Patent Application
20190024528
Embodiments of the invention relate to centrifugal compressor trains comprising one or more centrifugal compressors and a driver machine. Embodiments disclosed herein concern methods and systems for operating the start-up of centrifugal compressor trains.
Centrifugal compressors are used in several industrial applications to boost the pressure of process gas. In the field of oil and gas, for instance, centrifugal compressors are used to process refrigerant fluids in plants for liquefaction of natural gas (LNG plants). Refrigerant fluids, such as mixed refrigerant, propane and the like, are used in such plants to remove heat from a flow of natural gas which is extracted from a gas field to chill and liquefy the natural gas for transportation purposes. The centrifugal compressors are driven by a driver, which may include an electric motor or a gas turbine engine, for instance.
Single shaft gas turbine engines operating at quasi-fixed speed and fixed-speed electric motors have a reduced torque capability at start-up, which causes problems when the centrifugal compressor train must be started up following shut down, from pressurized conditions. The startup suction density can be much higher than the nominal one, and could cause startup issues also when variable speed drivers are employed, such as aero-derivative gas turbine engines, steam turbines and variable speed electric motors. In such situations, even if the driver has a nominal torque capability from low speed, the startup torque can be higher than the nominal torque, causing failure of the startup sequence.
The fluid circuit, whereof the centrifugal compressor train forms part, shall be emptied, or the amount of fluid therein shall be reduced, in order to limit the torque required to start-up the turbomachine. Emptying and subsequently refilling the fluid circuit is a time consuming and expensive operation.
A need therefore exists, for improvement of plants and systems involving centrifugal compressors, aimed at alleviating or solving drawbacks of the systems of the current art, especially arising at start-up.
According to a first aspect, a method for operating start-up of a compressor train is disclosed herein. The compressor train comprises a driver machine and at least a centrifugal compressor drivingly coupled to the driver machine. The centrifugal compressor is a further machine and not a part of the driver machine. The centrifugal compressor in turn comprises a plurality of compressor stages and at least a first set of variable inlet guide vanes (herein after shortly also IGVs) at an inlet of one of said compressor stages. The method comprises the following steps:
at least partly closing the first set of variable inlet guide vanes;
starting rotation of the centrifugal compressor and accelerating the centrifugal compressor up to a minimum operating speed, while the variable inlet guide vanes are at least partly closed;
opening the at least one set of variable inlet guide vanes to increase the gas flow through the centrifugal compressor once the minimum operating speed has been achieved.
According to some embodiments, the minimum operative speed can be the minimum allowable speed of the machine. The minimum allowable speed can be defined according to API 617 standard for “Axial and Centrifugal Compressors and Expander-Compressors for Petroleum, Chemical and Gas Industry Services”. The API 617 standard defines the minimum allowable speed as the lowest speed (in rpm) at which the manufacturer's design will permit continuous operation.
When the driver machine is an electric motor, the minimum operating speed can be the rated speed or the synchronization speed (i.e. the speed of synchronization of the electric motor to the electric grid frequency).
The partly closed inlet guide vanes reduce the gas flow through the compressor and thus the torque required to drive the compressor. A larger torque margin is available to accelerate the centrifugal compressor.
The variable inlet guide vanes can be arranged at the inlet of the most upstream one of the compressor stages. In some embodiments, additional variable inlet guide vanes can be arranged at the inlet side of one or more additional compressor stages. For instance, the compressor may have a back-to-back configuration, with variable inlet guide vanes at the inlet of the first (i.e. the most upstream one) compressor stage of each one group of compressor stages in back-to-back arrangement. This makes the configuration of the compressor particularly simple and affordable.
While according to some embodiments the step of at least partly closing the first set of variable inlet guide vanes can be performed prior to starting rotation of the centrifugal compressor, according to other embodiments, rotation of the compressor can be initiated prior to closing the variable inlet guide vanes. These latter are closed timely, once rotation has started and prior to reaching a steady state rotary speed, for instance, such that the torque margin does not drop under unacceptable values.
In some embodiments the driver machine comprises a single-shaft gas turbine engine and a starter. During the step of starting rotation of the centrifugal compressor and accelerating the centrifugal compressor, power generated by the starter is applied to the centrifugal compressor through the single shaft of the gas turbine engine.
