1. A basic pump system

A pump has two key characteristics. Fluid will be caused to flow at a particular rate, ie the fluid will be pushed along at a rate. Secondly, it will support a head of fluid.

There are two parts as seen in the diagram:

1. Static head.

2. Losses caused by friction due to the piping.

A pump selected for a duty, as the pump curve is superimposed on the system characteristic curve.

The curves cross over at the pump’s best efficiency point (BEP) at the desired flow rate.

Obtaining a perfect match between the system curve and the best efficiency point is difficult and not practical. In fact, the pump can become unstable. In the past the following methods were employed:

• adjusting the pump speed
  

• adjusting the pump impeller diameter
  

• changing the impeller design
  

• adjusting the system resistance
  

• modifying static head
  

• providing a system bypass flow route.
  

A preferred technique is to use a variable speed drive to drive the pump as this will enable the performance of the pump to be adjusted instantaneously. The VSD motor is a very efficient, low cost method of varying the motor speed.

Ideal pump performance curves against impeller diameter.
Ideal pump efficiency curves are superimposed.