Condition Monitoring Can Extend Life of Bearings, Turbines and Plants

Monitoring the operating condition of bearings in a hydropower unit is a vitally important component of an overall plant maintenance and reliability program. A number of manufacturers have developed on-line monitoring systems to prolong the life and functionality of a plant by maintaining the integrity of one of its smallest pieces.

By Bethany Duarte

When the value of a hydropower plant depends on the successful operation and functionality of all its individual parts, ensuring this is a plant owner/operator's top priority. Each part brings in an element of manageable risk and requires monitoring in one way or another for that risk to be mitigated and the well-being of the plant preserved.

When discussing machine condition monitoring, Drew D. Troyer with GE Energy categorizes five "rights" for proactive management of machine reliability:¹

— Right alignment: Angular or offset misalignment produces stress concentration, which leads to wear, fatigue and yield deformation;

— Right balance: Dynamic force imbalance produces stress concentration which leads to wear, fatigue and yield deformation;

— Right lubrication: Wrong type, wrong amount, degraded or contaminated lubricant leads to mechanical and corrosive wear;

— Right fastening: Looseness produces stress concentration, which leads to wear, fatigue and yield deformation; and

— Right operation: Operating machines incorrectly leads to wear and failure and can be a danger to the operator and others.

In order to ensure that bearings in hydroelecttric facilities remain properly aligned, balanced, lubricated, fastened, and operating, constant, accurate monitoring is a must.

Bearings play an intricate yet integral role in the operation of a hydropower plant and are found in a number of locations, including:

— Guide bearings located both above and below the generator;

— Thrust bearings below the generator; and

— Guide bearings on the turbine unit.

The five "rights" listed above provide a partial checklist for keeping them in good repair, which affects the entire plant.

To preserve these components and keep them at their optimal level of operation, systems are available to allow plant owners and operators to monitor the condition of their bearings while the turbine-generator units are operating. These systems are designed to monitor a variety of variables, including vibration, lubrication and oil thickness, alignment, temperature and more.

While relatively small in size, a bearing that is not operating properly can cause a plant shutdown and damage other valuable components of the facility, including the turbine unit itself. A thrust bearing that is out of alignment, has insufficient oil thickness between the thrust collar and thrust bearing pads, or is not maintaining the proper temperature or oil pressure can cause significant overheating within a turbine unit and affect the overall efficiency of the plant.

Condition monitoring serves to track changes in operation, structure, and all of the variables listed by Troyer and to provide specific measurements detailing how far off the unit is from optimal functioning. By giving plant operators diagnostic details of the issues within the unit, machine condition monitoring provides the information needed to make adjustments, repairs and replacements to keep the plant running, thus increasing efficiency and revenue and decreasing downtime. Knowing in advance that a piece of equipment is nearing failure allows plant personnel to plan the repair/replacement, giving time to order parts and schedule manpower.

In addition, use of this type of system can lower the facility's life-cycle and maintenance costs as well as insurance premiums thanks to its ability to help hydroplant operators make proactive decisions regarding their bearings.

Case studies

Monitoring equipment has proved useful in determining bearing performance and condition at many hydropower plants around the world.

For example, at the 1,020 MW Akosombo Generating Station on the Volta River in Ghana, the Volta River Authority installed equipment to monitor the average thrust bearing temperature during a turbine upgrade project.² Use of this equipment allowed the plant owner to determine that the upgrade increased bearing temperature by 10 degrees Celsius, confirming increased thrust load. The thrust bearings were modified, and continual monitoring indicated good operation of the units at an acceptable temperature.

Another utility, Hydro Tasmania, uses proximity probes installed on its hydropower units, along with vibration monitors, to check alignment of the turbine guide bearing and shaft.³ This is essential to ensure proper alignment of the stationary components to the rotating element.

These are just two examples of the value of installing and using bearing monitoring systems in hydroelectric facilities.

Understanding the technology

New monitoring technology is constantly being developed by turbine and bearing manufacturers. In this article, we provide information on technologies provided by three companies that represent some of the newest options for on-line bearing condition monitoring.

Bruel & Kjaer Vibro

Bruel & Kjaer Vibro of Germany offers a technology called Compass 6000, which is a new modular platform for safety, trending and diagnostic monitoring. This integrated monitoring system combines safety, condition and performance monitoring, with the ultimate goal of diagnosing problems before they result in downtime or additional damage to the plant. The system can be used to monitor air gap issues, turbine vibrations, flow disturbances and more. With regard to bearings specifically, the technology tracks any vibrations in the turbine that would indicate loose bearings or other failures.

