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Author Topic: Why does a gas flame not track back inside the supply pipe?  (Read 17192 times)

Offline hamza

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I know this sounds foolish but i really don't get it when the fire of my stove does not go through the gas pipe and way back to the where the gas is comming from.. I mean the gas immediately catches fire so WHY when we turn the gas on and light the fire, it stays over the stove and does.t move into the pipe that is bringing gas??


Moderator: Clarified thread title.
« Last Edit: 19/09/2010 22:57:54 by chris »


 

another_someone

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In order to burn gas, you need both gas and air.  The air does not travel back up the pipe, so the flame cannot travel back up the pipe because it would be starved of air back up the pipe.
 

Offline eric l

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There are a number of reasons, and this list will probably quite incomplete.
  • a fuel can only burn when it is mixed with air in a ratio between definte values (depending on the fuel), a mixture that is either too rich or to lean (in fuel) does not burn
  • the gas comes from the pipe through a nozzle, only after which it is mixed with air; the gas in the pipe does not contain enough oxygen to burn
  • there is also the speed of the flame front, if the speed of the mixture of gas and air is higher than this speed of the flamefront, the flame can not travel back to the nozzle
  • the flame is formed on the gas ring, a perforated tube.  The perforations make sure that the speed of the mixture remains high and the metal acts as a kind of heat sink so that the mixture inside remains cooler than the "flash point" (http://en.wikipedia.org/wiki/Flash_point)
See also :
- http://en.wikipedia.org/wiki/Bunsen_burner
- http://en.wikipedia.org/wiki/Gas_burner
- http://home.howstuffworks.com/how-to-repair-an-oven.htm
 

Offline Karen W.

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Thanks Alberto and George!  Whoops I did indeed mean Eric and George! (Thanks alberto!)

Hamza It is not a foolish Question! I being crazy over my gas appliances have often wondered about that and would never thought to have put the question in here, but what a great question!
« Last Edit: 31/10/2007 16:09:16 by Karen W. »
 

Offline lightarrow

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Thanks Alberto and George! 
Thanks Karen, but this time it was eric I to answer you!  :)

By the way, in a recent antifire course (I hope to have translated correctly) they said a curious thing: exploiting the fact gas needs air to ignite, technicians who repairs big gas pipes can weld them electrically while the high pressure gas comes out of the broken pipe!
Sincerely, I wouldn't advise anyone to try doing it!

(Can someone please tell me if the phrase I've written in blue is grammatically correct?
Thank you.)
« Last Edit: 30/10/2007 17:18:09 by lightarrow »
 

Offline eric l

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Alberto, I'm not trying to start a dynasty, so it's eric L (for the first character of my surname) rather than eric I.  But the mistake often does make me smile.

And as long as the gas does not build up around the leak, but disperses quickly, this electric welding may work - in the hands or real professionals.
 

Offline JimBob

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Thanks Alberto and George! 
Thanks Karen, but this time it was eric I to answer you!  :)

By the way, in a recent antifire course (I hope to have translated correctly) they said a curious thing: exploiting the fact gas needs air to ignite, technicians who repairs big gas pipes can weld them electrically while the high pressure gas comes out of the broken pipe!
Sincerely, I wouldn't advise anyone to try doing it!

(Can someone please tell me if the phrase I've written in blue is grammatically correct?
Thank you.)

It is grammatically correct.
 

Offline Atomic-S

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but pyrotechically risky.
 

Offline Karen W.

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Thanks Alberto and George! 

Hamza It is not a foolish Question! I being crazy over my gas appliances have often wondered about that and would never thought to have put the question in here, but what a great question!

It is as Jim said Gramatically correct.. Sorry ALBERTO MY BRAINS ARE KINDA MUSH LATELY.. I AM NOT SURE WHY I THOUGHT ERIC WAS YOU! THANKS ANYWAY!

ERIC THANKS FOR THE INFORMATION. *SMILES*
« Last Edit: 31/10/2007 04:23:05 by Karen W. »
 

Offline lightarrow

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but pyrotechically risky.
Very nice.:D
 

Offline hamza

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #10 on: 31/10/2007 14:10:18 »
Thanx everyone for the explainations.. It really helped alot.. and thanx in particular to Karen for appreciating my question
 

Offline Karen W.

