Abradable Seals for Gas Turbines and Other Rotary Equipment

Abstract

Abradable seals are typically used in compressors of aircraft and industrial gas turbines to decrease clearance between the stator casing and rotor blade tips, and hence, to increase compressor efficiency and decrease fuel consumption. The abradable seal concept originally developed for aircraft gas turbine applications is presently applied to other types of rotary equipment such as steam turbines and turbochargers. Among the most important variables influencing the choice of abradables for a particular section of a gas turbine are: application temperature, material of the rubbing element (titanium, nickel or iron alloy), the rubbing element geometry (blade, knife edge), seal durability (aircraft engines are refurbished more often than land based turbines), and of course the installed cost. In order to satisfy all the seal requirements, the chemical composition and structure of an abradable seal have to be carefully designed and controlled. The main challenge is to optimize the contradictory requirements such as durability and abradability. The process for choosing the best material starts with identifying the main variables and choosing the most suitable systems to fulfill all the requirements. This choice must be confirmed by a series of laboratory tests that simulate the actual application. The tests, test equipment and test results on a new seal system will be described to demonstrate the influence of all important parameters on seal performance. A new generation of abradable seals that, for example, can rub against titanium alloy blades at any temperature in the titanium alloy application range, will be presented. Emphasis will be put on the seal performance in “aged” conditions, i.e. after exposure to air at high temperatures for prolonged periods of time. Such results are necessary to evaluate the seal performance after extended service in the gas turbine when rubbing can still occur.