Multistage centrifugal pump shaft is also equipped with a series of two also on the impeller, it is relative to the general single-stage centrifugal pump, but also to achieve a higher head; relative to the piston pump, diaphragm pump reciprocating pump, but also Pump larger flow. Multistage centrifugal pumps have higher efficiency and can meet the needs of high-lift and high-flow conditions and are widely used in petrochemical, chemical, electric power, construction and fire protection industries. Due to its particularity, multi-stage centrifugal pumps have different and higher technical requirements in terms of design, use, maintenance and repair compared with single-stage centrifugal pumps. Often overlooked or neglected by people in some details, frequent failures such as abnormal wear, vibration, and axle holding after multistage centrifugal pumps are put into use have also caused the shutdown. A design 1.1 The basic structure Commonly used multi-stage centrifugal pump horizontal structure of the basic structure of open and multi-stage or multi-stage type in two forms. Horizontal open structure is characterized by upper and lower pump body through the axis of the horizontal split surface docking, import and export pipe, part of the volute and runner casting in the lower part of the pump housing, maintenance and repair more convenient, maintenance without disassembly Pump line can be removed directly on the pump housing. The segmental structure features each stage consisting of an impeller located in the diffuser housing. The diffuser is bolted to the connecting rod, and the stages are also connected in series by the mounting rod. The benefits are high pressure resistance , Difficult to leak, but must be removed in the maintenance of imported pipe, demolition assembly more difficult. Generally believed that the level of open multi-stage pump multi-stage multi-stage pump stiffness is good, low pump vibration. Inhalation chamber structure, the level of open multi-stage pumps are generally used semi-spiral, multi-stage pump mostly circular ring. The impeller of each stage of the pressure chamber, due to the volute manufacturing convenience, the liquid kinetic energy into pressure energy is high, the level of open multi-stage pump generally adopts the volute structure; but because of volute shape asymmetry, easy axis Curved, in the multi-stage pump segment is limited to the first and last sections can also be used volute, while in the middle section is used guide wheel device for the first stage impeller and secondary impeller energy conversion. The multistage pump's first-stage impeller is generally designed as a double-suction impeller. The remaining impellers at all levels are designed as single-suction impellers. This is especially true for media that have high temperatures, large flow rates and are prone to cavitation. For the very high pressure pump, with a single pump difficult to withstand the pressure of the shell, often using double-pump casing, the pump cylinder made into a barrel. Cylinder body to withstand high pressure pump, the cylinder installed horizontal open or segmented rotor. China's standard rules, high-pressure boiler feed pump with single-shell or double-shell structure of the cylinder type, 300MW and its generating units should be generally used on the pump double-shell cylinder structure. The inner shell of the double shell adopts segmented or horizontal open structure. 1.2 Axial force balance 1.2.1 Common axial force balancing measures Multi-stage centrifugal pump axial force balance measures are generally: impeller symmetrical arrangement, the balance drum device, balance disc device also balance drum, balance disc combination device Several. There are also double balanced drum balancing mechanism, such as some high-pressure boiler feed pump. Impeller symmetrical arrangement or the use of balance drum device, the axial force can not be completely balanced, still need to install thrust bearing to withstand the residual axial force, multi-stage centrifugal pump is more balanced with an automatic adjustment of axial force to balance the disc Axial force. In the design of multi-stage pump balance disc, balance drum and other devices, you must configure the appropriate balance of pipeline in order to make the axial force balance device to meet the design requirements. In the multi-stage pump bearing temperature is too high, the bearing burned accident, many are due to the balance of the flow area is too small, the pipeline resistance loss is too large, the balance of capacity failed to meet the requirements. Literature [1] also balancing drum device, for example, proposed a method of calculating the diameter of the balance pipe. In view of the multistage centrifugal pump, the balance disc and the pump are likely to be damaged due to the fitting of the balance disc and the balance disc seat. A multi-stage centrifugal wedge anti-friction balance disc [2] is designed as shown in FIG. This structure