Pump school: flow rate control

Question:

What are the various methods for controlling the rate of flow in centrifugal pumping systems? Is there an impact on efficiency?

Answer:

There are three basic ways of controlling flow rate from centrifugal pumps.

These are:

1. Throttling the discharge by closing a valve in the discharge line.

2. Controlled bypassing of liquid from the pump discharge back to the suction vessel.

3. Varying the speed of the pump.

Discharge throttling

A basic principle of pumping is that a pump operates at the intersection of the pump performance curve and the system resistance curve.

 

Throttling on the pump discharge by closing a valve (manual or automatic) has the effect of changing the system resistance curve. This changes the point of intersection with the pump performance curve with resultant change in flow. This is illustrated in the graph above.

Power consumption in a centrifugal pump is calculated from the formula:

Screen Shot 2015-01-24 at 2.00.14 pm

where

Q = flow rate in litres/sec
H = total head in metres
SG = specific gravity of the liquid
E = pump efficiency at duty point

Throttling wastes energy as pressure is broken down across the valve.

Bypassing

This is simply installing a line with a control valve (manual or automatic) which allows for the return of some of the discharge flow to the suction tank. Opening the valve will increase flow to the suction tank and at the same time reduce flow down the discharge line. This also has the result of wasting energy.

Speed variation

Changing the speed of a pump changes the pump performance curve. This changes the point of intersection with the system resistance curve with a resultant change in flow. This is illustrated in the graph below.

pump-school-graph-2

This is the most energy efficient method of controlling flow and becomes economically very attractive for larger power consumption installations.

Article courtesy of Kelair Pumps Australia – When Pump Knowledge Matters. For more information call 1300 789 466 or visit www.kelairpumps.com.au.

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