The fluid should be filled in the casing before the pump work. The impeller will be rotated by the bearing with high speed.
The fluid in the casing will be rotated at the same time.The fluid will absorb the energy when it be throw from the center of the impeller to the outside within the centrifugal force,then the fluid going into the volute pump casing directly , the speed of the fluid will be reduced as the diameter of the runner being more and more lager , meanwhile, some of the kinetic energytransfer to the static pressure energy ,therefore,the fluid will flow outside with high pressure to the location. there is a vacuum in the impeller of butterfly valve when the fluid flow outside from the center of the impeller ,As the pressure of the top tank is lager than the pressure of the inlet . The fluid will be press to the impeller constantly, therefore we can conclude that the fluid will inlet or outlet only if the impeller rotate constantly。