Which type of valves should not be used as primary shutoff valves?

The choice that indicates valves of cast iron or semi-steel complying with specific ASTM specifications should not be used as primary shutoff valves is based on their mechanical properties and suitability for handling high pressure and potential corrosive environments in natural gas applications. While cast iron and semi-steel valves may meet certain standards as per ASTM specifications, they can be more prone to failure under high pressure compared to other materials.

Primary shutoff valves are critical components in any gas handling system. They are responsible for isolating sections of the pipe or system quickly and efficiently in case of an emergency, maintenance, or a detected leak. The ability of these valves to withstand the demands of the pressure, stress, and environmental conditions they will face is paramount. Ductile iron, for instance, is more resilient and reliable under such conditions than cast iron or semi-steel.

Other options like valves made of ductile iron or those specifically designed for high-pressure systems take into account the necessary durability and pressure handling capabilities, making them suitable for critical shutdown roles. Furthermore, aluminum alloys with corrosion resistance are designed to function effectively in corrosive environments, which is another important aspect when considering materials for valves used in natural gas systems.

Thus, the focus on material properties and their intended use demonstrates