Although Germany still faces many unsolved problems in terms of implementing the energy turnaround, it has 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% of the electricity requirement is covered by hydroelectric power there – and according to the energy research institute, Empresa de Pesquisa Energética, a further 34 hydroelectric power stations will be connected to the grid by 2021.
Some of these plants are operated with know-how from Germany: Netzsch do Brasil, a company in the Netzsch Group, produces screw pumps for bearing lubrication and hydraulic units for the turbine blades of various plants that are already in existence or still being built. It is, above all, the hydraulic systems that are crucial in terms of the performance of the power stations, because they regulate the speed of the turbines.
Brazil: The Hydroelectric Hot Point
Brazil’s special geographical features make it seem as if it was made for hydroelectric power. Around 14% of global freshwater reserves are in its territory and the branching rivers are very well suited to energy generation.
These fundamental factors have resulted in some of the largest hydroelectric power stations in the world now being operated in Brazil. No expense was spared either with the latest plants: for example, 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 tonnes. Netzsch lubrication systems are being used at Estreito, so as to ensure the turbine support bearings and the guide bearing to the generators run consistently and without any wear or overheating.
High Load Requires Two-Part Lubrication
In addition to pumps, the systems developed for this also include filters, heat exchangers, instruments, control units and pipes and comprise two essential sub-areas: an injection unit and the actual lubrication oil circulation.
The enormous weight of the turbines that 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. From this point onwards, the continuous supply of lubricant and the feedback to cooling and filtration are taken over by special screw pumps.
These comprise two spindle-shaped rotors whose helical profiles are exactly attuned to one another and to the housing geometry. This means the screws intermesh as they rotate and sealed conveying chambers are formed in which the medium is conveyed from the inlet to the discharge side.
The capacity is substantially determined by the incline of the spirals, the diameter of the spindles and the shape of the spindle profile. Due to the hydraulic thrust compensation of the pump, the axial forces here are close to zero even for high pressures. In addition, an integrated spring-loaded overflow valve protects the system against overload.
Robust Pump Technology From Cast Iron
To achieve the longest possible service life and a high level of durability over the whole lifecycle, the screws are made of hardened nitrided steel and the pump housing from cast iron.
Furthermore, the precise adjustment of the interplay of the spindles is ensured over the long term by special bushes made of bronze. In addition, other materials are also available for special usage conditions, for example with chromium oxide or titanium coated screws or pump bodies and bushes made of cast steel, PTFE graphite or aluminium alloys. On the other hand, individual lubrication between the screw and housing is not required, as this is already provided by the conveyed oil.
As well as at Estreito, Netzsch systems are also in use in many other Brazilian hydroelectric power plants, for example at Santo Antônio and at Jirau on the Rio Madeira.
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.
Complex Drives Control Turbine Performance
However, screw pumps are not just used for bearing lubrication, but also for controlling turbine speed. This permanent control is essential 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 fitted at Estreito, Jirau and Santo Antônio, 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 and that then regulates the volume of water hitting the rotor. This not only determines the quantity of electricity generated; the technology is also needed for the start-up and fast acceleration of the turbine.
Netzsch do Brasil designed and supplied a complex speed regulation system for this, including a control unit. Lubricant oil according to ISO VG 68 is used as a hydraulic fluid and this is channelled into the drives on the blades and returned by screw pumps.
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 that this generates is sufficient for precise adjustment of the huge turbine blades while they are running. In this way every variation in speed that is recorded by the regulator can be compensated via the servo-unit by correcting the position of the blades.
Michael Groth is with Netzsch