According to a further aspect, disclosed herein is a gas compressor train comprising a centrifugal compressor comprised of a compressor inlet, a compressor outlet, a plurality of compressor stages sequentially arranged between the compressor inlet and the compressor outlet, and at least a first set of variable inlet guide vanes at an inlet of one of said compressor stages. The compressor train further comprises a driver machine drivingly coupled to the centrifugal compressor, and a controller, configured and arranged for at least partly closing the set of variable inlet guide vanes during a start-up and acceleration step of the centrifugal 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 embodiments 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 drawings. 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.
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:
The following detailed description of 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 described 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.
In other embodiments, the driver machine may include a gas turbine engine. In some exemplary embodiments, the driver machine includes a single-shaft gas turbine engine. In combination with a gas turbine engine, the driver machine can comprise a starter, for example an electric machine operating as a motor to start up the compressor train.
The multi-stage centrifugal compressor 1 can be one of a plurality of refrigerant compressors of an LNG plant. For instance, the multi-stage centrifugal compressor 1 can be a mixed refrigerant compressor or a propane compressor. The multi-stage centrifugal compressor 1 compresses the refrigerant fluid from a suction pressure to a delivery pressure and circulates the refrigerant fluid in a closed refrigerant circuit. The compressed refrigerant fluid is then expanded in an expander or in an expansion valve arrangement, to reduce the temperature thereof. The chilled expanded refrigerant fluid is placed in heat exchange relationship with a flow of natural gas to be chilled and liquefied, to remove heat from a natural gas flow and is subsequently cooled by heat exchange with a heat sink, prior to be processed again by the multi-stage centrifugal compressor 1.
One, some or all the centrifugal compressor stages 3-9 can be provided with variable inlet guide vanes. In the schematic embodiment of
One or more bleed lines, i.e. by-pass lines, can be provided, fluidly connecting the delivery side of one compressor stage to the suction side of another upstream compressor stage. In the exemplary embodiment of
The operating conditions of the multi-stage centrifugal compressor 1 can be controlled by acting upon the variable inlet guide vanes 17. These latter can be acted upon to modify the gas flow rate through the multi-stage centrifugal compressor 1, for instance.
As can be seen from
If the driver machine includes a single-shaft gas turbine engine, a separate starter, such as an electric motor or else a steam turbine, is required to start up the multi-stage centrifugal compressor 1. In such case, the rated power of the starter shall be dimensioned such that sufficient torque is available at start-up. The cost and dimension of the starter become overly high. Additionally, the high power rate provided by the starter cannot be transmitted through the shaft of the gas turbine engine, such that the starter shall be arranged on the end of the shaft line of the compressor train opposite the gas turbine engine. This causes drawbacks in terms of accessibility to the turbomachines and causes problems during maintenance of the centrifugal compressor(s) of the compressor train.
According to some embodiments of the subject matter disclosed herein, to reduce the torque margin drop, and thus cope with the required torque and power at start-up with a less performing starter, the variable IGVs 17 of multi-stage centrifugal compressor 1 are at least partly closed at start-up. This causes a reduction of the process gas flow across the multi-stage centrifugal compressor 1 and consequently a reduction of the torque required driving the multi-stage centrifugal compressor 1 into rotation.
In terms of torque margin, this is reflected in a minimum torque margin at a given compressor speed (see
Additional benefits in terms of torque reduction can be achieved by bleeding part of the process gas from one of the compressor stages back to the suction side of an upstream compressor stage. In the embodiment of
The torque reduction at start-up is achieved by reducing the process fluid flow acting upon the variable IGVs 17. In embodiments, the variable IGVs 17 are closed prior to starting rotation of the multi-stage centrifugal compressor 1. However, this is not strictly necessary. In fact, as shown by the head curves of
The variable IGVs 17 can be re-opened once the multi-stage centrifugal compressor 1 has achieved a minimum operative speed. In case of a turbomachine train operating at fixed or quasi-fixed speed, the minimum operative speed can be the rated speed of the driver machine. In some embodiments, the minimum operative speed can be set as the minimum allowable speed under API 617 standard (see API 617 standard, 2002 edition, paragraph 1.5.22).
