The Development of High Performance Leaf Seals

Abstract

Recently, from the environmental point of view, demand for combined cycle plant is increasing, and superior gas turbine performance is being rapidly promoted at the same time. As one of the key technologies for superior performance, reduction of secondary air leakage which is necessary for blade cooling and bearing sealing is required. Especially, reduction of air leakage through rotating parts and stationary parts clearance is critical problem. Hitherto, a non-contact type labyrinth seal has been widely used as a seal between rotating parts and stationary parts. However, this seal requires a large clearance to avoid contact, and this causes reduction of performance due to large amount of air leakage. Currently, application of brush seal is rapidly increasing as an improvement, however, a brush seal maintains contact not only at shut down, but also during operation, thereby wear of wire is accelerated during prolong operation, and reducing the sealing performance. In addition, since stiffness of the wire itself is low, differential sealing pressure is also low. In order to overcome these short comings, so-called “Leaf seal” which is MHI’s patent has been developed over the years. Leaf seal has a structure in which multi layered flexible leaves are arranged in the circumferential direction. In this seal, tip of the leaf is lifted up from the rotor surface by the hydrodynamic effect as the rotating speed is accelerated. As a result of this mechanism, wear of the seal is reduced during operation. The clearance generated by the leaf tip lifting up is negligibly small and, therefore, sealing performance is better. Moreover, because the leaf has an axial width, it can endure seal differential pressure several times that of the brush seal. The effect of the leaf lifting up and the leakage air amount were verified by rig tests. After these verification tests, this leaf seal is now being used for M501G gas turbine, installed at *T-point, in order to confirm sealing performance and durability. The leaf seal can sustain high differential pressure even if it is used in single stage, and has very good durability. To conclude it can be said that the leaf seal is the next generation seal replacing the brush seal. * MHI constructed the long-term in-house verification plant known as T-point for verifying new gas turbine technology prior to commercialization. The facility at T-Point consists of a M501G gas turbine, steam turbine, HRSG and associated controls. The output of the M501G gas turbine at T-Point is 225MW.