Gas Turbine Compressor Blade Fouling Mechanisms

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Fouling is caused by the adherence of particles to airfoils and annulus surfaces. Particles that cause fouling are typically smaller than 2 to 10 μm.

Compressor fouling is due to the size, amount, and chemical nature of the aerosols in the inlet air flow, dust, insects, organic matter such as seeds from trees, rust or scale from the inlet ductwork, carryover from a media type evaporative cooler, deposits from dissolved solids in a water spray inlet cooling system, oil from leaky compressor bearing seals, ingestion of the stack gas or plumes from nearby cooling towers.

Fouling must be distinguished from erosion, the abrasive removal of material from the flow path by hard particles impinging on flow surfaces. These particles typically have to be larger than 10μm in diameter to cause erosion by impact. Erosion is probably more a problem for aero engine applications, because state-of- the-art filtration systems used for industrial applications will typically eliminate the bulk of the larger particles. Erosion can become a problem for engines using water droplets for inlet cooling or water washing.

Deterioration due to fouling is usually reversible, as the particles can be removed through compressor washing.(1) This distinction is important, because the economic implications of recoverable and non-recoverable degradation have different economic impacts.

Fouling can be removed by off line water washing and slowed down by online water washing. Theoretically, the engine can be kept at a very small degradation level at all times, if it is frequently washed on-line, and the cost (i.e. lost production) of shutting the engine down for off-line washing (typically a half day) is carried. The decision to shut the engine down for off-line washing is a balance between lost production due to the lower power vs. the lost production for shutting the engine down for a certain amount of time. The reversal of non-recoverable degradation requires the engine to be overhauled. Therefore, operators likely will allow much larger levels of non-recoverable degradation before they take action.

In this article, we will not address the other, more permanent consequence of particle ingestion that is the potential for hot corrosion as a result of salt particles entering the engine and reacting with sulfur from fuel or combustion air.

Industrial gas turbines can afford very effective inlet filtration systems. Modern systems can virtually eliminate the ingestion of particles into the engine compressor that can cause erosion (Figure 1). The trade-off for filtration systems lies in size, weight and cost on one side vs. filtration efficiency vs. low pressure loss as well as dust holding capacity. (3)

Figure 1: Comparison of fractional efficiency for filter elements from different suppliers and different face velocities in new and dirty conditions.(4)

Schroth et al.5 report on a comparison of GT power loss for two different air filtration systems used on 165MW gas turbines. The filtration systems are either a 2-stage or a 3-stage system. The 3-stage system causes a significant reduction in finer particles entering the engine. Power loss after 3,000 hours of operation was 4% with the 2-stage system and 2% with the 3-stage system.