K3L Bab#6 Fire and Explosion Hazards

transcript

Yulianto S Nugroho dan Tim

Fire and Explosion Hazards

Prof. Yulianto S Nugroho dan TimDepartment of Mechanical Engineering University of Indonesia

Flashover

©Dr. Ir. Yulianto S Nugroho, MSc3

Outline of talk

• Introduction• Heat of combustion and heat transfer• Fire triangle and tetrahedron• Fuel• Flammable limits• Ignition sources• Flame spread and flash over• Explosion hazards• Product of combustion• Fire protection systems

Reference :Charles A. Wentz, Safety, Health and Environmental Protection, MGH, 1998.

Introduction

Fire, which is a combustion process, is a complex chemical reaction between fuel, oxygen, and ignition sources. This exothermic reaction could involve a variety of fuels (solid, liquid or gas). The liquid and solid fuels normally vaporise before burning. These gases and vapor mix with oxygen or air to form the ignitable mixture. As these fuels oxidised there is an emission of heat. If the combustion process results in a rapid pressure rise, an explosion may occur.

Heat of combustion

Generally, hydrocarbon gases like methane and ethane have higher heats of combustion than coal or hydrocarbon liquids. The ratio of hydrogen tocarbon in the molecules is the critical factor in fuel’s heat content. The higher H/C ratio, the higher the heat content of the substance.

Heat transfer

There are three heat transfer mechanism.Heat loss by the conduction mechanism will increase proportionally with the refractory surface and the incineration temperature. Heat transfer by convection is related to the properties of the convection medium and the geometry of the system.Radiation is the most important mechanism for heat transfer in a fire.

Fire triangle

HeatFuel

Oxygen

The three elements of fuel, oxygen, and ignition are necessary to ignite ordinary burning and fires.

The fire triangle concept is also used to prevent fires or control them once they have begun.

Fire tetrahedron

The fire tetrahedron adds a forth component-chemical chain reaction - as a necessity in the prevention and control of fires. The chemical chain reaction involves free radicals, which are important intermediates in the initiation and propagation of combustion reaction. The removal of of any one of the for faces on the left diagram results in fire extinction.

FuelOxygen Chain

reaction

The flammability of hydrocarbon is greatly affected by substitution of, particularly, halogen for hydrogen.

For example, clorine atoms in hydrocarbons greatly reduce the flammability of the substance by interfering with the combustion chemical chain reaction.

The flash point is the minimum temperature that a liquid will give off sufficient vapor to ignite with oxygen in air.

Flammable and combustible liquids are characterised by flash points and boiling points.

Flammable limits : measurement

When gases or vapors form flammable mixtures with air or oxygen, there is a minimum concentration of gas or vapor below which the propagation of flame does not occur on contact with a source of ignition. In the measurement, aixture of gas and air is introduced to the tube. Then, an ignition source is introduced to lower end of the tube which is first opened by the removal of a cover plate. A mixture below the lower flammable limit (LFL) is too lean to burn or explode. A mixture above the upper flammable limit (UFL) is too rich to burn or explode.

Flammable limits (cont) : mixture

The LFL and UFL for a vapor mixture can be determined from the individual components of the mixture.The LFL of a flammable vapor can be estimated from its vapor pressure at its flash point at atmospheric pressure.

The Antoine equation can be useful in estimating the vapor pressure as a function of temperature.

Flammable limits (cont) : data

Flammable limits (cont) : MIE

Very small amounts of energy are sufficient to ignite flammable vapor/air mixture. The minimum of the left curve is known as the Minimum ignition energy (MIE).

Effect of temperature on the limits of flammability of a flammable vapour/air mixture at a constant initial pressure.

Flammable limits (cont) : diagram

Ignition sources

• Electric sparks• Smoking and matches• Frictional heat• Hot surfaces• Overheated materials• Open flames• Spontaneous heating• Welding and cutting• Combustion particles

RedFlammability

0 - 4Yellow

Reactivity0 - 4

White Special

BlueHealth0 - 4

NFPA 704 hazard identification

Flame spread : burning velocity

Propagation of premixed flame through a flammable mixture in a duct following ignition () at the closed end.

Flame spread

Flash over

EXPLOSION

Definition: A reaction that produces a change in the state of matter that results in a rapid and violent release of energy.

Types: - Mechanical- Chemical- Nuclear

Explosive

Definition: A material (chemical or nuclear) that can be initiated to undergo a very rapid, self propagating decomposition, resulting in:

a. formation of more stable materials b. the liberation of heat c. development of a sudden pressure effect.

Materials in the form of compound or mixture of compound which suddenly undergoes a very rapid chemical transformation with the simultaneous production of large quantities of heat and gases (CO, CO2, N2, steam, O2) and always accompined by a vigoros shock and an associated noise (brisance)

Explosion

Explosion: A very sudden release of energy resulting in a shock or pressure wave.

Shock, Blast or pressure wave: Pressure wave that causes damage.

Deflagration: Reaction wave speed < speed of sound.

Detonation: Reaction wave speed > speed of sound.

Speed of sound: 344 m/s, 1129 ft/s at ambient T, P.

Deflagrations are the usual case with explosions involving flammable materials.

Dust Explosion

Dusts of most combustible solids are an explosion hazard

Sufficiently small particle size Sufficient concentration dispersed in air Ignition source Secondary explosions often exceed initial explosion

The origin od Explosions

Sir Cyril Norman Hinshelwood (English, 1897-1967)

Investigation (1927) of the H2O2 reaction.discovery of the 1st and 2nd explosion limits

First experimental proof:Nikolay Nikolaevich Semenov (Russian, 1896-1986)Investigation (1926) of the phosphorus vapouroxygen reaction.Explosion occurs, if the partial pressure of O2 is between two limits. Interpretation via a branching chain reaction.

The Nobel Prize in Chemistry 1956: Semenov and Hinshelwood: "for their researches into the mechanism of chemical reactions"

Chain reaction

Chain carriers (also called chain centres, i.e. reactive intermediates) are generated in the initiation steps.

In the chain propagation steps the chain carriers react with the reactants, produce products and regenerate the chain carriers.

In the inhibition step the chain carriers react with the product, reactants are reformed, and there is no reduction in the number of chain carriers.

In the branching step two or more chain carriers are produced from a single chain carrier.

In the termination steps the chain carriers are consumed.

BLEVE: Boiling Liquid Expanding Vapor Explosion

Liquid

VaporVessel with liquid stored below its normal boiling point

Below liquid level - liquid keeps metal walls cool.

Above liquid level - metal walls overheat and lose strength.

Explosion

Fire radiation hazard

Blast wave

Fire radiation hazard

Product of combustion / fires

Air pollution caused by forest fires

Fire protection systems : fire action

Fire protection systems : fire extinguisher

K3L Bab#6 Fire and Explosion Hazards

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