How to balance the impeller of multistage series slurry pump
(1) Arrange impellers symmetrically.
As shown in figure 1-69, impeller with symmetrical arrangement is suitable for even number of stages. If the stage is odd, double suction impeller can be used for the first stage. Due to the different leakage and hub sizes of different stages of multistage pump, the impeller symmetrical arrangement can not completely balance the axial force, there is still a part of the axial force to be borne by the bearing. The advantage of this balance method is not to increase the volume loss, but the disadvantage is that the structure of the pump body is complex. It is mainly used in single suction two-stage Cantilever Pump and volute type multistage pump.
(2) Use balance drum. Figure 1-70 is a schematic diagram of the balancing drum, installed behind the last stage impeller. The balance chamber is behind the balance drum, which is connected with the suction chamber of the stage I impeller through the balance pipe. Therefore, the pressure in front of the balance drum is close to the discharge pressure of the last stage impeller, while the pressure behind the balance drum is equal to the sum of the pressure in the suction chamber and the resistance in the balance pipe. This creates a pressure difference before and after the balance drum to balance the axial force of the pump. The gap between the outer edge of the balance drum and the balance sleeve on the pump body is very small, which is 0.2~
0.3mm. Due to the frequent changes of pump working conditions, the balance state of balance drum will be affected, and the thrust bearing is still required to bear the remaining axial force.
(3) Automatic balancing plate is adopted. In centrifugal pumps, the automatic balancing disk is commonly referred to as the balancing disk. Its advantage is that it can automatically balance all axial forces under different working conditions, so it is widely used in multistage segmented centrifugal pumps. Its structure is shown in Figure 1-71. Its structure is characterized by a radial clearance B between the hub (or shaft sleeve) and the pump body, and an axial clearance Bo between the end face of the balance plate and the pump body. Behind the balance plate is the balance chamber which is connected with the suction port of the pump. The liquid pressure before the radial trace B is the pressure P on the general surface of the last stage impeller. After the liquid flows through the radial chamber B, the pressure drops to P. the pressure drop of the radial chamber is as follows:
After the liquid flows through the axial gap, the pressure drops to P0, and the pressure at both ends of the axial gap drops to,
In this way, a balance force is applied on the balance plate, and its direction is opposite to the axial force.
The principle of auto balance is as follows: when the axial direction is greater than the balance force, the rotor moves to the left, the axial clearance B0 decreases, and the resistance coefficient through the clearance increases. Of course, the total resistance coefficient of the whole balancing device also increases, but △ P does not change, so the leakage Q decreases. As a result, △ P1 decreases, while △ P2 increases, thus increasing the balancing force. The rotor moves to the left continuously, and the balance force increases continuously. When it reaches a certain position, the balance force and the axial force are equal and reach balance.
Similarly, when the axial force is less than the balance force, the rotor will move to the right, B0 increases, △ P2 decreases, and the balance force decreases. When the rotor moves to a certain position, the balance force is reduced to the same as the axial force, and the balance is achieved again. Therefore, the centrifugal pump equipped with balance plate device is generally not equipped with thrust bearing.
(4) The balance device is composed of balance plate and balance drum. Since the balance drum can balance 50% - 80% of the axial force, it can reduce the load of the balance plate, so a large axial clearance can be used to avoid the friction between the balance plate and the pump due to the rotor movement. It has been proved that this kind of balancing device has a good effect in large capacity and high parameter segmented multistage whiskers,