EP4609075A1

EP4609075A1 - Multi-stage compressor system with anti-surge arrangement, and method

Info

Publication number: EP4609075A1
Application number: EP23825610.1A
Authority: EP
Inventors: Olaf Stallmann
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2022-12-15
Filing date: 2023-12-08
Publication date: 2025-09-03
Also published as: IT202200025737A1; WO2024125819A1; AU2023397064A1; MX2025006726A

Abstract

The compressor system includes a low-pressure compressor section and a high-pressure compressor section. An intercooler and an inter-stage liquid-gas separator are arranged between the low-pressure compressor section and the high-pressure compressor section. A master anti-surge line including a master anti-surge valve is arranged in anti-parallel to the low-pressure compressor section and the high-pressure compressor section. An interstage anti-surge line including an interstage anti-surge valve is arranged in anti-parallel to the low-pressure compressor section.

Description

MULTI-STAGE COMPRESSOR SYSTEM WITH ANTI-SURGE ARRANGEMENT, AND METHOD

DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure concerns improvements to anti-surge systems for multi-stage compressors and relevant methods. Specifically, embodiments disclosed herein concern multi-stage compressors including an intercooler and an interstage liq- uid/gas separator.

BACKGROUND ART

[0002] Multi-stage compressors are used in several industrial applications, among others for the compression of mixed refrigerants in refrigeration cycles. Multi-stage compressors are often provided with an intercooler between two compressor sections arranged in sequence in an upstream-to-downstream direction with respect to the direction of flow of the gas processed by the multistage compressor. Especially if the process gas contains different chemical species having different dew points, for instance if the process gas is a mixed refrigerant (shortly MR), the gaseous components having the lowest condensation temperatures may partly or entirely condense in the intercooler (so-called interstage condensation). An interstage liquid/gas separator is then provided between the low-pressure compressor section and the high-pressure compressor section, to remove the liquefied fraction of the mixture from the gaseous fraction, which is delivered to the high-pressure compressor section for further compression.

[0003] In some cases, the multi-stage compressor may include more than two compressor sections one intercooler and respective liquid/gas separator. For instance, the multi-stage compressor can include a low-pressure compressor section, a high-pressure compressor section and one or more intermediate-pressure compressor sections between the low-pressure compressor section and the high-pressure compressor section. A respective intercooler and a respective liquid/gas separator are usually arranged between sequentially arranged sections of the multi-stage compressor. [0004] In multi-stage compressors of the prior art, an anti-surge line is arranged in anti-parallel to each compressor section. In “anti-parallel” means that the inlet of the anti-surge line is fluidly coupled at the discharge side of the compressor section and the outlet of the anti-surge line is fluidly coupled at the suction side of the compressor section, i.e. in the anti-surge line the process gas flows in a direction opposite to the direction of flow of the process gas in the compressor section. A respective anti-surge valve is positioned in each anti-surge line and is controlled by an anti-surge controller. The anti-surge controller opens the respective anti-surge valve when the operating point of the respective compressor section approaches the surge control line of the compressor section. Each anti-surge controller must ensure fast intervention of the respective anti-surge valve to prevent the respective compressor section from surging.

[0005] When used to process a gas mixture, such as a mixed refrigerant, the abovedescribed anti-surge control of the current art can cause a composition change in the gaseous mixture. This can result in a reduction in the amount of liquid formed in the interstage condensation and an increased condensation in the final stage discharge condenser. This effect can cause operational problems in connected refrigerant consumers like natural gas liquefaction systems for the production of liquefied natural gas.

[0006] An anti-surge control system alleviating or overcoming the above-mentioned problems would be welcomed in the art.

SUMMARY

[0007] According to one aspect, disclose herein is a compressor system, specifically for mixed process gases, including different gaseous species which have different liquefaction temperatures. The system is particularly beneficial for the compression of mixed refrigerants, instance. The system comprises a process gas path having a gas inlet and a gas outlet. Between the gas inlet and the gas outlet, along the process gas path, the system includes a low-pressure compressor section having a low-pressure suction side, fluidly coupled to the gas inlet, and a low-pressure discharge side; and a high-pressure compressor section having a high-pressure suction side; and a high-pressure discharge side fluidly coupled to the gas outlet. The compressor system further comprises an intercooler, between the low-pressure compressor section and the high- pressure compressor section and an interstage liquid/gas separator between the intercooler and the high-pressure compressor section. A master anti-surge line is provided in anti-parallel to the compressor sections. The master anti-surge line includes: an inlet fluidly coupled to the process gas path downstream of the high-pressure discharge side; and an outlet fluidly coupled to the process gas path upstream of the low- pressure suction side. A master anti-surge valve is arranged in the master anti-surge line. An interstage anti-surge line is further provided in anti-parallel to the low-pressure compressor section. More specifically, the interstage anti-surge line includes an inlet fluidly coupled to the process gas path downstream of the low-pressure discharge side and upstream of the high-pressure suction side, and an outlet fluidly coupled to the process gas path upstream of the low-pressure suction side. An interstage antisurge valve is arranged in the interstage anti-surge line.

[0008] According to embodiments disclosed herein, when the flowrate through the high-pressure compressor section drops at or below a pre-set minimum threshold value, the anti-surge controller which controls the interstage anti-surge valve is overridden by the anti-surge controller which controls the master anti-surge valve. The setpoint of the interstage anti-surge valve can be modified as function of the operating point of the high-pressure compressor section.

[0009] Embodiments disclosed below provide examples on how the set point of the interstage anti-surge controller can be modified depending upon the operating point of the high-pressure compressor.

[0010] In general terms, under the pre-set flowrate value mentioned above, the set point of the interstage anti-surge controller is modified such that the distance of the operating point of the low-pressure compressor section from the respective surge line is corrected and made equal to or proximate to the distance of the operating point of the high-pressure compressor section from the respective surge line.

