Referring back to Figure 10—1, remember that we have compared the actual gas compressor speed to the speed indicated by the curve that passes through point “A”. We calculated point “A” from the natural gas flow, and the observed suction and discharge pressure. We said that if the measured gas compressor speed exceeded the speed indicated by point “A” on Figure 10-1, then the gas compressor was deficient. This is not quite true. The following factors all raise the actual speed as compared to the speed calculated from Figure 10-1:
• Increased suction temperature.
• Increased gas compressibility
• Lower gas specific gravity
• Reduced impeller diameter
It is a relatively simple matter to reduce the diameter of the gas compressor impellers; they can be turned down on a lathe. For instance, on one centrifugal compressor, the impellers were trimmed down from a 12″ to an 11″ diameter. Other factors being equal, the speed of the gas compressor end of machine increased from 11,000 rpm to 12,000 rpm, while the speed of the combustion air compressor held constant at 13,200 rpm.
Figure 10—1 Actual speed vs. the predicted speed based on compression ratio and flow is a measure of centrifugal compressor efficiency.