AU2013204886A1

AU2013204886A1 - Compressor System and Method for Compressing

Info

Publication number: AU2013204886A1
Application number: AU2013204886A
Authority: AU
Inventors: Geoffrey Brian Byfield; Prabhu Pandalaneni
Original assignee: Woodside Energy Ltd
Current assignee: Woodside Energy Technologies Pty Ltd
Priority date: 2013-04-12
Filing date: 2013-04-12
Publication date: 2014-10-30
Anticipated expiration: 2033-04-12
Also published as: AU2013204886B2

Abstract

A compressor system is described. The compressor system comprises a first compressor 5 section arranged back-to-back with a second compressor section in a common compressor body; a first compression stage having an inlet for receiving a first gas stream at a first pressure and compressing the first gas stream to produce a first intermediate gas stream at a second pressure; a second compression stage having an inlet for receiving a second gas stream at a third pressure and compressing the second gas stream to produce a second 10 intermediate gas stream at a fourth pressure; a third compression stage having an inlet for receiving a third gas stream at a fifth pressure and compressing the third gas stream to produce a third intermediate gas stream at a sixth pressure; wherein either: (i) the first compression stage is arranged in the first compressor section and the second and third compression stages are arranged in the second compressor section; or (ii) the first and 15 second compression stages are arranged in the first compressor section and the third compression stage is arranged in the second compressor section; or, (iii) the first and third compression stages are arranged in the first compressor section and the second compression stage is arranged in the second compressor section. (I' w ~*1

Description

1 Compressor System and Method for Compressing TECHNICAL FIELD 5 The present invention is directed to a compressor system. The invention particularly, though not exclusively, directed specifically to a compressor system for a refrigerant compression circuit for use in the liquefaction of natural gas or other methane-rich gas streams. 10 BACKGROUND The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Liquefaction of large volumes of gas using a refrigerant circuit is energy and capital intensive. Broadly speaking, a plant for liquefying natural gas comprises a main heat exchanger in which a 15 hydrocarbon gas feed stream is liquefied by means of indirect heat exchange with evaporating refrigerant in one or more stages. The plant further comprises a refrigerant circuit in which evaporated refrigerant(s) are compressed, cooled and returned to the main heat exchanger. The refrigerant circuit typically includes a compressor train consisting of at least one compressor body driven by means of a mechanical driver (e.g., a gas turbine, a 20 steam turbine, or an electric motor) that is connected to the shaft of the compressor body via a common shaft or via a gearbox. The configuration of compressors and mechanical drivers in a gas processing plant greatly influences the physical capabilities of the equipment and/or the energy efficiency of the plant. 25 New gas liquefaction and other gas processing plants are being designed for ever increasing production rates in order to realize the favourable economic benefits associated with larger plants. These larger plants have larger refrigeration duties with higher refrigerant circulation rates, and therefore larger refrigerant compressors are required. The amount of natural gas which can be cooled per unit of time in the refrigerator is 30 proportional to the volumetric flow rate of the refrigerant through the refrigerator. However, there is a practical upper limit to the volumetric flow rate which can be handled by a single large compressor with the result that the maximum achievable production rate 2 is being limited by the maximum available compressor sizes for a given shaft rotational speed. Several alternative methods have been proposed in the art to compress large refrigerant 5 flows in a multi-level refrigeration system. US Patent Number 6,637,238 describes an apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant. The refrigeration circuit has an inlet, a first outlet for refrigerant at low pressure, a second outlet for refrigerant at intermediate pressure, a third outlet for refrigerant at high pressure and a fourth outlet for refrigerant at high-high pressure. The 10 apparatus comprises a first compressor and a separate second compressor. The first compressor has a main inlet connected to the first outlet, a side-inlet connected to the third outlet and an outlet connected to the inlet of the refrigeration circuit, and wherein the second compressor has a main inlet connected to the second outlet, a side-inlet connected to the fourth outlet and an outlet connected to the inlet of the refrigeration circuit. This 15 arrangement which relies on the use of two separate compressor bodies has not been used commercially. US Patent Number 6,962,060 describes a compressor system comprising a first compressor having a first stage and a second stage wherein the first stage of the first compressor is 20 adapted to compress a first gas and the second stage of the first compressor is adapted to compress a combination of a fourth gas and an intermediate compressed gas from the first stage of the first compressor. This compressor system includes a separate second compressor having a first stage and a second stage wherein the first stage of the second compressor is adapted to compress a second gas and the second stage of the second 25 compressor is adapted to compress a combination of a third gas and an intermediate compressed gas from the first stage of the second compressor. The first gas is at a first pressure, the second gas is at a second pressure higher than the first pressure, the third gas is at a third pressure higher than the second pressure, and the fourth gas is at a fourth pressure higher than the third pressure. This arrangement which relies on the use of two 30 separate compressor bodies has not been used commercially. Because gas liquefaction and other gas processing plants are being designed for ever increasing production rates in order to realize the favourable economic benefits associated 3 with larger plants, alternative methods are needed to eliminate the size and inlet velocity problems of single large compressors. Embodiments of the present invention, as described below and defined by the claims that follow, provide an alternative refrigerant compressor system for large gas liquefaction and processing plants. 5 SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a compressor system comprising: a first compressor section arranged back-to-back with a second compressor section 10 in a common compressor body; a first compression stage having an inlet for receiving a first gas stream at a first pressure and compressing the first gas stream to produce a first intermediate gas stream at a second pressure, said second pressure being greater than the first pressure; a second compression stage having an inlet for receiving a second gas stream at a 15 third pressure and compressing the second gas stream to produce a second intermediate gas stream at a fourth pressure, said fourth pressure being greater than the third pressure; a third compression stage having an inlet for receiving a third gas stream at a fifth pressure and compressing the third gas stream to produce a third intermediate gas stream at a sixth pressure, said sixth pressure being greater than the fifth pressure; 20 wherein either: (i) the first compression stage is arranged in the first compressor section and the second and third compression stages are arranged in the second compressor section; or (ii) the first and second compression stages are arranged in the first compressor section and the third compression stage is arranged in the second compressor 25 section; or, (iii) the first and third compression stages are arranged in the first compressor section and the second compression stage is arranged in the second compressor section. 30 In one form, the first compressor section and the second compressor section share a common outlet for discharging a final compressed gas stream comprising the first, second and third intermediate gas streams.

