1. Fire Hazard What does Fire Hazards mean? Fire hazards include:
Fire hazards involve the presence of flame or the risk of an uncontrolled fire.
Fire hazards include:
Live flames
Sparks
Hot objects
Flammable chemicals
Chemicals that can aggravate a fire The Fire Triangle
The essential elements for combustion are fuel, an oxidizer, and an ignition source.

2. Various fuels, oxidizers, and ignition sources commonly seen in the chemical industry are:
Gases: acetylene, propane, carbon monoxide, hydrogen
Liquids: gasoline, acetone, ether, pentane
Solids: plastics, wood dust, fibers, metal particles
Oxidizers
Gases: oxygen, fluorine, chlorine
Liquids: hydrogen peroxide, nitric acid, perchloric acid
Solids: metal peroxides, ammonium nitrite
Ignition sources
Sparks, flames, static electricity, heat

3. Types of Fire
Class A fires involve solid materials of an organic nature such as wood, paper, cloth, rubber and plastics that do not melt.
Class B fires involves liquids. They include petrol, diesel, thinners, oils, paints, wax, cooking fat and plastics that melt.
Class C fires involve electricity.
Class D fires involve flammable metals such as magnesium, aluminum, titanium, sodium and potassium.
Fires that involve cooking oils or fats are designated “Class K” under the American system, and “Class F” under the European/Australian systems.

4. Distinction between Fire and Explosion
The major distinction between fire and explosion is the rate of energy release.
Fire release energy slowly, whereas explosion release energy rapidly, typically on the order of microseconds.
Fire can also result from explosion, and explosion can result from fire.
A good example of how the energy release rate affects the consequences of an accident is a standard automobile tire.
The compressed air within the tire contains energy.
If the energy is released slowly through the nozzle, the tire is harmlessly deflated. If the tire ruptures suddenly and all the energy within the compressed tire releases rapidly, the result is a dangerous explosion.

5. Definitions:
Combustion or fire: Combustion or fire is a chemical reaction in which a substance combines with an oxidant and releases energy. Part of the energy released is used to sustain the reaction.
Ignition: Ignition of a flammable mixture may be caused by a flammable mixture coming in contact with a source of ignition with sufficient energy or the gas reaching a temperature high enough to cause the gas to auto-ignite.
Flash point (FP): The flash point of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air. At the flash point the vapor will burn but only briefly; inadequate vapor is produced to maintain combustion.
Fire point: The fire point is the lowest temperature at which a vapor above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.

6. 5) Flammability limits: Vapor-air mixtures will ignite and burn only over a well-specified range of compositions.
The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion.
The mixture is also not combustible when the composition is too rich; that is, when it is above the upper flammable limit (UFL).
A mixture is flammable only when the composition is between the LFL and the UFL.
Lower explosion limit (LEL) and upper explosion limit (UEL) are used interchangeably with LFL and UFL.
6) Explosion: An explosion is a rapid expansion of gases resulting in a rapidly moving pressure or shock wave. The expansion can be mechanical (by means of a sudden rupture of a pressurized vessel), or it can be the result of a rapid chemical reaction. Explosion damage is caused by the pressure or shock wave.

7. 7) Mechanical explosion: An explosion resulting from the sudden failure of a vessel containing high-pressure nonreactive gas. 8) Deflagration: An explosion in which the reaction front moves at a speed less than the speed of sound in the unreacted medium. 9) Detonation: An explosion in which the reaction front moves at a speed greater than the speed of sound in the unreacted medium. 10) Confined explosion: An explosion occurring within a vessel or a building. These are most common and usually result in injury to the building inhabitants and extensive damage. 11) Unconfined explosion: Unconfined explosions occur in the open. This type of explosion is usually the result of a flammable gas release. The gas is dispersed and mixed with air until it comes in contact with an ignition source 12) Shock wave: An abrupt pressure wave moving through a gas. A shock wave in open air is followed by a strong wind; the combined shock wave and wind is called a blast wave. The pressure increase in the shock wave is so rapid that the process is mostly adiabatic.

8. Ignition Energy:
The minimum ignition energy (MIE) is the minimum energy input required to initiate combustion. All flammable materials (including dust) have MIEs.
The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature.
The MIE of dusts is, in general, somewhat higher than combustible gases, and an increase in the nitrogen concentration increases the MIE.
Autoignition:
The Autoignition Temperature (AIT) of a vapor, sometimes called the spontaneous ignition temperature (SIT), is the temperature at which the vapor ignites spontaneously from the energy of the environment.
AIT is a function of the concentration of vapor, volume of vapor, pressure of the system, presence of catalytic material, and flow conditions.

9. Explosions
Explosion behavior depends on a large number of parameters like:
Ambient temperature
Ambient pressure
Composition of explosive material
Physical properties of explosive material
Nature of ignition source: type, energy, and duration
Geometry of surroundings: confined or unconfined
Amount of combustible material
Turbulence of combustible material
Concepts to Prevent Fires and Explosions
A twofold strategy is used to limit the potential damage from fires and explosions:
prevent the initiation of the fire or explosion
minimize the damage after a fire or explosion has occurred.

