On the feed water line, the valve closest to the boiler shell is a _____ valve.

The valve closest to the boiler shell on the feed water line is a stop valve. This valve is crucial in controlling the flow of feed water into the boiler. Its primary function is to provide a means to completely shut off the flow of water to the boiler when necessary, such as during maintenance or in emergency situations. By being located right next to the boiler, the stop valve ensures quick access to cut off water supply, which is essential for safety and operational integrity.

In addition to its shutting-off capabilities, the stop valve plays a significant role in preventing water hammer and maintaining the boiler pressure by allowing for the isolation of the boiler from the feed water system. This enhances the overall control of the system, ensuring that no unintended water flow occurs during repairs or inspections.

This capability distinguishes the stop valve from other types of valves, such as relief, control, or check valves, which serve different functions within the system. Relief valves are designed to release excess pressure, control valves regulate the flow but do not provide a full shut-off, and check valves prevent backflow, but they do not serve the same isolating purpose as a stop valve.