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.
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?
From your description, it looks like the bearing failure could be due to multiple factors. A sudden drop in the bearing gap voltage along with an increase in temperature suggests there might have been a sudden surge in load, possibly due to some discrepancies in the steam flow or excessive pressure on the bearing. Also, the wiped-off babbit could indicate a misalignment or instability - such issues can create uneven distribution of load among the pads and lead to wear. Finally, retrospective analysis of this unfortunate event underlines the importance of diagnostic tools like ADRE system which could've detected any abnormal patterns beforehand. Do also look into any changes in operating conditions or any maintenance work done prior to the incident. You may need to thoroughly inspect the condition of the entire bearing for any signs of similar damage.
From the information provided, it appears the sudden decrease in gap voltage and subsequent rise in bearing temperature could be the result of a variety of factors. A common cause may be bearing misalignment, which can result from physical impact, improper installation, or even normal wear and tear over time. This misalignment might lead to uneven load distribution, possibly explaining the wiped off babbit at the 10 o’clock position. Also, there could be issues related to lubrication failure or inadequate cooling. It's important to inspect the unit thoroughly, including the governor and extraction control system, especially considering the switch made in 2013 to Woodward 505E. Rigorous data analysis, regular maintenance, and good operating practices are keys to preventing such incidences.
It seems like your turbine may have experienced a problem with rotor dynamic instability, causing a thermal event. The sudden decrease in gap voltage coupled with a rise in temperature - especially up to 151degC - could indicate oil film breakdown in the bearing due to a significant increase in shaft vibration. As the vibrations escalate, it may also lead to the Babbitt layer of the pad being wiped. It's not uncommon under such conditions for the most load to occur at around the 10 o'clock position, leading to a higher rate of wear and eventually failure. I highly recommend taking a closer look at your oil pressure and lubrication system, as well your bearing design, and assess if it can withstand such operating conditions. You may need an upgrade to a more robust system or regular checks to avoid a recurrence.
Based on the sequence of events you've described, it seems plausible that the sharp decrease in the gap voltage could be due to a bearing failure, specifically the front bearing (VT-209). As you've pointed out, the babbit of the pad at the 10 o’ clock position was wiped off, which may suggest an excessive load or misalignment. This could lead to an increase in friction and heat, potentially resulting in a rising bearing temperature. The voltage drop may reflect these conditions as well. Another plausible explanation could be a system error or a fault with the Woodward505E governor, given it was installed relatively recently. Therefore, I would recommend a thorough investigation into both these areas.
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Answer: - The steam pressure is 101Kg/cm2, steam temperature is 500degC, turbine RPM is 13200, and KS flow is 320T/hr.
Answer: - The turbine front end bearing VT-209A has a tilting pad design with 05 pads load on pivot configuration.
Answer: - The governing system was originally PGPL Governor with extraction control, but it was changed to an electronic governor Woodward505E in 2013.
Answer: - The machine was operating at low load operation with HP at 65%, LP at 67%, and steam flow at 222 ton/hr.
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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