Case studies:Variable speed drives on a paper stock transfer pump

Case study 7: Variable speed drives on a paper stock transfer pump

Case study summary

Installing a VSD on a low-density stock pump saved energy and reduced pipework and valve maintenance cost by eliminating the flow control valve. The total annual saving was €/$ 30 000.

Other potential applications

Applications where flow control by valve can be eliminated (or minimized) by the use of variable speed control.

Investment cost

€/$ 17 500

Savings achieved

€/$ 17 000 energy cost and approximately €/$ 13 000 maintenance and repair costs, i.e. a total of €/$ 30 000 per annum.

Payback period

Less than one year.

Installation and operation details

The paper mill relied on a valve to control flow from the 150 kW (200 hp) fixed speed pump on the thermo-mechanical pulp (TMP) mill’s storage tower. Operators throttled a 250 mm (10 inch) control valve in the pump discharge to maintain a set level in the TMP low-density storage tower. Often the valve had to be nearly closed due to process demands. Not only did this waste considerable amounts of energy with the high head loss across the valve, it also resulted in severe cavitation across the valve, leading to valve and pipe damage. The pump also had to operate in a low flow region where pump reliability suffered.

A VSD was installed with sensors to monitor system parameters such that the controller could vary speed to maintain a set level in the storage tower and avoid an undesirable operating condition for the pump. The control valve was removed, being no longer required for flow control.

As a result, motor speed was reduced from 1150 r/min to generally around 450 r/min, saving about 75 kW (100 hp) which was equivalent to

€/$ 17 000 per annum. Eliminating the control valve and its associated repairs saved a further €/$ 13 000 per annum.

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