HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – BALANCING DISC AND COMBINATION OF BALANCING DRUM & BALANCING DISC

                Balancing disc is a balancing device used for hydraulic balancing of multi stage centrifugal pump which is fixed to the shaft and rotates with it.

It is separated by a small axial clearance from balancing disc head which is fixed to the pump casing as shown in the following figure.

 

 The leakage of the fluid through this axial clearance flows into the balancing chamber and then to the suction or suction vessel through a pipe which is also called balancing line or balance leak off line.

As the fluid shall flow to the suction from balancing chamber to suction, the pressure in the balancing chamber slightly maintained higher than the suction pressure. This balancing chamber pressure in balancing chamber can be maintained using restricted orifice. Also balancing chamber pressure is one of the important parameters of the pump to monitor. If balancing chamber pressure is not maintained within its limits then there will be a provision to damage of balancing disc and same thing applies even to balancing drum.

The forces acting on the balancing disc are as follows.

1.       Back side of the balancing disc = Balancing chamber back pressure.

2.       Front side of the balancing disc or disc face = Experiences some range of pressures.

The inner and outer disc diameters are designed in such a way that the difference between total force acting on disc face and that acting on disc back will balance impeller axial thrust.

If the axial thrust of the impellers exceed the thrust acting on the disc during operation then the disc is moved towards the disc head and thereby reducing the axial clearance between the disc and disc head.

From reducing clearance, the mechanism of the balancing disc can be shown in following steps.

Reducing the axial clearance between the disc and disc head 

Leakage to the balancing chamber is reduced

                    
Pressure difference between balancing disc and balancing chamber increases

Disc moves away from disc head

        

Clearance increases

      

 Fluid leakage to balancing chamber increases

      

 Back pressure in balancing chamber increases

Disc again moves towards disc head until equilibrium is achieved

 

    The main drawback with balancing disc is the pressure on seal chamber is variable which is injurious to seal. The life of seal decreases.

To avoid the above drawback of balancing disc and to retain the advantage of automatic compensation for axial thrust changes, the combination of balancing drum and balancing disc is used.

For this combination, the rotating portion of this balancing device consists of a long cylindrical body that turns with in a drum portion of the disc head. This rotating part incorporates a disc similar to one previously described.

In this design,

Radial clearance remains constant irrespective of disc position and

Axial clearance varies with the pump rotor position.

The combination of balancing drum & balancing disc can be shown in below figure.

 

 The forces acting on this balancing device is as follows.

1.       Towards discharge end = {(Discharge pressure * Area A) + (Average intermediate pressure * Area B)}

2.       Towards back side of the balancing device = Back pressure * Area C

Based on the applications and the requirement, balancing drum or balancing disc or combination of both can be selected.

While assembling the balancing device, all the bolts shall be tightened with recommended torque or else the scoring will be resulted on balancing drum or balancing disc with balancing drum head or balancing disc head.