DRY CHEMICAL EXTINGUISHING

 

OVERVIEW OF DRY CHEMICAL EXTINGUISHING AGENTS

 Dry chemical extinguishing agents are commonly listed for use on Class B and Class C fires. Dry chemical extinguishing agents are not effective on Class D fires. Multipurpose dry chemical because they are listed for use on Class A, B, and C fires.

The principal base chemicals used in the production of currently dry chemical extinguishing agents are sodium bicarbonate, potassium chloride, urea potassium bicarbonate, and mono-ammonium phosphate.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be non toxic. However, other physiological conditions such as minor skin and respiratory irritations may occur on exposure to the agents.

Toxicity.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be nontoxic and noncarcinogenic. However, other physiological conditions such as minor skin and respiratory irritations.

Extinguishing Properties

Smothering, cooling and radiation shielding contribute to the extinguishing efficiency of dry chemicals .but studies suggest that a chain-breaking reaction in the flame is the principal cause of extinguishment.

Cooling

Cooling action of the dry chemical cannot be substantiated as an important reason fot its ability to promptly extinguish fires. To be effective, any dry chemical must be heat sensitive and, as such, absorb heat in order to become chemically active.

Radiation Shielding.

Discharge of dry chemical produces a cloud of powder between the flame and the fuel; this cloud shields the fuel from some of the heat radiated by the flame.

Smothering Action

For special applications, such as kitchen range, hood, duct and fryer fire protection, the extinguishing mechanism for dry chemical is based on the process of saponification. Saponification is the process of chemically converting the fatty acid contained in the cooking medium to soap, or foam, and it accomplishes extinguishment by forming a surface coating that smoothers the fire.

Chain-Breaking Reaction.

The discharge of dry chemical into the flames prevents reactive particles from coming together and continuing the combustion chain reaction. The explanation is referred to as the chain-breaking mechanism of extinguishment.

Fixed systems are of two types:

• Total Flooding
• Hand Hose line/ Local Application

Uses and Limitations of Dry Chemical Systems

• Due to the rapidity with which dry chemical extinguishes flame, dry chemical is used on surface fires involving ordinary combustible materials (Class A Fires).
• There are several areas in the textile industry, notably opener-picker rooms and carding rooms in cotton mills, where regular dry chemical has been used effectively.
• Multipurpose dry chemicals  becomes sticky when heated, it is not recommended for textile card rooms or other locations where removal of the residue from fine machine parts may be difficult.
• Dry chemicals should not be used in installations where relays and delicate electrical contacts are located, as the insulating properties of dry chemical might render such equipment inoperative.
• Dry chemical extinguishing systems can be used in those situations where quick extinguishment is desired and where re-ignition sources are not present.
• Regular dry chemical will not extinguish fires that penetrate beneath the surface or fires in materials that supply their own oxygen for combustion.

In total flooding applications, a predetermined amount of dry chemical is discharged through fixed piping and nozzles into an enclosed space or enclosure around the hazard.

Hand hose line systems containing regular or ordinary dry chemical have been used to a limited extent for quick spreading surface fires on ordinary combustible material.