Oil-free operation2 March 2002
GE Hydro has developed technology to reduce or eliminate oil from inside Kaplan runner hubs. Kjell-Erik Värlind reports
The environmental impact of hydro power projects has been a topic for discussion over the past few years, with some of the most recent debates focusing on oil pollution.
Although oil discharges in the hydro industry are normally relatively small, the large quantities of oil found in a hydro station, particularly in Kaplan units, can mean that in some cases large discharges have occurred after equipment has become damaged.
Runner hubs of large Kaplan turbines have generally been completely oil-filled since they were introduced in the 1920s. Historically, oil has been responsible for the multi function of being the hydraulic fluid in the blade operating the servo motor, lubricating bearings and keeping water out.
Since the beginning of the 1990s the research and development programme within ge-hydro in Sweden has been focused on finding design solutions which are less harmful to the environment.
In 1994, a joint venture of Vattenfall, ABB Generation (now alstom) and Kvaerner Turbin (GE Hydro, Sweden) was formed to create the Porjus Hydropower Centre. Activities at the centre were to include training hydro power station personnel, as well as research and development into the area of hydroelectricity.
Porjus is the first power station on the Lule river, located north of the Arctic Circle in Sweden, and was built between 1910-1915. The old station has now been replaced. Originally the old station lodged nine horizontal shaft units and seven of these are now part of a museum. The new training and research units now replace the old units eight and nine.
The research and development unit has been built for future adaptation to new technology. The un-imbedded part of the unit can be replaced by new equipment when it is developed. The unit is also provided with extra equipment to allow necessary measurements and observations for research and development purposes.
The main areas to be studied and tested in Porjus were scale effects in the field of hydrodynamics. In the mechanical field, it was intended to focus around new materials, components and design features. The third area of study was interaction between different parts in complete systems under realistic conditions.
In June 1998, GE Hydro installed the first oil-less Kaplan runner at Porjus. The unit was designed to be filled with water in the spacing, where the runner blade operating mechanism is located. The blade operating servomotor was located in the upstream end of the hub (a common design for modern Kaplan runners) and the blade operating mechanism was provided with five different, permanently lubricated bearing materials:
• Deva CuSnPb8213/8E (A2).
• Tenmat FEROFORM T814.
• Deva CuSn8713/9P (A4).
• Thordon TRAXL.
The bearings were designed with relatively high surface pressures. Maximum surface pressure in blade bearings was calculated to more than 40Mpa and in link bearings to 75Mpa.
The links between the crosshead in the downstream part of the hub and the levers connected to the runner blades were provided with strain gauges to measure the link forces, making it possible to evaluate the friction in the different bearings.
The materials in the hub parts are chosen for their non-corrosive properties. The hub body is bronze, while the links, crosshead, levers, blade trunnions (integrated with the blades), piston rod and pins are made of stainless steal. The hub bottom is regarded as non-critical and is made of mild steel, protected by paint.
The unit was programmed to operate the runner blades extensively. Any signs of wear were investigated in December 2000 but no significant wear or difference between the bearing types could be measured. The friction in the bearings has been measured on a number of occasions to evaluate changes in the course of time. The results of the friction measures show that the friction is lower than the values given by the suppliers. The results also show that the friction decreases slightly after initial operation. Measured mean friction values are between 0.013 and 0.095.
After three years of operation, the main conclusion drawn is that a Kaplan runner can operate reliably with permanently lubricated commercial bearings, with the hub filled with water. The runner will however be dismantled in one or two years to examine the condition of the bearings and all other parts.
After the start up of the Porjus U9 runner, research and development efforts have been concentrated around preventing the development of corrosion in the hub. In the Porjus unit, most parts were made of stainless steel or bronze. To be able to design a runner at a competitive price, some of the parts should be manufactured from less expensive materials.
Together with the Swedish Corrosion Institute, GE Hydro Sweden has completed a research project on corrosion risks in Kaplan runners. The research discovered that with an efficient sealing technology, the runners could be filled with de-aerated water, which will prohibit the extent of corrosion.
To avoid local galvanic corrosion, it is important that large surface areas are left without surface treatment to consume the remaining small amounts of oxygen, which can be available inside the hub, according to research.
There are a number of methods for removing the oxygen from the water, primarily thermal or chemical. To prevent the introduction of toxic elements, chemicals should be avoided and instead the runner will be filled with thermal de-aerated water.
