Answer: The isentropic efficiency of the pump is nothing but the ratio of the isentropic work to the actual work.
The pump is a work consuming device. The pump can not produce its own thermodynamic work. Some amount of work is needed to be provided to the pump from outside so the pump increases the pressure of the fluid.
The actual work that needed to be provided to the pump from outside must be higher than isentropic work.
The actual work required by the pump is usually calculated by experiments.
OR
Most of the time, the isentropic efficiency of the pump is decided and accordingly, actual work is computed.
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