Wear of the balance disk in multi-stage centrifugal pumps and precautions for replacing the balance disk

Reasons for wear: 1. During start-up and stop-up conditions, instantaneous contact and friction between the balance disc and the balance plate may occur. 2. When the axial force exceeds the design value, the balance disc moves automatically toward the balance plate, reducing the operating clearance. Due to manufacturing errors, the two cannot be perfectly parallel, which results in friction. In pumps with a balance disk structure that use only sliding bearings and no thrust bearings, replacing the balance disk is simple; there is no need to consider the axial position – the part can be replaced directly. For detailed steps, please refer to the manufacturer’s instructions.

The damages to the balancing device during operation are mainly manifested as follows: ① An increase in shaft play leads to a disruption of alignment between the outlet of the pump impeller and the inlet of the guide vanes, resulting in increased vibration of the pump body; ② Friction between the impeller seal ring and the guide vane seal rings causes an increase in the gap, leading to higher leakage between different stages and a decrease in pump efficiency; ③ When the wear on the balancing device exceeds the clearance of the coupling, it exerts a large thrust on the motor, damaging its bearings; ④ In cases where the balancing device is severely damaged, friction occurs between the impeller inside the pump body, the guide vane seal rings, and the pump body itself, requiring an immediate shutdown of the pump, otherwise the pump will be severely damaged. The pump impeller is severely damaged; the magnitude of the axial force on the pump is calculated as follows: when the water pump is in operation, the pressure at the impeller outlet is P2, while the pressure at the inlet is P1. The pressure difference ΔP = P2 – P1 is greater than 0, and its direction is toward the inlet. Then the inlet area on both sides of the impeller is ΔS=π(DW2-D2h)/4. The presence of a pressure difference means that, in high-pressure applications, reciprocating pumps and various rotor pumps (i.e., rotary positive-displacement pumps) are generally used. In industrial production, centrifugal pumps are the most widely used type. Hongli Pump Industry’s centrifugal pumps offer advantages such as high speed, small size, continuous and stable flow rate, and easy operation; they account for about 80% of all pumps in use. In many application areas for reciprocating pumps, there is now a trend toward replacing them with high-speed multi-stage centrifugal pumps. 1 Maintenance of components in multi-stage centrifugal pumps – Pump shafts; Causes and consequences of pump shaft damage. The pump shaft is the main component used for transmitting power and torque. After the pump shaft has been in operation for a certain period of time, the shaft journals wear out due to relative movement with the bearings or from repeated disassembly; the keyways widen, become rough, or become misaligned as a result of long-term use. The areas where the shaft meets other rotating components also wear out due to frequent disassembly, and cracks may form inside the pump shaft. What’s more serious is that the pump shaft becomes bent due to reasons such as increased vibration caused by rotor imbalance, misalignment during installation, excessive twisting during disassembly, improper start-up and shutdown procedures, and uneven temperature conditions. Wear of the pump shaft, loose fit, or internal cracks can affect the proper operation of the pump, while a bent pump shaft has an even greater impact on its normal operation. It is mainly manifested by severe pump vibration, rapid wear of components such as bearing seals, and even the formation of a vicious cycle. Methods for straightening Hongli water pump shafts: (1) Mechanical pressing method; (2) Twisting method; (3) Local heating method; (4) Local heating and pressing method. 1.2 The impeller of a centrifugal pump is the main component that provides energy to the liquid; it comes in three types: closed, semi-open, and open.

1. The design is unreasonable; it does not take into account the self-balancing of the axial forces acting on the impeller in a back-to-back configuration, as a result of which the majority of these axial forces have to be balanced by the drum. Due to the small clearance of the balance disc, the high-pressure fluid delivered by multi-stage pumps inevitably causes erosion on the balance disc. Corrosion, gas content, or impurities in the medium will exacerbate this erosion process. 2. Scratching is inevitable during start-up and shutdown ； 3. Abnormal operation, such as vacuum creation, leads to pressure fluctuations, which in turn prevents the axial forces from being balanced and results in rubbing.

The balanced wear of multi-stage pumps is also related to the process conditions; during installation and adjustment, the total clearance is measured first, after which the axial clearance between the guide vanes is adjusted. In actual operation, the balance disc does not come into contact with the inner disc.

1. What is the gap between the balance discs, and is a liquid film formed between them while the pump is in operation?