posted
I have a scene in which people (pre-technological people) find a natural gas line, going between cities. It's sliced clean, so that you can look right into it. They accidentally set it afire.
If anyone knows:
* How big is an intercity gas line? * If someone lights a fire, how far away will people be burned?
I would like for the party to survive unharmed. I'd thought of having something fall across the hole to block it, but whether that's plausible depends on the size of a gas main.
I may be able to keep them from getting too close, if I know how close "too close" is; but maybe not.
posted
I am not an end-all source of information. They can be 18" to 24" diameter easily.
How is it sliced in half? likely the very reason it was sliced will cause the explosion. sparks and such will be caused as debris flies around from the gas pressure. The smell would drive them away, I think, or if the smell is added at the final end, it would still effect them, possibly suffocate them. The explosion would be big. I saw a program where a gas line blew up and took out a small group of campers a hundred yards away. the heat of the fire in another incident was felt several blocks away. Usually there are automatic shutoffs when the pressure drops, but there are times when debris built up and blocked the sensors.
posted
I've been told in the olden days of fifty or sixty years ago, kids would play in empty lots that were empty because the house that had been there blew up in a gas line explosion. There must be some stuff around about the dangers of cooking with gas---I remember press reports of the indicent "rstegman" describes.
Posts: 8809 | Registered: Aug 2005
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posted
Oops, I lost my entire post and now have lost interest in repeating myself.
Google "natural gas" safety. You'll find a lot of helpful websites. For information on major pipelines, you can check the American Gas Association site. They have links to documents with other information you can use.
I just want to point out that, in an environment free of sparks, there is no special reason that the gas released would ignite before dissipating safely (though the initial outgassing of a large high pressure pipeline would be pretty impressive by itself, and could generate sparks under the right conditions). Assuming that the party of humans wasn't already present, but was attracted to the scene later, the main danger would be gas remaining in the pipe.
This could be a very serious danger, particularly if the mixture of air/fuel in the pipe were just right. You could get a "for reals" directed high velocity blast rather than the flashier open air burn (still quite dangerous). In effect, it'd be like taking a point-blank blast from a battleship cannon. That sounds really cool, but I don't think it's what you're looking for here
Oops again! How's this?
[This message has been edited by Survivor (edited March 27, 2006).]
posted
Okay, looks good. Check the "How Does the Natural Gas Delivery System Work" link, it has all the info on pipelines you'll need.
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posted
I was watching an episode of This Old House one weekend, and a method has been developed that lets crews cut into live lines with absolutely no chance of ignition.
They clamp a special collar over the line that seals out all the oxygen and slices through the line. Even if there is a spark, there's no air to fuel the fire.
posted
That works for residential area supply lines, where the pressure difference is miniscule, but pipelines use very high pressures.
If they want to work on a pipeline, they first depressurize it using the compression stations at either end of that length of pipe. Then they can cut into it using inert gas shields or whatever. Anyway, this is off topic, our intrepid group of explorers in this case have no specialized tools of that sort, nor do they have control of the compression stations.
On point to consider, if this cut is in the pipeline between the city's local gas utility and the city, then the gas will be odorized. But if the utility is inside the city, the gas released may not be odorized. I'm sure that pre-tech peoples would smell it anyway before it reached lethal concentrations, but not before it reached explosive concentrations.
posted
Oh, darn. They don't put the smell in, when piping it between cities? There goes my best clue to the reader! Maybe I can think of something else that screams out "gas line."
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posted
I wrote the scene. I had a hole maybe 1 1/2 feet in diameter, spewing out gas at full force (the event just happened). No scent. Someone lights a fire, and is consumed; another man some 25 feet away is burned, but not badly; another, shielded by a tree, tries to recover the corpse, but can't get close enough to it. I hope that's reasonable!
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posted
Erm...well, the intercity pipelines are generally bigger than any of the pipelines inside a city, are at higher pressure, and have more length between pipes. So if you ignited the gas before a significant quantity of it dispersed....
Well, the upper limits of such an event would be something in the range of a very large fuel-air explosive device.
My suggestion, on careful consideration, would be to have the blast of compressed gas coming out of the pipe ignite as a result of throwing rocks around, then let that flame impress a party of humans a safe distance away. Because if they're close enough to light the flame themselves, they're all going to be killed.
posted
Well, you should ask yourself about your objectives in the scene. If the objective is simply to impress the natives without killing too many of them, then having a really big explosion far enough away that it doesn't kill them is probably your best bet.
It also depends a bit on how close the local utility is to the area where the cut occurs and how big a local utility we're talking about here. The amount of backpressure in the system is still going to be huge, no matter what. A minimum diameter pipeline supplying the local service is going to be more than a foot and a half, the pressure isn't going to be less than 15 atmospheres, and it'd be a real coincidence if your local utility were right near the border. You might be able to get a figure of slightly under a million cubic feet of gas being released.
That's still not a small amount of gas.
Given a few hours, a good breeze can disperse that much gas enough so that it isn't a serious hazard, but if you ignite it while the gas is still coming out under pressure, it will be spectacularly fatal for anyone within dozens to hundreds of yards, depending on particulars.
I think that having your intrepid natives spot a pillar of all consuming fire a kilometer or so away, then hiking over to the scene to find a scorched and scoured crater, would suit the needs of your scene just fine, insofar as I know them.
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