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.abyrinth Seals
The labyrinth is one of the simplest of sealing devices. It consists of a series of circumferential strips of metal extending from the shaft or from the bore of the shaft housing to form a cascade of annular orifices. Labyrinth sealleakage is greater than that of clearance bushings， contact seals， or film-riding seals. .onsequently， labyrinth seals are utilized when a small loss in efficiency can be tolerated. They are sometimes a valuable adjunct to the primary seal.
In large gas turbines labyrinth seals are used in static as well as dynamic applications. The essentially static function occurs where the casing parts must remain unjoined to allow for differences in thermal expansion. At thisjunction location， the labyrinth minimizes leakage. .ynamic labyrinth appli-cations for both turbines and compressors are interstage seals， shroudseals，balance pistons， and end seals.
The major advantages of labyrinth seals are theirsimplicity， reliability，tolerance todirt， system adaptability， very low shaft power consumption，material selection flexibility， minimal effect on rotor dynamics， back diffu-sion reduction， integration of pressure， lack of pressurelimitations， and tolerance to gross thermal variations. The major disadvantages are thehigh leakage， loss of machineefficiency， increased buffering costs， toler-ance to ingestion of particulates with resulting damage to other criticalitems such as bearings， the possibility of the cavity clogging due to low gas velocities orbackdiffusion， and the inability to provide a simple seal system that meets OSHA or EPA standards. Because of some of the foregoingdisadvantages， many machines are being converted to other types of seals.
Labyrinth seals are simple to manufacture and can be made from conven-tional materials. Early designs of labyrinth seals used knife-edge seals and relatively large chambers or pockets between the knives. These relativelylong knives are easily subject to damage. Themodern， morefunctional， andmore reliable labyrinth seals consist of sturdy， closely spaced lands. Some labyrinth seals are shown in Figure 13-21. Figure 13-21a is the simplest form of the seal. Figure 13-21b shows a grooved seal is more difficult to manu-facture but produces a tighter seal. Figures 13-21c and 13-21d are rotating labyrinth-type seals. Figure 13-21e shows a simple labyrinth seal with a buffered gas for which pressure must be maintained above the process gas pressure and the outlet pressure (which can be greater than or less than the atmospheric pressure). The buffered gas produces a fluid barrier to the process gas. The eductor sucks gas from the vent near the atmosphericend. Figure 13-21f shows a buffered， stepped labyrinth. The step labyrinth
Figure 13-21. .arious configurations of labyrinthseals.
gives a tighter seal. The matching stationary seal is usually manufactured from soft materials such as babbitt orbronze， while the stationary or rotating labyrinth lands are made from steel. This composition enables the seal to be assembled with minimal clearance. The lands can therefore cut into the softer materials to provide the necessary running clearances for adjusting to the dynamic excursions of the rotor.
To maintain maximum sealing efficiency， it is essential that the labyrinth lands maintain sharp edges in the direction of the flow. This requirement is similar to that in orifice plates. A sharp edge provides for maximum vena contractaeffect， and hence maximum restriction for the leakage flows. (Figure 13-22.)
High fluid velocities are generated at the throats of the constrictions， and the kinetic energy is then dissipated by turbulence in the chamber beyondeach throat. Thus， the labyrinth is a device wherein there is a multiple loss ofvelocity head. With a straight labyrinth， there is some velocity carry-over that results ina loss of effectiveness， especially if the throats are closelyspaced. To maximize the aerodynamic blockage effect of this carry-over， the diameters can be stepped or staggered to cause impingement of the expand-ing orifice jet on a solid， transverse surface. The leakage is approximately inversely proportional to the square root of the number of labyrinth lands.Thus， if leakage is to be cut in half at a four-point labyrinth， the number of lands would have to be increased to 16. The Elgi leakage formulae can be modified and written as
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