Non-Positive Displacement Pumps

There are many designs of impellers for different duties, e.g. for low viscosity fluids the impeller is normally shrouded for solids suspensions and slurries the impeller may be open-faced to prevent blockage. 

The delivery is continuous. Inlet and outlet valves are not required, but a non-return, or check valve is normally installed on the pump outlet line to prevent reverse flow when the pump is switched off. The outlet may be blocked (for short periods) without damage to the pump. 

Maximum head is attained at zero flow, when the maximum kinetic energy gained by the fluid within the impeller is converted into pressure. As the flow through the pump is increased, the fluid leaving the outlet takes with it kinetic energy. Therefore, less of the kinetic energy gained 

In a well-designed system, a centrifugal pump will normally operate at or close to its maximum efficiency point. Pump efficiencies, t], are typically in the range 60-80%, larger pumps having the higher efficiencies. 

---