HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – BALANCING DRUM

                In this post, the axial thrust forces distribution of the multi stage centrifugal pump and the concept of balancing drum will be discussed in detail. Below figure shows pressure distribution in a multi stage centrifugal pump.

 

To understand the concept of hydraulic balancing in multistage centrifugal pump, first we will consider the resultant pressure acting on the pump impellers can be understood by following cases.

Case I:  without installing any balancing devices.

From the above figure, it is clear that

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5. (Please not this is net pressure without installing any balancing devices)

To hydraulically balance on both suction side and discharge side, the net pressures acting on suction side and discharge side shall be made equal which can be done by installing balancing device.

Case II:  After installing balancing device

For this case also, the reference can be taken from above figure.

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5-Pb. (Pb is the balancing chamber pressure or pressure developed due to the design of balancing chamber and installation of the balancing device).

For perfect hydraulic balancing, P1 = Pb.

But in practical, Pb (balancing chamber pressure) shall be maintained in such a way that it is slightly greater than suction pressure as the fluid from balancing chamber will be going back to the suction side of the pump.

The balancing device shall balance this axial thrust and this hydraulic device may be balancing drum, balancing disc or combination of both.

The balancing drum is installed at last stage of the pump and is separated by a small radial clearance from the stationary portion of the balancing device called balancing drum head which is fixed to the pump casing.

Through the radial clearance, there will be fluid leakage and balancing chamber pressure is maintained based on the design parameters (dimensions) of the balancing drum.

The forces acting on the balancing drum are as follows.

1.       Towards the discharge end = Discharge pressure * Front balancing area (B) of the drum.

2.       Towards the suction end = Back pressure in balancing chamber * Back balancing area (C) of the drum.

Balancing drum diameter can be selected to balance the axial thrust completely or 90 to 95% depending on the desirability of carrying any thrust bearing loads.

The main drawback of the balancing drum is it lacks automatic compensation of any changes in axial thrust caused by varying impeller reaction characteristics.

For example, if axial thrust and balancing drum forces becomes unequal, then the rotating part will tend to move in direction of greater force. In this case, the thrust bearing must prevent excessive movement of rotating part.

Balancing drum performs no restoring function until such time as drum force again equals to axial thrust.

This drawback can be avoided using the design of the balancing disc.