Gas pipes - why does gas burn "outwards?"

Question

I've often wondered if a gas cylinder is connected to a hob (say, or a boiler or whatever) and someone turns the knob to allow gas to flow, why is that when a flame is held to light the gas it isn't possible for the newly lit gas (i.e. ignited gas) to travel back through the pipe to the source, hence igniting the gas in the cylinder?

Is it because the gas is held at pressure, so that the gas escapes the pipes with pressure and somehow burns “outward”, rather than returning down the pipe?

Half of this is physics - answered below. The other half is engineering. Since it would be unsafe if this happened, the burner is designed to prevent is. For instance, you have many little holes in the burner. This keeps oxygen out, because in a small channel you can't have two opposing gas flows. — MSalters, Sep 09 '19 at 08:11
Flashback arrestors are used to prevent spontaneous rapid disassembly events when gasses get into the wrong places. — Andrew Morton, Sep 09 '19 at 08:36
For other Americans here: Apparently, a hob is a British English word for a stove. Had to look that one up. — Hearth, Sep 09 '19 at 17:40
@AndrewMorton +1 these are especially useful for hho applications where the oxygen is already dangerously "everywhere" (even though the question is only limited to "hydrocarbon-only" tanks/storage scenarios) — ManRow, Sep 10 '19 at 03:38
There is a cool demonstration that you can do where you actually get a flame front to travel down a tube.
https://www.youtube.com/watch?v=tdLMB4Twm88

https://www.youtube.com/watch?v=8KSHcGHiY0M — Mike Vonn, Sep 10 '19 at 13:37
@Hearth - They're not exactly the same. A "stove" is used more for a flat heat source (like a metal plate). A "hob" is a gas burner. — Chris Melville, Sep 11 '19 at 08:40
@ChrisMelville Over here, that'd just be called a gas stove. — Hearth, Sep 11 '19 at 11:55
@Hearth - And over here, a "gas stove" would refer to the entire appliance, including the oven part. "Hob" exclusively refers to the bit at the top with the burners. Interesting Atlantic differences :) — Chris Melville, Sep 11 '19 at 14:49
Ironic to look up Hob_(hearth). Now spell Melville in Greenlandic. This is a — Rob, Sep 11 '19 at 14:52
@ChrisMelville Huh, I guess that's why "stovetop" and "stove" are synonyms here despite the "top" seeming completely redundant--it wasn't, before the word stove shifted to mean just the bit on the top and not the whole thing! — Hearth, Sep 11 '19 at 15:23

score 65 · Accepted Answer · edited Sep 09 '19 at 21:32

65

A gas flame is essentially a (chemical) reaction front, a (thin) layer in which a hydrocarbon (e.g. methane) is oxidised acc.:

$$\text{CH}_4(g) + 2\text{O}_2(g) \to \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$$

This oxidation reaction is colloquially known as burning or combustion. It's obvious from the equation the combustion needs requisite amounts of oxygen, $\text{O}_2$, coming from the air which contains about $20$ percent of it.

However, in order for the reaction to be viable, the ratio of combustant to oxygen, here:

$$\frac{\text{CH}_4}{\text{O}_2}$$

must fall within certain limits. Too much $\text{CH}_4$ and combustion is not possible. Similarly too much $\text{O}_2$ and the reaction does not proceed.

The ratio is optimal in the reaction front, but not outside of it. Inside the pipe there's not enough oxygen to sustain combustion.

There is also such a thing as a reverse (or inverse) flame. In this video, oxygen burns inside a propane atmosphere:

Photo above: oxygen burning in propane.

Below for comparison, propane burning in air:

edited Sep 09 '19 at 21:32

Volker Siegel

3,883

answered Sep 08 '19 at 17:38

Gert

35,289

Thanks for the additional details. Does this mean, then, that if there were a small hole in either the pipe or the tank that did allow sufficient oxygen to enter (say there was little gas left) then it could be problematic? – nox Sep 08 '19 at 17:40
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@lux: potentially yes. But remember that the line is under slight pressure, so leaking gas out is what would happen. This is very dangerous when the amount of gas in a room enters into the combustion limits discussed! – Gert Sep 08 '19 at 17:49
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It'd be nice to add, that there are gases that do have a rather wide range of reaction regarding their respective partial pressures. Silane, Acetylene, Hydrogen all have very very interesting ranges, and blowback is a serious engineering issue because of it - and these gases can have very very interesting results when the engineering issues are ignored. – Stian Sep 09 '19 at 14:42
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I could have sworn that my propane tank (for a gas grill) has some kind of device that is supposed to avoid the air rushing back in. Did I misunderstand that? If there is such a device ("Rückschlagventil" - "non return valve"?), you might add it to the otherwise very fine answer. – AnoE Sep 09 '19 at 16:02
The image was of the first flame in the video, which is the reference flame of the experiment, propane in air. I added the second flame in the same video, which shows oxygen burning in propane. – Volker Siegel Sep 09 '19 at 21:24
@VolkerSiegel: Thank you! – Gert Sep 09 '19 at 22:00
Many thanks for adding the photos and extra details!! – nox Sep 10 '19 at 01:42
@AnoE You don't need a device. Its called positive pressure. The propane wants to escape 100 times harder than the oxygen wants in. There probably is one, depending on precise circumstance being addressed (see leftaroundabout's answer for one). – Draco18s no longer trusts SE Sep 10 '19 at 16:54
@Draco18s Well, you might want to have a device that seals the pipe when there isn't a pressure gradient. A propane tank "run to empty" will have little to no pressure gradient, so if open oxygen could flow in. The inside of the tank could then be filled with a fuel-oxygen mixture. Which seems like a bad plan, unless you are building a bomb. – Yakk Sep 10 '19 at 17:58
@Yakk that sounds like a "precise circumstance being addressed" which I mentioned. – Draco18s no longer trusts SE Sep 10 '19 at 17:59
Unfortunately, this is not true for typical gas appliances. All the gas appliances that I know use pre-mixing of the gas with air. Basically they shoot a thin, fast stream of gas into the open end of a much larger tube, relying on the momentum of the gas to suck in appropriate amounts of air. The mixing happens inside the tube as the gas travels to the burner. The burner needs to be specifically designed to hold the actual combustion zone in place. The point of pre-mixing is a better (cleaner and hotter) flame. – cmaster - reinstate monica Sep 10 '19 at 23:57

