Patent Application
20200056702
The invention relates to mechanical seals, and more particularly, to mechanical seals used to seal high pressure compressible fluids.
Mechanical seals are commonly used in many applications to minimize the leakage of a pressurized fluid through the gap between rotating and stationary components in rotating shaft equipment such as pumps and compressors. A mechanical seal typically comprises a rotating “sealing” ring located parallel with and closely adjacent to a stationary sealing ring, such that only a very small amount of the contained fluid passes through the gap between them. The gap between the adjacent faces of the sealing rings is controlled to a very tight tolerance, typically on the order of less than 10 microns and in some cases less than 1 micron. This small gap induces large viscous stresses on the fluid that enters the gap, thereby reducing the fluid pressure as the fluid travels through the gap between the seal faces. Reducing the fluid pressure in this way results in a very small fluid flow rate through the gap, thus effectively sealing the fluid without requiring physical contact between the sealing rings.
A major byproduct of the viscous stress imposed on the fluid in the gap between the sealing rings is viscous heating, which can lead to overheating of the seal and chemical breakdown of the fluid. This heating typically has a significant influence on the design of the seal face in terms of material selection, geometry, and manufacturing.
This traditional approach to mechanical seals becomes less efficient as the pressure of the sealed fluid increases, and its viscosity decreases, making it very difficult to seal high pressure and ultrahigh pressure compressible fluids.
What is needed, therefore, is a mechanical seal that is more efficient and effective in sealing high pressure and ultrahigh pressure compressible fluids than traditional mechanical seals.
The present invention is a rotating shaft seal that is structurally analogous to a conventional mechanical seal, but which functions according to a fundamentally different physical principle that enables it to be more efficient and effective than traditional mechanical seals in sealing high temperature compressible fluids. As with a traditional mechanical seal, the seal of the present invention comprises a rotating “sealing” ring located parallel with and closely adjacent to a stationary sealing ring, such that the pressurized fluid can only escape from the sealed volume by passing through the gap between the seal faces. As with a conventional mechanical seal, there is a significant reduction of the fluid pressure within the gap, thereby ensuring that only a very small amount of the contained fluid passes through the gap.
However, instead of relying on viscous stresses to reduce the pressure of the fluid that leaks between the seal faces, the seal of the present invention reduces the pressure of the compressible fluid within the gap by causing it to be accelerated to supersonic conditions and then to undergo shockwaves that reduce the stagnation pressure of the fluid. This approach substantially reduces viscous heating, and enables large pressure differentials, and thereby effective seals, to be established over distances within the seal face gap that are much shorter than for conventional mechanical seals.
Specifically, the high pressure compressible fluid is caused to pass through a converging segment where the gap narrows, followed by a diverging segment where the gap widens. The converging and diverging segments can be either axial or radial, and can be provided on the seal faces and/or on other seal parts such as bushings. Within each section, the fluid is thereby accelerated to a supersonic condition, such that a shockwave is induced shortly after the fluid enters the diverging segment. The stagnation pressure of the fluid is consequently decreased by the action of the shockwave, causing a backpressure that significantly reduces leakage of the fluid through the gap.
The number and the dimensions of the converging/diverging sections included in each embodiment are determined according to the fluid pressure within the sealed volume and the desired relative pressure drop.
In various embodiments, the converging-diverging geometry of each section is imposed on one or both of the seal faces. In embodiments, the converging-diverging sections are located near the edges of the seal faces where the pressurized fluid from the sealed fluid volume enters the gap between the seal faces.
In some embodiments, the seal faces and/or other seal parts that provide the converging/diverging sections are configured axially, for example in a configuration typical of a bushing, while in other embodiments they are configured radially, for example in a configuration typical of a mechanical seal. In various axial embodiments, the stationary seal face is the inner diameter of a stationary component that surrounds a shaft, while the rotary seal face is the outer diameter of a rotating component that surrounds and is fixed to the shaft. In various radial embodiments, the seal faces are axially adjacent to each other, with one sealing ring being attached and sealed to the rotating shaft, and the other being attached and sealed to the stationary assembly.