4 In one form, each of the first, second and third compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and a shared impeller is arranged at the common outlet. 5 In one form, the system further comprises a fourth compression stage having an inlet for receiving a fourth gas stream at a seventh pressure and compressing the fourth gas stream to produce a fourth intermediate gas stream at an eighth pressure, said eighth pressure being greater than the seventh pressure. 10 In one form, the first and fourth compression stages are arranged in the first compressor section and the second and third compression stages are arranged in the second compressor section. In one form, the first and third compression stages are arranged in the first compressor section and the second and fourth compression stages are arranged in the second compressor section. In one form, the first and second compression stages are 15 arranged in the first compressor section and the third and fourth compression stages are arranged in the second compressor section. In one form, the first compression stage is arranged in the first compressor section and the second, third and fourth compression stages are arranged in the second compressor section. In one form, the first, third and fourth compression stages are arranged in the first compressor section and the second 20 compression stage is arranged in the second compressor section. In one form, the first, second and fourth compression stages are arranged in the first compressor section and the third compression stage is arranged in the second compressor section. In one form, the first, second and third compression stages are arranged in the first compressor section and the fourth compression stage is arranged in the second compressor section. 25 In one form, the first compressor section and the second compressor section share a common outlet for discharging a final compressed gas stream comprising the first, second, third and fourth intermediate gas streams. In one form, each of the first, second, third and fourth compression stages is provided with one or more impellers arranged on a common 30 shaft driven by a driver and a shared impeller is arranged at the common outlet. In one form, the first compressor section has a first outlet for discharging a first final compressed gas stream and the second compressor section has a second outlet for discharging a second final compressed gas stream and the compression system further comprises a piping means 5 for receiving the first final compressed gas stream from the first outlet and the second final compressed stream from the second outlet and combining the first and second final compressed streams downstream of the common compressor body to produce a final compressed gas stream. 5 In one form, any of the first, second, third or fourth gas streams is a refrigerant gas provided from a refrigeration system. In one form, the final compressed gas stream is provided to a refrigeration apparatus adapted to receive the final compressed gas stream and provide refrigeration in a plurality of temperature ranges to produce one or more of; 10 the first gas stream at the first pressure, the second gas stream at the third pressure, the third gas stream at the fifth pressure, and the fourth gas stream at the seventh pressure. In one form, the refrigeration apparatus is adapted to precool natural gas prior to liquefaction. In one form, the common compressor body has a horizontal or vertical split 15 configuration. According to one aspect of the present invention there is provided a method for gas compression comprising: arranging a first compressor section back-to-back with a second compressor section 20 in a common compressor body; providing a first gas stream at a first pressure at an inlet of a first compression stage and compressing the first gas stream to produce a first intermediate gas stream at a second pressure, said second pressure being greater than the first pressure; providing a second gas stream at a third pressure at an inlet of a second 25 compression stage and compressing the second gas stream to produce a second intermediate gas stream at a fourth pressure, said fourth pressure being greater than the third pressure; providing a third gas stream at a fifth pressure at an inlet of a third compression stage and compressing the third gas stream to produce a third intermediate gas stream at a 30 sixth pressure, said sixth pressure being greater than the fifth pressure; and, either: (i) arranging the first compression stage in the first compressor section whilst arranging the second and third compression stages in the second compressor section; or 6 (ii) arranging the first and second compression stages in the first compressor section whilst arranging the third compression stage in the second compressor section; or, (iii) arranging the first and third compression stages in the first compressor section whilst arranging the second compression stage in the second compressor section. 5 In one form, the first compressor section and the second compressor section share a common outlet and the method comprises the step of discharging a final compressed gas stream comprising the first, second and third intermediate gas streams. 10 In one form, each of the first, second and third compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and the method comprises the step of arranging a shared impeller at the common outlet. In one form, the method further comprises the step of providing a fourth gas stream at a 15 seventh pressure at an inlet to a fourth compression stage and compressing the fourth gas stream to produce a fourth intermediate gas stream at an eighth pressure, said eighth pressure being greater than the seventh pressure. In one form, the method comprises the step of arranging the first and fourth compression 20 stages in the first compressor section whilst arranging the second and third compression stages in the second compressor section. In one form, the method comprises the step of arranging the first and third compression stages in the first compressor section whilst arranging the second and fourth compression stages in the second compressor section. In one form, the method comprises the step of arranging the first and second compression 25 stages in the first compressor section whilst arranging the third and fourth compression stages in the second compressor section. In one form, the method comprises the step of arranging the first compression stage in the first compressor section whilst arranging the second, third and fourth compression stages in the second compressor section. In one form, the method comprises the step of arranging the first, third and fourth compression 30 stages in the first compressor section whilst arranging the second compression stage in the second compressor section. In one form, the method comprises the step of arranging the first, second and fourth compression stages in the first compressor section whilst arranging the third compression stage in the second compressor section. In one form, the method 7 comprises the step of arranging the first, second and third compression stages in the first compressor section whilst arranging the fourth compression stage in the second compressor section. 5 In one form, the first compressor section and the second compressor section share a common outlet and the method comprises the step of discharging a final compressed gas stream comprising the first, second, third and fourth intermediate gas streams. In one form, each of the first, second, third and fourth compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and the method comprises the 10 step of arranging a shared impeller at the common outlet. In one form, the first compressor section has a first outlet for discharging a first final compressed gas stream and the second compressor section has a second outlet for discharging a second final compressed gas stream and the method further comprises 15 providing a piping means for receiving the first final compressed gas stream from the first outlet and the second final compressed stream from the second outlet and combining the first and second final compressed streams downstream of the common compressor body to produce a final compressed gas stream. 20 In one form, any of the first, second, third or fourth gas streams is a refrigerant gas provided from a refrigeration system. In one form, the method further comprises the step of providing the final compressed gas stream to a refrigeration apparatus adapted to receive the final compressed gas stream and 25 providing refrigeration in a plurality of temperature ranges to produce one or more of; the first gas stream at the first pressure, the second gas stream at the third pressure, the third gas stream at the fifth pressure, and the fourth gas stream at the seventh pressure. In one form, the refrigeration apparatus is adapted to precool natural gas prior to 30 liquefaction. In one form, the common compressor body has a horizontal or vertical split configuration.

