Rank: Guest 
|
Posted By holmezy
Please educate me. On page 15 of the H+S at work magazine in an article re fire risk assessment it says that static discharge from human body will ignite petrol, however a burning cigarette end will not. I seem to remember someone explaining why a cigarette end won't but can't recall the details. If it is correct then why is it unsafe to smoke on petrol station forecourts or is vapour a different hazard? And what about all those films where the bad guy slops petrol around then nonchalantly flicks his (in slow motion!!) cigareete into the pool of petrol?
Can any of you "experts" out there enlighten me?.......on the petro bit, not the static!!
thanks
Holmezy
Soon be Friday!!
|
|
|
|
|
|
Rank: Guest
|
Posted By Jeffrey Watt
Holmezy
I was taught that the risk from static discharge igniting vapours at a petrol station was very low. If I recall correctly due to the difficulty getting the right conditions of low humidity in the UK. By that I mean it is hard to get a static spark of significant energy cos the air is so bloody wet all the time. This humidity dissipates the energy, I think.
Cone of a drawn cigarette reaches 700 degrees C (two generations of Watts in the ciggy industry). I believe the trick of flicking lit cig into bucket of petrol has something to do with the restrictivly tight LEL and UEL of petrol. By that I mean big open can of petrol provides so much vapour that explosion cannot occur because there is not enough oxygen available for rapid combustion.
Or I could have dreamt the above? Either way do not try at home.
Jeff
|
|
|
|
|
|
Rank: Guest
|
Posted By Peter Longworth
I've seen the trick done where a lit cigarette is dipped into a beaker full of petrol. I think the mechanism is that the liquid petrol extinguishes the cigarette before the vapour can ignite. However I wouldn't want to be around if someone was smoking in the presence of an explosive mixture of petrol vapour and air.
|
|
|
|
|
|
Rank: Guest
|
Posted By gham
the only gas capable of creating a flame from a glowing ember is pure oxygen of which there is not enough in the atmosphere to do this with a ciggy or they wouldn't last too long, nor would your eyebrowns if your a smoker.
The engergy in the glowing part of the ciggarette i think is too low to ignite the fuel oxygen mixture in vapour where as in a static dischards although smaller that a fag end has all it's energy concentrated at the point of discharge.
But the conditions have to be exactly righ for this to happen, I'll go back to my books and figure it out for sure unless some kind persons puts us right before then
|
|
|
|
|
|
Rank: Guest
|
Posted By JM82
I'd use a lighter personally...
|
|
|
|
|
|
Rank: Guest
|
Posted By Chris Packham
Liquid petrol does not burn. It is the vapour, mixed with oxygen, that burns. If the mixture is within the right concentration limits, then you will get an explosion.
I have never tried it with petrol, but, many more years ago than I care to remember, I taught trainee LPG fitters in Austria. We used to do a demonstration. We filled a glass tank with propane gas, then lit the surface, where there was an interface with the oxygen. (Note that a match plunged into the centre of the tank would have no effect as no oxygen was present.) You would get a layer of flame, apparently suspended in clear air. As this slowly burned down into the tank the gas and tank would warm up. The result was convection. Eventually this would create the right oxygen/gas mix and there would be a bright flash and the flame would then go out. PLEASE DON'T TRY THIS AT HOME. I DECLINE ANY RESPONSIBILITY!
|
|
|
|
|
|
Rank: Guest
|
Posted By gham
HOW BIG A TANK AND WHERE CAN I GET ONE?
|
|
|
|
|
|
Rank: Guest
|
Posted By Jeffrey Watt
I find an old SMA milk powder tin and a can of hairspray is big enough for demo purposes. SMA tin has a push fit lid that gets blown off as the explosion relief.
Also don't try taking apart those mini draught barrels that Stella Artois comes in as they are still under pressure when nominally empty. The ringing noise in my ears dissipated eventually.
|
|
|
|
|
|
Rank: Guest
|
Posted By steve e ashton
I suspect the answer to this question is connected, not with the LEL/UEL balance (somewhere in a disippating vapour cloud, you will find a zone where the vapour is between its flammable limits....)
The issue is the ignition or 'initiation' energy. Any vapour/air mix within its flammable limits)requires a source of energy to start the flame. A static discharge spark can be extremely energetic (nb not concerned with corona discharges). A cigarette end MAY not have sufficient energy - a spark is 'hotter' than a fag end.
I have never seen or tried the fag /petrol experiment. The fact that a petrol engine uses a spark to ignite the fuel / air mix, and does not normally 'run-on' when the spark is taken away (unless there's burning soot in the cylinder) suggests that the hot internal engine surfaces 'may' not ignite a petrol/air mix.
BUT - A WARNING - I do recall from NEBOSH diploma studies many many moons ago a case where a fag-end in a harbour fuel spill resulted in a fire (Any recent students with clear memories or notes confirm this?). Personally, I wouldn't risk it!
Steve
Steve
|
|
|
|
|
|
Rank: Guest
|
Posted By Incolumitas
I don't think we have to look any further than Buncefield so see what petrol vapour CAN do when ignited under optimum conditions!!
However, continuing with the car engine analogy, it can be demonstrated that those conditions need to be within a fairly narrow range. Too lean a mixture and your engine won't run, too rich and it won't run either. And that's with a high energy spark.
With a low energy dog-end the conditions are even narrower. It is most likely to ignite a "thin" spill - not too much fuel so not too much vapour at ground level. This is the sort of spill that could flash a long distance along the ground without causing a real explosion.
But I'm not going to try it at home.
|
|
|
|
|
|
Rank: Guest
|
Posted By Jeffrey Watt
Folks
Having grown up in Norn Iron I can tell you from watching the news every night from the age of 6 that the lit fag and the lit rag both have sufficient initiation energy to light a bottle of petrol in the right conditions i.e. over a Landrover.
|
|
|
|
|
|
Rank: Guest
|
Posted By Ken Taylor
The cigarette ignites the wick (rag) which then ignites the petrol vapour in air. Another reason to give up smoking!
|
|
|
|
|
|
Rank: Guest
|
Posted By David McGuire
To Jeffery, I know what you mean as I have been on the receiving end of the petrol bomb in NI many a time all good fun though when you think back, will say one thing you lot in NI have a wicked throwing arm !!!!!
|
|
|
|
|
|
Rank: Guest
|
Posted By Jeffrey Watt
David
You'd think we would be better at cricket considering all the practice that was got in.
Jeff
|
|
|
|
|
|
You cannot post new topics in this forum.
You cannot reply to topics in this forum.
You cannot delete your posts in this forum.
You cannot edit your posts in this forum.
You cannot create polls in this forum.
You cannot vote in polls in this forum.