Water & Wastewater Treatment

30 | january 2015 | WWT | www.wwtonline.co.uk Digging deeper Reducing the hammer effect There are three methods of starting and stopping a pump; direct on-line (DOL), solid-state reduced voltage (SSRV) starting; and so• starter control- lers with pump control option. The main concern with DOL starting is that the motor torque output more than exceeds the requirement of the pump during the start cycle. Locked rotor torque (LRT) is the torque devel- oped by the motor the instant that full voltage is seen at the motor terminals at zero speed. LRT can be as high as 180 per cent of the torque the motor produces at full speed. Breakdown torque (BT) is the highest amount of torque the motor can develop. BT can be as high as 250 per cent of full load torque. The difference between the torque produced by the motor and that required by the load is called ac- celerating torque. If the period of time in which the flow reaches 100 per cent can be increased, then hammering can be reduced. This can be achieved by reduc- ing the amount of accelerating torque (the torque that causes the motor to rotate the connected load) delivered by the motor. Less accelerating torque means less force to turn the load and therefore more time required to change the speed of the pump. This can be done using a solid-state reduced voltage (SSRV) starter to slowly ramp the voltage applied to the motor from zero to full voltage over some pre-set time, usually adjustable from two to thirty seconds. With the SSRV starting method, the accelerat- ing torque is greatly reduced compared to the direct on-line method. At the end of the ramp, however, there is a sudden and excessive acceleration torque which generates a corresponding burst of speed and this may result in ham- mering as the pump motor rapidly approaches 100 per cent speed. This is a result of the breakdown torque that is still present when using an SSRV starter. This sudden surge in pump motor torque at the end of the start cycle results in a flow surge. The sudden surge in torque is due to the characteristics of the motor. It occurs because "These pressure surges can stress the walls of the pipe and cause an audi- ble noise. The sound is as if the pipe was struck with a mallet repeatedly, termed 'water hammering'." SSRV starting ramps the voltage up without regard to the motor's performance. So although SSRV starting improves starting torque charac- teristics of the pump motor, it cannot control breakdown torque that causes surges. With the pump control option, the surge pro- duced during DOL and SSRV is greatly reduced. This can be achieved by using the micropro- cessor in the so• start controllers to carefully control the torque output to the motor. As there are no sudden changes in torque, this delivers smooth acceleration of the motor and reduces surges or hammering (Figure 1). Stopping problems However it is not just starting the pump that can be problematic, stopping it smoothly is equally as important. When a DOL starter is applied, the pump motor will coast when a stop command is initiated. The system head will quickly overcome the motor inertia, and the pump will come to a rapid stop. The fluid, which is in motion and has momentum, must come to a complete halt as well. This action causes pressure surges on the pipes and valves (Figure 2). In an attempt to curb this, many control manufacturers are recommending an SSRV starter with a so• stop as a solution to surge. The problem with that approach is that in many applications a so• stop cannot prevent sudden changes in motor torque required on pumping applications. When a so• stop is initiated, the voltage drops to zero over a time selected by the user. This reduction in voltage does reduce the torque, causing the pump to slow down. However, a point is quickly reached where the load torque demand exceeds the motor torque supply, and the motor stalls. The effect, though not as severe, is the same as slamming a valve closed, and hammering occurs. This can be avoided by using so• starter con- trollers with pump control option. This will allow control of the pump's deceleration similar to the control achieved during the acceleration phase. When a pump is stopped the controller reduces the motor speed to prevent any sudden changes in torque and minimises any surges. The so• starter controllers continue to reduce the torque of the pump motor to ensure there are no sudden changes in flow that can cause hammering. So• starter controllers with pump control option produce the most desirable flow charac- teristics when starting and stopping centrifugal pump motors. There are no sudden peaks or breaks in flow that result in surges or hammer- ing in the system. When analysing what is to be done about a hammering problem, an electrical solution should be considered before a mechani- cal solution. The initial cost for the electrical solution tends to be less than that of a specialised control valve as well as being less complex. All things considered, there can be little doubt that so• starter controllers with pump control are the optimum solution for centrifugal pump systems. Figure 3. Pump Control vs. Flow

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