8 DESCRIPTION OF THE DRAWINGS In order to facilitate a more detailed understanding of the nature of the invention embodiments will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 5 FIG. 1 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; FIG. 2 is a schematic illustration of a three-stage compressor system according to 10 one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; FIG. 3 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in 15 which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; FIG. 4 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second 20 compressor section compresses the second gas stream and the third gas stream; FIG. 5 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; 25 FIG. 1 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; FIG. 2 is a schematic illustration of a three-stage compressor system according to 30 one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the third gas stream and the second compressor section compresses the second gas stream; 9 FIG. 3 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second gas stream and the second compressor section compresses the third gas stream; 5 FIG. 4 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the fourth gas stream and the second compressor section compresses the second gas stream and the third gas stream; 10 FIG. 5 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the third gas stream and the second compressor section compresses the second gas stream and the fourth gas stream; 15 FIG. 6 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second gas stream and the second compressor section compresses the third gas stream and the fourth gas stream; 20 FIG. 7 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream, the third gas stream and the fourth gas stream; 25 FIG. 8 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream, the third gas stream and the fourth gas stream and the second compressor section compresses the second gas stream and the third gas stream; 30 FIG. 9 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in which the first compressor section compresses the first gas stream, the second gas stream 10 and the fourth gas stream and the second compressor section compresses the third gas stream; FIG. 10 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a common outlet and a shared impeller in 5 which the first compressor section compresses the first gas stream, the second gas stream and the third gas stream and the second compressor section compresses the fourth gas stream; FIG. 11 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which 10 the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream and the third gas stream; FIG. 12 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the second gas stream and 15 the second compressor section compresses the third gas stream; FIG. 13 is a schematic illustration of a three-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the third gas stream and the second compressor section compresses the second gas stream; 20 FIG. 14 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the fourth gas stream and the second compressor section compresses the second gas stream and the third gas stream; 25 FIG. 15 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the third gas stream and the second compressor section compresses the second gas stream and the fourth gas stream; 30 FIG. 16 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the second gas stream and the second compressor section compresses the third gas stream and the fourth gas stream; 11 FIG. 17 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream and the second compressor section compresses the second gas stream, the third gas stream and the fourth gas stream; 5 FIG. 18 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream, the third gas stream and the fourth gas stream and the second compressor section compresses the second gas stream; FIG. 19 is a schematic illustration of a four-stage compressor system according to 10 one embodiment of the present invention having a first outlet and a second outlet in which the first compressor section compresses the first gas stream, the second gas stream and the fourth gas stream and the second compressor section compresses the third gas stream; FIG. 20 is a schematic illustration of a four-stage compressor system according to one embodiment of the present invention having a first outlet and a second outlet in which 15 the first compressor section compresses the first gas stream, the second gas stream and the third gas stream and the second compressor section compresses the fourth gas stream; FIG. 21 is a schematic illustration of the final compressed gas stream from a three stage compression system being directed to a refrigeration system; and, FIG. 22 is a schematic illustration of the final compressed gas stream from a four 20 stage compression system being directed to a refrigeration system. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in greater detail with reference to the accompanying drawings wherein several preferred embodiments of the present invention 25 are set forth. Those skilled in the art will recognize that the accompanying drawings are schematic representations only and therefore, many items of equipment that would be needed in a commercial plant for successful operation have been omitted for the sake of clarity. Such items might include, for example, compressor controls, flow, level, temperature and pressure controls, pumps, motors, filters, additional heat exchangers, and 30 valves, etc. It will be readily appreciated that a person skilled in the art would be able to include such items in accordance with standard engineering practice.

