Face Seal With Automatic Face Convergence Means

This invention relates generally to a mechanical seal assembly for sealing a rotating body relative to a housing of a fluid machine in a manner preventing leakage of fluids from the housing. More particularly, it relates to a mechanical seal wherein the effectiveness of the sealant relationship between the two seal faces of the seal assembly may be modulated as a function of machine operation.

Industry has used mechanical seals to replace packing as a means of sealing a rotary shaft or body passing through the wall of a vessel containing liquid or gas. As will be appreciated by those skilled in the art, a mechanical seal is basically formed of two elements or rings. One of the seal rings may be connected to the rotating body while the other ring is sealed to the housing. The radial end faces of the seal rings are arranged in an opposed relationship relative to one another and, thus, such seals are often referred to as end face seals. To allow for wear, at least one seal face may be spring loaded and axially movable relative the other seal face. In many applications, one of the seal faces is comprised of a softer self-lubricating material, such as carbon, while the other seal ring face will be a hard material, such as metal, ceramic, or metal carbide. Moreover, to accommodate high pressure applications, the faces of the seal rings are usually hydraulically balanced.

Because of the seal rings' relatively high cost, wear of the seal faces is an important consideration. Therefore, in recent years, many seal rings are designed such that both sealing faces are made from hard materials to accommodate more severe service. Because such hard materials are less yielding and have poor self-lubricating qualities, it is vital that a very thin lubricating fluid film, measurable in microinches, must be maintained between the seal faces.

The end face type seal has proven very effective and industry has widely adopted its use. Difficulties, however, have been encountered when end face type seals are used under high pressure and particularly under variable high pressures. Excessive leakage and erratic performance have been experienced even when the seals are manufactured with the highest possible manufacturing precision. As machine operating pressure increases, distortion of the seal rings can, and often does, result. Seal ring distortion, although measurable in only microinches, causes the lubricating fluid film between the seal rings to either collapse, resulting in zero film thickness, or such distortion causes the film to be too thick, thus causing leakage between the seal faces.

Tests have revealed a corrolation between fluid film thickness and the relative relationship of the seal faces. In particular, if the opposed seal faces are flat and parallel, the movable seal face will be urged into contact with and across the interface of the stationary or fixed seal ring. Consequently, the lubricating fluid film required therebetween will collapse resulting in zero film thickness and rapid seal wear. To prevent this from occuring, the opposing faces of the seal rings are specifically designed in a nonparallel relationship relative to one another. That is, the seal faces converge radially in the direction from the high pressure side to the low pressure side of the seal. By such construction, the pressurized product fluid has access to the convergent opening between the seal faces. The pressurized product fluid between the seal faces provides a sufficiently large opening force tending to move the resiliently mounted seal face away from the fixed seal face. The magnitude of this opening force will be a function of the size of the gap between the seal faces.

Because of the above, mechanical seals are often designed and manufactured with their sealing faces having a predetermined angle of convergence relative one another during steady state operation. As may be appreciated, heretofore conventional mechanical seals have been designed very carefully. In designing seals, it is the art of the seal designer to anticipate thermal and mechanical seal ring deformation and distortion, seal ring cross section, as well as width and balance of the faces to produce an acceptable film thickness under conditions of steady state operation. Additionally, seal designs are often specifically suited to predetermined operating conditions regarding temperature pressure, speed, load and fluid characteristics.

Today's industrial applications, however, vary considerably. Some applications often involve pumping unusual liquids which verge on being gases at similar densities. That is, the operating parameters of a pump regarding pressure speed, load and other considerations often changes. To accomodate such changing conditions, the faces and seal ring cross sections have to be repeatedly modified to adapt the pump to the particular operation. Even with carbon seal rings, such changes are costly. The rings made from the new and harder materials are even more expensive to modify and are far less forgiving of slight variations from optimum shape.

U.S. Pat. 3,275,330, issued to E. N. Rein on Sept. 27, 1966, discloses a face-type mechanical seal which is specifically designed to minimize distortion in a seal ring. To effect that end, the seal ring cross-section of the Rein device attempts to cancel any appreciable unbalance of forces which tend to distort the contacting seal faces out of a true flat condition. To achieve the desired cross-section is both a delicate and costly procedure.

Therefore, there remains a need in the art for a controllable seal wherein the fluid film thickness between the separated seal faces can be controlled despite seal ring deformation and without separate and specific design configurations for each pump operating condition. By accepting seal ring deformation and controlling same, the angle of convergence between the seal faces can be optimized for a wide range of operating conditions.

