Question about Propane - how is it explosive?

[beadysam]

This may be a stupid question - I don't know, but me and hubby were discussing propane last night and wondered about this.

I know propane must be stored outside in case of leakage, thats fine, I get that. But when there is a propane explosion, what actually happens? Say the gas is stored indoors(!) and there is a leak, then a spark would ignite the gas, there would be a big bang, but presumably the gas would be used up quickly in the explosion? Obviously things that get in the way would ignite, but the initial gas explosion although large would be limited in the amount of time it takes to happen - also it would quickly use up available o2 in a sealed room so that would be somewhat self limiting too wouldn't it? If there were open doors etc than obviously o2 would be sucked from this source. The force from the explosion (depending on the amount of gas) would be strong and destuctive? But what about the actual gas container, thats the bit I can't figure out? If the container gets too hot - it explodes, yes? But if the explosion of the gas leak is quick, there would be no time for the canister to get hot would there? In this case would the canister stay intact or would the fire track into thebottle somehow and ignite and explode the whole thing?

I'm sorry for loads of questions. I do understand and comply with the safety recommendations I just would like to understand the rationale behind what I am doing a bit better.
Thanks in advance
SAM xx

[MikeAurelius]

There are a couple of different answers based on your questions...

As far as the tank itself getting hot:


From the MSDS for propane:

"DANGER! Fires impinging (direct flame) on the outside surface of unprotected pressure storage vessels of Propane can be very dangerous. Direct flame exposure on the container wall can cause an explosion by BLEVE (Boiling Liquid Expanding Vapor Explosion). This is a catastrophic failure of the vessel releasing the contents into a massive fireball and explosion. The resulting fire and explosion can result in severe equipment damage (meaning fire trucks!) and personnel injury or death over a large area around the vessel. For massive fires in large areas, use unmanned hose holder or monitor nozzels; if this is not possible, withdraw from the area and allow fire to burn."



This is direct from the MSDS on propane.

Now, for the rest, what usually happens with a propane leak leading to an explosion is that the leak gradually fills up the area until it reaches its "ignition concentration", which for propane, I believe it is about 3-5% (meaning that 3 to 5% of the "air" in a given space is gaseous propane.

At ignition, it becomes a fuel/air explosion, there is a concussion blast, then flame effects.

At lower than ignition concentration percentages, there is usually a flash fire followed by secondary fires lit from the flash fire.

I was personally involved in the second type (lower than ignition concentration) about 20 years ago while lighting a propane hot water heater. I was lucky and only got a couple of flash burns.

The first type is of course the most dangerous, and what leads to most deaths. And of course, is the reason why, especially if you are using a hot head or similar torch without a regulator on the tank, a leak can cause ignition concentration very quickly (usually in under a minute).

[Dale M.]

Where the violence and trauma come in is when propane explodes, its expansion rate is 270 time the initial volume of the fuel/air mix... So if you have a cube (room) that is 10x10x10 feet it is actually a 1,000 cubic feet and it reaches ignition point, its going to try to instantaneously expand to 270,000 cubic feet, or 270 times original size and it is so fast that anything the pressure wave hits in its path its like having a solid object hitting it...

Dale

[one-eared pig]

Where the violence and trauma come in is when propane explodes, its expansion rate is 270 time the initial volume of the fuel/air mix... So if you have a cube (room) that is 10x10x10 feet it is actually a 1,000 cubic feet and it reaches ignition point, its going to try to instantaneously expand to 270,000 cubic feet, or 270 times original size and it is so fast that anything the pressure wave hits in its path its like having a solid object hitting it...

Dale
WOW. OH WOW. great thread, thanks for the question and the answers!

[sandra j ziolkowski]

Its one thing to keep your propane out side with the hose coming into the house. But what if you forget to turn the tank off after use and somehow the hose or the valve leaks in the house? As soon as it builds up in the house ..Boom!! So If you don't turn the tank all the way off after use then your in just as much danger. Right? The tank might not blow but your house is full of gas...Kaboom

[EmbellishYourself]

I use to work for the Gas Company and one of the Supervisors there told me about a propane explosion. It was from a bbq grill sized tank, it leaked in the garage and was ignited by the water heater. It knocked the house off its foundation. He said the clothes came out of the closets on the second floor. He said he had never seen anything like it. He also said there was still gas in the tank so it could have been even worse. Pretty scary if you ask me. Propane is a heavy gas and pools on the ground so I imagine it takes alot before it reaches the level of the water heater to ignite. Jenny

[MikeAurelius]

Yes, it is heavy, however, remember that it only takes 3-5% concentration, and that the burner for most hot water heaters is only 3-4" off the ground. It only takes a slight air current to "slosh" the propane around, such as the furnace running, or someone simply walking by.

[beadysam]

Thanks for the answers - this is a really interesting thread and I'm glad I asked. I felt a bit stupid really as though I was missing something that was glaringly obvious](*,) but the answers have clarified things for me. Sorry I've not checked back sooner - it was my sons birthday this weekend.
SAM xx