DESCRIPTION TECHNICAL FIELD

The present disclosure relates to compressor control methods and systems. Embodi- ments disclosed herein specifically relate to wet gas compressors, in particular centrifugal wet gas compressors, which process gas that can contain a liquid phase, e.g. heavy hydrocarbons, water or the like.

BACKGROUND ART

Centrifugal compressors have been designed to process a so-called wet gas, i.e. gas that can contain a certain percentage of a liquid phase. Wet gas processing is often required in the oil and gas industry, where gas extracted from a well, such as a subsea well, can contain a liquid hydrocarbon phase, or water.

The presence and percentage amount of a liquid phase in a gas may affect the operation of the compressor and in particular may have an impact on the surge limit, which de- termines the range of safe operation of the compressor. Usually, the liquid volume fraction in the gas flow at the suction side of the compressor, however, is not known. Flowmeters capable of determining the liquid volume fraction are cumbersome and expensive and might not be suitable in certain applications in extreme environmental conditions. A need therefore exists, for reliably and efficiently controlling the operation of a wet gas compressor, in particular as far as anti-surge is concerned.

SUMMARY

According to a first aspect, a method for anti-surge protection of a compressor under wet gas conditions is disclosed herein. The compressor comprises a suction side and a delivery side. An anti-surge system is arranged between the delivery side and the suction side of the compressor. According to embodiments disclosed herein the method comprises the following steps: calculating a surge limit line in a compression ratio vs. corrected power diagram; determining a compressor operating point in said compression ratio vs. corrected power diagram; detecting a distance between the operating point and the surge limit line; acting on the anti-surge system of the compressor if the distance is below a minimum safety distance.

According to a further aspect, disclosed herein is a wet gas compressor system comprising: a compressor having a suction side and a delivery side; an anti-surge control arrangement; a control unit, functionally coupled to the anti-surge control arrangement. The control unit is configured and arranged for performing a method as above defined.

The compression ratio vs. corrected power diagram is a diagram wherein the compressor performances are represented as a function of the relationship between the compression ratio over the compressor and the corrected power of the compressor. Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the draw- ings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig.1 illustrates a compressor system; Fig.2 illustrates a wet gas compressor operating diagram;

Fig.3 illustrates a flow chart of methods disclosed herein.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic de- scribed in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. Fi . 1 schematically illustrates a system 1 comprising a driver 3 and a load 5. The load 5 includes a compressor 7, for instance a centrifugal compressor. A shaft 9 driv ingly connects the driver 3 to the load 5. The driver 3 can be an electric motor, a gas turbine engine, a steam turbine or any other suitable driver.

The compressor 7 comprises a compressor suction side 7S and a compressor delivery side 7D. The compressor 7 is further provided with an anti-surge system. In the schematic of Fig. 1 the anti-surge system is comprised of a line or duct 1 1 that is fluidly coupled to the delivery side 7D and to the suction side 7S. Furthermore, the anti-surge system comprises an anti-surge valve 13 arranged on the anti-surge line 1 1. The antisurge valve 13 can be controllably opened to recirculate gas from the del ivery s ide 7D to the suction side 7S of compressor 7, to prevent surge phenomena in the compressor, if the operating point of the compressor approaches a surge limit line.

In some embodiments a pressure transducer 17 and a temperature transducer 19 are arranged at the suction side 7S of compressor 7, to measure the gas suction pressure Ps and the gas suction temperature Ts of the gas at the suction side IS. Moreover, a further pressure transducer 19 and a further temperature transducer 21 are arranged at the deliv ery side 7D of compressor 7, to measure the gas delivery pressure Pd and the gas delivery temperature Td.

The system 1 further comprises a control unit 23 , which can be functionally coupled to the pressure and temperature transducers 15, 17, 19, 21 to collect measured values of the gas temperature and pressure at the del iv ery side 7D and suction side 7S of compressor 7. The control unit 23 can be further functionally coupled to an actuator 13A configured and arranged for selectively opening and closing the anti-surge valve 13. Reference number 25 generally designates storage memory resources for the control unit 23, which can store data useful for an anti-surge control of the compressor 7, as will be explained in greater detail herein after.

The control unit 23 can be configured and arranged for receiving further input information, such as data on the gas processed by compressor 7. Block 27 schematically represents a data input, for instance providing information on the mean molar mass Mw of the gas being processed by compressor 7.