In line with the above, the present invention provides an end face mechanical seal for an apparatus having a body which rotates relative to a housing. The mechanical seal of the present invention includes a pair of seal rings having opposed seal faces which are urged into sealing relationship relative to one another to seal a high pressure fluid from a protected space. The seal faces of the rings, at a given operating condition of the machine, have a predetermined angle of convergence relative to one another. One of the rings is supported on its outer cylindrical surface, intermediate its radial end surfaces, by a seal ring support member. Unlike other mechanical seals, the present invention uses unbalanced forces for controlling the angle of convergence between the seal faces and thus the thickness of lubricating film therebetween. That is, by selectively arranging the support member in a predetermined lateral position on the outer cylindrical surface of one of the seal rings, the end portions or regions of the seal ring which are exposed to the pressurized pump product are subjected to differential pressures which cause the associated seal ring to distort. By selectively controlling the magnitude of seal ring distortion, the face orientation of the supported seal ring relative to the other seal ring face may be modulated in a manner thereby controlling the film thickness therebetween. Means may also be provided for monitoring the effectiveness of the seal and automatically controlling the angle of convergence as a function of the monitoring process.

In line with the above, a primary object of this invention is the provision of a more effective high pressure seal which will remain effective under variations in machine operating conditions.

Another object of this invention is to provide a mechanical seal assembly having a pair of seal rings, one of which is supported on unique structure which permits the angle of convergence between the seal faces of the rings to be modulated as a function of the seal's effectiveness.

A further object of this invention is the provision of a mechanical seal assembly having a pair of seal rings whose seal faces define a predetermined angle of convergence therebetween and wherein one of the seal rings are subjected to pressure differentials created by selectively controlling the affect of the machine product pressure in distorting the seal ring faces.

Having in mind the above objects and other attandant advantages that would be evident from an understanding of this disclosure, the invention comprises the devices, combination and arrangement of parts as illustrated in the presently preferred form of the invention which is hereinafter setforth in detail to enable those skilled in the art to readily understand the function, operation, construction and advantages of same when read in conjunction with the accompanying drawings in which:

FIG. 1 is a partial longitudinal sectional view of an improved mechanical seal constructed in accordance with the principles of the present invention;

FIG. 2 is an enlarged cross-sectional view schematically illustrating the present invention in one mode of operation; and

FIG. 3 is a cross-sectional view similar to FIG. 2 and schematically illustrating the present invention in another mode of operation.

Referring now in detail to the drawings, wherein like reference numerals indicate like parts throughout the several views, in FIG. 1, there is schematically illustrated a pump assembly 10. While the drawings illustrate application of the present invention to a pump assembly, it will be appreciated that the general teachings of the invention equally apply to other types of fluid machinery. For purposes of simplicity, only a portion of the pump 10 has been illustrated. Suffice it to say, the pump or fluid machine includes a chambered housing 12 which is penetrated by a shaft 14. One end of the shaft 14 passes through an opening 11 in the housing 12 and is rotatably driven relative to the housing 12 by a motor (not shown), or other suitable means, operably connected to one end of the shaft. The other end of the shaft 14 has an impeller 16 connected thereto. The impeller is enclosed in a chamber of the housing wherein a pressurized fluid flow is created as a result of impeller action.

Referring to FIG. 1, the controllable mechanical seal 20 of the present invention is constructed and arranged to substantially retard passage of pumped fluid from a high pressure environment to a low pressure environment while concurrently permitting the shaft to rotate with respect to the housing. That is, the seal assembly of the present invention provides an essentially fluid-tight dynamic seal which restrains the passage of pump product between a first zone or area 22 wherein there exists elevated pressures and a second zone or area 24 wherein relatively low pressure exists and which extends along the shaft and may be open to atmosphere or environment surrounding said housing.

In the illustrated embodiment, the mechanical seal assembly 20 includes a pair of coaxially mounted seal rings 26 and 28 arranged in juxtaposed relation and which surround the shaft 14. The seal rings are preferably constructed of silicon or tungsten carbide. The outside cylindrical surface 30 of each ring is exposed to the pressure media being sealed while the inner cylindrical surface 32 is exposed to the protected space fluid. Seal ring 26 is shown to be rotatable with and sealed to the shaft, while seal ring 28 is shown to be nonrotating or relatively stationary. Any suitable means (not shown) may be provided in conjunction with the non-rotating seal ring to resist torque imparted thereto by the rotating element. It should be understood, however, that either element or ring could be rotatable with the other being nonrotatable. Similarly, while ring 26 is schematically illustrated to be floating or resiliently biased as seen in FIG. 1 by a resilient member or spring 33 and ring 28 is shown to be fixed, either could be the floating or movable ring, with the other being fixed.

