MechChem Africa March-April 2022

⎪ 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, besure first todeterminethesystemcurveand itsstaticandfric - tionheadcomponents.Then, bysuperimposingthesystemcurveontothe 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 VSDwill deliver the benefits you expect. www.tasonline.co.za

averydangerousareaintermsof thelifeof thepump. Ifapumpisoperated at closed valve head for anythingmore than a fewminutes 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 Sobeforecommissioningthe installationofaVSDonanexistingpumping

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