Turbine technology | Monitoring and diagnostics
Figure 5. Compass runup (left) and DCS plot (right) for the third startup, highlighting shaft eccentricity. The IP shaft shows the highest eccentricity, at 56.60 μm, compared to the HP shaft at 31.71 μm and the LP shaft at 12.17 μm
(performed every 500 running hours due to prior cracking). Shortly after startup, increased vibrations at the bearing pedestals were observed, though they remained below alarm limits (Figure 4, right). On May 27, the unit tripped, but the cause was a boiler fault, not high vibrations. The boiler issue was resolved, and the unit was restarted.
During the second startup, two runup holds were bypassed in the DCS as no alarm limits were exceeded.
The unit synchronised and was loaded to 130 MW, but BP vibrations remained elevated (Figure 3, left, Figure 4, right). The operator contacted the Predictive Maintenance Engineer, who remotely accessed the Compass system. Analysis revealed elevated BP and 1X velocity vibrations for HP and IP turbine bearings (#1 to #4), with the 1X velocity vibration in an alarm state (not displayed on the DCS). Consequently, the Senior Operator and Predictive Maintenance Engineer decided to shut down the unit due to these findings.
Given market conditions, there was pressure to return the unit online quickly.
Diagnostics
From 27 May to 2 June, the unit was inspected to identify the cause of the high vibrations in the HP and IP turbine pedestal bearings. Checks for possible rubs in the outer gland housings and axial key wear revealed no significant issues. Meanwhile, the Predictive Maintenance Engineer analysed Compass system data, including eccentricity (disabled by the DCS above 600 RPM but monitored by Compass),
bearing vector displacement (Figure 4, left), and pedestal vector velocity vibration.
Compass data showed a significant increase in BP vibration at bearings #1 to #5 before shutdown, although these values were below DCS alarm limits.
Additionally, the 1X displacement magnitude (Figure 4) for HP and IP bearings exceeded alarm limits, which could not be viewed in the DCS, similar to eccentricity above 600 RPM.
The findings pointed to a possible issue in the HP turbine affecting the IP turbine or vice versa, or a concurrent problem in both turbines.
Root cause analysis
The issue may have been caused by a bent rotor, either permanently or temporarily during operation.
Potential causes include:
partial rub leading to uneven rotor heating; delay in barring the rotor while still hot after shutdown;
Imbalance caused by erosion, corrosion, deposit buildup/detachment, or blade loss; bearing wear or damage; axial keyway wear or damage; a combination of the above factors. During the initial inspection, axial key wear was ruled out, and the possibility of a permanently bent shaft was dismissed since shaft eccentricity normalised during barring. Other potential causes listed typically develop gradually, which was not observed in this case, simplifying further analysis.
Additional diagnostics were required to pinpoint the issue and restore unit 2 to operation as quickly as possible.
A controlled startup of unit 2 was conducted to further investigate the issue, with a portable diagnostic analyser connected to the Compass system’s buffered outputs for higher resolution data. During the test, the unit was run at manual speed holds of 500, 1174, 1500, 2000, and 3000 RPM. These speeds were selected to avoid generator critical speeds and turbine run backs. The startup was only to proceed if vibrations remained within acceptable levels. Since high vibrations were not observed, the turbine was brought to full speed. Figure 5 summarises the results. The Compass data revealed that the IP turbine shaft eccentricity remained high throughout the run- up. This confirmed the issue stemmed from the IP turbine section, not the HP section. The eccentricity issue was initially unnoticed in the DCS trend display due to its use being primarily for assessing turbine shaft straightness at barring speeds. Eccentricity measurements are disabled above 600 RPM in the DCS to avoid dynamic effects caused by imbalance or misalignment. When enabled above this threshold, eccentricity can function as a backup for 1X displacement measurements, capturing both static and dynamic signals.
After confirming the IP turbine section as the problem source, the outer and inner casings were removed for inspection, revealing significant damage (Figure 6): Missing blade package in the row-one stationary blades of the top diaphragm.
Figure 6. IP casing bottom section (left), top stationary blades (middle), IP shaft (right) 18 | July/August 2025|
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