Hydro Review

Identifying a Guide Bearing Rub Using Vibration Analysis

When commissioning a hydro unit, proper use of vibration monitoring can help identify problems such as a rub on the guide bearings. Catching issues early can allow the problem to be fixed quickly, preventing units from operating inefficiently and limiting unplanned outages.

By Bernard F Boueri

Improper installation of hydroelectric generating units - such as misalignment, generator air gap inequality or improper bearing clearances - could lead to rubs on the guide bearings. These rubs could occur on the bearing seals and/or bearing guide shoes. Rubs on the guide bearing pads could lead to damage of the guide shoes and/or shaft, while heavy rubs on the seals could damage the seals and the shaft. Symptoms of this type of rubbing manifest themselves as an increase in overall and 1x rpm vibration levels over time and/or an increase in bearing temperatures in the case of contact between the turbine shaft and guide shoes. (1x rpm vibration component is the speed of the rotating unit. The level of this component is usually used to determine, among other things, the balancing of the unit.)

These symptoms can be identified using proper vibration measurements and diagnosis. Rubs on shoes also can be determined by an increase in temperature on the guide pads, but it could be too late by the time there is a rise in temperature. Other issues - such as misalignment, unbalance, turbulence and cavitation - can only be determined using vibration analysis. All of the fleet protection systems installed at Canadian provincial utility Ontario Power Generation's (OPG) hydro plants are based on vibration monitoring.

After a rewind was performed on a hydroelectric generator, OPG asked its machine dynamic and component integrity department (MDCI) to perform a baseline vibration assessment of the unit to determine whether balancing was required before it was returned to service. This department performs baseline vibration measurements on the entire OPG fleet of 234 units, as well as provides diagnostics and support of all auxiliary equipment at the utility's 65 hydro plants.

This article provides a good example of how proper vibration assessment of a hydropower unit can help detect potentially harmful problems. In this case, both a rub on the generator seals and an air gap problem were identified. However, proper vibration monitoring of hydroelectric units could also identify unbalance, misalignment, runner cavitation, draft tube pressure pulsations and other problems.

Equipment setup and results

The unit under investigation is a full spiral school case vertical 15-blade Francis turbine (25 Hz blade pass frequency) with 112,000 hp, rated power of 85 MW and speed of 100 rpm (1.67 Hz). The hydropower station in which it is installed consists of two units with total intake of 15,200 cubic feet per second. The generator guide bearing and turbine guide bearing clearances were 20 mils and 18 mils diametrical, respectively.

Two proximity probes were mounted in the north and west directions on the generator and turbine. A temporary optical keyphasor supplied by General Electric Bently Nevada was installed on the turbine cover on the north side, in line with the north proximity probe, to measure rotational speed. A target consisting of reflective tape was installed in line with the rotor leads. The signals were input into a Bently Nevada ADRE 408 data acquisition system.

Preliminary testing was done at speed no load, with and without field, to determine the severity of the vibrations occurring in the turbine-generator unit and under which conditions they occur, as well as whether balancing is required to reduce the vibration levels.

Figures 1 and 2 show the 1x rpm and phase trend measured at the generator and turbine guide bearing, respectively. Relative shaft displacement levels at the generator guide bearing exceeded 34 mils peak-to-peak and were trending upwards. Relative shaft displacement levels at the turbine were about 6 mils peak-to-peak and trending upward as well. This behavior is evidence of a possible rub on the bearing seals. Normally in vibration trends the levels should be constant. If the levels increase over time, that is an indication of a possible rub due to the fact that during rotation the shaft is hitting the static component, which would cause an increase in vibration levels as heating increased due to the rub. In addition, a closer look at the orbit and time waveform of the generator shaft (see Figure 3 on page 11) shows that the shaft movement is restricted during a portion of the orbit, indicating a high possibility of a rub.

Personnel in the MDCI department requested that station personnel inspect the upper generator bearing seals on this unit. Visual inspection of the seals indicated a rub at that location, identified by the shaft discoloration. Clearances were measured using feeler gages and found to be set incorrectly. The upper generator bearing seals were readjusted to achieve proper clearances.

Before additional testing was performed on this unit, two proximity probes were installed on the generator shaft below the generator thrust and guide bearings. The original proximity probes on the generator will be referred to as "upper," while the ones installed below the thrust and guide bearings will be referred to as "lower."

Figures 4 and 5 on page 14 show the relative shaft displacement trends measured at the upper and lower locations. There was minimal improvement in the relative shaft displacement levels after the seals were adjusted at the upper location, and the shaft displacement trends were still increasing over operating time, up to 36 mils peak-to-peak at the upper location. It was evident that the rate of increase in relative shaft displacement levels was more severe at the lower location. This indicates that the bearing rub is still present.

