Expert: Mike Smith - 11/11/2009

Question
QUESTION: In Industry ,we install a flame arrestor on the vent of tanks containing flammable liquids,so that external fire or spark doesn't propagate back  to the tank through the tank.
How does the fire does not travel back to the cylinder or the pipe in case of a domestic cooking gas stove.

ANSWER: Hi Kantilal,
Venting gas back through stacks has been around for a long time. And it works.
The idea is that the cylinder you refer to is under pressure until it reaches the top of the stacks where it is ignited. If we remember back to fire behavior we know that when the gas is in the lines it is to rich to burn. It only burns when the air mixture (At the top of the stacks) reaches it's flammable range. In order for fire to burn it must have 1. Fuel 2. Heat 3. Oxygen or 4. A chemical chain reaction. When the gas (Fuel) mixes with air (Oxygen) and the spark arrester (Heat) come together we burn it off. If the (Heat) trys to go back into the lines, it becomes to rich (Removes Oxygen)to burn.
I hope this helps. It is a complex process.

Goos Luck,
Mike

---------- FOLLOW-UP ----------

QUESTION: Thanks Mike

When the cylinder is used and is about to be near empty, the pressure in the cylinder will  be low and also the pipe will be partially filled, that is, the gas concentration will be in the flammable range.At this point of time can  a  flashback occur.
Are there any  other  provisions in the gas stove in addition to the explanation given by you, which prevent the occurrence of the flashback .

Answer
Hi Kantilal,
That is a great question. I don't know what system you are using but the ones I am familiar with will not allow the spark to ignite if the pressure in the stack falls below a specific level. Others have a monitor that registers the flammable limit and locks out the spark if the gas falls below the LFL. Then the stack simply vents the gas into the atmosphere.
That is not to say that the system will not malfunction, or be in override from someones perception of what should be occuring. Many plants have had large explosions because some decided that they know more than the computer.
If you are talking about a simple system such as a BBQ or something like that, then yes, flashback can occur.
Does that help you? I hope so.
Good luck,
Mike