The most economical flow control valve for use with large diameter pipes is a

ANS:A - butterfly valve

The most economical flow control valve for use with large diameter pipes is a butterfly valve.

Explanation:

1. Butterfly Valve:
   • A butterfly valve is a type of quarter-turn valve that controls flow by rotating a disc (or "butterfly") in the fluid path.
   • It is well-suited for large diameter pipes because it requires less material and is lighter compared to other types of valves, such as globe valves or gate valves.
   • The reduced material and weight contribute to lower manufacturing costs and easier installation.
2. Advantages for Large Diameter Pipes:
   • Cost-Effective: Butterfly valves are generally less expensive to manufacture and install compared to globe valves or gate valves.
   • Low Pressure Drop: They have a low pressure drop across the valve, which minimizes energy losses in large diameter pipelines.
   • Maintenance: Butterfly valves are relatively easy to maintain and operate due to their simple design and fewer moving parts.
3. Other Valve Types:
   • Globe Valve: Globe valves are better suited for throttling and controlling flow where precise regulation is required, but they are generally more expensive and have higher pressure drops compared to butterfly valves.
   • Needle Valve: Needle valves are used for fine control of flow, typically in small diameter pipes or for precise applications, and are not typically economical for large diameter pipes.
4. Conclusion:
   • Butterfly Valve: Given its simplicity, lower cost, and suitability for large diameter pipes, a butterfly valve is considered the most economical choice for flow control in such applications. It strikes a balance between cost-effectiveness and efficient flow regulation in industrial and large-scale piping systems.