Anti-surge control in a centrifugal compressor working with a fluid in supercritical conditions
The method calculates the compressor operating point using an isentropic exponent to graphically represent the surge limit line, addressing the instability in anti-surge control for centrifugal compressors with supercritical fluids by providing a stable and efficient solution independent of fluid temperature and pressure variations.
Patent Information
- Application Number
- PCT/EP2025/059018
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-04-02
- Filing Date
- 2025-04-02
- Publication Date
- 2025-10-09
AI Technical Summary
The control of anti-surge in centrifugal compressors working with supercritical fluids, particularly CO2, is unstable due to the dependence of surge limit line calculations on fluid temperature and pressure variations, necessitating frequent recalculations.
A method and system that calculates the compressor operating point using an isentropic exponent independent of fluid temperature and pressure, allowing for an offline determination of the surge limit line, represented graphically as a function of reduced head and corrected mass flow, enabling stable anti-surge control.
The method provides a stable and efficient anti-surge control by eliminating the need for real-time recalculations of the surge limit line, ensuring consistent operation independent of fluid conditions.
Smart Images
Figure EP2025059018_09102025_PF_FP_ABST
Abstract
Description
TITLE ANTI-SURGE CONTROL IN A CENTRIFUGAL COMPRESSOR WORKING WITH A FLUID IN SUPERCRITICAL CONDITIONS DESCRIPTION TECHNICAL FIELD
[0001] The subject matter disclosed herein relates to a method for performingan anti-surge control in a centrifugal compressor, in particular working with a fluid in supercritical condition, more in particular working with CO2 in supercritical conditions (=sCO2). BACKGROUND ART
[0002] The problem of “surge”, which is an unstable operation state, is a well-known problem in centrifugal compressors.
[0003] In general, a centrifugal compressor receives a fluid to be compressedthrough an inlet duct (suction) and delivers a compressed fluid through an outlet duct (discharge).
[0004] Typically, a modern compression system comprises a centrifugalcompressor and an anti-surge recirculation loop, which fluidly couples the outlet duct and the inlet duct, in order to avoid compressor surge. In general, the anti- surge recirculation loop is fluidly coupled to the compressor outlet duct and to the compressor inlet duct in order to recirculate partially or totally the compressed flow from the discharge to the suction of the compressor.
[0005] In particular, the anti-surge recirculation loop comprises an anti-surgevalve controlled by a control unit. The control unit calculates the operation point of the compressor, depending on the distance of the calculated operation point with respect to the surge limit line (=SLL), opens / closes the anti-surge valve. In particular, when there in the risk of surge, i.e. when the calculated operating point (=OP) is near the SLL, the control unit opens (fully or partially) the anti- surge valve to establish a recirculation flow in the recirculation loop and to return (fully or partially) the fluid discharged from the compressor to the inlet of the compressor, in order to reduce the pressure ratio of the compressor and avoid compressor surge.
[0006] From an operational point of view, after having calculated the surge limitline and plotted it on a graph as a function of the pressure ratio and the corrected mass flow, the control unit continuously (i.e. with very short time intervals, such as every 40 milliseconds) calculates the compressor operating point and plot it on the graph with the surge limit line. Finally, the control unit calculates the distance between the SLL and the operating point in order to regulate the opening of the anti-surge control valve.
[0007] However, when the fluid to be compressed is in supercritical condition,the calculation of the surge limit line is dependent from temperature and pressure of the fluid and therefore it requires to be recalculated at each fluid temperature / pressure variation, making the control of the anti-surge recirculation control very unstable.
[0008] Therefore, it would be desirable to have methods and systems forovercoming the problem of surge specifically for centrifugal compressors working with a fluid in supercritical conditions, in particular with sCO2, in aneasy and effective way.
