Method for operating a compression system, and system performing the method

Abstract

The method comprises the following steps: (a) when a plurality of compressor units are in an online state and operating mode, responsive to a reduction in an inlet flow rate: bringing one of said compressor units in an offline state; wherein the offline compressor unit rotates at a minimum operative speed and in full recycle mode; and (b) when at least one compressor unit is in the offline state, responsive to an increase in the inlet flow rate: bringing said at least one compressor unit in offline state back in the online state and operating mode.

Description

METHOD FOR OPERATING A COMPRESSION SYSTEM, AND SYSTEM PERFORMING THE METHODDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure relates to compression systems comprising a plurality of compressor units arranged in parallel between a common suction header and a common discharge header, for processing a process gas supplied at a flow rate that may vary over time.BACKGROUND ART

[0002] Dynamic compressors, such as axial compressors and centrifugal compressors, can operate under variable operating conditions to process variable flow rates. However, the flow rate that can be processed by the dynamic compressor cannot be below a lower threshold to prevent surging phenomena, and cannot be above an upper threshold, to prevent choking and to exceed the maximum rotational speed. In some applications, the flow rate of the process gas to be compressed by the compression system may vary significantly. Exemplary situations where strong flow rate fluctuations may occur are those where the process gas is supplied from a source with a highly variable gas flow output. This presents a serious challenge with regard to compressor control.

[0003] Renewable energy sources are currently being explored to reduce the use of fossil fuels, whose combustion produces carbon dioxide, a greenhouse gas considered to be a major contributor to climate change and global warming. One of the issues with renewal energy sources is the fluctuation of the electrical power generated thereby. For instance, solar energy is absent at nighttime and can strongly varies during daytime due to weather conditions. Power generated by wind farms also strongly depends upon whether conditions.

[0004] Various systems are therefore currently being studied to store and accumulate energy produced inconsistently or intermittently by renewable energy plants. Storage facilities accumulate energy during periods of high production and release it duringperiods of low or no production from renewable sources.

[0005] A particularly effective method of storing energy produced from renewable sources is based on the production of hydrogen, a highly flammable gas that can later be used as fuel. Its combustion produces water and, therefore, does not cause pollution.

[0006] Hydrogen can be produced in a number of ways. So-called green hydrogen is produced by electrolysis using electrical energy generated from renewable sources. Unless high-pressure electrolysis is used, conventional electrolyzers typically operate at ambient pressure and produce hydrogen at a pressure of about 1 barA. The hydrogen produced in this way must be compressed in a compression system. Due to the intermittent nature of most renewable energy sources, the hydrogen flow rate at the suction header of the compression system typically varies. This can cause problems in the operation of the compression system as flow rate of the process gas (hydrogen) may vary to the point where the compressors in the compression system approach or exceed the surge control line or the choke control line. To prevent the occurrence of choking or surge phenomena, it may be necessary to either turn on or turn off one or more compressors depending on the variation in process gas flow rate.

[0007] In some cases, fluctuations in inlet flow rate can be compensated by adjusting the rotational speed of the compressor units, or by acting upon control devices, such as variable inlet guide vanes (IGVs) or anti-surge valves, without the need to shut down or start up compressors. If the inlet flow rate strongly decreases, anti-surge valves are opened to prevent the compressors from operating too close to the surge limit line (SLL). However, these interventions result in a significant loss of system efficiency. Specifically, opening of the anti-surge valve keeps the flow rate within each compressor at a sufficiently high value to avoid crossing the surge limit line (SLL). However, this is achieved by recirculating the process gas. Consequently, the gas is compressed multiple times in a closed loop, leading to energy waste and reduced efficiency. Also, operating at a reduced rotational speed, lower than the design speed reduces the efficiency of the compressor and thus of the compression system as a whole.

