Centrifugal compressor recovers energy from recirculation line

Abstract

A centrifugal compressor (21) is described and disclosed with an anti-surge return line (2), a radial expansion impeller (22) configured downstream of a compressor discharge (23), one or more flow regulators configured between the compressor discharge (23) and the radial expansion impeller (22), and a radial expansion impeller discharge (29) is connected to the anti-surge return line (2). A method for controlling surge in a compressor is also described, the method including directing at least a portion or a constant volume of a continuous flow of fluid from the compressor (21) to the radial expansion impeller (22) and the return line (2).

Description

[Technical field]

[0001] The present disclosure relates to centrifugal compressors, and in particular to centrifugal compressors having an anti-surge control system. More particularly, the present disclosure relates to centrifugal compressors having a recirculation line for returning a portion of the discharge gas from the outlet of the compressor to the inlet, and a means for recovering energy from said portion of the discharge gas. The embodiments disclosed herein particularly relate to a centrifugal compressor comprising a recirculation line and a means for recovering energy arranged along said recirculation line, together with suitable equipment for monitoring and control. [Background technology]

[0002] Centrifugal and axial compressors can experience a potentially destructive condition known as surge.

[0003] Surge is defined as the operating point where the peak head capacity and minimum flow limit of a centrifugal compressor is reached. This condition occurs when the amount of gas being processed by the compressor is insufficient for the size of the compressor and the blades lose their ability to transfer energy from the shaft to the fluid.

[0004] The working principle of a centrifugal compressor is to create a pressure rise by adding kinetic energy / velocity to the continuous flow of fluid through the rotor, and then convert this kinetic energy into potential energy / increase in static pressure by slowing down the flow through the diffuser. The pressure rise in the rotor is almost always approximately equal to the pressure rise in the diffuser. The fluid flow from the diffuser is then collected by the collector and delivered downstream at the required pressure and flow rate.

[0005] It is known that during start-up / emergency shutdown, if the fluid flow through the compressor is significantly reduced, surge may occur and the compressor may not be able to meet the downstream flow demands. The collector pressure may become higher than the compressor outlet pressure and the fluid tends to backflow or even reverse in the compressor. As a result, the collector pressure decreases, the inlet pressure increases and the flow reverses again. This phenomenon occurs repeatedly in cycles with frequencies varying from 1Hz to 2Hz. The compressor loses the ability to maintain peak head and the whole system becomes unstable.

[0006] Surging can overheat the compressor to the point where it exceeds the maximum allowable temperature of the unit. Surging can also cause damage to the rotor thrust bearings as the rotor shifts back and forth from the working to the non-working side. This is defined as the compressor's surge cycle.

[0007] To avoid surge, an anti-surge control system is usually made available with the compressor to detect when a process compression stage is approaching surge and then take action to maintain stable operating conditions by reducing collector pressure and increasing flow through the compressor. Referring to FIG. 1, a schematic diagram of a centrifugal compressor including an anti-surge control system according to the prior art is shown, which is usually achieved by providing the compressor 1 with a recirculation line 2 (also called an anti-surge line), which returns a portion of the discharge gas from the outlet 3 to the inlet 4 of the compressor 1. The anti-surge control system 5 is connected to temperature, pressure and flow measuring instruments 6 on the outlet line 7 of the compressor 1 and temperature, pressure and flow measuring instruments 8 on the inlet line 9 of the compressor 1. Such an anti-surge control system 5 is operated by opening a control valve 10 (also called an anti-surge valve) in the recirculation line 2 via a valve control subsystem 10'. By returning a portion of the compressor discharge gas to the inlet 4 of the compressor 1, a minimum required flow rate is always guaranteed at the compressor inlet 4. The gas flow returned to the compressor 1 typically passes through a scrubber 11 arranged upstream of the inlet 4 of the compressor 1. The scrubber 11 is typically necessary to prevent liquids from entering the compressor 1. The gas flow returned to the compressor 1 also passes through a cooling system 12 (chiller), which can be arranged upstream or downstream of the anti-surge valve 10, and in some cases may be bypassed depending on the dynamics that need to be applied. FIG. 1 also shows a driver 13 for the compressor and a cooling system 14 for the portion of the gas flow that is not returned to the compressor 1 but is sent to a downstream service 15 or process. The driver 13 drives the compressor 1 in rotation by providing the necessary mechanical power. The driver 13 can be an electric motor. In other embodiments, the driver 13 can be a mechanical power generating turbomachine, such as a gas turbine engine or a steam turbine. In yet another embodiment, the driver 13 can include a reciprocating internal combustion engine.

