⎪ Water and wastewater processing ⎪
Static Head (m) m 3 /h
Speed (%)
Power (kW)
m 3 /kWh
Es= kWh/m 3
0
800
100
79.5
10.1
0.099
(1) The all fric- tion (steep) pump curve (2) A mixed static plus frictional head scenario (3) The constant static head low friction scenario
0
400
50.0
10.4
38.5
0.026
20
800
100
79.5
10.1
0.099
20
400
78.5
34.6
11.6
0.087
27.5
800
100
79.5
10.1
0.099
27.5
400
86.5
44.7
8.9
0.112
Table 1: The effect of a 50% change in pump speed on the Pump Specific Energy and power drawn for three different pump curve scenarios: (1) The all-friction (steep) pump curve; (2) A mixed static plus frictional head scenario; (3) A constant static head low friction scenario. Note: The increase in Es (kWh/m 3 ) for Scenario 3 when the speed reduces from 800 to 400 m 3 /h.
system, be sure first to determine the system curve and its static and fric - tion head components. Then, by superimposing the system curve onto the family of pump curves at different speeds, have a look at how the system will respond to varying the speed. To complete the picture, calculate the motor speed, power draw and specific energy for several different pump speeds. Only then can you be 100% sure that the VSD will deliver the benefits you expect. www.tasonline.co.za
a very dangerous area in terms of the life of the pump. If a pump is operated at closed valve head for anything more than a few minutes at a time, the resultant loading on the bearings and the increased temperature of the fluid can quickly lead to bearings seizing, mechanical seals failing, shafts snapping and even the possibility of the pump casing exploding.
Conclusion So before commissioning the installation of a VSD on an existing pumping
A closed loop cooling water system pumping through heat exchangers and back to pump. The net static head of this system is zero and the steep system curve is dominated by friction losses.
March-April 2022 • MechChem Africa ¦ 9
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