Fire Understanding

The Phenomenon of Fire

Fire is the visible heat energy being released from rapid oxidation of afuel.  Something is
'on fire' when the exothermal release of heat from the oxidation reactionreaches visible light level.

Basically, combustion takes place within two modes-flaming andflameless. The flaming mode can be viewed as a tetrahedron, with each siderepresenting one of the four basic
components of flaming combusion: fuel, heat, oxygen and uninhibited chainreaction. Here the practical emphasis is on extinguishing an existingfire.

The classic fire triangle illustrates the flameless mode, the three legsrepresenting fuel, oxygen and heat. Air provides oxygen forcombustion. The heat required for ignition varies with the characteristicsof the fuel. Fuel is any substance that will sustain combusion after theinitial application of heat to start it. Here the practical emphasis is onpreventing a fire from starting by prohibiting the formation of thetriangle.    

How Flammable Liquids Safety Equipment Defeats Fire

Safety equipment for use in storing, transporting or dispensingflammable liquids is designed to control one or more of the legs of the FireTriangle.

Containment of the liquid fuel to prevent it from spreading in event offire is a primary function of all safety containers. These containers includesafety cans, rinse and wash tanks, bench and plunger cans, rinse and washtanks, bench and plunger cans, drip cans, storage cabinets, waste containersand others.

Dissipation of heat to prevent flammable liquid vapor from reachingignition temperature is another function built into certain types of safetyequipment. This is accomplised by the flame arrester which is common tosafety cans and faucets, bench cans and other equipment. The flame arrester, inthe form of a wire mesh screen or perforated baffle plate, permits escapingvapor to burn but dissipates heat so that vapor inside the container will notignite or explode.

Closing out oxygen is still another function of certain safetycontainers. For example, when the lids of self-closing rinse and cleaningtanks shut, they snuff out fire by closing off the oxygen supply.

Characteristics of Flammable Liquids

In order to best understand the hazards of flammable liquids and thecontrol procedures and to interpret the tabular material, the followingdefinitions will prove useful.

Underwriters Laboratories, Inc., Flammability Hazard Classification.

In addition to the NFPA, this is also a system for grading the relativeflammability hazards of various liquids. This is a useful classificationbecause actual hazard in use of a flammable liquid is dependent on a number offactors in addition to flash point. These are the UL classes of relativeflammability hazards:

Ether : class 100 (max hazard)
Gasoline : class 90-100
Alcohol (ethyl) : class 60-70
Kerosene  : class 30-40
Paraffin Oil : class 10-20

Classifications, Flammable and Combustible Liquids.  

A flammable liquid is one having a flash point below 100 degree F andhaving a vapor pressure not exceeding 40 lb/sq in. absolute at 100 degree F. Acombustible liquid is one having a flash point at or above 100 degree F.

Vapor. In any liquid there is a constant movement ofmolecules. As temperature increases, the molecules speed up, someacquiring enough energy to escape from the liquid surface as a vapor.When vaporescapes from a flammable liquid into the air, a flammable or explosivesituation can occur, dependent upon the proportions of the air/vapor mixture.

Flash point is the lowest temperature of a flammable liquid at which itgives off vapor sufficient to form an ignitable mixture with the air near thesurface of the liquid or within the vessel used. Combusion is not continuous atthe flash point. Flash point is the basic characteristic used by NFPA toclassify the relative hazards of liquids.

Ignition temperature is the minimum temperature to which flammableliquid vapor in air must be heated in order to initiate or cause self-sustainedcombustion independently of the original heat source.One might tend to discountthe high hazards of flammable liquids when noting the relatively high ignitiontemperatures (gasolines, for example, are in the 500 to 800 degree Frange). But what must be recognised is that an extremely small area andduration of temperature contact is all that's needed to set flammable vaporaflame. A static spark with the duration of a few thousandths of a second,contacting a few molecules of the vapor/air mixture is enough to raise thetemperatures above the ignition point.

Flammable (explosive) range of flammable liquids is the percentage rangeof liquid vapor in air, by volume, within which ignition canoccur. Gasoline, for example, has an explosive range between 1.4% and7.6%. This indicates that any concentration of gasoline vapor in airbetween these percentage limits will ignite at any temperature above -45 degreeF (flash point) when an ignition source provides a contact temperature in therange of 500 degree to 800 degree F (ignition temperature, depending on type ofgasoline). Auto engine carburetion is designed to keep the gas/air mixturefed to the cylinders within this explosive range. Too 'lean' a mixture (near orbelow 1.4%) or too 'rich' (at or above 7.6%) causes engine firing failure orfaulty operation because the gasoline vapor/air mixture is at the limits of theexplosive range.

Explosive range figures are based on normal atmospheric temperatures andpressures.  There may be a considerable variation in explosive range whereother temperatures and pressure are present. Increases in temperature willwiden the explosive range.  Pressure differences depend on theflammable liquid involved but substantial vacuum (pressure decrease) willgenerally narrow the explosive range.

Specific gravity of flammable liquids is important in fire preventionplanning to anticipate behavior of hazardous materials where water or otherliquids are present under fire conditions. Many flammable liquids withspecific gravity below 1 (lighter than water) are also insoluble inwater. In the event of fire with such liquids present, water may beineffective as an extinguishing agent.

Water solubility refers to the degree to which a flammable liquid issoluble in water. This is useful in determining effective extinguishingagents and methods. Alcohol-resistant type foam, for example, is usuallyrecommended for water soluble flammable liquids.

Vapor pressure of liquids is below 40 pounds per square inch absolute,at 100 degree F, by definition. Materials with higher vapor pressures aredefined as gases at 100 degree F to establish a convenient technical separationbetween the two forms of matter. Vapor pressure is the pressure exerted byvapor above the surface of a liquid in a closed container. It is caused byevaporation and is stabilized by confinement in a closed container to apressure characteristic of the specific liquid.As temperature increases, thevapor pressure of a liquid increases. At the point where the vaporpressure equals atmospheric pressure, the escape of molecules from the liquidsurface is greatly accelerated and boiling takes place.

Vapor pressures of flammable liquids are an important consideration infire prevention.They give the relative speed of evaporation: the higher thevapor pressure, the greater the evaporation rate and the more vapor escapepotential every time a safety container is opened.

Boiling point of a liquid is the temperature of the liquid at which itsvapor pressure equals the atmospheric pressure.

Vapor density, as commonly used in fire protection, is the weight of avolume of pure gas compared to the weight of an equal of volume of dry air atthe same temperature pressure.  A figure greater than 1 indicates that agas is heavier than air. This means that any escaped vapor will settledownward onto floors and flow with air currents, around corners and down stairsor elevator shafts to pool in low spots. If the source liquid is open and acontinuous supply of vapor is flowing, a spark anywhere along the vapor trail -perhaps hundreds of feet or several floors away - will set off an explosion andfire that may envelop an entire building almost instantly.