[0008] As a result of the use of the anti-surge control system, the operating condition of the compressor remains stable since it has the dual effect of increasing the flow rate and reducing the delivery pressure which actually changes the resistance curve of the system.

[0009] Additionally, in some applications, anti-surge lines can be used to extend the operating range of the compressor towards lower flow rates. Thus, the anti-surge system can operate either during shutdown and start-up or during normal operation.

[0010] With regard to the effect of surging on the compressor thrust bearings, the following must also be considered:

[0011] Centrifugal compressors process a volume of gas to increase its pressure. In particular, in multi-stage compressors, the pressure increases progressively with each stage. As a result, in normal operation, the pressure downstream of the compressor is higher than upstream, and a resulting effect of the compressor operation is the generation of axial forces on the rotor. In addition to pressure differentials, the axial forces are essentially due to contributions from momentum variations in various parts of the rotor.

[0012] With reference to FIG. 2, a schematic diagram of a longitudinal section of a centrifugal compressor according to the prior art is shown. In FIG. 2, the same reference numbers represent parts, elements or components that are the same or correspond to those already illustrated in FIG. 1 and described above, and will not be described again here. In particular, FIG. 2 shows a cross section of half of the compressor 1 above the axis of rotation z of the rotor 16. FIG. 2 also shows the stator 17, the gas flow inlet 4 and the outlet 3 of the compressor. The axial forces on the rotor 16 are counterbalanced by a thrust bearing 18. In order to minimize the size of the thrust bearing 18, the compressor structure includes an axial thrust balancing system that reduces its size. The balancing system consists of a balance drum 19 attached to the rotor 16 and joining a stator seal 20 arranged downstream of the compressor outlet. A pressure drop occurs at the interface between the balance drum 19 and the stator seal 20 so that in normal operation the balance drum 19 opposes a higher pressure upstream (usually the discharge pressure of the compressor 1) and a lower pressure downstream (usually the suction pressure of the compressor 1). The pressure difference across the front of the balance drum 19 balances the other thrust contributions and creates a force that can be adjusted by selection of the diameter of the balance drum 19. The fluid leaving the balance drum 19 is directed through a pipe to a low pressure source, which is usually the compressor suction, but can be any other low pressure point inside or outside the machine.

[0013] Therefore, an improved anti-surge control system to address the problem of instability and risk of damage to the thrust bearings of current technology compressors would be beneficial and would be welcomed in the art. More generally, it would be desirable to provide a system adapted to more efficiently address the problems associated with surging. Summary of the Invention

[0014] In one aspect, the subject matter disclosed herein is directed to a centrifugal compressor having a radial expansion impeller configured on an end of a rotor shaft downstream of a compressor discharge outlet, the outlet of the radial expansion impeller being connected to a recirculation line to return a portion of the compressor discharge gas to an inlet of the compressor.

[0015] In another aspect, the subject matter disclosed herein relates to a centrifugal compressor arrangement having a radial expander connected via a gas path and a mechanical device, the expander capable of performing pressure expansion of a working fluid to generate useful mechanical power. [Brief description of the drawings]

[0016] A complete understanding of the disclosed embodiments of the present invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which: [Figure 1] FIG. 1 is a schematic diagram of a centrifugal compressor including an anti-surge control system according to the prior art. [Diagram 2] FIG. 2 shows a schematic diagram of a longitudinal section of a centrifugal compressor including a balance drum according to the prior art. [Diagram 3] FIG. 3 shows a schematic diagram of a longitudinal section of a centrifugal compressor including a radial expansion impeller and a discharge gas recirculation line according to one embodiment. [Figure 4] FIG. 4 shows a schematic diagram of a centrifugal compressor including an anti-surge control system according to the present disclosure. [Diagram 5] FIG. 5 shows the compressor operation map. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] According to one aspect, the present subject matter is directed to a centrifugal compressor in which a centrifugal expansion impeller is configured and positioned downstream of the compressor discharge to convert potential energy / static pressure of the compressor discharge gas into kinetic energy and balance the axial thrust of the compressor.

