MULTI-PHASE GAS-ENHANCED LUBRICANTS FOR PERFORMANCE CONTROL IN ROTATING MACHINERY BEARINGS AND RELATED METHOD THEREOF

Abstract

A method and system for lubricating at least one moving part with a lubricating medium includes a chamber configured to house at least one moving part, a lubricant reservoir configured to house a liquid lubricant, a gas reservoir configured to house a gas, and a controller configured to supply a first amount of the gas in the form of bubbles to a second amount of the liquid lubricant to entrain the bubbles of gas in the liquid lubricant. The controller is also configured to regulate proportions of the liquid lubricant and the gas as a function of at least one operating condition, including at least one of pressure, temperature, and flow rate, a solubility relationship between the liquid lubricant and the gas, and a size of the bubbles of gas, and supply the liquid lubricant with entrained bubbles of gas to at least one moving part.

Description

BACKGROUND

Lubricants are used in many applications where an unacceptable amount of mechanical friction would otherwise occur between moving parts. One example of a typical application for lubricants is in bearings that are configured to support the weight of a shaft under loaded conditions. The selected lubricants must provide suitable stiffness and damping when the bearing is incorporated into a rotor system, and must protect the mechanical components from wear. Both the mechanical (e.g., viscosity, density) and thermal (e.g., heat capacity, conductivity) properties of the lubricant are important to ensure proper system function. High viscosity and/or density fluids typically provide more stability and protection for the bearing. A lubricant viscosity that is too high can result in undesirable power losses and unacceptably high temperatures generated at the intersection between the moving parts. The thermal properties of the lubricant must be selected so that friction in the bearing does not lead to temperature build-up and viscosity changes caused by the degradation of the lubricant. Selecting an application-specific lubricant generally solves this trade-off between bearing stability and bearing efficiency. The result can be a bearing that operates optimally in a narrow window of speeds and loads but is less robust if system conditions change.

Gas-expanded lubricants (GELs), such as fully dissolved, single-phase mixtures of lubricant oil and carbon dioxide, have been previously developed to improve bearing efficiency and reduce operating temperatures in rotating machinery by controlling the properties of the lubricant in real time. A reduction in lubricant viscosity that results from the dissolution of a gas such as CO₂ in a liquid lubricant such as a synthetic lubricant allows for reduced shear of the fluid in the bearing, which results in lower power loss and less heat generation. The ability to control the concentration of CO₂ in the mixture using pressure would impart a tunable nature to the lubricant that would provide control over bearing stiffness and damping forces and subsequently machine vibration and stability. This capability could be useful as the fluid properties could be altered prior to start-up and shutdown to maximize machine stability and then optimized while operating at steady state to maximize efficiency and reduce operating temperatures.

A limitation of existing GELs that are single-phase mixtures of lubricant oil and carbon dioxide is that the dissolution of a gas into lubricant oil results in a reduction of the lubricant viscosity. There may be applications where it would be desirable to raise the viscosity level of the lubricant rather than lower the viscosity level as is the case when dissolving a compressed gas into the liquid lubricant. Many machine process fluids that also serve as lubricants within the machine, or even fluids within which the machine is submerged, may have viscosities that are lower than desired for certain applications. In some situations, traditional oil-based, or other relatively high viscosity lubricants may not be available or may be prohibitively expensive.

A gas-enhanced lubricant according to various implementations of the present disclosure addresses the above-described characteristics and/or other problems of the prior art.

SUMMARY

One aspect of the present disclosure is directed to a medium for lubricating at least one moving part. The medium may include a liquid lubricant and a discrete gas phase in the form of bubbles of the gas entrained in the liquid lubricant.

Another aspect of the present disclosure is directed to a method of lubricating at least one moving part with a lubricating medium. The method may include supplying a first quantity of a liquid lubricant and a second quantity of a gas, wherein the relative amounts of the liquid lubricant and the gas are controlled based on at least one operating condition including pressure, temperature, and flow rate, and a solubility relationship between the liquid lubricant and the gas. The method may further include introducing the gas in the form of bubbles into the liquid lubricant in an amount sufficient to increase the viscosity of the liquid lubricant by at least 10%, and providing the liquid lubricant with entrained bubbles of the gas to at least one moving part as the lubricating medium.

