What is the only allowable use case for a check valve in a bilge pump system?

In a bilge pump system, the primary function of a check valve is to prevent back-flow from the discharge line. When a bilge pump operates, it is designed to move water out of the bilge area. If there is no check valve in place, once the pump stops, water could potentially flow back into the bilge through the discharge line due to gravity or pressure differences. This back-flow could negate the pump's efforts and lead to water accumulation in the bilge, which is problematic for the vessel’s safety and integrity.

The check valve ensures that once the water is pumped out, it cannot return to the bilge, thereby maintaining the effectiveness of the bilge pump system. This use case is crucial as it directly relates to the safety and operational efficiency of the vessel, making it the correct answer.

Other options, while relevant in different contexts, do not align with the primary purpose of a check valve in this specific application. For example, improving flow rates is generally achieved through the pump design and piping rather than through a check valve. Similarly, controlling water intake is not a function of check valves, and redundancy in pump operation pertains more to system design than to the role of a check valve within any single pump's operation.