Make	Dongfang (Schematic Attached)
Power	335MW
Rated Speed	3000 RPM
Type	N330-16.67/538/538
Model	Subcritical, Reheat, 2-casing, 2-exhaust, Single Shaft, water-cooled, condensing steam turbine
Rated Steam Parameter	16.67MPa / 538 ºC
Steam Stages	31 Total: HP – 12, IP – 7, LP - 12
Coupling	HP – IP – Common Shaft HP/IP – LP – Rigid LP – Generator -Rigid
Vibration Monitoring System	Bentley Nevada
Vibration Probes	2 x Proximity Probes (µm) on each bearing.
Bearings	1 – 2 Tilting Pad (On Load Pad) 6 Pads 3 – 6 Elliptical
Exhaust Steam Pressure	6.9 KPa

Problem Statement:

After an outage at Plant in September (01-09-24 to 11-09-24), Bearing 1 vibrations have become erratic. Overall vibrations have seen a slight increase, but peculiar trend has been back and forth switching of X / Y Values. DCS Trend graphs have been attached below as well.

- Turbine overhauling was carried out in March - April 2023 (called A Type Outage); All bearings were inspected including Bearing 1. As found clearance was found slightly below limit due to carbon layer inside, condition otherwise was satisfactory and same re-installed back after cleaning (clearance as installed was within design limit). No maintenance on turbine or any bearing has been performed since.

- Alignment of complete train was performed and set within design limit.
- Bearing 1 bottom drain hole plug (2nd from left in the picture) was removed during A Type outage in Mar - Apr 23, previously only 1 (left most) was open. This resulted in decrease in temperature of Bearing Left Side sensor, thereby creating a Delta T of 10-12 degrees between left and right sensor. The trend has remained the same since then and no change has been observed.


Since A Type, vibrations have seen a gradual increase in trend with baseline vibrations increasing slightly after each shutdown. Shutdown reasons have varied, i.e: scheduled plant outages, National Grid power demand etc. Trend attached. X/Y Fluctuations were observed in August 2024 but have become more frequent post outage. Load variations (Grid Demand varies from 175MW to 330MW) increase fluctuations as well (10 12 fluctuations per day). No abnormal shut downs or operational situations have been faced during the period May 2023 - October 2024.

- Electrical team has inspected the sensors and associated connections for tightness and accuracy.
- Thermography of bearing was done to detect heat zones, no abnormality observed.

Vibrations are within allowable limit but the sudden and random spikes coupled with probe switching have become a concern. (Alarm = 127um, Trip = 250um). Currently, maximum Peak value registered was 95um. Min, Max and Avg vibration values table for past 1 week is also attached herewith.

Due to the steady full load, fluctuations have been less in the past 3 4 days.

Following operational parameters were checked and compared especially during switching periods, but no change in any other parameter has been observed:

• Lube Oil Inlet and Return Temperature
• Lube Oil Pressure
• HP Casing Absolute expansion Left
• HP Casing Absolute Expansion Right
• HP Differential Expansion
• LP Differential Expansion
• Bearing 2-6 Vibrations
• Axial Shaft Position
• Steam Inlet Temperatures and Pressures
• Exhaust (Condenser) Pressure
• Gland Steam Temperature and Pressure

The vibration average (and peak) values have remained steady in last one week but the overall rising trend and fluctutions have brought uncertainty as to the reliability of machine.

Support and opinion of the experts here is requested to help diagnose and troubleshoot root cause of the issue before next outage for proper corrective action planning.

Thank you.
Regards,
Syed Osama

 

Use a borescope inspection to check for signs of excessive wear, pitting, or any mechanical damage to the bearing. If wear or insufficient clearance is found, re-machining or replacing the bearing may be necessary.

 

Use a borescope inspection to check for signs of excessive wear, pitting, or any mechanical damage to the bearing. If wear or insufficient clearance is found, re-machining or replacing the bearing may be necessary.

Thank you Bidrohini for the response.
We actually performed vibration analysis of the System Data extracted through ADRE and the plots / graphs did indicate that there may be rubbing happening inside the bearing. .

But thats not the major concern right now. Concern is to know what else can be a possible cause of such an issue so that we can inspection on those lines as well during our outage. Lest we replace the bearing and the vibrations remain the same or increased soon after because we could not address the real root cause behind it .

Any case history or experience of such nature elsewhere would greatly help.