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Tuesday, September 7, 2010

Hydraulic Balancing Devices

If all the single suction impellers of a multistage pump face the same direction, the total theoretical hydraulic axial thrust acting toward the suction end of the pump will be the sum of the individual impeller thrusts. The thrust magnitude will be approximately equal to the product of the net pump pressure and the annular unbalanced area. Actually the axial thrust turns out to be about 70 to 80 percent of this theoretical value.

Some form of hydraulic balancing device must be used to balance this axial thrust and to reduce the pressure on the seal chamber adjacent to the last stage impeller. This hydraulic balancing device may be a balancing drum, a balancing disk, or a combinations of the two.
Balancing Drums
The balancing chamber is at the back of the last stage impeller is separated from the pump interior by a drum that is usually keyed to the shaft and rotates with it. The drum is separated by a small radial clearance from the stationary portion of the balancing device, called the balancing drum head, or balancing sleeve, which is fixed to the pump casing.

The balancing chamber is connected either to the pump suction or to the vessel from which the pump takes its suction. Thus the back pressure in the balancing chamber is only slightly higher than suction pressure, the difference between the top being equal to the friction losses between the chamber and the point of return. The leakage between the drum and the drum head is, of course, a function of the differential pressure across the drum and of the clearance area.

Balancing Disks
The disk is fixed to and rotates with the shaft. It is separated by a small axial clearance from the balancing disk head, or balancing sleeve, which is fixed to the casing. The leakage through this clearance flows into the balancing chamber and from there either to the pump suction or to the vessel from which the pump takes its suction. The back of the balancing disk is subject to the balancing chamber back pressure, whereas the disk face experience a range of pressure. These vary from discharge pressure at its smallest so that the difference between the total force acting on the disk face and that acting on its back will balance the impeller axial thrust.

If the axial thrust of the impeller should exceed the thrust acting on the disk during operation, the latter is moved toward the disk head, reducing the axial clearance between the disk and the disk head. The amount of leakage through the clearance is reduced so that the friction losses in the leakage return line are also reduced, lowering the back pressure in the balancing chamber. This lowering of pressure automatically increases the pressure difference acting on the disk and moves it away from the disk head, increasing the clearance. Now the pressure build up in the balancing chamber, and the disk is again moved toward the disk head until an equilibrium is reached.

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