12 The terms "compressor", "compressor body" and "compressor casing" as used herein refer to a rotary device having a shaft and which is used to increase the pressure of an incoming fluid by decreasing its volume. A compressor may have one or more compressor sections with each section having one or more compression stages with each stage having an inlet 5 for receiving the fluid to be compressed and means to let the compressed fluid exit the stage. Where more than one compression stage is present in a compressor section, the mass flow through each compression stage will be equal to the mass flow entering the compressor section at that stage plus the mass flow of all gas streams entering the compressor section at lower pressure stages. 10 The term "back-to-back compressor" refers to a compressor body having two compressor sections within a single compressor body, each compressor section having a high pressure end and a low pressure end with the high pressure end of each of the two compressor sections being arranged adjacent to each other. Each of these compressor sections can 15 have one or more compression stages. The term "compression string" is used to describe one or more compressor bodies, the rotors of which are connected together and driven by a common driver or common drivers. 20 The term "driver" as used herein refers to a mechanical device such as a gas turbine, a steam turbine, an expander, an electric motor or a combination thereof which is used to cause rotation of a shaft upon which the impeller (s) of a compressor body or compression string is/are mounted. 25 Referring to FIG. 1, there is provided a compressor system (10) comprising a first compressor section (12) arranged back-to-back with a second compressor section (14) in a common compressor body (16). The common compressor body is of either horizontal or vertical split configuration. The compressor system includes a first compression stage (18) having an inlet (20) for receiving a first gas stream (22) at a first pressure and compressing 30 the first gas stream to produce a first intermediate gas stream (24) at a second pressure, said second pressure being greater than the first pressure. In the embodiment illustrated n FIG. 1, the first compression stage (18) is arranged in the first compressor section (12). The compressor system also includes a second compression stage (26) having an inlet (28) 13 for receiving a second gas stream (30) at a third pressure and compressing the second gas stream to produce a second intermediate gas stream (32) at a fourth pressure, said fourth pressure being greater than the third pressure. In the embodiment illustrated in FIG. 1, the second compression stage (26) is arranged in the second compressor section (14). The 5 compressor system also includes a third compression stage (34) having an inlet (36) for receiving a third gas stream (38) at a fifth pressure and compressing the third gas stream to produce a third intermediate gas stream (40) at a sixth pressure, said sixth pressure being greater than the fifth pressure. In the embodiment illustrated in FIG. 1, the third compression stage (34) is arranged in the second compressor section (14). In one 10 alternative embodiment illustrated in FIG. 2, for which like reference numerals refer to like parts, the first compression stage (18) and the third compression stage (34) are arranged in the first compressor section (12) with the second compression stage (26) being arranged in the second compressor section (14). In another alternative embodiment illustrated in FIG. 3, for which like reference numerals refer to like parts, the first compression stage (18) 15 and the third compression stage (34) are arranged in the first compressor section (12) with the second compression stage (26) being arranged in the second compressor section (14). In the specific embodiment illustrated in FIG. 1, the fifth pressure is the same as the fourth pressure, since the third gas stream (38) mixes at the same pressure with the second 20 intermediate gas stream (32), and the sum of these two streams pass through to the third compression stage (34). In an analogous manner, the sixth pressure in this specific embodiment is the same as the second pressure, as the first intermediate gas stream (24) mixes at the same pressure with the third intermediate gas stream (40). 25 In each of the embodiments illustrated in FIG. 1, FIG. 2 and FIG. 3, arranging the first compressor section (12) back to back with the second compressor section (14) facilitates the sharing of a common outlet (42) in the common compressor body (16) for discharging a final compressed gas stream (44) comprising the first, second and third intermediate gas streams (24, 32 and 40, respectively). 30 Referring to FIG. 1, each of the first, second and third compression stages (18, 26 and 34, respectively) is provided with one or more impellers (50) arranged on a common shaft (52) driven by a driver (54). For clarity purposes, the common shaft and driver are not shown 14 in FIG. 2 or FIG. 3. The number of impellers used in a given stage may vary depending on such relevant factors as the fluid composition, mass flow rate, inlet pressures and temperatures of the first, second and third gas streams, the degree of compression required and the rotational speed of the shaft. By way of example only, in the embodiment 5 illustrated in FIG. 1, the first compression stage (18) is provided three impellers, the second compression stage (26) is provided with a single impeller and the third compression stage (34) is provided with a single impeller. In this embodiment, a shared impeller (56) is arranged at a common outlet (42) with the shared impeller (56) representing one of the impellers (50) used for the first compression stage (18) and the third compression stage 10 (34). The term 'shared impeller' as used throughout this specification refers to the last of the impellers of each of the first compressor section (12) arranged back-to-back with the second compressor section (14). The shared impeller can be formed from two impeller sections which are butted together or manufactured as a single entity with each side of the shared impeller providing the functionality of the impeller for each of the respective first 15 compressor section and second compressor section. Using this arrangement, the mass flow through the first compressor section (12) is the mass flow of the first gas stream (22) while the mass flow through the discharge of the second compressor section (14) is the sum of the mass flow of the second gas stream (30) and the mass flow of the third gas stream (38). 20 By way of further example only, in the embodiment illustrated in FIG.2, the first compression stage (18) is provided with two impellers (50, the second compression stage (26) is provided with two impellers (50) and the third compression stage (34) is provided with a single impeller. In this embodiment, the shared impeller (56) arranged at the common outlet (42) is one of the two impellers used for the second compression stage (26) 25 and is the single impeller used for the third compression stage (34). Using this arrangement, the mass flow through the discharge of the first compressor section (12) is the sum of the mass flow of the first gas stream (22) and the mass flow of the third gas stream (38), while the mass flow through the second compressor section (14) is the mass flow of the second gas stream (30). 30 By way of further example only, in the embodiment illustrated in FIG.3, the first compression stage (18) is provided with a single impeller (50), the second compression stage (26) is provided with two impellers (50) and the third compression stage (34) is 15 provided with a single impeller. In this embodiment, the shared impeller (56) arranged at the common outlet (42) is one of the two impellers used for the second compression stage (26) and is the single impeller used for the third compression stage (34). Using this arrangement, the mass flow through the discharge of the first compressor section (12) is 5 the sum of the mass flow of the first gas stream (22) and the mass flow of the second gas stream (30), while the mass flow through the second compressor section (14) is the mass flow of the third gas stream (38). In each of the alternative embodiments illustrated in FIGS. 4 to 10, the compressor system 10 (10) includes a fourth compression stage (60) having an inlet (62) for receiving a fourth gas stream (64) at a seventh pressure and compressing the fourth gas stream to produce a fourth intermediate gas stream (66) at an eighth pressure, said eighth pressure being greater than the seventh pressure. In each of the embodiments illustrated in FIGS. 4 to 10, arranging the first compressor section (12) back to back with the second compressor 15 section (14) facilitates the sharing of a common outlet (68) in the common compressor body (16) for discharging a final compressed gas stream (70) comprising the first, second, third and fourth intermediate gas streams (24, 32, 40 and 66, respectively). Without limiting the generality of the foregoing description of embodiments of the present invention, the first pressure of the first gas stream (22) may be lower than the third 20 pressure of the second gas stream (30) which, in turn, may be lower than the fifth pressure of the third gas stream (38), which in turn may be lower than the seventh pressure of the fourth gas stream (64). Referring to FIG. 4, each of the first, second, third and fourth compression stages (18, 26, 25 34, and 60, respectively) is provided with one or more impellers (50) arranged on a common shaft (52) driven by a driver (54). For clarity purposes, the common shaft and driver are not shown in FIG. 5 or FIG. 6. As for previous embodiments, the number of impellers used in a given compression stage may vary. By way of example only, in the embodiment illustrated in FIG. 4, the first compression stage (18) is provided with three 30 impellers, the second compression stage (26) is provided with a single impeller, the third compression stage (34) is provided with two impellers and the fourth compression stage (60) is provided with a single impeller. In this embodiment, a shared impeller (56) is arranged at the common outlet (68) with the shared impeller (56) representing one of the 16 impellers (50) used for the third compression stage (34) and the single impeller used for the fourth compression stage (60). By way of further example only, in the embodiment illustrated in FIG. 5, the first compression stage (18) is provided with two impellers, the second compression stage (26) is provided with two impellers, the third compression stage 5 (34) is provided with two impellers, and the fourth compression stage (60) is provided with a single impeller. In this embodiment, the shared impeller (56) arranged at the common outlet (68) is one of the two impellers used for the third compression stage (34) and is the single impeller used for the fourth compression stage (60). By way of further example only, in the embodiment illustrated in FIG. 6, the first compression stage (18) is provided 10 with a single impeller, the second compression stage (26) is provided with three impellers, the third compression stage (34) is provided with a single impeller, and the fourth compression stage (60) is provided with a single impeller. In this embodiment, the shared impeller (56) arranged at the common outlet (68) is one of the three impellers used for the second compression stage (26) and is the single impeller used for the fourth compression 15 stage (60). In each of the four-stage compression systems illustrated in FIGS. 4 to 6, the first compressor section (12) comprises two compression stages and the second compressor section (14) compressors two compression stages. In the embodiment illustrated in FIG. 4, 20 the first compressor section (12) provides the first compression stage (18) and the fourth compression stage (60) while the second compressor section (14) provides the second compression stage (26) and the third compression stage (34). In the embodiment illustrated in FIG. 5, the first compressor section (12) provides the first compression stage (18) and the third compression stage (34) while the second compressor section (14) comprises the 25 second compression stage (26) and the fourth compression stage (60). In the embodiment illustrated in FIG. 6, the first compressor section (12) provides the first compression stage (18) and the second compression stage (26) while the second compressor section (14) comprises the third compression stage (34) and the fourth compression stage (60). 30 In the four-stage compression system illustrated in FIG. 7, the first compressor section (12) provides the first compression stage (18) while the second compressor section (14) provides the second compression stage (26), the third compressor stage (34) and the fourth compression stage (60). In the four-stage compression system illustrated in FIG 8, the first 17 compressor section (12) provides the first compression stage (18), the third compression stage (34) and the fourth compression stage (60), while the second compressor section (14) provides only the second compression stage (26). In the four-stage compression system illustrated in FIGS. 9, the first compressor section (12) provides the first compression stage 5 (18), the second compression stage (26) and the fourth compression stage (60) while the second compressor section (14) provides only the third compression stage (34). In the four-stage compression system illustrated in FIGS. 10, the first compressor section (12) provides the first compression stage (18), the second compression stage (26) and the third compression stage (34), while the second compressor section (14) provides only the fourth 10 compression stage (60). All of the embodiments described above rely on the use of a common outlet from the common compressor body. In each of the alternative three-stage compression system embodiments illustrated in FIGS. 11 to 13 and in each of the four-stage compression 15 system embodiments illustrated in FIGS. 14 to 20, the first compressor section (12) has a first outlet (72) for discharging a first final compressed gas stream (74) and the second compressor section (14) has a second outlet (76) for discharging a second final compressed gas stream (78) and the compression system further comprises: a piping means (80) for receiving the first final compressed gas stream (74) from the first outlet (72) and the 20 second final compressed stream (78) from the second outlet (76) and combining the first and second final compressed streams downstream of the common compressor body (16) to produce a final compressed gas stream (82). The three-stage compression embodiments illustrated in FIGS 1 to 3 all rely on the use of a common discharge outlet. The use of a separate first outlet (72) and second outlet (76) instead of the common outlet (42), as well 25 as removing the reliance on the use of a shared impeller (56), provides increased flexibility to customise each of the impellers in each compression stage to match the required performance of that impeller. Referring to FIG. 21, any of the first, second, and third gas streams (22, 30 and 38, 30 respectively) may be a refrigerant gas provided from a refrigeration system (84) whereby the final compressed gas stream (44) is a compressed refrigerant gas provided to a refrigeration apparatus (86). The refrigeration apparatus (86) is adapted to receive the final compressed gas stream (44) and provide refrigeration in a plurality of temperature ranges 18 to produce the first gas stream (22) at the first pressure, the second gas stream (30) at the third pressure and the third gas stream (38) at the fifth pressure. Referring to FIG. 22, when a four-stage compression system is used, any of the first, second, third and fourth gas streams (22, 30, 38 and 64, respectively) may be a refrigerant gas provided from a 5 refrigeration system (84) whereby the final compressed gas stream (82) is a compressed refrigerant gas provided to a refrigeration apparatus (86). In this embodiment, four vaporized refrigerant streams, each at a different pressure, are withdrawn from the refrigerant system and are introduced into the stages of a multistage compressor at the appropriate locations depending on the pressure of each stream. The refrigeration 10 apparatus (86) is adapted to receive the final compressed gas stream (70) and provide refrigeration in a plurality of temperature ranges to produce the first gas stream (22) at the first pressure, the second gas stream (30) at the third pressure, the third gas stream (38) at the fifth pressure and the fourth gas stream (64) at the seventh pressure. 15 The refrigeration system can be any type of refrigeration system in which multiple refrigerant streams are vaporized at different pressure levels to provide refrigeration in multiple temperature ranges. The refrigeration apparatus may be adapted to cool fluids in any chemical, petrochemical, fertiliser or gas plants. By way of example, the refrigeration apparatus may be adapted to pre-cooling of a natural gas stream prior to 20 liquefaction. When the refrigeration system is used in the context of producing liquefied natural gas (LNG), the refrigeration apparatus (86) is adapted to precool natural gas prior to liquefaction. In that context, the refrigerant gas may be a mixed refrigerant gas containing two or more components selected from the group consisting of nitrogen and hydrocarbons having from one to five carbon atoms. 25 Now that embodiments of the present invention have been described in detail, a number of advantages over the prior art can now be described. Compared to using prior art compressor systems which rely on compressing all of the mass flow of the plurality of gas streams in a single compressor section with all of the mass flow occurring in the same 30 direction through a single compressor body, the use of back-to-back compressor sections in the compressor system of the present invention permits higher mass flows to be compressed. When used in the context of an LNG plant, this increased capacity for refrigerant mass flow through the compressor system of the present invention facilitates a 19 proportional increase in the capacity of the LNG liquefaction train. Increased capacity of in excess of 20% may be achieved. The mass flow through the associated refrigerant compressor system is traditionally the 5 limiting factor in determining the size of the LNG train. Using a conventional prior art single compressor section three-stage compression system, the third compression stage would have to compress the combined mass flow of the first, second and third gas streams whilst the second compression stage would have to compress the combined mass flow of the first and second gas streams. Referring to the three-stage compressor system illustrated 10 in FIG.1, the third compression stage is only required to compress the combined mass flow of the second and third gas streams whilst the second compression stage compresses only the mass flow of the second gas stream. For an equally sized LNG train, the mass flows through the second and third compression stages are reduced. Consequently, using the compressor system of the present invention, the LNG train size may be increased 15 compared with the prior art. It is noted that the first compression stage using the compressor system of the present invention is required to compress only the mass flow of the first gas stream in an analogous manner to prior art compressor systems. However, the first compression stage is not normally the bottleneck for optimising the size of an LNG train. In the event that a reduction in the mass flow of the first gas stream is desired, the 20 pressure of the second gas stream can be reduced, resulting in an increase in the mass flow of the second gas stream. Advantageously, the compressor system of the present invention is able to handle such an increase in the mass flow of the second gas stream. Referring to the specific embodiment of a three-stage compressor system illustrated in 25 FIG.2, the third compression stage compresses the combined mass flow of the first gas stream and the third gas stream. Assuming that the mass flow of the first gas stream is lower than that of the second gas stream, then the sum of the mass flows of the first gas stream and the third gas stream will be lower than the sum of the mass flow of the second gas stream and the third gas streams. Where this is the case, the embodiment illustrated in 30 FIG. 2, allows for even more expansion of the total LNG train capacity compared with the embodiment illustrated in FIG. 1.

