US20090066033A1

US20090066033A1 - Barrier sealing system

Info

Publication number: US20090066033A1
Application number: US11/853,404
Authority: US
Inventors: Roderick Mark Lusted; Nitin Bhate; Eric John Ruggiero; Kripa Kiran Varanasi; Gabriele Mariotti; Paolo Susini
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2007-09-11
Filing date: 2007-09-11
Publication date: 2009-03-12

Abstract

A barrier seal system comprises a rotatable shaft disposed within an outer housing. The rotatable shaft axially extends from a high-pressure region to a low-pressure region within the outer housing. At least a portion of the rotatable shaft, typically the portion disposed adjacent to the low-pressure region, comprises an oleophobic surface. The system further comprises a barrier seal that radially extends from the outer housing to the rotatable shaft and defines a gap between the barrier seal and the rotatable shaft. A barrier gas injector is provided for injecting a barrier gas into the gap to flow from the high-pressure region to the low-pressure region.

Description

BACKGROUND

The invention relates generally to barrier sealing technologies and more specifically to methods and apparatus for low-flow oil barrier sealing technologies with improved surface properties.
Many current applications, including for example rotating machinery, use a barrier seal to separate lubricant from the internals of a respective machine to prevent damage and degradation of the various internal machine parts. Barrier seals, or tertiary seals, are often used in conjunction with dry gas seals in many applications, including for example, compressor technology like gas centrifugal compressors. Ever greater demands are being placed on dry gas seals and their support systems, requiring continual improvements in the design of the dry gas seal environment, both internal and external to the compressor. Contamination is the leading cause of dry gas seal operational degradation and reduced reliability.
A barrier seal is typically required on the outboard side of a dry gas seal, between the gas seal and the compressor bearing housing area. This seal is typically buffered with air or nitrogen. The primary function of the barrier seal, in this application, is to prohibit the flow of bearing lubrication oil or oil mist into the dry gas seal. Contamination of the dry gas seal from the lubrication oil can occur when the barrier seal fails to function as intended. Even with the application of a purge gas to prevent the migration of the lubrication oil through the barrier seal, the lubrication oil frequently wicks or leaks along the shaft into the internals of the machine and to the dry gas seal. If the purge or barrier gas is applied at a sufficient flow rate, for example, greater than about 16 feet per second over the life of the seal, the velocity boundary layer that forms is typically thin enough to preclude oil migration no matter how thin the lubricant film becomes. It is important, however, to minimize the quantities of the purge or barrier gas required to preclude the oil migration.
Accordingly, there is a need in the art for an improved barrier seal that can prevent the migration of lubricants into the internals of a machine while minimizing the quantities of separation gas required to do so.

BRIEF DESCRIPTION

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of one embodiment of the instant invention.

FIG. 2 is a schematic illustration of another embodiment of the instant invention.

FIG. 3 is a schematic illustration of another embodiment of the instant invention.

FIG. 4 is a schematic illustration of an aspect of another embodiment of the instant invention.

FIG. 5 shows photographs of an oil droplet on silicon posts with different b/a ratios.

FIG. 6 is roll-off droplet radius as a function of b/a for silicon post structures.

FIG. 7 is a plot of post sizes for impact resistant textures as a function of b/a.

FIG. 8 is a side-view pictorial representation of drag force on a droplet.

FIG. 9 is a top-view pictorial representation of drag force on a droplet.

FIG. 10 depicts a textured surface comprised of an array of square posts.

FIGS. 11-14 are plots demonstrating drag force reduction on a variety of surfaces.

