Dynamic (centrifugal, and axial) and rotary compressors generally exert much smaller dynamic forces than reciprocating compressors. Nevertheless, these forces should be accounted for to avoid a potentially serious vibration problem during operation of the compressor. A fault in the design of a concrete foundation is extremely difficult to correct after the concrete has been poured. There is no easy way to add mass, alter the stiffnesses, or adjust damping to change the natural frequency of a concrete foundation in an effort to move the system away from a condition of resonance. In a few extraordinary cases, it has been necessary to break out an existing foundation and pour a redesigned foundation to solve a serious vibration problem. Obviously, such instances are exceedingly expensive and time consuming.
While guidelines have been developed over the years for the allowable vibration of the foundation itself, criteria for defining the forces to be used in foundation design have been lacking.
A misunderstanding between the foundation designer and the compressor manufacturer regarding the unbalanced forces to be allowed for in the design has contributed to many foundation vibration problems. These problems have commonly been
caused by not designing for the actual dynamic forces, but rather for some lower value, due to communication problems between the foundation designer and the machine manufacturer.
Depending on how the question about unbalanced force is asked, the manufacturer might respond with the rotor’s residual unbalance from the dynamic balancing machine. This balancing-machine tolerance is an extremely small number which might be only 1/20th of the actual force at rated speed. At other times, arbitrary values are assumed for foundation design, yet they may not be representative of actual machine operation.