Yet another aspect of the present disclosure is directed to a system for lubricating at least one moving part with a lubricating medium. The system may include a chamber configured to house at least one moving part, a lubricant reservoir configured to house a liquid lubricant, and a gas reservoir configured to house a gas. The system may also include a controller configured to supply a first amount of the gas in the form of bubbles to a second amount of the liquid lubricant to entrain the bubbles of gas in the liquid lubricant in proportions that are a function of at least one operating condition, including pressure, temperature, and flow rate, and a solubility relationship between the liquid lubricant and the gas. The controller may be further configured to supply the liquid lubricant with entrained bubbles of gas to the at least one moving part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of an exemplary system for applying a gas-enhanced lubricant accord to this disclosure;

FIG. 2 is a graphical illustration showing effective viscosities of mixtures of liquid lubricant and various amounts of discrete gas bubbles;

FIGS. 3A and 3B are graphical illustrations showing potential impacts of increased viscosity multi-phase lubricant mixtures according to exemplary embodiments of this disclosure on bearing direct and cross-coupled stiffness values;

FIGS. 4A and 4B are graphical illustrations showing potential impacts of increased viscosity multi-phase lubricant mixtures according to exemplary embodiments of this disclosure on bearing damping and inertial forces; and

FIG. 5 is a graphical illustration showing potential impacts of increased viscosity multi-phase lubricant mixtures according to exemplary embodiments of this disclosure on stability of rotating machinery using the lubricant mixture.

DETAILED DESCRIPTION

As shown in FIG. 1, one aspect of an embodiment of the present disclosure may include a system for lubricating at least one moving part 24 with a multi-phase, gas-enhanced lubricating medium. The system may include a chamber 22, which may or may not be pressurized, and chamber 22 may be configured to house at least one moving part 24. In one exemplary implementation of this disclosure, moving parts 24 may include a tilting pad journal bearing rotatably supporting a shaft. However, one of ordinary skill in the art will recognize that various embodiments of the disclosed multi-phase, gas-enhanced lubricating medium may be applicable in any type of fluid film bearing or other moving parts wherein the interacting surfaces are oriented radially or axially relative to each other.

As shown in FIG. 1, an aspect of an embodiment of the present disclosure may include a lubricant reservoir 20 configured to house a liquid lubricant, and a gas reservoir 18 configured to house a gas. The gas may be introduced into the liquid lubricant either before entering chamber 22, or within chamber 22 in the form of bubbles of the gas that become entrained in the liquid lubricant to form a multi-phase, gas-enhanced lubricating medium. The system may include a controller 16 (or one or more processors) to provide the transfer of liquid lubricant from the lubricant reservoir 20 and gas from the gas reservoir 18 to chamber 22 and to interacting surfaces of the moving part 24. The controller 16 may function by determining the respective proportions of the liquid lubricant and gas in forming the multi-phase mixture 26 in response to input conditions 14 such as environmental or loading conditions. The system may include an oil/gas separator 28 configured to separate the gas bubbles from the liquid lubricant and vent the gas from the chamber 22, thereby decreasing the pressure and/or impacting the viscosity, density, thermal conductivity, temperature, and/or other properties of the multi-phase mixture in the chamber 22. The system may also include a return channel 30 configured to scavenge surplus lubricant from the chamber 22 and to return the lubricant to the lubricant reservoir 20.

Moving part 24 may be a rotor, bearing, gear, a combination of these and/or other interacting parts with a fluid film of a multi-phase, gas-enhanced lubricant according to various embodiments of this disclosure provided between the interacting parts. The moving parts 24 may be housed in a chamber 22 that may or may not be pressurized above ambient pressure or subjected to a partial vacuum. The liquid lubricant used in forming the multi-phase, gas-enhanced lubricant may be a liquid with a viscosity lower than typical oils or synthetic lubricants. Typical oils and synthetic lubricants may have a viscosity at ambient temperatures that is greater than or equal to approximately 32 milliPascals-second (mPa·s). Viscosity may be measured with various types of viscometers and rheometers. A rheometer is used for those fluids that cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. Close temperature control of the fluid is essential to acquire accurate measurements, particularly in materials like lubricants, whose viscosity can double with a change of only 5 degrees Centigrade. Newtonian fluids exhibit a viscosity that remains essentially constant over a wide range of shear rates. Lubricants without a constant viscosity over a range of shear rates (non-Newtonian fluids) cannot be described by a single number and may exhibit a variety of different correlations between shear stress and shear rate. Nevertheless, when referring to “low viscosity” liquid lubricants herein, viscosities that are less than approximately 30 mPa·s at ambient temperatures are intended. Other examples of low viscosity liquid lubricants may include water, seawater, ionic liquids, refrigerants, and other machine process fluids that may be used as operating fluids performing various functions within a machine, or as fluids within which a machine may be submerged. One example may include seawater-lubricated bearings in a subsea compressor that operates while being fully submerged in seawater. Other machine process fluids that may be used in forming the multi-phase, gas-enhanced lubricants according to various embodiments of this disclosure may include refrigerants or other low viscosity liquids.