DETAILED DESCRIPTION

A barrier seal system 10 comprises a rotatable shaft 12 (with or without a shaft sleeve (not shown)) disposed within an outer housing 14 and a barrier seal 16 that radially extends from the outer housing 14 to the rotatable shaft 12. The rotatable shaft 12 axially extends from a high-pressure region 18 to a low-pressure region 20. The barrier seal 16 and the rotatable shaft combine to define a gap 22 therebetween. The system 10 further comprises a barrier gas injector 24 for injecting a barrier gas 26 into the gap 22 to prevent oil migration from the low-pressure region 20 to the high-pressure region 18.
Conventionally, the barrier gas injector 24 injects the barrier gas 26, for example air or an inert gas like nitrogen, at a sufficient flow rate, for example, greater than 16 feet per second over the life of the seal. At this flow rate, the velocity boundary layer is typically thin enough to preclude oil migration no matter how thin the lubricant film becomes. It is important, however, to minimize the quantities of the purge gas required to preclude the oil migration. Accordingly, in one embodiment of the instant invention, at least a portion of the rotatable shaft 12 (or shaft sleeve), typically adjacent to the low-pressure region, comprises an oleophobic surface 28.
As used herein, the term “oleophobic surface” means any surface that reduces the tendency for an oil to attach to that surface or form a film on that surface, including all superoleophobic surfaces. Oleophobic surfaces are characterized by reduced build-up and more facile removal of oils from the surface, compared to surfaces that are not oleophobic in nature.
Accordingly, because a portion of the rotatable shaft 12 adjacent the low-pressure region 20 comprises oleophobic surface 28, the quantities of the injected barrier gas 26 required to preclude the oil migration are greatly reduced because any oil that contacts that portion of rotatable shaft or sleeve will bead up or have steep wetting angles and the barrier gas will be able to push the oil away from the barrier seal easier. By reducing the quantities and flow rate of the required barrier gas 26, the barrier seal system 10 is greatly improved over a conventional system, making the system 10 more resistant to oil leaking or wicking and reducing the costs associated with the system's operation.
The oleophobic surface 28 on the rotatable shaft 12 increases the wetting angle of the oil contacting the shaft such that the oil cannot maintain a film thickness that is small enough to stay inside a low velocity boundary layer of the barrier gas 26. Therefore, the higher velocity in the bulk stream of the barrier gas 26 can force the oil away from the barrier seal 16 easier and preclude oil migration and reduce the quantities of barrier gas 26 required to do so.
As shown in FIG. 2, the barrier seal system can include additional types of sealing arrangements, for example brush seals, labyrinth seals, carbon face seals and combinations thereof. In FIG. 2, barrier seal system 50 includes multiple labyrinth seals 52 and multiple brush seals 54. In this embodiment, a centrally located barrier gas injector 56 is used to inject the barrier gas into the gap 58.
As used herein, the “contact angle” or “static contact angle” is the angle formed between a stationary drop of a reference liquid and a horizontal surface upon which the droplet is disposed, as measured at the liquid/substrate interface. Contact angle is used as a measure of the wettability of the surface. If the liquid spreads completely on the surface and forms a film, the contact angle is 0 degrees. As the contact angle increases, the wettability decreases.
Referring to the drawings in general and to FIG. 3 in particular, it will be understood that the illustrations are for the purpose of describing a particular embodiment of the invention, and are not intended to limit the invention thereto. FIG. 3 is a schematic cross-sectional view of a surface of the rotatable shaft 12 coated with an oleophobic surface 28, according to one embodiment of the invention. Article 100 comprises a surface 102. As used herein, the term “surface” refers to that portion of the article 100 that is in direct contact with an ambient environment surrounding the article 100. The surface may include the substrate, the features, or the surface modification layer disposed over the substrate, depending on the specific configuration of the article. Surface 102 has low oil wettability. One commonly accepted measure of the oil wettability of a surface 102 is the value of the static contact angle 104 formed between surface 102, and a tangent 106 to a surface of a droplet 108 of a reference oil at the point of contact between surface 102 and droplet 108. High values of contact angle 104 indicate a low wettability for the reference oil on surface 102.
