Torsional Analysis Resolves Pump Motor Failure


Torsional Analysis Resolves Pump Motor Failure

A six-stage horizontal centrifugal pipeline shipping pump driven by a 10,000 BHP variable frequency electric motor failed after only four years of operation. Inspection of the failed unit revealed that the rotor bars were badly deformed with points too close to the stator windings. Two identical pumps were inspected and the same problem was detected, though not as severe.

DYNA, a vibration analysis consultant, was called in to investigate the three systems. Utilizing TorqueTrak instrumentation and Micro-Measurements strain gages, test data was collected over the full range of shaft speeds from dead stop on up to the maximum operating speed of 4,100 rpm. Two speeds were found to produce specific torsional vibration amplitudes, indicating a resonance condition, on each of the three pumps. The first (at 720 rpm) was the typical Slow Roll speed. Maximum dynamic torque values as much as 30% of the supplied torque were measured at this speed. The second was at 3,375 rpm, which was within the normal operating range, with peak loads as great as 10% of the supplied torque detected. For an electric motor, the maximum allowable dynamic torque is 1% of the supplied torque. At these shaft speeds, the 6th motor harmonic was exciting resonance in the system and causing high lateral vibration leading to premature failure.

As a result of the analysis, the suggested solutions were to shift the Slow Roll speed up or down 10% and to add inertia (a weighted disk) close to the coupling to move the natural frequency node below the normal operating range. Once implemented, the vibration was eliminated and the reliability of the pumps restored.

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