When comparing ball valves and butterfly valves, which type is more leak-tight when fully closed under differential pressure?

The choice of ball valves as more leak-tight when fully closed under differential pressure is due to their design and sealing mechanisms. Ball valves utilize a spherical disc (the ball) that pivots to open or close the flow. When in the closed position, the ball makes direct contact with the valve seats, which are usually made from materials designed to create a reliable seal. This contact is uniform across the surface of the ball, leading to a high level of tightness and significantly reducing the chance of leaks even under high pressure.

In contrast, butterfly valves usually consist of a disc that rotates within the flow, which has a different sealing configuration. While they can provide adequate sealing in many applications, they are generally considered less effective at preventing leaks compared to ball valves, especially under higher pressures.

Other types of valves mentioned, such as check valves and globe valves, serve different functions and are not specifically designed to provide the tight seal that ball valves do in this context. Hence, the selection of ball valves for situations requiring superior leak tightness under differential pressure is well justified.