Sealing in extreme conditions

Future seals for gas turbines and jet engines are expected to be able to run hotter without allowing the passage of oil mist. No solid materials are suitable because of the speeds and temperatures, so it is to air bearing type concepts that engineers are turning. Brian Roche, a design engineer with Stein Seal, based in Kulpsville, Pennsylvania, told Eureka at the Farnborough Air Show that he is involved in developing what he described as an “advanced aspirating seal” based on research work undertaken by Nasa at the Glenn Research Center. Labyrinth seals, which are widely used at present, do not seal sufficiently well and allow the passage of oil mist. Graphite seals are limited to temperatures of 375 deg C, above which they start to oxidise. In aircraft engines, there are particular problems because of changing operating conditions. Roche explained the new device: “It’s a metal-on-metal seal, but aerodynamics keeps a small clearance between the surfaces. We are doing analytical work to ensure that the system has sufficient stiffness to prevent rubbing.” The Stein Seal aspirating seals are being developed as face seals for GE Aircraft Engine. The first to be made was a 14.7 inch (374mm) seal, followed by a full-sized 36 inch (914mm) seal. The requirements were: a shaft speed of 365 ft/s (111m/s), a pressure difference of 100psid (6.9bar), an air temperature of 750 deg F(399 deg C) and a leakage rate of only 1.25 scfm/psid . Applications areas were to be turbine rim seal and compressor discharge applications. The intention was to replace brush seals – which have thousands of tiny metal tubes or wires and are an improvement over labyrinth seals. During start up, the aspirating seal is retracted by springs. As the engine runs up and the pressure drop increases, the seal starts to close towards the rotor. As the speed and pressure differential increases, the seal approaches the rotor but is kept separate from it by air bearing force with a clearance of 1.5 to 2.0 mils (38 to 51 microns). The seal has been demonstrated in the GE90 engine. Development forms part of NASA’s Ultra Efficient Engine Technology Project. Brush seals, which remain the present state of the art, can reduce leakage by 80% compared with labyrinth seals, are available from a number of manufacturers such as Bath-based Cross Manufacturing. They can accommodate shaft excursions, thermal movements and misalignments that would instantly reduce the efficiency of a labyrinth seal. Cross can supply seals from 20-1200mm diameter in one-piece rings or diameters greater than 1200mm in multiple segments. Working temperatures are up to 600 deg C and pressure drop up to 20 bar per stage. Shaft surface speeds can be up to 384m/s. The Thermo Fluid Systems University Technology Centre, part of the University of Surrey, is developing mathematical and computational aeromechanical models in order to assist the development of improved seals. The position of the bristles in a brush seal is influenced by the pressure distribution, which is itself influenced by the configuration of the bristle pack. The work is being sponsored by Alstom Power and the EPSRC. The Centre is supported by Rolls Royce. Stein Seal Cross Manufacturing Pointers * Brush seals generally perform better than labyrinth seals in gas turbines and jet engines but are still being improved * Advanced aspirating seals, which could replace brush seals, have been demonstrated on the GE90 engine