Development of Abradable and Rub Tolerant Seal Materials for Application in Centrifugal Compressors and Steam Turbines

The use of abradable and rub tolerant polymer seals to reduce leakage between rotating and stationary members in centrifugal compressors and steam turbines has increased over the years. Abradable materials such as mica-filled tetrafluoroethylene (TFE), nickel graphite, and aluminum alloy containing silicon/polyester are the most commonly used and have gained acceptance in the marketplace. This paper highlights the development of a new abradable silicon rubber adhesively bonded to a metal substrate. The abradability of the material was found to be excellent with no damage to the rotating member. For rub tolerant polymer seals little data are available on the effects of an aggressive rub with relation to deflection and wear at various temperatures. A series of tests was done on labyrinth seals manufactured from various forms of carbon filled polyetheretherketone (PEEK) and a polyamideimide (PAI). These tests evaluated their mechanical performance at low temperatures, room temperatures, and high temperatures. The results of the tests indicated that at room temperature and low temperatures, the seals exhibited acceptable deflection properties without damage to the rotating shaft and minimal material loss from the labyrinth teeth. However, at higher temperatures of 150°F, the PAI material had greater deflection values than the two types of PEEK, which resulted in the PAI exhibiting greater tooth height loss compared to the PEEK materials. For steam turbines, nickel chrome and nickel graphite were tested in a steam turbine and results are highlighted. This paper also shows the efficiency improvement for applying abradable seals to centrifugal compressors and steam turbines. INTRODUCTION In today’s turbomachinery business great emphasis is placed on improving the efficiency of the rotating equipment. Efficiency improvements lead to energy savings, which has become one of the important factors in today’s marketplace. One way of accomplishing an efficiency improvement is by reducing or eliminating gas leakages. Since clearances effectively control the gas leakages between rotating and nonrotating components, the application of abradables, rub tolerant, and honeycomb seals have found great attention in application for compressor and steam turbines. This paper will review the three types of seals and discuss their application in both centrifugal compressor and steam turbines. The three types of seals are: 95 DEVELOPMENT OF ABRADABLE AND RUB TOLERANT SEAL MATERIALS FOR APPLICATION IN CENTRIFUGAL COMPRESSORS AND STEAM TURBINES by Phillip Dowson General Manager, Materials Engineering Michael S. Walker Senior Materials Engineering Technician and Andrew P. Watson Senior Design Engineer Elliott Company Jeannette, Pennsylvania • Abradable seals • Rub tolerant seals • Honeycomb seals ABRADABLE SEALS The use of abradable seals is one way of accomplishing reduced clearances and limits the risk of damage to the rotating/stationary member if a rub occurs. The concept of abradable seal was first used in aircraft engines. However, over the years, the use of abradable seals has become more common in centrifugal compressors and steam turbines. The abradable seals that are most common in centrifugal compressors are mica-filled tetrafluoroethylene (TFE), nickel graphite (oxygen fuel gun sprayed), and plasma sprayed aluminum alloy containing silicon/polyester resin. Dowson, et al. (1991), presented a paper on abradable seal materials where the basic concept for abradable seals in centrifugal compressor application was highlighted. In conventional seal application the stationary static aluminum labyrinth contracts the gas as it flows through the close clearance gaps under the teeth and then expanding it between the teeth. This repeated contraction and expansion reduces the flow of the gas and lowers its flow rate. Leakage through the seal is proportional to the clearance (Figure 1). For an abradable seal, the labyrinth teeth are now on the rotating component and the smooth abradable seal is the stationary component (Figure 2). Figure 1. Conventional Seals Before and After Rub. Figure 2. Abradable Seals Before and After Rub. In the case of the abradable seal much tighter clearances can be achieved, since if contact should occur between rotor and seal, the rotating teeth will cut into the seal and remove the abradable material leaving grooved markings where contact had been made. In new designs, the rotating teeth are an integral part of the impeller (Figure 3) for both shaft and impeller eye seals. Another application for abradable seals in centrifugal compressors is in the balance piston. This component is used to balance the axial thrust of the rotor so that the size of the thrust bearing can be reduced. The balance piston (Figure 4) rotates with the shaft, and, if contact does occur with the abradable seal, negligible effect will result. Compared to a nonabradable material, contact can lead to Figure 3. Centrifugal Compressor Abradable Seal Locations (A). serious consequences for the compressor where the labyrinth teeth could be damaged leading to increased leakage. This would lead to a pressure drop causing an unbalance, which in turn would increase the loading on the thrust bearing. The benefit of applying these abradable seals is shown in Table 1. Figure 4. Centrifugal Compressor Abradable Seal Locations (B). Table 1. Benefit of Applying Abradable Seals. PROCEEDINGS OF THE THIRTY-THIRD TURBOMACHINERY SYMPOSIUM • 2004 96 Inlet Flow (ICFM) Impeller Eye and Shaft Seal Clearance Reduction (%) Stage Efficiency Gain (%)