[0008] US 11,994,135 discloses a method and apparatus for compressing a gas feed with a variable flow rate. According to this known method and system, the number of active compressors can vary depending upon the gas feed received, i.e. the gas flowrateat a common suction header. During periods when the gas feed is equal to the total maximum capacity of a given number of active compressors, all said active compressors are run at full load. During periods when the flowrate of the gas feed is in a range from less than the total maximum capacity of the active compressors to a total turndown capacity of the active compressors, the active compressors are run at a speed which varies as a function of the flow of gas feed, at a reduced load. The total turndown capacity is the sum of the turndown capacities of the active compressors. The turndown capacity of a compressor is the minimum capacity of a compressor, i.e. the minimum flowrate which the compressor can process without surging. Only when the total flow of the gas feed drops below the total turndown capacity, one of the active compressors is switched off and the total number of active compressors is reduced. This method results in an inefficient operation of the system, as when the gas feed drops below the maximum capacity of the active compressors, all compressors operate in an inefficient off-design condition for a fairly long period of time until the gas feed drops below the turndown capacity.

[0009] An object of embodiments disclosed herein is to address or mitigate the aforementioned drawbacks of existing compression systems that may arise under fluctuating flow rate conditions.SUMMARY

[0010] According to one aspect, disclosed herein is a method for operating a compression system comprising a plurality of compressor units in parallel, each compressor unit including an anti-surge line and an anti-surge control valve along the antisurge line. The several compression units in parallel can be fluidly coupled to a common suction header and to a common discharge header.

[0011] According to the method of the present disclosure, when a plurality of compressor units are in an online state and operating mode, responsive to a reduction in an inlet flow rate (i.e. the total flow rate entering a common suction header), a control unit brings one of said compressor units in an offline state. In the offline state, the compressor unit rotates at a minimum operative speed and in full recycle mode. Conversely, when at least one compressor unit is in the offline state, responsive to an increase in the inlet flow rate, the control unit brings said at least one compressor unit,which is in offline state, back in the online state and operating mode.

[0012] Advantageously, the steps of bringing one of the compressor units in an offline state and of bringing one of the compressors units from an offline state to an online state are performed based on a predictive approach, such that the change of total number of online compressor units results in an increase of an overall efficiency of the compression system, and each online compressor unit operates between admissible flow rate limits.

[0013] Unless differently indicated, as understood herein the flow rate of the process gas is the total flow rate of the gas processed by the compression system which enters the common suction header, whereto the suction sides of the compressor units are fluidly coupled.

[0014] As generally understood herein, the flow rate is the mass flow rate (e.g. expressed in kg / s).

[0015] Under steady-state conditions, when the flow rate delivered to the compression system 1 is constant, the flow rate entering the common suction header is equal to the sum of the flow rates entering the active compressor units, i.e. the compressor units that are in an online state and in an operating mode.

[0016] If the flow rate delivered to the suction header changes over time, e.g. because the production of process gas from a process gas source changes (increases or reduces), two different conditions may occur. If the flow rate decreases, i.e. the total flow rate entering the common suction header becomes less than the sum of the flow rates processed by the compressor units, the pressure in the common suction header decreases. If the incoming flow rate increases and becomes higher than the sum of the flow rates processed by the online compressor units, the pressure in the common suction header increases. Under these circumstances, the method provides for a change in the operating conditions of the compressor units and can decrease or increase the number of processing compressor units, i.e. of compressor units that are in the online state and operating mode.

[0017] As will become clearer later, based on the detailed description of embodiments, the method can operate as a predictive method, based on the performance mapsof the compressor units and can switch one or more compressor units from an online to an offline state and vice versa, if this results in an increase of the overall efficiency of the compression system and the online compressor units all operate between admissible flow rates through each compressor unit, i.e. all online compressor units operate between the surge limit line and the choke limit line, as well as between the maximum continuous speed line and the minimum operative speed line.

[0018] Specifically, the predictive method can be performed as follows. Responsive to an inlet flow rate reduction, a compressor unit which is in an online state and operating mode is brought in an offline state if the following conditions are met: (1) the total power absorbed by the compression system prior to bringing said online compressor unit in the offline state is higher than the total power absorbed by the compression system with said one compressor unit in the offline state; and (2) when said one compressor unit is in the offline state, each remaining compressor unit in the online state and operating mode processes a flow rate equal to or lower than a choke-safe flow rate threshold.

[0019] Similarly, according to embodiments of the predictive method, responsive to an inlet flow rate increase an offline compressor is brought back in the online state and operating mode if the following conditions are met: (1) a total power absorbed by the compression system prior to bringing said compressor unit in the offline state back in the online state and operating mode is higher than the total power absorbed by the compression system with said compressor unit brought back in the online state and operating mode; and (2) when said compressor unit in the offline state is brought back in the online state and operating mode, each compressor unit in the online state and operating mode processes a flow rate equal to or higher than a surge-safe threshold.