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #11 on: 31/10/2007 16:31:22 »
Your Welcome.. I still have the same question though..! LOL

Please bare with me I may be missing something!

There are a number of reasons, and this list will probably quite incomplete.
  • a fuel can only burn when it is mixed with air in a ratio between definte values (depending on the fuel), a mixture that is either too rich or to lean (in fuel) does not burn
  • the gas comes from the pipe through a nozzle, only after which it is mixed with air; the gas in the pipe does not contain enough oxygen to burn
  • there is also the speed of the flame front, if the speed of the mixture of gas and air is higher than this speed of the flamefront, the flame can not travel back to the nozzle
  • the flame is formed on the gas ring, a perforated tube.  The perforations make sure that the speed of the mixture remains high and the metal acts as a kind of heat sink so that the mixture inside remains cooler than the "flash point" (http://en.wikipedia.org/wiki/Flash_point)
See also :
- http://en.wikipedia.org/wiki/Bunsen_burner
- http://en.wikipedia.org/wiki/Gas_burner
- http://home.howstuffworks.com/how-to-repair-an-oven.htm

Eric, The wiki links were great about telling how the air to gas ratio burns and the bunsen burner one helped a great deal but somehow I still don't get or see that it answered why the fire once lit does not travel back down the tube and just blow everything to bits!  How is the flame only burning the gas coming out the end? WAS THIS TO DO WITH THE GAS AIR RATIO COMING OUT OF THE TUBE???I am confused because I still don't get it.. what is to prevent the flame from traveling down the tube? Is it because the tube is narrow and maybe the gas heavy enough to prevent oxygen mix sufficient for ignition or what...???

OH......Dear wait.. Thats where Alberto comes in Gas needs air to ignite so the tube allows so much gas though to the opening where it is mixed further with the air making it igniteable.... whats in the tube will not ignite because of lack air...Air mix with gas + ignition sourse + flame on exit but not down tube as gas coming through has no air yet! Is that correct?? Is that right???

« Last Edit: 31/10/2007 16:37:06 by Karen W. »
 

lyner

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #12 on: 31/10/2007 16:47:41 »
The shape of the flame front, at the top of the Bunsen tube, is conical.
If it starts to burn quicker, the cone gets smaller and, so, the area of burning gets less - this slows the rate at which gas can be burned and then tend to make the cone bigger - this produces a stable situation in which the flame is advancing downwards at the same rate as the gas/air mixture is blowing it upwards and produces a steady flame (usually - although you can get a 'guttering flame' when the pressure is too low for the burner design. There is a maximum size of  Bunsen 'chimney' tube for which this can work and the flame will just travel down a wide enough bore tube and burn at the mouth of the gas jet, which is called a blowback.
It was possible to get a 'blow-back'  on bunsens at school; you could make it happen, sometimes, by punching the rubber rube and interrupting the gas supply. Or you can light it at the air inlet hole and get a flame,  sometimes.
I haven't actually added anything to the previous posts but I've tried to put it more simply.
 

Offline lightarrow

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #13 on: 31/10/2007 17:10:14 »

Eric, The wiki links were great about telling how the air to gas ratio burns and the bunsen burner one helped a great deal but somehow I still don't get or see that it answered why the fire once lit does not travel back down the tube and just blow everything to bits!  How is the flame only burning the gas coming out the end? WAS THIS TO DO WITH THE GAS AIR RATIO COMING OUT OF THE TUBE???I am confused because I still don't get it.. what is to prevent the flame from traveling down the tube? Is it because the tube is narrow and maybe the gas heavy enough to prevent oxygen mix sufficient for ignition or what...???

OH......Dear wait.. Thats where Alberto comes in Gas needs air to ignite so the tube allows so much gas though to the opening where it is mixed further with the air making it igniteable.... whats in the tube will not ignite because of lack air...Air mix with gas + ignition sourse + flame on exit but not down tube as gas coming through has no air yet! Is that correct?? Is that right???
yes, yes, yesss! Guessed!

