A 4kg powder stored pressure (ABC type) fire extinguisher primarily contains monoammonium phosphate and is pressurized with nitrogen gas.

 A 4kg powder stored pressure (ABC type) fire extinguisher primarily contains monoammonium phosphate and is pressurized with nitrogen gas. It is highly versatile, effective against most common fires, and has a general lifespan of 10 to 12 years with proper maintenance. 

Composition

The extinguishing agent in an ABC dry powder extinguisher is a fine, yellow powder that is specially fluidized and siliconized to prevent caking and moisture absorption. 

Active Ingredients: The primary active ingredient is monoammonium phosphate (MAP), which makes up a significant percentage of the mixture (commonly 40-90% depending on standards). Other agents like ammonium sulfate, mica, melamine, and silicone oil might also be included to enhance its effectiveness and flow properties.

Expellant: The extinguisher is a "stored pressure" type, meaning the extinguishing agent is stored under constant pressure, typically using nitrogen gas (N2). 

Significance

The main significance of this type of extinguisher lies in its versatility and rapid knockdown capability across a wide range of fire classes. 

Multi-Class Effectiveness: It is effective against Class A (ordinary combustibles like wood, paper, and cloth), Class B (flammable liquids like gasoline, oil, and paint), and Class C fires (flammable gases like LPG and methane). It is also safe to use on most electrically started fires because the powder is a non-conductor of electricity.

Mechanism: The powder extinguishes fire by creating a barrier that smothers the flames, separating the fuel from the oxygen supply. The heat causes the powder to melt and form a coating that prevents re-ignition. It also works by interrupting the chemical reaction of the fire itself.

Ideal Use Cases: It is best suited for outdoor environments, large workshops, commercial boiler rooms, garages, and facilities storing flammable liquids where visibility and post-fire cleanup are less of a concern. 

Lifespan and Maintenance

A 4kg dry powder extinguisher generally has a long lifespan, but requires regular checks to remain operational. 

Total Lifespan: Most rechargeable dry chemical extinguishers have a total life expectancy of 10 to 12 years, and some up to 15 years, provided they are properly maintained. Non-rechargeable units typically last around 12 years and should be replaced at that point.

Monthly Inspections: The user should perform monthly visual inspections to ensure the pressure gauge needle is in the green zone, the pin is intact, the seals aren't broken, and there are no signs of physical damage or corrosion. Shaking the extinguisher periodically helps prevent the powder from compacting.

Professional Servicing: A certified professional must conduct an annual inspection.

Six-Year Maintenance: Rechargeable dry chemical extinguishers require an internal inspection and an agent refill every six years.

Hydrostatic Testing: The cylinder must undergo hydrostatic testing (pressure testing) every 12 years to ensure its integrity.

After Use: The extinguisher must be recharged immediately after any use, even if only a small amount of powder was discharged.