Gas not used for 2 years

[Roger Mills]

Not ideal, I agree, but the OP isn't in the UK (near Screwfix) and is virtually certain *not* to have the proper stuff to hand.

[Roger Mills]

working on my mas installation get anywhere near following all that!

[Ed Sirett]

Well I confess that if I'm fitting a modern boiler I'll probably skip the purge step as there is no chance that a flame could propagate back through the premix fan and zero governor gas valve back into a combustible mix in the pipes.

If I'm fitting a gas fire, say, I'd have to wait a very very long time for the gas to come through via the pilot hole. In these circumstances it's a lot quicker to open the pipework. In which case you need to make sure enough gas and not too much gas comes through. OK, there's another short cut, the timbre of the hissing changes between air and gas.

[Ed Sirett]

Indeed so. Most pre-pay key meters have all sorts of abuse trips.

[Andy Champ]

Being a cynic I wonder if that was because they thought you weren't safe

- or because as gas is measured by volume you were getting twice the amount of gas you were paying for?

Andy

[Andy Wade]

Err, doubling the gauge pressure from 20 to 40 mb (over atmospheric pressure of ~1000 mb) would only increase the volume of gas delivered by ~2%. To double the volume, the gauge pressure would have to rise to around 1 bar, at which point things would get a little dangerous, as the residents of (was it) Hemel Hempstead (?) once found out.

[Andrew Gabriel]

It's one of the specific incidents they have to respond to in a short time (hours).

It was Royston where the gas company connected a high pressure gas trunk network supply to the low pressure local supply network and instantly destroyed a very large number of gas meters, appliances, regulators, etc.

[Andy Wade]

Yes, you're right, it was Royston, in March 1991:

3 refers to 56 explosions being caused.

[Andy Champ]

Ah! A Gasman's double isn't a physicist's double. I stand corrected.

Andy

[Ed Sirett]

Yes, indeed so. Whenever 'pressure' is referred to in gas, plumbing, heating etc. It is a synonym for Gauge Pressure.