[0009] Accordingly, it would be desirable to have a graphical representationof the surge line limit independent from temperature and pressure of the fluid so that the calculation of the surge limit line can be done offline, corrected using experimental results, but it is not necessary to recalculate the surge line limit for each fluid temperature / pressure variation. SUMMARY
[0010] According to a first aspect the subject matter disclosed hereinrelates to a method for performing anti-surge control in a centrifugal compressor working with a fluid in supercritical conditions comprises: calculating an isentropic exponent in volume ^^^at a compressor suction; calculating a compressor operating point OP, at a compressor suction or at a compressor discharge, as a function of the isentropic exponent in volume ^^^; and calculating a distance between the compressor operating point OP and the surge limit line.
[0011] According to a second aspect, the subject matter disclosed hereinrelates to an anti-surge control system for a centrifugal compressor working with a fluid in supercritical conditions; the system being configured to carry out the method and comprising a centrifugal compressor, an anti-surge recirculation loop, and a control unit. BRIEF DESCRIPTION OF THE DRAWINGS.
[0012] A more complete appreciation of the disclosed embodiments ofthe invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanying drawings, wherein: Fig. 1 illustrates a flow diagram of an embodiment of innovative method in a compressor; Fig.2 illustrates a flow diagram of an embodiment of the steps to calculate a compressor operating point OP of innovative method, when a flow element is inserted at a compressor suction; Fig. 3 illustrates a simplified schematic view of a compressor with a flow element inserted at a compressor suction; Fig. 4 illustrates a flow diagram of an embodiment of the steps to calculate a compressor operating point OP of innovative method when a flow element is inserted at a compressor discharge; Fig.5 illustrates a simplified schematic view of a centrifugal compressor with a flow element inserted at a compressor discharge; Fig.6 illustrates a diagram of an example of representation of surge limit line SLL and the operating compressor points OP. DETAILED DESCRIPTION OF EMBODIMENTS
[0013] According to the subject matter disclosed herein, the problem ofsurge in centrifugal compressors working with fluid in supercritical conditions is solved through an innovative method based on measuring the distance from the compressor operating point to the new graphical representation of the surge limit line as a function of an isentropic exponent ^^^.
[0014] The method is able to recover surge limit line (SLL), independentfrom the values of the temperature and pressure of the fluid. The compressor operating point OP is calculated starting from a combination of non-dimensionalparameters so that result is invariant, and the surge limit line is invariant.
[0015] The surge limit line is a graphical representation of therelationship between the reduced head ℎ^^^and the corrected mass flow ^^^^as a function of the isentropic exponent ^^^at a suction.
[0016] The invariance of the surge limit line, with the new graphicalrepresentation of surge limit line, is recovered which then no longer depends on the values of pressure and temperature. So, the calculation of the SLL can be done offline, it does not need to be recalculated at each condition at a suction, and at a discharge.
[0017] Referring now to the drawings, Fig. 1 shows a flow chart of anembodiment of an anti-surge control method 1000 that may be used for example in the centrifugal compressor 100 of Fig. 3, working with a fluid in supercritical conditions, preferably the fluid is ^^^in supercritical conditions, implemented by a programmable logic controller.
[0018] The flow chart has a start block 1100 and an end block 1400; thephases corresponding to block 1200 to block 1300 are typically iterated every 40 milliseconds during anti-surge control of the centrifugal compressor 100, through programmable logic controller, the phases are:- calculating (see block 1200) a compressor operating point OP, at acompressor suction 120 or at a compressor discharge 130, as a function of an isentropic exponent in volume ^^^;- calculating (see block 1300) a distance between the compressor operatingpoint OP and a surge limit line 200.
[0019] Referring to Fig. 2 e Fig. 3 a flow element device 110, for examplea Venturi flow meter, is inserted at a compressor suction 120, and the step of the calculation of at least compressor operating point OP 1200 of the method 1000 comprises the steps of:- acquiring 1210 the values of pressure and temperature of the fluid at thecompressor suction ^^, ^^ and values of pressure and temperature of the fluid atthe compressor discharge ^^,^^;- acquiring 1220 a value of differential pressure at the compressor suction^^^- calculating 1230 a compressibility factor at the compressor suction ^^and a compressibility factor at the compressor discharge ^^;- calculating 1240 the isentropic exponent in volume ^^^ at a suction;- calculating 1250 a polytropic exponent;- calculating 1260 an operating point reduced head ℎ^^^;- calculating 1270 an operating point corrected mass flow ^^^^;- obtaining 1280 the compressor operating point OP as a function ofreduced head ℎ^^^and corrected mass flow ^^^^.
