What type of valve should be positioned closest to the boiler on a feedwater line?

The stop valve is ideally positioned closest to the boiler on a feedwater line because its primary function is to completely shut off the flow of water when necessary. This is crucial for maintenance and safety; in the event that repair work or inspections need to be performed on the boiler system, being able to isolate the boiler from the feedwater supply without disrupting the entire system is essential.

Stop valves are designed to handle the high-pressure conditions commonly found in boiler systems, ensuring that the flow can be controlled effectively. In emergencies, having a stop valve immediately accessible allows operators to swiftly halt water supply, preventing overfilling or other potential hazards.

Other types of valves, while useful in their own right, serve different specific purposes. For instance, check valves prevent backflow and are more suited for ensuring one-directional flow, while throttle valves are employed to regulate flow rate, which is not the immediate priority in the context of isolation. Pressure relief valves focus on discharging excess pressure to protect the system, and they are usually placed at points where pressure management is critical rather than directly on the feedwater line to the boiler.