[0020] The method allows therefore to modulate the number of compressor unit which are operative, i.e. are processing gas, based on the flow rate of the incoming process gas. This allows the compression system to cope with highly variable flow rate of incoming gas under optimum efficiency conditions.

[0021] As understood herein, a compressor unit in the online state and operating mode is a compressor unit which receives gas from the suction header, pressurizes the gas and delivers the pressurized gas to a discharge header. An offline compressor unitin full recycle mode is a compressor unit which rotates at a minimum operative speed and processes a gas flow which is fully recycled from the delivery side to the suction side of the compressor unit, through the anti-surge line, for instance. In the offline state no process gas flows from the suction header to the discharge header. The compressor unit rotates under minimum power absorption and is ready to be brought in an operating mode and online state as soon as this is required, responsive to an increase in process gas flow rate.

[0022] Additional features and embodiments of the method according to the present disclosure are described below and outlined in the annexed claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Reference is now made briefly to the accompanying drawings, in which:Fig.1 illustrates a schematic of a compression system according to the present disclosure in one embodiment;Figs. 2 and 3 illustrate head-vs-flow rate diagrams showing the step of turning one compressor unit from an operating, online state into an offline state;Figs. 4 and 5 illustrate head-vs-flow rate diagrams showing the step of turning one compressor unit from an offline state into an online state; andFig.6 illustrates a schematic of a further compressions system according to the present disclosure in another embodiment.DETAILED DESCRIPTION

[0024] In the present disclosure, reference is made to a compression system integrated with a green hydrogen production plant. However, those skilled in the art of gas processing will understand that novel and useful features of the compression system disclosed herein can also be applied to other types of compression systems when similar issues arise, particularly when the process gas flow rate at the suction header varies over time.

[0025] Referring now to the drawings, a first embodiment of a compression system 1 according to the present disclosure is illustrated in Fig. l. The compression system 1 is integrated with a source of process gas 3. In this exemplary embodiment, the source of process gas 3 is a hydrogen source. In the embodiment of Fig.l, the source ofprocess gas 3 comprises a green hydrogen production plant 4. In the exemplary embodiment of Fig.1 the green hydrogen production plant 4 uses solar energy from a photovoltaic field 5, which supplies electrical energy generated from solar energy to an electrolyzer 7. Hydrogen (H2) produced by the electrolyzer 7 is fed to the compression system 1 through a hydrogen feed line 9.

[0026] The compression system 1 comprises a plurality compressor trains or compressor units, referred to as 10.1, 10.2, 10.3. The reference 10 is used in the present description to indicate a generic one of said compressor units. While in Fig.1 the compression system 1 comprises three compressor units 10, the actual number of compressor units may vary. The minimum number of compressor units is two, but the compression system 1 may comprise more than three compressor units, e.g. four, five or even more compressor units. The compression system 1 can be designed such that the number of compressor units and their rated flow rate can process the maximum expected hydrogen flow rate with all the compressor units operating at full capacity

[0027] Each compressor unit 10 comprises, for instance, a driver 13 and a compressor 15. A shaft line 17 drivingly couples each driver 13 to the respective compressor 15.

[0028] In some embodiments the compressor units lO.j (with j=l,2,3) are identical to one another. But this is not mandatory. In some embodiments, not shown, the compression system 1 can include two or more different kinds of compressor units. For instance, compressor units of different dimension, and adapted to process different hydrogen flow rates can be combined in the same compression system 1. However, since the compressor units are fluidly coupled to a common suction header 11 and to a common discharge header 12, i.e. the suction pressure and the delivery pressure of the compressor units is the same, it is preferred that all compressor units have a maximum efficiency for the same head across the compressor units 10.

[0029] Each compressor 15 can consist of, or include, a dynamic compressor, such as an axial compressor or a centrifugal compressor. In some embodiments, each compressor 15 can be or include a set of dynamic compressors connected in series. Each compressor can be a single-stage or a multi-stage compressor.

