What is a relief valve used for in fire apparatus?

A relief valve is an essential component in fire apparatus that maintains a consistent and set pump discharge pressure. Its primary function is to divert excess water flow back to the pump or tank when the pressure exceeds a predetermined limit. This is crucial for preventing damage to the pump and the entire hydraulic system caused by overpressure.

When the discharge pressure reaches the set point, the relief valve opens to allow a controlled amount of water to recirculate, ensuring that the pressure remains stable within the desired range. This operation helps protect equipment and ensures optimal performance, allowing firefighters to operate effectively under varying conditions.

The other options refer to different functions that are not served by a relief valve. For example, compensating for decreased pressure relates more to how systems can adjust to maintain operational pressure rather than specifically regulating it. Engine temperature regulation is a separate function typically managed by cooling systems. Increasing water flow through the pump does not align with the role of a relief valve, which is focused on managing pressure rather than directly affecting flow rates.