IOtech products used in this application:

• ZonicBook/618E Vibration Analysis and Monitoring System • eZ-TOMAS Machine Vibration Monitoring Software

The ZonicBook/618E with eZ-Series software and your PC makes a real-time, portable vibration analysis monitoring system.

eZ-TOMAS is the economical solution to rotating machine monitoring and analysis. eZ-TOMAS will continuously monitor your machines. eZ-TOMAS records the change in vibration condition, and quickly provides you with the information you need to make critical decisions.

Many different kinds of problems can cause turbine rotor vibrations. Some common ones are called oil whirl, oil whip, and rub. Their symptoms are similar, and maintenance people often find difficulty distinguishing one from the other. Oil whip is a condition arising from a more basic problem called oil whirl, which in turn results from an uneven oil distribution (oil wedge) around the shaft in the journal bearing. Machine misalignment, improper oil viscosity, or an incorrectly designed bearing can cause this anomaly. It often generates a vibration at a frequency that is a subharmonic of the full rotor speed.

Rub is a condition where the turbine rotor contacts the stationary components including seal rings and the inside diameters of the bearings. Running the machine for a short time usually clears the problem, but in some cases, the wear continues to generate larger amplitudes that may become unstable and damage the machine in a short time.

Potential Solution
Engineers and technicians typically use data acquisition systems to measure, monitor, and record minute time vs. amplitude waveforms of the turbine-rotor shaft displacements within the journal bearings. The wave shape characteristics of the resulting plots help them determine which fault is the cause of the vibration condition.

Nelson Watson, president of Watson Engineering, Inc., Baton Rough, La., has been consulting in the utilities and process industries for many years. One of his clients was faced with exactly this situation recently, and Watson was called upon to diagnose and remedy the problem.

The test gear that resided at the site of a turbine compressor train was a type widely used by maintenance people and engineers in the process industry, but in this case, some of the permanently embedded sensors used to monitor vibration and generate orbit and phase plots were defective.

IOtech’s Solution
A 12,000-hp steam turbine driving a centrifugal compressor had been running without a problem for several years. Then, two years ago, the machine developed a vibration that damaged the bearings, which were subsequently replaced with a modified version.

The bearing malfunction came from a discharge check-valve failure that caused the compressor to rotate in reverse while the system was being switched to another unit. Soon after the incident, the turbine developed another high-amplitude vibration. The machine would shut down due to high vibration before reaching operating speed each time it was energized.

Watson set up his IOtech ZonicBook to record signals from radial proximity sensors located at both turbine and compressor bearings. A tachometer signal was connected to the governor speed sensor and calibrated to the turbine speed using a ratio function in the ZonicBook software. “Because the client’s embedded sensors were not operating,” says Watson, “I was unable to perform the typical orbit and phase analysis normally done under these conditions. In addition, the speed sensor stopped operating when the turbine vibration became excessive near the maximum running speed.”

So Watson connected the ZonicBook to the eight proximity sensors, two on each of the four bearings, and four tachometer signals, one at each of the four main bearings. He says, “The total recording time was about 10 minutes. The ZonicBook was able to record vibration signals that ranged from about 0.8 mils to 10 mils from a slow roll to a maximum speed of 4200 rpm. The initial vibration amplitude during the slow roll was about 0.8 mils, then it went through a critical speed where the amplitude reached 3 mils, and finally, it backed down to less than 2 mils.” As the speed increased, the amplitude reached approximately 10 mils.

Watson used the ZonicBook’s software to print a waterfall plot of vibration frequencies and amplitudes that characterized the turbine’s behavior. The first critical speed appears to be about 2100 rpm, although the manufacturer claims the critical speed should be 2850 rpm. The constant speed lines start at 1569 rpm and show zeros above 3824 rpm. The zeros appeared after the speed sensor stopped functioning when the vibration amplitude suddenly increased. The diagonal row of peaks is recorded at running speed and was the only significant vibration until it passed 3824 rpm. Beyond that point, the speed approached 4200 rpm (70 Hz) and the sudden vibration appears at 2100 rpm (35 Hz).

The oil whirl condition usually precedes the oil whip condition. Spectral and orbit analysis can be used to identify either situation. When this occurs, usually a sub-synchronous frequency can be measured in a range less than half of rotor speed. “Oil whirl produces a distinctive orbit pattern,” says Watson, “but because the client’s phase-sensitive transducers were not working, the orbit could not be displayed. It would have shown metal-to-metal contact within a bearing. Because the critical speed is close to one half running speed, it is difficult to determine if the problem is due to oil whip or a rub.”

The bearings were inspected, and one packing bearing rub was evident in both the compressor and turbine. Says Watson, “The most likely condition is oil whip, which caused the rub when the vibration amplitude increased dramatically. The turbine inboard bearing instability appears to have been caused by the compressor bearing damage. The instability occurred after the bearing failure and was apparently mitigated after the bearing repair.” The two problems were corrected and the machine ran smoothly.

Conclusion
Nelson Watson, president of Watson Engineering, Inc., uses an IOtech ZonicBook to troubleshoot large turbine and compressor problems that originate from a variety of sources, including unbalanced rotors and bearing problems due to oil whip, and rub. In a typical test, he connects eight proximity sensors to the bearings and records the vibration frequencies and amplitudes. The eZ-TOMAS software helps him analyze the data and pinpoint the problem’s source.

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