How can I guess if an outdoor expanse contains flammable gas?

Question

As someone who frequents valleys, I was unhinged to learn of Dax Cowart's calamity beneath.

1. What types of alfresco terrain ought I most beware? It obviously can happen only in small hollows in the land (that I'll call dale), but how small?

2. Please see the titled question. I know that propane is denser than air.

In July 1973, Cowart, then a pilot in the Air Force reserve, and his father were visiting a tract of land that his father was thinking of purchasing. The land lay in a small valley and, unbeknownst to the Cowarts, a gas leak had caused the valley to become filled with propane gas. After surveying the land, the Cowarts returned to their car, and the sparking of the ignition set the gas on the floor of the valley ablaze, severely burning both men. According to Cowart, "I was burned so severely and in so much pain that I did not want to live even in the early moments following the explosion. A man who heard my shouts for help came running down the road, I asked him for a gun." He said, "Why?" I said, "Can't you see I am a dead man? I am going to die anyway. I need to put myself out of this misery." In a very kind and compassionate caring way, he said, "I can't do that."

Afterword: I learned of this on p. 63 of Better Never to Have Been (2008 1 edn).

Answer 1

This is rare. I have a BS in chemical engineering, worked in natural gas transportation, and went to fire school at Texas A&M.

Natural gas is mainly methane. It is odorless and colorless. It is lighter than air and disperses fairly rapidly. Mercaptan (odor) is only added when the natural gas goes into retail distribution.

Propane is also odorless and at the retail level mercaptan is typically added. Propane is heavier than air and will pool but still disperses fairly rapidly.

It is odd to transport propane by pipe. Propane alone is not naturally occurring. It is a byproduct of a lot of processes. Mainly it gets pulled out of gasoline as propane has too much vapor pressure.

Or, it could have been natural gas and was misreported. I know from working in oil and gas industry that details are often wrong. An editor could have thought propane was more descriptive.

A leak in a high pressure gas line is loud.

Finally to answer the question propane is odorless. You are not going to smell it. You would be short of breath on high concentration (according to a link in a comment it would not cause shortness of breath). To have a mixture that does not cause asphyxiation, still be flammable, and far enough away not to hear the leak is unlikely. Clearly it can happen but is rare.

It's more common to be killed by the blast that can go maybe a mile. I know of windows that were taken out a mile away.

There are equipments to sense flammable gas and asphyxiants (e.g. CO and CO2). Some oil and gas companies require these to be used when working in the field but they are not OSHA required. A guy I water skied with had his on and it sensed the CO2 while he was at the back of the boat pulling in the rope.

For sure you don't want to test for flammable gas with a spark.

There has been at least one similar incident since.

I think I take back asphyxiation and ignition. According to this propane only burns in mixture between 2.15 and 9.6%. I figured that number would like 2 - 40%. I doubt 10% would cause asphyxiation but just a guess.

Answer 2

The best way to tell an area's at risk of a gas explosion is it smells like gas. Another good way is posted warnings of flammable gas, like you might see on a propane tank. These are way more valuable than any analysis of which terrain might make a better gas trap. Gas explosions can happen an any terrain

It is true terrain can trap hazardous gasses. But if you have reason to believe gas is present in an area, you should just treat the entire area as dangerous. If there's no reason to believe a gas leak is present in an area, then don't worry about buildup. (Your house is a much better trap than any topography, do you worry this much when you're at home?)

If you're not sure what gas smells like, contact your local gas company. Many gas companies give out example scratch and sniff cards (I'm assuming you're in the US)