Resonance and rotor dynamics
Resonance conditions can result in excessive vibration levels, which in turn are potentially harmful to equipment and environment, particularly with PO, vertical and large rotodynamic pumps.
Pumps, their support structure and piping are subject to a variety of potential structural vibration problems (resonance conditions). Fixed speed applications often miss these potential resonance situations, because the common excitation harmonics due to running speed, vane passing frequency, plunger frequency etc., do not coincide with the structural natural frequencies. For VSD applications, the excitation frequencies become variable and the likelihood of encountering a resonance condition within the continuous operating speed range is greatly increased.
Pressure pulsations are the common excitation mechanism. These pressure pulsations may be further amplified by acoustic resonance within the pump or the adjacent piping.
Precautions There are a number of analyses that can be performed in order to predict potential resonance situations:
• Simple hydraulic resonance calculations.
• Passing frequency analysis.
• Using finite element analysis to investigate structural resonance.
• Modal testing of the physical machine can supplement the regular vibration test. Very often a pump intended for variable speed operation will only be tested at one single speed.
The risk of the rotating element in the pump or motor encountering a lateral critical speed increases with the application of a VSD. Lateral critical speeds occur when running speed excitation coincides with one of the rotor’s lateral natural frequencies. The resulting rotor vibration may be acceptable or excessive, depending on the modal damping associated with the corresponding mode.
Additionally, drive-induced torque harmonics may result in resonance conditions with torsional rotor dynamic modes.
• Lateral and torsional rotor dynamic analyses allow the calculation of rotor natural frequencies under operating conditions over the entire applicable speed range. Lateral modes are typically damped and the acceptability of a particular design depends on a combination of available modal damping and separation between natural and excitation frequencies.
• Torsional modes have little damping. The acceptability of the design may be proven by means of a forced response analysis and comparison of calculated stresses to allowable stresses.
• Three basic corrective actions, or a combination thereof, can be applied if a resonance situation exists and associated vibration levels are excessive:
• Locking out of certain speeds or speed ranges from the continuous operating speed range can be achieved in the drive software. However this may limit the operational flexibility.
• Detuning a resonance condition by means of changing the structural natural frequency or changing the excitation frequency.
• Reducing excitation levels by means of improving balance, alignment etc. or by means of changing impeller/diffuser or impeller/volute configurations.