The present disclosure relates, generally, to turbomachines and other mechanisms and, more particularly, to mechanisms for avoiding surge in multi-stage centrifugal compressors.
Turbomachines, such as centrifugal flow compressors, axial flow compressors, and turbines may be utilized in various industries. Centrifugal flow compressors and turbines, in particular, have a widespread use in power stations, jet engine applications, oil and gas process industries, gas turbines, and automotive applications. Centrifugal flow compressors and turbines are also commonly used in large-scale industrial applications, such as air separation plants and hot gas expanders used in the oil refinery industry. Centrifugal compressors are further used in large-scale industrial applications, such as refineries and chemical plants.
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The performance of a centrifugal compressor is typically defined by its head versus flow map bounded by the surge and stall regions. This map is critical in assessing the operating range of a compressor for both steady-state and transient system scenarios. Specifically, the centrifugal compressor performance map (head or pressure ratio versus flow rate) with the corresponding speed lines indicates that there are two limits on the operating range of the compressor.
Global aerodynamic flow instability, known as surge, sets the limit for low-flow (or high-pressure ratio) operation, while, the condition of maximum allowable flow or choke or “stonewall” sets the high flow limit. The exact location of the surge line on the map can vary depending on the operating condition and, as a result, a typical surge margin is established at 10% to 15% above the stated flow for the theoretical surge line. Surge margin is usually defined as: SM(%)=((QA−QB)/QA)×100. QA is the actual volume flow at the operating point, and QB is the flow at the surge line for the same speed line of the compressor. Most centrifugal compressor manufacturers design the machine to have at least a 15% surge margin during normal operation and set a recycle valve control line at approximately a 10% surge margin. That is, once the surge margin falls below 10%, the recycle valve is opened to keep the compressor operating at the above 10% surge margin line.
Therefore, every compressor has a surge limit on its operating map, where the mechanical power input is insufficient to overcome the hydraulic resistance of the system, resulting in a breakdown and cyclical flow-reversal in the compressor. Surge occurs just below the minimum flow that the compressor can sustain against the existing suction to discharge pressure rise (head). Once surge occurs, the flow reversal reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume until the pressure rise again reaches the surge point. This surge cycle continues at a low frequency until some changes take place in the process or the compressor conditions. The frequency and magnitude of the surge flow-reversing cycle depend on the design and operating condition of the machine, but, in most cases, it is sufficient to cause damage to the seals and bearings and sometimes even the shaft and impellers of the machine. Surge is a global instability in a compressor's flow that results in a complete breakdown and flow reversal through the compressor.
The current state of the art for centrifugal compressor surge control is to utilize a global recycle valve to return flow from the discharge side of a centrifugal compressor to the suction side to increase the flow through the compressor and thus avoid entering the surge region. This is conventionally handled by defining a compressor surge control line that conservatively assumes that all stages must be kept out of surge all the time. Specifically, a flow return line provides additional flow through all stages, as opposed to individual stages, of the compressor regardless of whether only one impeller stage of the compressor is in surge or all of them are in surge. This makes recycle operation highly inefficient since the fluid that the compressor has worked on at the expense of energy is simply returned to the compressor's suction for re-working. In compressors with multiple stages, the amount of energy loss is disproportionally large since the energy that was added in each stage is lost during system level (or global) recycling.
In view of the foregoing problems with the current art of centrifugal compressor surge control, there is a current need in the art for a mechanism or arrangement for centrifugal compressors that provides a more controlled flow recycling to affect only those stages that may be on the verge of surge.
According to a particular example of the present disclosure, a turbomachine is provided. The turbomachine comprises a casing having an inlet end opposite an outlet end along a longitudinal axis of the casing; a shaft assembly provided within the casing, the shaft assembly extending from the inlet end to the outlet end; a plurality of rotating impellers extending radially outward from the shaft assembly; and a communication channel defined between two adjacent impellers to permit a backflow of fluid from a diffuser channel of a downstream impeller to a return channel of an adjacent upstream impeller.
The communication channel may be defined in the casing between the two adjacent impellers.
According to an example, the two adjacent impellers are positioned directly next to each other on the shaft assembly without an additional impeller positioned therebetween.
The communication channel may be a borehole defined in the casing between the two adjacent impellers.
The turbomachine may be a single-stage or multi-stage centrifugal compressor.
