This part of the project will examine the Seals of a rotary system on an air craft. Investigation will be carried out on Labyrinth Seals in terms of functions, material, and size. Within this chapter implementations would be made to improve the functions of seal. A close look at how Labyrinth Seals have improved the efficiency and the components that make up the seal.
Seals are used to keep substances such as grease and lubricant in, while keeping dirt, fluids, and other contaminants out. An advantage of a seal is that when small amounts of leakage occur across the seal, as is inevitable, contaminants from the outside still can't get all the way inside, and the materials being sealed may not make their way all the way to the other end.
A labyrinth seal is a mechanical seal that fits around an axles to prevent the leakage of fluids; it has a number of grooves which interlock with other grooves or teeth. The labyrinth seal differs from most traditional seals; they use a single lip in to try to seal the opening. With the labyrinth seal, each groove acts as its own lip, distributing pressure along the seal rather than engaged in one area.
The labyrinth seal design also reduces friction, making it suitable for devices such as turbines. These seals can withstand conditions which might cause other seals to break down, and as long as they are well maintained, they should hold up very well. Materials such as metal is use to produce labyrinth seals, in a gas turbine titanium is used as it is less likely to corrode, titanium seals can withstand higher temperature than normal metals like steel as a gas turbine outlet temperature can increase up to 2000 °C. Titanium seals give a much longer life time than other metals this is very useful for particular applications like a gas turbine.
(Smith, 2007)
Seals fall into two main categories first being a non-contacting seal, and the secong being a face seal.
Non contacting seals are used extensively in high-speed turbo machinery and have good mechanical reliability. There are two types of non-contacting seals (or clearance seals): labyrinth seals and ring 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 seal leakage is greater than that of clearance bushings, contact seals, or film-riding seals. labyrinth seals are utilized when a small loss in efficiency can be tolerated. They are sometimes a valuable adjunct to the primary seal.
(Boyce, 2006)
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 this junction location, the labyrinth minimizes leakage. Dynamic labyrinth applications for both turbines and compressors are inters stage seals, shroud seals, balance pistons, and end seals.
The major advantages of labyrinth seals are their simplicity, reliability, tolerance to dirt, system adaptability, very low shaft power consumption, material selection flexibility, minimal effect on rotor dynamics, back diffusion reduction, Integration of pressure, lack of pressure limitations, and tolerance to gross thermal variations.
Figure Cut view of labyrinth seals
(Qualiseal Technology (QT))
The major disadvantages are the high leakage, loss of machine efficiency, increased buffering costs, tolerance to ingestion of particulates with resulting damage to other critical items such as bearings, the possibility of the cavity clogging due to low gas velocities or back diffusion, and the inability to provide a simple seal system that meets OSHA or EPA standards. Because of some of the foregoing disadvantages, many machines are being converted to other types of seals.
The leakage of a labyrinth seal can be kept to a minimum by providing:
(1) minimum clearance between the seal lands and the seal sleeve, (2) sharp edges on the lands to reduce the flow discharge coefficient, and (3) grooves or steps in the flow path for reducing dynamic head carry-over from stage to stage.
The labyrinth sleeve can be flexibly mounted to permit radial motion for self aligning effects. In practice, a radial clearance of under 0.008 is difficult to achieve, except with very small high-precision machines. On larger turbines, clearances of 0.015-0.02 are generally used. During machine construction, it is important to measure and record these clearances because mechanical seizure or loss in aerodynamic efficiency can often be traced to incorrect labyrinth seal clearances.
(Boyce, 2006)
A labyrinth seal is composed of many straight grooves that press tightly inside another axle, or inside a hole, so that the fluid has to pass through a long and difficult path to escape.
The purpose of labyrinth seals is to prevent air from entering turbine during start up on the HP end and the LP end all the time if under vacuum. Labyrinth seals on rotating shafts provide non-contact sealing action by controlling the passage of fluid through a variety of chambers by centrifugal motion, as well as by the formation of controlled fluid vortices. At higher speeds, centrifugal motion forces the liquid towards the outside and therefore away from any passages. Similarly, if the labyrinth chambers are correctly designed, any liquid that has escaped the main chamber, becomes entrapped in a labyrinth chamber, where it is forced into a vortex-like motion. This acts to prevent its escape, and also acts to repel any other fluid. Because these labyrinth seals are non-contact, they do not wear out.
(Dorf, 2004)
Labyrinth seals are in gas turbines and compressors separate the high pressure region from the low ones and minimize the leakage of the high pressure gas. This leakage, which depends on a great variety of parameters such as geometry of the teeth, number of cavities, pressure differences, temperature and type of gas, etc., is inevitably present even in the case of abradable seals. There are a few types of labyrinth seals which are being used. The most common one is the straight through labyrinth seals.