20 Referring to the specific embodiment of a three-stage compressor system illustrated in FIG. 3, the mass flow through the third compression stage is even further reduced compared with the embodiments illustrated in FIG. 1 and FIG. 2. However, in this embodiment, the mass flow through the second compression stage is the combined mass 5 flow of the first gas stream and the second gas stream. In this embodiment, the second compression stage may become the bottleneck rather than the third compression stage. That being said, the embodiment illustrated in FIG. 3 will still provide an increased capacity compared with prior art compression systems, which prior art compression systems are usually restricted by the combined mass flow of the first, second and third gas 10 streams being compressed by the third compression stage. In the event that a reduction in the mass flow of the second gas stream is desired, the pressure of the third gas stream can be reduced, resulting in an increase in the mass flow of the third gas stream and a reduction in the mass flow of the second gas stream. Advantageously, the compressor system of the present invention is able to handle such an increase in the mass flow of the third gas 15 stream. Whilst the embodiment illustrated in FIG. 3 does not offer the same level of mass flow improvement as the embodiments illustrated in FIG. 1 and FIG. 2, the embodiment illustrated in FIG. 3 still represents an improvement over prior art compressor systems. Relative to the embodiments illustrated in FIG. 1 and FIG. 2, the embodiment in FIG. 3 represents a reduction in the overall number of impellers used, which reduces the size of 20 the compressor system. Using the single compressor section four-stage compressor system of the prior art, the third compression stage handles the combined mass flow of the first gas stream, the second gas stream and the third gas stream. Referring to the specific embodiments of a four-stage 25 compressor system illustrated in FIGS. 4 to 9, the mass flow through the third compression stage and the fourth compression stage are, for an equivalent size LNG train, lower than the mass flow through the a conventional prior art single compressor section four-stage compressor system. In the embodiments illustrated in FIG. 4 and 7, the third compression stage compresses the combined mass flow of the second gas stream and the third gas 30 stream. In the embodiments illustrated in FIG. 5 and FIG. 8, the third compression stage compresses the combined mass flow of the first gas stream and the third gas stream. In the embodiments illustrated in FIG. 6 and FIG. 9, the third compression stage compresses 21 only the mass flow of the third gas stream. As a result the LNG train size may be increased whilst staying within the same impeller flow limitations. Referring to the specific embodiment of a four-stage compressor system illustrated in FIG. 5 10, the third stage compression system compresses the combined mass flow of the first gas stream, the second gas stream and the third gas stream in an analogous manner to a conventional prior art single compressor body four-stage compressor system. However, using the compressor system of the present invention, the first, second and third compression stages are arranged in the first compressor section whilst the fourth 10 compression stage is arranged in the second compressor section, the second compressor section being arranged back-to-back to the first compressor section in a single compressor body. This separate flow path of the fourth compression stage facilitates the opportunity to increase the mass flow of the fourth gas stream by decreasing the pressure of the fourth gas stream as a mechanism for reducing the mass flow of the third gas stream. Reducing the 15 mass flow of the third gas stream in this manner, reduces the combined mass flow compressed by the third compressor stage, which in turn allows the LNG train size to be increased whilst staying within the same impeller flow limitations. Thus, the embodiment illustrated in FIG. 10 is particularly advantageous if the conditions are such that the mass flow of the fourth gas stream through the fourth compression stage is the bottleneck. 20 A further advantage of the embodiment illustrated in FIG. 10 is that it reduces the number of impellers required. The number of impellers that may be installed in large compressors may be restricted to between only 5-7 impellers, depending upon factors such as gas composition, impeller diameter and rotational speed. The lower number of impellers used 25 in the embodiment illustrated in FIG. 10 allows for the use of a shorter compressor shaft than the embodiments illustrated in FIGS 4 to 9. In an analogous manner, the number of impellers used in the embodiments illustrated in FIG. 6 and FIG. 9 is one less than the number of impellers used in the embodiments illustrated in FIGS 4, 5, 7 and 8. In applications where it is desirable to keep the number of impellers and shaft length to within 30 the mechanical limitations, the embodiments illustrated in FIG. 6 and FIG. 9 are preferred. In this regard, the number of impellers and limitations on the shaft length are a more significant consideration for a four-stage compression option than for a three-stage compression option.

