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ABB Softstarters: Prevention of water hammering with torque control


25.05.2017

One of the biggest difficulties in pumping applications comes when the pump stops. The combination of water pressure, a high coefficient of friction in the pump system and a low moment of inertia of the pump leads to the fact of very fast motor stop when a direct stop is used. This will result in the return of the water column, collapsing into the closing valve, and creating additional pressure waves in the pipeline system. That is water hammering phenomenon. You can even hear it as a bursting sound with vibrations throughout the system.

Day by day, water hammering at every stop will quickly wear down the valves, gaskets and pipe joints. This may lead to maintenance and repair or even to unplanned downtime. Slowing down of the water flow in the system can solve the problem. It may not be sufficient simply to perform a conventional soft stop using a linear voltage change beacause of the complexity of piping systems. The engine torque needs to be regulated in order to properly slowing down the flow of water. That's why the torque control was developed.

The soft starter normally performs a smooth stop by linearly changing the line voltage from the full voltage to the end voltage. However, when the voltage decreases, the motor self-compensates using a larger current to keep in its speed. Then, when the voltage drops enough, the engine will become too weak and the speed will reduce too fast. Such way of stopping is much better than a direct stop, but  this is not good enough and it may still cause a water stroke. Torque control uses a very advanced algorithm and controls the torque of the engine both at start-up and when stopped. This function is based on closed-loop control, where both voltage and current are measured. Using this data, the engine torque can be continuously calculated with high accuracy.

The voltage applied to the motor is then adjusted so that the resultant engine torque is maintained exactly at the required level. It is criticalto have the ability to control a torque of the engine  to prevent water hammering. In addition, a thorough understanding of the of engine and pump working and how to reduce torque is necessary in order to prevent a hydraulic shock. ABB has worked closely with pump manufacturers for a long time to get this knowledge. Thousands of tests and experiments with different combinations of pumps and motors have led to an excellent way to stop pumps. Regardless of how complex the function is, it still needs to be easily configured. Using the torque control function in the PSE and PST (B) program modules from ABB, there is no need for speed feedback from the motor, and there is no need in additional motor data. You only have to decide whether torque control should be activated or not. Using the preset application setting in the PST (B) program module, torque monitoring will be used automatically when you select your application for the pump.

Using softstarters  from ABB, you can always find a good initial solution regardless of the engine  size  and the required functionality. You can find variety of ABB softstarters on our website Eltra-trade including an cost efficient compact PSR range, the advanced PST (B) range and a new effective PSE range.