OEL Group have several dedicated service centres, serving five continents, allowing us to provide expert field service anywhere in the world. Our experienced field engineers offer on-site consultancy, alignment, monitoring and where possible, bearing repair.
Hydro Power Station.
Client experiencing high bearing temperatures, causing the machine to trip.
CONSEQUENCES OF PROBLEM
Lost generation and lost revenue.
“It looks like the thrust temperature issue on unit 4 has been resolved. I don’t think the measured values are directly comparable to the pre-outage values, but the trend looks good.
Thanks for your help with this.”
Mechanical Engineer, Hydro Power Station
OEL Group were asked to perform on site services to establish the possible cause for high thrust pad temperatures on a ring lubricated horizontal thrust & journal pedestal bearing assembly. The bearing was located at the drive end of generator between the Flywheel and Turbine.
The active thrust face of this bearing is installed with 2 thrust thermocouples located at a 10 O’clock and 2 O’clock positions. The probes are installed into the same face. A temperature difference between these two positions of 20ºC was present during operation. Under normal operating conditions it would be expected that both temperatures should be similar if the load is equally distributed across all the pads and the thermocouples are installed in a similar manner.
OEL Group were employed by the client to carry our a full strip down and inspection of the bearing assembly in order to determine the root cause of the temperature differential across the working face and suggest and implement any improvements or modifications that may be required.
• Produce Risk assessment and method statements
• Provide on-site engineers and equipment
• Inspection existing instrumentation
• Full and comprehensive dimensional inspection during strip down
• Safe method of turbine jacking and temporary shaft support
• Removal of bearing assembly from client site to OEL Group workshop for refurbishment and modification
• Full reassembly on-site, including hydraulic torquing of bolts.
• Technical manual documenting as received general condition, anomalies identified, solutions implemented and commissioning.
JOINT FACING SEALING
Joint faces were found to be in a good condition and the flatness verified by carrying out an engineering blue check. During dis-assembly there was a noticeable variation of joint bolt torques. Uniform bolt loading is essential to ensure a leak free joint during operation. For this particular unit OEL Group recommended a torque setting of 2200-2400 Nm should be used during re-fit.
A phenomenon known as thermal ratcheting was present on the active thrust pads. We would not expect to see this issue on components operating at the temperatures measured during operation. Typically, this occurs on components which are subject to thermal cycling where operating temperatures are in excess of 90°C. The microscopic examination showed a large grain structure was present across all the thrust pads which is typical of a slow cooled casting process. These large grains are more susceptible to movement during thermal cycles and can lead to premature migration of the white metal and early bearing failure.
Electrical pitting had been detected on both the thrust pads and journal bearing and apparent in regions where you would expect to see the smallest operating clearances. This suggested a current built up within the rotating shaft was arcing into the bearing. This it not typical of this type of bearing installation and suggested that another issue existed with the current shaft earthing set up on this machine.
No manufacturing dimensional or geometric issues were detected with the thrust pads that would account for a high temperature being detected during operation. However, the clearance between the thermocouples and the hole drilled into the pad was found to be restrictive and could prevent the instrumented thrust pads from moving freely during operation. Considering general manufacturing tolerances and the potential for positional movement of the spherical bush during assembly we recommended that the clearance should be increased by increasing the drilled hole size and installing a reduced diameter probe.