[0011] The above concept can be duplicated for any number of sequentially arranged compressor sections. Thus, disclosed herein is also a compressor system including a plurality of compressor sections placed in sequence along a gas flow path. Between at least two adjacent compressor sections an intercooler is provided, adapted to cause interstage condensation, in combination with a liquid/gas separator. In anti-parallel to the plurality of compressor sections, a master anti-surge line including a master anti- surge valve in said master anti-surge line is arranged. Moreover, at least one interstage anti-surge line and a respective interstage anti-surge valve are provided. The interstage anti-surge valve has an inlet between two sequentially arranged compressor sections and an outlet upstream of the most upstream of said plurality of compressor sections. Preferably, an interstage anti-surge line is provided for each compressor section upstream of the last compressor section, i.e. the most downstream compressor section, where the highest pressure of the process gas s achieved.

[0012] Each interstage anti-surge controller provided for controlling the respective interstage anti-surge valve can be inter-related to the master anti-surge controller as mentioned above, such as to adapt the set point thereof to the operating condition of the most downstream compressor section, when the flowrate through the most downstream compressor sections drops at or below the pre-set threshold flowrate value.

[0013] According to a further aspect, a method for operating a compressor system as outlined above is disclosed herein, specifically when processing a gaseous mixture, including components having different liquefaction temperatures, such as mixed refrigerants. In embodiments disclosed herein, the method comprises the following steps: controlling the master anti-surge valve through a master anti-surge controller; controlling the interstage anti-surge valve through an interstage anti-surge controller; adapting a set point of the interstage anti-surge controller as a function of an operating condition of the high-pressure compressor section when a flowrate through the high-pressure compressor section drops below a pre-set value.

[0014] The system and method disclosed herein achieve the objective of harmonizing the distance from the surge point for each stage or section to the same value or around the same value, such that the master anti-surge valve and relevant controller can be used to adjust the compressor load. This results in an overall more agile and reactive stem if compared with the prior art. In the system disclosed herein already the master anti-surge controller and relevant valve can react to changes in the compressor load demand and adjust the complete system. This results also in a reduced lean-out effect during part-load operation. The capacity of each anti-surge valve of the interstage anti-surge line(s) is considerably reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Reference is now made briefly to the accompanying drawings, in which:

Fig. l is a schematic of a compressor system according to the present disclosure in a first embodiment;

Figs.2, 3, 4, 5, 6, 7, 8 and 9 are diagrams illustrating the anti-surge control operation according to the present disclosure for the compressor system of Fig.1; and

Fig.10 is a schematic of a compressor system according to the present disclosure in a further embodiment.

DETAILED DESCRIPTION

[0016] To enhance anti-surge control in a multi-stage compressor arrangement, including intercooling and interstage condensation, a main or master anti-surge line is arranged in anti-parallel with the sequentially arranged compressor stages or sections, such that the inlet of the master anti-surge line is fluidly coupled to a gas path downstream of the discharge side of the most downstream compressor section or compressor stage, while the inlet of the master anti-surge line is fluidly coupled to the gas path upstream of the suction side of the most upstream compressor section or compressor stage. A master anti-surge valve is positioned along the master anti-surge line and is controlled by a master control loop to protect the compressor system, and specifically the most downstream compressor section, against surging. One, some or preferably all compressor sections upstream of the last (most downstream) compressor section are provided with individual interstage anti-surge lines. Each interstage anti-surge line has an inlet fluidly coupled to the gas path between two sequentially arranged compressor sections, between which an intercooler and a liquid/gas separator are provided. The outlet of the interstage anti-surge line is flui dly coupled to the gas path upstream of the sucti on side of the must upstream one of the compressor sections. An interstage antisurge valve is positioned in each interstage anti-surge line and is controlled by a respective interstage anti-surge controller. The set point of each interstage anti-surge controller is adapted during operation as a function of the position of the operation point of the most downstream compressor section with respect to the relevant surge control line of the master anti-surge controller.

[0017] Turning now to the figures, Fig.l illustrates a first embodiment of a multistage compressor system 1 according to the present disclosure. The compressor system 1 comprises a driver 3, for instance a gas turbine engine, a steam turbine, an electric motor or the like. The driver 3 drives into rotation two or more compressor stages or compressor sections.

[0018] In the embodiment of Fig. l, the compressor system 1 comprises two compressor sections or compressor stages, 5 and 7 arranged in sequence in an upstream- to-downstream direction, with respect to the direction of flow of a process gas processed by the compressor system 1.

[0019] Herein, the first compressor section 5 will be referred to as low-pressure compressor section 5 and the second compressor section 7 will be referred to as high-pressure compressor section 7. As will become clearer from the description of further embodiments, the compressor system can include more than two compressor sections 5, 7, for instance a low-pressure compressor section (or must upstream compressor section), a high-pressure compressor section (or most downstream compressor section) and one or more compressor sections in-between, which can be referred to as intermediate-pressure compressor sections.

[0020] By way of example, in Fig.l the low-pressure compressor section 5 and the high-pressure compressor section 7 are represented as separate compressors of a compressor train. Reference number 9 indicates a shaft line drivingly connecting the compressors of the compressor train to one another and to the driver 3. It shall be understood, however, that two or more compressor sections can be featured as stages or group of stages within one and the same compressor. For instance, the low-pressure compressor section 5 can include a first group of one or more impellers supported for rotation in a casing of a multi-stage compressor, and the high-pressure compressor section 5 can include a second group of one or more impellers supported for rotation in the same casing of the multi-stage compressor.

[0021] The low-pressure compressor section 5 has a suction side 5.1 and a discharge side, aka delivery side, 5.2. Similarly, the high-pressure compressor section 7 has a suction side 7.1 and a discharge side or delivery side 5.2.

[0022] The compressor system 1 of Fig.1 is an intercooled compressor system featuring an interstage condensation, as follows. An intercooler 11 is provided between the discharge side 5.2 of the low-pressure compressor section 5 and the suction side

7.1 of the high-pressure compressor section 7. Partially compressed process gas delivered by the low-pressure compressor section 5 is cooled in the intercooler 11 and can partly condense. Specifically, when the process gas is a gas mixture, e.g. a mixed refrigerant containing hydrocarbons having different liquefaction temperatures, the heavier hydrocarbons will condense in the intercooler 11.