Within each converging-diverging section of the disclosed seal, the compressible fluid is first accelerated within the converging segment as the gap narrows, until it reaches the minimum gap clearance. At this location, the fluid reaches a “choked” condition, where the fluid velocity cannot increase beyond Mach 1. As the fluid enters the diverging segment, the gap widens, further accelerating the flow to a supersonic condition, whereupon back-pressure imposed by downstream sections induce a standing normal shockwave in the diverging segment. Although the static pressure of the fluid after the shockwave is much higher than the static pressure immediately upstream of the shockwave, the stagnation pressure of the fluid is greatly reduced, and is a function of the maximum Mach number that is reached in the diverging segment before the shock wave. The amount of pressure drop in each section is therefore limited by the maximum Mach number that is achievable in each section.
In some embodiments, a Mach number of 4 or higher can tend to result in dissociation of the fluid within the shockwave and along the walls, and may begin to cause significant viscous heating effects on the walls. Accordingly, embodiments are designed such that the maximum Mach number of each section is 3 or less. Under these conditions, each section results in a pressure drop of approximately 50%. Thus, to achieve a desired pressure drop across the seal faces, the number of converging-diverging sections is selected according to the requirements of the embodiment, with higher sealed fluid pressures typically requiring a greater number of converging-diverging sections.
Of course, the requirement to achieve supersonic acceleration depends upon the fluid remaining compressible, and does not, for example, condense into in incompressible liquid or solid. As such, the number of converging-diverging sections may be limited if the fluid pressure drops below the minimum pressure and temperature that are required to maintain compressibility. In embodiments, the amount of fluid leakage through the seal is determined by the first converging-diverging section's minimum gap width and area.
If it is desirable to combine the shockwave pressure reduction method disclosed herein with viscous-effect pressure reduction, then the seal faces can be extended downstream of the converging-diverging sections so as to further reduce the fluid pressure due to viscous dissipation. Similarly, a seal operating according to the principles disclosed here can be used as a “pre-conditioner,” much like a throttle bushing, in combination with a conventional downstream fluid seal. In this way, a seal of the present invention can be used to enable existing seal technology that is well proven for sealing fluids at lower pressures to be applied in ultra-high pressure applications.
The present invention provides at least two significant technical advantages as compared to prior art mechanical seals. The first is the significant reduction in viscous heating associated with the action of the shockwaves that reduce the fluid pressure. Although some viscous heating is unavoidable due to the small gap clearances required by the invention, the principle by which the invention operates does not depend upon viscous effects (other than the shockwaves) to reduce the fluid pressure, and so embodiments can be designed to minimize viscous heating. The second advantage is the short length scale of the converging-diverging seal features, which can allow the seal faces to be scaled to very small sizes, reducing material costs and product envelopes.
The present invention is a mechanical seal configured for sealing a pressurized, compressible fluid. The seal includes a rotating sealing ring surrounding a rotating shaft, the rotating sealing ring being rotationally cooperative with and sealed to the rotating shaft, a stationary sealing ring cooperative with and sealed to a housing that is penetrated by the rotating shaft, a stationary seal face of the stationary sealing ring being substantially parallel to a rotating seal face of the rotating sealing ring, the rotating and stationary seal faces being separated by an annular seal gap, and at least one annular shock inducing section included in the seal gap, the shock-inducing section including an annular converging segment within which a radial cross section of the seal gap narrows with increasing radius and an annular diverging segment within which the radial cross section of the seal gap widens with increasing radius, the diverging segment being adjacent to and radially outward from the converging segment, so that a local minimum in seal gap width, referred to herein as a “choke point,” is formed between the converging segment and the diverging segment, the converging and diverging segments being configured to cause the compressible fluid when flowing radially outward through the seal gap to be accelerated within the shock-inducing section into a supersonic state, so that a shockwave is formed by the fluid within the diverging segment, thereby reducing a stagnation pressure of the fluid.
In embodiments, the seal gap includes a plurality of adjacent shock-inducing sections.
In any of the preceding embodiments, at least one of the narrowing and widening of each segment of each shock-inducing section can be due to shaping of at least one of the rotating seal face and the stationary seal face.
In any of the preceding embodiments, a combined width of the shock-inducing sections can be between 0.004 inches and 0.008 inches.
