The secret to decreasing water hammer for pump applications with a soft starting is not only to "make the motor run slowly." More importantly, it is used to control the starting and stopping operation of the pump to avoid the sudden change in the water flow velocity in the pipeline.
Water hammer is typically induced by the sudden starting or stopping of a pump or by the quick closing of valves or check valves. These motions can produce an instantaneous acceleration or deceleration of water flow inside the pipeline and result in pressure shock.
A soft starter can help to prevent water hammer mainly in the following ways:
1. Gradual voltage increase during startup to prevent sudden water impact
This provides high torque instantly since the motor is direct-on-line starting. The pump impeller speeds up quickly. This causes a sudden surge of water flow in the pipeline, which might easily result in a pressure shock.
The thyristors are controlled by a soft starter to gradually boost the motor terminal voltage. This enables to build up the water flow stepwise and so reduce the pipeline impact at commencement by a steady increase of the motor torque and speed.
This can be interpreted as:
Direct start: the water is "pushed out" with force, abruptly.
Soft start: "pushed ahead" of the water slowly.
2. Soft stop during shutdown to reduce backflow and check valve impact
Water hammer is more likely to occur in pump systems after shutdown. In the case of the conventional shutdown, the motor is suddenly turned off and the pump speed lowers quickly, but the water flow in the pipeline still retains inertia. This can produce back flow and when the check valve closes rapidly it can induce impact noise and pressure waves.
The soft stop function of the soft starter can reduce the voltage of the motor gradually, so as to reduce the speed of the pump slowly. Backflow and water hammer created when the check valve closes suddenly can be reduced when the flow of water is progressively slow down.
3. Pump control performs better than ordinary ramp-down stopping
Ordinary soft stop only reduces voltage in a simple ramp-down manner. However, pump loads have a square torque characteristic, and the change in flow at low speed is not completely linear.
A soft starter with a dedicated Pump Control function can optimize the deceleration curve according to the characteristics of pump loads. This makes the reduction of flow and pressure smoother, making it more suitable for pump systems than ordinary voltage ramp-down stopping.
Efficiency leap Precision and stability
By limiting the starting current, a soft starter can reduce mechanical impact on the motor and pump shaft. It helps reduce instantaneous stress on couplings, impellers, and pipeline supports, making the entire pumping system more stable and reliable.
