Which type of valves must NOT be used as primary shutoff valves?

The focus on which type of valves should not be used as primary shutoff valves is critical for safety in systems involving compressed natural gas (CNG). Cast iron valves are not appropriate for primary shutoff applications primarily due to their inherent brittleness under stress and their susceptibility to corrosion. While they may be suitable for various applications, their mechanical properties don't hold up well in high-pressure situations found in CNG systems.

In contrast, steel, plastic, and brass valves are more commonly accepted for these roles. Steel valves offer strength and durability under pressure, while brass is known for its corrosion resistance and ability to perform well in gas applications. Plastic valves, when designed specifically for gas services, can also be used in certain low-pressure applications. Therefore, the use of cast iron as a primary shutoff valve in CNG systems poses a significant risk and is specifically avoided.