6.9 Jacking oil systems
When a turbine-generator shaft is rotating very slowly, the normal lubricating oil supply cannot ensure that a hydrodynamic oil wedge will be maintained. Also, additional lubrication measures are required to prevent bearing damage and to minimise breakaway torque when starting a turbine-generator from standstill.
In both circumstances the jacking oil system provides high pressure oil to the base of the bearing and floats the rotors on a film of oil. Until the turbine shaft speed is capable of sustaining an adequate bearing oil wedge (above 200 r/min), the jacking oil system and the normal lubricating oil system are run in parallel.
The oil pressure used to lift the rotors is around 300 bar and with the use of mineral oil in close proximity to high temperature components, this represents a fire hazard unless special precautions are taken. Older units used a single pump with long pipelines to feed each bearing and with this arrangement oil leakage does occur. On modern units, the high pressure pumps are mounted in or on the bearing pedestals (see Fig 2.66). Details of the pumps used are given in Section 6.2 of this chapter. A pressure relief valve is fitted in the discharge line from each pump in order to prevent over pressurisation and overload damage. Jacking oil is also provided to the turning gear journal bearings during start-up and shutdown.
The pipework systems use Class 1 welded pipe joints and all the high pressure pipework is contained within the pedestal. Figure 2.67 shows the arrangement used for the turning-gear jacking oil pump, but is typical of these installations.