In the illustrated embodiment the multi-stage centrifugal compressor 1 comprises a first set 31 of compressor stages and a second set 33 of compressor stages, in a back-to-back arrangement. The gas flow in the first set 31 of compressor stages is generally in the direction of arrow F31, while the gas flow in the second set 33 of compressor stages is generally in the direction of arrow F33. The delivery side of the most downstream compressor stage of the first set 31 is fluidly coupled to the suction side of the most upstream compressor stage of the second set 33.
In some embodiments, a first set of variable inlet guide vanes 17 is arranged at the suction side of the multi-stage centrifugal compressor 1, and a second set of variable inlet guide vanes 18 is arranged at the suction side of the most upstream compressor stage of the second set 33 of compressor stages. One or both sets of variable inlet guide vanes 17, 18 can be controlled to reduce the process gas flow at start-up, thus reducing the total torque required to start-up rotation of the multi-stage centrifugal compressor 1.
In some embodiments, as shown in
The reduction of power obtained at start-up of the multi-stage centrifugal compressor 1 results in the possibility of reducing the rated power of an electric motor used to drive the multi-stage centrifugal compressor 1, or in the possibility of reducing the rated power of a starter used in combination with a prime mover, such as a gas turbine engine, the starter and the gas turbine engine forming part of the driver machine.
A lower power rate results in a smaller and less expensive machine, but results in further advantages, which will become more apparent referring to the arrangement of
The system of
The multi-stage centrifugal compressors 1A, 1B can be both vertically-split, or both horizontally-split compressors. For instance, compressors 1A, 1B can both be MCL series compressors, or both be BCL series compressors. If one of the two compressors is a vertically split compressor, e.g. a BCL series compressor, in some arrangements, said vertically split compressor will be arranged at the end of the compressor train 2, i.e. opposite the driver machine 30.
The driver machine 30 can be comprised of a gas turbine engine 43. For instance, the gas turbine engine 43 can include a single shaft gas turbine engine 43. The gas turbine engine 43 can be comprised of an air compressor section 45, a combustor section 47 and a turbine section 49. Power generated by the turbine section 49 drives the air compressor section 45 and the compressor train 2.
Reference number 51 schematically designates the single shaft of the gas turbine engine 43. Shaft 51 has a first shaft end 51A at the suction side, or cold side, of the gas turbine engine 43, and a second shaft end 51B at the pressure side, or hot side, of the gas turbine engine 43. Contrary to commonly used configurations of the current art, wherein an electric starter is arranged at the end of the compressor train 2 opposite to the gas turbine engine 43, in the disclosed configurations an electric machine 53 operating as a starter is drivingly coupled to the first end 51A of the shaft 51. The electric machine 53 can be a reversible electric machine, capable of operating selectively in a generator mode or in a motor mode. The electric machine 53 can thus operate as a starter and as a helper when operating in the electric motor mode, or alternatively as an electric generator. The second end 51B of the shaft 51 is drivingly coupled to the compressor train 2.
The same driver machine 30 can be used in combination with the configurations of
By operating upon the IGVs 17 and/or 18 of one or more of the multi-stage centrifugal compressors 1A, 1B at start-up, the gas flow rate through the gas compressor train 2 at start-up is reduced. A smaller amount of power is thus needed, if compared to centrifugal compressor train of the current art, to start the compressor train. Thus, the total power rate of the electric machine 53 can be lower than in plant configurations of the current art.
The electric machine 53 can thus be arranged on the side of the gas turbine engine 43 opposite the compressor train 2, thanks to the reduced mechanical power, which is required to be transmitted through the single shaft 51 across the gas turbine engine 43.
By arranging the starter at the side of the gas turbine engine 43 opposite the compressor train 2, maintenance of the compressor train is made easier. In particular, opening of the vertically split multi-stage centrifugal compressor 1B and access to the interior thereof is less critical and can be obtained without dismantling the line shaft.
While the above described embodiments are specifically referred to LNG applications, innovative features disclosed herein can be embodied also in other systems where similar problems are encountered at compressor start-up.
Additionally, while the above description specifically refers to driver machines including single-shaft gas turbines, advantages can be achieved by the use of variable IGV at start-up also when the driver machine comprises an electric motor. The rated power, size and thus cost of the electric motor can be reduced, if a lower power at start-up is required.
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 connection 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.
This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102016000006952 | Jan 2016 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/051319 | 1/23/2017 | WO | 00 |
