Pump Performance

 The performance characteristics of a pump can be represented in terms of overall efficiency.

PUMP PERFORMANCE

The performance characteristics of a pump can be represented in terms of overall efficiency. Overall efficiency, in turn, has two components: 'volumetric efficiency' and 'mechanical efficiency'. These three efficiencies are presented below.

1. Volumetric Efficiency

• Definition: It is the ratio between the actual flow rate produced by the pump and the theoretical flow rate that the pump should produce. 

• Formula:

• Significance: The volumetric efficiency indicates the amount of leakage within the pump. The lower the internal slip losses, the higher the volumetric efficiency. For zero slip, the volumetric efficiency is 100%.

2. Mechanical Efficiency

• Definition : It is the ratio between the theoretical power required to operate the pump and the actual power delivered to the pump.

• Formula :

Mechanical efficiency can also be calculated in terms of torques as follows: 

• Significance: The mechanical efficiency indicates the amount of energy lost due to friction in bearings and other mating parts, and energy lost due to fluid turbulence. In other words, the mechanical efficiency indicates the amount of energy losses that occur due to reasons other than leakages.

• Since the amount of power required to overcome friction rises with increased liquid viscosity, mechanical efficiency decreases as liquid viscosity decreases.

• Power losses in timing gears, bearings and seals reduce mechanical efficiency.

3. Overall Efficiency

• Definition: It is the ratio between the actual power delivered by pump and the actual power delivered to pump.

• Formula:

Mathematically, the overall efficiency can also be written as

• Significance: The overall efficiency indicates the amount of energy losses by all means.

Note 

In our calculations, we may use the terms 'brake power' and 'hydraulic power'. The actual power delivered to a pump from a prime motor (like electric motor) via a rotating shaft is often called by the term 'brake power'. Similarly the actual power delivered by a pump to the fluid is called 'hydraulic power'.

4. Comparison of Various Positive Displacement Pumps

Table 4.9 compares the various types of positive displacement pumps.

Table 4.9. Comparison of various positive displacement pumps