BOILER FEED WATER BALANCING LINE

In any centrifugal pump, each impeller tends to produces some amount of thrust because of different pressures and different geometries on the two sides of the impeller. In a high pressure multi-stage pump (such as BFW) the number of impellers is high, thus the net thrust would be large unless something is done to balance it out.

In Multistage centrifugal pump operation, Suction side will have less pressure and discharge side will have high pressure. Due to this pressure difference the impeller with pump shaft and bearing will move from discharge side to suction side. This movement is called axial trust. The axial trust will Damage the pump internals and bearings.The two main ways to reduce the net thrust are to oppose the impellers or to use a balance disk/drum.For axial split pumps, it is usually most economic to oppose the impellers.  About half of the stages are oriented with the suction pointing toward the coupling and the rest are oriented with the suction toward the thrust bearing.  The thrust of the stages pointed in opposite directions tend to cancel out.  The net thrust that the thrust bearing must take is much smaller than it would be if they all pointed in the same directionBut axial split cases tend to have an upper pressure limit.  At very high pressures, barrel pumps are used since they can handle the very high pressures better.  With a barrel pump, it is much more difficult to find a good way to direct the flow path through a set of opposed impellers.  So instead, they point all the impellers in the same direction and use a balance disk or drum on the end.The balance disk is just after the last stage so it has full discharge pressure on one side.  A line is routed from the other side of the balance disk back to the suction. Since there is a very high pressure differential across the balance disk/drum, it is critical that the clearances are correct or else excessive flow will be diverted back to suction and the thrust balancing force will be lost.  In other words, if the balance drum/disk fails, a thrust bearing failure is likely to follow.  

Balancing Line and Balance Drum  is used to balance the axial trust . The balance Drum is attached to the shaft in the Non drive side The balancing chamber will be in the Discharge cover. The discharge Cover will have the balance sleeve or bush. There will be gap between the Balance Disk and the Balance Bush.Balance line  is a tapping  taken from the pump discharge line and connected to the pump suction.

During operation the water from the last stage of the impeller passes between the drum and drum bushes and enters the balance chamber at pressure equal to the suction pressure. there will be two pressures. pressure before the balance drum and after the balance drum. the difference in pressure causes the balance drum with shaft to move and create a clearance between bush and  the drum. 

If the axial trust on the impeller is higher the shaft will move to non drive side reducing clearance between the drum and bush. so that the amount of leakage to balance chamber also reduced causes pressure in balance chamber to be reduced. Thus lower the pressure in balance chamber causes difference in pressure and the disk tends to move away from the disk head causing more clearance and the pressure builds up in the balance chamber and moves the disk towards the drum.

by this way the shaft of the pump is hydraulically balance under tension by the balance drum. The small amount of residual forces will be taken by the trust bearing