8.3   Pedestals

 

The main purpose of the bearing pedestals is to support the turbine rotor, via the journal bearings, in a fixed relationship to the cylinders so that gland clearances are maintained in all phases of operation. To this end, all pedestals and covers are rigidly fabricated in steel, amply stiffened by ribs and gusset plates, to support the bearings in a fixed relation to the cylinder supports.

Fabrication of pedestals has been adopted on large turbines because of the advantages offered in terms of increased support stiffness, whilst maintaining a compact overall pedestal size with good resistance to impact loading.

Improved cast materials, such as spheroidal graphite iron, may find future application with improved casting techniques.

Pedestals in the LP area of the turbine are normally firmly bolted and do welled to the foundations. It is, however, common practice at the high temperature end of the turbine to make provision either for the cylinders to expand at sliding mounting points on top of their pedestals or for the pedestal to slide relative to the foundations. For details of cylinder support and expansion arrangements refer to Section 6 of this chapter.

The pedestals incorporate facilities for the attachment of lubricating and jacking oil supply pipework and the associated bearing oil drain pipework. Mounted within (or on) the enclosure are all necessary instrumentation connections, e.g., bearing temperature, differential expansion pick-ups, together with eccentricity and vibration detectors. A manometric level system is attached to the pedestals adjacent to each bearing to detect misalignment due to support structure settlement, for details of this system refer to Chapter 2.

Particular care is taken to ventilate around the pedestals, keeping them cool so that any vertical thermal expansion effects, which might disturb the overall vertical alignment of the turbine, are minimised. For example, many pedestals contain couplings which are surrounded by coupling guards; these guards are themselves cooled by oil sprays tapped-off the lubricating oil supply to remove windage heat generated by coupling rotation.

In addition, those pedestals adjacent to the high temperature components of the turbine are frequently protected by thermal radiation shields, with provision for air circulation in the space between the shield and the pedestal structure, also to minimise thermal expansion effects.

 

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