Gas turbines are used to power various applications, including electricity generation in power plants and propulsion in airplanes, but their performance can quickly degrade without the right care. The objective of gas turbine maintenance is to maximize performance, improve maintainability, reduce downtime and improve reliability. This can only be achieved by preventing turbine operational deterioration by monitoring critical turbine systems continually. This condition monitoring program must pay particular attention to these five specific gas turbine operational systems:

The thermodynamic gas path.

Turbine vibration and instability.

The lubricant and lubrication system condition.

Parameters for online condition monitoring.

Scheduled inspection and proactive maintenance activities.

The five operational considerations affect each other, and the components related to each system must all be in excellent condition to ensure optimal performance of the turbine. Maintenance considerations for each of these systems are described briefly below.

1. Thermodynamic gas path: Includes the condition and cleanliness of the compression intake air, fuel quality and the integrity and quality of the combustion process and turbine exhaust.

a) Inlet air must be free of contaminants, such as dust, sand, salt, agricultural air-borne chemicals, snow, ice and air-borne pollen, all of which will contribute to turbine erosion, corrosion, fouling and sulphidation which attacks turbine blade and nozzle materials and can render the turbine useless in a year or less, if proper air filtration is not provided and maintained. High-efficiency filters capable of capturing more than 95 per cent of the problematic contaminants down to one micron in size must be used. Some filter systems have a “self-clean” feature that uses high-pressure air to reverse flow through the filters and blow excess contaminants from the filter elements. This feature is highly recommended. A decrease in turbine power output, a decrease in compressor efficiency, or an increase in the frequency of cleaning the compressor are indications that the filter system is not functioning satisfactorily.

b) Fuel quality and the combustion and exhaust process. There are many fuel types used in gas turbines. Fuels range from liquids such as distillate and residual oils to various fuel gases, such as propane, natural gas, methane, or gas produced from biomass processes. Pollutants such as hydrocarbons, carbon monoxide, sulphur oxides and nitrogen oxides (NOX) are all products of the gas turbine combustion process and these pollutants can be reduced or eliminated by using properly designed combustors, reducing combustion temperature with water or steam injection into the combustion process, or by using a selective catalytic converter in the turbine exhaust. In addition, turbine exhaust temperature is a critical condition that should be monitored continually.

2. Turbine vibration and instability of rotors, bearings and gear box components can directly affect the efficiency and reliability of the process. Specifically, resonance, rotor imbalance, misalignment, mechanical looseness, oil whirl in hydrodynamic bearings, gear problems and defects in ball and roller bearings should be regarded as potential problems and continuously monitored with an online vibration analysis program. Handheld vibration monitoring devices should also be used when a specific problem is suspected or indicated.

3. Lubricant and lubrication system condition: includes the condition of the lubricant itself and reliable operation of the lubrication system. The lubrication system must be periodically inspected for oil leaks at connections, piping and fittings. Oil levels should be monitored daily and oil pressure and temperature must be part of the condition monitoring program. Oil pressure is indicative of the pressure drop across filters and can indicate internal leaks if external leaks are not present, but the oil level is dropping. Internal leaks are difficult to detect and can result in oil leaking into the hot gas path. This may or may not be indicated by either a gradual or sudden appearance of exhaust smoke. If internal leaks are suspected, an inspection of the combustor, the exhaust duct, the compressor discharge, or air bleed discharge may be possible using a borescope or other method to look for traces of burned oil in these components.

Monitoring oil temperature can be carried out using one (or all) of three methods; measure the temperature of the oil leaving the bearings, measure the actual oil temperature at the return line to the reservoir, or measure the bearing metal temperature using contact thermocouples or resistance temperature detectors (RTDs). An increase in oil temperature can occur quickly if leaking seals allow hot gases to leak into the oil. Oil temperature will also increase if the cooler is inefficient or plugged, or if the cooler doesn’t have the correct capacity for the system.

4. Parameters for online condition monitoring of gas turbines should include, but are not limited to, the following list depending on the design:

? Ambient air temperature and barometric pressure.
? Inlet air pressure at the compressor.
? Low-pressure compressor “out” pressure.
? High-pressure compressor “out” pressure.
? RPM, single shaft.
? RPM, dual shaft.
? RPM, free power turbine.
? Fuel flow and pressure.
? Exhaust total pressure and exhaust gas temperature.
? Overall vibration levels.
? Lubricant oxidation and contaminant levels.

5. Scheduled inspections and proactive maintenance activities should include annual fuel nozzle inspection and/or replacement, governor control system inspection and testing, water or chemical washing procedures and borescope inspections. Borescope inspections provide the final step in the identification of an internal turbine problem and this equipment is limited by the turbine design, borescope accessibility locations and the inspector’s knowledge and capability. Borescope ports are provided by many aero-derivative turbine manufacturers but may not be provided by the makers of heavy industrial turbines. In these cases, the inspector must use every available access point, such as removing fuel nozzles, pressure or temperature probes. If accessible, the borescope can be used to view the combustion liner, turbine nozzles and blades and compressor vanes and blades.

If a borescope inspection reveals a problem such as compressor fouling, water or chemical washing may be required. Washing procedures and access instructions are available from the turbine manufacturers, whose procedures include detailed internal washing.
Inadequate or incomplete washing may actually cause additional problems or increase existing problems, such as moving the debris deposits to a more critical component. This is a particular concern when cleaning compressors of salt contamination. The cleaning process may simply move the salt into the turbine area where sulphidation corrosion may be accelerated. Maintenance personnel must be fully knowledgeable of their turbine designs and the washing procedures recommended.

In conclusion, effective gas turbine maintenance relies on a comprehensive monitoring program that addresses the critical systems affecting turbine performance. By focusing on the thermodynamic gas path, vibration and instability, lubrication systems, online condition monitoring, and scheduled proactive inspections, operators can significantly reduce downtime, enhance reliability, and extend the life of the turbine. Regular maintenance, attention to detail, and advanced monitoring techniques are essential in preventing operational deterioration and ensuring that turbines continue to perform at optimal levels. Proper care and preventive strategies are key to maintaining the efficiency and longevity of these complex machines.