Multi-stage centrifugal pump: suction seal pressure, discharge seal pressure, and pressure balance relationship

I don’t understand what you mean – is it the pressure difference before and after the balance disc?

The pressure in the sealing chamber at the discharge end is slightly higher than that in the sealing chamber at the suction end; if the balance tube becomes blocked, the pressure in the sealing chamber at the discharge end increases, and it equals the outlet pressure when it is closed

Under what circumstances will the suction seal pressure be higher than the discharge seal pressure?

The pressure in the inlet seal chamber is approximately equal to the pump inlet pressure, while the pressure in the outlet seal chamber is slightly higher than that in the inlet seal chamber. In the self-flushing 11 scheme, a flow-limiting hole should be provided at the inlet of the sealing chamber; otherwise, the sealing chamber at the outlet will not be properly flushed, causing the mechanical seal to overheat and get damaged

The balancing force in a multi-stage centrifugal pump is controlled by a balance drum or balance disk. The pressure at the pump outlet is higher than that at the inlet; this outlet pressure is directed to the other side of the balance disk through balance pipes, thereby balancing the pressure from the pump outlet toward the inlet and ensuring that the rotor of the multi-stage pump remains in the central position of the pump. However, due to changes in pressure, the position of the rotor is not fixed – it moves back and forth around the center of the pump, which is what is referred to as the axial movement of a multi-stage centrifugal pump.

The discharge pressure is higher than the suction pressure; axial forces are balanced through balance discs, balance pipes, etc

1. First, you need to understand how a multi-stage pump works: that is, the liquid to be transported enters the pump’s suction inlet at a given pressure. Due to the action of the impeller, both the kinetic and potential energy of the liquid increase. After entering the guide vanes, part of the kinetic energy is converted into potential energy (each impeller is equipped with guide vanes), and the vanes guide the liquid toward the inlet of the next stage of impellers under favorable hydraulic conditions. Due to the repeated occurrence of this process from one stage to another, the same pressure is added at each stage; after passing through the final guide vanes, the fluid enters the annular chamber of the cylindrical body, and finally exits through the discharge port into the discharge pipeline. II. Working principle of the balance device: The inlet of the impeller faces the drive end of the pump. The liquid pressure acting on the rear cover plate of each stage of the impeller is greater than that on the front cover plate, and this axial force is very large, typically ranging from several dozen tons to several hundred tons. Therefore, the pump must have a balance device to counteract this axial force directed toward the suction end of the pump rotor. The pressurized liquid exiting the final impeller flows, through the radial gap between the balance seat and the adjustment sleeve, into the water chamber located between the balance disc and the balance seat, thereby keeping the water chamber under high pressure. Behind the balance disc, there is a balance pipe connected to the pump inlet, whose pressure is approximately equal to the pump inlet pressure. In this way, since the pressures on both sides of the balance disk are not equal, a backward axial balancing force is generated. The magnitude of the axial balancing force changes as the axial displacement varies, and by adjusting the axial gap between the balancing disc and the balancing seat (i.e., by changing the pressure in the chamber between them), balance is achieved. But this balance is often a dynamic balance. III. How is the axial force balanced? The common methods for balancing the axial force in multi-stage centrifugal pumps include symmetric arrangement of impellers, use of balance drum devices, balance disc devices, as well as combinations of balance drums and balance discs. There are also (high-pressure water pumps) that use a dual-balanced drum mechanism. Whether the impellers are arranged symmetrically or a balance drum is used, the axial forces cannot be completely balanced; therefore, thrust bearings are still required to accommodate the residual axial forces. In multi-stage centrifugal pumps, balance discs that can automatically adjust the axial forces are more commonly used to balance such forces. The balance drum is a cylinder that is mounted behind the final impeller and rotates together with the rotor. A radial gap is formed between the outer circular surface of the balance drum and the pump body; one end of this gap is the high-pressure area of the final impeller, while the other end is the low-pressure area connected to the suction inlet. The pressure difference acting on the balance drum thus creates a balancing force that opposes the axial force on the impeller, and its magnitude is determined by the diameter of the balance drum. Its function is only to reduce the axial force, but it cannot completely balance it. I hope this can help you