4 replies to this topic

[NoobMi]

I am quite confused over how fire starts. Hope someone will be able to give me some advice/directions.

Provided u have all 3 things in the fire triangle:

For a flammable vapor, the condition for fire to start will depends on its LEL and UEL, where LEL<x<UEL will fulfil the requirements for fire

So what about the conditions required for flammable liquid to catch fire? Is it the heat energy required to be provided to the liquid (is there a term for this)?
i.e. if u have a flammable liquid but its vapor is below LEL is it still ignitable?

[latexman]

"So what about the conditions required for flammable liquid to catch fire?"

The vapor from the flammable liquid burns, not the liquid itself. Below LEL, the vapors will not burn.

[Steve Hall]

Some materials can detonate on their own, without the need for external oxygen, but I think you are asking about commonly encountered liquids that have a flash point.

What this means is that in the vapor above the liquid level the concentrations of oxygen and flammable material are within the fire triangle. If you start a pool of flammable liquid on fire what you are really doing is vaporizing liquid into the air and that is what is burning. Examine a candle flame to see this is true: a pool of liquid wax sits at the bottom of the wick which transports it up to the flame; this is where the wax vaporizes and burns. There is a gap between the flame and the liquid pool.

The temperature of the liquid determines the vapor concentration above the liquid. For a closed-cup flash point, standard vapor-liquid equilibria apply and the temperature that results in the LEL concetration is what's needed to result in a flammable (or, technically "inflammable") condition. Open cup means that some of the vapors can dissipate so the temperature needs to be a bit higher to achieve the LEL concentration. These are standardized test conditions. In real life the actual temperature needed is likely to be even higher than the open cup temperature, but for engineering purposes you should use the most conservative value which is the closed cup test.

[kkala]

A few more notes in addition to the useful posts already sent:
1. For a flammable vapor, the condition for fire to start will depends on its LEL and UEL, where LEL<x<UEL will fulfil the requirements for fire.
The vapor can catch fire, if its concentration in atmospheric air is between LEL (low explosion limit) and UEL (upper explosion limit), as you said. This is a possible, not certain event. Fire shall be created as soon as the vapor meets an ignition source (e.g. flame, electric spark).
I think above refers to vapor - air mixtures of approximately atmospheric pressure and ambient temperature.
2. So what about the conditions required for flammable liquid to catch fire? Is it the heat energy required to be provided to the liquid (is there a term for this)? i.e. if u have a flammable liquid but its vapor is below LEL is it still ignitable?
No, the liquid will not catch fire (even through an ignition source) as long as its vapor pressure is low enough, that is its temperature remains below its flash point. See previous post of Steve Hall.
But any flammable substance at its own autoignition temperature (or higher) shall spontaneously get fire, as soon as it contacts air. Autoignition temperature of a flammable liquid is higher than flash point temperature (=lowest temperature that the liquid can catch fire).
The heat energy required for the fire to start is the activation energy for the combustion.
Note: For instance, naphtha roughly has -21 oC flash and 225 oC autoignition points. It can have a temperature well above 225 oC in hydro desulfurization, any such leakage to air means fire. Worst refinery local accident occurred when naphtha vapors (apparently below 225 oC) excaped to atmosphere and were ignited in a few seconds (1992, 14 deaths, http://www.medbc.com/annals/review/vol_7/num_1/text/vol7n1p36.htm).

Edited by kkala, 18 September 2012 - 11:25 AM.

[eurekaignem]

Allow me to address the questions...although I apologize for arriving so late to this thread. I only just registered moments ago.

*** begin questions***
I am quite confused over how fire starts. Hope someone will be able to give me some advice/directions.
Provided u have all 3 things in the fire triangle:
vapor phase: or a flammable vapor, the condition for fire to start will depends on its LEL and UEL, where LEL<x<UEL will fulfil the
requirements for fire
So what about the conditions required for flammable liquid to catch fire? Is it the heat energy required to be provided to the liquid (is
there a term for this)?
i.e. if u have a flammable liquid but its vapor is below LEL is it still ignitable?
****end questions****

In the real world, the answer to this question is "YES." In the real world, either at the liquid surface or as some distance above the liquid surface, there will be enough vapor present to sustain burning.


For a liquid to qualify for the name "flammable liquid" that liquid will display high vapor pressures which in effect produces its low flash point. In the real world, if we measured the vapor concentration at 2 meters above the surface of a flammable liquid, we may find that the vapor concentration is < LEL. But at the surface of a flammable liquid, the likelihood is that the vapors are present in sufficient (or excessive [which would be concentrations > UEL/UFL]) quantity to support combustion for most normally conditions exhibited on this planet. And in the parenthetical case above, even if vapors were present at levels > UEL/UFL, at some point above the liquid surface these concentrations of vapor fall under the UEL/UFL, and at that point we could get ignition as well.

Now, where this ignition source comes from is a heretofore unaddressed topic. In the real world, for outdoor tanks, the ignition source is often lightning, or man-mad hot work. In the real world, a lightning strike brings enough energy to the initiating event to vaporize a flammable liquid, in the unlikely event that vapor was < LFL/LEL. Different sources of ignition effect the ability of the same vapor concentrations to ignite, in different manners: ignition of flammable liquid vapors is rather easy with piloted or flame ignition; these same vapors are more difficult to ignite with purely radiative sources.

The long and short of it is, if you have a flammable liquid stored in a closed container where oxygen and an ignition source can not reach it, the flammable liquid is safe, but probably not as safe as you think. Sure, it is safe as long as the storage container is closed [and no static electrical charges arc], but history has shown there is a valid reason for categorizing and identifying liquids as "flammable". The history shows that these flammable liquids are relatively easy to ignite, and due care should be taken in providing fire/explosion protection for them.

Regarding the 2nd-to-last question: heat energy required to ignite a flammable liquid. Yes, if you had the flammable liquid cold enough that the vapor at or near the surface was < LEL/LFL, then yes, we would need to add heat to the liquid until such point that vaporization rate increased to a level > LEL/LFL. Large, impulsive heat inputs accomplish this necessary heating and vaporization instantly (e.g. lightning or electrical arc); slow heat inputs could create vapor > LEL very slowly (e.g. average temperatures warming from winter into spring time).


Scot Deal
Excelsior Fire/Risk Engineering
eurekaignem@gmail.com