Pump efficiency

The pump does not completely convert kinetic energy to pressure energy since some of the kinetic energy is lost in this process. Primarily, there are three areas where this energy is dissipated and not converted to useful work. Pump efficiency is a factor that accounts for these losses. Pump efficiency is a product of the following three efficiencies:

1. Hydraulic efficiency
(primarily, disk friction, which is the friction of the liquid with the impeller shrouds. This is a function of speed and impeller geometry. Other losses are shock losses during rapid changes in direction along the impeller and volute)

2. Volumetric efficiency
(recirculation losses at wear rings, interstage bushes and other)

3. Mechanical efficiency
friction at seals or gland packing and bearings). Some texts call the product of the first two efficiencies as internal efficiency of the pump. Every pump is designed for a specific flow and a corresponding differential head, though it is possible to operate at certain percentage points away from the designed values. However, the efficiency of the pump at the designed point is maximum and is called as the BEP. Efficiency at flows lower or higher than this design point is lower. The efficiency of the pump has a close relationship to an important pump number called as the specific speed.