Varnish’s Effect on Gas Turbine Vibration: A Case Study

Varnish is one of the main factors causing oil degradation in gas turbines. It’s common practice for refineries to analyze oil as part of their condition-monitoring routine, which indicates varnish and premature oil degradation.

Varnish is characterized by sludge-like deposits resulting from oil degradation and oxidation, which in turn causes further oil degradation. Varnish clogs filters, blocks lubrication lines, causes inadequate heat transfer in oil coolers, increases bearing temperature in turbines and compressors, and jams servo valves.

It also impacts vibration levels. The lubrication system plays a major role in this process. Its main functions include:

• keep shaft vibration at an acceptable limit
• lubricate and generate hydrostatic pressure to maintain shaft stability during normal operation
• reduce bearing temperature
• remove contaminants from the bearings
• protect metal surfaces against rust and chemical corrosion
• reduce friction

Oil’s primary function in a turbine is to keep shaft vibration within acceptable limits during normal operation and to prevent damage during start-up and shutdown when passing through natural frequencies. Journal bearings must also be properly lubricated to pass through natural frequencies under controlled conditions.

The clearance between rotating and stationary components is a crucial parameter of journal-bearing and labyrinth-seal design. Any reduction or increase in this clearance can affect bearing vibration levels—varnish deposits between the shaft and bearing pads can significantly alter clearance.

CASE STUDY: SGT-600 INDUSTRIAL GAS TURBINE

Siemens Energy’s SGT-600 gas turbine consists of two main sections: gas generator (GG) and power turbine (PT). The two main sections are not mechanically interconnected.

• The GG comprises an axial-flow air compressor, combustion chamber, and compressor turbine. The axial-flow compressor and compressor turbine have 10 stages and two stages, respectively. The GG rotor is supported by two journal bearings.
• The PT consists of a turbine with two stages. Its rotor is supported on two journal bearings.

FIGURE 1: SGT-600 gas turbine bearings

In the refinery, the SGT-600 gas turbines drive methane export compressors. These compressors transfer methane to the Iranian Gas Trunk (IGAT) line. In recent months, bearing No. 2’s vibration level had relatively increased in one of the gas turbines to a minimum magnitude between 6 mm/s rms and 7 mm/s rms; its maximum level reached up to 12 mm/s rms. The alarm and trip limits were 7 mm/s rms and 15 mm/s rms, respectively (FIGURE 2).

FIGURE 2: Vibration trend of bearing No. 2

FIGURE 2 shows the vibration levels increased sharply within one to two hours and then decreased sharply. There were no significant changes in operating speed, pressure, or other process parameters. The following factors impeded an accurate picture of the dynamic and vibration behaviors that were occurring during sharp vibration changes:

• Lack of online vibration analysis system
• Lack of proximity probes on the bearing
• Installing only one accelerometer on the bearing

Therefore, the only way to analyze vibration characteristics was the frequency spectrum from the accelerometer installed on the bearing (FIGURE 3).

FIGURE 3: Frequency spectrum of bearing No. 2

As seen in FIGURE 3, the dominant peak is at rotational speed, i.e., 1X, a primary sign of unbalance. Due to the lack of another accelerometer and the inability to perform orbit and phase analyses, a definitive conclusion could not be drawn. However, if the unbalance was the only problem, the vibration levels would continuously be high and not have severe fluctuations, which means a more significant factor affected the bearing’s vibration. To obtain more information, the oil was analyzed.

OIL ANALYSIS

An oil analysis revealed a rising trend in the membrane patch colorimetry (MPC) test (FIGURE 4), indicating the onset of oil degradation.

FIGURE 4: MPC trend due to the oil analysis results

As varnish forms, particles gradually deposit on metal surfaces and cause issues, particularly in low-clearance areas. MPC had increased in the past 18 months, providing sufficient time for varnish deposits to form on the shaft, bearings pads, and labyrinth seals. Since bearing No. 2 is close to the combustion chamber, it receives the highest heat. Consequently, oil in this bearing is more exposed to thermal degradation, so varnish formation is more probable than in other bearings.

On the other hand, as mentioned previously, severe vibration fluctuations were only observed in bearing No. 2. Hence the hypothesis that varnish deposits reduce clearance and cause partial rub between rotating and stationary parts, e.g., the shaft and bearing pads (FIGURE 5).

FIGURE 5: Varnish formation in journal bearing decreases clearance and finally partial rub

This partial rub generates local heat, leading to thermal bending of the rotor locally, resulting in a thermal unbalance—due to temperature gradient in part of the rotor—and ultimately increased vibration. The number of deposits decreases gradually as the rotating and stationary parts touch. Therefore, the clearance increases, partial rub stops, and rotor temperature gradually decrease. Consequently, the thermal unbalance is resolved and vibration decreases.

To evaluate the above hypothesis, the varnish must be removed or minimized, and the vibration examined.

VIBRATION CONDITION AFTER CORRECTIVE ACTIONS

An oil flush was performed, and after, the oil was changed. After start-up, an oil sample was taken and the oil condition, especially the MPC parameter, was evaluated. The oil analysis showed a significant decrease in the MPC level from 23.9 to 1.4 (FIGURE 6).

FIGURE 6: MPC before and after oil change

FIGURES 7 and 8 show a trend of vibration and frequency spectra of the bearing (before and after the oil change), respectively. The bearing’s vibration significantly decreased after the oil change, fluctuating between 4 mm/s rms and 5 mm/s rms, which is an acceptable range. This proves the hypothesis that varnish caused the increased vibrations.

FIGURE 7: Bearing No. 2 vibration trend after oil change

FIGURE 8: Frequency spectrum of bearing No. 2 before (top) and after (bottom) oil change

CONCLUSION

Here are the results from our investigation:

• The bearing's vibration behavior showed intermittent peaks over several hours. If mechanical issues such as unbalance occur, the vibration is continuously high.
• Unbalance was initially observed in the frequency spectrum analysis. However, precise evaluation and definitive conclusions were not possible. A more accurate evaluation of GG and PT's bearing vibrations requires another accelerometer and two proximity probes on each bearing. Currently, no proximity probes are installed, making it difficult to assess the shaft's dynamic behavior and perform analyses such as orbit and phase. This complicates the troubleshooting process.
• The results before and after the oil change indicate a significant reduction in varnish and consequently a vibration reduction.
• Varnish formation in gas turbines is inevitable. Oil flushing and substituting new oil cannot completely remove varnish—varnish-removal equipment is required.

About the Authors

Mohammadreza Galeban is a Condition Monitoring Supervisor at the Persian Gulf Bidboland Gas Treating Co.

Moslem Kamaie Monfared is a Maintenance Manager at the Persian Gulf Bidboland Gas Treating Co.

Abed Khateri and Hasan Mokhtaripour are Senior Condition Monitoring Experts at the Persian Gulf Bidboland Gas Treating Co.