R. Mishra, Reliance Industries Ltd., Mumbai, India
Rotating machinery generate different types of problems, including misalignment, structural issues, bearing faults, internal faults and process-related issues, among others. Consequently, these conditions produce various signature vibrations that can be used to diagnose machinery health and recommend required corrective actions.
One such vibration signature is the condition with an increased 1X component. 1X component vibrations are the most typical to diagnose accurately, as they arise from different sources (e.g., unbalance, resonance, structural issues, misalignment). These vibrations can be distinguished by measuring phase data at different machine locations. An unbalanced rotor, rotor resonance and an eccentric rotor have similar vibration characteristics, including phase data. Machines running on a variable-frequency drive (VFD) or turbines are useful while eliminating resonance related issues by changing the forcing frequency. Sometimes, phase measurement is limited due to design/constructional issues. The bottom line is that it takes significant effort to diagnose simple 1X vibration issues. Sometimes, the source of vibrations cannot be identified; however, the vibrations can be suppressed by other means.
Observation. A similar case of 1X vibration was observed at a petrochemical plant. The machine train was comprised of a motor with a rating of 650 kW, 3,000 rpm and 3.4 kV, and a single-stage integrally geared compressor. The compressor was a Sundyne make single-stage, open impeller, integrally geared compressor with a single-stage gearbox that increased the speed from 3,000 rpm to 38,500 rpm. All motor and gearbox bearing points contained proximity probes. No thrust bearing was present on this motor. All proximity probe data was available on an online monitoring system for diagnosis. Forced lubrication was used for the compressor and gearbox, while sump lubrication was used for the motor.
The motor’s drive-end (DE) relative shaft vibrations were stable and running in the range of 15 microns–17 microns since commissioning in 2014. However, the motor’s non-drive end (NDE) side vibrations were slightly high and running in the range of 44 microns–38 microns pp since 2014. Motor overhaul and balancing were carried out in 2022 during a major overhaul of the compressor. After the overhaul, the same vibration readings were observed, ruling out the unbalance issue at the motor’s NDE side.
The motor’s NDE side orbit plot (FIG. 1) shows a highly preloaded condition with 1X component of approximately 90% of the overall value. The full spectrum plot shows all forward precision peaks. Bearing temperatures were stable and no significant change was observed. Since the motor was running, the sleeve bearings casing vibrations were very low to capture a stable phase data. The proximity probe phase data was approximately 100° relative phase at the DE side and 180° relative phase at the NDE probes.
The NDE side phase data was shifted approximately 90°. Also, during normal operating condition, the sump level of the motor’s NDE bearing housing dropped to 50% due to some leakage—during this time, the bearing temperature increased by 15°C and vibrations dropped from 50 microns to 35 microns. It was determined that this is related to some stiffness and damping issues.
Coast-up and coast-down data was reviewed from the last available data and it was observed that during the coast-up and coast-down of the motor, vibrations were stable up to 2,700 rpm to a maximum of 25 microns. From 2,700 rpm−2,980 rpm, there was a sharp increase in vibrations with a corresponding 90° phase change. This phenomenon was indicated by some natural frequency being excited. The motor’s bottom chock plate was checked for any abnormalities that might lead to resonance. All motor base bolts were loosened one at a time to observe any changes in vibration—no significant changes were observed. Motor NDE bearing blue matching was done to check the loading condition on the bearings.
Surprisingly, the motor’s NDE bearing bottom half was indicating a blue contact of approximately 30%. The bearing housing was checked, and it was determined that a clearance had been generated between the bearing shell and the housing. The NDE bearing and bearing housing were replaced. The concentricity was checked, but due to the limited time window and site workshop limitations, this activity could not be performed. After these possible corrections at the site, a motor trial run was undertaken. During this trial run, the vibrations remained high at 50 microns pp at the NDE bearing and 15 microns pp at the motor DE bearing.
Actions taken. After performing all the above-mentioned activities, the author’s company was disappointed to remain on the same platform where it started. As the company planned its next steps, it realized that the motor faced resonance issues in the range of 2700 rpm–2,980 rpm (FIG. 2). FIG. 2 shows the motor NDE bearing Bode plot after maintenance. The natural frequency shifted below running speed and vibration dropped at steady-state condition. The motor NDE orbit plot after maintenance at 2,600 rpm matches the orbit before maintenance at 2,980 rpm. This indicates that the problem frequency is moved away from the running speed. The motor full speed orbit plot after maintenance matches with the slow roll orbit. This indicates that the motor is running with minimum vibration at the NDE side bearing.
Resonance is a combined effect of stiffness, speed and mass. The company was unable to change the motor’s speed and the stiffness of the bearing as there was no forced lubrication. However, the shift of the natural frequency that was being excited could be altered. It was determined that the high vibration was at the NDE side bearing, while the DE side was experiencing much less vibration—this provided the margin for the cross effect of adding mass at the NDE side. The company also had the phase data from the online vibration monitoring system and, therefore, the angle required for correction. Finally, there were holes at the motor cooling fan hub where weights could be added.
Finally, the vibrations at the motor’s NDE bearing were reduced to 27 microns and the DE bearing vibrations increased to 27 microns in a motor solo run. After the coupling of the motor, vibrations further dropped to 16 microns at both the DE and NDE bearings.
Takeaways. This article has addressed the problem statement of high vibration and corresponding phase change at a motor running speed of 2,950 rpm. A mass of almost 80 grams was added at the motor cooling fan at the NDE end of the motor. This mass addition shifted the natural frequency being excited down by 200 rpm (~2,700 rpm) and avoided the running speed. Mass addition also generated the cross effect on the DE side bearing, resulting in an acceptable margin in vibrations at the DE side. HP
Rohit Mishra is the Senior Manager, Rotary Reliability-Condition Monitoring, for Reliance Industries Ltd., India. He has 11 yr of experience in the field of condition monitoring and holds a BE degree in mechanical engineering and Vibration ISO CAT-III qualifications.