Development of non-contacting, film-riding face seals for large-diameter gas engines

Abstract

A new design tool, incorporating a commercial finite element program ADINA, has been developed for advanced analysis of film-riding face seals. This code is capable of modeling transient fluid-structure interaction inside a general seal configuration, enables designer to predict seal responses to speed, temperature and pressure changes in a timedependent manner. Therefore, through evaluation of influences of different seal parameters, the application envelopes of conventional spiral groove face seal can be extended to larger diameters, higher speed ranges. For applications that experience large seal surface coning due to high speed and large diameter, a new double spiral groove design has been developed with significantly increased angular film stiffness. Test rig results for some large-diameter seals are presented and compared with numerical simulation. INTRODUCTION TM^TMTM TM^TMTMTM« Significant advancement of non-contacting, filmriding face seal technology has been made over the past three decades. Today, film-riding mechanical face seals are used in almost every sector of industrial pumping applications. Because of their extremely low leakage, low friction and low wear, considerable efforts have been made to adopt them in aerospace application in the past few years [1,2]. Research and rig tests have shown that the aerospace sealing applications are much more demanding than the industrial applications. First of all, seals in gas engines experience frequent speed change, startup and shutdown. There are also times that the seals are operating at altitude conditions characterized by high rotational speed and low pressure. It is well known that higher speed requires higher film stiffness so that the stator can track the rotor to avoid face contact or blow open in case of axial rotor runout. But at the low-pressure (near vacuum) condition, conventional groove design will not be able to generate the required film stiffness due to the low air density which limits the hydrodynamic effectiveness. On the other hand, for large diameter applications, the thermal deflection from high speed can cause a divergent or convergent flow path between the seal faces and lead to film breakdown. In 1999, Menendez and Cunningham of PerkinElmer Centurion Mechanical Seals (formerly EG&G Sealol) reported the development of liftoff technology for air/oil axial sealing applications in aircraft APU main shaft, APU & propulsion engine gearbox and engine bearing locations [3]. With help of theoretical study, design code development and test rig investigation, today, the design of small and middle size of filmriding face seal for those applications is not presented as a challenge any more. However, there are still difficulties in hightemperature, high speed, large diameter applications. First, controlling the flatness of the seal faces becomes difficult as the size increases. Second the seal face coning of both the rotor and stator due to the thermal deflection grows proportionally to the fourth power of the seal diameter and second power of the rotational speed [2]. A negative deflection causing a divergent flow path can be disastrous for a standard hydrodynamic face seal, since it tends to cut off the flow of gas into the region between the faces. With standard hydrodynamic face seals, the deflection is expected to be much larger than the film thickness that the'face seal runs on [2]. Large positive coning can also result in failure for large diameter face seals because the resulting weak film stiffness increases * Engineering Specialist, AIAA member f Program Manger, AIAA member Copyright © 2001 by Xiaoqing Zheng and Gerald Berard. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. the chance efface contact. Since early 1990s, GE and Stein Steel have been developing a non-contacting aspirating face seal for large-diameter application [4,5]. It is a clever device which is essentially a hydrostatically balanced seal working solely on pressure difference. At nonor small-pressure difference conditions, the retraction coil springs will hold the seal face open to avoid face contact during startup and shutdown. Like other hydrostatic face seals, operating at lower pressure conditions than designed leads to reduced film stiffness. The weakest point of this seal is at a pressure condition such that the closing force from aspirator tooth and seal face is equal or close to the spring force. Then the film stiffness will be largely