Figure 200-39 represents the pressure profile acting on a centrifugal compressor impeller, showing net pressure and net thrust pattern. This pressure pattern on the impeller results in a net thrust force towards the suction end of the machine. The total net thrust is the sum of the thrusts from all the individual impellers.
The rotor’s thrust is handled by the thrust bearing. However, in most multistage compressors, a very large, if not impractical, thrust bearing would be required to handle the total thrust load, if not otherwise compensated. Therefore a thrust compensating device, or balance piston (or balancing drum) is normally provided as part of the rotating element.
As shown in Figure 200-40, compressor discharge pressure acts on the inside end of the balance piston. The area on the discharge side (outside) is vented, usually to suction pressure. The resulting differential pressure across the balance piston develops a force which opposes the normal thrust force, thus greatly reducing the net thrust transmitted to the thrust bearing.
Thrust compensation can be regulated by controlling the balance piston diameter. However, there are usually physical and design limitations. Normally a balancing force less than the total impeller thrust (approximately 75%) is selected to maintain the rotor on one face of the thrust bearing for all operating conditions. Otherwise, the rotor could bounce back and forth between the thrust faces as process conditions