[0030] In some embodiments, the drivers 13 are adapted to operate the compressors 15 at variable speed. Each driver 13 can include, or can be a variable speed electric motor. If required, a gearbox (not shown) can be positioned along the shaft line 17 which drivingly couples the electric motor 13 with the corresponding compressor 15.

[0031] In some embodiments, the compressors 15, or some of them, can be provided with variable inlet guide vanes, schematically shown at 19. The variable inlet guide vanes are adapted regulate the gas flow at the inlet of the compressors 15 and can be adjusted to compensate the gas flow rate.

[0032] Each compressor unit 10 further includes an anti-surge line 21, which has an inlet fluidly coupled with a delivery side 10D of the respective compressor unit 10 and an outlet fluidly coupled with a suction side 10S of the respective compressor unit 10. The anti-surge line 21 is therefore arranged in anti -parallel with the respective compressor 15. An anti-surge valve 23 is positioned in each anti-surge line 21. An aftercooler 25 can be positioned between the delivery side of each compressor 15 and the inlet of the anti-surge line 21. A scrubber 27 can be positioned between the outlet of each anti-surge line 21 and the suction side 15S of the respective compressor 15.

[0033] A controller 31 is functionally coupled to the anti-surge valve 23 of each compressor unit 10. In the embodiment of Fig.1, a respective controller is provided for each compressor unit 10. In other embodiments, not shown, a common controller can be used to control two or more, possibly all, anti-surge valves 23.

[0034] As will be explained in more detail, each compressor unit 10 can be in an online state and operating mode, or an offline state. In the online state and operating mode, process gas (in this exemplary embodiment, hydrogen) flows through the compressor unit and is pressurized by the respective compressor 15. The flow rate through each compressor unit 10 can be adjusted according to the total inflowing process gas entering the common suction header 11 from the hydrogen source 3.

[0035] Each compressor unit 10 can be placed in an offline state under certain conditions, as described below, specifically if the hydrogen flow rate is below the maximum expected flow rate which the compression system 1 can process. In the offline state, the compressor unit 10 is fluidly decoupled from the common suction header andthe common discharge header. To achieve this, each compressor unit 10 can be equipped with non-return valves (check valves).

[0036] In some embodiments, a suction-side shut-off valve 41 and a delivery-side shut-off valve 43 can also be present, to separate the compressor from the circuit, if required.

[0037] In the offline state, the compressor unit rotates at lower speed, in particular at a minimum operative speed (MOS), and in a full recycle mode, i.e. the entire gas processed by the compressor is recycled through the anti-surge line 21. The anti-surge valve 23 is at least partly open to allow hydrogen recycling. Compression heat is removed from the recycling hydrogen through the after-cooler 25.

[0038] The number of compressor units 10 which are placed in the offline, full recycle mode depends upon the hydrogen flow rate supplied from the hydrogen source 3.

[0039] With continued reference to Fig.1, Figs.2 and 3 illustrate a flow rate vs. polytropic head diagram of each compressor unit 1 in two different operating states of the compression system 1. The polytropic head is plotted on the vertical axis and the mass flow rate is plotted on the horizontal axis.

[0040] In this embodiment, it is assumed that all compressor units lO.j (j=l, 2, 3) are identical to each other. The flow rate (in kg / s) plotted on the horizontal axis is the flow rate of each individual compressor unit 10. If the compressor units 10 are identical to one another, they can be run at the same conditions and will therefore process the same flow rate of process gas. The sum of the flow rates of all operating and online compressor units 1 is the flow rate delivered to the common suction header 11.

[0041] The difference between the operating condition illustrated in Fig.2 and the operating condition illustrated in Fig.3 is the number of compressor units 10 which are in the online state and operating mode. Fig.2 shows the operating point of each compressor unit in a situation where N compressor units 10 are operating, Fig.3 shows the operating point of the compressor units 10 after one of the N compressor units has been brought in the offline, full recycle mode, i.e. with (N-l) compressor units in operation, the total flow rate processed by the compression system 1 being the same.

[0042] As mentioned, for the sake of clarity in this embodiment, it is assumed that all compressor units 10 are identical to one another, but the general teaching disclosed herein can easily be applied to systems where the compressor units are not identical to one another.

