(1) For the low-power positive displacement pumps, large reserve factor is reserved for the capacity selection of the drive machine, no start-up problem exists no matter whether the bypass connection is started without load or with load. For large reciprocating pumps, the bypass connection method is recommended to start. At this time, the motor mainly overcomes the friction torque generated by the pump drive mechanism and the hydraulic torque caused by the pressure drop when the fluid passes through the pump assembly and the bypass system. In this case, There is no start-up problem; if the process requires a start-up with load, the start-up conditions should be carefully accounted. (2) Centrifugal pump start Centrifugal pump needs a small start-up torque, the use of motor as the driver, as long as the power is sufficient, there is generally no start-up problems. For a very powerful centrifugal pump, check the start-up of the pump. Figure 1 ~ 25 is the centrifugal pump axial flow resistance during start-up process and flow changes in the schematic. In the figure: A point is the start point of start, D, E, F is the resistance moment when the speed reaches 100% speed, and G point is the normal operation point. ABCDG is the centrifugal pump at the exit of the shut-off valve to start the transition process curve; open at point D l21 shut-off valve, pump to G stable operation. ABCF (CG) is the start-up curve of the centrifugal pump with the outlet shut-off valve open. At point C, the one-way valve after the shut-off valve is opened and the pump begins to deliver fluid to the system until the pump reaches point F at point G Resistance torque (rated flow) oC point can be determined according to the characteristics of the pump and pipe network characteristics. ABEG is the axial flow pump in the outlet shut-off valve starts the process of transition curve; open outlet at E cut-off valve, pump to G stable operation. Point A is the static friction drag torque of the pump mechanical system. For a horizontal pump with a sliding bearing, the drag torque MA is about 15% of the rated drag torque MG; for a horizontal pump with a rolling bearing, the drag torque MA is about the rated drag torque MG 10%. At point B, the rated speed of l5% ~ 20%, drag torque to a minimum. D, E points for the pump off point, the rotational speed of l00% of the rated speed, the point of resistance torque can be obtained by the shaft power of the dead point: (1-12) where Md - D point of the resistance torque , N · m; η - 100% of rated speed, r / min; η Off-point - shaft power at pump-off point in kW. The ratio of MD to MG is related to the specific speed of the pump. The higher the specific speed is, the higher the ratio of MD / MG. For the axial flow pump MF greater than MG, this is also why the axial flow pump requires valve opening. The resistance moment at any point on the BCD curve can be determined by the following formula according to a similar law: (1-13) where: resistance moment at a point on MX - BCD curve, N · m; resistance moment at MD - D point, N · m; ηx - the speed of a point on the BCD curve, r / min; η - l00% of the rated speed, r / min. Figure 1-26 Motor starting torque curve and pump resistance torque curve Figure 1-26 is a comparison of motor starting torque curve and pump resistance torque curve. As can be seen from the figure, for a large motor, around 80% of rated speed , The difference between the motor starting torque and the pump resistance torque is the smallest, which is an area that needs to be checked. In order to overcome static friction, the starting torque of the motor usually needs to reach 10% ~ 25% of the torque of the pump at the best efficiency point. For vertical long axis pump, due to the bearing more, start-up torque should be carefully checked.