As used herein, “oil” is to be understood as having its common meaning to cover a wide variety of unctuous substances not miscible with water. Examples include oils of animal, vegetable, or mineral origin, as well as synthetic oils. Particular examples of oils include petroleum-based products, such as crude oil and products distilled therefrom, such as kerosene, gasoline, paraffin, and the like. In some embodiments, the oil comprises an industrial lubricant such as bearing oil or light turbine oil. In one embodiment of the invention, the oleophobic surface 28 (FIG. 1, FIG. 2) is a surface that generates a static contact angle with oil of at least about 30 degrees.
In one embodiment, a surface 150 comprises a material having a nominal liquid wettability sufficient to generate, with reference to an oil, a nominal contact angle of at least about 30 degrees, as shown in FIG. 4. For the purposes of understanding the invention, a “nominal contact angle” 152 means the static contact angle 152 measured where a drop of a reference oil 154 is disposed on a flat, smooth (<1 nm surface roughness) surface 150 consisting essentially of the material. This nominal contact angle 152 is a measurement of the “nominal wettability” of the material. In one embodiment, the nominal contact angle, with reference to an oil, is at least about 50 degrees. In one embodiment, the nominal contact angle, with reference to an oil, is at least about 70 degrees. In one embodiment, the nominal contact angle, with reference to an oil, is at least about 100 degrees. In yet another embodiment, the nominal contact angle, with reference to an oil, is at least about 120 degrees.
Surface 28 (FIGS. 1 & 2) and surface 102 (FIG. 3) comprise at least one material selected from the group consisting of a ceramic, an intermetallic, and a polymer. Suitable ceramic materials include inorganic oxides, carbides, nitrides, borides, and combinations thereof. Non-limiting examples of such ceramic materials include aluminum nitride, boron nitride, chromium nitride, silicon carbide, tin oxide, titania, titanium carbonitride, titanium nitride, titanium oxynitride, stibinite (SbS₂), zirconia, hafnia, and combinations thereof. In certain embodiments, the surface comprises an intermetallic. Examples of suitable intermetallic materials include, but are not limited to, nickel aluminide, titanium aluminide, and combinations thereof. Polymer materials that may be used in surface 102 include, but are not limited to polytetrafluoroethylene, fluoroacrylate, fluoroeurathane, fluorosilicone, modified carbonate, silicones and combinations thereof. The material is selected based on the desired contact angle and the fabrication technique used.
In another embodiment, surface 102 further comprises a texture comprising a plurality of features 110 (FIG. 3). A surface 102, comprising a material of comparatively high nominal wettability, with a specific texture, as described in detail below, has a significantly lower wettability than that inherent to the material from which the surface is made. In particular, surface 102 has an effective wettability (that is, wettability of the textured surface) for the reference oil sufficient to generate an effective contact angle greater than the nominal contact angle. The effective contact angle depends, in part, on the feature shape, dimensions, and spacings, as will be described in detail below.
As described above, in one embodiment, surface 102 has a texture comprising a plurality of features 110. The plurality of features 110 may be of any shape, including at least one of depressions, protrusions, nanoporous solids, indentations, or the like. The features may include bumps, cones, rods, wires, channels, substantially spherical features, substantially cylindrical features, pyramidal features, prismatic structures, combinations thereof, and the like. Numerous varieties of feature shapes are suitable for use as features 106. In some embodiments, as shown in FIG. 1, at least a subset of the plurality of features 110 protrude above the surface 102 of the article. In some embodiments at least a subset of the plurality of features 110 is a plurality of cavities 112 disposed in the surface 102. In some embodiments, at least a subset of the features 110 has a shape selected from the group consisting of a cube, a rectangular prism, a cone, a cylinder, a pyramid, a trapezoidal prism, and a hemisphere or other spherical portion. These shapes are suitable whether the feature is a protrusion 110 or a cavity 112.