[0020] In the acquiring step 1220 the value of differential pressure atcompressor suction ^^^, is obtained by measuring the pressure difference at two different points in the compressor 100. The value of differential pressure ^^^is a flow element measurement at compressor suction, a function of flow rate calculation.
[0021] The step of calculating the compressibility factors 1230 allows toobtain the compressibility factor at the compressor suction ^^and thecompressibility factor at the compressor discharge ^^ from a fluid state equationas CO2 state equation: ^=^^^^^^ wherein ^ is the density of the fluid, wherein P is the pressure, wherein Mwis the molecular weight, wherein Z is the compressibility factor, wherein ^ is the temperature, wherein R is the universal gas constant.
[0022] The isentropic exponent ^^^ depends on the values of pressure andtemperature of the fluid at the compressor suction ^^, ^^, wherein the values of pressure and temperature of the fluid at the compressor suction ^^, ^^are obtained from sensors located at the compressor suction, and it is calculated preferably from a CO2 state equation. The step of calculating the isentropicexponent ^^^ 1240 allows to obtain the isentropic exponent ^^^ as a function ofthe values of pressure and temperature of the fluid at the compressor suction ^^, ^^as follow:wherein V is the specific volume of fluid. wherein P is the pressure of fluid.
[0023] The step of calculating the polytropic exponent 1250 allows toobtain the polytropic exponent as a function of the compressibility factor at the compressor suction ^^and of the compressibility factor at the compressor discharge ^^, as follow:
[00024] The step of calculating the operating point reduced head ℎ^^^1260 allows to obtain the operating point reduced head ℎ^^^as a function of: the polytropic exponent; the value of the pressure of the fluid at the compressor suction ^^; the value of the pressure of the fluid at the compressor discharge ^^; and the isentropic exponent ^^^, as follow :
[00025] The step of calculating the operating point flow corrected ^^^^1270 at the compressor suction 120 allows to obtain the operating point flow corrected ^^^^as a function of: the value of differential pressure at the compressor suction ^^^; the value of the pressure of the fluid at compressor suction ^^; and the isentropic exponent ^^^as follow:
[00026] Therefore, in the step obtaining the compressor operating point1280, the calculation of the operating point corrected mass flow ^^^^, and the operating point reduced head ℎ^^^allows to obtain coordinates of at least operating point of compressor 100 ,and to graphical representation of at leastoperating point as function of the corrected mass flow ^^^^and of the reduced head ℎ^^^as shown in the Fig.6.
[0027] Referring to Fig. 4 e Fig. 5 the flow element device 110, forexample a Venturi flow meter, is inserted at a compressor discharge 130, and the step of the calculation of the compressor operating point OP 1200 of the method 1000 comprises the steps of:- acquiring 1210 the values of pressure and temperature of the fluid at thecompressor suction ^^, ^^ and values of pressure and temperature of the fluid atthe compressor discharge ^^,^^;- acquiring 1225 a value of differential pressure at the compressordischarge ^^^;- calculating 1230 a compressibility factor at the compressor suction ^^and a compressibility factor at the compressor discharge ^^;- calculating 1240 the isentropic exponent in volume ^^^ at a suction;- calculating 1250 a polytropic exponent;- calculating 1260 an operating point reduced head ℎ^^^;- calculating 1275 an operating point corrected mass flow (^^^^);- obtaining 1280 the compressor operating point OP as a function ofreduced head (ℎ^^^) and corrected mass flow (^^^^).
[0028] In the acquiring step 1225 the value of differential pressure atcompressor discharge ^^^, is a flow element measurement at compressor discharge 130, obtained by measuring the pressure difference at two different points in a compressor.
