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A labyrinth seal

EPRI Electric Power Research Institute

(http://www.my.epri.com.)


     A labyrinth seal is a mechanical seal that fits around a rotating shaft to prevent the leakage of oil or other fluids. A very small clearance must exist between the tips of the labyrinth threads and the running surface. A labyrinth seal is composed of many straight threads that press tightly inside another shaft, or stationary hole, so that the fluid has to pass through a long and difficult path to escape. Sometimes 'threads' exist on the outer and inner portion. These interlock, to produce the long characteristic path to slow leakage.

     Turbines use labryinth seals due to the lack of friction which is necessary for high rotational speeds. They are also found on pistons, which use them to store oil and seal against the explosion of combustion.

     · A labyrinth seal is a non-contacting circumferential seal utilizing a tortured path for flow between the stationary and rotating parts. The design utilizes a series of pressure drops to reduce the leakage.

     · The labyrinth seal is effective while the pump is running, but when the pump stops you are trying to seal with a hole unless you have purchased a design with a static sealing feature. In most cases a positive face seal would be a better choice in most of these applications.

     Efficient performance and long life of the roller bearing depend to a large extent upon the exclusion of foreign matter from the internal bearing surfaces.

     Grease, or oil, serve the dual purpose of lubricating these surfaces and protecting them from corrosion. Thus the seal must prevent dust, grit and moisture from entering the bearing and at the same time, prevent grease or oil from escaping.

     The aligning feature of the Cooper split roller bearing is integral with the cartridge housing. Any shaft misalignment that may exist tends to move the cartridge housing, seal and bearing together maintaining a parallel axis with the shaft.

     Cartridges up to 300mm are usually supplied with a general purpose felt seal. The felt groove will also accommodate high temperature packing seals, lipped rubber seals or suitable blanking plates.

     Triple labyrinth seals have precision non-rubbing characteristics. Extremely close tolerances can be maintained between the housing and the seal resulting in an effective sealing element which is one of the best of its type in the anti-friction bearing industry.

     Triple labyrinth seals are readily available on request to suit the more difficult sealing environments.

     For special applications seals may be made from other materials and alternative special seals can be supplied to suit specific conditions.

     Illustrates an aluminum triple labyrinth seal on an 01E Series bearing and cartridge.

     The twin 'O' rings are clearly visible. The compression of the 'O' rings causes the seal to rotate with the shaft, but the amount of compression is so designed that the shaft moves through the seal when axial expansion occurs

     This report evaluates the design advantages of a number of new labyrinth seal concepts compared to an existing seal for specific locations along steam turbine shafts (such as the N2 packing on a combined high pressure-intermediate pressure (HP-IP) shaft). The physics of fluid flow in selected new labyrinth seals along with the flow pattern and its effect on the dynamics of the turbine shaft have been examined, and the results are presented in this report. The new seals have specific features that are designed to minimize two basic and critical seal design performance issues: leakage through the seals and shaft dynamics effects on the leakage. In recent years, interest in increased efficiency of various turbines, including steam power-generating turbines, has grown. As the unit price of energy increases, efficiency of the production and consumption units becomes more relevant to the producers and to the consumers. Seals are one of the key components for improving the efficiency of steam turbines. Improving the

     seal is accomplished through better leakage reduction that will reduce wasted steam, improving the efficiency of the turbines. In addition to reduced leakage, today's power producers demand reliability and seek a reduction in unscheduled maintenance caused by seal damage.

     Objective

     This report is useful for the power generation technology and plant managers, engineers, designers, and maintenance supervisors associated with the operation of steam turbines. The report describes a number of innovative axial-flow labyrinth seal designs. Two-dimensional axisymmetric flow analyses were performed to optimize leakage reduction with these seal design concepts. Additional studies are recommended, and those with current seal design or operational concerns may be particularly interested in reading about, following, and supporting this effort.

     Approach

     This report contains the results of an initial study in which two-dimensional and axisymmetric computational fluid dynamics (CFD) was used to optimize the geometric characteristics of a new labyrinth seal design concept based on the criterion of reduced leakage.