[0023] To prevent liquid (i.e. condensed gas) from entering the high-pressure compressor section 7, an interstage liquid/gas separator 13 is provided downstream of the intercooler 11, between the intercooler 11 and the suction side 7.1 of the high-pressure compressor section 7. Reference number 13.1 designates the inlet of the liquid/gas separator 13, while 13.2 and 13.3 represent the liquid outlet and the gas outlet, respectively. The gas outlet 13.3 of the liquid/gas separator 13 is fluidly coupled to the suction side 7.1 of the high-pressure compressor section 7.

[0024] Downstream of the high-pressure compressor section 7, a high-pressure gas cooler 15 and a high-pressure gas condenser 17 are positioned in sequence along the process gas path. The gas cooler 15 cools the compressed process gas at a temperature above the liquefacti on point of the compressed process gas. If the process gas is a gas mixture, e.g. a mixed refrigerant, the temperature at the outlet of the gas cooler 15 is such that none of the gas mixture components condenses in the gas cooler 15. The gas condenser 17 condenses the compressed process gas at least in part. The compressed and partly condensed process gas is delivered to a liquid/gas separator 19, whereof

19.1 is the inlet, 19.2 is the liquid outlet and 19.3 is the gas outlet.

[0025] The components described above are part of a process gas path, which may include a scrubber 20 upstream of the suction side 5.1 of the low-pressure compressor section 5.

[0026] To protect the multi-stage compressor, and specifically the high-pressure compressor section 7 thereof, against surging, a master anti-surge system 22 is provided, including a master anti-surge line 21 connected in anti-parallel to the sequence of compressor stages or sections 5, 7. The master anti-surge line 21 has an inlet 21.1 fluidly coupled to the process gas path downstream of the discharge side 7.2 of the high-pressure compressor section 7. The outlet of the master anti-surge line 21 is shown at 21.2 and is arranged upstream of the suction side 5.1 of the low-pressure compressor section 5.

[0027] The master anti-surge line 21 is defined as “in anti-parallel” to the compressor sections 5, 7, because the process gas path and the master anti-surge line 21 are connected in parallel between the two end points 21.1 and 21.2, but the flow of gas recycling through the master anti-surge line 21 flows in a direction opposite to the process gas flow through the compressor sections 5, 7.

[0028] Preferably, to avoid overheating of the process gas when process gas is recycled through the master anti-surge line 21, the inlet 21.1 of the master anti-surge line 21 is fluidly coupled to the process gas path downstream of the high-pressure gas cooler 15. In other embodiments, the inlet 21.1 of the master anti-surge line 21 can be fluidly coupled upstream of the high-pressure gas cooler 15 and the gas recycled through the master anti-surge line 21 can be cooled by an auxiliary cooler (not shown) positioned along the master anti-sure line 21, for instance.

[0029] By arranging the inlet 21.1 of the master anti-surge line 21 between the high- pressure gas cooler 15 and the high-pressure gas condenser 17, ingress of condensed gas in the master anti-surge line 21 is avoided. This prevents damages to a master antisurge valve 23 positioned along the master anti-surge line 21.

[0030] The master anti-surge valve 23 is controlled by a master anti-surge controller 25.

[0031] Additionally, an interstage anti-surge system 30 is provided, to protect the low-pressure compressor section 5 against surging. The interstage anti-surge system 30 comprises an interstage anti-surge line 31 having an inlet 31.1 fluidly coupled to the process gas path downstream of the delivery side 5.2 of the low-pressure compressor section 5 and an outlet 31.2 fluidly coupled to the process gas path upstream of the suction side 5.1 of the low-pressure compressor section 5. The interstage anti-surge line 31 is therefore arranged in anti-parallel to the low-pressure compressor section 5.

[0032] In preferred embodiments, the inlet 31.1 of the interstage anti-surge line 31 is fluidly coupled downstream of the intercooler 11 and of the interstage liquid/gas separator 13. This prevents overheating of the gas, since gas recycling through the interstage anti-surge line 31 is cooled in the intercooler 11 before entering the anti-surge line 31. Moreover, ingress of liquefied gas in the interstage anti-surge line 31 is prevented, since any condensed gas is removed in the liquid-gas separator 13 .

[0033] An interstage anti-surge valve 35 is arranged along the interstage anti-surge line 31. The interstage anti-surge valve 35 is controlled by an interstage anti-surge controller 37.

[0034] Operation of the two anti-surge systems 22 and 30 is described hereafter.

[0035] Fig.2 illustrates a diagram showing the volumetric flowrate at the suction side of the high-pressure compressor section 7 on the horizontal axis versus the compression ratio or head of the high-pressure compressor section 7 on the vertical axis. The surge line of the high-pressure compressor section 7 is shown at SLH. The surge control line of the high-pressure compressor section 7 is shown at SCLH. The diagram of Fig.2 also shows an additional curve, referred to as activation line AL, the meaning and purpose whereof will be clarified here below. The activation line AL is approximately parallel to the surge control line SCLH and at a suitable distance therefrom. In some embodiments, the distance of the activation line AL from the surge control line SCLH can be selected in an interval between 0.1% and 5% of the maximum flowrate of the high-pressure compressor section 7.

[0036] The curve nl represents the characteristic curve of the compressor at a given rotational speed nl. The intersection between curve nl and the anti-surge control line SCLH is the set point SPH of the anti-surge controller 25. Reference PH indicates a generic operating point of the high-pressure compressor section 7 along the curve nl.

[0037] Fig.3 illustrates a similar diagram for the low-pressure compressor section 5, in which the volumetric flowrate at the suction side 5.1 of the low-pressure compressor section 5 is plotted on the horizontal axis and the pressure ratio across the low-pressure compressor section 5 is plotted on the vertical axis. The surge line of the low-pressure compressor section 5 is shown at SLL and the surge control line of the low-pressure compressor section 5 is shown at SLCL.

