Gas purge required after property left empty

[ChrisR]

TT
where do differential gas pressure like 500mbar (half a bar) come into the equation?
I don't see where you other figs come from either.

My guess is that the guy did just turn the gas off, (that's what he said) so the pressure in the pipe would go to about atmospheric + maybe a few mbar, and then only weather variations would be the driving force.

We're talking 6 weeks, not 6 months, too.

nam - 100% gas burns in air perfectly well!

Still not convinced.

 

[namsag]

As i put chris gas needs air to burn

 

[FatGit]

nam - 100% gas burns in air perfectly well!
.

If it is 'in air' then it cannot be 100% gas by definition as there is air in there! You couldn't light a match in 100% gas as there would be no oxygen for it to burn!

 

[ChrisR]

You couldn't light a match in 100% gas as there would be no oxygen for it to burn!

Yes we know that but it isn't the scenario here so not relevant.

Sure, burning IS oxidation. It's coming out of the pilot jet here. 100% gas coming out of a jet burns perfectly well. If it was a mix of gas and air, what % gas would be required for it to burn?

The only figure I can find for the lower (in)flammability limit is the same as the LEL, 5%.
SO for it NOT to burn, nearly ALL the gas would have had to come out of the pipe and be replaced with air? This is with holding a flame at the pilot outlet - it's not burning.
If it were premixed with say 30% air, I would have thought it would still burn, just needing less from the atmosphere.

 

[FatGit]

Ah! I see what you are saying. I was confused with people talking about 100% gas, which as you have clarified refers to the gas comming out of the jet rather than the combustion mixture (which cannot be 100% gas for it to burn). I've no idea what the ratio of gas to air would have to be for it to burn, sorry.

 

[croydoncorgi]

Probably safer to assume that to burn at all at a jet the gas mix in the pipe must be ABOVE the UEL (15%) (otherwise, it would probably 'pop' and the flame would not sustain). I reckon in practice for a pilot flame to sustain reliably, you need maybe 40 - 60% gas in the pipe, otherwise the flame will be too unstable.

 

[sooey]

So we've got two pages of waffle which amounts to "if there's gas in the pipe the pilot will light, if there's no gas it won't " or is it all going over my head?

 

[namsag]

Chris you are asking how much of a premix would be req for gas to burn without taking it from the surrounding air , this isn`t going to happen, because the second it leaves the pilot jet and an ignition source is added it is just going to pull instantly whatever oxygen it requires to burn...

The only time you will get it to light in your sort of circumstance is flash back within a pipe (as it is not taking air from around it) and then that is has to have perfect conditions.. So as ignition can only occur when the gas concentration is between 5/15% in air you would need between 85/95% air in that pipe aswell not the 30% premix you are thinking.

When a burner uses premixed gas and air this is generally between 40and 50% of total air requirements the rest being supplied by the surrounding atmosphere.... 1m3 of gas requires 10m3 of air to burn.


EDIT . The micro second gas leaves the injector it stops being 100% gas and pulls air along with it this is why it burns when an ignition source is added completing the reaction (ignition source must be above 704c)

 

[TicklyT]

TT
where do differential gas pressure like 500mbar (half a bar) come into the equation?
I don't see where you other figs come from either.

My guess is that the guy did just turn the gas off, (that's what he said) so the pressure in the pipe would go to about atmospheric + maybe a few mbar, and then only weather variations would be the driving force.

We're talking 6 weeks, not 6 months, too.

nam - 100% gas burns in air perfectly well!

Still not convinced.

Even at 0 millibar gauge pressure, the gas in the pipe is still at 1 bar absolute pressure, which is the same as the atmosphere outside the pipe. However the partial pressure of methane in the atmosphere at sea level is about 3/10ths of nothing, as there is very little present (except near dairy farms that haven't been replaced by non-dairy creamer, or plumbers' apprentices after a night out on the beer and a vindaloo on the way home), so the difference in partial pressures is about 1 bar. Similarly, there's about 800mb of nitrogen and 200mb of oxygen making up the atmosphere outside and none inside the pipe. As the gas 'rushes' out of the leak, it is replaced by air 'rushing' in as the partial pressures try to equalise inside and outside the pipe.

