Patent Application
20170130730
The present disclosure relates to subsea fluid processing machines. More particularly, the present disclosure relates to techniques for offloading a thrust bearing in rotating fluid processing machines such as subsea pumps and compressors.
Known turbo compressors and pumps generate a differential pressure across one or more impeller stages attached to a rotating shaft. The generated differential pressure acts on the impeller and shaft arrangement, creating a thrust force in the axial direction pointing from the discharge end towards the suction end.
An axial thrust bearing is normally attached to the shaft to counteract this thrust force and keep the shaft in a stable position during various operating and load conditions. In some applications, a differential pressure is created across a balancing piston arranged such that both ends of the shaft arrangement are exposed to a similar pressure that can be either the suction or discharge pressure. In such cases, the balancing piston and shaft areas are exposed to the pressure so as to more or less balance, or counteract, the thrust force. Normally such an arrangement is combined with a smaller thrust bearing to counteract the thrust force that may be un-balanced under some conditions.
However, balance piston arrangements as described involve tight clearances that are exposed to the process medium. Therefore, balance piston arrangements may not be well suited in applications where the process medium is not a clean single phase fluid. Examples include where the process medium contains wear particles, such as sand or other solid particles, or where phase changes may occur in the process fluid such as the formation of ice or hydrates. In such cases the tight clearances of the balancing piston may be at risk of wear, partially blocking and/or fully blocking which would jeopardize the intended function of the balance piston.
An axial thrust bearing can be designed to compensate the entire thrust force without the use of a balance piston. However, such bearings may have to be very large and therefore suffer from disadvantages such as: (1) large associated frictional losses; (2) adverse effects on shaft rotor dynamics; (3) utilization of large amounts of space; and (4) substantial increases to the overall system weight. In such cases, the size of the axial thrust bearing required may constrain the effective differential pressure that can be provided by the turbo compressor or pump.
This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to alter or limit the scope of the claimed subject matter.
According to some embodiments, a fluid pressure increasing machine, such as a compressor and/or pump is described. The machine includes: a fluid processing chamber configured to contain a process fluid, including an inlet and an outlet; a first member rotatable about a central longitudinal axis; and a motor system mechanically engaged to the first member so as to rotate the member about the longitudinal axis in a rotation direction. A plurality of impellers are fixedly mounted to the first member and exposed to the process fluid within the fluid processing chamber, such that when the first member is rotated in the rotation direction the impellers act on the process fluid thereby increasing pressure of the process fluid towards the outlet. A reaction force is also imparted on the first member in an axial direction from the outlet toward the inlet. A first rotating element is mounted to the first member and is surrounded and lubricated by a barrier fluid. The rotating element has a higher pressure surface exposed to the barrier fluid at a higher pressure and a lower pressure surface exposed to the barrier fluid at a lower pressure. The difference between the higher and lower pressure barrier fluid acting on the respective higher and lower pressure surfaces generates a force on the first member that at least partially counteracts the reaction force.
According to some embodiments, the rotating element is a thrust disk having a bearing surface configured to bear at least a part of the reaction force that is not counteracted by said the force generated by the pressure differential of the barrier fluid.
According to some embodiments, the pressure differential of the barrier fluid is at least partially caused by structures, such as impellers, on the first rotating member configured to increase the barrier fluid pressure by rotating the rotating member. Other examples of differential pressure generating structures are impellers, vanes, pump rings, labyrinths, grooves, etc. affixed to the thrust disc.
According to some embodiments, the pressure differential of the barrier fluid across the thrust disk is at least 10 bars, and the generated force on the thrust disk due to the pressure differential counteracts at least 25% of the reaction force from the main impellers of the fluid processing machine.
According to some embodiments, the machine is a subsea wet gas compressor, and the process fluid is a wet hydrocarbon gas being produced from a subterranean rock formation. The machine can also be a multiphase pump configured to be deployed in a subsea environment and the process fluid can contain additional constituents such as solid particles and/or hydrates. According to some embodiments the machine is an electrical submersible pump deployable within a wellbore. According to some embodiments the machine is a contra-rotating wet gas compressor.
According to some embodiments, the difference between the higher and lower pressure clean barrier fluid is at least partially caused by a separate barrier fluid pump. The barrier fluid pump can be attached elsewhere to the first rotating member, for example using a plurality of impellers fixedly attached to the first member. In other cases, the barrier fluid pump can be powered by a second motor that is not part of the motor system.
According to some embodiments, a method of increasing pressure of a process fluid is described that includes rotating with a motor system a first member including a shaft and a hub about a central longitudinal axis. A plurality of impellers mounted to the hub are caused to engage and increase fluid pressure of the process fluid along a first axial direction which causes a reaction force to be imparted on the impellers, hub and shaft in a second axial direction opposite to the first axial direction. The first member also includes a thrust disk surrounded and lubricated by a barrier fluid that has a surface facing towards bearing elements that bear at least part of the reaction force. The thrust disk may also have structures, such as impellers, vanes, pump rings, labyrinths, groves, etc., that increase barrier fluid pressure by rotating the first member thereby causing a pressure differential in the barrier fluid. A higher pressure surface of the thrust disk is exposed to a higher pressure barrier fluid and a lower pressure surface of the thrust disk is exposed to a lower pressure barrier fluid. The pressure differential in the barrier fluid acting on the respective higher and lower pressure surfaces generates a force on the thrust disk that partially counteracts the reaction force and off-loads the bearing elements.
