SEALING APPLICATIONS 19-05-2011

The major benefits of clearance control in industrial gas turbine (IGT) coatings are increased engine efficiency and reduced fuel consumption. Although there are many types of coatings and methods to manufacture them, the operation of refurbishment of the seals must be simple, reliable and efficient. Thermal spray coatings meet these requirements and are becoming the most popular choice for sealing applications. Thermal sprayed coatings are easily applied and can be removed by stripping, with comparative ease.

NEW HIGH-TEMPERATURE COMPRESSOR ABRADABLE
Plasma spray applied abradables are superior to other processes because of lower blade wear, lower operating cost, extended temperature operating capability, greater spray process control, and more process reliability. This results in fewer part rejections during manufacturing, and less downtime during operation. A typical plasma gun for spraying coatings with powdered materials is seen in Figure

A major application area of clearance control coatings in gas turbines is the high-pressure section of the compressor. Gas temperatures in this section can typically extend up to 750 °C. A recent study carried out by Sulzer Metco using a newly developed and patented plasma sprayed Cobalt-Nickel-Chromium-Aluminum-Yttrium- hexagonal boron nitride 15- weight polyester (CoNiCrAlYhBN polyester) coating confirmed the suitability of this advanced material for use in compressors.

EROSION RESISTANCE AND ABRADABILITY
Clearance control material generally consists of three components:
metal matrix, solid lubricant, and polyester.

The importance of the metallic CoNiCrAlY matrix is to give the coating its oxidation and hot-corrosion resistance, while the polyester is used as a means of controlling the porosity within the coating. The size and volume percentage of this polyester component are critical for optimizing both the abradability and erosion resistance performance. Typically, the erosion resistance and interparticle
cohesive strength is reduced with increasing polyester levels, while the abradability characteristics
of the coating improves. The boron nitride addition is as an entrapped solid lubricant and helps to minimize the coating’s Benefits of Clearance Control Thermal Spray Coatings

In response to customers’ concerns regarding increasing demands on turbine operating
temperatures and output efficiencies, Sulzer Metco has further developed its line of reliable
thermal spray materials and coating systems. One result of this work are abradable coatings
that are being used successfully in turbine engine applications for clearance control seals
of gas leakage over the tips of the blades. Sealing the Gap Sulzer Metco 9MB, a typical plasma gun used for spraying abradable coatings.

Thanks to Sulzer!


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