On Wed, 23 Dec 2009 16:36:17 -0000, "mark"
wrote:

"DavidM" wrote in message
.. .
Been reading the Eurostar thread with interest.

Similar question then ocurred about the failure of gas supplies in
Luton and Barnet. Why the hell do they think it will take until after
christmas to sort out a couple of broken pipes? Maybe there's some bit
of high tech complexity in gas pipes that escapes me.



The fixing of the gas pipes isn't the bit that takes the time, a few hours
at most.
Excavation, repair, reinstatement. The main will have been turned off at
the nearest Pressure Reduction Station, (PRS) to facillitate repair.

This will be the bit that escapes you:
Not high-tech but complex.
When gas mains fail the pressure over the entire district supplied will
fall dramatically and with the deliberate isolation of said main there won't
be many millibars around.
Before pressure can be restored the district affected has to be clearly
defined. Every house that has a gas supply has to be identified and
visited. If N/A, revisited etc. If people are away, lots are, or the
property vacant then it has to be isolated, and if the meter is not external
then the service has to be cut off by excavating it first. The reason for
all of this is that some people still have gas appliances with non-cut off
pilot lights. If pressure was restored to such appliances, uncontrolled gas
would fill the property. Missing even a single property is not allowed.

Gas and air must not be allowed to mix within the gas pipes or houses will
be going bang all over the place. So if air has got into the pipes then a
nitrogen purge is necessary to flush it out first, then the nitrogen
flushed out with gas.

So once pressure has been restored with gas only in the mains then every
house service has to be purged to get rid of any air nitrogen.

It is more complex than a water or electric main failing.


mark

blimey, tis indeed quite a palaver!
"I know more now than I did before"
Thanks, Mark.