Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | january 2015 | 29 Digging deeper So Starters: Smoothing the Surge T he damage and noise that are caused by flow changes in centrifugal pump- ing systems is a well-known challenge for engineers. These flow changes or surges can occur when starting or stopping a pump or opening or closing valves. There are a number of mechanical surge reduc- tion techniques, but these tend to be costly and complex – which is why electronic starting and stopping of the pump motor is increasingly being considered a cost-effective solution to reduce surge or hammering problems. Centrifugal pumps are generally coupled directly to the sha• of an electric motor. When applying full line voltage to start the motor, the pump is rapidly accelerated to full speed, o•en in less than a quarter of a second. Thus, the flow out of the pump also increases to total capacity in that short time frame. Because fluids are only slightly compressible and have momentum, this large change in flow over such a short period of time results in high and low pressure surges as the system seeks equilibrium. This results in many undesirable effects. These pressure surges can stress the walls of the pipe and cause an audible noise. The sound is as if the pipe was struck with a mallet repeat- edly, termed 'water hammering'. But although annoying, the sound created is trivial when com- pared to the physical damage that pressure surg- es can cause. Extremely high-pressure transients can cause the pipe to burst while extremely low transients can cause pipes to collapse. So if starting and stopping the pump abruptly and the subsequent rapid changes in flow cause the water hammer, it is apparent that some form of control over this sequence is desirable. By controlling the acceleration and deceleration of the pump motor, the water hammer effect can be minimised. Centrifugal pumping systems are at most of risk of damage when starting and stopping. Can greater electronic control provide the solution? Stuart Greenwood ProduCT MarkeTing Manager, indusTrial ConTrol & auToMaTion, eaTon Figure 1. starting Flow vs. Time Figure 2. Pump stop Flow vs. Time

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