[0038] The curve nl represents the characteristic curve of the low-pressure compressor section 5 at a given rotational speed nl. The intersection between curve nl and the anti-surge control line SCLL is the set point SPL of the anti-surge controller 37. Reference PL indicates a generic operating point of the low-pressure compressor section 5 along the curve nl.

[0039] As known to those skilled in the art of compressor control, surging of a compressor is avoided by recycling process gas from the delivery side to the suction side of the compressor when the operating point of the compressor in the flowrate/com- pression ratio diagram moves along the characteristic curve nl until the set point SP on the surge control line is reached. In other words, to prevent the compressor from surging, i.e. to prevent the flowrate to drop until the operating point reaches the surge line SL, when the gas flowrate drops to the value corresponding to the surge control line, further reduction of flowrate through the compressor is prevented by recycling part of the compressed process gas towards the suction side through the anti-surge line and the anti-surge valve, by controllably opening the anti-surge valve.

[0040] The method disclosed herein provides that when the operating point PH of the high-pressure compressor section 7 is on the right side of the activation line AL (Fig.2), i.e the flowrate processed by the high-pressure compressor section 7 is higher than a pre-set value that is defined for each operating curve (i.e. for each rotational speed of the compressor) by the point of the activation line AL corresponding to the actual rotational speed of the compressor, each anti-surge system 22 and 30 will operate as known from the prior art. Namely, the master anti-surge valve 23 is maintained closed by the master anti-surge controller 25, since the operating point of the high-pressure compressor section 7 is far on the right of the surge control line SCLH, i.e. the volumetric flow rate across the high-pressure compressor section is largely above the set point SPH.

[0041] The interstage anti-surge controller 37 will control the interstage anti-surge valve 35 as in systems of the current art, i.e., will keep the interstage anti-surge valve 35 closed if the operating point PL of low-pressure compressor section 5 is on the right side of the surge control line SCLL and will open the interstage anti-surge valve 35 if the operating point PL reaches the set point SPL or moves on the left thereof (flowrate lower than flowrate at set point SPL). Opening of the interstage anti-surge valve 35 causes recirculation of process gas through the interstage anti-surge line 31 and thus an increase of the total flowrate through the low-pressure compressor section 5, until the operating point of the low-pressure compressor section 5 moves back to the surge control line SCLl or on the right side thereof.

[0042] In the condition plotted in Fig.3, the operating point PL is on the right side of the relevant surge control line SCLL, and therefore the interstage anti-surge valve 35 remains closed.

[0043] When the operating point PH of the high-pressure compressor section 7 reaches the activation line AL and/or when said operating point is on the left side of the activation line AL(Fig.4), i.e. when the flowrate processed by the high-pressure compressor section is at or below the pre-set value defined by the activation line, the interstage anti-surge control system 30 will shift to a different mode of operation, wherein the set point given by the intersection of the surge control line SCLL can be over-ridden by a value that depends upon the operating conditions of the high-pressure compressor section 7, as follows.

[0044] As a general principle, if the distance of the operating point PH of the high- pressure compressor section 7 from the respective surge control line SCLnis larger than the distance of the operating point PL of the low-pressure compressor section 5 from the respective anti-surge control line SCLL, the interstage anti-surge controller 37 is over-ridden, in the sense that the set point SPL of the inter-stage anti-surge controller 37 is moved to a new position, distanced from the surge control line SCLL and on the right side thereof. The distance of the modified set point SPL of the interstage anti-surge control 37 from the surge control line SCLL is a function of the distance of the operating point PH of the high-pressure compressor section 7 from the respective surge control line SCLH.

[0045] The distances mentioned above are referred to the horizontal axis and are, therefore, proportional to the volumetric flowrate. In order to properly compare the distances mentioned above, it is required to convert the volumetric flowrate at the inlet of the high-pressure compressor section 7 to a corresponding volumetric flowrate at the suction side pressure of the low-pressure compressor section 7. In other words, the value read on the horizontal axis of the diagram (Figs. 2, 4) referred to the high-pressure compressor section 7, which is the volumetric flowrate at the suction side of the high-pressure compressor section 7, must be converted into the equivalent volumetric flow-rate at the sucti on pressure of the low-pressure compressor section 5.

[0046] For a better understanding of the above-mentioned over-riding criterion, Figs 4 and 5 show the same diagrams of Figs. 2 and 3 in an operating condition wherein the operating point PH of the high-pressure compressor section 7 has moved along curve nl to the left side of the activati on line AL. For easy of understanding and by way of exemplary, non-limiting embodiment, the volumetric flowrate at the set point SPH of master anti-surge controller 25 and the volumetric flow-rate at the operating point PH are indicated along the horizontal axis of the diagram in Fig.4. For practical reasons the values shown along in Fig.4 are already converted into flowrates at the suction pressure of the low-pressure compressor. Thus, the set point SPH of the master antisurge controller 35 at rotational speed nl corresponds to a volumetric flowrate of 375, which is however not the actual volumetric flowrate at the suction side of the high- pressure compressor section 7, but rather the flowrate at the suction pressure of the low-pressure compressor section 5. The volumetric flowrate at the suction pressure of the low-pressure compressor section 5 can be obtained from the volumetric flowrate at the suction side of the high-pressure compressor secti on 7 using the gas state equation.

[0047] In Fig.5 the volumetric flowrate at the suction side of the low-pressure compressor section 5 is shown to be 450 at the set point SPL. The low-pressure compressor section 5 is operating at a flowrate of 452, i.e., the abscissa of the operating point PL of the low-pressure compressor section 5 is at 452.

[0048] The numerical values of flowrates on the horizontal axis of Figs. 4, 5 as well as in Figs. 6 to 9 are by way of example only. They can be expressed in k Nm³/h (normal cubic meter) per hours divided by 1000. Thus, e.g., 450 means 450,000 Nm³/hr.

[0049] The distance of operating point PH from set point SPH is 5 (380-375), while the distance of the operating point PL from set point SPL is 2. Thus, the first distance is larger than the second distance (expressed, as said above, in terms of volumetric flowrate at the suction pressure of the low-pressure compressor section 5).

[0050] The set anti-surge controller 37 is over-ridden in the sense that the set point thereof is now shifted from SPL to SPL’, by 5-3=2.

