On Mon, 03 Dec 2007 18:31:27 -0000, (Chris
Lewis) wrote:

According to Oren :

Recently the gas company was out to put a new meter on a new home. She
installed the meter and opened a gas line inside the garage to purge
the line. About thirty feet away was a lit cigarette.

The tech told me that the cigarette was "not hot enough" to ignite the
gas. I was surprised by this comment.

A smouldering cigarette isn't very hot - puffing it makes it a lot
hotter, and apparently methane has a rather high ignition energy compared
to other gasses.

However, the explanation is probably not that.

Methane has a LFL (lower flammability limit) of 5.3% and a UFL
(upper flammability limit) of 15%. It also has LEL (lower explosive
limit) of 5.53%.

Which means that unless the methane concentration in air is between LFL
and UFL, it won't ignite.

Gas lines past the regulator are also fairly low pressure, so opening
it doesn't spew gas very quickly. Secondly, methane is lighter than
air, so it goes up, rather than stick around.

An residential gas line venting in open air, is quite unlikely to
ignite from something 30' away. Even enclosed in an open garage,
it'd take a while to build up the concentration to the danger
point - if there's any ventilation at all, it might not ever reach
that concentration more than a few feet away from the vent point.

You still have to be careful - there could be wind patterns
that push a plume to exactly the right place. And it depends on
the gas. Propane is heavier than air. It will "puddle" in
depressions and stick around for quite a while in low wind
conditions. There was a propane tank car derailment around 40
years ago in the north end of Toronto, where the tank fully
vented. Everything was pretty much under control, until the
propane flowing down a small creek valley (very still air
conditions) finally reached an open flame about half a mile away.

Can't happen with methane.

Thanks. IIRC, she did state that striking a match would be hot enough
to ignite the gas, but the concentration would have to build up.