In the illustrated embodiment, an apertured closure plate 42, fabricated of easily machineable metal material, is affixed to the open end of the pump housing behind the seal ring 28 by means of suitable, circularly arranged fasteners 44. The annular space between the closure plate 42 and the pump casing or housing 12 is sealed by means of an O-ring 46 seated in a suitable groove. The end surface 48 of the closure plate 42 is stepped to provide an annular chamber 49 behind the radial end surface 50 of the seal ring 28; the chamber 49 being arranged in fluid communication with the high pressure media in chamber 22 of the housing. An O-ring 54, disposed in a suitable groove, is provided in the stepped portion of the closure plate 42 to seal the plate 42 to the seal ring 28 thus preventing the passage of pressurized pump product thereby. By such construction, a portion of radial end surface 50 of seal ring 28 is exposed to the high pressure fluid in chamber 22 and thus the net axial pressure affect on the seal ring 28 is extremely low. By providing the groove for the O-ring 54 in the easily machineable material of the closure plate 42, the net axial pressure on the seal ring 28 may be controlled as a function of the radial dispostion of the O-ring 54 relative to the outside diameter of the seal ring.

In the illustrated embodiment, a seal ring support assembly 60 is associated with the stationary seal ring 28. It will be appreciated, however, that with slight modification the seal ring carrier assembly could be associated with the other or both of the seal rings. The seal ring support assembly 60 includes a seal ring carrier element 62 which is mounted for movement relative to the end plate 42. The free-end of the carrier element 62 is formed as a cylindrical member or housing which is telescopically arranged about the outside cylindrical surface of the seal ring which is carried thereby. The outer cylindrical surface 30 of the supported seal ring defines with or capacity of which is defined by a spaced pair of O-rings 66 and 68. The cavity 64 operatively divides or separates the outer cylindrical surface 30 of the supported ring into three sections; namely a medial section 70 defined by the lateral width of the cavity 64 and two end portions or regions 72 and 74 outwardly extending from the medial region to the respective radial end surface of the supported seal ring. A radial passageway 76 extends through and permits communication of the cavity or chamber 64 with the protected space fluid. As a skilled artisan will appreciate, alternative structure could be provided to communicate the chamber 64 with the protected space fluid. In the illustrated embodiment, the radial passageway 76 extends substantially through the centroid of the seal ring cross section. The cylindrical bracket or housing 62 is provided with one or more radial passages 78 which permit exposure of the end portion 74 of the supported seal ring to the high pressure media. Moreover, the passageway 78 allows fluid communication between annular chamber 49 and the high pressure media for purposes described above.

The seal ring carrier element 62 is mounted for lateral shifting movement relative to the supported seal ring. To effect these ends, an annular groove 81 provided in plate element 42. The inside and outside diameters of the extension are sealed to the plate 42 by sealant means 82 and 84, respectively. The sealant means 82 and 84 comprise suitable o-rings. Separate keys or pins (not shown) may engage the element 62 to prevent it from rotating. Three or more operator accessible regulating screws 86 are disposed in equally spaced locations about the plate 42 for positioning the carrier 62. Linear displacement of the regulating screws 86 ultimately result in lateral displacement of the seal ring carrier 62 and thereby a change in the lateral disposition of the cavity 64 relative to the supported seal ring. A change in the lateral position of cavity 64 will result in a change in the exposed surface ratio of the end portions 72 and 74.

As schematically illustrated in FIG. 1, sensing or monitoring means 91 may be suitably embedded in one of the seal rings. The sensing means 91 provides an output signal indicative of various parameters such as temperature, pressure and leakage in the area of the seal faces. A control system 92 is responsive to the output of the sensor means 91 and develops a response thereto when the monitored effect of the seal ring is different from established predetermined expectancies. The response of the control system effects or actuates a motor 94 which, in turn, is operably connected to the regulating screw 86 to effect the lateral disposition of member 62.