[0051] The result of shifting the set point SPL of the interstage anti-surge controller 37 from SPL to SPL’ is that the interstage anti-surge valve 35 opens and causes process gas to recirculate from the discharge side 5.2 to the suction side 5.1 of the low-pressure compressor section 5, moving the operating point PL along curve nl in Fig.5 to the right till the flowrate of 453 at the suction side 5.1 of the low-pressure compressor section 5 is achieved.

[0052] Thus, when the operating point of the high-pressure compressor section 7 moves in a range between the activation line AL and the surge control line SCLH, the master anti-surge control system 22 can modify the set point SPL of the interstage antisurge system 30.

[0053] The set point SPL of the interstage anti-surge controller 35 is changed as a consequence of the operating point PH approaching the surge control line SCLH only in some conditions, namely if the distance of the operating point PH from the surge control line SCLH is higher than the distance of the operating point PL from the surge control line SCLL.

[0054] A situation in which the set point SPL is not modified by the operating conditions of the high-pressure compressor section 7 is depicted in Figs. 6 and 7. In this situation the operating point PH of the high-pressure compressor section 7 has moved on the left of the surge control line SCLH, i.e., on the left of the set point SPH. Consequently, the master anti-surge valve 23 opens and process gas recirculates from the delivery side of the high-pressure compressor section 7 towards the suction side of the low-pressure compressor section, to increase the volumetric flowrate through the high- pressure compressor section 7 and move the operating point PH back to the right of surge control line SCLH. The distance between the operating point PH and the set point SPH is in this case 375-374=-!. Conversely, the distance between the operating point PL and the set point SPL is 452-450=2. Thus, the difference between the two distances is -3, which is lower than the distance between SPL and PL. The position of the set point SPL remains unaffected and the interstage anti-surge valve 35 remains closed.

[0055] In Figs. 8 and 9 a situation is depicted, wherein both the operating point PH of the high-pressure compressor section 7 and the low-pressure compressor section 5 are on the left side of the respective surge control line SCLH and SCLL. Specifically, the distance of the operating point PH from the surge control line SCLH is -1 (at the suction pressure of the low-pressure compressor section 5) and the distance between the set operating point PL from the surge control line SCLL is -3. The set point SPL of the interstage anti-surge controller 37 is moved to SPL’, at a distance (- 1 -(-3))=2 from the interstage surge control line SCLH. Under these conditions, both anti-surge control valves 23 and 35 are opened, but by shifting the set point SPL to the modified position SPL’, due to the overriding of the interstage anti-surge controller 37 by the master antisurge controller 25, the interstage anti-surge valve 35 is opened more.

[0056] As described above, the overriding of the interstage anti-surge controller 37 by the master anti-surge controller 25 takes place when the operating point PH moves on the left of the activation line AL. To make the transition between the two operative modes soother, in some embodiments the difference between the distance between PH and SCLH and the distance between PL and SCLL can be weighed by a weighing factor which is a function of the position of the operating point PH with respect to the activation line. For instance, said weighing factor can gradually increase from 0 to 1 when the operating point PH moves from the activation line AL towards the surge control line SCLH, and may become 1 for instance in an intermediate point between AL and SCLH.

[0057] The embodiment of Fig.1 includes a compressor system with two sequentially arranged compressor stages or sections 5 and 7. The above-described criteria for optimized anti-surge control can be extended to a compressor system with intercooling and interstage condensation including any number of compressor stages or sections. By way of example, Fig.7 illustrates a compressor system 1 including three compressor sections.

[0058] As in Fig.1, the compressor system 1 of Fig.7 comprises a driver 3, for instance a gas turbine engine, a steam engine, an electric motor or the like. The driver 3 drives into rotation two or more compressor stages or compressor sections. And three compressor sections or compressor stages, 5, 8 and 7 arranged in sequence in an up- stream-to-downstream direction, with respect to the direction of flow of a process gas processed by the compressor system 1.

[0059] As in the preceding description of Fig.1, the first compressor section 5 will be referred to as low-pressure compressor section 5 and the third compressor section 7 will be referred to as high-pressure compressor section 7. The intermediate compressor section 8 will be referred to as the “intermediate-pressure compressor section 8”. While in Fig.7 only one intermediate-pressure compressor section is provided between the most upstream (low-pressure) compressor section 5 and the most downstream (high-pressure) compressor section 7, in other embodiments, not shown, more than one intermediate compressor section can be provided.

[0060] As in the embodiment of Fig.1, the low-pressure compressor section 5 has a suction side 5.1 and a discharge side, aka delivery side, 5.2. Similarly, the high-pressure compressor section 7 has a suction side 7.1 and a discharge side or delivery side 5.2. The intermediate-pressure compressor section 8 includes a suction side 8.1 and a delivery side 8.2.

[0061] The compressor system 1 of Fig.7 is an intercooled compressor system with interstage condensation, as follows. An intercooler 11 is provided between the discharge side 5.2 of the low-pressure compressor section 5 and the suction side 8.1 of the intermediate-pressure compressor section 8. Partially compressed process gas delivered by the low-pressure compressor section 5 is cooled in the intercooler 11 and can partly condense. Specifically, when the process gas is a gas mixture, e.g. a mixed refrigerant containing hydrocarbons having different liquefaction temperatures, the heavier hydrocarbons will condense in the intercooler 11.

[0062] To prevent liquid (i.e., condensed gas) from entering the intermediate-pressure compressor section 8, an interstage liquid/gas separator 13 is provided downstream of the intercooler 11, between the intercooler 11 and the suction side 8.1 of the intermediate-pressure compressor section 7. Reference number 13.1 designates the inlet of the liquid/gas separator 13, while 13.2 and 13.3 represent the liquid outlet and the gas outlet, respectively. The gas outlet 13.3 of the liquid/gas separator is fluidly coupled to the suction side 8.1 of the intermediate-pressure compressor section 8.