is similar to the dry gas seal principle of centrifugal compressors: when the balance disc is close to the balance disc seat, the power wedge can generate a huge opening force, thus preventing the balance disc from being fitted with the balance disc seat. After nine months of operation test, the balance disc is working normally with no wear and scratches on the work surface. This shows that the new type of dynamic wedge wear balance disc can effectively prevent the balance disc from being fitted with the balance disc seat. The dynamic wedge balancing disc can not only prolong the service life of the balance disc, but also reduce the amount of leakage of the balance disc gap and save energy and reduce consumption. Some people also based on multistage pump axial force is due to the impeller at all levels are one side of the water absorption, proposed by improving the pump body, impeller and interstage plate structure so that both sides of the impeller water, to achieve axial force balance, so Do not need to set the balance plate, balance drum and other institutions, do not need to consider the amount of axial play. 1.2.2 Balance plate, balance drum mechanism limitations a) variable conditions: Pump start and stop, the moment of axial force by the balance plate and balance plate seat to withstand direct contact, the friction may cause the balance plate, seat bite Death, dry burning, and even pump shaft was broken accident; load mutation, the axial force changes, the rotor also axial movement, resulting in balance plate, the gap between the sudden change, prone to cavitation and vibration . b) Liquid-solid two-phase flow media: The pressure of the medium entering the balancing disk, balancing drum and other balancing mechanisms is the output pressure of the pump. The pressure after the throttle is the inlet pressure of the pump, and the medium forms when flowing from the high pressure zone to the low pressure zone Jet erosion, liquid-solid two-phase flow of solid particles in the media will soon wear bad balance mechanism balance plate, seat and other dynamic and static parts, the final pump can not be normal operation. 1.3 Axis deflection Multistage centrifugal pump shaft deflection is too large, easily lead to abnormal vibration, shaft, mechanical seal sealing surface also caused by failure of failure, etc., should be designed to control the radial force generated to minimize the pump shaft Deflection value during operation. Generally considered in the design of the measures are: a) spiral structure with diversion and energy conversion of multi-stage pump, the spiral shape of the asymmetry in the operation of the shaft is easy to bend, the two adjacent volute should be staggered 180 ° Arrangement to reduce radial force. b) The pump impeller series not too much, if necessary by increasing the lift of each impeller to ensure that the total head, so by reducing the pump impeller series to minimize the length of the pump shaft. c) When selecting multi-stage centrifugal pump shaft material, consider the material suitable for the type of medium, temperature and other needs at the same time, give priority to the selection of materials with good mechanical strength and stiffness. d) Consider the factors such as transmission power, starting method, radial force, shaft deflection and related inertia load when designing the pump shaft diameter; consider the radial force that may be generated during non-design flow operation to resist bending deformation of the shaft Need. e) Reasonable choice of pump shaft support points. 1.4 Anti-vibration and vibration-proofing Considerations The design of multi-stage pump anti-vibration and vibration reduction measures are as follows: a) Control the pump shaft deflection within the regulation range. b) Clearly require the pump shaft, impeller and other dynamic and static balance test. c) Multi-stage pump shaft should be designed according to the rigid shaft, the working speed should be less than or equal to 0.75 times the first-order critical speed. d) Independent positioning of the impeller and the pump shaft in single-stage, the impeller and the pump shaft are fitted by interference fit and also increase the rigidity and critical speed of the rotor assembly. e) Selection of materials such as pump shafts and impellers, selection of material with good quality uniformity, selection of material supply status and processing method to ensure uniform cross-sectional quality of the material. f) Design appropriate shaft and radial clearance to avoid vibration caused by abnormal friction and axial movement of rotor and stator. g) Multistage pumps with balanced discs to balance axial forces to properly and correctly design the balance disc mechanism. 1.5 vertical multistage pump For vertical multistage centrifugal pump, the general design of the axial force under normal operation to consider the downward direction, but in the early stages of the car, because the outlet pressure has not risen, the leaves