[0043] In Figs.2 and 3, SLL represents the surge limit line, SCL denotes the surge control line, and CLL represents the choke limit line. MCS is the maximum continuous speed of the compressor 15, and MOS is the minimum operative speed of the compressor.

[0044] For instance, the maximum continuous speed can be 105% of the rated design speed of the compressor 15, while minimum operative speed can be as low as 20% or 30% of the rated speed. The minimum operative speed represents the minimum speed at which the compressor 1 can rotate. Lower speeds may result in damage to the dry gas seals of the compressor, for example, or other inconveniences.

[0045] If all compressor units 10 are identical to one another, the operating map of Figs.2 and 3 and the operating point OP shown therein are the same for each compressor unit 10. It is assumed that all compressor units 10 in operative condition process approximately the same flow rate, i.e. they rotate at the same speed and with the same position of the variable IGV, if present.

[0046] Thus, if Qtot is the total mass flow rate processed through the compression system 1, i.e the flow rate to the common suction header 11, and N is the number of compressor units 10 which are in an online state and operating mode, each compressor unit 10 will process a flow rate equal to Qtot / N.

[0047] Line LL represents the load line of the compressor 15. The load line LL depends upon the features of the compressor 15, and of the hydraulic circuit, whereof the compressor 15 forms part. The load line LL can be determined knowing the curves of the compressor(s) forming the compressor units and the characteristic curve of the circuit in which the compressor units are positioned. The load line LL can also be determined with a series of tests on site.

[0048] Qm max is the maximum mass flow rate which can be processed by each compressor 15, Qm min is the minimum flow rate which can be processed remainingon the right side of the surge control line SCL. In Fig.2 OP indicates the operating point which can move along the load line LL as a function of the flow rate. Qm_0 is the flow rate of the compressor 15 in operating point OP.

[0049] Let N be the total number of compressor units which are in the operating mode in a certain operating condition, where the operating point OP is positioned as shown in Fig.2. It is noted that, under the operating conditions depicted in Fig.2, the flow rate Qm_0 is low and the compressor 15 is rotating at a speed substantially lower than the rated speed. The efficiency of the compressor system 1 is low, because the compressors operate far from the design point. Moreover, under this condition, a further reduction in flow rate supplied from the hydrogen source 3 could induce unstable behavior of the compressors 15 (operating point OP moving towards the left of the SCL). This situation must be avoided to prevent surging phenomena, by opening the anti-surge valves 23, which would cause further loss of efficiency.

[0050] Starting, for instance, from a full load condition (maximum flow rate from the hydrogen source 3 and compressor units 10 operating at or near the maximum continuous speed line MCS), a reduction in the flow rate moves the operating point OP along the load line LL.

[0051] The reduction in the flow rate at the common suction header 11 causes a reduction in the suction pressure, which can be detected by a pressure sensor. Alternatively, if the suction pressure is a controlled parameter, a flow rate reduction may result in a reduction of the suction pressure and consequently causes a reduction of the rotational speed of the compressor units 10 and a consequent displacement of the operating point towards the surge limit line. Responsive to a flow rate reduction, a control system 20 will act upon the compressor units 10 changing one or more operation parameters thereof. In some embodiments, a drop of the flow rate will be compensated for by a reduction of the rotational speed of those the compressor units 10 that are in the online state and operating mode. Alternatively to, or in combination with a speed reduction, the angular position of the variable IGV 19 (if present) can be modified to regulate the operating conditions of the compressor units. Alternative or additional operating parameters can be adjusted in combination with, or alternatively to the IGV and the rotational speed. For example, a throttle valve on the suction side of the compressor units10 can be partially closed, in response to the reduction in the flow rate and consequent suction pressure drop. This will modify the characteristic curve of the circuit and consequently change the position of the operating point OP of the compressor 15.

[0052] If the flow rate continues to decrease, bringing the compressors 15 close to the surge control line SCL and no further speed reduction or regulation is possible, the anti-surge valves 23 will open to prevent surging.