(From now on, you won't need to worry about it anylonger!) ;)
« Last Edit: 31/10/2007 17:13:03 by lightarrow »
 

another_someone

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #14 on: 31/10/2007 17:16:26 »
It was possible to get a 'blow-back'  on bunsens at school; you could make it happen, sometimes, by punching the rubber rube and interrupting the gas supply. Or you can light it at the air inlet hole and get a flame,  sometimes.

Bunsens are quite different from domestic gas fires because bunsens introduce air ahead of the flame front, through a hole in the base of the bunsen, so as you point out, basically control the location of the flame by ensuring the flame cannot travel back against the flow of gas.

The other thing that I don't think has been mentioned (maybe I just have not read it), is that hot gasses travel upwards, and thus inevitably will tend to draw most of the heat away from gasses coming from beneath (turn the whole thing upside down, and it will be more difficult to control).  Heat can still travel down through radiation, but not through convection.
 

Offline eric l

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #15 on: 31/10/2007 18:49:30 »
Bunsens are different in the sense that you can regulate the air flow with them, and that they do not have this little holes where the flame is formed.  (That's why you can have this blow back in a Bunsen.)
But take a standard gas stove.  The flame is formed above a circular tube, with holes in the top side.  This tube is linked to the gas supply by a straight section of tube, that ends a couple of mm from the gas tube.  There you have the jet or nozzle, through which the gas flows.  As there is some free space between that jet and the tubing, air flows in with the gas (venturi effect).
If you change jets, e.g. because you switch from butane to natural gas, you may have to change the distance between the end of the pipe and the jet.  Too short means not enough air and a yellow flame; if too long you have irregular burning because the flame tends to be blown away from that ring. You don't need to change jets when you switch from butane to propane (or vice versa); and if changing is needed, have it done by a qualified installer.  That will save you a lot of trouble and time.
 

lyner

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #16 on: 01/11/2007 10:53:21 »
Quote
Bunsens are quite different from domestic gas fires because bunsens introduce air ahead of the flame front, through a hole in the base of the bunsen,
Gas fires and gas rings are just the same as bunsens, in that respect.
If they were not, the flame would be yellow and sooty.  The gas jet is just by the tap, at that end of the tube, air is admitted  through a hole  and the mixture goes to the burner. There is no need for a mixture adjustment.
The only difference is that the burner has lots of small holes instead of one big one. This makes the flame more stable and spreads the heat out under pans or 'candles'- the ceramic glowing bits.
BTW, there are bunsens with  a perforated plate at the top of the chimney, with lots of small flames - more stable again but more susceptible to getting grott (from kids) down them.
 

Offline victoria19

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #17 on: 30/08/2010 06:46:17 »
I also experienced that when I was cooking. I thought it was cause fire the entire house. But suddenly the fire stop suddenly. My mother explains me what the appliances done, the gas of the stove leaks.
 

Offline maffsolo

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #18 on: 30/08/2010 22:55:59 »
The shape of the flame front, at the top of the Bunsen tube, is conical.
If it starts to burn quicker, the cone gets smaller and, so, the area of burning gets less - this slows the rate at which gas can be burned and then tend to make the cone bigger - this produces a stable situation in which the flame is advancing downwards at the same rate as the gas/air mixture is blowing it upwards and produces a steady flame (usually - although you can get a 'guttering flame' when the pressure is too low for the burner design. There is a maximum size of  Bunsen 'chimney' tube for which this can work and the flame will just travel down a wide enough bore tube and burn at the mouth of the gas jet, which is called a blowback.
It was possible to get a 'blow-back'  on bunsens at school; you could make it happen, sometimes, by punching the rubber rube and interrupting the gas supply. Or you can light it at the air inlet hole and get a flame,  sometimes.
I haven't actually added anything to the previous posts but I've tried to put it more simply.

I recognize this when using different types in gas jet burning, essential protection.

http://www.wittgas.com/EN/flashback_arrestors_interview.html 
 

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Re: Why does a gas flame not track back inside the supply pipe?
« Reply #18 on: 30/08/2010 22:55:59 »

 

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