Dry Chemical Powders (DCP) have emerged as an important fire extinguishing agent due to their ease of use and their role in fire extinguishing. Their role in firefighting is well understood. It extinguishes fire both physically and chemically.
o Physically: The fine powder generated by dry chemical powder deposits as a layer and works as a blanket. This surface layer stops the oxygen from attacking and extinguishing the fire. I.e., the “blanketing” or “smothering” effect is in place
o Chemically: The fine powder generated by dry chemical powder reacts with the free radicals generated during the combustion process and aids in the termination of chain propagation.e., the “free radical inhibition”
The particle size of the powders plays an important role in determining the fire knockdown properties. The finer the particle size the more effective would be the fire knockdown properties, but as the particle size becomes finer (smaller), it would be very difficult to project as a continuous stream of jet, hence a balance of finer and coarser particles is done by manufacturers to make sure that powder ejection happens properly. A combination of fineness and dispersibility of powders is necessary for formulating an effective powder that works against the elements of fire and extinguishing it.
There are very useful in extinguishing three-dimensional fires(liquid fuels under fire getting discharged from an elevated surface vertically and creating a pool of fire on the surface and spreading horizontally). Dry chemical powders are used in all types of portable fire extinguishers, mobile type fire extinguishers and fixed installations. Dry chemical powders are categorized typically into three types depending on the class of fire they are used against. They are
1. Dry chemical powder (ABC)- These are used in all three types of fires, solid fires (A-class), liquid fires (B-class), gaseous fires (C- class) and live electrical equipment. Hence these are called “multi-purpose” powders. These powders are compatible with all types of foam and display pale yellow colour. These are generally composed of
a. Mono ammonium phosphate
b. Ammonium polyphosphate and Ammonium sulphate mixtures
2. Dry chemical powder (BC)- This is used to extinguish fires of liquid fires (B class), gaseous fires (C class) and live electrical equipment. These are generally composed of
a. Sodium bicarbonate
b. Potassium bicarbonate
c. Potassium chloride
3. Dry chemical powder (Metal) – This is used to extinguish metal fires (D class). Dry chemical powders of type “ABC” or “BC” are not suitable to tackle D class fires. Hence special materials are used to tackle these kinds of fires. They are generally composed of
a. Ternary Eutectic chlorides
b. Ammonium phosphate and sodium chloride mixtures
Ternary Eutectic chlorides (TEC) are generally used on metal fires like uranium, titanium, powdered aluminium. Sodium chloride-based powders are used on metal fires like Magnesium, sodium and potassium. The application of powder does not cause thermal shock
QUALITATIVE REQUIREMENT OF DRY POWDERS :
The effectiveness of the dry chemical powder in stopping the fire depends upon qualitative parameters to which the Dry chemical powder should meet the required specification range. The qualitative parameters which are taken for testing are.
1. Apparent density
2. Chemical content
3. Particle size distribution
4. Hygroscopicity
5. Resistance to Caking
6. Water repellent nature
7. Moisture content
8. Heat resistance nature
9. Free-flowing characteristics
10. Foam compatibility
11. Fire knockdown properties
MANUFACTURING REQUIREMENT OF DRY POWDERS :
The following information is provided on each pack of powder containing dry chemical powder
• Manufacturer's name
• Quantity of the powder, in kg
• Ordinary or foam compatible
• Date of manufacture/Batch No
• Percentage of the base material.
SAFETY GUIDELINES WHILE USING DRY CHEMICAL POWDERS :
• The discharge of powder can create hazards like reduced visibility, temporary asphyxiation, irritation to the eyes, and can cause toxicity if inhaled in large quantities. This problem is more aggravated when used in confined spaces. The presence of personnel and evacuation before use of the powder to be considered and proper first aid measures to be taken when accidentally exposed through contact or inhalation of these powders.
o Inhalation: Inhalation irritates the respiratory tract and can cause temporary asphyxiation depending upon the time and quantity of exposure to Dry chemical powder cloud. In such cases the victim to be moved to fresh air. Medical attention to be immediately sought for any persisting breathing difficulty.
o Ingestion: Exposure through this route is rare, still if it happens the effect to be reduced by diluting the material by drinking more quantities of water and immediately obtaining medical attention
o Skin Contact: Exposure can cause skin irritation. The exposed area to be washed with soap and water and to checked for any signs of irritation that is going away or persisting. If the irritation persists immediately medical attention is sought
o Eyes: Contact of powder with eyes can result in irritation and pain. The eyes should be washed with plenty of water with eyes open for at least 15 minutes. If any redness or irritation persists immediate medical attention to be sought.
• In places where electronic equipment is used, the dry chemical powder usage will result in powder residue which can adversely affect their functioning.
• The acidic nature and basic nature of powders to be considered while using different types of powders as they can corrode metals, for example, steel and iron in presence of moisture can get corroded due to the acidic nature of potassium chloride. Aluminium and bronze can get corroded due to the basic nature of sodium and potassium bicarbonate
• They are used either in “stored” or “cartridge” operated fire extinguishers. Their storage should be avoided in places with high temperatures and direct sunlight. The storage area should be cool, well ventilated, dry and protected from direct sunlight. The powder’s stability and integrity depend on the storage conditions that are maintained at end-users premises.
• During usage of these powders on fire, hazardous decomposition products will occur like oxides of carbon, nitrogen and phosphorus and ammonia release. The use of appropriate PPE’s while handling these powders is essential.
Article by PJ Mohan
Sr.Faculty, NIFS
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