Reciprocating Compressors – Unbalanced Forces and Moments

Reciprocating machines generate primary and secondary forces and couples as a result of unbalanced rotating masses and unbalanced reciprocating masses which accelerate and decelerate each revolution (see Section 300). These forces and couples react at the main bearings, and the resultant forces are transmitted to the foundation via the frame and bolting. These forces and moments are sometimes called shaking forces. Primary refers to the frequency of running speed; secondary means the frequency of twice running speed.

Forces arising from differential gas pressure acting on the piston and piston rod have no effect on the unbalanced forces and moments transmitted to the foundation.

The magnitude of these forces and moments can sometimes be extremely large, and they must be accounted for in the foundation design. There is no “rule of thumb” to predict the magnitudes as reciprocating machine designs vary widely in terms of the number and sizes of cylinders and crankshaft configuration. Data Sheet CMP-DS-875 may be sent to the vendor with the Company’s quotation request, and preliminary values are provided by the vendor so that the forces and moments of the various machines proposed can be evaluated. Preliminary foundation design can begin with this information, and be refined as the actual forces are furnished by the vendor.

Other Considerations
Any natural frequency of the foundation should be at least 30% above or below the primary and secondary frequencies. For instance, on a 450 RPM unit, ideally any natural frequency should not be anywhere in the range of 315 to 1170 cycles per minute (cpm). Sometimes it is impractical to keep natural frequencies out of a range that wide. Accordingly, it may be possible to place a natural frequency between the primary and secondary frequencies. A natural frequency in the range of 585–630 cpm would satisfy the criterion of 30% separation margins in the above example.

For reciprocating units, as a rule of thumb, the weight of the foundation should be a minimum of five times the combined weight of the frame, cylinders, and driver.

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