Aims and objectives of maintenance:
The purpose of maintenance is to maintain the steam turbine in order to achieve as high a plant availability as possible at the minimum cost. Maintenance of steam turbines used in power station may be classified broadly in three categories:
- Rectification of defects: The defects may or may not be urgent. If immediate repairs are needed, these may be carried out on running limits or the units should be shut down. If the repairs are not urgent, it may be carried out in planned program of future work.
- Planned preventive maintenance on running units: A system of routine maintenance is adopted to prevent the defects and breakdowns. The planning of such system needs careful consideration in order to arrive at the optimum level of maintenance. The actual work content of each maintenance routine can only be determined after experience with the plant over a period of time. After carrying out the preventive maintenance, it is recorded on a record card with full particulars or the items attended.
- Planned preventive maintenance on shut down units: In order to minimize the maintenance cost of shut down units, it is necessary to plan the work to be done carefully in advance so that the duration of the plant shut down is reduced to a minimum. This planning process should start some weeks or months before the shutdown is due to take place.
Steam turbine overhaul:
Following is the sequence of events of a typical turbine overhaul:
- Remove pipe works mounted on turbine and all top half cylinders and covers.
- Remove coupling bolts and bearings covers leaving thrust bearing assembled.
- Measure all relevant blade and gland clearances on the horizontal joints with fillers or tapered gauges.
- Remove the thrust bearings and then the turbine rotors to have a detailed examination of fixed and moving blades and diaphragms gland segments, casings, bearings, bolts,etc. concerning any type irregularities and damages.
- Re-form all gland and baffle segment knife-edge and restore all radial clearances. Examine the bedding of all bearings to journals and measure oil clearances. Check the adhesion of wright metal of journal bearings.
- After cleaning the blade deposits by water washing, chemical washing or blasting process, refit the rotors and measure clearances on the bottom points of glands and blades with lead or plastic strip. Measure similar clearances on the horizontal joint and compare for eccentricity of the shaft in the casing.
- After cleaning the bearings, check the wear down of bearings with the help of appropriate bridge gauge and filler gauges and compare the figures with those taken on the irection of previous overhaul
- Check the alignment of shaft by taking readings on all couplings and also record the level of all journals.
- Refit top half cylinders and before bolting up take a further set of top blade and gland clearances to confirm the concentricity of the rotor within the cylinder.
- Remark the horizontal casing joints and refit all heavy parts. After this, take the readings for final coupling alignment and adjust it if necessary.
- After refitting the coupling bolts, refit the thrust bearings and all other bearing covers.
- Inspect turbine governor gear with stop, throttle and intercept valves.
- Dismantle other accessories and mountings if necessary and inspect service and reinstall these.
- Clean or replace the oil filters and other items of lubricating systems. Replace the lubricating oil if necessary.
- Inspect all the measuring instruments installed on the turbine and replace if necessary.
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