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Early problem detection on rotating machines

Condition monitoring systems help prevent damage from resonant vibrations
Early problem detection on rotating machines

Everyone is familiar with the catastrophic effect that resonant vibrations can have on bridges when winds cause the structure to oscillate violently to the point of destruction. Resonant vibrations can have similarly dramatic effects in the industrial environment. Fortunately, these can be prevented by monitoring vibration trends with a condition monitoring system and taking corrective measures before damage can result.

When Prüftechnik was called in to assess the damage to a supply air fan in a large pharmaceutical company, the scene was one of destruction and devastation (Fig. 1). Both pedestal bearings had been torn from the fan shaft and hurled away from the machine. The 65 mm shaft had been bent 30° on both sides of the impeller. The impeller itself had struck the left and right suction cones, completely destroying them and leaving it badly dented. All the fan belts were torn. In short, the fan was a total loss. What had happened?

One important clue was the operator’s information that the damage had occurred as the fan was starting up. Since an identical fan was in operation nearby, it was possible to record a so-called Bode diagram, or ramp-up curve, where the amplitude of the rotational vibration was plotted as a function of the rotational speed. Rotational vibrations are usually caused by rotor imbalance. The resulting curve provided the evidence necessary to reconstruct what had happened. At approximately 1130 min-1, and thus below the operating speed of 1300 min-1, there was a sudden rise in rotational vibration amplitude to over 80 mm/s, which abruptly declined above the critical speed. This signal curve corresponds exactly to the characteristics of a rotor as it passes through its first natural bending vibration. Not only the marked rise in rotational vibrations but also the phase rotation of 180° were typical.
This made it clear that the only possible underlying cause of the destruction was rotor resonance. However, the catastrophe would actually have been triggered by excessive imbalance in the fan impeller. This imbalance may well have been acceptable at operating speed, but would have caused excessive vibration excitation as the impeller passed through the resonance point – with fatal consequences for the fan.
Design improvements
The degree to which vibration increases at the resonance point depends mainly on the damping of the rotor and the dwell time of the rotor in the resonance region. For example, the heavy-duty rotors of large steam turbines take longer to reach large vibration amplitudes than lightweight fan wheels. Consequently, in addition to ensuring that the rotor is well balanced, it is important that it passes through the resonance point quickly before excessive vibrations can build up in the resonance region.
If the resonance point of the rotor has to be shifted, modifications must be made to the fan design. The most important parameters here are rigidity, mass and bearing spacing. However, such changes are usually too costly to permit extensive redesign. As a minimum measure in variable speed fans, the speed in the resonant region must be blocked in the converter control unit.
Online monitoring
More effective than making constructional changes to the rotor is monitoring the fan during operation. In the present case, the operator decided to install an online condition monitoring system – a decision grounded to no small degree on the extent of machine damage already incurred. Increasing demands for high machine and process availability were the major motivating factors. Operator safety was also a primary concern. Maintenance personnel are continually exposed to a significant hazard when performing manual measurements in the vicinity of enclosed fans. Yet another reason for the decision – which at the same time placed a demand on the future online system – was the vibration fluctuations caused by the process-related speed variations of the unit.
The Vibronet Signalmaster from Prüftechnik Condition Monitoring was selected to handle this task. Figure 2 shows the Signalmaster arrangement. The most important key data for the overall online system is as follows:
  • Online monitoring of 46 fans and 15 pumps
  • 172 online vibration measurement locations (vibration and bearing monitoring)
  • 42 speed measurement locations
  • 49 alarm states are output to the existing building control system
  • 4 Signalmaster units
As standard network stations, all Signalmaster units deliver their measurement data to an Oracle server via TCP/IP. Visualisation and evaluation of the measurement data are performed by the Omnitrend PC software. This software offers an extensive selection of tools and aids for precise condition diagnosis of the monitored systems. Omnitrend is an important element of the product philosophy, as it represents a platform solution for offline and online prod-ucts alike. This feature is particularly beneficial in combined monitoring concepts (deployment of hand-held measurement devices and online monitoring), where all participating measurement systems are able to deliver their measurement data to the same evaluation software. Figure 2 also shows an integrated remote access point over which external diagnostics experts can provide support without accruing travel and on-site expenses. In addition, Prüftechnik offers system maintenance agreements for remote maintenance of online systems.
Keeping cabling to a minimum
Figure 3 shows the structural configuration of a Vibronet Signalmaster. All Vibronet multiplexers are connected in series via a string line. This cabling strategy is highly advantageous in terms of the investment budget for field installation. Likewise, to keep cabling between the multiplexer and the machine or measurement location to a minimum, these are installed as close as possible to the fans and pumps, resulting in an average cable length of 10 m between the multiplexer and accelerometer. The raw vibration acceleration signal provided by the accelerometer is the basis for all subsequent analyses. It alone provides all the required data for the entire scope of analyses – ranging from an evaluation of the running smoothness of the vibration velocity according to DIN 10816-3 to complex roller bearing analyses using the envelope method. Further process parameters such as temperature, pressure, rotational speed and air quantity can naturally be measured via the multiplexers in the field or passed to the Signalmaster as existing process variables. If necessary, process-related fluctuations in the acceleration values can be automatically normalised in the Signalmaster by correcting them on the basis of the associated process parameters, thereby making them available for analysis and diagnostics.
All measured and monitored measurement variables are equipped with warning and alarm thresholds. If values reach or exceed the threshold values, status messages are output to the existing control system via the digital alarm outputs of the Vibronet Signalmaster.
In summary, this online system provides thorough condition monitoring of process machines. It facilitates early detection of potentially catastrophic damage, thereby preventing dramatic machine failure as seen in the case described above. Machine breakdown and unscheduled production downtime are now a thing of the past.
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