A discussion of the operation of the controllable mechanical seal 20 will now be described with reference to FIGS. 2 and 3. Each seal ring has opposed radial end faces. Specifically, seal ring 26 has radial end faces 96 and 98 (FIG. 1) while seal ring 28 has radial end faces 100 and 50. As is evident from the drawings, the lapped seal end faces 98 and 100 are biased together to establish the dynamic seal therebetween. Depending on pump operation, the seal faces 98 and 100 have a predetermined angle of convergence relative to one another. Assuming that the high pressure media is restrained to the outer diameter of the seals and the protected fluid area is on the inner diameter, the seal surfaces 98 and 100 may converge from the outer diameter to the inner diameter as illustrated in FIG. 2 or in the opposite extreme as illustrated in FIG. 3 depending on various conditions. It will be appreciated that the illustrated angles of convergence and tipping of the supported seal ring have been exaggerated for purposes of illustration. The actual movement of the seal face is on the order of 1.times.10-5 inches.

Being disposed in the pressurized environment of a fluid machine, the outer diameter of the seal rings will have fluid forces acting thereon. Besides the fluid force applying inwardly directed force components to the outside diameter of the seal ring in a direction extending substantially perpendicular to the center line of the shaft 14, the axially directed force components are applied to the end faces of the seal rings by the pressurized medium. The axially directed fluid forces acting on the seal rings 26 and 28 includes closing forces applied to the radial end surfaces 96 (FIG. 1) and 50, respectively, which tend to urge the rings toward one another and opening forces which are applied to the radial end surfaces 98 and 100, respectively, tending to urge these elements apart from one another. The resilient biasing force of spring 33 (FIG. 1) is negligible as compared with the magnitude of these fluid forces. During steady state operation of the fluid machine, the net opening and closing forces must substantially balance one another for the system to be in equilibrium. Although the sealant means of the invention may be considered essentially fluid-tight, a successful mechanical seal has a thin lubricating film between the seal faces 98 and 100. The thickness of this film must be large enough to reduce wear and/or seizure of the rings, but small enough to prevent excessive leaksage. The resultant force balance between the rings determines the thickness of this fluid film.

Assuming that machine 10 is operating in stable condition, the angle of convergence between the surfaces will correspond to some film thickness having an optimum value for this particular operating condition. The predetermined angle of convergence may remain substantially constant when the opening and closing force vectors applied by the pressurized fluid remain in equilibrium. If there is a change in some operating condition, such as start up, shut down, speed change during operation, temperature variation, pressure changes, change in the characteristics of the particular fluid used in the machine and the like, the effectiveness of the seal may change. This change in the effectiveness of the seal may be sensed by the monitoring means 91. As a result, the control means 92 causes actuation of the motor 94 thereby effecting a change in the lateral disposition of the seal ring carrier 62 relative to the supported seal ring.

As described above, the lateral disposition of the seal ring carrier relative to the supported seal ring determines the exposed surface area ratio of one end portion 72 relative to the other end portion 74. That is, the selected lateral disposition of the seal ring carrier 62 relative to the supported seal ring determines the pressure differential which may be applied to the supported seal ring and thereby provides a desired degree of deflection of the sealing surface of the associated seal ring. Should the seal ring carrier be disposed in the position shown in FIG. 2, the resultant of the external forces applied to the supported seal ring will theoretically create or apply a force vector directed substantially perpendicular to the axis of rotation and against the exposed surface area of end portion 74 which lies to the right of the seal ring centroid. The other exposed end portion 72 having a lesser exposed surface area, will have a proportionately lesser force vector applied thereto. Seal ring end portion 74 having a greater exposed surface area will accordingly respond to such a radially directed resultant force by rotation of that section around the seal ring centroid. Such action causes distortion or deflection of the seal ring and thereby modulation of the disposition of the contact face 100 and thereby the angle of convergence between the seal faces. Of course, if the seal ring carrier 58 is shifted in the opposite direction such that a resultant force is moved to the opposite side of the seal ring centroid, the ring will distort or deflect in a different manner thereby moving the contact face 100 into a different convergent relationship with the other seal ring face. Understandably, if the clockwise and counterclockwise moments applied to the end portions of the supported ring are in balance, there is no tendency for the end sections 72 and 74 to rotate and thus the surfaces 100/96 may assume contact across their entire width if such is desired for the particular operation. By such construction, the angle of convergence between the seal faces may be modulated either automatically or manually, as a function of pump operation.

Thus, there has been provided an IMPROVED MECHANICAL SEAL which fully satisfies the objects, aims and advantages setforth above. While the invention has been described in connection with a specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.