[0018] According to another aspect, the discharge of the radial expansion impeller is connected to a recirculation line to return a portion of the compressor discharge gas to the compressor inlet, acting as an anti-surge system.

[0019] According to one aspect, the present subject matter includes a radial compressor in which the balance drum of a prior art centrifugal compressor is replaced with a radial expansion impeller.

[0020] According to another aspect, the nozzle system is configured to regulate the passage of gas through the expansion impeller to allow for the passage of some or all of the compressor discharge flow.

[0021] According to yet another aspect, the diffuser is configured to convey gas processed by the radial expansion impeller downstream to a discharge volute connected to a return line that is routed to the compressor suction.

[0022] According to a further aspect, the diffuser is alternatively axially or radially oriented.

[0023] According to another aspect, the radial expansion impeller is disposed along the rotor, for example overhanging the outside of the journal bearing, this arrangement being particularly convenient for configurations where the fluid may be vented to atmosphere.

[0024] According to yet another aspect, the machine includes a gas journal bearing or an active magnetic bearing.

[0025] Reference will now be made in detail to the embodiments of the present disclosure, one or more examples of which are illustrated in the figures. Each example is provided as an explanation of the present disclosure, not as a limitation of the disclosure. Indeed, it will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the scope or spirit of the disclosure. References throughout this specification to "an embodiment" or "one embodiment" or "some embodiments" mean that a particular feature, structure, or characteristic described with respect to an embodiment is included in at least one embodiment of the disclosed subject matter. Thus, the appearance of the phrases "in an embodiment" or "in one embodiment" or "in some embodiments" in various places throughout this specification are not necessarily all referring to the same embodiment. Moreover, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0026] When presenting elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the element. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0027] Referring now to the drawings, and in particular to Figure 3, there is shown a schematic longitudinal section of a centrifugal compressor according to an embodiment, with the same reference numbers indicating the same or corresponding parts, elements or components already shown in Figures 1 and / or 2 and described above, which will not be described again. In particular, Figure 3 shows a cross section of half of the compressor 21 above the axis of rotation z of the rotor 16. Figure 3 also shows the stator 17 and thrust bearing 18, as well as the gas flow inlet 4 and outlet 3 of the compressor.

[0028] The radial expansion impeller 22 is configured and arranged downstream of the compressor discharge 23 and receives the gas flow directly from the compressor discharge volute 24 through a deswirler 25. The passage of the gas flow through the expansion impeller 22 is regulated by a system including one or more adjustable nozzles 26 configured to adjust the amount of gas flow passing through the radial expansion impeller 22. Thus, the one or more nozzles 26 may be adjusted to allow 100% of the gas flow (i.e., the entire gas flow) to pass through the impeller 22, or to allow any amount less than 100% of the total gas flow (i.e., a portion or volume of the total gas flow) to pass through the impeller 22. The one or more nozzles 26 may also control the flow rate of the gas flow, and in some embodiments where multiple nozzles 26 are used, the amount of gas flow through each nozzle and / or the associated gas flow rate through each nozzle may be the same for all nozzles 26 or may differ between any of the nozzles. When only a partial gas flow is allowed to pass through the impeller 22, the remaining amount, portion, or volume of the gas flow, i.e., the gas flow that does not pass through the expansion impeller 22, is delivered at the compressor design delivery pressure towards a downstream service or process 15 (e.g., a pipeline, a tank, a chemical reactor, a cavity, a reservoir, a heat exchanger, etc.).