For this to be able to happen in 6 weeks instead of 6 months, the leakage must be at least four times as big as I thought, so it may even be as much as a millionth of a millibar per minute in a world where little significance would be attached to a rate of a tenth of a millibar per minute.

There may be a flammable gas/air mixture inside the pipe, but the size of a pilot jet (and the proximity of the walls of the jet) would absorb the heat energy of the pilot flame and snuff it out in much the same way as the gauze in a safety lamp works, so no flash back.

 

[croydoncorgi]

Silly me!
I'd forgotten that partial pressures related to absolute pressure, not relative. Doh!

 

[ChrisR]

Nam:

Chris you are asking how much of a premix would be req for gas to burn without taking it from the surrounding air

No I don't mean that, I mean how much air would have to get into the pipe before it won't light when it comes out.
I mean at 70% gas it would I think still light, but 30%, I don't know.

TT I'm following more of what you're saying, but I don't get the same figures as you at all.

Crucailly, the "forces" causing the pp's to equalise, aren't very strong. Propane doesn't even easily make it out of a boat!
If the partial pressure of the gas in the pipe is maintained at 1.020 bar, air doesn't make it in at all - or are you saying it does?
Not sure whose Law it would be...
POint is, I don't see where you're differentiating between the force pushing the gas out of a pipe when it's at 1 atmosphere (pp), and that when it's say 1.020 atm. Clearly, it comes out an awful lot faster when it's at 20mbar over, not just 1.02 times as fast.
One thing that means, is that temperature and pressure variations due to weather would become much more significant. The pressure in the pipe would lag that in the pipe so there would be a constantly varying and alternating, equalising pressure.

So, although the pp thing may be part of the issue, I don't see it can be equated with the passage of gas though a leak at test pressure.

 

[croydoncorgi]

Obviously (?), the area of any interface between the different gas mixtures (atmosphere, with negligible CH3, and inside pipe,initially with 100% CH3) makes a helluva difference to the diffusion rate due to differing partial pressures. If we're talking tiny, tiny holes, the rate of diffusion per week is going to be tiny too. A second factor is the volume of the pipe from meter to appliance gas valve. Obviously (? - oh dear, there it is again!), the greater the enclosed volume of 100% CH3 that's there to begin with, the less the effect of whatever volume of other gases that migrate in according to partial-pressure effect. Say there's 5 metres of 22mm tube = .38 litres of CH3. I'd guess that to have any measurable effect on combustion, at least 20% of the CH3 will have to 'exchange-out' - nearly 0.1 litres. That's a big ask!

 

[chrishutt]

CH3

Sorry to nit-pick but isn't methane CH4?

 

[ChrisR]

CC's always been something of a free radical....

I imagine TT used 2 litres because of the volume of the gas meter, tho I forget it in litres.

I do take TT's point that over 6 weeks a lot of gas could come and go.

I've got to go now, packing, but I think 0.1litres in 6 weeks is 0.3 cubic millimeters in a test period.
I do hope that the matter is resolved by the time I return!

 

[FatGit]

When a burner uses premixed gas and air this is generally between 40and 50% of total air requirements the rest being supplied by the surrounding atmosphere.... 1m3 of gas requires 10m3 of air to burn.

Only for complete combustion to CO2 + H2O.
If there was less air the gas (methane) could still burn, but incompletely leading to the formation of CO.
I think what is being asked is what is the minimum % oxygen required in a gas/air mixture for there to be any combustion at all, however incomplete. I.E. assuming the pilot light is is air, and there has been, for whatever reason, a mixing of gas and air in the supply pipe to the jet what is the minimum % gas that must be in the supply pipe mixture in order for ignition (combustion) to occur even if incomplete?