What operation involves each impeller of a multi-stage centrifugal pump discharging into a common outlet?

In a multi-stage centrifugal pump, when each impeller discharges into a common outlet, this configuration is indicative of parallel operation. In this setup, the fluid output from each stage is collected and combined, allowing for a higher total flow rate while maintaining the pressure characteristics of each impeller. This is beneficial for applications where a significant volume of fluid is required without needing to significantly increase discharge pressure beyond a certain point.

This is different from series operation, where the output of one impeller feeds into the next, resulting in a cumulative increase of pressure across the stages but not necessarily an increase in flow rate. The other options, such as single and combined operations, do not accurately describe the configuration of a multi-stage pump where all impellers discharge together into a single outlet, thus making parallel operation the correct choice in this context.