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CXS490 - Characteristic of Fire

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Other Resources•NFPA Standards (available through Seneca

Libraries Link

•NFPA Handbook

•Manufacturer’s Technical Information

•UL listed installation and maintenance manuals

•US EPA “SNAP” list

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Fire CharacteristicsBasic Characteristics of Fires

A knowledge of the basic facts about combustion, sources of ignition and fire hazards of materials is necessary to understand the "Why and How" of suppression agents and systems.

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Theory of CombustionCombustion is basically an oxidation reaction involving an oxidizing agent (usually oxygen), and a reducing agent (fuel).

Example: Combustion of methane:

Methane Oxygen Carbon Water

Dioxide VapourCH4 + 2O2 ---> CO2 + 2H2O

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Definition - Fire

Fire is a rapid, self-sustaining oxidation process accompanied by the evolution of heat and light in varying intensities.

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Combustion - Types

The types of combustion can be separated into two types:

Flaming (including explosions)

Flameless surfaces (glow and deep seated glowing embers)

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Fire TriangleThree elements are required for combustion to take place: •something to burn (fuel),•a source of ignition (heat) •and an oxidizing agent (oxygen).

These three elements are represented by the "Fire Triangle".

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Fuel & IgnitionThe fuel can be in solid, liquid and/or gaseous form.

Sources of ignition (heat) may be:•open flame (such as a pilot flame)

•chemical (spontaneous heating),

•electrical (arcing, sparking, resistance),

•mechanical (friction).

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Fire Tetrahedron

Further studies of fire phenomena showed the need to add a fourth element to characterize a flaming fire as a series of

"uninhibited chain reactions".

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Combustible Solids

Most common combustible solids (such as wood, paper products, etc.) contain•complicated compounds of carbon and hydrogen with•other elements such as oxygen, sulphur, etc. present.

In order to burn, these complex molecules must•break down, or pyrolyze under the•influence of heat to produce simple, combustible gases and solids that will react with the oxygen in air.

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Combustible Solids

The flames that occur during the combustion of all solids are always the result of the rapid oxidation of the gases and vapour that are produced during the pyrolysis process.

The size and the shape of a solid are determining factors in the ease of ignition and the rate of burning.

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Combustible Solids - Metals

Metals require special consideration.

Some metals burn when heated to high temperature by friction or when exposed to external heat.

Others burn from contact with moisture or in reaction with other materials .

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Liquids - DefinitionsFlash Point: "the minimum temperature at which a liquid within a container gives off vapours in sufficient concentration to form an ignitable mixture with air near the surface the liquid. The flash point is a distinctive characteristic of each flammable or combustible liquid.

Fire Point: Temperature at which the vapours are emitted in sufficient quantity to continue to burn even after the source of ignition has been removed.

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Liquids - Definitions

Vapour pressure: The vapour pressure characterizes the degree to which the molecules of the liquid escape and therefore determines the amount of combustible vapour available for burning at a certain liquid temperature. The higher the evaporation rate, the higher is the vapour pressure

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Liquid - Classification

Flammable liquids: any liquid having a flash point below 37.8 C (100° F) and a vapour pressure not exceeding 275.8 kPa absolute (40 psi) at 37.8° C.

Combustible liquids: any liquid having a flash point at or above 37.8° C (100° F).

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Flammable Gases & Vapours

Except for inert gases (nitrogen, helium, carbon dioxide, etc.), most gases are capable of burning. Flammable gases and vapours are the basic elements of flames. All solid and liquids must either decompose or vapourize before combustion can take place.

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Combustible gases, vapours and dusts have limits of concentration in air below which, or above which they will not propagate or sustain flame. These limits are called lower and upper limits of combustibility or explosive limits

Flammable Gases & Vapours

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Extinguishment MethodsFour basic methods of extinguishment:

•Extinguishment by cooling

•Extinguishment by oxygen dilution

•Extinguishment by fuel removal or•separation from oxygen

•Extinguishment by chemical flame inhibition.

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Special SystemsSpecial Fire Suppression Systems

The special extinguishing systems fall into two categories:

•Water based agents, such as foams

•Non-aqueous agents such as carbon dioxide, halons and dry chemical

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Requirements• The special extinguishing systems are

mainly used for property protection.

• They are usually not required by building or fire codes.

• However, the codes require that supplementary protection be installed in accordance with

• recognized standards i.e. NFPA standards.

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ApplicationThe special extinguishing systems are used for.......hazards where extinguishment cannot be accomplished with water, e.g. flammable liquid storage tank protected by a foam system

For hazards where unacceptable damage would result from the use of water. e.g. printing press protected by carbon dioxide or computer room protected by “clean agent” system

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Application

Where a special extinguishing system would be more cost effective than water, e.g. dry chemical system for a remote outdoor power transformer.

Where water would freeze, e.g. a dry chemical system for an off-the-road vehicle.

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Application

Where sudden expansion of water from a liquid to a vapour could cause damage, e. g. a total flooding dry chemical system for a dog house (a unit used to heat a flammable oil used for heat transfer in a refining process)

Where water would react with the burning material, e.g. dry power system for a combustible metal such as sodium.

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Common Characteristics• They have a fixed supply of extinguishing agent

based on specific design for a determined hazard.

• They are designed to extinguish fire in specific hazards

• The agent must be applied simultaneously to all hazards which could be involved in fire as the result of a single incident

• The agent must be applied at a sufficient rate to achieve an adequate concentration by volume (total flooding) or quantity per area (local application)

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Components

Automatic special extinguishing systems consist of theses groups of components:

•Detection and control equipment•Agent release device (s)•Agent storage container (s)•Agent distribution (pipe and nozzles)•Ancillary devices (door closures, damper releases, etc.)

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Application Methods

Three basic methods are used to apply these extinguishing systems

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Total Flooding Systems• sufficient quantity of extinguishant is

discharged into• an enclosure to provide a• uniform fire extinguishing concentration of

agent throughout the entire enclosure.• openings in the enclosure can sometimes be

compensated for by adding extinguishing agent,

• most often, by providing automatic closure devices controlled by the fire protection system.

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Local Application Systems

• agent is discharged directly onto the burning metal or object

• without relying on an enclosure to retain the agent

• typically used to protect printing presses, dip and quench tanks, oil filled electric transformers, etc.

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Hand Hose Line Systems• consist of a supply of extinguishing agent• with one or more hand hose lines• to allow manual delivery of the agent to the

fire• are connected to the agent container either• directly or• by means of intermediate piping.

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Design of SystemsAn engineered system is one in which individual calculations or engineering by the selling company and design are required to determine the agent flow rate, the size of piping., nozzle pressures, etc.

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Design of Systems

A pre-engineered system, sometimes called a package (kit) system is one in which minimum and maximum parameters have beenpre-determined engineered by manufacturerand confirmed by a third party testing agency. Installation within the listed limits assure adequate flow rate, pressure and pattern coverage without requiring individual calculations.

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ReliabilitySprinkler systems 95%Criteria:

• Control of Fire• Most fails due to shutoff 2.5% (i.e. 50%)

Special Extinguishing 60% Criteria:

• Extinguishment of Fire• Containment issues• Room changes• Changes in fuel• Others - Discussion?