Introduction:
Dry chemical is a powder mixture which is used as a fire extinguishing agent. It is intended for application by means of portable extinguishers, hand hose line systems, or fixed systems. The following types of dry chemical powder are generally used:
a) Borax and sodium bicarbonate based dry chemicals were the first such agents developed. Sodium bicarbonate became the standard because of its greater effectiveness as a fire extinguishing agent.
b) Now sodium bicarbonate base dry chemical was modified to render it compatible with protein based low expansion forms to permit a duel agent attack.
c) Multipurpose (mono-ammonium phosphate base) and "purple-K" (potassium bicarbonate base) dry chemicals then were developed for fire extinguishing use.
d) Shortly there after, "Super-K" (Potassium chloride base) was developed to equal "purple-K" in effectiveness.
Dry chemical is unusually efficient in extinguishing fires in flammable liquids. It can also be used in fires involving some types of electrical equipment. Regular dry chemical powder has certain limited applications in Extinguishment of ordinary combustibles. But multipurpose dry chemical can be used on fires in flammable liquids, fires involving energised electrical equipments, as well as, fires in ordinary combustible materials.
2.Physical Properties of Dry Chemicals:
The principal base chemicals, used in the production of currently available dry chemical extinguishing agents as already stated are sodium bicarbonate, potassium bicarbonate, potassium chloride, urea-potassium bicarbonate and mono-ammonium phosphate. Various additives are mixed with these base materials to improve their storage, flwo and water repellency characteristics. The most commonly used additives are metallic stearates, tricalcium phosphate, or silicons, which coat the particles of dry chemical to make them free flowing and resistant to the caking effects of moisture and vibration.
A) Stability:
Dry chemical is stable at both low and normal temperatures. However, since some of the additives may melt and cause sticking at higher temperatures, an upper storage temperature limit of 120°F(49°C) is recommended for dry chemical. Upto 150°F(66°C) may be acceptable for very short duration. At fire temperatures, the active ingredients either dissociate bor decompose while performing their function in fire Extinguishment. Extreme importance is the danger caused by indiscriminate mixing of various dry chemicals. For example, mixing multipurpose (mono ammonium base) dry chemical, which is acidic, with an alkaline dry chemical (most of other dry chemicals) will result in an undesirable reaction that release free carbon dioxide gas and causes caking.
b) Toxicity:
The ingredients used in dry chemical powder are non-toxic. However the discharge of large quantities may cause temporary breathing difficulty during and immediately after discharge and may seriously interfere with visibility.
c) Particle Size:
Particles of dry chemical range in size and are from less than 10 microns upto 75 microns. Particle size has a definite effect on Extinguishing efficiency, and careful control is necessary to prevent particles from exceeding the upper and lower limit of this performance range. The best results are obtained by a heterogeneous mixture with medium particles in the order of 20-25 microns.
3.Extinguishing Properties:
Fire tests on flammable liquids have shown that-
I) Potassium bicarbonate base dry chemical to be more effective than sodium bicarbonate base dry chemical in Extinguishment.
II) Similarly mono ammonium phosphate has been found equal to or better than sodium bicarbonate in Extinguishment effectiveness.
III) Urea potassium bicarbonate exhibits the greatest effectiveness of all the dry chemicals tested.
When introduced directly to the fire area, dry chemical causes the flame to go out almost at once. Smothering, cooling and radiation shielding contribute to the extinguishing efficiency of dry chemical, but studies suggest that a chain breaking reaction in the flame is the principal cause of Extinguishment.
A) Smothering Action:
For many years it was widely held that regular dry chemical extinguishing properties relied primarily on the smothering action of the carbon dioxide released when sodium bicarbonate was heated by fire. However, tests have generally disapproved the belief that these gases are a major factor in Extinguishment.
When multipurpose dry chemical is discharged into burning ordinary combustibles, the decomposed mono ammonium phosphate leaves a sticky residue (meta phosphoric acid) on the burning material. This residue seals glowing materials, from oxygen, thus helping to extinguish the fire and prevent re-ignition.