[0029] Referring also to FIG. 4, which shows a schematic diagram of a centrifugal compressor including an anti-surge control system according to one embodiment, the adjustable nozzle 26 is automatically controlled to keep the operating point within the compressor stable operating map (as described below with reference to FIG. 5) by an anti-surge control system 27 connected to temperature, pressure and flow meters 6 on the outlet line 7 of the compressor 21, temperature, pressure and flow meters 8 on the inlet line 9 of the compressor 21, and a nozzle guide vane control subsystem 27', which is connected to a nozzle guide vane actuator 28 to adjust the nozzle 26 of the expansion impeller 22 as needed.

[0030] The discharge outlet 29 of the radial expansion impeller 22 is connected to a diffuser 30 and then to the inlet 4 of the compressor 21 via a recirculation line 2 .

[0031] A centrifugal compressor including an anti-surge control system according to the present disclosure provides faster system response time and more effective control since the input signal (compressor discharge) is at the same location as the control actuation (compressor discharge and recirculation inlet are in the same location) eliminating the delay due to the gas volume (piping and devices) between the compressor outlet and the anti-surge control valve of the prior art.

[0032] 5, which shows the compressor operating map, i.e. the compressor operating state, i.e. the discharge pressure / suction pressure ratio as a function of the inlet flow rate x, as points representing the compressor head y, the centrifugal compressor 21 of the present disclosure operates as follows: The compressor operating map shows several curves 31, typical of any compressor for different rotation speeds, and is divided into three regions, namely a stable operating region 32 to the right of a surge control line 33, a margin region 34 included between the surge control line 33 and a surge limit line 35, and an unstable operating region 36 to the left of the surge limit line 35. During normal operation, i.e. when the operating point is within the stable operating region 32 of the compressor operating map, the gas flow (G cc ) is the flow at the compressor outlet 3 (G out ) and the flow to the expansion impeller 22 (G exp ) and G cc =G out +G exp

[0033] During normal operation, the flow rate at the compressor outlet 3 is much higher than the flow rate handled by the expansion impeller 22, and G out >>G exp (For example, cc 96% and 4%) of the total.

[0034] If the anti-surge control system 27 detects, by continuously monitoring the compressor inlet flow rate x and the relative delivery head y, that the process compression stage is approaching a surge condition or situation, for example when the operating point 37 is on the surge control line 32, the anti-surge control system 27 communicates to the nozzle control subsystem 27' to operate the nozzle actuator 28, which in turn opens the adjustable nozzle 26, causing a larger portion or volume of the total gas flow from the compressor discharge port 23 to pass through the deswirler 25 to the expansion impeller 22, converting the potential energy / static pressure of the expansion impeller 22 into kinetic energy.

[0035] During this stage, the flow rate handled by the expansion impeller 22 may vary from a small amount (minimal recirculation) to the entire compressor flow (full recirculation) (e.g., G cc 0% and 100%).

[0036] Energy E recovered by the expansion impeller 22 rec is the capacity G of the expansion impeller 22 exp and its efficiency η exp is proportional to.

number

[0037] In particular, assuming the expansion impeller 22 has an efficiency of 0.8 and 10% of the process flow is recirculated to the inlet of the compressor 21, the power lost by the expansion impeller 22 is advantageously only 2% of the total compression power, in comparison to the power wasted in a conventional anti-surge valve, which is 10% of the total compression power.

[0038] In addition, since the gas flow pressure downstream of the expansion impeller 22 is reduced to the value of the pressure at the compressor inlet 4, the expansion impeller 22 effectively eliminates the pressure difference between the pressure downstream and upstream of the rotor 16 and the axial force generated on the rotor 16.

[0039] The gas processed by the radial expansion impeller 22 is then conveyed through a diffuser 30 and finally returned to the compressor inlet 4 through the return line 2 .

[0040] As an advantageous result, the radial expansion impeller 22 effectively performs the dual functions of balancing the axial thrust of the compressor and recovering energy from the processed flow.

[0041] The power generated by the radial expansion impeller 22 contributes to driving the operating machine, thus reducing the power absorbed from or the power output required by the primary driver 13 during normal operation.

