Hydro-Turbine Main Shaft Axial Seals of Elastic Polymer - Principle and Practice

摘要

The main shaft seal in a hydro-turbine is one of the key components of the turbine system. Ideally, the seal should seal completely but due to the size of typical units such an arrangement is prohibitively expensive. The main shaft seal therefore functions not to eliminate leakage but more to control the leakage to an acceptable amount. The requirements on such seals include, but are not limited to, effective operation, long wear life, easy maintenance and low initial cost. The main shaft seal often operates under harsh working conditions. Two key challenges for the design are that the seal must provide high wear resistance against abrasives within the water and be able to operate at high rubbing velocities with low leakage. Seals of elastic-polymer prevail in terms of combined benefits to meet these requirements. They are economical, non-brittle and able to provide a long service life - comparable to or longer than non-polymer seals in abrasive water environments. With careful design of factors such as seal surface finish, seal profile, installation and water pressure balancing, leakage rates comparable to or less than from other materials can be achieved. There are generally two main types of seals in widespread service, differentiated by the orientation of the sealing face to the shaft axis -- these are radial and axial seals. For the larger shaft diameters (over 1000 mm), axial type seals are receiving more preference in recent years and after a brief overview of the different types, this paper focuses on axial seals only and discusses their working principle, design criteria and a case study.