How many valves should be installed on main intersections?

The correct answer is that three valves should be installed on main intersections. This is due to the need for effective operational management and maintenance of the distribution system.

Having three valves allows for the isolation of different segments of the main line. This capacity is crucial during repairs or maintenance activities, as it enables operators to shut off flow in one section while maintaining service in other areas. It also enhances the system's overall reliability and safety by providing multiple points of control.

In practice, the configuration typically includes one valve for the main flow direction and two additional valves to isolate the line from different branches or connections, ensuring that the system can function efficiently with minimal impact on service to customers.

While fewer valves might reduce initial installation costs, having the appropriate number of valves is important for long-term operational efficiency, system flexibility, and minimizing disruptions in service during maintenance or emergencies. Thus, installing three valves at main intersections aligns with best practices in distribution system management.