Hydraulic piston pump with what preparation work

First of all should do the preparatory work before the pump, such as the pre pump etc.. Based on the parameters of outlet flow, petrol dispenser pump current, pressure, liquid level and switched to each other, the principle is to start standby pump, after each part is normal, the pressure up, slowly open the outlet valve, and slowly close outlet valve switch hydraulic piston pump, pump outlet valve switch until it is completely closed, stop by the switching of the pump, but should try to reduce the flow rate fluctuation causing by switching. Why not start centrifugal pump wheel does not train: centrifugal gas transfer pump wheel does not train, that the pump has fault, the fault may be the impeller is what stuck or pump shaft bending over.