Pumps come in standard sizes and discharge rates. The actual flow required in a system may differ from the designed discharge rate. For example, a filter may have a flow rate which is lesser than the discharge rate of the pump connected to its output. In such situations, it may be necessary to reduce the discharge rate.A commonly used practice is to throttle the pump with a valve. Sometimes, orifice plates are also used. Throttling creates an obstruction in the flow. This leads to a drop in the efficiency of the pump. The backpressure developed due to the restriction placed on the flow can stress the pump affecting its life.
Throttling is, therefore, not advisable.
Alternatives to Throttling
Variable Frequency Drives
Variables Frequency Drives or VFDs are devices which control the speed of the motor (the prime mover) by varying the frequency applied to the motor terminals. VFDs can be used to regulate the speed. VFDs also help save energy. VFDs can make quick changes in speed and thereby the discharge pressure. VFDs are gaining in popularity in speed control. However, they may be expensive to install.
Trimming the Impeller
This method can be used if the discharge rate is to be permanently reduced. The impeller is trimmed by reducing its circumference. This affects the output pressure of the pump. This method may not be suitable if the discharge pressure has to be constantly adjusted.
Installing multiple pumps
In this method, more than one pump can be connected in parallel. These pumps can have discharge rates. The pumps can be brought on line and taken offline depending on the required pressure. This method, however, is expensive and may require modification in the system which may not be possible.
Sometimes, a combination of the three methods is used.