To find the system curve the calculations were repeated for the velocities shown in the table below:
Plotting these values on the pump characteristic gives the operating point as 18.5 m at
CHAPTER 6
VELOCIS SOLUTIONS @2018
12
Capacity control
Generally, pumping system demands to work under capacity range rather than at fixed capacity.
For maintaining controlled flow rate the change in one or both pump curves is required.
The traditional method for controlling flow rate is utilizing throttling valve at the pump discharge
line. Doing so system head is artificially increased to give the desired flow in accordance with
performance curve. However, this gives rise some basic mechanical faults that leads to drop in
pump efficiency. Variable speed driver is therefore used mainly to prevent efficiency problem.
Pump head curves are adjusted with varying frequency driver in accordance with the affinity law
(explained in following subsection).
Variable speed system has other advantages. Since there is no closed valve at the discharge of the
pump, soft start of pump is possible without consuming high amount of energy from motor. At
constant speed system, shaft deflection increases with pressure imbalance around the impeller.
However, within variable speed driver pressure is relatively equal all around the impeller and
therefore, shaft deflection is less. Hydraulic noise is decreased as well and there is no maintenance
of throttling valve concern.
a) change speed b) throttle
Figure 7.9: Capacity control of a centrifugal pump