The gas that may be introduced as bubbles into the liquid lubricant may be a single gas such as one of Carbon Dioxide, Nitrogen, Oxygen, Argon, Air, and Helium, or a combination of these and/or other gases of similar characteristics as desired or required. The oil/gas separator 28 may be a valve, vent, egress, or a combination of these and/or other membranes or devices downstream of chamber 22 and moving parts 24 capable of extracting the gas in bubble form from the liquid lubricant. The multi-phase mixture 26 may have properties, such as viscosity, thermal conductivity, and/or temperature, which are adjusted by adding and/or releasing at least some of the gas from chamber 22. The input conditions 14 may be defined by a user 10 and/or controller 16 in response to some external factors. The input conditions 14 may be configured in real time, continuously, and/or over intervals of time.

An aspect of an exemplary embodiment of the present disclosure includes a method of lubricating at least one moving part with a lubricating medium. The medium may include a multi-phase mixture of liquid lubricant and gas in the form of bubbles entrained in the liquid lubricant. The relative amounts of liquid lubricant and gas may be controlled in forming the multi-phase mixture in response to input conditions. A user and/or external factors received and interpreted by controller 16 may be used to determine the input conditions. In response to the input conditions the amount of liquid lubricant and gas may be delivered to moving part 24 housed in chamber 22. The properties of the multi-phase mixture can be adjusted, whereby the properties may include, but are not limited to: viscosity, density, temperature, and thermal conductivity. This adjustment to the multi-phase mixture may be accomplished, for example, by releasing gas from chamber 22 in an amount to adjust the properties. In a further approach, lubricant may be scavenged from chamber 22 by returning surplus lubricant to its original source or other designated location.

Another aspect of an exemplary embodiment of the present disclosure provides a new concept and application of multi-phase, gas-enhanced lubrication, whereby the addition of a discrete gas phase in the form of gas bubbles to a base liquid lubricant results in an increase in the viscosity of the multi-phase mixture as compared to the viscosity of the base liquid lubricant without the addition of the gas bubbles. The gas phase may include any single gas or mixture of gases such as air. The base lubricant may include water, seawater, refrigerants, ionic liquids, machine process fluids, and fluids being used as lubricants due to the machine being submerged in the fluid. A significant benefit, among others, of the described controllable multi-phase lubrication approach is to facilitate the use of low viscosity lubricants, such as water or seawater in applications where the use of more traditional lubricants is unavailable. As discussed above, reference to “low viscosity” in this disclosure is a viscosity that is below approximately 30 mPa·s at ambient pressure and temperature. By adding gas bubbles to the base lubricant, the effective viscosity of the mixture can be increased, providing greater load capacity for the rotating shaft or other moving part and greater stiffness and damping forces to the system to increase rotordynamic stability. In various implementations of this disclosure, the addition of gas bubbles to the base lubricant may result in the multi-phase mixture having a viscosity that is at least 10% greater than the viscosity of the base lubricant alone. The controlled addition of a gas phase to liquid lubricants impacts bearing and rotordynamic performance. FIG. 2 provides a graphical illustration of the effects of adding different amounts of CO₂ to a liquid lubricant on the resulting viscosity of the multi-phase mixture.

Various factors that may be controlled in order to produce a mixture with the desired increase in viscosity and desired lubricating characteristics may include operating conditions such as pressures, temperatures, and flow rates of one or both of the liquid lubricant and the gas, and the types of liquid lubricants and gases that are used together based at least in part on the compatibility and solubility relationships of the fluids. As one example of the type of compatibility and solubility relationships of the liquid lubricant and the gas that may be taken into account to produce the multi-phase mixture, some gases may dissolve more readily into certain liquids than others. Therefore, various exemplary implementations of this disclosure may include the selection of gases that are not as easily dissolved in the chosen liquid lubricant, such that the chosen gas remains in gaseous form as bubbles entrained in the liquid lubricant and provides the desired increase in viscosity of the multi-phase mixture. Other factors that may be controlled in order to retain the desired benefits of the multi-phase mixture may include the pressure of the chamber containing the bearing or other interacting surfaces. In some situations, it may be desirable to actually lower the pressure in the chamber such that the gas remains in gaseous form as bubbles entrained in the liquid lubricant. Alternatively or in addition, the temperature in the region of application of the multi-phase lubricant may be controlled at least in part as a function of the solubility relationship of the liquid lubricant and the gas added to the liquid lubricant. Still further controls that may affect the characteristics of the multi-phase mixture may include control of flow rates of one or both of the liquid lubricant and the gas, the purging or separation of the gas bubbles from the liquid lubricant using a separator, and control of the size of bubbles of the gas introduced into the liquid lubricant.