[0053] To improve the efficiency of the compression system 1, e.g. in a situation such as that shown in Fig.2, one of the N compressor units 10 that are currently in an online state and operating mode, may be turned into an offline state, without shutting down the compressor unit, but isolating said compressor unit from the system, such that the incoming flow rate is processed by the remaining operating compressor units 10. If N compressor units 10 are operative in the situation depicted in Fig.2, , when one compressor unit is turned into an offline state, the total incoming flow rate (N*Qm_0) will be distributed among the remaining (N-l) compressor units 10 which are maintained in the online state and operating mode, such that each of said operating compressor units 10 processes a flow rate equal toN*Qm_0 / (N-l)

[0054] The increased flow rate processed by each of the N-l compressor units 10 which remain in the online state and operating mode causes the rotational speed of the compressor units to increase and the operating point OP thereof to move along the load line LL toward the maximum continuous speed (curve MCS).

[0055] The compressor unit which has been turned into the offline state will operate in a full recycle mode, i.e. no flow from the common suction header 11 reaches the offline compressor unit 10, the anti-surge valve 23 is partially or fully open and the compressor unit rotates at minimum operative speed. This minimizes the power needed to keep the offline compressor rotating, such that it can be returned in an inline operating condition without delay when the flow rate increases again.

[0056] The diagram of Fig.3 is the same as the diagram of Fig.2, but shows the operating point OP of the compressor units 10 after one of the previously operating compressor units 10 has been turned into the offline state. The operating point OP of eachone of the (N-l) compressor units 10, which remains in operation and in the online state and operating mode, has moved along the load line LL towards the maximum continuous speed line MCS. The flow rate processed by each one of the (N-l) online compressor units 10 (plotted on the horizontal axis) is increased with respect to Fig.2.

[0057] One compressor unit 10 can be turned into the offline state when the flow rate drops to such an extent that the operating point OP reaches the surge control line SCL. However, it may be preferable to turn said operating compressor unit 10 into the offline state and full recycle mode at an earlier stage, before reaching the SCL line, when a speed reduction caused by a flow rate reduction is detected, with the operating point OP moving towards the surge control line SCL, in order to maximize the efficiency of the compression system 1. On the other hand, a compressor unit 10 shall not be turned into the offline state too early, if by so doing the compression system 1 would operate beyond the choke control line.

[0058] In other terms, one (or more) compressor unit(s) are turned into the offline state based on a predictive approach, namely if by so doing the efficiency of the compression system 1 increases, and if the operating point of the compressor units which remain in the online state and operating mode falls within the admitted area of the performance map, i.e. between the surge limit line and the choke limit line, as well as between the maximum continuous speed line and the minimum operative speed line.

[0059] Therefore, according to embodiments disclosed herein, turning one (or more) compressor units 10 into the offline state can be performed when two pre-conditions are met.

[0060] A first condition is that the total power absorbed by the compression system 1 prior to bringing the operating compressor unit 10 in the offline state is higher than the total power absorbed by the compression system 1 with said operating compressor unit turned into the offline state. A second condition is that, when said compressor unit 10 is offline, each remaining compressor unit, i.e., the compressor units remaining in the operating state, processes a flow rate equal to or less than a choke-safe flow rate threshold, approximately Qmax_tot / (N-1).

[0061] By controlling these two conditions, and knowing the compressorperformance maps, the controller, or control unit 20 is able to place one (or more) compressor unit(s) 10 in the offline state well before the operating point OP reaches the surge control line SCL, but not so much earlier as to interfere with the proper operation of the compression system 1.

[0062] These conditions can be checked based on the following parameters: Hmo: compressor polytropic head in the current operating point OP,Qmo: flow rate of each compressor 15 in the current operating point OP, r|mo: efficiency of the compressors in the current operating point OP,Hm: compressor head after turning of one compressor unit 10 offline,Qm: flow rate of each compressor 15 after turning of one compressor unit 10 offline, r|m: efficiency of the compressor units after turning one compressor unit 10 offline,HmmaX: maximum polytropic head when the operating point OP is on the choke limit line CLL or on the MCS line, depending on whether the load line crosses theMCS line or the CLL,Qm max • maximum flow rate when the operating point OP is on the choke control line or on the MCS line, and r|mmax: efficiency of the compressor 15 when the operating point OP is on the choke control line or on the MCS line.

[0063] The flow rate parameters mentioned above are plotted on the horizontal axis in the diagrams of Figs. 2 and 3.

