Civil Engineering Handbook

Balance Piston

U is desirable to have additional axial-load control on the multistage compressor. A balance piston, also referred to as the balance drum, can be located at the discharge end (see Figure 5-46). The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston (opposite the last stage location) is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston area to develop a thrust force opposite that generated by the impellers. The pressure on the

Figure 5-46. Balance piston. (Courtesy of Elliott Company)

low pressure side of the balance piston is higher than the reference pressure by an amount equal to the resistance of the balance line, the line taking the flow from the low pressure cavity to the reference point. Line resistance, of course, is a function of the flow in the line. To permit efficient balance, an effective seal must be used at the rim of the balance piston because the leakage also represents parasitic power loss. In the earlier paragraph on sizing, a target value of 1% was used as a base value, recognizing that for higher pressure applications this value would tend to be greater. While full control of the thrust can be developed by controlling the diameter, limits are in order. Generally, the balance force is kept less than that developed by the impellers, with the thrust bearing taking the remainder of the load. This keeps the rotor on one face of the thrust bearing for all load conditions and is the recommended practice. An alternative philosophy overbalances the thrust with the balance piston, arguing that balance piston seal deterioration will unload the thrust bearing for more conservative design. The problem with this approach is that the rotor will tend to shift its operating position from one side of the thrust bearing to the other for varying loads and conditions. Because the thrust bearing has .012 to .015 inches of float, the rotor will not be in a fixed position, making instrumentation for rotor position difficult to judge. Also, overling the balance piston means a larger seal diameter, making the potential seal leakage greater. Besides the ramifications of the higher leakage, the method tends to be somewhat self-fulfilling in that the deterioration will tend to increase at a higher rate.

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