The correct prediction and control of this leakage is crucial for efficient and economic operation of turbomachinery. The gas flow through a labyrinth seal may be briefly described as follows. Swirling gas at the high pressure enters through the clearance between the first tooth of the labyrinth seal and wall opposite to it to first cavity of labyrinth seal, expanding and altering its rotational momentum by the first friction of cavity walls which may rotate at speeds quite different from the inlet swirl. This rotation is in general non-axisymmetric and time dependent due to small but nevertheless important vibration of the rotor.
Once the gas crosses several such cavities it emerges at the other end of the labyrinth seal at significantly reduced pressure. A significant assumption which facilities the semi analytic treatment of this very complex three dimensional unsteady flow is that the gas pressure in each labyrinth cavity as well as the circumferential velocity in each cavity are independent of the radial and axial coordinates within the cavity.
(Dorf, 2004)
Ring (Bushing) Seals
The restrictive ring seal is essentially a series of sleeves in which the bores
form a small clearance around the shaft. Thus, the leakage is limited by the flow resistance in the restricted area and controlled by the laminar or turbulent friction.
The API 617 codes characterize this type of seal. Most of the restrictive type seals are of the floating type rather than the fixed type. The floating rings permit a much smaller leakage.
Because of the minimal contact between the stationary ring and the rotor, these
seals, when properly designed, are ideal for high-speed rotating machinery.
When adequate lubrication and cooling fluid is available, the seal ring, manufactured from babbitt-lined steel, bronze, or carbon, will function satisfactorily.
When the medium to be sealed is air or gas, carbon seal rings must be used.
Carbon has self-lubricating properties. Cooling of the seal is provided by the leakage
flow through the seal. Depending on the operating temperature and environment,
aluminum alloys and silver are also used in the manufacture of the seal rings.
Leakage limitation depends upon the type of flow and type of bushing. There are four types of flow compressible and incompressible, each of which maybe either laminar or turbulent. Ring seals are divided into two categories: fixed breakdown rings and floating breakdown rings, according to whether or not they are fixed with respect to the stationary housing.
(Boyce, 2006)
Fixed seal rings
The fixed seal ring consists of a long sleeve affixed to a housing in which the shaft rotates with small clearance. It is an inexpensive assembly. However, since it is fixed, the seal behaves like a redundant bearing when rubbing occurs and, like the labyrinth, requires large clearances. Therefore, long assemblies must be used to keep leakage within reasonable limits. Since long seal assemblies aggravate alignment and rubbing problems, sturdier shafts are required to keep operating speeds in a subcritical region. The fixed-bushing seal almost always operates with appreciable eccentricity.
(Boyce, 2006)
The primary function of labyrinth seals is to either minimize or control leakage, while a secondary but equally important purpose is to provide (or at least not to deteriorate) rotordynamic stability. The fluid within the seals generates reaction forces acting on the rotor. For small rotor displacements about the center position, the reaction forces Fx and Fy can be modeled as a linearized set of equations:
Where
x, xi, xii and y, yi, yii are rotor displacements, velocities, and accelerations in the x and y directions; K and k are the direct and cross-coupled stiffness coefficients; C and c are the direct and cross-coupled damping coefficients; and M and m are the direct and cross-coupled inertial coefficients, respectively. Among the above force coefficients, the tangential components k and C are important in determining rotor dynamic stability. The combined effect of these two coefficients forms the effective damping Ceff [= C − k/Ω].
Figure : (Rhode, 2006)
Geomatric and kinematic relationship of circular rotor whirl about the stator center at t = 0, ω: rotating speed; Ω whirling speed
Rotors in gas turbines experience noticeable thermal axial growth during transient operations such as the start-up process. The rotor axial shifting caused by the thermal expansion could significantly alter the performance and rotordynamic forces of the sealing labyrinth. The seal forces could contribute to the rotordynamic instability of the rotor-bearing-seal system even though the force magnitude is smaller than that of the bearing fluid film forces. As the rotating speed increases or the seal clearance decreases, the labyrinth-seal-excited problems often become more and more critical in the system design. It is therefore necessary to predict these forces accurately for both reliable operations and the future design of high-performance steam turbines.