     Results

     A new labyrinth seal has been conceptually developed that has a number of advantages and allows some flexibility in the implementation of the seal at various stages of axial-flow machines. The new seal also has the potential for reducing leakage and improving rotor dynamic stability. Durability of the design is a key requirement that will be addressed in a subsequent phase of research. Leakage flow results obtained to date are based on numerical simulation and would need to be validated through physical experimentations. This numerical investigation is justified primarily as an initial effort due to the simplicity of the new seal and its future potential. The seal cross-section is curved, and -- if constructed from a material that allows elastic deformation -- it would be compliant if rubbed by the turbine shaft. In this manner, the seal clearance with respect to the shaft could initially be small. During operation, as the shaft-seal clearance is decreased, the local pressure would increase and have a stabili

     zing effect on the shaft dynamics.

     Application, Value and Use

     The types of seals examined in this report can potentially reduce steam leakage or operate at increased nominal clearance so that rubs are less likely to occur during startups. In addition, there is potential for improved shaft stability with these labyrinth concepts. The potential applications would then be for all seals, where long-term durability and improved performance are desirable.

     EPRI Perspective

     Steam turbines in many of today's generating stations use interstage labyrinth designs that are essentially unchanged from those employed more than 30 years ago. Despite the significant numerical analysis capabilities offered by computational fluid dynamics (CFD), few improvements have been made in the basic axial flow labyrinth seal design. Parametric analyses such as those presented in this report can be used to design improved retrofit seals that are economical, improve operating efficiency, and extend the time between turbine overhauls.

     ABSTRACT:

     A three-step labyrinth seal with 12, 11, and 10 labyrinth teeth per step, respectively, was tested under static (nonrotating) conditions. The configuration represented the seal for a high-performance turbopump (e.g., the space shuttle main engine fuel pump). The test data included critical mass flux and pressure profiles over a wide range of fluid conditions at concentric, partially eccentric, and fully eccentric seal positions. The seal mass fluxes (leakage rates) were lower over the entire range of fluid conditions tested than those for data collected for similar straight and three-step cylindrical seals, and this conformed somewhat to expectations. However, the pressure profiles for the eccentric positions indicated little, if any, direct stiffness for this configuration in contrast to significant direct stiffness reported for the straight and three-step cylindrical seals over the range of test conditions. Seal dynamics depend on geometric configuration, inlet and exit parameters, fluid phase, and rotation

     . The method of corresponding states was applied to the mass flux data, which were found to have a pressure dependency for helium. Data for helium corresponded to the parahydrogen and nitrogen data but required an empirical correction for reduced pressure. In comparison with the straight and three-step cylindrical seals the three-step labyrinth seal offers the poorest dynamic stability and the lowest forces for restoring an out-of-balance dynamic shaft to the concentric position.

     >> What are labyrinth seals? Do you mean some kind of interlocking hub >> to seal setup that won't leak or wear out?

     Most seal designs use some form of rubber wiper or "lip" seal. These seal pretty well when they're new, but the contact pressure leads to wear at the crucial part of the seal, and then they stop sealing so well. A really waterproof lip seal requires a lip pressure that might even generate noticeable drag.

     Seals cannot separate two fluids with a single seal lip because the fluid retained is the lubricant of the seal lip. If two fluids meet at such a lip (like water and oil) the milky suspension intrudes into the bearing and causes rust. Typically the seal lip runs dry at some point and gets burned by friction. Then it becomes a capillary suction device that attracts water into the bearing. A text on seals will clarify this but the old axiom that: "The seal that doesn't leak, leaks." is correct. If the seal lip doesn't weep to lubricate itself, it fails and becomes a big leaker later on.

     > A labyrinth seal takes a different approach. Instead of using contact pressure over a small area, it reduces the pressure and increases the contact. Rather than trying to seal with a single strong barrier, a labyrinth seal uses a long path to accomplish the same reduction in flow. Labyrinths are usually made of two hard surfaces with carefully matched diameters such that they barely contact. Not only is seal friction reduced, but more importantly so is seal wear. Labyrinth seals are non contacting rotating labyrinths through which splash water cannot enter and gravity drains them. Sturmey Archer hubs are classic examples of such seals. They are more than 40 years old and used in the rain, yet the hubs are clean and dry inside with no wear or frictional drag.


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