According to an example, a control valve is positioned within the communication channel to control a volume of fluid that is directed through the communication channel. The control valve may be a check valve. The control valve may be configured to permit the fluid to flow upstream while preventing the fluid from flowing downstream between the two adjacent impellers. The control valve may be configured to permit the fluid to flow upstream between the two adjacent impellers only after a predetermined pressure is achieved with the fluid.
According to another particular example of the present disclosure, a turbomachine is provided. The turbomachine comprises a casing having an inlet end opposite an outlet end along a longitudinal axis of the casing; a shaft assembly provided within the casing, the shaft assembly extending from the inlet end to the outlet end; a plurality of rotating impellers extending radially outward from the shaft assembly; a communication channel defined between two adjacent impellers to permit a backflow of fluid from a diffuser channel of a downstream impeller to a return channel of an adjacent upstream impeller; and a disk member rotatably positioned on the shaft assembly between the two adjacent impellers.
According to an example, the disk member defines at least one opening that is configured to be rotated between a first position in which the at least one opening is in line with the communication channel and a second position in which the at least one opening is rotated away from the communication channel.
According to an example, the turbomachine further comprises a control mechanism configured to rotate the disk member.
The communication channel may be defined in the casing between the two adjacent impellers.
According to an example, the two adjacent impellers are positioned directly next to each other on the shaft assembly without an additional impeller positioned therebetween.
The communication channel may be a borehole defined in the casing between the two adjacent impellers.
According to an example, the turbomachine is a multi-stage centrifugal compressor.
The disk member may define a plurality of circumferentially spaced openings.
According to another particular example of the present disclosure, a method of reducing a surge in a turbomachine is provided. The method comprises directing fluid through an inlet of the turbomachine; directing the fluid through at least one stage of the turbomachine; recycling a portion of the fluid upstream from a downstream impeller to an adjacent upstream impeller via a communication channel defined in the turbomachine between the two adjacent impellers; and directing the recycled fluid downstream in the turbomachine.
A control valve may be positioned within the communication channel.
A disk member may be provided between the adjacent impellers to control a flow of fluid through the communication channel.
Further preferred and non-limiting embodiments or aspects will now be described in the following numbered clauses.
Clause 1: A turbomachine, comprising: a casing having an inlet end opposite an outlet end along a longitudinal axis of the casing; a shaft assembly provided within the casing, the shaft assembly extending from the inlet end to the outlet end; a plurality of rotating impellers extending radially outward from the shaft assembly; and a communication channel defined between two adjacent impellers to permit a backflow of fluid from a diffuser channel of a downstream impeller to a return channel of an adjacent upstream impeller.
Clause 2: The turbomachine of Clause 1, wherein the communication channel is defined in the casing between the two adjacent impellers.
Clause 3: The turbomachine of Clause 1 or Clause 2, wherein the two adjacent impellers are positioned directly next to each other on the shaft assembly without an additional impeller positioned therebetween.
Clause 4: The turbomachine of any one of Clauses 1-3, wherein the communication channel is a borehole defined in the casing between the two adjacent impellers.
Clause 5: The turbomachine of any one of Clauses 1-4, wherein the turbomachine is a single-stage or multi-stage centrifugal compressor.
Clause 6: The turbomachine of any one of Clauses 1-5, wherein a control valve is positioned within the communication channel to control a volume of fluid that is directed through the communication channel.
Clause 7: The turbomachine of Clause 6, wherein the control valve is a check valve.
Clause 8: The turbomachine of Clause 6 or Clause 7, wherein the control valve is configured to permit the fluid to flow upstream, while preventing the fluid from flowing downstream between the two adjacent impellers.
Clause 9: The turbomachine of any one of Clauses 6-8, wherein the control valve is configured to permit the fluid to flow upstream between the two adjacent impellers only after a predetermined pressure is achieved with the fluid.
Clause 10: A turbomachine, comprising: a casing having an inlet end opposite an outlet end along a longitudinal axis of the casing; a shaft assembly provided within the casing, the shaft assembly extending from the inlet end to the outlet end; a plurality of rotating impellers extending radially outward from the shaft assembly; a communication channel defined between two adjacent impellers to permit a backflow of fluid from a diffuser channel of a downstream impeller to a return channel of an adjacent upstream impeller; and a disk member rotatably positioned on the shaft assembly between the two adjacent impellers.
