MadSci Network: Other
Query:

Re: How is a strike back dangerous?

Date: Fri Jun 20 11:15:32 2003
Posted By: Carlin Gregory, , Chemistry (BA and MS) in Synthetic , Williams Gas Pipelines - Texas Gas
Area of science: Other
ID: 1056115348.Ot
Message:

Charlotte,
You are correct about a strike back occurring when you light a Bunsen 
burner with the air vent open.  The reason this occurs is due to the right 
mixture of air and natural gas.  There are two physical properties of all 
flammable gases, an upper explosive limit (UEL) and lower explosive limit 
(LEL).  For natural gas the LEL is 3.8-6.5% and UEL is 13-17%.  This means 
that natural gas will only burn when its concentration is between 3.8 and 
17%.  When you close off the air vent, you do not allow air to enter the 
Bunsen burner, thus you allow the concentration of the natural gas to 
exceed the UEL and burning will not occur.  When you light it and open the 
air vent you allow a flow of air in with the natrural gas and it mixes 
before coming out the top where the burning is now occurring.  There is a 
flame at the jet but it will not enter the jet becasue the concentration 
of natural gas immediately inside the jet above the UEL.  It is dangerous 
in that you expect the flame to come out of the burner tip and instead you 
get a pop at the base.  If you hands or other obects are close by you may 
get burned or cause a fire.  Hope this answers your question.

Carlin Gregory
Staff Chemist
Texas Gas Transmission


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