[0063] A second intercooler 12 is provided between the discharge side 8.2 of the intermediate-pressure compressor section 8 and the suction side 7.1 of the high-pressure compressor section 7. Partially compressed process gas delivered by the intermediate-pressure compressor section 8 is cooled in the second intercooler 12 and can partly condense, as in the intercooler 11.

[0064] To prevent liquid (i.e., condensed gas) from entering the high-pressure compressor section 7, a second interstage liquid/gas separator 14 is provided downstream of the second intercooler 12, between the second intercooler 12 and the suction side 7.1 of the high-pressure compressor section 7. Reference number 14.1 designates the inlet of the second liquid/gas separator 14, while 14.2 and 14.3 represent the liquid outlet and the gas outlet, respectively. The gas outlet 14.3 of the liquid/gas separator is fluidly coupled to the suction side 7.1 of the high-pressure compressor secti on 7.

[0065] Downstream of the high-pressure compressor section 7, a high-pressure gas cooler 15 and a high-pressure gas condenser 17 are positioned in sequence as shown in Fig.1 and described above, for the same functions as mentioned above, followed by a further liquid/gas separator 19.

[0066] The components described above are part of the process gas path, which may include a scrubber 20 upstream of the suction side 5.1 of the low-pressure compressor section 5 and which further includes the low-pressure compressor section 5, the intercooler 11, the liquid/gas separator 13, the intermediate-pressure compressor section 8, the second intercooler 12, the second liquid/gas separator 14, the high-pressure compressor section 7, the cooler 15, the condenser 17 and the liquid-gas separator 19.

[0067] To protect the multi-stage compressor system 1 against surging, a master antisurge system 22 is provided, including a master anti-surge line 21 connected in antiparallel to the sequence of compressor stages or sections 5, 8 and 7. The master antisurge line 21 has an inlet 21.1 fluidly coupled to the process gas path downstream of the discharge side 7.2 of the high-pressure compressor section 7. The outlet of the master anti-surge line 21 is shown at 21.2 and is arranged upstream of the suction side 5.1 of the low-pressure compressor section 5.

[0068] As described above with reference to Fig.l, preferably, to avoid overheating of the process gas, the inlet 21.1 of the master anti-surge line 21 can be fluidly coupled to the process gas path downstream of the high-pressure gas cooler 15. In other embodiments, the inlet 21.1 of the master anti-surge line 21 can be fluidly coupled upstream of the high-pressure gas cooler 15 and the gas recycled through the master antisurge line 21 can be cooled by an auxiliary cooler (not shown) positioned along the master anti-sure line 21, for instance.

[0069] By arranging the inlet 21.1 of the master anti-surge line 21 between the high- pressure gas cooler 15 and the high-pressure gas condenser 17, ingress of condensed gas in the master anti-surge line 21 is avoided. This prevents damages to a master antisurge valve 23 positioned along the master anti-surge line 21.

[0070] The master anti-surge valve 23 is controlled by a master anti-surge controller 25.

[0071] Additionally, a first interstage anti-surge system 30 is provided, to protect the low-pressure compressor section 5 against surging. The interstage anti-surge system 30 comprises an interstage anti-surge line 31 having an inlet 31.1 fluidly coupled to the process gas path downstream of the delivery side 5.2 of the low-pressure compressor section 5 and an outlet 31.2 fluidly coupled to the process gas path upstream of the suction side 5.1 of the low-pressure compressor section 5. The interstage anti-surge line 31 is therefore arranged in anti-parallel to the low-pressure compressor section 5.

[0072] In preferred embodi m ents, the inlet 31.1 of the interstage anti-surge line 31 is fluidly coupled downstream of the intercooler 11 and of the interstage liquid/gas separator 13. This prevents overheating of the gas, since gas recycling through the interstage anti-surge line 31 is cooled in the intercooler 11 before entering the anti-surge line 31 and condensed gas is removed in the liquid-gas separator 13, thus preventing ingress of liquefied gas in the interstage anti-surge line 31.

[0073] An interstage anti-surge valve 35 is arranged along the interstage anti-surge line 31. The interstage anti-surge valve 35 is controlled by an interstage anti-surge controller 37. [0074] Furthermore, a second interstage anti-surge system 60 is provided, to protect the intermediate-pressure compressor section 8 against surging. The second interstage anti-surge system 60 comprises a second interstage anti-surge line 61 having an inlet 61.1 fluidly coupled to the process gas path downstream of the delivery side 8.2 of the intermediate-pressure compressor section 8 and an outlet 61.2 fluidly coupled to the process gas path upstream of the suction side 5.1 of the low-pressure compressor section 5. The second interstage anti-surge line 61 is therefore arranged in anti-parallel to the low-pressure compressor section 5 and to the intermediate-pressure compressor section 8.

[0075] In preferred embodiments, the inlet 61.1 of the second interstage anti-surge line 61 is fluidly coupled downstream of the intercooler 12 and of the interstage liq- uid/gas separator 14. This prevents overheating of the gas, since gas recycling through the interstage anti-surge line 31 is cooled in the intercooler 11 before entering the antisurge line 31 and condensed gas is removed in the liquid-gas separator 13, thus preventing ingress of liquefied gas in the interstage anti-surge line 31.

[0076] A second interstage anti-surge valve 65 is arranged along the second interstage anti-surge line 61. The interstage anti-surge valve 65 is controlled by a second interstage anti-surge controller 67.

[0077] The master anti-surge system 22, interstage anti-surge system 30 and the second interstage anti-surge system 60 of Fig.7 are controlled in the same way as the master anti-surge system 22 and the interstage anti-surge system 30 of Fig.1.

[0078] The compressor system and anti-surge control method thereof described above achieve the objective of harmonizing the distance from the surge point of each stage or section to approximately the same value and use the master anti-surge control valve 23 to adjust the compressor load. An overall more agile and reactive system, compared to the systems of the current art, is obtained since already one anti-surge controller, i.e., the master anti-surge controller 25, can react to changes in the compressor load demand and adjust the entire system. Smaller anti-surge valves and smaller piping for the anti-surge lines can be used.

[0079] The system and method described so-far are particularly useful e.g. in mixed refrigerant compressors, such as those used in natural gas liquefaction systems.

