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Causes of Sudden Steam Turbine Front Bearing Gap Voltage Drop: Analysis and Solutions

Question:

Dear Experts, We are managing a high-pressure steam turbine system that drives a synthesis gas compressor train. The steam pressure is 101Kg/cm2, steam temperature is 500degC, turbine RPM is 13200, and KS flow is 320T/hr. The turbine rotates clockwise when viewed from the steam inlet with a bearing design of tilting pad (05 pads load on pivot configuration). For further details on the machine layout, please refer to Annexure-1. The governing system was originally PGPL Governor with extraction control, but it was changed to an electronic governor Woodward505E in 2013. In August 2016, the machine was operating at low load operation with HP at 65%, LP at 67%, and steam flow at 222 ton/hr. There was a sudden decrease in gap voltage at the turbine front end bearing VT-209A, along with a rise in bearing temperature to 151degC. The shaft vibration levels were monitored throughout the event, with a jump to 136 microns at the time of tripping. Unfortunately, ADRE was not installed at the time of tripping, but vibration data was recorded from the Bentley Navada 3500 display panel. Upon dismantling the front bearing (VT-209), it was observed that the babbit of the pad at the 10o’ clock position had been wiped off. The main question arising from this situation is: What could have caused the sharp decrease in the gap voltage from 8.44 to 7.1 in just 1 minute? For more information and visual references, please see the attached annexures.

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Hello John and other industry experts, I would greatly value your expert insights on the sudden decrease in gap voltage and issue with bearing damage. Looking forward to hearing from you. Thank you.

The drop in voltage coincided with a rise in bearing temperature, indicating potential issues during pad wiping. Of particular concern is the bearing temperature reaching 116 degrees Celsius before the incident. Is this a common high temperature for the bearings, or had it been increasing prior? Additionally, it would be expected for the RTD to be located on the bottom pads if fitted. This pattern of behavior suggests that bearing oil temperature may be a factor. What was the oil temperature for the bearings?

Dear Vibramac, Thank you for your response. It was really informative and helpful. In our analysis, we found that there are two thermocouples in place: one on the bottom pad and the other at the 7 o'clock position (TI_18). TI_18 is referenced in Annexure-IV of my previous post. Prior to the load reduction at 12:12 PM, the bearing temperature (TI_18) was recorded at 98.7°C. After the load reduction began, we observed an increase in the bearing temperature to 109.4°C, remaining at this level for approximately 4 hours until 4:10 PM. Within the next 27 minutes until 4:37 PM, the temperature slowly rose to 117°C, with a gap voltage of 8.46 at that time. By 4:38 PM, the temperature had spiked to 151°C. Subsequently, at 4:39 PM, the gap voltage reached a minimum of 7.1V and the bearing temperature dropped to 146°C. Attached is a table with detailed data for your reference. The decrease in gap voltage and the discovery of bearing wipe-off at the 10 o'clock position suggest an abnormal rotor position during low load operation. This sudden drop in gap voltage is unprecedented. For additional context, the weight of the steam turbine rotor is only 440kg. We are eager to hear insights and advice from fellow experts in the forum regarding this intriguing issue. Please refer to the attached tabular data for more information. Attachment(s): Tabular Data.pdf (132 KB) - 1 version

In my opinion, the data, especially the temperature spike, indicates a classic bearing failure (specifically a pad) that was likely supporting the journal. The abnormal journal position under light load could be attributed to misalignment (possibly related to a gear coupling issue), steam admission path, or casing distortion (specifically contraction in this case). It would be beneficial to examine past journal and temperature patterns under similar load conditions. It would also be informative to view the SCL data over the same 4-hour period and the direct orbit leading to the bearing failure. Kudos on a well-documented case study with valuable data and a clear timeline of events.

What was the condition and performance of the lubricating oil system? Did you observe any pressure variations? How frequently are the lube oil strainers altered and maintained?

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During the transient event, ADRE was not installed, which means that SCL cannot be verified. However, the machine has been in operation since August 2016 with steam valves fully open. As a precautionary measure, the specific HP lift opening where the issue occurred has been noted to prevent similar incidents in the future. The lubricating oil system, as assessed by MICHAEL MEEHAN, CMRP, CR, is in good condition. The oil was completely replaced in March 2016, and the strainers and oil filters were also cleaned and replaced at that time. Each bearing oil supply has a local pressure indicator, and no variations were noted before or after the event. Additionally, the lube oil header pressure is monitored in the DCS, and there have been no fluctuations in its trend.

Frequently Asked Questions (FAQ)

FAQ: 1. What are the operating conditions of the steam turbine system in question?

Answer: - The steam pressure is 101Kg/cm2, steam temperature is 500degC, turbine RPM is 13200, and KS flow is 320T/hr.

FAQ: 2. What type of bearing design does the turbine front end bearing VT-209A have?

Answer: - The turbine front end bearing VT-209A has a tilting pad design with 05 pads load on pivot configuration.

FAQ: 3. What was the change in the governing system of the turbine in 2013?

Answer: - The governing system was originally PGPL Governor with extraction control, but it was changed to an electronic governor Woodward505E in 2013.

FAQ: 4. What were the operating conditions of the machine during the incident in August 2016?

Answer: - The machine was operating at low load operation with HP at 65%, LP at 67%, and steam flow at 222 ton/hr.

FAQ: 5. What were the observed symptoms during the incident at the turbine front end bearing VT-209A?

Answer: - There was a sudden decrease in gap voltage at the turbine front end bearing VT-209A, along with a rise in bearing temperature to 151degC. Shaft vibration levels jumped to 136 microns at the time of tripping.

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