Which types of shells qualify for dry chemical extinguishers that follow a 12-year test interval?

The appropriate type of shells that qualify for dry chemical extinguishers following a 12-year test interval includes mild steel, brazed brass, or aluminum shells. These materials are specifically chosen for their durability and resistance to both corrosion and pressure, which are important characteristics for pressure vessels like fire extinguishers.

Mild steel is commonly used because it offers a good balance of strength and weight, while aluminum is favored for its lightweight characteristics and resistance to corrosion. Brazed brass provides additional corrosion resistance and durability, making it another suitable option.

Moreover, the 12-year test interval is crucial for these materials as it requires thorough inspections and testing to ensure that they maintain structural integrity and effectiveness over time. Selecting materials that meet these criteria helps in ensuring the reliability and safety of the extinguishers when they are needed in emergency situations.