By far the most significant factors affecting the maintenance interval are the type of operation (base load, peak load, trips, starts), the type of fuels used, the method of NO x control and the quality of air entering the turbine.
Peak load operation will have in excess of a six fold effect on the life of the blades compared with base load operation. Similarly, a trip from a full load will be equivalent to 8 normal starts in terms of cyclic life consumption.
Operation at part load, say 80% load, will reduce the turbine inlet temperature on an open cycle plant by as much as 10%. However, for combined cycle re-heat plant, the inlet temperature needs to be maintained by controlling the variable guide vanes, and therefore part load operation does not significantly influence the turbine inlet pressure until load reduces to around 70%.
Heavy hydrocarbon fuel such as residuals release considerably more radiant heat than natural gas and refined distillates. This higher radiant heat substantially reduces the life of combustion components.
In addition, heavy oils frequently contain corrosive elements such as sodium, potassium, lead and vanadium which will accelerate the rate of corrosion on turbine nozzles and blades. Natural gas fuel provides considerably reduced corrosive related problems.
Steam or water injection
For NO x control, operation with fuels other than natural/liquid gas will require either steam or water injection. Such injections increase the thermal conductivity and specific heat both of which increase the temperature of the nozzle and blade material.
Particles of dust, salt and oil can cause erosion, corrosion and fouling of the compressor. Erosion is cased by abrasive removal of material from the flow path, while corrosion is loss of material from the flow path due to chemical reaction with airborne contaminants. Fouling is caused by caked-on dust on compressor blades. These effects can be substantially reduced by filtration and manufacturers normally recommend the most suitable filters depending on the geographical location of the gas turbine. Filter cleaning or replacement becomes essential when the pressure drop across the filter exceeds the manufacturer's recommendation.
However, despite filtration, fouling of a compressor is a common occurrence which affects the efficiency and load output. By monitoring the performance of the gas turbine, the fouling of the compressor can be assessed and the performance of the compressor may be restored by compressor washing.
Each manufacturer provides a schedule of inspections which determines the condition of the components and, depending on the condition, the repairs/replacements activities which should be carried out to enable continued safe operation for a specified period of time. These inspections normally comprise:
- Combustor inspection
- Hot gas component inspection
- Major inspection
Inspection and maintenance intervals
The recommended maintenance intervals are similar for most makes of gas turbines. These are:
- Combustor inspection at 8,000 equivalent operating hours (EOH)
- Hot gas path component at 24,000 EOH
- Major inspection at around 48,000 to 50,000 EOH
However, different manufacturers apply different rules with respect to arriving at the equivalent operating hours (EOH) which determines the intervals between the inspections and the type of the inspection to be carried out.
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