10. How to prevent fires and explosion
1) Inerting:
Inerting is the process of adding an inert gas to a combustible mixture to reduce the concentration of oxygen below the limiting oxygen concentration (LOC).
The inert gas is usually nitrogen or carbon dioxide, although steam is sometimes used.
Inerting begins with an initial purge of the vessel with inert gas to bring the oxygen concentration down to safe concentrations.
A commonly used control point is 4% below the LOC, that is, 6% oxygen if the LOC is 10%.
After the empty vessel has been inerted, the flammable material is charged.

11. 2) Using the Flammability Diagram To Avoid Flammable Atmospheres
A general way to represent the flammability of a gas or vapor is done with the help of flammability diagram.
Concentrations of fuel, oxygen, and inert material (in volume or mole %) are plotted on the three axes.
Each apex of the triangle represents either 100% fuel, oxygen, or nitrogen.
The tick marks on the scales show the direction in which the scale moves across the figure. Thus point (A) represents a mixture composed of 60% methane, 20% oxygen, and 20% nitrogen.
The zone enclosed by the dashed line represents all mixtures that are flammable. Because point (A) lies outside the flammable zone, a mixture of this composition is not flammable.

13. The air line represents all possible combinations of fuel plus air
The air line represents all possible combinations of fuel plus air. The air line extends from the point where fuel is 0%, oxygen is 21% and nitrogen is 79% to the point where fuel is 100%, oxygen is 0% and nitrogen is 0%.
The stoichiometric line represents all stoichiometric combinations of fuel plus oxygen. The combustion reaction can be written in the form:
Fuel + z O2  combustion products (where z is the stoichiometric coefficient for oxygen)
The stoichiometric line extends from a point where the fuel is 100/(1 + z), oxygen is 100z/(1 + z) and nitrogen is 0%, to a point where fuel is 0%, oxygen is 0% and nitrogen is 100%.
The shape and size of the flammability zone on a flammability diagram change with a number of parameters, including fuel type, temperature, pressure, and inert species.
Thus the flammability limits and the LOC also change with these parameters.

14. How to understand the flammability diagram:
If two gas mixtures R and S are combined, the resulting mixture composition lies on a line connecting the points R and S on the flammability diagram. The location of the final mixture on the straight line depends on the relative moles in the mixtures combined: If mixture S has more moles, the final mixture point will lie closer to point S.
The LOC can be estimated by reading the oxygen concentration at the intersection of the stoichiometric line and a horizontal line drawn through the LFL

15. The flammability diagram is useful for tracking the gas composition during a process operation to determine whether a flammable mixture exists during the procedure.
For example, consider a storage vessel containing pure methane whose inside walls must be inspected as part of its periodic maintenance procedure.
For this operation the methane must be removed from the vessel and replaced by air for the inspection workers to breathe.
The first step in the procedure is to depressurize the vessel to atmospheric pressure. At this point the vessel contains 100% methane, represented by point A in Figure 6-7. If the vessel is opened and air is allowed to enter, the composition of gas within the vessel will follow the air line in Figure 6-7 until the vessel gas composition eventually reaches point B, pure air.

17. Note that at some point in this operation the gas composition passes through the flammability zone.
If an ignition source of sufficient strength were present, then a fire or explosion would result.
The elimination of ignition sources alone is not enough to prevent fires and explosions; ignition sources are too plentiful to use as the primary prevention mechanism.
A more robust design is to prevent the existence of flammable mixtures as the primary control, followed by the elimination of ignition sources as a secondary control.
The flammability diagram is important for determining whether a flammable mixture exists and for providing target concentrations for inerting and purging procedures.
So, the objective is to avoid the flammable region.

18. 3) Static Electricity
A common ignition source within chemical plants is sparks resulting from static charge buildup and sudden discharge.
Static electricity is perhaps the most indefinable sources of ignitions.
Despite considerable efforts, serious explosions and fires caused by static ignition continue to plague the chemical process industry.
A charged object must be discharged to a ground or to an oppositely charged object
4) Ventilation
Proper ventilation is another method used to prevent fires and explosions.
The purpose of ventilation is to dilute the explosive vapors with air to prevent explosion and to confine the hazardous flammable mixtures.
Open-Air Plants
Open-air plants are recommended because the average wind velocities are high enough to safely dilute volatile chemical leaks that may exist within a plant.
Although safety precautions are always practiced to minimize leaks, accidental releases from pump seals and other potential release points take place.
Plants Inside Buildings
Frequently, processes cannot be constructed outside. In this case local and dilution ventilation systems are required.

19. 5) Sprinkler Systems
Sprinkler systems are an effective way to contain fires.
The system consists of an array of sprinkler heads connected to a water supply.
The heads are mounted in a high location (usually near ceilings) and disperse a fine spray of water over an area when activated.
The heads are activated by a variety of methods.
A common approach activates the heads individually by the melting of a fusible link holding a plug in the head assembly.
Once activated, the sprinklers cannot be turned off unless the main water supply is stopped.
These systems are used for storage areas, laboratories, control rooms, and small pilot areas.

20. Types of Fire Extinguishers