Compressor degradation is the major cause of output and efficiency loss in a gas turbine. "Degradation reduces the air mass flow and pressure ratio, thus reducing power output,” said Jean-Pierre Stalder, head of new product development for Turbotect Ltd. Fouling is a major contributor of degradation. According to Stalder, it accounts for much higher losses than aging. He pointed out that enough air passes through the compressor on a 172 MW turbine in one year to make up a column that is the size of a football field, rising 1,320 miles high. The massive volume of air ingested into the compressor contains on an average 139 metric tons of foulant, assuming 10 ppm ambient concentration.
However, fouling is not the only element to consider when compressor performance declines. Erosion, corrosion, abrasion and damage also play their roles. This includes viscous hydrocarbons from flares and exhausts, dust, sand, cement dust, shot blast, iron oxides, salt crystals as well as cyclic wet and dry conditions. Degradation can also be caused by changes in airfoil and flow path geometry, altered surfaces and shift of machine clearances.
Much of this performance loss is recoverable, and the available tools include online and offline washing, engine adjustments, filtration, component replacement and repairs, said Rainer Kurz, Manager of Systems Analysis at Solar Turbines (and a Myth Buster columnist for Turbomachinery International). "The particle size most responsible for fouling is 1 to 5 microns,” said Kurz. "Therefore, fouling can be controlled by the quality of the air filtration system."
As gas turbines consist of many interacting turbomachinery units, the picture can become complex due to various contaminants attacking different parts. Bad air filtration, for example, tends to have more impact on the compressor while bad fuel affects the hot section. "The amount of power or efficiency lost for a given amount of component degradation differs for various ambient conditions," said Kurz. "Therefore, it is not possible to establish a universal rate of engine degradation that is valid for any condition."
Kurz offered tips on how to deal with component degradation: The impact is more severe on the full load power output than on the full load efficiency or heat rate of the engine. Except for a reduction in compressor flow capacity or a reduction in gas generator turbine flow, all other component degradations have a larger impact on engine performance at higher ambient temperatures. Airflow reacts distinctly to most types of degradation. Monitoring compressor discharge pressure is the most effective way to keep track of fouling. Apply traditional safeguards, such as air filtration, fuel, water, steam treatment, process gas strainers, knock-out drums, online and offline washing and cleaning.
(This is an excerpt from the cover story of the July-August 2013 issue of Turbomachinery International)