Further investigation of the shaft orbit at the lower location confirmed the presence of a possible rub (see Figure 6 on page 16). MDCI department personnel requested that the upper generator guide bearing on this unit be disassembled to check for any rubs.

It was also evident during the second run that when the field was applied on the unit, there was:

- An increase in the relative shaft displacement levels from about 20 mils peak-to-peak to 30 mils peak-to-peak, with a shift in the 1x rpm phase of about 60 degrees at the upper location (see Figure 4 on page 14); and

- An increase in levels from 15 mils peak-to-peak to 20 mils peak-to-peak, with a shift in the 1x rpm phase of about 85 degrees at the lower location (see Figure 5 on page 14).

The increase in vibration levels and shift in the 1x RPM phase is in most cases an indication of air gap issues, and machine dynamic and component integrity department personnel requested that air gap measurements be performed. Investigation of the air gap by plant's electrical group revealed that the air gap was not concentric (it was off by about 4 mils in one direction) and needed adjustment as well.

Investigation of the generator guide bearing revealed a heavy rub causing discoloration of the shaft. All seals on the generator guide bearings were reset. In addition, after disassembly of the generator guide bearing, measurements of the guide shoe clearances were taken and it was found that some guide shows did not have the proper clearances.

Following reassembly, the unit trends of the relative shaft displacement levels at the generator and turbine guides were constant, indicating that the rub was cleared. MDCI personnel performed a balancing of the unit to reduce the overall relative shaft displacement levels. The turbine-generator unit was returned to service after balancing with overall relative shaft displacement levels of 7 mils on the generator guide and 8 mils on the turbine guide.

Conclusion

Vibration monitoring of hydroelectric units should be performed during commissioning (whether of a new unit or after a rehabilitation) to identify any problems that could be detrimental to operation of the unit. Proper probe installation and data collection is a must, in addition to the use of qualified personnel who are capable of determining the cause of the problem based on the vibration data acquired. In this case, MDCI department personnel were able to identify the problem early and prevent further damage to the hydropower unit's shaft and guide bearing. In addition, air gap problems were detected and corrected that would have resulted in possible mechanical and electrical problems.

OPG incorporates vibration measurements during outages and return to service as part of its schedule, to ensure that the unit will be acceptable for long-term operation. This practice has been proven to reduce unplanned outages of its hydroelectric generating units.

Editors' Note: Issues surrounding operations and maintenance of hydroelectric units and powerhouses are always hot. Recognizing this, our HydroVision International event features an entire seven-session panel presentation track on O&M topics. Come join us in Nashviille, Tenn., July 22 through 25 and hear experts speak on a variety of O&M-related topics, including generator, transformer and substation maintenance; the impact of intermittent renewables on O&M; bearing failures and shaft alignment issues; and turbine triage. For more information on this track and other offerings at HydroVision International, visit www.hydroevent.com.


Finding seal suppliers and installers, fast

The HydroWorld Buyers' Guide is a great way to tap into the expertise of all the product and service providers in the hydroelectric industry. This online resource, at http://buyersguide.hydroworld.com/index.html, features products and services in eight broad categories: dams/civil works; engineering and design; environmental; marine hydrokinetics; new project development and construction; operations, maintenance and rehabilitation; regulation/policy; and technology/equipment.

So, what if you are looking for a specific product? Simply type that product into the search box at the top right (above the Browse box). For example, if you are looking for seals, the guide features 48 companies with this term in their names or descriptions.

The companies listed hail from around the world. Twenty one of the companies are based in the U.S. and 11 in Canada. The rest come from Austria, Brazil, France, Germany, India, Switzerland, Turkey and the United Kingdom.

These companies provide a wide variety of products and services related to seals. The buyers' guide features two product and service categories directly related to seals: gate hoists/seals and seals/packings/gaskets. Under these categories, the companies provide rubber gate, mechanical, inflatable, segmented shaft and bronze seals; fluid sealing devices; gate sealing solutions; turbine shaft sealing; and polymeric sealing.

Information on a total of 21 specific products is available using the buyers' guide. These include:

- Axial turbine seals supplied by Fugesco Inc.;
- Butterfly valve seals supplied by SKF Sealing Solution Austria GmbH;
- Inflatable tunnel boring machine seals supplied by Obermeyer Hydro Inc.;
- Kaplan turbine seals supplied by PXL Seals;
- Radial turbine seals from Fugesco;
- Ring gate seals supplied by SKF Sealing Soutions Austria;
- Shaft/pump seals supplied by Wartsila Defense Inc.;
- Split mechanical seals supplied by AW Chesterton Co.; and
- Wear pads and seals supplied by Lignum Vitae North America LLC.

The HydroWorld Buyers' Guide is available year-round and updated regularly by the companies featured. So take advantage of it as a valuable to resource to help you do your job better.


Bernard Boueri, PhD, P.Eng., is senior engineer in the machine dynamic and component integrity department of Ontario Power Generation in Canada.

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