A system according to various implementations of this disclosure may be used for lubricating at least one moving part 24 with a lubricating medium, and enhancing control in rotating machinery bearings through the use of multi-phase, gas-enhanced lubricants. The system may include chamber 22 configured to house at least one moving part 24, lubricant reservoir 20 configured to house a liquid lubricant, and gas reservoir 18 configured to house a gas. The system may also include controller 16 configured to supply a first amount of the gas in the form of bubbles to a second amount of the liquid lubricant to entrain the bubbles of gas in the liquid lubricant in proportions that are a function of at least one operating condition, including pressure, temperature, and flow rate, and a solubility relationship between the liquid lubricant and the gas. Controller 16 may be further configured to supply the liquid lubricant with entrained bubbles of gas to the at least one moving part.

In one exemplary implementation of this disclosure, controller 16 may determine the proportions of each of the liquid lubricant and the gas to be mixed in chamber 22 at least in part by receiving feedback on the operating condition of moving parts 24, such as temperature, speed, vibration levels, and load. Controller 16 may be configured to determine the viscosity needs of the one or more moving parts 24 based on an algorithm that utilizes this feedback. Viscosity control may also be aided by in-line viscosity sensors in the supply lines of the liquid lubricant and the gas, or in an output line from a mixing zone before the multi-phase mixture is supplied to chamber 22 and moving parts 24. Flow rates needed for cooling or for adequate lubrication can also be determined using a similar data processing method. After performing its lubricating function, the multi-phase lubricant in chamber 22 may flow through separator 28. The liquid lubricant may be returned to lubricant reservoir 20 through line 30, and the gas may be discharged to atmosphere or collected to be optionally filtered or otherwise processed and returned to gas reservoir 18 for re-use.

To demonstrate the potential for this new concept of multi-phase, gas-enhanced lubricants, an example case for a lightly loaded, seawater-lubricated bearing supporting a subsea compressor is provided. A bearing model developed by Dousti et al. (Dousti, S., Cao, J., and Younan, A., 2012, “Temporal and Convective Inertia Effects in Plain Journal Bearings with Eccentricity, Velocity and Acceleration,” Journal of Tribology, 134(3), pp. 031704-1-031704-8) was used to simulate this bearing as the model developed was designed specifically for the analysis of bearings lubricated with low viscosity fluids such as water. The fluid property cases analyzed for their performance were pure seawater, a multi-phase, gas-enhanced lubricant with a 50% increase in viscosity, and a multi-phase, gas-enhanced lubricant with a 100% increase in viscosity. The effective density of the multi-phase lubricants was also decreased as the viscosity was increased as is typical for bubbly liquid flows. The pressure in the bearing was based on a machine submergence depth of 300 meters as is typical for subsea compressors. The bearing size, speed, and light load were also chosen to reflect the typical conditions of a subsea compressor bearing.

Results for this case study show that the increase in mixture viscosity can result in significant effects on the bearing forces applied to the shaft. FIGS. 3A and 3B illustrate potential impacts on direct bearing stiffness and cross-coupled hearing stiffness values, showing an increase in these forces with lubricant viscosity for this case. FIGS. 4A and 4B illustrate how the changes in lubricant viscosity can affect bearing damping forces and inertia forces, showing an increase in damping and decrease in mass coefficients for this case with increasing viscosity values. The effects of these changing bearing forces on rotor stability are graphically illustrated in FIG. 5. For this exemplary case study it was found that the increase in lubricant viscosity resulted in a shift from a negative log decrement (indicating an unstable rotor motion) to a positive log decrement (indicating a stable operating regime). Hence, it was verified that the disclosed novel multi-phase, gas-enhanced lubrication may provide significant benefits in controlling and improving hearing and rotordynamic performance in rotating machinery.

