Which type of design must valves that are three inches or larger have?

Valves that are three inches or larger typically require a slow closing design to minimize the risk of water hammer, which is a pressure surge that can occur when fluid in motion is forced to stop or change direction suddenly. A slow closing design allows the valve to gradually close, controlling the flow of water and helping to maintain a steady pressure within the system. This is particularly important in larger diameter piping systems, where abrupt changes in flow can create significant pressure fluctuations that may lead to damage or failure of pipes, fittings, and valves.

In contrast, quicker opening designs might lead to immediate surges in pressure, which could be detrimental in larger systems. Automatic and spring-loaded designs focus more on the device's ability to react to pressure changes or the need for immediate closure, rather than managing the flow dynamics in large systems. Hence, for valves of this size, a slow closing design is the preferred choice in ensuring safety and maintaining system integrity.