22 The four-stage compression embodiments illustrated in FIGS 14-20 include both a first outlet (72) and a second outlet (76) which will require the use of a longer shaft compared with the four-stage compression embodiments illustrated in FIGS 4 to 10 which all rely on the use of a common outlet (42) and a shared impeller (56). However, the separation of the 5 final impellers in each compressor section allows for additional flexibility to customise each impeller to match the required performance of that impeller. It will be apparent from the description of the embodiments of the present invention above that the compressor system of the present invention allows for compression of higher mass 10 flows through a single compressor body by way of providing a first compressor section arranged back-to-back with a second compressor section in a common compressor body of either horizontal or vertical split configuration. Prior art conventional compressor systems all rely on the use of two separate compressor bodies. 15 The invention allows for increased flows through a single compressor body. Several other methods would allow similar flow increases by using two compressor bodies, such as the compressor systems described in US Patent Number 6,637,238 and US Patent Number 6,962,060. It is also known to use two identical parallel compressor bodies, each compressor body arranged to receive half of the mass flow at each pressure level. The 20 present invention allows a cost reduction compared to these prior art approaches in that the present invention allows for the use of a single compressor body instead of two compressor bodies. The ability to use just a single compressor body is particularly important when considering an LNG train in which the cryogenic mixed refrigerant compression is shared across two compression strings. For example, the well-known SplitMRTM liquefaction 25 process features a propane compressor and high pressure mixed refrigerant compressor on the same compression string. Unlike the alternatives, this invention will permit the increase in the capacity of a SplitMRTM LNG train without having to employ three separate compressor bodies on the same compression string. 30 It will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. All such modifications and variations are considered to be within the scope of the present invention, 23 the nature of which is to be determined from the foregoing description and the appended claims.

