catch fire or explode.

Brian