6 Lubrication systems
6.1 Lubrication requirements and typical arrangements
Forced lubrication of the turbine bearings is essential in order to prevent damage due to wear or excessive white metal bearing temperature. Before starting to turn the main rotating elements, it is necessary to lift the rotors clear of the bearing surface: the jacking oil system used to achieve this is described later in Section 6.9 of this chapter.
The purposes of the forced lubrication of the main bearings are:
(a) To provide a hydrodynamic oil wedge between the bearing surface and the journal bearing of sufficient thickness so that the particles of debris normally present in the oil will not damage the bearing surfaces.
(b) To provide a flow of cooling fluid sufficient to maintain the bearing white metal temperature below 110°C, dissipating the heat at the bearing due to:
- Thermal conduction
- Friction between the journal, oil film and white metal
- Turbulence with the oil itself
The majority of the oil flow is needed to cool the bearing. Because of the problems of accurately measuring the white metal temperature at the point of maximum hydrodynamic pressure in the oil wedge, experience has shown that limiting the oil temperature leaving a bearing to 71°C, gives satisfactory bearing life.
On older units, the lubricating oil system was integrated with the turbine control and protection oil system, with the lubricating oil being supplied through a pressure control valve. A typical arrangement is shown on Fig 2.52.
On modern units, the control system (see Section 1.6 of this chapter) normally requires a fire-resistant fluid (FRF) system with a pressure between 70 and 175 bar. The lubricating oil system on these units (Fig 2.53) is provided using a directly-driven centrifugal pump delivering oil at around 11 bar. The oil from the pump passes through an oil turbine which drops the oil pressure to around 3 bar; the oil turbine drives a booster pump which supplies oil from the main oil tank to the centrifugal oil pump suction. This arrangement allows the oil tank location to be optimised with regard to oil drainage under gravity, whilst meeting the centrifugal pump suction-head requirements. To protect the system against overpressurisation, a connection is taken from the bearing oil supply line to a relief valve mounted on the oil tank.
The directly-driven main oil pump provides an extremely secure source of lubricating oil under normal running conditions. For normal start-up and shutdown, an AC auxiliary oil pump provides the lubricating oil supply. For emergency shutdown, when the AC supplies are not available or the AC pump fails to start when required, a DC auxiliary oil pump is provided. Automatic sequential starting of these pumps is arranged upon falling lubricating oil pressure to ensure that the unit can be brought to a standstill without damage to the bearings, and to ensure that the bearings do not overheat due to conducted heat.
Lubricating oil is also used for the generator hydrogen seals and the supply is taken from the 70-170 bar oil pressure manifold. On some modern units, a separate seal-oil system (see Chapter 6, Section 5) is provided to prevent hydrogen transfer to the main oil system; on these units, the supply from the main lubricating oil system is used as a back-up.
On modern units, the lubricating oil system supplies:
- Each journal bearing on the turbine/generator/exciter rotating line.
- The main thrust/surge bearing.
- The generator hydrogen seals, either as the sole supply or as a back-up system.
- The bearings on the turbine-driven boiler feed pump, where this plant is provided.
Oil filters and strainers, oil cooler, tank vents and oil purifier connections are provided on the lubricating oil system. Details of these features are described below.
The unit lubricating oil tank is connected to the station clean and dirty oil tanks, and also to the oil purification system, with the pipework and pump necessary to permit the transfer of oil:
- From the station clean-oil tank to the unit oil tank.*
- From the station used-oil tank to the unit oil tank.*
- To drain the whole of the oil inventory from the unit oil system into the unit oil tank, and from the unit oil tank into the station used-oil tank.
- From a road oil-tanker into the station clean-oil tank.
- From the station used-oil tank into a road oil-tanker.
- Through the oil purifier to process either the oil in the unit tank or the oil in the station clean-oil tank.
- Through a portable unit oil purifier to process the oil in either the unit tank or the station clean-oil tank.
* These transfers are made using the oil purification unit.
A schematic arrangement of the systems used is shown in Fig 2.54.