VSD in centrifugal pump systems
Various methods of pump capacity control are in Appendix below. The representation of the
system curve and actual operating capacity and head for various control options is also
explained. Please read the Appendix before answering the questions.
The pump was installed with design capacity of 240 m3/hr and 53 m total head of which 10 m is
static head. The system curve is shown by blue line which is not going to change. The installed
pump has characteristics as shown in Fig 3.1 below. Pumped fluid is water with density of 1000
kg/m3. The required flow is now 160 m3/hr only. Note that the static head will not change, only
frictional head will change due to capacity.
Exact match of desired Q and H can only be achieved by VSD. For all other cases either H will be
high or Q will be high thus consuming more power.
Following methodology discussed above, draw operating curves for the four possible flow control
options on the pump curve and calculate power consumption Ph for all options. Represent four
options on 4 different plots by copying the pump curve. You may need to extend the pump curve
Where, Hydraulic Power Ph – kW, flow Q – m3
/hr, Density ρ – kg/m3
, g – 9.81 m/sec2
, Head h – m
3.1 Estimate actual power (BHP = Ph/ηp) using above formula and pump efficiency ηp at
operating point as read from the graph.
3.2 Using a clean copy of the pump curve provided, plot new operating points for each of 4
cases similar to that shown in the appendix (throttling, bypass, on-off control 67% of time
ON and VSD control, all at desired net flow rate of 160m3
3.2 Compute actual head and actual power for all new operating point to meet desired net
flow rate of 160 m3
/hr. Summarize your results in table 3.1 below.