Causes of Lost Performance
in Gas Turbines
§axial compressor fouling
§turbine blade erosion
§increased tip clearances
§worn seals
§fuel nozzle erosion, misalignment
§excessive inlet filter pressure drop (dirt, ice)
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Excess fuel consumption and loss of throughput can be very costly.
compressor fouling - is caused by adhesive materials like oil vapour, smoke, sea salt, or industrial vapours. Compressor fouling reduces compressor efficiency, lowers compressor discharge pressure, and results in reduced turbine output. This is the single most frequent cause of gas turbine performance loss. Approximately 70% of the total gas turbine performance loss can usually be attributed to compressor deterioration due to blade fouling.
The head produced by the impeller is the product of the impeller tip speed and the tangential velocity. The tangential velocity is reduced when the blades are fouled. This means that the compressor head capacity and efficiency is also reduced.
turbine blade erosion - is attributed to airborne particles and “blunts” the leading blade edge and changes the angle of the air. It also thins the blade trailing edge which can lead to blade fatigue.
Performance loss due to poor air quality can be minimized by proper design and maintenance of inlet air filters and frequent schedule of blade washing (either chemical/water or an abrasive for severe fouling).
Increased tip clearances
At certain operating speeds and conditions, the engine may at times undergo compressor stall. The stall can cause rotating blades to bend cyclically due to excessive aerodynamic loads. If the blades bend far enough, they will actually contact the stationary vanes of the stator assembly. Minor damage to the blades and vanes will reduce compressor efficiency. In severe cases, the blades can be completely sheared off, resulting in damage to the rest of the engine.
Fuel nozzles
Turbine performance suffers if the fuel is contaminated. Fuel contamination can cause clogging (or erosion) and coking from improper combustion. Performance is also reduced if the fuel nozzle is not aligned or assembled properly.
This paper focuses on natural gas fuel for industrial turbines. Machines that use good quality natural gas tend to operate better than similar units run on liquid fuels. The gas should be dry and free of particulates or hydrogen sulfide. Many manufacturers suggest limiting the variation in BTU content to 10% of the specified value. Variations beyond this level may require changes to the fuel gas control valves and/or fuel nozzle orifice sizes.