Non-Contacting Seal With Centering Spring Mounted in Dovetailed Grooved

This invention relates to sealing devices for rotating shafts where fluid is employed to generate pressure forces between interacting face-type sealing elements, in which one is stationary and the other rotating. These forces provide for slight separation and non-contacting operation of the above sealing elements, thereby minimizing face wear and friction losses while maintaining low fluid leakage.

Non-contacting face seals are usually applied to high-speed high-pressure rotating equipment, where the use of ordinary mechanical face seals with face contact would result in excessive generation of heat and wear. Non-contacting operation avoids this undesirable face contact when the shaft is rotating above a certain minimum speed, which is called a lift-off speed.

As with ordinary mechanical seals a non-contacting face seal consists of two sealing rings, each of which is provided with a very precisely finished sealing surface. These surfaces are perpendicular to and concentric with the axis of rotation. Both rings are positioned adjacent to each other with the sealing surfaces in contact at conditions of zero pressure differential and zero speed of rotation. One of the rings is normally fixed to the rotatable shaft, the other located within the seal housing structure and allowed to move axially. The seal housing structure is normally stationary and fixed to the body of the compressor, turbine or pump to which the seal is applied.

To achieve non-contacting operation of the seal, one of the two sealing surfaces in contact is normally provided with shallow surface recesses, which act to generate pressure fields that force two sealing surfaces apart. When the magnitude of the forces resulting from these pressure fields is large enough to overcome the forces that hold seal faces closed, the sealing surface will separate and form a clearance, resulting in non-contacting operation. The character of the separation forces is such that their magnitude decreases with the increase of face separation. Opposing or closing forces, on the other hand, depend on sealed pressure level and as such are independent of face separation. They result from the sealed pressure and the spring force acting on the back surface of the axially movable sealing ring. Since the separation or opening force depends on the separation distance between sealing surfaces, unlike the closing force, during the operation of the seal or on imposition of sufficient pressure differential, equilibrium separation between both surfaces will establish itself. This occurs when closing and opening forces are in equilibrium and equal to each other.

Equilibrium separation constantly changes within the range of gaps. The goal is to have the low limit of this range above zero. Another goal is to make this range as narrow as possible, because on its high end the separation between the faces will lead to increased seal leakage.

Since non-contacting seals operate by definition with a clearance between sealing surfaces, their leakage will be higher then that of a contacting seal of similar geometry. Yet, the absence of contact will mean zero wear on the sealing surfaces and therefore a relatively low amount of heat generated between them. It is this low generated heat and lack of wear that enables the application of non-contacting seals to high-speed turbomachinery, where sealed fluid is gas. Turbocompressors are used to compress this fluid and since gas has a relatively low mass, they normally operate at very high speeds and with a number of compression stages in series. This requires relatively long shafts that have to operate frequently above their critical speeds. Neither seals nor bearings can contact at these high speeds so shafts are therefore made to float on fluid cushions, both at the bearings and the seals.

In the above situation, it becomes important to make sure that the shaft does not vibrate excessively during operating speeds as well as during acceleration or deceleration. This means that the shaft inertia axis must be as close to its geometrical axis as possible, since the bearings will be forcing shaft rotation around its axis of geometry rather than around its axis of inertia.

Should there be a significant distance between the inertia and geometry axes, the shaft would be prone to vibration, making it more likely to contact stationary parts in its vicinity. Such contact may result in overheating, wear and perhaps a failure at the bearing or the seal. To prevent the above problem, shafts are normally balanced by a selective removal of shaft material or by a redistribution of weights at its periphery. This balancing thus eliminates the effects of machining inaccuracy and/or material non-uniformity.

Since the shaft rotates with components attached to it, such as compressor impellers, bearing sleeves and seal rings, it is necessary to balance it with these components in place. While many can be fixed to the shaft during its manufacture and therefore balanced in the manufacturer's shop, rotating parts of the non-contacting seal normally come as a part of the seal cartridge and are mounted onto the shaft on site in the process plant and thus away from the manufacturer's shop. Special care, therefore, has to be exercised to keep the unbalance of the rotating seal parts to a minimum. A key requirement in this regard is that rotating parts remain concentric to the shaft at all times.

This is not easy to accomplish, because seal components are often of diverse materials with widely different coefficients of thermal expansion and moduli of elasticity. As the machine goes through thermal and pressure cycles, clearances between concentric components may vary.

One way of maintaining the concentricity of components despite variation in clearance is to place flexible elements between both concentric components. As the clearance changes, the flexible element deforms elastically an equal amount all around and thus maintains concentricity.

One example of such a solution is prior art to U.S. Pat. No. 5,039,113. This patent shows how a rotating sealing ring can be supported by an elastic strip at its inner diameter. It also illustrates how two such elastic strips can be placed close to both ends of a relatively long sleeve, between its inner diameter and the shaft.

This invention is aimed at the improvement of the centering property of an elastic element between rotating parts. Improvement is directed at problem points of the prior art design, one of which is the absence of a positive retention method for the elastic centering element in its groove.

