Which valve generally offers the least resistance to flow when fully open?

The ball valve offers the least resistance to flow when fully open due to its design. It features a spherical disc (the ball) with a hole in the center that aligns with the pipe when the valve is open. This creates a straight passage for the fluid to flow through, minimizing turbulence and pressure drop. As a result, ball valves are often preferred for applications that require high flow rates and low resistance.

In contrast, other types of valves, such as gate valves and globe valves, have designs that can create more resistance. Gate valves, while designed for minimum resistance when fully open, still have some obstruction due to the gate's position. Globe valves, designed primarily for regulating flow, have a profile that restricts flow by creating turbulence, resulting in higher resistance compared to a ball valve. Check valves also introduce resistance due to their internal mechanisms that prevent backflow, which can disrupt the continuous flow of fluid.

Therefore, the unique design of the ball valve allows for the least resistance, making it the most efficient choice for maximizing flow in a water distribution system.