The subject disclosure is further described in the following detailed description, in reference to the following drawings of non-limiting embodiments of the subject disclosure. The features depicted in the figures are not necessarily shown to scale. Certain features of the embodiments may be shown exaggerated in scale or in somewhat schematic form, and some details of elements may not be shown in the interest of clarity and conciseness. Like reference numbers and designations in the various drawings indicate like elements.
One or more specific embodiments of the present disclosure will be described below. The particulars shown herein are by way of example, and for purposes of illustrative discussion of the embodiments of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details of the subject disclosure in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Additionally, in an effort to provide a concise description of these exemplary embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” Also, any use of any form of the terms “connect,” “engage,” “couple,” “attach,” or any other term describing an interaction between elements is intended to mean either an indirect or a direct interaction between the elements described. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. The use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components.
Certain terms are used throughout the description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function.
According to some embodiments, techniques are described for off-loading the axial thrust bearing without the use of a conventional balancing piston that is exposed to the process. In many compressor and/or pumping applications, the process medium is not a clean single phase fluid, but may include wear particles such as sand or other solid particles, and/or may be prone to phase changes such as the formation of ice or hydrates. In such cases, the design may involve techniques for keeping critical and exposed components such as bearings and couplings separate from the process fluid. One way to achieve this is to have the critical components submerged in a barrier fluid at a pressure higher than the process fluid (at an “overpressure”). For many such designs it is desirable to omit a balancing piston and let the axial thrust bearing counteract the full thrust force created by the generated differential pressure.
According to some embodiments, impeller blades or a pump ring are provided on the outside end of a thrust disc that forms part of the thrust bearing and is attached to the rotating shaft.
The impeller blades or pump ring affixed to the outside end of the thrust disc will, when the shaft rotates, create a differential pressure in the barrier fluid across the thrust disc. By arranging this differential pressure such that the low-pressure side of the thrust disc is towards the process outlet end, a thrust force that off-loads the axial thrust bearing is created.
The compressor section 210 has interleaved rows of impellers mounted to the inner hub and outer sleeve that are stacked successively to each other and rotate in opposite directions. The interleaved rows impellers mounted on the inner hub and outer sleeve are shaped to successively increase the pressure in the process fluid as the fluid is moved upwards. The resultant axial force on both the upper shaft 244 and lower shaft 254 is therefore in the downwards direction. Each shaft 244, 254 has associated with it a thrust disk having a lower surface thereon that forms a bearing surface. In particular, upper shaft 244 has an upper thrust disk 246 and the lower shaft 254 has a lower thrust disk 256. Each thrust disk 246 and 256 is surrounded by barrier fluid that is maintained at an overpressure with respect to the process fluid pressure. In general, barrier fluid acts as a barrier against an outside environment and/or process fluid. Barrier fluid can also serve other functions such as lubricating various bearing surfaces and seals, cooling of various elements, and electrical insulation. Barrier fluid is typically an oil, and is “clean” when compared to the process fluid in that it contains far lower levels of wear-inducing matter such as sand and other solid particles. According to some embodiments, each of the thrust disks 246 and 256 have structures such as impellers, that are shaped to increase barrier fluid pressure on the lower side of the thrust disk such that the barrier fluid pressure is higher on the bottom side of the thrust disk than on the top side.
Similarly, the upper shaft 244 is fixed to outer sleeve 340 and is driven about axis 300 in the direction shown in arrow 312. Note that the upper shaft 244 and the lower shaft 254 are driven in opposite, contra-rotating directions. The outer sleeve 340 has multiple rows of impellers 342 mounted thereon and interleave with the hub-mounted impellers 352. The sleeve-mounted impellers 342 are shaped and positioned to increase fluid pressure in the process fluid and move the process fluid in an upwards direction, as shown by dotted arrows 302 and 304. A reactionary force is imparted on the upper shaft 244 in the downward axial direction as shown by the dashed arrow 306. Upper thrust disk 246 is mounted to the shaft 244 and is configured to counteract at least a part of the downward force through engagement with thrust bearing pads 344 on the upper surface of the thrust bearing. According to some embodiments, the thrust disk 246 includes impellers 346 mounted on its outer edge as shown (i.e., the radially outer edge of disk 246). The impellers 346 are shaped and positioned so as to increase fluid pressure of the surrounding barrier fluid in the downwards direction such that the lower surface of thrust disk 246 is exposed to a higher barrier fluid pressure than the upper surface of thrust disk 246. Due to the relative surface areas and the differential barrier fluid pressure, an upward force is imparted on the thrust disk 246, which partially counteracts the downward force from the main sleeve-mounted impellers 342. In this way a significant portion of the downward imparted force on the shaft 244 and thrust disk 246 can be offloaded from the thrust disk bearing surfaces.
Thus, according to some embodiments, a means for off-loading an axial thrust bearing is provided without the use of a process exposed balance piston. The disclosed techniques allow for: a smaller thrust bearing used for a given differential pressure; a larger effective differential pressure; or a combination of both. In this way an axial thrust bearing can be off-loaded without the use of a process exposed balance piston. The benefits of the disclosed techniques include the use of a smaller thrust bearing for a given differential pressure, and/or the ability to safely achieve larger effective differential pressures. This has been found to be particularly beneficial for applications where the process medium is not a clean single phase fluid but may include wear particles such as sand or other solid particles or where phase changes may occur in the process fluid, such as the formation of ice or hydrates and therefore the use of a process exposed balance piston is not practical.
While the subject disclosure is described through the above embodiments, modifications to and variations of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. These and other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.
