Your analyst team has found a vibration problem with your reciprocating compressor. Operators are uncomfortable with the vibration, things look scary, “I can see things shaking!” Now what is the next course of action?
The balance of force and response is the key to keeping vibration levels in check on any piece of machinery, and more so with a reciprocating compressor where there are more complex forces. For vibration to change there must be a change in one of the two factors in the vibration equation.
Vibration= Dynamic Force x Dynamic Flexibility.
The key to the equation is the dynamic components of each factor. The forces and flexibility of the components change with frequency. This is an important key to keep in your mind when vibration problems, particularly with reciprocating compression, crop up from nowhere. Something has to have changed in the whole system of components and the associated forces for this vibration level to change.
Field Pulsation Vibration Analysis - Temp bracing.
From the start, getting a handle on the forces generated from the sources, be it the compression of gas or the balance of the reciprocating components will determine where the force input may have changed. Was there a change in unit operating conditions? Was the loading sequence changed? These factors will then guide the rest of the investigation if they are unchanged.
Next would be the flexibility of the units support structures such as mounting bases, chocks, supports, clamps or any other significant structure. This will require some expertise in to how the unit is supported or how they are constructed to identify common failure points and areas to inspect. The failure of a weld or crack in a support structure will adversely change the Mechanical Natural Frequency of the system. So now components that would not be responsive to common input forces, can become amplified to the point of failure. Simple repairs to these failed structures will improve the response to the vibration. However, if speed is variable, dynamic force and dynamic flexibility are important to recognize as your repairs may align these resonant frequencies with other common forces.
Accurate testing of the Mechanical Natural Frequency will be paramount in determining the course of action to rectify the problem. If the history of the machine has MNF data from previous analyses, the comparison can be made to see if the repairs to the support are making a positive impact.
The important point to remember when vibration levels change is which of the two factors have been affected. Is it the dynamic force or the dynamic flexibility? Once you have determined this, the solution is not far away.
Jason Hoffman C.E.T at EMS Inc.
