Which of the following is a function of a stop valve in a boiler system?

In a boiler system, a stop valve serves the essential function of isolating the boiler from the feedwater supply. This means that when maintenance or repairs are necessary, the stop valve can be closed to prevent water from entering the boiler, ensuring safety and allowing work to be carried out without risk of flooding or water flow issues.

The other functions mentioned, such as regulating flow rate, controlling pressure levels, and increasing efficiency of combustion, involve different types of valves and components within the system. For example, flow rate regulation is typically handled by control valves that adjust the amount of water or steam moving through the system, whereas pressure control is managed by pressure relief valves or pressure regulating valves designed to maintain an optimal pressure level. Efficiency of combustion is influenced by factors like burner design and fuel-air mixture, rather than the operation of the stop valve itself.