Reference number 29 schematically designates one or several further process parameter transducers, which prov ide additional information to the control unit 23, such as for instance the rotational speed N of compressor 7, the driving power W required to drive the compressor 7 into rotation and any additional information which may be useful or necessaiy for controlling the system 1.

Anti-surge control of the compressor 7 can be performed using the diagram of Fi .2. The compression ratio, or pressure ratio, PR of compressor 7 is plotted on the vertical axis of the diagram of Fig. 2. A dimensionless parameter depending upon the absorbed power, i.e. the power required to drive the compressor 7 into rotation, is plotted on the horizontal axis of the diagram of Fig. 2. The dimensionless parameter is a function of the actual driving power W, the suction pressure Ps and the suction temperature Ts of the gas, and can further depend upon parameters of the gas being processed and of characteristics of the compressor.

According to some embodiments, on the horizontal axis of the diagram in Fig. 2 the dimensionless corrected power Wcorr is plotted, defined by the following formula: wherein:

W is the actual measured power absorbed by the compressor 7;

Ps, Ts are the gas pressure and temperature at the suction side of the compressor 7;

Mw is the mean molar mass of the gas processed by compressor 7; Zs is the compressibility of the gas at the compressor suction side; R is the gas constant; kvs is the isentropic volume exponent of the gas at the compressor suction side;

D is the impeller diameter. It has been discovered that for a given set of operating parameters a suction limit line SLL can be plotted on the diagram of Fig. 2, which allows anti-surge control of the compressor 7 without requiring knowledge of the actual liquid mass fraction (LMF) or liquid volume fraction (LVF) of the gas, i.e. the mass or volumetric percentage of liquid phase in the wet gas. For a given compressor 7, the SLL is a function of the gas conditions at the suction side 7S of compressor 7, i.e. of the suction temperature Ts and the suction pressure Ps. Additionally, the SLL is a function of the rotational speed of compressor 7, as well as of the mean molar mass Mw of the gas and of the compressibility Zs of the gas at the suction side 7S of compressor 7. Thus, the SLL can be expressed as follows: SLL = f(Ts, Ps, Zs, Mw, N) (2)

Some of the parameters appearing in the function which defines the surge limit line SLL, specifically the mean molar mass Mw and the compressibility Zs at the suction side depend upon the chemical composition of the gas. The chemical composition of the gas processed by compressor 7 usually varies very slowly during time and can be considered quasi-constant over relatively long time spans, e.g. 24 hours. The chemical composition of the gas can be analyzed in-line by flowing a portion of gas through a gas chromatograph. In other embodiments, the gas composition can be analyzed offline, e.g. by taking a gas sample from the gas duct. Irrespective of how the gas is analyzed, the mean molar mass and the compressibility of the gas can be determined. The remaining parameters can be detected by the transducers o system 1 during operation of the compressor 7. The surge limit line SLL extends from a first end point corresponding to a dry gas condition (Liquid Mass Fraction, LMF = 0%) to a second end point corresponding to the maximum l iquid content (LMF= LMFmax).

During operation of the system 1 , therefore, the current SLL can be determined, based on features of the compressor, parameters of the gas being processed and operating parameters of the system 1 , which are detected by the transducers functionally coupled to the control unit 23. Based upon the detected values of suction pressure (Ps), suction temperature (Ts), angular speed (N), mean molar mass (Mw) and compressibility (Zs), the control unit 33 calculates the current suction limit line SLL, based on store data, e.g. in table form, and/or by interpolation. The data for the calculation of the SLL can be stored in the storage memoiy resources 25. Additionally, based on the above mentioned data and on the actual power W currently absorbed by compressor 7, the corrected power Wcorr is calculated with formula (1). The actual operating point of compressor 7 is determined, the operating point having the coordinates [Wcorr; PR=Pd/Ps] in the diagram of Fig. 2. The distance between the actual operating point and the calculated SLL is then determined. Based on said distance, an anti-surge control routine is started, if needed, to control the opening of the anti-surge valve. The anti-surge valve can be controlled according to current art methods. In general, if the distance is less than a safety value, the anti-surge val ve 13 is opened. If the distance is equal to or greater than a safety value, the anti-surge valve 13 is maintained in the closed condition.

The control method described so far is summarized in the flow chart of Fig. . The last block of the flow chart represents an anti-surge valve control.

While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in con- nection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.