[0064] The conditions mentioned above are expressed as follows: (Hmo*Qmo / Hmo )* N > (Hm*Qm / l]m)*(N-l) (1)Qmo*N = Qm*(N-l) < Qm_max*(N-l) (2)

[0065] Condition (1) ensures that turning of the compressor unit into the offline state improves the overall efficiency of the compression system. Condition (2) ensures that the compressor units which remain in use do not reach a choke condition.

[0066] Conversely, when at least one of the compressor units 10 of the compression system 1 is in an offline state and full recycle mode, the compression system 1 canreact to an increase in the flow rate (which corresponds to an increase in the pressure in the common suction header 11) by bringing the offline compressor unit back in an operating, online state and operating mode.

[0067] Similarly to what has been described above with regard to the step of reducing the number of compressor units which are in an online state, the opposite step prevents unstable operation of the compressor system 1, and specifically prevents the compressor system from approaching or reaching the choke control line CLL. As mentioned above, regarding turning an online compressor unit 10 into an offline state, also the opposite procedure is not necessarily performed only when the compressor units are approaching the choke control line.

[0068] Rather, assuming that N’ compressor units are in online state, with N’<Ntot, where Ntot is the total number of compressor units (i.e. at least one compressor unit 10 is offline), the compression system 1 can respond to a flow rate increase by bringing said offline compressor unit (or more than one offline compressor unit) into an online state whenever this is possible and convenient. Turning the offline compressor unit 10 into the online state is possible if the change does not bring the operating point OP of the online compressor units 10 on or beyond (on the left of) the surge control line SCL. Turning the offline compressor unit 10 back into the online state is convenient, and is performed, if this results in an improved efficiency of the compression system 1, i.e., in a reduction in the total power required to run the system.

[0069] These conditions are expressed as follows:(Hmo*Qmo / r|mo )* N’ > (Hm*Qm / qm)*(N’+l) (3)Qmo*N’ = Qm*(N’+l) > Qm-min*(N’+l) (4) whereinHm min is the minimum polytropic head of the compressor units when the operating point OP is on the surge control line (SCL),Qm min is the minimum flow rate processed by one compressor when the operating point OP is on the surge control line (SCL), and r|m min is the efficiency of one compressor unit 10 when the operating point OP is on surge control line (SCL).

[0070] The diagram of Fig.4 illustrates the operation point OP prior to turning the offline compressor unit 10 into an online state, i.e., with N’ compressor units 10 in the online state) and the diagram of Fig. 5 illustrates the operating point OP after turning the offline compressor unit back into the online state, i.e. with (N’+l) compressor units 10 in the online state and operating mode.

[0071] In general terms, therefore, also turning an offline compressor unit 10 back into the online state and operating mode is done based on a predictive approach, i.e. if this results in an increase of the overall efficiency of the compression system 1 and the operating point of all compressor units which are finally in the online state and operating mode falls between the surge limit line and the choke limit line, as well as between the minimum operative speed and the maximum continuous speed.

[0072] As described with regard to the procedure for turning offline one compressor unit in response to a reduction in the flow rate, also in the case of an increase in the flow rate, prior to turning an offline compressor back into the online state, one or more operating parameters of the currently online compressor units can be regulated to compensate for the increase in flow rate. Specifically, one or more of the following parameters can be acted upon prior to resuming operation of one of the offline compressor units that are in a full recycle mode: rotational speed, position of the variable inlet guide vanes, opening of a throttle valve upstream of the compressor units.

[0073] The procedures described above for turning one operating compressor unit into the offline state and recycle mode, and vice versa, can be repeated as many times as allowed by the number of compressor units 10 included in the compression system 1. Operation of the compression system 1 can thus be adapted to the available incoming flow rate of process gas (hydrogen in the present exemplary embodiment) with the aim of maximizing the efficiency of the compression system 1 while avoiding shutdown of compressor units.

[0074] Fig. 6 illustrates a further embodiment of a compressor system of the present disclosure. The same reference numbers designate the same or functionally equivalent components, which are not described again in detail herein.

[0075] The main difference between the embodiment of Fig.1 and the embodimentof Fig.6 is that in Fig.2 each compressor unit 10 comprises a first compressor 15A and a second compressor 15B arranged in series. In this embodiment, each compressor 15A, 15B comprises a respective anti-surge line 21A, 21B, in which an anti-surge valve 23 A, 23B is positioned. Each anti-surge valve 23 A, 23B can be controlled by a respective controller 31 A, 3 IB. In other embodiments, not shown, a common antisurge line containing a respective anti-surge valve can be positioned in anti-parallel with both serially arranged compressors 15 A, 15B. A scrubber 27A, 27B is arranged upstream of each one of said first compressor 15 A and second compressor 15B.

