While designers of dynamic systems rotating at high speed do everything possible to ensure concentric rotation, over time slight eccentricities can occur. In addition, inertial forces of rotation can cause component growth. Therefore, it is usually necessary to design a gap between the rotating and stationary components of the system.
For gas turbine engines, such gaps in the gas path represent a loss of efficiency. The tighter the gap can be, the better the operational efficiency of the engine. It has been determined that a 0.125 mm (0.005 in.) space between the engine compressor blades and the outer casing can increase fuel consumption by 0.5%. Clearance control technologies permit the design of the smallest possible gap size using a surface that will abrade preferentially when contact is made with a mating part.
Sulzer Metco offers a number of different clearance control coating types, catering to various dynamic conditions, operating temperatures, corrosive environments and mating materials.
In rotating equipment such as compressors, gas turbines and turbochargers, dimensional changes take place between the rotor and stator components because of thermal and mechanical effects during operation. These dimensional changes affect sealing by opening gaps between the blade tips and the casing in gas path systems, and between the seal and housing in labyrinth seal systems. In these applications, clearance control systems can be installed that consist of a sacrificial element and a cutting component. Special thermal spray coatings, called abradables, and honeycomb seals form effective sacrificial systems.
Thermal sprayed abradable coatings have low structural integrity (e.g. good friability), so they are readily abraded when they contact a moving surface of higher structural integrity without damaging the mating surface. The coatings are uniform in structure with additives such as polymers, graphite, bentonite and boron nitride that provide the necessary friability, as well as aid in dry lubrication. Other thermal sprayed coatings can also be used on the ‘cutting’ side of the clearance control system, and can be used when the dynamic member of the system is too soft to cut without a coating.
In pumps, thermal spray, diffusion and thin film coatings with high resistance to galling are applied on wear ring surfaces to enable tight clearance control.
In many situations, honeycomb seals are a time-tested method used to control clearances. Made from a wide range of metals and tailored to the specific application, honeycomb seals are used in all sections of the turbine engine.
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