The size of features 110 (FIG. 3) can be characterized in a number of ways. Features 110 comprise a height dimension (h) 114, which represents the height of protruding features above the surface 102 or, in the case of cavities 112, the depth to which the cavities extend into the surface 102. Features 110 further comprise a width dimension (a) 116. The precise nature of the width dimension will depend on the shape of the feature, but is defined to be the width of the feature at the point where the feature would naturally contact a drop of liquid placed on the surface of the article. The height and width parameters of features 110 have a significant effect on wetting behavior observed on surface 102.
Feature orientation is a further design consideration in the engineering of surface wettability in accordance with embodiments of the present invention. One significant aspect of feature orientation is the spacing of features. Referring to FIG. 3, in some embodiments features 110 are disposed in a spaced-apart relationship characterized by a spacing dimension (b) 118. Spacing dimension 118 is defined as the distance between the edges of two nearest-neighbor features. Other aspects of orientation may also be considered, such as, for instance, the extent to which top (or bottom for a cavity) deviates from being parallel with surface 102, or the extent to which features 110 deviate from a perpendicular orientation with respect to the surface 102.
The plurality of features 110 (FIG. 3) making up texture 110 need not be confined to the surface 102 or a region immediately proximate to the surface 102. In some embodiments, article 100 further comprises a bulk portion 120 disposed beneath surface 102, and the plurality of features 110 extend into bulk portion 120. Distributing features 110 throughout the article 100, including at the surface 102 and within the bulk portion 120, allows surface 102 to be regenerated as the top layer of surface erodes away.
In certain embodiments, the surface comprises a surface energy modification layer (not shown). In certain cases, the surface energy modification layer comprises a coating disposed over a substrate. The substrate may comprise at least one of a metal, an alloy, a plastic, a ceramic, or any combination thereof. The substrate may take the form of a film, a sheet, or a bulk shape. The substrate may represent article 100 in its final form, such as a finished part; a near-net shape; or a preform that will be later made into article 100. Surface 102 may be an integral part of the substrate. For example, surface 102 may be formed by replicating a texture directly onto the substrate, or by embossing the texture onto the substrate, or by any other such method known in the art of forming or imparting a predetermined surface texture onto a substrate surface. Alternatively, surface 102 may comprise a layer that is disposed or deposited onto the substrate by any number of techniques that are known in the art.
Example—Making silicon articles with oleophobic properties: Silicon substrates were provided via lithography with right rectangular prism features about 3 micrometers in width (a) and having various b/a ratios. The substrates were then placed in a chamber with a vial of liquid fluorosilane, and the chamber was evacuated to allow the liquid to evaporate and condense from the gas phase onto the silicon substrate, thereby creating a film on the surface. The contact angle was recorded as a function of b/a ratio. FIG. 5 shows the photographs of oil droplets 62 on silicon posts 60 with different b/a ratios. The figure lists static contact angle of oil (a light turbine oil in this case) on different textures. The ease of roll-off was measured by determining the angle of tilt from the horizontal needed before a drop will roll off of a surface. A drop that requires a near vertical tilt is highly pinned to the surface, whereas a drop exhibiting easy roll-off will require very little tilt angle to roll off the surface. In some embodiments, the drop will roll off of the surface at the point where the force of gravity pulling on the drop equals the force pinning the drop to the surface. This situation can be represented by the following expression:
ρVg sin α=2πμβr (1);
where ρ is the liquid density, V is the volume of the drop, g is the gravity constant, α is the angle of inclination from the horizontal, μ is the pinning parameter, β is the fraction of the contact line that is pinned, and r is the radius of the contact area of the drop with the substrate. μ, the pinning parameter, is a material constant that is independent of the surface texture, but β and r are functions of the texture. The texture, in some embodiments, is represented by the parameters a, b, and h of the features. Based on the oil roll-off on smooth silicon with fluorosilane, the pinning parameter μ was calculated to be 0.029 N/m. For water, the pinning parameter is of the order of 0.013 N/m. Table 1 lists the contact angles for different b/a ratios.

