Centrifugal pump generally driven by the motor, before starting the pump, the pump body and the suction pipe filled with liquid. When the impeller rotates at a high speed, the impeller drives the liquid between the blades to rotate together. Due to the centrifugal force, the liquid is thrown to the outer edge of the impeller from the center of the impeller (the flow rate can be increased to 15-25 m / s) and the kinetic energy also increases. When the liquid enters the pump housing, due to the gradual enlargement of the flow passage in the volute-shaped pump housing, the liquid flow rate gradually decreases and a portion of the kinetic energy is converted into static pressure energy. As a result, the liquid flows out at a higher pressure along the discharge outlet. At the same time, a certain vacuum is formed at the center of the impeller due to the liquid being thrown out, and the pressure Pa at the liquid level is higher than the center of the impeller. Therefore, the liquid in the suction line enters the pump under the pressure difference. Impeller rotation, the liquid is also continuously being inhaled and pressed out. As the reason why the centrifugal pump can transport liquid, mainly by the role of centrifugal force, it is called centrifugal pump. So, the centrifugal pump to close the outlet valve after the start of what are the reasons? 　　 As the motor in the start-up phase, the rotor speed from slow to fast, to reach the rated value, it's stator current is from large to small, to normal current. Motor starting current generally up to rated current value of 4 to 7 times. In order to operate safely, it is necessary to try to start the motor under light load conditions and then add the load to the rated value after entering normal operation. When the pump is started, the flow rate is determined to be about 1/3, so that the maximum current can be avoided, so the centrifugal pump is closed and the valve is started. Standby unit speed is stable, open the valve, the opening time to be as short as possible, generally not more than 3 to 5min, or pump friction heat will cause pump parts damaged.