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Gas Leaks and Smells

From: andrew cooke <andrew@...>

Date: Wed, 28 Mar 2012 17:01:36 -0300

Our apartment building has two vertical shafts that contains the utilities for
each side of the building.  Last week a couple on the ground floor (our
vertical neighbours) started to smell gas - you could smell it around their
apartment, but it was particularly strong at the bottom of the shaft.

The gas company was called, but after showing that it wasn't a leak in their
own side (ie before the individual gas meters), left.  They were called again,
but again failed to find anything.  On the third attempt, several days later,
they found a leak on the third (Chilean/US style numbering; ground is first)
floor.

What's interesting here is the likely physics involved.

As far as I understand things, the gas company did *not* detect gas on the
ground floor.  At one point they claimed that the smell was from refuse.

And the principal components of natural gas are lighter than air.  So any gas
escaping from the third floor should have gone upwards.  Someone from the gas
company suggested that the smell on the ground floor only happened once the
shaft had "backed up".

But that seems crazy.  The smell would have been worse on the higher floors -
it wasn't (even when the access doors to the shaft were opened).

Instead, I believe that the gas was trapped, stagnant, in the shaft for
sufficient time for the odorant (the bad smell) to separate out, to some
extend.  Unlike natural gas, the odorant gas *is* heavier than air.  And while
it will normally be carried along in a cloud of gas it will, over time, in
static conditions, tend to gravitate out, and fall downwards.

So the bad smell fell to the bottom of the shaft - where it pooled - as the
gas from the leak slowly leaked away from the top.

Andrew

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