I’ve listened to PG&E engineers on the "gas side of the house" try to convince me that natural gas leaks are not really that dangerous. They have argued that you need more than just a leak to end up with a fire or explosion.
First, they say, you need the proper mixture of gas and air. That mixture is 5 to 15% gas, and the rest air. If the gas content is less than that, the mixture is too lean to ignite. If it is more than that, it is too rich. To ignite, the mixture must be just right.
Second, before a combustible mixture can explode it must come in contact with an ignition source. Say, for example, an open flame.
Everything has to coincide just right, they say, for an explosion to occur. So leaks shouldn’t always be considered that dangerous. Leaks are not always an emergency.
Some of what they say is true. But only to a point.
Yes, only a combustible mixture of natural gas and air can ignite. At the source of the leak, the "mixture" is almost 100% gas. That’s too rich to combust. Far from the leak, the mixture is almost pure air. That’s too lean. However, there is always an area between the source of the leak and the air far from the leak where the mixture will be just right. And that combustible mixture will move as the leak continues and the gas cloud grows.
Sooner or later, the combustible mixture will encounter an ignition source. An open flame is not required. A car cranking over its engine, someone flipping on a light switch, or someone answering a cell phone is enough to trigger an explosion.