The improvement this invention provides is a dovetail-shaped groove to positively retain edges of the elastic centering element. This is important, since during assembly process there are significant drag forces acting on the centering element in the axial direction and these often displace it out of the groove causing unbalance, even fracture of the element centered, if it is of a brittle material. Once assembled, it is difficult or impossible to check, whether centering strip is in the right position within its groove. When the sides of the groove are slanted to direct the edge of the centering strip towards the bottom of the groove, it will not have a tendency to slip out.

Another improvement is a ladder-like design of the centering element itself. With continuous edges at its sides interconnected by flexible links, centering element installs easily and minimizes changes in gap width between its ends when deflected. The centering element to prior art of the U.S. Pat. No. 5,039,113 provides for continuous longitudinal section near its centerline with sides which are discontinuous. Such element is not as easy to install and its end gap will vary more with deflection, creating residual unbalance.

FIG. 1 is a cross-sectional view of a non-contacting seal, constructed in accordance with this invention, taken along the longitudinal axis thereof.

FIG. 2 is an enlarged fragmentary cross-sectional view of a centering arrangement used on the seal.

FIG. 3 is a view, partially broken away, of a centering strip taken generally in a plane indicated by line 3--3 of FIG. 2.

Referring first to FIG. 1, there is shown the invention and its environment. This environment comprises a housing 10 and a rotatable shaft 12, extending through said housing. The invention is applied to seal a fluid within the annular space 14 and to restrict its escape into the fluid environment at 16. The basic components of the invention comprise an annular, axially movable sealing ring 18, having a radially extending face 20 in a sealing relationship with a radially extending face 24 of an annular rotatable sealing ring 22. The sealing ring 18 is located within cavity 26 of housing 10 and held substantially concentric to rotatable sealing ring 22. Between housing 10 and the sealing ring 18 is a plurality of springs 28, spaced equidistantly around the cavity 26 of housing 10. Springs 28 urge the sealing ring 18 into engagement with the sealing ring 22. An O-ring 30 seals the space between the sealing ring 18 and the housing 10. The sealing ring 22 is held concentric to a shaft sleeve 40 by an annular centering strip 42 which is elastically deformed between the inner diameter of ring 22 and sleeve 40 and retained within sleeve 40 by slanted sides of an outwardly facing dovetail-shaped radial groove 44. Groove 44 is located radially inwardly from the sealing ring 22 along an exterior peripheral surface 31 of the shaft sleeve 40. The sealing ring 22 is also retained in axial position against a radial extension of shaft sleeve 40 by a spacer sleeve 32. O-ring seal 36 precludes leakage between the sealing ring 22 and the shaft sleeve 40. Shaft sleeve 40 is located axially against a step on the shaft 12 by a locknut 34, which is threaded on the shaft 12 as shown. Concentricity of the shaft sleeve 40 with respect to the shaft 12 is provided by a pair of centering strips 42' situated at opposite ends of sleeve 40 in axial direction, each strip 42' being located and retained within an inwardly facing dovetail groove 44'. Grooves 44' are located radially outwardly from the shaft 12 along opposite ends of an interior peripheral surface 33 of the shaft sleeve 40. In operation, the radially extending face 24 of the sealing ring 22 and the radially extending face 20 of the sealing ring 18 are in a sealing relationship, maintaining very narrow clearance, generated by a shallow and concentric hydrodynamic groove pattern 38, where the subject grooves extend from the outer periphery of the face 24 inward, having in many cases a shape of the logarithmic spiral and ending at a diameter larger than that of the inner extent of sealing face 20. Pattern 38 can be electro-plated, etched or otherwise fabricated into face 24 of the sealing ring 22 or alternatively into face 20 of the sealing ring 18. Said narrow clearance prevents generation of friction heat and wear, yet limits outflow of the sealed fluid, present at space 14.

FIG. 2 shows an enlarged sectional view of the centering strip 42 within the dovetail groove 44 of the shaft sleeve 40. Strip 42 is in its relaxed shape prior to assembly with sealing ring 22. Sides or edges 50 of the strip 42 are retained within the groove 44 by groove undercuts or sidewalls 43, each defining an acute angle A with the bottom surface or wall of the groove 44. The angle must measure less than 90 degrees and preferably measure in the range of about 30 to 70 degrees in order that the centering strip is securely retained within the groove during the assembly process of ring 22 and sleeve 40 and during seal operation. Strip 42 is installed into the groove 44 for example by compression, where its sides 50 are squeezed together close enough for the strip to pass through the narrow part of the groove 44. Strip 42 is then expanded and assumes its shape within the wide part of the groove 44, formed by undercuts 43.

FIG. 3 shows the plane view of the centering strip 42 as installed within the groove 44 and retained. Elongated perforations 46 provide for bridges 48 between both continuous sides 50 of the strip 42. Bridges 48 have a function of curved beams, which are loaded in their center by centering forces and the geometry of which is determined from the centering force and elastic deformation required. Through this ladder-like design of strip 42 it is then possible to have smooth edges 50 for ease of installation and positive retention as well as minimize gap variation at its ends when deflected.