Hydraulics and lubrication for turbines weighing many tons in Brazilian hydroelectric power stations
While Germany still faces many unsolved problems in terms of implementing the energy turnaround, they have moved significantly further on in South America. Of all the countries, the developing nation of Brazil is proving itself to be the role model when it comes to renewable energies.
Around 75 percent of the electricity requirement is covered by hydroelectric power there – and there are more sites under construction. Some of these plants use NETZSCH screw pumps for bearing lubrication and hydraulic units for the turbine blades of various plants which are already in existence or still being built. It is above all the hydraulic systems which are crucial in terms of the performance of the power stations, because they regulate the speed of the turbines.
The largest Kaplan turbines in the country so far are running at the new Estreito power station on the Rio Tocantins. Their rotor alone weighs 470 tons. The enormous weight of the turbines which rests on the bearings means that the lubricant has to be applied at very high pressure, above all when starting up and switching off, so as to be able to create any lubricating film at all between the rotating and static components. Gear pumps are used for this which can generate 100 to 250 bar. This means the moving components of the dry bearing can easily be raised and reliably lubricated. Injection is stopped as soon as a stable film of oil is established.
Supply of lubricant
From this point onwards, the continuous supply of lubricant and the feedback to cooling and filtration are taken over by special screw pumps.To achieve the longest possible service life and a high level of durability over the whole lifecycle, design pays attention to compensation of the hydraulic thrust of the pump, so that the axial forces are kept close to zero even for high pressures. In addition, an integrated spring-loaded overflow valve protects the system against overload.
The screws are made of hardened nitride steel and the pump housing from cast iron. The pumping capacity of the pumps that have been installed varies from site to site, depending on the volume of lubrication oil required and the size of the turbine bearing and the generators. The spectrum of the various projects ranges from 25 l/min to 3,000 l/min at pressures of 3 to 8 bar in the recirculation circuit. The oil temperature can rise to up to 60°C due to the friction heat in the bearing, which means that pump configurations were chosen which are designed for up to 120°C.
Turbine performance control
Screw pumps are also used for controlling turbine speed. This permanent control is essential in order to be able to react to changes in the quantity of water in the river being used and it also prevents spinning when there is falling torque on the generator shaft. With the Kaplan turbines the appropriate settings are carried out using adjustable blades on the impeller and on the idler. These can be set via hydraulic drives, either making them flatter or steeper, entirely according to requirements and operating conditions to regulates the volume of water hitting the rotor, thereby determining the quantity of electricity generated; the technology i NETZSCH designed and supplied a complex speed regulation system for this, including a control unit. The pumps involve a model with three screws, one of which functions as a drive spindle. A three-phase motor is connected and this spindle transmits the power of the motor to the two auxiliary spindles. The pressure of 68 bar which this generates is sufficient for precise adjustment of the huge turbine blades while they are running.