Centrifugal pumps have an important feature: when the pressure (head) is very low, the flow rate will be large, as can be seen from the pump's characteristic curve. The power of the pump is proportional to the flow rate. When the pump starts, there is no pressure in the pipeline, which causes the flow of the pump to be very large. The power of the pump is very large, and the rotating part of the motor and the pump runs from stationary to high speed. The acceleration, so it is bound to cause a large starting current, so the method of closing the outlet valve is adopted, so that the pump does not output water when starting, so that the pump power is minimized. When the pump reaches the rated speed, slowly open the outlet valve and gradually increase the water. The flow rate causes the motor current to gradually increase to the rated current. On the other hand, the water on the pump inlet pipe is stationary before the pump is turned on. If it suddenly accelerates, the back water “cannot keep up”, the inlet pressure will drop suddenly, the water will vaporize, and the centrifugal pump will be evacuated. (Because the centrifugal pump uses the pump impeller to drive the rotation of the water to generate centrifugal force, and the mass of the steam (gas) is very light, according to F = ma, the centrifugal force is small, so that the pump can not send water).
