Lighting a gas pipe

[Uncle Peter]

No it wasn't me. It's the person posting as bod in here. What is the group's opinion on this?

"That reminds me of the time that my mate and me were installing C/Heating in an empty flat. It was way below freezing in the flat, so I lit the open gas pipe from the meter (safely) so we had at least some heat. About an hour later their was furious banging and shouting coming from the front door. It was the Fire Brigade. I opened the door and they came running in like they were on speed :-) A neighbour had called them and said they could see a flame in the empty flat."

[Onetap]

On Thursday, December 19, 2013 11:42:37 AM UTC, Uncle Peter wrote: It's possible, but it would chuck out loads of CO and have a big yellow flame, as with a Bunsen burner with the air inlet completely shut off.

[Tim Watts]

On Thursday 19 December 2013 11:42 Uncle Peter wrote in uk.d-i-y:

That's a tall tale, even by your standards...

[Uncle Peter]

What part of "No it wasn't me. It's the person posting as bod in here." did you not understand?

[Graham.]

I can't see a post from "bod" since 7/9/2013 17:25

[Uncle Peter]

And your point is?

[Mr Pounder]

The point is that Bod did not post it as he would have a good idea of what sort of replies it would invite.

[Uncle Peter]

He said "I have no interest in doing that. I have more important clicking to do :-)"

[harryagain]

Stupid idea. It's possible to get an explosion resulting from a flashback.

[Uncle Peter]

That's what I thought, but he claims it's safe as there's no oxygen in the pipe.

[SteveW]

That depends upon the diameter of the pipe, air can be drawn in.

Early gas lighting (before the introduction of the mantle) effectively used a flattened pipe, giving a narrow slot for gas to emerge from and burn.

SteveW

[Tim+]

That looks like a huge load of bollocks arranged to sell flashback preventers. With gas flowing freely through the meter where is the O2 going to appear in the meter to make an explosive mixture? The fact that comments aren't allowed is telling in my opinion.

Tim

[Onetap]

How would that happen? The gas is under positive pressure, so how does it suck in air, at atmospheric pressure.

The air mixture device /educator is designed to draw air into the gas stream, but a simple leak in a pipe won't do that.

Flashbacks usually happen where there's a pressurized air/oxygen supply.

[Onetap]

It isn't. It's an oxygen/natural gas torch, the oxygen is under pressure and can force it's way into the gas pipe.

The bit that is bollocks is that a flashback arrestor won't work at that pressure and they shouldn't be using mains gas. I'd thought they were a passive device with a gauze screen, like a Davey lamp. I need to read more.

[SteveW]

I phrased that badly. A smll pipe full of gas will tend to stay that way except near the open end and will purge quickly anyway when turned on, while a larger pipe will allow air to flow in and mix much more easily and will take longer to purge too. The risk is at the point of lighting, not once it is burning steadily.

SteveW

[Bod]

You're spot on. It IS a load of old codswallop.

[Bod]

[The Natural Philosopher]

We used to blow down the bunsen burners to do that.

[harryagain]

It's a very well understood (by non-numbskulls) cause of gas explosions. The flame travels down the pipe faster than the gas is emerging. The flame speed depends on the gas/air ratio so if it is faster than the gas is emerging it can travel back down the pipe drawing it's own air/oxygen down after it.

If there is a container (eg gas meter) full of gas at some point, an explosion can occurr if conditions happen to be right.

Gas appliances are carefully designed to prevent this. You can perform the experiment on a bunsen burner by turning the air up until a flashbck occurrs and the gas burns at the gas jet. Old gas cookers used to do it regularly.

It's the main reason it's hard to design gas boilers with a high turndown ratio.

[harryagain]

Somebody elese willing tocome to conclusions from a zero knowledge base.