Claims

1. A compressor system comprising: a first compressor section arranged back-to-back with a second compressor section in a common compressor body; 5 a first compression stage having an inlet for receiving a first gas stream at a first pressure and compressing the first gas stream to produce a first intermediate gas stream at a second pressure, said second pressure being greater than the first pressure; a second compression stage having an inlet for receiving a second gas stream at a third pressure and compressing the second gas stream to produce a second intermediate gas 10 stream at a fourth pressure, said fourth pressure being greater than the third pressure; a third compression stage having an inlet for receiving a third gas stream at a fifth pressure and compressing the third gas stream to produce a third intermediate gas stream at a sixth pressure, said sixth pressure being greater than the fifth pressure; wherein either: 15 (i) the first compression stage is arranged in the first compressor section and the second and third compression stages are arranged in the second compressor section; or (ii) the first and second compression stages are arranged in the first compressor section and the third compression stage is arranged in the second compressor section; or, 20 (iii) the first and third compression stages are arranged in the first compressor section and the second compression stage is arranged in the second compressor section.

2. The compressor system of claim 1 wherein the first compressor section and the 25 second compressor section share a common outlet for discharging a final compressed gas stream comprising the first, second and third intermediate gas streams.

3. The compressor system of claim 2 wherein each of the first, second and third compression stages is provided with one or more impellers arranged on a common shaft 30 driven by a driver and a shared impeller is arranged at the common outlet. 25

4. The compressor system of any one of the preceding claims further comprising a fourth compression stage having an inlet for receiving a fourth gas stream at a seventh pressure and compressing the fourth gas stream to produce a fourth intermediate gas stream at an eighth pressure, said eighth pressure being greater than the seventh pressure. 5

5. The compressor system of claim 4 wherein the first and fourth compression stages are arranged in the first compressor section and the second and third compression stages are arranged in the second compressor section. 10

6. The compressor system of claim 4 wherein the first and third compression stages are arranged in the first compressor section and the second and fourth compression stages are arranged in the second compressor section.

7. The compressor system of claim 4 wherein the first and second compression stages 15 are arranged in the first compressor section and the third and fourth compression stages are arranged in the second compressor section.