B) Cooling Action:
It can not be substantiated that the cooling action of dry chemical is an important reason for its ability to promptly extinguish fires. The heat energy required to decompose dry chemicals plays an undeniable role in contributing towards their individual extinguishing abilities. To be effective, any dry chemical power must be heat sensitive and as such absorbs heat in order to become chemically active.
C) 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 radiation by the flame.
D)Chain Breaking Reaction:
The chain reaction theory of combustion has been advanced by some investigators to provide the clue to the identity of this unknown extinguishing factor. This theory assumes that free radicals are present in the combustion zone and that the reactions of these particles with each other are necessary for continued burning. The discharge of dry chemical into the flame prevents reaction particles from coming together and continuing the combustion chain reaction. The explanation is referred to as the chain breaking mechanism of Extinguishment.
4.Uses and limitations:
Dry chemical is primarily used to extinguish flammable liquid fires. Because it is electrically non-conductive, it can be used on flammable liquid fires involving live electrical equipments. Regular dry chemical extinguishers have been tested and found suitable for use on flammable liquid and electrical fires.
Due to its rapidity with which dry chemical extinguishes flame, dry chemical is used on surface fires involving ordinary combustible materials. However, when regular dry chemical is provided for use on class A fires, it should be supplemented by water spray for extinguishing smoldering embers or in case the fire gets beneath the surface.
Dry chemical does not produce a lasting inert atmosphere above the surface of a flammable liquid; consequently, it's use will not result in permanent Extinguishment if re-ignition sources, such as hot metal surfaces, or persistent electrical arcing are present.
Dry chemical should not used in installations where relays and dedicate electrical contacts are located, e.g. in telephone exchanges and computer equipment rooms, etc since the insulating properties of dry chemical might render such equipments inoperative. Because some dry chemicals are slightly corrosive, they should be removed from all undamaged surfaces as soon as possible after fire Extinguishment.
Regular dry chemicals will not extinguish fires that penetrate beneath the surface, or fires in materials that supply their own oxygen for combustion. Dry chemical powder may be incompatible with mechanical (air) foam unless the dry chemical has been specially prepared to be reasonably foam compatible.
5.Methods of Applications:
The two basic, types of dry chemical systems are referred to as fixed systems and hand hose like systems. Other method of applying dry chemical are by portable or wheeled type extinguishers.
A) Fixed Systems:
Fixed dry chemical systems consists of supply or dry chemical, an expellant gas, an actuating method, fixed piping and nozzles through which the dry chemical can be discharged into the hazard area. Fixed dry chemical systems are of two types- total flooding and local application.
I) Total Flooding System:
In total flooding, a predetermined amount of dry chemical is discharged through fixed piping and nozzles into an enclosed space or enclosure around the hazard. Total flooding is applicable only when the hazard is totally enclosed or when all openings surrounding a hazard can be closed automatically when the system is discharged. Total flooding can be used only where no reigniting is anticipated be cause the Extinguishing action is transient.
II) Local application System:
Local application differs from total flooding in the nozzles are arranged to discharge directly into the fire. Local application is practical in those situations where the hazard can be isolated from other hazard so that fire will not spread beyond the area protected, and where the entire hazard can abe protected. The principal use of local application system is to protect open tanks of flammable liquids. As with total flooding system, local application is ineffective unless Extinguishment can be immediate and there are no reigniting sources.
B) Hand Hose Line System:
Hand Hose like System consist of a supply of dry chemical and expellant gas with or more hand hose lines to deliver the dry chemical to the fire. The hoses are connected to the dry chemical container either directly or indirectly by means of intermediate piping. They can provide a large quantity of Extinguishing agent for quick knockdown and Extinguishment of relatively large fires such as might be experienced at gasoline loading racks, flammable liquid storage areas, diesel and gas turbine locomotives, aircraft hangers, etc.
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