Higher rotating speed increases the number of cyclic events (such as stress reversals) that occur in a given period of time. For instance, the number of valve openings and closures increase directly with speed. Valve life is related to the number of cycles of operation, as are the lives of other components. For a given stroke, an increase in rotating speed directly increases the average piston speed.
Average piston speed is an index of the rubbing speed and wear of the piston and its rings. Average piston speed, in feet per minute (FPM), may be calculated with the following formula:
A.P.S. = SN/6
S = stroke in inches
N = RPM
For critical service heavy-duty Class A compressors, a speed limit of 360 RPM is recommended. The average piston speed should be limited to about 860 FPM for lubricated machines, and 700 FPM for non-lubricated machines.
Class B, high-speed machines used in medium- to light-duty applications in the oil field typically have upper limits of 1000 RPM. Machines will work at these levels, but significant improvement in reliability can be achieved by running these units at 900 RPM or slower. Although the slower speeds result in larger cylinder sizes and increased rod loading, this disadvantage is usually overbalanced by the reduced