TABLE 1

Contact angles for different b/a ratios.

			Contact angle (with
Sample	a (micrometers)	B/a	reference to oil)

1	3	0.33	110
2	3	0.5	151
3	3	0.75	149
4	3	1	137
5	3	1.5	144
6	3	2	132
7	3	4	90
8	3	5	83
9	3	7.5	103
10	3	10	81

Table 1 shows the effect of varying b/a on the contact angle. The contact angle measured on a control specimen having a smooth (non-textured) surface coated with fluorosilane was about 88 degrees. Above b/a of 4, the contact angle decreases as the drop settles into a wetting state under its own weight.
As a practical matter, design considerations are applied to arrive at a surface design that promotes a high contact angle and easy drop roll-off. FIGS. 6 and 7 show the results of work aimed at validating the above analysis, and the plots illustrated in these figures may be used to select suitable textures for a range of applications, for a given combination of oil type and surface material. FIG. 6 shows the plot 200 of maximum diameter of the drop required for the drop to roll off of a texture comprised of posts described above. FIG. 7 gives plot 300, the maximum diameter of the drop required for the drop to roll off on a texture comprised of pore structures.
FIG. 8 (side-view) and FIG. 9 (top-view) are pictorial representations that depict the drag force on a droplet. The drag force is calculated using the following equation:
$\frac{F_{d}}{μ V^{1 / 3}} = {β (24 π^{2})}^{1 / 3} (\frac{\sin θ}{{(2 - 3 \cos θ + \cos^{3} θ)}^{1 / 3}})$
where Fd is the drag force, μ is the hysteresis coefficient, θ is the contact angle, β is the texture coefficient, and V is the volume of the droplet.
FIG. 10 depicts a textured surfaces comprised of an array of square posts. The texture coefficient is represented by the following equation:
β=1/(1+b/a)
where b is the distance between adjacent posts and a is the width of a respective post. The contact angle on a textured surface is represented by the following equation:
cos(θ_t)=−1+[(1+cos(θ_S))/(1+(b/a))²]
where θ_tis a contact angle on a smooth surface.
For an oil droplet sitting on a textured oleophobic material, θ will be greater than 90 degrees and β will be less than 1 (assuming the droplet hasn't penetrated the texture). As a result, the drag force on the oil droplet will be significantly less for a textured oleophobic material compared to that on a smooth non-oleophobic surface. The drag force can be reduced further by choosing a material with low hysteresis coefficient, μ. This has been illustrated in FIG. 11-14. All the plots are based on Exxon Teresstic GT 32 Turbine Oil interacting with a Silicon surface.
FIG. 11 is a graphical depiction of a non-dimensional drag force versus the contact angle for a smooth surface. As one can see from FIG. 11, the drag force can be substantially reduced by employing a smooth oleophobic surface thereby increasing the oil droplet contact angle.
FIG. 12 is a graphical depiction of a non-dimensional drag force versus the contact angle for textured surface. FIG. 12 illustrates that the drag force reduction can be further enhanced by using a textured oleophobic material. This example is based on Exxon Teresstic GT 32 Turbine Oil on a textured Silicon surface coated with flurosilane. The contact angle of the Exxon Teresstic GT 32 Turbine Oil on a smooth silicon surface is 83 degrees and the hysteresis coefficient is equal to 0.029 N/m.
FIG. 13 is a graphical depiction of drag force versus contact angle for a 1 micro liter droplet on a smooth surface.
FIG. 14 is a graphical depiction of drag force versus contact angle for a 1 micro liter droplet on a textured surface. This example is based on Exxon Teresstic GT 32 Turbine Oil on a textured Silicon surface coated with flurosilane. The contact angle of the Exxon Teresstic GT 32 Turbine Oil on a smooth silicon surface is 83 degrees and the hysteresis coefficient is equal to 0.029 N/m.
FIGS. 13 and 14 depict the actual drag force reduction with the change in oil contact angle and in the presence of texture.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

1. A barrier seal system comprising:

a rotatable shaft disposed within an outer housing and axially extending from a high-pressure region to a low-pressure region, wherein at least a portion of said rotatable shaft disposed adjacent said low-pressure region comprises an oleophobic surface;

a barrier seal that radially extends from said outer housing to said rotatable shaft and defines a gap between the barrier seal and the rotatable shaft;

a barrier gas injector for injecting a barrier gas into said gap to flow from said high-pressure region to said low-pressure region.

2. A barrier seal system in accordance with claim 1, wherein said barrier seal system is used within rotating machinery.

3. A barrier seal system in accordance with claim 1, further comprising a lubricating element within the low-pressure region.

4. A barrier seal system in accordance with claim 3, wherein said barrier gas injector injects a barrier gas through said gap to prevent migration of said lubricating element from said low-pressure region to said high-pressure region.

5. A barrier seal system in accordance with claim 4, wherein said oleophobic surface prevents film formation of said lubricating element adjacent to said gap.

6. A barrier seal system in accordance with claim 1, wherein said oleophobic surface is an oleophobic coating.

7. A barrier seal system in accordance with claim 1, wherein said oleophobic surface is an oleophobic texturing.

8. A barrier seal system in accordance with claim 3, wherein said lubricating element is oil.

9. A barrier seal system in accordance with claim 1, wherein said barrier seal is selected from the group consisting of brush seals, labyrinth seals and carbon face seals.

10. A barrier seal system in accordance with claim 1, wherein said barrier seal is selected from the group consisting of contacting seals and non-contacting seals.

11. A barrier seal system in accordance with claim 1, wherein said barrier gas is injected at a mass flow rate of less than about 16 feet per second.

12. A barrier seal system in accordance with claim 3, wherein said oleophobic surface increases the wetting angle of the lubricating element and correspondingly decreases the required mass flow rate of said injected barrier gas to prevent migration of said lubricating element through said gap.

13. A barrier seal system in accordance with claim 1, wherein said shaft further comprises a shaft sleeve.

14. A barrier seal system in accordance with claim 13, wherein at least a portion of said shaft sleeve disposed within said high-pressure region comprises an oleophobic surface;

15. A barrier seal system in accordance with claim 1, wherein said barrier gas is an inert gas.

16. A barrier seal system in accordance with claim 15, wherein said inert gas is nitrogen.

17. A barrier seal system in accordance with claim 3, wherein the static contact angle of the oleophobic surface is greater than about 30 degrees.

18. A dry gas seal assembly comprising:

a dry gas seal; and

a barrier seal assembly spaced apart from said dry gas seal comprising: