8.4 Oil sealing arrangements
The ends of the pedestals where the turbine rotors emerge are fitted with oil catchers. The baffle fins on the oil catchers, in conjunction with oil throwers on the shaft, prevent egress of bearing lubricating oil from the pedestal.
Having regard for the speed and size of the bearing and coupling assemblies within the pedestals, the production of oil mist is inevitable. The escape of this mist is prevented by the main oil tank vapour extractors which, since the bearing drain lines do not run full, maintain the inside of the pedestal at a slightly lower air pressure than the turbine hall. There is thus a small flow of air through the baffle ring and across the surface of the shaft; this purges the oil mist back into the pedestal and via the bearing drains to the main oil tank, whence it is discharged at a safe location. For details of the lubricating oil system see Chapter 2.
Oil leakage from the horizontal joint of the pedestals is commonly prevented by machining a groove all round the joint flange inside the line of the half-joint bolts; any seepage from the ends of these grooves is collected and piped away to a dirty-oil drains tank.