[0080] During partial load operation in the mixed refrigerant cycles of the current art, the recirculation through the anti-surge line causes a compositional change of the mixed refrigerant. By recirculating the light MR species, not condensed in the interstage coolers, will be recirculated back to the suction side. The result is a drop in the dew point temperature of the gaseous mixture. The effect is a so-called “lean-out” effect. Associated with such drop is a reduction in the amount of liquid formed in the interstage condensation vessel, and increased condensation in the final stage discharge condenser, i.e., in the condenser downstream of the last compressor stage or section. This means, that more than proportionally the condensation rate drops during part load operation. As the draw off from the vessels is not always driven by level control but based on production capacity, in part load the reduced condensation rate and the drawoff rate do not match anymore and cause the interstage vessel liquid inventory to diminish. The not condensed fraction from the current interstage is subsequently condensed in the next stage interstage cooler, resulting in a shift in the mixed refrigerant species hold-up location from one stage to the next and finally an accumulation in the last stage MR condenser.

[0081] The system described above suggests using minor anti-surge recycling from each discharge of each interstage back to the suction side of the first stage section, and an overall or master anti-surge line from the last discharge of the last stage or section of the compressor system to the first stage suction side. The master anti-surge valve is controlled by a master anti-surge controller. The interstage anti-surge valves are controlled by respective inter-stage anti-surge controllers that align the stage operating point distance from the respective surge line to the one of the last stage or section. This results in only one anti-surge controller (the master anti-surge controller) to act on the overall compressor capacity and multiple smaller anti-surge controllers (one for each compressor section except the most downstream one) to compensate variations in the mixed refrigerant during operation.

[0082] Recirculation during part-load of only a minor amount of gas from the interstage, necessary to align the stage capacity, will result in a reduced impact on the mixed refrigerant composition. [0083] A larger amount of liquefied process gas is obtained in each interstage liq- uid/gas separator (e.g., liquid/gas separator 13 in Fig. l and liquid/gas separators 13, 14 in Fig.7). This can be beneficial in that liquid is available in the interstage vessel for a longer time, i.e., more time is available till interstage vessel drainage when the anti-surge system is active.

[0084] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A compressor system comprising: a process gas path having a gas inlet and a gas outlet; between the gas inlet and the gas outlet, along the process gas path: a low-pressure compressor section having: a low-pressure suction side, fluidly coupled to the gas inlet, and a low-pressure discharge side; and a high-pressure compressor section having: a high-pressure suction side; and a high-pressure discharge side fluidly coupled to the gas outlet; an intercooler, between the low-pressure compressor section and the high-pressure compressor section; an interstage liquid/gas separator between the intercooler and the high-pressure compressor section; a master anti-surge line having: an inlet fluidly coupled to the process gas path downstream of the high-pressure discharge side; and an outlet fluidly coupled to the process gas path upstream of the low-pressure suction side; wherein a master antisurge valve is arranged in the master anti-surge line; an interstage anti-surge line having an inlet fluidly coupled to the process gas path downstream of the low-pressure discharge side and upstream of the high-pressure suction side, and an outlet fluidly coupled to the process gas path upstream of the low- pressure suction side; wherein an interstage anti-surge valve is arranged in the interstage anti-surge line; wherein: the interstage anti-surge valve is functionally connected to an interstage anti-surge controller; the master anti-surge valve is functionally connected to a master anti-surge controller; and the interstage anti-surge controller and the master anti-surge controller are functionally inter-related such that under some conditions a set point of the interstage anti-surge controller is modified as a function of an operating point of the high-pressure compressor section

2. The compressor system of claim 1, wherein the interstage anti-surge controller and the master anti-surge controller are functionally inter-related such that, when a flowrate through the high-pressure compressor section is lower than a pre-set value, a set point of the interstage anti-surge controller is modified as a function of an operating point of the high-pressure compressor section.

3. The compressor system of claim 2 wherein the interstage anti-surge controller and the master anti-surge controller are adapted to modify a set point of the interstage anti-surge controller such that the distance of an operating point of the low- pressure compressor section from a surge line of the interstage anti-surge controller is modified as a function of a distance of the operating point of the high-pressure compressor section from a surge line of the master anti-surge controller.

4. The compressor system of one or more of the preceding claims, wherein the inlet of the interstage anti-surge line is fluidly coupled to the process gas path downstream of the interstage liquid/gas separator.

5. The compressor system of one or more of the preceding claims, wherein the inlet of the master anti-surge line is fluidly coupled to the process gas path downstream of a high-pressure gas cooler adapted to cool compressed gas delivered by the high-pressure discharge side.

6. The compressor system of claim 5, further comprising a high-pressure gas condenser arranged downstream of the high-pressure gas cooler and adapted to condense cooled high-pressure gas exiting the high-pressure gas cooler at least partly.

7. The compressor system of one or more of the preceding claims, further comprising: at least one intermediate-pressure compressor section having an intermediatepressure suction side and an intermediate-pressure discharge side, positioned between the liquid/gas separator and the high-pressure compressor section; a further intercooler, fluidly coupled to the intermediate-pressure discharge side and to a further interstage liquid/gas separator; a further interstage anti-surge line having an inlet fluidly coupled to the process gas path downstream of the intermediate-pressure discharge side and upstream of the high-pressure suction side, and an outlet fluidly coupled to the process gas path upstream of the low-pressure suction side; wherein a further interstage anti-surge valve is arranged in the further interstage anti-surge line.

8. The compressor system of claim 7, wherein: the further interstage anti-surge valve is functionally connected to a further interstage anti-surge controller; and the further interstage anti-surge controller and the master anti-surge controller are functionally inter-related such that under some conditions a set point of the further interstage anti-surge controller is modified as a function of the operating point of the high-pressure compressor section.

9. The compressor system of claim 7, wherein: the further interstage anti-surge valve is functionally connected to a further interstage anti-surge controller; the further interstage anti-surge controller and the master anti-surge controller are functionally inter-related such that, when a flowrate through the high-pressure compressor section is lower than a pre-set value, a set point of the further interstage antisurge controller is modified as a function of the operating point of the high-pressure compressor section.

