In fact, in order to answer this question, we should first understand an important characteristic of the centrifugal pump: when the pressure (head) is very low, the flow will be large, which can be seen from the pump performance curve. And pump power is proportional to the flow, when the pump starts, there is no pressure in the pipeline, the pump flow is large, so does the pump power and the rotation of the motor and pump parts need a great deal of acceleration from static to high-speed operation, which certainly will cause large starting current.
Taking the measure of closing the outlet valve, so the water pump won’t output water during starting to minimizing the power of the pump. After the pump reaching the rated speed, slowly open the outlet valve, gradually increasing water flow, to gradually increase electrical current to the rated current.
When start the centrifugal pumps, there is no water in the outlet pipeline, so there is no exist pipeline and ascension obstruction.After the centrifugal pump is started, head is very low, flow capacity is very large, the pump motor( shaft power )output is very large( according to the pump performance curve), it is easy to overload, which will damage the pump, motor and circuit. Therefore we need to close the outlet valve before centrifugal water pump starts to make sure the the normal operation of pump.