The following design features are intended to be used in an oil-free unit. The servomotor is located in the downstream end and allows inspection from the runner inspection platform. All bearings will be permanently lubricated, and all parts with large surface areas, such as the hub body and trunnion, will be made of mild steel. Only minor components will be stainless. All seals will be of an efficient well-proven type.
The first runners of this design are expected to be manufactured during next year.
The Traryd project
In January 2001, power company Sydkraft ordered a refurbishment of one unit at Traryd power station on the river Lagan in southern Sweden. It was agreed that the runner in this unit should be reconstructed to an oil-free design to serve as a test and reference point for this technology.
Nohab, one of the antecedents of GE Hydro Sweden, originally commissioned Traryd power station in 1946. The station is furnished with two vertical Kaplan units with a capacity of 6.6MW each.
The turbine was previously refurbished in 1964, when the discharge ring was exchanged for a stainless design, and the runner was fitted with new blades and blade seals.
The runner servomotor was originally located in the main shaft at the interface between the turbine and generator shaft, and the runner blade regulation movement was transferred to the runner through a rod in the centre of the turbine shaft. The runner was oil-filled through a channel in the piston rod in the shaft centre. The hub bottom was closed, which meant that there was a displacement change inside the hub when operating the blades. The blade trunnion was originally an integrated part of the stainless steel blade.
Consequently, there were many challenges to overcome in order to reconstruct the runner to an oil-free design. The runner blades were in a relatively good condition, so it was decided to keep the blades from 1964. Blade and link mechanism bearings were exchanged to permanently lubricated bearings of composite material, with PTFE dispersed in the material as a lubricant. The counter parts in the link mechanism bearings were exchanged to stainless steel.
The hub bottom was exchanged to one with an opening, allowing the piston rod to go through to eliminate the displacement change inside the hub. The piston rod was provided with stainless surfaces at seal and bearing positions. The rod was also extended with a stainless part to go through the hub bottom. The hub was filled with de-aerated water.
Dry sump lubrication
A new design solution has also been developed by GE Hydro to minimise the volume of oil in the hub and to supervise the function of the seal.
The idea is that instead of filling the runner completely, the oil will be let to the bearings inside the hub for lubrication. Once this is completed, the oil will be pumped out again through the shaft. This will be achieved by installing a combined hydraulic motor and pump in the runner.
All bearings will be provided with permanently lubricated bearings, so the absence of oil lubrication will not lead to breakdown.
Oil will be taken from a separate hydraulic system. A magnetic valve will open for the oil when the turbine is at normal speed. This will help ensure that the pump does not run dry for a long period. With a constant flow valve, the flow and speed will be directed exactly where it is needed. The control tubes in the shaft will be modified by adding two tubes or channels, which will provide the motor with oil and take care of the pumped-up return oil. The channel intended for static oil pressure in previous designs will connect the runner to the atmospheric pressure. The two oil channels should be suitably connected to the motor/pump unit with hoses through a swivel, similar to the type used for small turbines.
The hydraulic motor and pump will be installed in the hub cone where there is space and where it is suitable for the pump to obtain the oil. The suction point for the pump will determine the volume of the oil in the hub, which will be less than 10% of the full oil volume. The existing centrifugal forces must be taken into consideration when locating the pump (suction head) and for installation of the tubes. By having hose connection, the unit can be easily inspected and repaired by lowering the hub cone.
The return oil from the motor will then be led to the runner blade bearings in order to lubricate them.
Other bearings in the blade operating mechanism will not be provided with a separate oil supply and must therefore rely on the permanent lubrication in the bearing.
In old runner blade seals, there were often drainage holes on the suction side of the runner blade to get rid of water leaking in on the pressure side. The GE Hydro design has reversed this role. The water will now be let in between the seals on the pressure side of the runner blade, thereby ensuring that there is water outside the inner sealing which has a higher pressure than the atmospheric pressure inside the hub upcoming from centrifugal forces.
When the pressure level in the waterway is low, water with defined pressure can be led from another direction, for example from the sealing box. This method is used to ensure that oil is not leaking out, but water is leaking into the hub, where the leakage can be indicated and the penetrating water can be taken care of.
However, the surface outside the seal must be resistant to corrosion, as there will always be water between the seals.
According to GE Hydro, which has applied for a patent for this method, dry sump lubrication is an effective alternative to reconstructing old runners .