With environmental policies and legislations at every level of government becoming more stringent, the potential environmental and financial costs of an oil leak are causing many energy producers to consider water lubrication of bearings over conventional oil systems.
by Scott Groves
Hydroelectric turbines operating in remote areas all around the world are producing enormous amounts of clean energy. Nonetheless, a large number of these generating stations have the potential of posing an environmental threat to local ecosystems. Oil and grease have long been considered one of the primary methods of lubricating the lower turbine guide bearing. As environmental regulations become more stringent, any accidental or operational loss of oil or grease to the environment is not acceptable and can have very grave repercussions. This can be a serious issue in areas where detection, monitoring and spill clean-up can be difficult due to the poor accessibility or general remoteness of the site.
Converting to water lubrication
Converting a lower turbine guide bearing from an oil-lubricated system to a water-lubricated system potentially provides the same operational reliability of an oil-lubricated system without any risk of accidental or operational oil discharge to the environment.
In this example, initial conversations began in mid-2009 with an important electricity producer on the subject of converting an oil-lubricated turbine main guide bearing. The first unit considered for such a project was located in a mountain region using runoff from a lake at a higher elevation. This particular unit was originally built in 1927 and is located in the first of two single-unit powerhouses connected in series. Factors considered in selection of the first conversion site were the age of the unit and ease of access to the powerhouse. The chosen unit is the older of the two and was determined to be the greatest risk for oil leakage, which would have devastating effects on the complete ecosystem downstream of the station. It is also the more accessible of the two units at this particular site.
In early 2010, conceptual designs were developed and initial concepts were arrived at for two options.
With the design finalized, production began in late spring 2010. Water-lubricated Thordon SXL bearings use an elastomeric polymer material offering the combination of strength/stiffness and flexibility/elasticity. The design also considers the amount of water absorption, thermal expansion and bore closure due to the interference fit. Water absorption will occur during a certain period of time that typically ranges from two to three months. However, the specific water absorption rate also depends on the temperature.
To facilitate the conversion process, a custom carrier was designed to interface with the existing register fits and mounting features in the head cover. The carrier was of fabricated/welded design and was split so the unit could be installed in position without requiring the removal of the shaft.
The seal housing assembly has connections for water injection for cooling and lubrication of the seals and also the bearing. The arrangement is designed as a two-ring segmented seal assembly with the two rings above the water inlet and the bearing below.
Since start-up of the water lubricated system, the unit was monitored actively. The SXL bearing was operating smoothly and not showing any signs of overheating or other malfunction. Indeed, the same customer placed another order in late 2011 for a second unit conversion for the powerhouse located downstream. This conversion package was installed in April/May 2012.
With the large amount of hydroelectric turbines in operation all around the world and with many of them being multiple decades old, the risk of operational oil leaks is a big concern. The financial costs of an oil leak are causing many energy producers to seriously consider water lubrication.
Scott Groves is global hydro business development manager for Thordon Bearings Inc.