Method for adjusting the balance disk clearance in multi-stage pumps

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I. Working principle of the balance disk in multi-stage pumps: The balance disk mechanism of multi-stage pumps consists of a balance plate and a balance disk. The pressurized liquid coming out of the last impeller flows through the radial gap between the balance plate and the balance disk into the chamber located between them, thereby keeping this chamber under high pressure. Behind the balance disc, there is a balance tube connected to the pump inlet, whose pressure is approximately equal to the pump inlet pressure. In this way, the pressures on both sides of the balance disc are not equal, resulting in a backward axial balancing force. 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 balance plate (i.e., by changing the pressure in the water chamber between them), equilibrium is achieved. But this balance is often a dynamic balance. The liquid under pressure that emerges from the final stage flows, through the radial gap between the balance plate and the balance disc, into the cavity in front of the balance disc; this cavity is at a high pressure. Behind the balance disc, there is a balance pipe connected to the pump inlet, whose pressure is approximately equal to the inlet pressure. In this way, the pressures on both sides of the balance disc are not equal, which in turn generates a backward axial thrust, that is, the balancing force. The balancing force is opposite to the axial force, thereby automatically balancing the axial thrust of the impeller. When the axial thrust of the impeller is greater than the balancing force of the balance disk, the pump rotor moves toward the inlet side. Due to inertia, this movement does not stop immediately at the balanced position; instead, it goes beyond that limit, resulting in an excessive reduction in the axial clearance of the balance disk. This leads to a decrease in leakage volume and an increase in the pressure in the cavity in front of the balance disk. As a result, the balancing force on the balance disk increases and exceeds the axial thrust of the impeller, pulling the rotor back toward the outlet side. Similarly, this process has inertia, which increases the axial clearance of the balance disk; as a result, the balancing force becomes smaller than the axial thrust, causing the rotor to move back toward the inlet side, and the above process repeats itself. This process is automatic; while the pump is operating, the rotor continuously moves axially within a certain balanced position, though the amount of movement is extremely small, making it difficult to detect visually. The balance disk is installed behind the final impeller of a multi-stage pump. Apart from a gap b between the hub (or shaft sleeve) and the pump casing, there is also an axial gap b0 between the disk and the pump casing. Behind the balance disk lies the balance chamber through which the inlet pipe passes. The high-pressure liquid behind the final impeller flows toward the radial gap b, with its pressure dropping from P to P’. Since P’ is greater than P0 (the pressure in the balance chamber), a pressure difference is created on either side of the balance disk. The liquid at pressure P’ pushes the balance disk backward and flows through gap b0 toward the balance chamber. This force that pushes the balance disk is the balancing force, and it acts in the opposite direction to the axial thrust of the rotor. II. Method for adjusting the clearance of the balance disk in multi-stage pumps: High-pressure water exerts a force to the right on the balance disk through the clearance b0 (an axial force), causing the rotor components to move to the right; this results in the balance disk opening up and creating a clearance of b. The high-pressure water enters the balance chamber through this clearance b, and then returns to the pump inlet via the balance tube. When the net force acting on the impeller to the left balances the force acting on the balance disc to the right, the rotor no longer moves. It has reached a stable operating state. In traditional D-type horizontal multi-stage centrifugal pumps, the impellers are arranged in the same direction, and the axial force acting on each impeller points in the same direction. To balance these axial forces, a specialized balancing mechanism is used, which includes a balance disc, balance rings (balance sleeve), and balance pipes. Method for adjusting the clearance of the balance disk in multi-stage pumps: 1. Press the balance disk tightly against the balance seat, then use a dial indicator on the shaft to record the reading. Adjust according to the values indicated on the drawing for the clearance before assembly and after assembly. 2. Install the thrust bearing to push the rotor toward the low-pressure side (push it in one direction); the reading on the dial indicator at this point represents the clearance of the balance disk. 3. The gap size can be adjusted by adding or removing shims on the inside of the thrust bearing or thrust disc.