[0029] The value of the differential pressure ^^^ calculated at thecompressor discharge 130 multiplied by a relation between a density of fluid at discharge and a density of fluid at suction, is equivalent at the value of differential pressure ^^^calculated at a compressor suction 120:
[0030] The step of calculating the operating point corrected mass flow^^^^1275 at the compressor discharge 130 allows to obtain the operating point corrected mass flow ^^^^as follow:whereinwherein
[0031] In this case the operating point corrected mass flow ^^^^ isexpressed as a function of the value of differential pressure at the compressor discharge ^^^, the value of the temperature at the compressor suction ^^, thevalue of temperature at the compressor discharge ^^, the compressibility factorat the compressor suction ^^, the compressibility factor at the compressordischarge ^^, the value of the pressure of the fluid at compressor suction ^^, the value of the pressure of the fluid at the compressor discharge ^^and the isentropic exponent ^^^as follow:
[0032] Therefore, in the step obtaining the compressor operating point1280, the calculation of the operating point corrected mass flow ^^^^, and the operating point reduced head ℎ^^^allows to obtain coordinates of at least operating point of compressor 100 and to graphical representation of at least operating point as function of the corrected mass flow ^^^^and of the reduced head ℎ^^^as shown in the Fig.6.
[0033] In particular the compressor operating point OP calculate with theinnovative method 1000, when the flow element device 110 is inserted at compressor suction 120, corresponds to the compressor operating point OP calculate with the innovative method 1000, when the flow element device 110 is inserted at compressor discharge 130.
[0034] Referring to Fig.1 the phase of calculating distance 1300 allowsto calculate at least a value of distance between the compressor operating point OP and a surge limit line 200.
[0035] Such as possibility is shown in Fig.6 a graphic with correctedmass flow ^^^^along the x-axis, and reduced head ℎ^^^along the y-axis, show a graphical representation of the surge limit line 200 as a function of corrected mass flow ^^^^and reduced head ℎ^^^, and a graphical representation of at least characteristic curve 210 of a compressor 100 as a function of a centrifugalcompressor speed with a plurality of compressor operating points.
[0036] According to Fig.6 the centrifugal compressor 100 is endowed bya surge limit line 200 independent from the values of the temperature and pressure of the fluid, the surge limit line 200 is a graphical representation of the relationship between the reduced head ℎ^^^and the corrected mass flow ^^^^as a function of the isentropic exponent in volume ^^^.
[0037] An anti-surge control system for the centrifugal compressor,working with a fluid in supercritical conditions, is configured to carry out the innovative method 100, in particular is able to evaluate at least the value of distance calculated with the phase of calculating distance 1300 between the at least operating point OP and the surge limit line 200 of the innovative method 1000.
[0038] In particular the anti-surge control system comprising:- the centrifugal compressor,- an anti-surge re-circulation loop, it includes anti-surge valve fluidlycoupled to the compressor outlet duct and to the compressor inlet duct;- the control unit, it is able to do a control of opening / closing of the anti-surge valve as a function of the at least value of the distance between the at least operating point OP and the surge limit line 200, calculated through the innovative method 1000.
[0039] In fact the control unit is able to evaluate :- if the value of distance calculated is below a fixed value, the control unitinduces the opening (totally or partially) of the anti-surge valve, in order to recirculate partially or totally the compressed flow from the discharge to thesuction of the compressor;- if the value of distance calculated is above a fixed value, the control unitinduces the closing (totally or partially) of the anti-surge valve, in order to avoid compressor surge.
