August 15, 2008

Field application of compressor coatings saves big dollars

Mark F. Mosser and Paul Brooks, Sermatech International Inc.

Turbine compressors historically have been coated as an afterthought. Initially, silicone aluminum paint and diffused nickel cadmium were the only coatings available for application to both rotating components and vanes/cases. But today, the addition of a sealer can enhance the protection of centrifugal compressors in gas-processing applications in ways that a coating alone cannot. To take advantage of coating process improvements, and because some installed equipment isn’t coated by the original equipment manufacturer (OEM), on-site coating is typically the only way to go.

Coating conundrums

The goal of any coating on a turbine compressor is to maintain the surfaces of all airfoils at a low surface roughness and eliminate corrosion and fouling. But choosing a coating is a complex decision, because turbines operate in environments that often combine high humidity, high salinity, severely acidic or alkaline pollution (in chemical plants), and sand erosion. On the other hand, many turbines operate in low-humidity conditions without any extreme environmental influences.

Because of these many variables, the OEM may apply protective coatings suitable for “average” or “typical” conditions. Such coatings may perform well in some environments and fail prematurely in others, but the OEM cannot economically provide for coating protection in every different application. In fact, driven centrifugal compressors (any that aren’t integral to a gas turbine) are often not coated at all and are therefore more subject to corrosion and fouling.

In addition, the time between overhauls of all gas turbine compressors has increased, which means that compressors must function efficiently for significantly longer periods of time. That makes the quality of compressor coatings even more important.

 

Coating failure can quickly lead to blade failure due to stress corrosion, pitting, and catastrophic failure (Figure 1). Fatigue failures invariably originate with corrosion pits forming a stress riser from which a crack propagates—ultimately resulting in blade failure (Figure 2).

Though fatigue failure is a serious issue, there are other consequences of coating failure. Any increase in surface roughness results in a decrease in compressor efficiency and increased fuel consumption, which raises operating costs. In addition, aerodynamic losses in the compressor may increase exhaust gas temperatures and increase wear and hot corrosion, further increasing costs and decreasing time between overhauls.

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