(Rhode, 2006)
In a gas turbine a plurality of rotors is essential for a relatively long, trouble-free operative life to provide means for cooling the rotor discs or hubs of the turbine. Conventionally, this rotor disc cooling has been achieved by providing wall means, partition means or diaphragm means cooperating with the housing means, including stator vane assemblies, to form chambers adjacent the rotor discs into which chambers cooling fluid such as air, is introduced To render the chambers reasonably fluid-tight, a labyrinth seal is conventionally provided between the diaphragm means and rotor disc, one part of each of the labyrinth seals being carried by a diaphragm means while the other parts is carried by a rotor disc. It has been found that in such installations the integrity or efficiency of the labyrinth seals cannot be maintained because of the differential, radically directed, expansion and contraction between the turbine housing and the stator vanes which, in turn, through the diaphragm means causes one part of the labyrinth seal to move relative to the other part.
A labyrinth seal for the rotary part of the engine situated at the combustion chamber level, having a labyrinth seal provides a sealing between the high pressure compression outlet gas flow and the void under the combustion chamber contained by the diffusion housing immediately downstream of the high-pressure compressor and the engine shaft.
A labyrinth seal consists of a cylindrical part integral with the rotor and coaxial with the axis of rotation of the engine. A number of components are installed on this part, shaped as circumferential blades or teeth placed perpendicular to the engine shaft with a narrow free edge, arranged parallel to each other. The teeth work together with a cylindrical part integral with the stator. Each tooth is held a short distance from the stator cylindrical part and forms a throttle for any fluid flow caused by a pressure difference of the seal. The clearance between the two parts determines the leak flow through the labyrinth seal. To prevent the rotary teeth from being damaged in case of contact, the stator cylindrical element has a wearing part of a material that is liable to deform, preferentially with respect to the material forming the teeth. For instance, this can be a honeycomb material or material of a type known in the field under the term "abradable".
(Sylvie, 2007)
In the case of the labyrinth seal mentioned above, this means can be put to good advantage to control the air flow from the compressor passing through it and that is directed towards the ventilation resources of the turbine disk immediately downstream of the combustion chamber. This air comes from the void between the bladed disk of the high-pressure compressor rotor and the diffuser. The pressure prevailing in the void immediately downstream of this sampling point is determined by the throttle formed by the labyrinth. It will be seen that the pressure may fluctuate and this can be damaging to the rotor if the clearance is affected by variations caused by uncontrolled expansion of the seal teeth. Therefore, it is necessary to control the radial clearance of the labyrinth.
(Sylvie, 2007)
During some engine operating phases, for instance, acceleration, air from the compressor may be affected by a considerable temperature increase in a very short period of time. The state or section of the seal is exposed to high temperature whereas the rotor elements of the seal deeper down in the engine are less exposed. Therefore, in so far as the two parts of the seals do not expand in the same way, there is a tendency for the clearance to open up considerably. Accordingly, an attempt is made to control the radial clearance of the labyrinth
The invention achieves these objectives by means of a labyrinth sealing device for a gas turbine engine including both upstream and downstream a high-pressure compressor, a diffuser and a fixed wall element forming at least one part of the combustion chamber internal flange envelope said seal including a stator part mounted on the wall element by a flanged attaching device and supporting an annular wearing part, said wearing part working together with a rotor element having circumferential teeth and that is integral with the compressor rotor in forming the labyrinth seal, characterized by the fact that at least one of the flanges of the attaching means has a thermal inertia such that the dilatation response time of the stator part is increased with respect to the dilatation of the rotor element during the engine acceleration phases in order to reduce a leakage flow rate of the labyrinth seal.
(Sylvie, 2007)
In this way, by a simple arrangement, the invention resolves the problem of checking the clearance in the labyrinth seal.
In a preferred embodiment, the wall elements have ventilation holes immediately next to the attaching means. By drawing air in immediately downstream of the last compressor stage and arranging the calibrated orifices in a suitable manner, it is easy to control the dilatation of the stator element during the transient operating phases of the engine.
In an advantageous arrangement, the seal stator can include a first part enclosing the wearing part, extending downstream into a second annular part integral with a radial attaching flange on its outer face, said outer attaching flange working together with an inner attaching flange integral with the section of the wall to form the attaching means.
The second part of the annular stator, the means of attachment and the wall element, together define a first cavity. In particular, the first cavity is fed through the ventilation holes.
In conformity with another characteristic, the first part of the annular stator part, with this element, forms the wall of an open passage towards the upstream end whereby the air from the ventilation holes is fed into the first cavity then into this passage before being exhausted upstream of the labyrinth seal. By this means, a continuous sweep of the first part is obtained forming a support for the wearing parts, and contributing to stabilizing this part with respect to the temperature variations resulting from overheating due to possible friction.
In conformity with another characteristic, the second annular part extends downstream of the flange into a third part, the said second and third parts forming a channel guiding the air leakage from the compressor away from the wall of the combustion chamber envelope. More particularly, the third part forms a second cavity with the combustion chamber envelope wall element.
(Sylvie, 2007)