Clause 11: The turbomachine of Clause 10, wherein the disk member defines at least one opening that is configured to be rotated between a first position in which the at least one opening is in line with the communication channel and a second position in which the at least one opening is rotated away from the communication channel.
Clause 12: The turbomachine of Clause 10 or Clause 11, further comprising a control mechanism configured to rotate the disk member.
Clause 13: The turbomachine of any one of Clauses 10-12, wherein the communication channel is defined in the casing between the two adjacent impellers.
Clause 14: The turbomachine of any one of Clauses 10-13, wherein the two adjacent impellers are positioned directly next to each other on the shaft assembly without an additional impeller positioned therebetween.
Clause 15: The turbomachine of any one of Clauses 10-14, wherein the communication channel is a borehole defined in the casing between the two adjacent impellers.
Clause 16: The turbomachine of any one of Clauses 10-15, wherein the turbomachine is a multi-stage centrifugal compressor.
Clause 17: The turbomachine of any one of Clauses 10-16, wherein the disk member defines a plurality of circumferentially spaced openings.
Clause 18: A method of reducing surge in a turbomachine, comprising: directing fluid through an inlet of the turbomachine; directing the fluid through at least one stage of the turbomachine; recycling a portion of the fluid upstream from a downstream impeller to an adjacent upstream impeller via a communication channel defined in the turbomachine between the two adjacent impellers; and directing the recycled fluid downstream in the turbomachine.
Clause 19: The method of Clause 18, wherein a control valve is positioned within the communication channel.
Clause 20: The method of Clause 18 or Clause 19, wherein a disk member is provided between the adjacent impellers to control a flow of fluid through the communication channel.
Clause 21: A method of reducing surge in a turbomachine, comprising: providing a turbomachine according to any one of Clauses 1-17; directing fluid through the inlet of the turbomachine; directing the fluid through at least one stage of the turbomachine; recycling a portion of the fluid upstream from a downstream impeller to an adjacent upstream impeller via a communication channel defined in the turbomachine between the two adjacent impellers; and directing the recycled fluid downstream in the turbomachine.
Clause 22: The method of Clause 21, wherein a control valve is positioned within the communication channel.
Clause 23: The method of Clauses 21 or Clause 22, wherein a disk member is provided between the adjacent impellers to control a flow of fluid through the communication channel.
Clause 24: The turbomachine according to any one of Clauses 1-9, further comprising: a disk member rotatably positioned on the shaft assembly between the two adjacent impellers.
Clause 25: The turbomachine of Clause 24, wherein the disk member defines at least one opening that is configured to be rotated between a first position in which the at least one opening is in line with the communication channel and a second position in which the at least one opening is rotated away from the communication channel.
Clause 26: The turbomachine of Clause 24 or Clause 25, further comprising a control mechanism configured to rotate the disk member.
Clause 27: The turbomachine of any one of Clauses 24-26, wherein the communication channel is defined in the casing between the two adjacent impellers.
Clause 28: The turbomachine of any one of Clauses 24-27, wherein the two adjacent impellers are positioned directly next to each other on the shaft assembly without an additional impeller positioned therebetween.
Clause 29: The turbomachine of any one of Clauses 24-28, wherein the communication channel is a borehole defined in the casing between the two adjacent impellers.
Clause 30: The turbomachine of any one of Clauses 24-29, wherein the turbomachine is a multi-stage centrifugal compressor.
Clause 31: The turbomachine of any one of Clauses 24-30, wherein the disk member defines a plurality of circumferentially spaced openings.
Clause 32: The turbomachine of any one of Clauses 10-17, further comprising a control valve positioned within the communication channel to control a volume of fluid that is directed through the communication channel.
Clause 33: The turbomachine of Clause 32, wherein the control valve is a check valve.
Clause 34: The turbomachine of Clause 32 or Clause 33, wherein the control valve is configured to permit the fluid to flow upstream while preventing the fluid from flowing downstream between the two adjacent impellers.
Clause 35: The turbomachine of any one of Clauses 32-34, wherein the control valve is configured to permit the fluid to flow upstream between the two adjacent impellers only after a predetermined pressure is achieved with the fluid.
These and other features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and the claims, the singular forms of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
For purposes of the description hereinafter, the terms “end”, “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, and derivatives thereof shall relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are simply exemplary embodiments or aspects of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments or aspects disclosed herein are not to be considered as limiting.