[0076] An inter-cooler 25 A is positioned between the delivery side 10D of the first compressor 15A and the suction side 10S of the second compressor 15B. An aftercooler 25B is positioned downstream of the delivery side 10D of the second compressor 15B.

[0077] Operation of the compression system 1 of Fig.6 is the same as described above.

[0078] 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

CLAIMS1. A method for operating a compression system comprising a plurality of compressor units in parallel, each compressor unit including an anti-surge line and an anti-surge control valve along the anti-surge line; the method comprising the following steps: when a plurality of compressor units are in an online state and operating mode, responsive to a reduction in an inlet flow rate: bringing one of said compressor units in an offline state; wherein the offline compressor unit rotates at a minimum operative speed and in full recycle mode; and when at least one compressor unit is in the offline state, responsive to an increase in the inlet flow rate: bringing said at least one compressor unit in offline state back in the online state and operating mode; wherein the steps of bringing one of the compressor units in an offline state and of bringing one of the compressors units from an offline state to an online state are performed based on a predictive approach, such that the change of total number of online compressor units results in an increase of an overall efficiency of the compression system, and each online compressor unit operates between admissible flow rate limits.

2. The method of claim 1, wherein the step of bringing the compressor unit in offline state responsive to the reduction in the inlet flow rate is performed if the following conditions are met:- a total power absorbed by the compression system prior to bringing said operating compressor unit in the offline state is higher than the total power absorbed by the compression system with said one compressor unit in the offline state; and- when said one compressor unit is in the offline state, each remaining compressor unit in the online state and operating mode processes a flow rate equal to or lower than a choke-safe flow rate threshold.

3. The method of claim 1 or 2, wherein the step of bringing said compressor unit from the offline state back in the online state responsive to the increase inthe inlet flow rate is performed if the following conditions are met:- a total power absorbed by the compression system prior to bringing said compressor unit in the offline state back in the online state and operating mode is higher than the total power absorbed by the compression system with said compressor unit brought back in the online state and operating mode; and- when said compressor unit in the offline state is brought back in the online state operating mode, each compressor unit in the online state and operating mode processes a flow rate equal to or higher than a surge-safe threshold.

4. The method of any one of the preceding claims, further comprising the step of: responsive to the reduction in the inlet flow rate, before bringing said one of said compressor units offline, regulating at least one operating parameter of the compressor units to compensate for the reduction in the inlet flow rate prior to bringing said one of said compressor units offline.

5. The method of any one of the preceding claims, further comprising the following step: responsive to the increase in the inlet flow rate, before bringing said at least one offline compressor unit back in the online state and operating mode, regulating at least one operating parameter of the compressor units to compensate for the increase in the inlet flow rate prior to bringing said compressor unit in offline state back in the online state and operating mode.

6. The method of claim 4 or 5, wherein the at least one operating parameter is selected from the group comprising: a rotational speed of the compressor units; an inlet guide vane geometry of the compressor unit; a throttle valve partializa- tion; a combination thereof.

7. A compression system for compressing gas according to the method of any one of claims 1 to 6, the system comprising: a source of process gas;a plurality of compressor units in parallel, each compressor unit having a suction side fluidly coupled to a common suction header, adapted to receive process gas from the source of process gas, and a delivery side fluidly coupled to a common discharge header; wherein each compressor unit comprises an anti-surge line comprising an inlet fluidly coupled with the delivery side thereof and an outlet fluidly coupled with the suction side thereof; and wherein each anti-surge line comprises an anti-surge valve; a control system for switching each compressor unit selectively in an online state and operating mode and in an offline and full recycle mode, responsive to a change of incoming process gas flow rate, based on a predictive approach, such that the change of total number of compressor units in an online state results in an increase of an overall efficiency of the compression system, and each online compressor unit operates between admissible flow rate limits.

8. The compression system of claim 7, wherein the source of process gas comprises a green hydrogen production plant.

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