By Allan R. Budris
Excessive vibration is a good indicator that some damaging phenomenon could be occurring within a pump, or the immediate pump system, which is why many pump users monitor pump vibration on a regular basis. However, once the vibration level is perceived to reach an unacceptable level (see June 2008 Column), the next question becomes: "what is the root cause of this excessive vibration, and how can it be corrected"?
You cannot just assume that the rotor is out of balance (which could be the case). There are many other potential culprits. Machinery vibration problems result from the interaction between an exciting force (hydraulic or mechanical) and the associated structural and/or hydraulic resonance frequencies. The stronger the exciting force and/or the closer these exciting force(s) are to natural frequencies, the greater the vibration amplitude. Although resonance response problems are most often encountered on new installations, they can also pop up on existing installations due to some system changes, such as the addition of a variable speed drive, a new replacement pump, and/or different pump piping.
Causes of Excessive Vibration Excitation
There are many potential vibration excitation sources. Fortunately, many of the sources do have specific frequency signatures (as multiples of the pump running speed), which can help in their identification, as shown in Table 1 below. Figure 1 shows some of these filtered vibration peaks, as multiples of the pump running speed and number of impeller vanes. It should further be noted that "diffuser stall", "recirculation" and "cavitation" sources shown below are hydraulic in nature, with the remaining being mechanical.