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With continued reference to
Working fluid, such as a gas mixture, moves from an inlet end (suction end) 206 to an outlet end (discharge end) 208 of the compressor 200. A diffuser channel 212 is provided at the outlet of the rotating blades of the impeller 205 for homogenizing the fluid flow coming off the rotating blades. The diffuser channel 212 optionally has a plurality of diffuser vanes extending within the casing 204. In a multi-stage compressor 200, a plurality of return channels 214 are provided at the outlet end of a fluid compression stage for channeling the working fluid to the rotating blades of the next successive stage. The last impeller 205 in a multi-stage turbomachine typically only has a diffuser channel 212, which may be provided with or without the diffuser vanes. The last diffuser channel 212 directs the flow of working fluid to a discharge casing (generally volute) having an exit flange for connecting to the discharge pipe.
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The recycled fluid enters the impeller 205 downstream of the return channel 214 and thus increases the impeller through flow and moves impeller operating conditions away from the surge phenomenon. In another example, the communication channel 216 includes a control valve 218 housed within an aperture defined in the casing 204 of the compressor 200. The control valve 218 may be a check valve or any other valve that is configured to control the flow of working fluid therethrough. In one example, the check valve 218 may only permit the working flow to move from the diffuser channel 212 to the upstream return channel 214 but not from the upstream return channel 214 to the downstream diffuser channel 212. The control valve 218 may only permit the working fluid to pass therethrough after a predetermined pressure has been reached by the working fluid. While only a single communication channel 216 is shown in
With continued reference to
The internal stage-wise recycling of the working fluid provides a much more controlled flow recycling to affect only those stages of the compressor 200 that may be on the verge of surge. The amount of working fluid flow needed for such an arrangement is much smaller than highly conservative global recycling arrangements. Furthermore, the working fluid flow does not leave the compressor casing 204 and, therefore, does not cross the pressure boundary. In comparison to global recycling arrangements, the currently disclosed internal stage-wise recycling arrangement has less pressure loss depending on the application and specific control design.
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In one position of the disk member 220, the communication channel 216 is completely blocked off by the disk member 220, thereby providing a complete stoppage of working fluid flow between the two stages of the compressor 200. A suitable sealing arrangement is also provided between the disk member 220 and the casing 204 of the compressor 200 to prevent unintentional leakage. In this position, the openings 224 of the disk member 220 are not aligned with the respective communication channel 216. In another position of the disk member 220, at least one opening 224 of the disk member 220 is aligned with the communication channel 216, thereby permitting a working fluid flow through the communication channel 216 to be directed from the downstream stage of the compressor 200 to the adjacent upstream stage of the compressor 200 to avoid surge in the compressor 200. This use of the disk member 220 provides an improved stage-to-stage surge control arrangement that utilizes stage return flow control valves to control the volume of working fluid that is directed from a downstream stage of the compressor 200 to an upstream stage of the compressor 200. The disk member 220 may be housed in the diaphragm 221 between adjacent stages of the compressor 200, such that the compressor 200 will include a corresponding number of disk members 220 and diaphragms 221. For example, a five-stage compressor would include four rotatable disk members 220. It is also contemplated that the number of openings 224 defined in the disk member 220 would correspond to the number of communication channels 216 defined in the casing 204 of the compressor 200 at the corresponding stage. By using the disk member 220, only a single moving component and one penetration to the exterior of the compressor casing 204 is required for the recycling process. This present stage-to-stage recycling arrangement provides a wider operating range for the compressor 200 and a faster response to changing operating conditions within the compressor 200.
In another example of the present disclosure, a method of recycling working fluid within the compressor 200 to avoid surge in the compressor 200 is also provided. Using this method, the working fluid is recycled between adjacent impeller stages instead of from the outlet or discharge end 208 of the compressor 200 all the way back to the inlet end 206 of the compressor 200 (see
It is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the specification are simply exemplary embodiments or aspects of the invention. Although the invention has been described in detail for the purpose of illustration based on what are currently considered to be the most practical and preferred embodiments or aspects, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments or aspects, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope thereof. For example, it is to be understood that the present invention contemplates that to the extent possible, one or more features of any embodiment or aspect can be combined with one or more features of any other embodiment or aspect.
The present application claims priority to U.S. Provisional Patent Application No. 62/911,697, filed on Oct. 7, 2019, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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62911697 | Oct 2019 | US |