Gas purge required after property left empty

[ChrisR]

But I don't know why. Only two burners, both with thermocouples which would have shut gas valves when the gas was turned off at the meter.
(J&S Warm air + circulator)

After 6 weeks the pilots wouldn't light, due, apparently to air in the pipe. How would it get there?

Bet namsag knows!

 

[gas4you]

Would it actually be air Chris? Or perhaps just the gas pipes empty due to the pilots using up what gas was left in them after the gas was turned off at the meter?

 

[Agile]

The gas pressure would have fallen to zero as it was burnt up by the pilot. Then the T/C would have shut off the valve supplying the pilot within 60 seconds.

The gas pipe would then be at sero pressure so the gas inside would not be able to get out.

Assuming gas turned on six months later then it should light pretty quickly as it will be gas in the pipe UNLESS there was a small leak which would allow the gas to be replaced by air over several months.

Tony

 

[namsag]

Chrisr i would go along same lines as agile. micro leak that would never show on a gauge , causing air/gas mixture in pipes over long period of time.

 

[ChrisR]

There was something coming out!
No leak when all working, pilots off, at all.
A temperature or air pressure change could have applied some pressure relative to the pressure left in the pipe whan the pilots cut off, but I can't see it being much.
The new owner knew about lighting boilers, and had tried for a pretty long time to light them, and I disconnected the pilot tube on one GV (smelt of gas of course), so a fair bit of the contents of the pipe had been purged

A couple of ARGI people guessed the problem, one said it was common, but no suggestion why.

 

[chrishutt]

If there was mainly air in the pipe it might still smell strongly of gas due to the residual smelliness of old gas pipes. Still hard to imagine how a small leak could possibly cause such a loss of gas in favour of air unless there was something causing substantial alternating pressure differentials (inside and outside pipe) - say the gas pipe being heated by something.

 

[TicklyT]

Even with the pressure at zero, gas tends to leak out of microleaks over time because the molecules in both the gas inside and the air outside behave almost as if the other isn't there. The gas will continue to flow out, and air flow into the pipe until the mixture of gas and air in the pipe is the same as the mixture of gas and air in the room, and eventually the earth's atmosphere.

 

[ChrisR]

Not convinced, the pressure didn't drop 0.001mbar in 2 mins! Jumped around a bit as per usual, but no trend down.

Ok TT, assuming any micro leak in the pipe behaves like a semi-permeable membrane in an osmotic world;

How low a percentage of gas in air will still burn?
We know the EL is 5 - 20%, but just as 100% gas burns, I assume some lower concentration than 80% would too.
Or would it? Must have it in a book somewhere.

 

[namsag]

Chris so where did the gas go. explosive limit is 5 %to 15% by the way , and 100% percent gas doesn`t burn which is why if it was possible to light a match in a gas holder it would not explode it needs oxygen around it to burn,. If it took up to 6 months for gas to dissappear out of pipe why would you expect to see this on any gauge no matter how accurate.. especially over a couple of minutes . I doubt you would see any movement in 24 hours.

Other possibilty is the gas was there all the time and the ignition was faulty

 

[sambotc]

Anything which could cause the gas to go 'stale' therefore un-ignitable after sitting in the pipe for so long maybe?

 

[chrishutt]

The methane of natural gas has been sitting deep underground for millions of years before we use it. If it could "go stale" then it would have done so already, surely?

Nor can I see how the gas could have been replaced with air without there being quite a significant leak somewhere. If no trace of leak with a differential pressure of 20 mbar, how much interchange could one expect with no pressure differential? Or is TT saying that the air outside at 1 bar will leak in as if there were a vacuum in the pipe, and vice versa?

Incidentally, methane in the atmosphere, where it mixes freely with oxygen (specifically OH radicals), has a half life of seven years, meaning that every seven years, half of the methane present is converted to carbon dioxide and water.

 

[Agile]

Incidentally, methane in the atmosphere, where it mixes freely with oxygen has a half life of seven years, meaning that every seven years, half of the methane present is converted to carbon dioxide and water.

That sounds like a solution to global warming! Just wait seven years!

Cows, which produce a lot of methane, could be replaced with powdered non dairy creamer!

 

[TicklyT]

Have a look at 'partial pressures'.

For the sake of an illustration, assume the volume of the pipework is 10 Litres.

The average rate of leakage required for the partial pressure of methane to fall to, say, 500 millibars inside the pipe (probably enough to move the gas/air mixture at the burner outside the limits for combustion) in 6 months would be something of the order of 0.000,002 (two millionths) litres per minute at an average partial pressure of, say, 600 millibars over a 6 month period.

Testing the same piece of pipe at a working pressure of, say, 20 mb above atmospheric, it would leak at a rate of about 0.000,000,06 (six one hundred millionths) litres/ minute. That would give a pressure drop of something less than one ten millionth of a bar per minute.

That's a pretty small leak!

 

[croydoncorgi]

Pressure inside the pipe relative to atmospheric is only going to be +20 - 24 millibar to start with.
Presumably, with this very small over-pressure, silly things like weather-related changes in atmospheric pressure may have an effect...?
What about partial-pressure calcs at VERY low net pressures??

 

[namsag]

I think people find it hard to work out how air can get in and replace the gas,, its all to do with size of molecules, Think of barrier pipe this is designed to help stop oxygen molecules passing through the pipe walls and into the water contained in those pipes.And that is without any tiny micro leaks.

As for the air getting in to gas pipes it may not be a micro leak in the pipes themselves but getting into pipes through pilots/burners and passing past seals in gas valves cooker taps etc etc