score 52 · Answer 2 · answered Sep 09 '19 at 10:06

52

Gert explained why the flame can't travel back into the cylinder (because there's no oxygen). However, that actually doesn't explain why the flame doesn't travel into the burner itself, because burners (in particular Bunsen/Teclu burners, but hob/camping burners as well) actually pre-mix the gas with air inside the burner tube, but still it only burns outside!

The reason this works is that the burner output forms a flame arrester. It is made of thermally conductive metal, which keeps the gas that streams out of the burner openings cool. Moreover, the holes of some (most? not sure) burners are terminated with a bit of wire mesh, which also prevents heat front from eating “into the interior” of the gas stream. And the gas, even when mixed with oxygen, can only burn at sufficiently high temperature. Normally the flame sustains this temperature by its own exothermic reaction, but by seeping away that heat sufficiently fast is is possible to prevent burn in the tube.

The reason to pre-mix gas and air is that it leads to a cleaner – no soot – and also hotter flame.

answered Sep 09 '19 at 10:06

leftaroundabout

14,903

4

I believe that all stoves have such a thing (just a ball of wire stuffed in the pipe usually), for safety in case oxygen enters the hose or pipe. You don't want a gas explosion inside a gas pipe. The wire mesh prevents the flame from passing through, similar to the wire gauze mat in this short video. I was looking in vain for a better video cum explanation but my google fu failed me. For example, one can capture the flame between two mats, cutting it off above and below and having an even more strikingly "levitated" flame. – Peter - Reinstate Monica Sep 09 '19 at 10:22
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Bunsen burners can "flashback", when the flame moves from the top of the wide tube of premixed gas air to the bottom, where it burns on the very narrow gas nozzle, – user20637 Sep 09 '19 at 19:04
Blowtorch burners typically operate without a wire mesh. They also typically use much higher gas pressures than stoves, which means much quicker gas flow. Typical design of a blow-torch's combustion chamber applies a twist to the gas-air mix as it enters the chamber, creating a low-pressure zone in the center that allows hot gases to recirculate, igniting the fresh mixture as it enters the chamber. Without this recycling of gases, the flame would just be blown out of the combustion chamber. – cmaster - reinstate monica Sep 11 '19 at 00:08

score 11 · Answer 3 · answered Sep 08 '19 at 17:15

Combustion requires the ratio of fuel and oxygen in the air to fall within a specific range, which is different for different types of fuel. There are different cutoff points for ignition, for sustained burning, and for deflagration. Too far from the gas source, there's not enough gas to burn. Inside the pipe, there's too much gas and not enough oxygen.

score 8 · Answer 4 · answered Sep 11 '19 at 00:49

I'm assuming that you are talking about pre-mixed burners. If you have no pre-mixing of the gas with oxygen/air, the answer to your question is trivial: No combustion without both burnable gas and oxygen. However, all good burners use pre-mixing: The gas is mixed with air in a separate mixer before it enters the actual burner. And for those, the answer is no that trivial.

The trick of pre-mixed burners is, that every combustible gas mixture has a certain flame speed. If the gas mixture moves faster than this flame speed, the flame front is basically blown away with the mixture.

Now, when you put a combustible gas mixture through a tube at a speed that exceeds its flame speed, and let it exit this tube, tiny vortexes will be created at the edge of the gas stream and the gas at the edges will be slowed down by the surrounding air. This slowdown in combination with recirculation of hot gases by the turbulences allows the flame front to attach to the edge of the tube, and then eat its way progressively into the unburned mixture flow. In other words, the edge of the tube acts as a flame holder.

Flame holders may be constructed in different ways: They may be simple wire meshes (some stoves), or they may be tube edges (typical cooking stoves), or they may be more elaborate like vortex combustion chambers (blow-torches). But they all operate on the same principle that the flow-rate of the pre-mixed gas is faster than its flame speed, making it impossible for the flame to enter the openings of the gas supply backwards.

The wikipedia article on pre-mixed flames seems to have more details on this.

(Btw, that's why every gas stove has a certain minimum flame size: If you regulate a gas stove down too much, the speed of the mixture becomes lower than the flame speed at the flame holder, and a flash-back occurs. Stove designers don't want this for obvious reasons, so they construct the valves to avoid this condition.)

Gas pipes - why does gas burn "outwards?"

4 Answers4