[0042] The anti-surge control system can open the nozzle 20 during normal operation or during equipment shutdown. In the first case, the power generated by the radial expansion impeller 22 is used to assist the operator 13 or to charge the battery system. In case of anti-surge opening during a trip or shutdown, the power generated by the radial expansion impeller 22 may not be used to contribute to the drive of the compressor, since it is no longer needed, but rather can be stored by the battery for further use.

[0043] It should be noted that the configurations shown in Figures 3 and 4 can be combined with each other in various ways.

[0044] While aspects of the invention have been described in terms of various specific embodiments, it will be apparent to one of ordinary skill in the art that many modifications, changes, and omissions are possible without departing from the spirit and scope of the claims. In addition, unless otherwise specified herein, the order or sequence of any process or method steps may be changed or rearranged according to alternative embodiments.

Claims

1. A centrifugal compressor (21), comprising a compressor rotor (16), a compressor stator (17), a working fluid compressor inlet (4), a working fluid compressor outlet (3), and a driver (13), the driver (13) rotatably driving the compressor rotor (16) through a rotor shaft end (16'), a radial expansion impeller (22) configured and arranged on the rotor shaft end (16') downstream of the compressor rotor (16), the radial expansion impeller (22) having a radial expansion impeller inlet and a radial expansion impeller outlet (29), and one or more flow regulators (26) configured between the working fluid compressor outlet (23) and the radial expansion impeller inlet to regulate an amount of the working fluid that can pass through the radial expansion impeller (22). a working fluid compressor outlet (3) and a driver (13) arranged in a radial expansion impeller (22) and the radial expansion impeller discharge (29) is connected to the working fluid compressor inlet (4) through a return line (2), the centrifugal compressor (21) further comprising an anti-surge control system (27) in communication with temperature, pressure and flow meters (6) on an outlet line (7) of the compressor (21) and with temperature, pressure and flow meters (8) on an inlet line (9) of the compressor (21), wherein by using the one or more flow regulators, the anti-surge control system (27) is configured to adjust the one or more flow regulators to increase and / or decrease the amount of the working fluid that can pass through the radial expansion impeller (22).

2. 2. The centrifugal compressor (21) of claim 1, wherein a compressor discharge volute (24) is arranged downstream of the working fluid compressor discharge (23), and a deswirler (25) is arranged between the compressor discharge volute (24) and the radial expansion impeller inlet.

3. 3. The centrifugal compressor (21) according to claim 1 or 2, wherein a diffuser (30) is arranged between the radial expansion impeller discharge (29) and the return line (2).

4. The centrifugal compressor (21) of any one of claims 1 to 3, wherein the one or more flow regulators are adjustable nozzles (26).

5. The centrifugal compressor (21) of any one of claims 1 to 4, wherein a nozzle guide vane actuator (28) is configured to actuate the adjustable nozzle (26) and operate the compressor (26) in a recirculation mode.

6. 1. A method for controlling surge in a compressor, the compressor inducing a pressure rise against a continuous flow of a working fluid; The method comprises: providing a radial expansion impeller (22) downstream of the compressor discharge; providing one or more flow regulators (26) between the compressor discharge and the radial expansion impeller (22); providing a return line (2) downstream of the radial expansion impeller (22) for connecting the radial expansion impeller (22) to the compressor; The method comprises: The method further includes adjusting the one or more flow regulators (26) to direct at least a portion or a constant volume of the continuous flow of fluid through the radial expansion impeller (22) and into the return line (2), wherein the one or more flow regulators (26) are adjusted to direct 100% of the continuous gas flow through the radial expansion impeller (22) and into the return line (2).

7. The method of claim 6, wherein the one or more flow regulators (26) are adjusted to direct less than 100% of the continuous gas flow through the radial expansion impeller (22) to the return line (2).

8. 8. The method of claim 7, wherein a portion of the continuous gas flow not directed through the radial expansion impeller (22) is delivered to a downstream service or process (15) at a compressor design delivery pressure.

9. The method of any one of claims 6 to 8, further comprising passing the continuous gas flow through a deswirler (25) before the continuous gas flow is directed through the radial expansion impeller (22).

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