Fire Behavior

OBJECTIVES:

Upon completion of this lesson, you will be  able to:

1. Describe the phases of fire and explain each phase of fire.
2. Define the flashover, its characteristics/ symptoms and the preventive measures.
3. Define the phenomenon of Back draft characteristics/ symptoms of Back draft and its preventive measures.
4. Describe the key terms related to “BLEVE”.
5. Define BLEVE
6. Describe the main characteristics of “BLEVE”.
7. List the Safety measures to prevent/ avoid BLEVE.

Fire Behavior | Phases of Fire | Flashover | Backdraft | BLEVE

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1. Phases of Fire:

There are four phases of fire according to Its magnitude and development:

1. Ignition.
2. Growth.
3. Fully Developed.
4. Decay.

1.1. Description of Phases of Fire:

 

1.1.1 Ignition Phase:

The phase of fire development in which the fire limited to the immediate point of origin.

1.2 Growth Phase:

The phase of fire development in which the fire starts spreading beyond the point of origin and beginning to involve other fuels in the adjoining area.

1.3 Fully Developed Phase:

The phase of fire development in which all available and heat is produced at the maximum rate.

1.4 Decay Phase:

In the development phase, fire has consumed either the available fuel or oxygen and starts to come down and ultimately extinguishes.

2. Flashover:

The simultaneous ignition of all combustible materials in an
enclosed area when the surfaces in a space are heated to the
temperature at which the flammable gases are hot enough to ignite.

2.1 Flashover Temperature:

Flashover normally occurs at 500 C (930 F) OR 1100 F for ordinary combustibles.

2.2 characteristics/symptoms of Flash-over:

2.3 Sequence of a Room Fire

Room fire at 5 minutes is still limited to paper in wastebasket; HRR 100 kW.

Fire at 7 minutes has spread to draperies by direct flame contact and radiant heating from sofa. HRR increasing very rapidly as vertical flame spread dominates; HRR _ 1,000 kW (1MW).

Steps and Phases of Flashover in Room.

Fire at 9 minutes. Radiant heat ignition of tops of furniture, and the flames in the smoke layer become continuous (rollover); HRR _ 2.5 MW.

Fire at flashover transition (10.75 minutes). All contents are on fire, and there is a large plume exterior to room; HRR _ 5.2 MW.

2.4 Preventive Measures:

• Use of appropriate PPE.
• Careful entry (Sitting beside the door instead of in front of it).
• To be ready to fight shooting flames.

Back draft:

A situation where fire has burned out all the Oxygen in a room, leaving only fuel, gases and smoke at a high temperature. Reintroduction of Oxygen will restart combustion often in form of an explosion.

3.1 Characteristics of Back draft:

The  following conditions may indicate a backdraft or smoke explosion condition:

• Smoke under pressure exiting through small building openings
• Black smoke becoming dense, greyish yellow
• Confinement and excessive heat
• Little or no visible flame
• Smoke leaving the building in puffs and being drawn back in
• Smoke stained and/or rattling windows
• Muffled sounds
• Sudden, rapid movement of air and smoke inward when an opening is made
• Hot or warm outside walls with little or no fire evident

 

 

3.2 Preventive Measures of Backdraft:

• immediate Evacuation.
• Proper PPE
• Boundary Cooling.
• Careful Entry.
• Ventilation.

 

 

4. Key terms related to “BUEVE”:

4.1 Vapor Pressure-.

The pressure exerted by the gaseous form of a substance when the liquid and the vapor are in equilibrium.

4.2 Fire Balls:

Fire ball are created from the Ignition of flammable vapor clouds, Such clouds burn with great Intensity, with burmup rates measured In tones of fuel per second.

4.3 Explosions:

Means rapid or violent processes in which a large amount of energy is released. Explosions have great disaster potential and cause great loss of life and damage to property.

5. BLEVE (Boiling Liquid Expanding Vapor Explosion):

“An explosion caused by rapid expansion of flammable gas stored in a container resulting in sudden release of huge quantities of atomized burning liquid which appear as a fire ball and radiating intense heat all around”.
An explosion that occurs when a tank containing a volatile liquid is created (NFPA 2nd Edition, 2009).

6. Characteristics of BLEVE:

 

• Typically a BLEVE occurs after a metal container been heated above 538 ‘C.
• Metal Is unable to withstand the high vapor pressure.
• Most BLEVE, s occurs when containers are less than V* to 1/3 full of liquid.
• Fire balls may occur at the time of rupture.
• Fire balls result in intense heat exposure in the premises.
• Studies have shown that BLEVE occurs in 30 minutes after the fire has started.

7. Safety Measures to Avoid/ Prevent BLEVE:

The container carrying LPG must be filled more than half and ideally to its full mark.
No personnel should be allowed to enter in the premises j where a potential of BLEVE exists.
Fire fighting must be done from a safe distance of 40-50 meters, which is a minimum standard.
Foam should be used as fire fighting medium as it does smothering as well as cooling actions and tends to lower down the temperature.