A schematic of the counterweight locations is shown in Figure 11. The original design utilized four rotating counterweights as shown.
Minimizing the main bearing loads required a total of 13 rotating counterweights (also shown in Figure 11).
Field Measurements
To verify the acceptability of the super balanced system, field data were compared before and after adding the rotating counterweights. The most significant improvement was in the frame and cylinder vibration levels. Figure 13 shows the differential frame vibration at the crankshaft centerline elevation. Maximum differential vibration levels were reduced by more than a factor of two.
Operating Deflection Shape (ODS) measurements showed similar reductions in frame motion and base-to-grout differential motion (Figure 14). The oil leaks have been considerably reduced and no main bearing problems have recurred.
The addition of the rotating counterweights had a significant influence on the torsional natural frequencies of the system. To maintain approximately the same first torsional natural frequency, the flywheel was trimmed to reduce the WR2 to compensate for the added WR2 from the counterweights. The measured torsional response showed that the first torsional natural frequency for the super balanced system is 24.8 Hz. This agreed well with the predicted first torsional natural frequency, which was 24.7 Hz. The torsional amplitudes also agreed well. It should be noted that this degree of correlation required careful consideration of the exact operating conditions (load step, speed and pressures).
This case history illustrates the need for detailed engineering whenever modifications to compressors are undertaken. The main bearing loads, as well as inertial unbalance forces and moments should be evaluated whenever the operating speed is increased. Controlling these forces often involves the use of both rotating and reciprocating counterweights. Re-configuring cylinders and counterweights can also have a significant influence on the torsional response of the system.
