In Variable Speed Drivein Centrifugal Pump How We Can Save the Energy?

It depends entirely on the system curve and the pump curve and how much your conditions/requirements vary.

If the flow and head required remain relatively steady and the pump has been correctly sized for the duty and the system then you will get no energy saving from the use of variable speed pumps.

Go to this web site and download "Variable Speed Drives make Cents. Make sure to take note of the section that tells you when not to install them, because the salesman won't. Also look at the power savings against the extra capital cost and complexity.

You may also want to download their software which allows you to play with the systems and see the effects on the pump.

In the same way as any electrical device: turn the damn thing off when it is not needed.