10. The compressor system of claim 8 or 9, wherein the further interstage anti-surge controller and the master anti-surge controller are adapted to modify a set point of the interstage anti-surge controller such that the distance of an operating point of the intermediate pressure compressor section from a surge line of the interstage anti-surge controller is modified as a function of a distance of an operating point of the high-pressure compressor section from a surge line of the master anti-surge controller.

11. A method for controlling a compressor system comprising: a low-pressure compressor section; a high-pressure compressor section; an intercooler, between the low-pressure compressor section and the high-pressure compressor section; an interstage liquid/gas separator between the intercooler and the high-pressure compressor section; a master anti-surge line having: an inlet fluidly coupled to the process gas path downstream of a high-pressure discharge side of the high- pressure compressor section; and an outlet fluidly coupled to the process gas path upstream of a low-pressure suction side of the low-pressure compressor section; wherein a master anti-surge valve is arranged in the master anti-surge line; an interstage anti-surge line having an inlet fluidly coupled to the process gas path downstream of a low-pressure discharge side of the low- pressure compressor section and upstream of a high-pressure suction side of the high-pressure compressor section, and an outlet fluidly coupled to the process gas path upstream of the low-pressure suction side; wherein an interstage anti-surge valve is arranged in the interstage anti-surge line; wherein the method comprises the following steps: controlling the master anti-surge valve through a master anti-surge controller; controlling the interstage anti-surge valve through an interstage anti-surge controller; adapting a set point of the interstage anti-surge controller as a function of an operating condition of the high-pressure compressor section when a flowrate through the high-pressure compressor section drops below a pre-set value.

12. The method of claim 11, wherein the step of adapting the set point of the interstage anti-surge controller comprises the step of modifying the set point of the interstage anti-surge controller such that the distance of an operating point of the low-pressure compressor section from a surge line of the inter-stage anti-surge controller is modified as a function of a distance of the operating point of the high-pressure compressor section from a surge line of the master anti-surge controller.

13. The method of claim 11 or 12, wherein the compressor system further comprises: an intermediate-pressure compressor section; a further intercooler, between the intermediate-pressure compressor section and the high-pressure compressor section; a further interstage liquid/gas separator between the further intercooler and the high-pressure compressor section; a further interstage anti-surge line having an inlet fluidly coupled to the process gas path downstream of an intermediate-pressure discharge side of the intermediate-pressure compressor section and upstream of the high-pressure suction side of the high-pressure compressor section, and an outlet fluidly coupled to the process gas path upstream of the low-pressure suction side; wherein a further interstage anti-surge valve is arranged in the interstage anti-surge line, the method further comprising the following steps: controlling the master anti-surge valve through a master anti-surge controller; adapting a set point of the further interstage anti-surge controller as a function of the operating condition of the high-pressure compressor section when the flowrate through the high-pressure compressor section drops below the pre-set value.

14. The method of claim 13, wherein the step of adapting the set point of the further interstage anti-surge controller comprises the step of modifying the set point of the further interstage anti-surge controller such that the distance of the operating point of the intermediate- pressure compressor section from a surge line of the further inter-stage anti-surge controller is modified as a function of the distance of the operating point of the high-pressure compressor section from the surge line of the master anti-surge controller.

15. A compressor system comprising: a plurality of compressor sections placed in sequence along a gas flow path; between at least two adjacent compressor sections an intercooler adapted to cause interstage condensation and a liquid/gas separator; in anti-parallel to said plurality of compressor sections, a master anti-surge line; a master anti-surge valve in said master anti-surge line; at least one interstage anti-surge line having an inlet between two sequentially arranged compressor sections and an outlet upstream of the most upstream of said plurality of compressor sections; an interstage anti-surge valve in the interstage anti-surge line; wherein: the interstage anti-surge valve is functionally connected to an interstage antisurge controller; the master anti-surge valve is functionally connected to a master antisurge controller; and the interstage anti-surge controller and the master anti-surge controller are functionally inter-related such that under some conditions a set point of the interstage anti-surge controller is modified as a function of an operating point of the most downstream compressor section

16. The compressor system of claim 15, wherein: the interstage antisurge controller and the master anti-surge controller are functionally inter-related such that, when a flowrate through the high-pressure compressor section is lower than a preset value, a set point of the interstage anti-surge controller is modified as a function of an operating point of the most downstream compressor section.

17. The compressor system of claim 15 or 16 wherein the interstage antisurge controller and the master anti-surge controller are adapted to modify a set point of the interstage anti-surge controller such that the distance of an operating point of the compressor section, at the discharge side whereof the interstage anti-surge line is connected, from a surge line of the interstage anti-surge controller is modified as a function of a distance of the operating point of the most downstream compressor section from a surge line of the master anti-surge controller.

18. A method for controlling a compressor system comprising: a plurality of compressor sections placed in sequence along a gas flow path; between at least two adjacent compressor sections an intercooler adapted to cause interstage condensation; in anti-parallel to said plurality of compressor sections, a master anti-surge line; a master anti-surge valve in said master anti-surge line; at least one interstage anti-surge line having an inlet between two sequentially arranged compressor sections and an outlet upstream of the most upstream of said plurality of compressor sections; an interstage anti-surge valve in the interstage anti-surge line wherein the method comprises the following steps: controlling the master anti-surge valve through a master anti-surge controller; controlling the interstage anti-surge valve through an interstage anti-surge controller; adapting a set point of the interstage anti-surge controller as a function of an operating condition of the most downstream compressor section when a flowrate through the most downstream compressor section drops below a preset value.

19. The method of claim 18, wherein the step of adapting the set point of the interstage anti-surge controller comprises the step of modifying the set point of the interstage anti-surge controller such that the distance of an operating point of the compressor section, at the discharge side whereof the interstage anti-surge line is con- nected, from a surge line of the inter-stage anti-surge controller is modified as a function of a distance of the operating point of the most downstream compressor section from a surge line of the master anti-surge controller.