Analysis of the Reason for the Pressure Fluctuation of the Balance Pipe of the Feed Water Pump of the Multi-stage Boiler

Analysis of the Reason for the Pressure Fluctuation of the Balance Pipe of the Feed Water Pump of the Multi-stage Boiler

Function of the balancing pipe for boiler feed pump:

The balancing pipe is a connecting pipe from the pump's outlet seal ring to its inlet end. Its primary function is to balance the axial thrust of the pump, reduce the axial movement of the rotor, and prevent friction between the impeller and the casing.

During operation, the boiler feed pump discharges high-pressure liquid from the impeller outlet. A portion of this liquid flows behind the impeller, equalizing the pressure there with the outlet. Meanwhile, the front cover plate acts as the suction end, maintaining low pressure. This creates a significant pressure differential across the impeller, generating an axial thrust parallel to the shaft that directs the rotor toward the suction side. In severe cases, this may cause friction or impact between the impeller and pump casing, jeopardizing safe operation. Therefore, balancing measures must be implemented to mitigate these effects.

     Diagram of the structure of the balance pipe of boiler feed pump

Multiple methods exist to balance axial thrust, including dual-suction impellers, symmetrically arranged impellers (for multi-stage pumps), and components like balance holes, balance discs, or balance drums. The balance pipe serves as a primary method to equalize axial thrust by diverting the pressure fluid behind the impeller to the inlet side, thereby achieving pressure equilibrium. While structurally simple, this approach cannot fully balance axial thrust. The residual axial thrust must be absorbed by dedicated thrust bearings and balance devices.

The principle of balance disc is similar to that of thrust bearing in steam turbine, and the balance pipe is similar to the return oil pipe of thrust bearing.

Analysis of the Pressure Fluctuation of the Balance Pipe of Boiler Feed Water Pump

1. As a balance pipe, its pressure should remain relatively stable unless it becomes clogged or leaks.

2. The balance pipe is used to eliminate axial thrust. When the pump outlet valve is closed or the downstream line is blocked, the pressure in the balance pipe becomes high; during pump siphoning, the pressure in the balance pipe is low. Under normal conditions, the pressure remains constant.

3. The balancing tube pressure of the high-pressure feed pump is slightly higher than the inlet pressure. If the pressure increases, it indicates that the gap between the balancing drum and its sleeve has widened. If the pressure reaches 2-3 times the inlet pressure, it is advisable to disassemble and inspect the system.

4. The pressure of the balance pipe is changed greatly because of the wear of the sealing ring, the balance disc and other wear parts.

5. The pressure difference of the balancing tube changes due to inter-stage leakage and the motor's frequency conversion (compared with the original speed).

6. When the external import pressure changes, the pressure difference of the balance pipe fluctuates accordingly.