The Compass 6000 system utilizes an adaptive monitoring strategy that is sensitive to small changes in vibration, which assists with predictive monitoring. Additionally, the system automatically adapts where there are changes in the mode of operation, adjusting alarm levels to the new standards.

The software packages provide tools for monitoring different plant modules, trending and detection, diagnostics, performance monitoring and data analysis. The modules are designed to meet specific requirements depending on the input and output of each machine, such as input of displacement, velocity, acceleration and process sensors in condition monitoring modules and output over direct current relays. Modules also exist for generic sleeve bearing and rolling-element bearing machineries.

GE's Bently Nevada 3500 Monitoring System offers continuous, on-line monitoring for bearings in the turbine unit. (Photo courtesy of GE.)

An example of a hydroelectric facility using the Compass 6000 technology is the 455 MW Novosibirskaya plant on the Ob River in Siberia. This facility was built in the 1950s and is being refurbished and modernized with new turbines and control systems and upgraded to a capacity of 504 MW. The goal of this work is to improve plant reliability. Plant owner HydroOGK is using the Compass 6000 system to monitor all seven Kaplan turbine-generator units for the parameters of vibration (shaft/bearing, casing and stator core), cavitation, air gap, magnetic flux and partial discharge.

GE Industrial

GE's Bently Nevada Asset Condition Monitoring group in the USA provides condition monitoring products that measure data corresponding to three common bearing malfunctions: overload, fatigue and insufficient lubrication. These failures can be determined and often prevented by measuring guide bearing vibration (runout), thrust bearing oil film thickness, thrust bearing temperature and guide bearing temperature.

The SKF Insight system integrates diagnostic tests, monitoring, and communication into the bearing itself. (Photo courtesy of SKF.)

One of these products, the 330505 Low-Frequency Seismic Sensor, can be installed at bearing supports, where vibration often indicates major machinery issues. It is often installed on the bearing housing and measures vibrations within the range of 0.5 Hz to 1.0 kHz.

This sensor works with the company's 3500 Series Machinery Protection System, which has modules for vibration, bearing temperatures, thrust position, air gap, speed and process variables. For bearings, it utilizes radial vibration measurements for guide bearing runout. The system allows for up to eight individual modes of operation to be programmed, including time delays and varying alarm setpoints.

SKF

Sweden-based SKF recently released a new technology for bearing condition monitoring. The wireless SKF Insight system is located within the bearing. With this system, the plant operator is immediately made aware of changes to the operating conditions of the bearing that could lead to damage, which allows for quick remediation and risk mitigation.

The SKF Insight technology is made up of sensors located along the bearing, each designed to monitor and measure specific pieces of data — such as revolutions per minute, velocity, load, temperature, vibration and more. These sensors are linked wirelessly to an independent network to communicate the change as quickly and effectively as possible.

SKF says the integrated bearings, currently under trial in several industries, could prove useful for more than just what they were made for, especially in the hydropower sector. One measurement taken by SKF Insight is the load the bearing experiences, rather than the load the bearing was designed for.

In addition, SKF has introduced the SKF Machine Condition Advisor, which records vibration and temperature, and the SKF Advanced Bearing Analysis Kit, which analyzes the data collected by the advisor. A smaller solution is the SKF MicroVibe P, a hand-held vibration monitoring tool that links and communicates with Windows mobile operating system. The SKF Infrared Thermometer and Oil Check Monitor both complement this suite of tools.

All these tools, as well as on-line surveillance and protection and support on proactive decisions, integrate to build a program called SKF @ptitude Asset Management System. The @ptitude system connects all the individual lines of communication and pieces of data into a usable chunk of information that can assist in making decisions to improve plant health.

Coming full circle

Rotating machinery bears with it a certain measure of risk in that a number of things can go wrong, causing damage and, at worst, plant failures. Condition monitoring systems can help mitigate these risks by providing data to assist plant operators in making informed decisions about the health of their equipment. Not only do these systems help prevent problems, they can in turn increase revenue by ensuring the unit is available to operate when needed and decreasing time offline not generating power.

Notes

1. Troyer, Drew D., "The Dynamic Duo of Machine Condition Monitoring," www.ge-mcs.com/en/online-learning-center/machine-condition-monitoring.html.

2. Arthur, Kenneth, and Michael A. Dupuis, "Upgrading Thrust Bearings at Akosombo," HRW-Hydro Review Worldwide, Volume 18, No. 5, October 2010, pages 20-22.

3. Zulovic, Enes, "Improving the Shaft Alignment Process," HRW-Hydro Review Worldwide, Volume 16, No. 5, October 2008, pages 20-25.

Bethany Duarte is associate editor of HRW-Hydro Review Worldwide.