8. The compressor system of claim 4 wherein the first compression stage is arranged in the first compressor section and the second, third and fourth compression stages are 20 arranged in the second compressor section

9. The compressor system of claim 4 wherein the first, third and fourth compression stages are arranged in the first compressor section and the second compression stage is arranged in the second compressor section. 25

10. The compressor system of claim 4 wherein the first, second and fourth compression stages are arranged in the first compressor section and the third compression stage is arranged in the second compressor section. 30

11. The compressor system of claim 4 wherein the first, second and third compression stages are arranged in the first compressor section and the fourth compression stage is arranged in the second compressor section. 26

12. The compressor system of any one of claims 4 to 11 wherein the first compressor section and the second compressor section share a common outlet for discharging a final compressed gas stream comprising the first, second, third and fourth intermediate gas streams. 5

13. The compressor system of claim 12 wherein each of the first, second, third and fourth compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and a shared impeller is arranged at the common outlet. 10

14. The compressor system of any one of the preceding claims wherein the first compressor section has a first outlet for discharging a first final compressed gas stream and the second compressor section has a second outlet for discharging a second final compressed gas stream and the compression system further comprises a piping means for receiving the first final compressed gas stream from the first outlet and the second final 15 compressed stream from the second outlet and combining the first and second final compressed streams downstream of the common compressor body to produce a final compressed gas stream.

15. The compressor system of any one of the preceding claims wherein any of the first, 20 second, third or fourth gas streams is a refrigerant gas provided from a refrigeration system.

16. The compressor system of any one of the preceding claims wherein the final compressed gas stream is provided to a refrigeration apparatus adapted to receive the final 25 compressed gas stream and provide refrigeration in a plurality of temperature ranges to produce one or more of; the first gas stream at the first pressure, the second gas stream at the third pressure, the third gas stream at the fifth pressure, and the fourth gas stream at the seventh pressure. 30

17. The compressor system of claim 16 wherein the refrigeration apparatus is adapted to precool natural gas prior to liquefaction. 27

18. The compressor system of any one of the preceding claims wherein the common compressor body has a horizontal or vertical split configuration.

19. A method for gas compression comprising: 5 arranging a first compressor section back-to-back with a second compressor section in a common compressor body; providing a first gas stream at a first pressure at an inlet of a first compression stage and compressing the first gas stream to produce a first intermediate gas stream at a second pressure, said second pressure being greater than the first pressure; 10 providing a second gas stream at a third pressure at an inlet of a second compression stage and compressing the second gas stream to produce a second intermediate gas stream at a fourth pressure, said fourth pressure being greater than the third pressure; providing a third gas stream at a fifth pressure at an inlet of a third compression 15 stage and compressing the third gas stream to produce a third intermediate gas stream at a sixth pressure, said sixth pressure being greater than the fifth pressure; and, either: (i) arranging the first compression stage in the first compressor section whilst arranging the second and third compression stages in the second compressor section; or 20 (ii) arranging the first and second compression stages in the first compressor section whilst arranging the third compression stage in the second compressor section; or, (iii) arranging the first and third compression stages in the first compressor section whilst arranging the second compression stage in the second compressor section. 25

20. The method of claim 19 wherein the first compressor section and the second compressor section share a common outlet and the method comprises the step of discharging a final compressed gas stream comprising the first, second and third intermediate gas streams. 30

21. The method system of claim 20 wherein each of the first, second and third compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and the method comprises the step of arranging a shared impeller at the common outlet. 28

22. The method of any one of claims 19 to 21 further comprising the step of providing a fourth gas stream at a seventh pressure at an inlet to a fourth compression stage and compressing the fourth gas stream to produce a fourth intermediate gas stream at an eighth pressure, said eighth pressure being greater than the seventh pressure. 5

23. The method of claim 22 comprising the step of arranging the first and fourth compression stages in the first compressor section whilst arranging the second and third compression stages in the second compressor section. 10

24. The method of claim 22 comprising the step of arranging the first and third compression stages in the first compressor section whilst arranging the second and fourth compression stages in the second compressor section.

25. The method of claim 22 comprising the step of arranging the first and second 15 compression stages in the first compressor section whilst arranging the third and fourth compression stages in the second compressor section.

26. The method of claim 22 comprising the step of arranging the first compression stage in the first compressor section whilst arranging the second, third and fourth 20 compression stages in the second compressor section.

27. The method of claim 22 comprising the step of arranging the first, third and fourth compression stages in the first compressor section whilst arranging the second compression stage in the second compressor section. 25

28. The method of claim 22 comprising the step of arranging the first, second and fourth compression stages in the first compressor section whilst arranging the third compression stage in the second compressor section. 30 29. The method of claim 22 comprising the step of arranging the first, second and third compression stages in the first compressor section whilst arranging the fourth compression stage in the second compressor section.

29

30. The method of any one of claims 19 to 29 wherein the first compressor section and the second compressor section share a common outlet and the method comprises the step of discharging a final compressed gas stream comprising the first, second, third and fourth intermediate gas streams. 5

31. The method of claim 30 wherein each of the first, second, third and fourth compression stages is provided with one or more impellers arranged on a common shaft driven by a driver and the method comprises the step of arranging a shared impeller at the common outlet. 10

32. The method of any one of claim 19 to 31 wherein the first compressor section has a first outlet for discharging a first final compressed gas stream and the second compressor section has a second outlet for discharging a second final compressed gas stream and the method further comprises providing a piping means for receiving the first final compressed 15 gas stream from the first outlet and the second final compressed stream from the second outlet and combining the first and second final compressed streams downstream of the common compressor body to produce a final compressed gas stream.

33. The method of any one of any one of claims 19 to 32 wherein any of the first, 20 second, third or fourth gas streams is a refrigerant gas provided from a refrigeration system.

34. The method of claim 33 further comprising the step of providing the final compressed gas stream to a refrigeration apparatus adapted to receive the final compressed 25 gas stream and providing refrigeration in a plurality of temperature ranges to produce one or more of; the first gas stream at the first pressure, the second gas stream at the third pressure, the third gas stream at the fifth pressure, and the fourth gas stream at the seventh pressure. 30

35. The method of claim 34 wherein the refrigeration apparatus is adapted to precool natural gas prior to liquefaction. 30

36. The method of any one of claims 19 to 35 wherein the common compressor body has a horizontal or vertical split configuration.