Claims
CLAIMS 1. A method (1000) for performing anti-surge control in a centrifugal compressor (100) working with a fluid in supercritical conditions, wherein the centrifugal compressor (100) is endowed by a surge limit line (200) as a function of an isentropic exponent ^^^, comprising the phases of:- calculating a compressor operating point OP, as a function of theisentropic exponent in volume (^^^);- calculating a distance between the compressor operating point OP andthe surge limit line (200), wherein calculating compressor operating point OP comprises a step of calculating an operating point reduced head (ℎ^^^), and a step of a calculating the isentropic exponent in volume (^^^) at a compressor suction; wherein the step of calculating an operating point reduced head (ℎ^^^), allows to obtain the operating point reduced head (ℎ^^^) is a function of: a polytropic exponent, a value of the pressure of the fluid at the compressor suction (^^), a value of the pressure of the fluid at the compressor discharge (^^) and the isentropic exponent (^^^) as follow:
2. The method of claim 1, wherein calculating compressor operating point OP comprises moreover the steps of:- acquiring the values of pressure and temperature of the fluid at thecompressor suction (^^, ^^), and values of pressure and temperature of the fluid at the compressor discharge (^^,^^);- acquiring a value of differential pressure at the compressor suction (^^^)or acquiring a value of differential pressure at the compressor discharge (^^^);- calculating a compressibility factor at the compressor suction (^^) and acompressibility factor at the compressor discharge (^^);- calculating a polytropic exponent;- calculating an operating point corrected mass flow (^^^^);- obtaining the compressor operating point OP as a function of reducedhead (ℎ^^^) and corrected mass flow (^^^^).
3. The method of claim 1 wherein the surge limit line is a graphical representation of the relationship between a reduced head (ℎ^^^) and a corrected mass flow (^^^^) as a function of the isentropic exponent in volume (^^^) at a suction.
4. The method of claim 1, wherein the isentropic exponent (^^^) depends on the values of pressure and temperature of the fluid at the compressor suction (^^, ^^), wherein the values of pressure and temperature of the fluid at the compressor suction (^^, ^^) are obtained from sensor(s) located at the compressor suction (120) as follow:wherein V is the specific volume of the fluid 5. The method of claim 1, wherein the method is implemented by a programmable logic controller.
6. The method of claim 1, wherein if the distance between the operatingpoint OP and the surge limit line (200) exceeds is below a fixed value, an anti- surge valve is totally or partially opened.
7. The method of claim 6, wherein a control opening / closing of the anti- surge valve is carried out as a function of the distance.
8. The method of claim 1, wherein the fluid is ^^^in supercritical conditions.
9. The method of claim 1, wherein steps are iterated, in particular every 40 milliseconds.
10. The method of claim 2, wherein the compressibility factor at the compressor suction (^^) and the compressibility factor at the compressor discharge (^^) are calculate preferably from state equations of the fluid.
11. The method of claims 1 and 2, wherein the calculate polytropic exponent depends on the compressibility factor at the compressor suction (^^) and on the compressibility factor at the compressor discharge (^^) as follow.
12. The method of claim 2, wherein the operating point corrected mass flow (^^^^) at the compressor suction (120) is a function of: the value of differential pressure at the compressor suction (^^^), the value of the pressure of the fluid at compressor suction (^^) and the isentropic exponent (^^^) as follow:
13. The method of claim 2, wherein the operating point corrected mass flow (^^^^) at the compressor discharge (130) is a function of the value of differential pressure at the compressor discharge (^^^), the value of the temperature at the compressor suction (^^), the value of temperature at the compressor discharge(^^), the compressibility factor at the compressor suction (^^), thecompressibility factor at the compressor discharge (^^), the value of the pressure of the fluid at compressor suction (^^), the value of the pressure of the fluid at the compressor discharge (^^) and the isentropic exponent (^^^)as follow:
14. The method of claim 2, wherein the value of the differential pressure (^^^) calculated at a compressor discharge (130) multiplied by a relation between a density of fluid at discharge and a density of fluid at suction, is equivalent at the value of differential pressure (^^^) calculated at the compressor suction (120).
15. An anti-surge control system for a centrifugal compressor working with a fluid in supercritical conditions, the system being configured to carry out the method of any of the preceding claims, wherein the anti-surge control system comprising a centrifugal compressor, an anti-surge recirculation loop, and a control unit.