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Experience has shown that axial compressors will foul in most operating environments; be they industrial, rural or marine. There are a wide range of industrial pollutants and a range of environmental conditions (fog, rain, humidity) that play a part in the fouling process.
Compressor fouling is typically caused by:
• Airborne Salt
• Industrial Pollution – hydrocarbons, fly ash, smog, exhaust emissions from traffic etc. This causes a grimy coating on the early compressor stages and can get "baked on" in the latter stages because of the high compressor discharge temperatures (This is especially true of high pressure ratio compressors).
• Ingestion of Gas Turbine Exhaust or Lube Oil Tank Vapors.
• Mineral Deposits such as limestone, coal dust and cement dust.
• Airborne Materials - soil, dust, sand, chemical fertilizers, insecticides, and plant matter.
• Insects – This can be a serious problem in tropical environments
• Internal Gas Turbine Oil Leaks – Leakage from the front bearing of the axial compressor is a common cause. Oil leaks combined with dirt ingestion cause heavy fouling problems.
• Impure Water from Evaporative Coolers (carryover) • Spray Paint that is ingested.
• Vapor plumes from adjacent cooling towers.
Often the inlet struts and IGVs become severely fouled. Hand cleaning the IGVs and first stage will restore a considerable amount of performance. Ambient air can be contaminated by solids, liquids and gases. Air loadings can be defined in mg/m3 , grains/1000 ft3 or ppm (mass of contaminant per unit mass of air).
In general, particles up to 10 microns cause fouling, but not erosion. Particles above 10 to 20 microns cause blading erosion. Some typical air loadings are as follows:
• Rural 0.01 - 0.1 ppm by weight
• Coastal 0.01 - 0.1 ppm by weight
• Industrial 0.1 - 10 ppm by weight
• Desert 0.1 - 700 ppm by weight Felix and Strittmatter have detailed the type of analysis that should be done at a gas turbine plant site. In most industrial areas, the air quality can create quite acidic conditions in the axial compressor. The importance of climatic conditions, rain showers, relative humidity etc cannot be overemphasized.
Several operators have reported dramatic drops in gas turbine output coincident with rain showers. Often air filters will exhibit a sudden growth in differential pressure as the filters get saturated with water due to high humidity. Under certain conditions, the filter may suddenly unload into the airflow causing rapid compressor fouling.