In any of the preceding embodiments, the shock-inducing sections can be located proximal to a boundary of the seal gap where the pressurized fluid enters the seal gap.
In any of the preceding embodiments, the mechanical seal can be axially configured, such that the seal faces are parallel to the rotating shaft, the rotating seal face being radially inward of the stationary seal face, or it can be radially configured, such that the seal faces are perpendicular to the rotating shaft, the rotating and stationary faces being axially offset from each other.
In any of the preceding embodiments, the shock-inducing sections can be configured to cause fluid flowing outward through the seal gap to be accelerated into a supersonic state having a Mach number of less than 4.
Any of the preceding embodiments can further include a concentric, viscosity-inducing section included in the seal gap outward from the at least one shock-inducing section, the seal faces in the viscosity-inducing section being flat and parallel to each other, and being configured to induce viscous stresses on the fluid within the seal gap, thereby further reducing the stagnation pressure of the fluid.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.
The present invention is a rotating shaft seal that is structurally analogous to a mechanical seal, but which functions according to a fundamentally different physical principle that enables it to be more efficient and effective in sealing high temperature fluids than traditional mechanical seals. With reference to
According to the present invention, the seal is configured such that the pressurized compressible fluid can only escape from the sealed volume 106 to the surrounding, lower pressure environment 108 by passing through the gap 110 between the seal faces 112, 114. The high pressure fluid entering the gap 110 between the seal faces 112, 114 passes through one or more annular sections 116, 118, 120. The structure of one of these sections 116 is enlarged in
The dimensions and the number of converging/diverging sections included in each embodiment are determined according to the fluid pressure within the sealed volume 106 and the desired relative pressure drop. For example, the embodiment of
In the embodiment of
The embodiment of
With reference again to
In some embodiments, a Mach number of 4 or higher can tend to result in dissociation of the fluid within the shockwave and along the walls, and may begin to cause significant viscous heating effects on the walls. Accordingly, embodiments are designed such that the maximum Mach number of each section is 3 or less. Under these conditions, each section results in a pressure drop of approximately 50%. Thus, to achieve a desired pressure drop across the seal faces, the number of converging-diverging sections is selected according to the requirements of the embodiment, with higher sealed fluid pressures typically requiring a greater number of converging-diverging sections.
In embodiments, the amount of fluid leakage through the seal is determined by the first converging-diverging section's minimum gap width and area.
Referring again to
Similarly, a seal operating according to the principles disclosed herein can be used as a “pre-conditioner,” much like a throttle bushing, in combination with a conventional downstream fluid seal. In this way, a seal of the present invention can be used to enable existing seal technology that is well proven for sealing fluids at lower pressures to be applied in ultra-high pressure applications.
The present invention provides at least two significant technical advantages as compared to prior art mechanical seals. The first is the significant reduction in viscous heating associated with the action of the shockwaves that reduce the fluid pressure. Although some viscous heating is unavoidable due to the small gap clearances required by the invention, the principle by which the invention operates does not depend upon viscous effects (other than the shockwaves) to reduce the fluid pressure, and so embodiments can be designed to minimize viscous heating. The second advantage is the short length scale of the converging-diverging seal features, which can allow the seal faces to be scaled to very small sizes, reducing material costs and product envelopes.
In the embodiment of
The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application.
The invention illustratively disclosed herein suitably may be practiced in the absence of any element which is not specifically disclosed herein and is not inherently necessary. However, this specification is not intended to be exhaustive. Although the present application is shown in a limited number of forms, the scope of the invention is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof. One of ordinary skill in the art should appreciate after learning the teachings related to the claimed subject matter contained in the foregoing description that many modifications and variations are possible in light of this disclosure. Accordingly, the claimed subject matter includes any combination of the above-described elements in all possible variations thereof, unless otherwise indicated herein or otherwise clearly contradicted by context. In particular, the limitations presented in dependent claims below can be combined with their corresponding independent claims in any number and in any order without departing from the scope of this disclosure, unless the dependent claims are logically incompatible with each other.
This application claims the benefit of U.S. Provisional Application No. 62/473,535, filed Mar. 20, 2017, which is herein incorporated by reference in its entirety for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US17/68095 | 12/22/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62473535 | Mar 2017 | US |
