What type of valve is used on overboard overflows to prevent backflow of water from the sea?

The use of a check valve on overboard overflows is important because it is specifically designed to allow fluid to flow in one direction while preventing backflow. In the context of an overboard overflow system, this characteristic is crucial to ensure that seawater does not enter the vessel through the overflow. The check valve operates automatically, using the pressure of the fluid to open and close, thus ensuring that the water can only exit the ship to the sea and cannot re-enter.

In contrast, other types of valves, such as ball valves, gate valves, and butterfly valves, are primarily designed for on/off control rather than controlling flow direction. They can seal against backflow when closed, but they do not provide the automatic function of preventing reverse flow that is needed for overboard applications. Therefore, when it comes to applications where backflow prevention is critical, such as in overboard overflow situations, a check valve is the appropriate choice.