Valve sizes should be based on which two factors?

The rationale for selecting valve sizes based on capacity and fluid-friction losses stems from the need to ensure optimal flow and efficiency within a piping system. Capacity refers to the maximum flow rate that the valve can handle without causing significant pressure drop or flow restrictions, which is crucial for maintaining system performance. It's important to match the valve's capacity to the system's operational requirements to avoid bottlenecks and ensure smooth operation.

Fluid-friction losses are also a critical factor because they impact how much energy is required to move the fluid through the system. Every component in a fluid system, including valves, introduces some level of resistance, which can lead to pressure drops. By considering fluid-friction losses alongside capacity, engineers can select valves that minimize these losses, thereby improving overall system efficiency and reducing operating costs. Properly sized valves help in maintaining the desired flow rates while minimizing the risk of cavitation and ensuring the longevity of the valves and the entire system.