An aspect of the method and system according to various embodiments of this disclosure may provide the use of multi-phase, gas-enhanced liquid lubricant/gas mixtures that can be tuned in response to changing speed or loading conditions and solubility relationships between the liquid lubricant and the gas to improve the energy efficiency of tilting pad journal bearings and other fluid film bearings. These multi-phase, gas-enhanced lubricants may provide the foundation for a novel type of smart lubricant with viscosity that is adjustable in real time. The ability to increase the viscosity of low viscosity liquid lubricants such as water, seawater, refrigerants, ionic fluids, and other machine or environment-driven lubricants by the entrainment of gas bubbles in the liquid lubricant may also facilitate the use of readily available and inexpensive liquid lubricants while still achieving desired bearing stiffness, damping, and inertia and maintaining the rotating stability of machines in which the multi-phase, gas-enhanced lubricants are used.

It will be apparent to those skilled in the art that various modifications and variations can be made to the methods and system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the various implementations disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalent.

Claims

1. A medium for lubricating at least one moving part, said medium comprising a liquid lubricant and a discrete gas phase in the form of bubbles of a gas entrained in the liquid lubricant.

2. The medium of claim 1, wherein the liquid lubricant has a viscosity that is below approximately 32 mPa·s.

3. The medium of claim 2, wherein the liquid lubricant comprises at least one of a machine process fluid and a fluid in which a machine is submerged.

4. The medium of claim 3, wherein the liquid lubricant comprises water.

5. The medium of claim 3, wherein the liquid lubricant comprises seawater.

6. The medium of claim 3, wherein the liquid lubricant comprises a refrigerant.

7. The medium of claim 2, wherein the liquid lubricant comprises an ionic liquid.

8. The medium of claim 1, wherein the gas comprises one or more gases selected from the group consisting of carbon dioxide, nitrogen, oxygen, argon, helium, and air.

9. The medium of claim 1, wherein the medium with the discrete gas phase has a viscosity that is at least 10% greater than a viscosity of the liquid lubricant alone.

10. A method of lubricating at least one moving part with a lubricating medium; the method comprising: supplying a first quantity of a liquid lubricant and a second quantity of a gas;introducing the gas in the form of bubbles into the liquid lubricant in an amount sufficient to increase a viscosity of the liquid lubricant by at least 10%; andproviding the liquid lubricant with entrained bubbles of the gas to the at least one moving part as the lubricating medium.

11. The method of claim 10, wherein the liquid lubricant has a viscosity that is below approximately 32 mPa·s.

12. The method of claim 10, wherein the liquid lubricant comprises at least one of a machine process fluid and a fluid in which a machine is submerged.

13. The method of claim 12, wherein the liquid lubricant comprises water.

14. The method of claim 12, wherein the liquid lubricant comprises seawater.

15. The method of claim 10, wherein the liquid lubricant comprises a refrigerant.

16. The method of claim 10, wherein the liquid lubricant comprises an ionic liquid.

17. The method of claim 10, wherein the gas comprises one or more gases selected from the group consisting of carbon dioxide, nitrogen, oxygen, argon, helium, and air.

18. The method of claim 10, further including controlling proportions of the liquid lubricant and the gas as a function of at least one operating condition for the at least one moving part, including at least one of pressure, temperature, and flow rate; a solubility relationship between the liquid lubricant and the gas; and a size of the bubbles of gas.

19. A system for lubricating at least one moving part with a lubricating medium, the system comprising: a chamber configured to house at least one moving part;a lubricant reservoir configured to house a liquid lubricant;a gas reservoir configured to house a gas; anda controller configured to: supply a first amount of the gas in the form of bubbles to a second amount of the liquid lubricant to entrain the bubbles of gas in the liquid lubricant;regulate proportions of the liquid lubricant and the gas as a function of at least one operating condition, including at least one of pressure, temperature, and flow rate; a solubility relationship between the liquid lubricant and the gas; and a size of the bubbles of gas; andsupply the liquid lubricant with entrained bubbles of gas to the at least one moving part.

20. The system according to claim 19, wherein the liquid lubricant comprises at least one of a machine process fluid and a fluid in which a machine is submerged, the liquid lubricant having a viscosity of less than approximately 32 mPa·s, and the liquid lubricant being selected from the group consisting of water, seawater, a refrigerant, and an ionic liquid; wherein the gas is one or more gases selected from the group consisting of carbon dioxide, nitrogen, oxygen, argon, helium, and air; and wherein entrainment of bubbles of the gas in the liquid lubricant results in an increase in the viscosity of the liquid lubricant by at least 10% over the viscosity of the liquid lubricant alone.