Description

The present invention relates to a method for providing pressurized gas at different pressure levels from a source of liquefied gas to different pressure level consumers and a corresponding compressor arrangement lt is of particular reference and benefit to the supply of fuel gas from a source of liquefied gas.

The invention is of particular relevance to the supply of fuel gas from a source of liquefied natural gas (LNG), especially in ocean-going tankers and is primarily described herein with the reference to this application lt is, however, to be understood that it is also applicable to other cryogenic liquids and liquid mixtures.

State of the Art

While natural gas is conveniently stored and transported in liquid state, it is generally used, however, in the gaseous state, e. g. for propulsion of the tanker. To this end, a flow of LNG can be vaporized and/or boil-off gas, i. e. evaporated LNG from the ullage space of the container can be used. Such vaporized gas is supplied from the source of liquefied gas through a main input line to a compressor for pressurizing the vaporized gas. Over the past decades, fuel gas supply to LNG carrier propulsion has namely being achieved using multi-stage compressors (stage number ranging from 2 to 6 stages), in which typically each stage is integrated in one single gear box including several high speed shafts (for low pressure (LP) and medium pressure (MP) fuel gas centrifugal compressors) or in pistons mounted in series entailed by a low speed shaft (typically pistons compressor used for high pressure (HP) fuel gas supply of about 300 bars) ln the present application, different pressure fuel gas consumers can roughly be divided in three groups, namely high pressure (HP) consumers requiring fuel gas supply of some 100 bar, particularly about 300 bar, medium pressure (MP) consumers requiring fuel gas supply of some 10 bars, particularly 17 bar or 40 bar, and low pressure (LP) consumers requiring fuel gas supply at 15 bar or less, particularly at about 6 bar.

For example, 4-stage compressors have progressively replaced 2-stage

compressors for DFDE (Dual Fuel Diesel Electric) 4-stroke propulsion, since 4- stage compressors are able to maintain the required fuel gas (FG) pressure (6 bara) even with warm boil off gas (BOG) at suction. Recently, 6-stage compressors have been developed to cope with 2-stroke dual fuel propulsion requirements for 17 bara fuel gas pressure level (XDF). A 2-stage compressor is mainly used in laden voyage when BOG is cold (typically -90°C). However, when the BOG temperature warms-up (especially during ballast voyage), performance limitations are reached and it becomes difficult to maintain the required fuel gas pressure. 4-stage compressors can be used either in cold (laden) or in warm (ballast and heel-out) BOG conditions. Thus, different BOG conditions (laden, ballast or heel-out) and different consumers (2 or 4-stroke dual fuel engines) require different muli-stage compressors leading to a cumbersome and costly compressor arrangement.

Fuel gas pressure systems on floating LNG units are becoming more and more sophisticated and are requiring a high degree of flexibility. On the one hand, the variability of potential operating modes (ballast, laden, loading, unloading, with our without reliquefaction plant service) has an impact on suction conditions the fuel gas compressor has to cope with (wide ranges of pressure, temperature and composition). On the other hand, an important factor of flexible Fuel Gas Supply Systems (FGSS) lies in the pressure levels required by the different types of fuel gas consumers (LP DFDE/Genset/GCU, MP XDF, HP MEG1). Very often, a standard approach selected during ship design is to provide one single FG (Fuel Gas) compressor (with a spare one) sized to supply gas to the consumers requiring the highest feed pressure ln that case, either the FG compressor is equipped with side-streams to feed lower FG pressure consumers or high pressure fuel gas taken at compressor discharge is expanded thanks to a valve to feed lower FG pressure consumers. This often leads to fuel gas systems with poor efficiency in some operating modes lndeed, in the first approach, even without any high pressure fuel gas consumption, the whole compressor

arrangement is to be maintained in operation which leads to unnecessary power consumption ln the second approach, the poor efficiency is due to the fact that MP or HP fuel gas is produced and expanded to LP levels. Again, this leads to unnecessary power consumption.

Another approach might be to supply a fuel gas compressor per type of consumer (HP, MP, LP). With this method, each consumer is fed by a dedicated compressor at an adapted pressure level, so that without consumption by a specific consumer, the dedicated compressor can be stopped. This approach, however, leads to a high number of installed compressors with an impact on the overall fuel gas system foot print and costs involved.

The typical composition of BOG is ranging from pure methane to a C1/N2 mixture containing up to 20 % mol N2. BOG from the tanks is usually at about - 90 °C, at a pressure ranging from 1.03 to 1.2 bara. LP consumers usually require FG at around 6 bara and 20/40 °C. MP consumers usually require FG at a pressure level of 15 or 40 bara and 20/40 °C. HP consumers usually require FG at a pressure above 100 bar (up to 400 bara), at a temperature range 20/40 °C. lt is therefore and object of the present invention to provide an efficient method for providing pressurized gas from a source of liquefied gas to different pressure level consumers where the above disadvantages are avoided, especially in the case of providing fuel gas from an LNG source to different pressure level consumers. Summary of the present invention

According to the present invention there is provided a method for providing pressurized gas at different pressure levels from a source of liquefied gas to different pressure level consumers, wherein vaporized gas is supplied from the source of liquefied gas through a main input line to a compressor arrangement for pressurizing the vaporized gas, and a corresponding compressor arrangement according to the independent claims. Preferred embodiments are given in the respective dependent claims and in the following description.

According to the present invention, in the above method the compressor arrangement comprises a plurality of compressor modules connected in series and/or in parallel, each compressor module being able to operate independently from any other compressor module of the compressor arrangement, and gas flows of different pressure levels are generated by conducting the respective gas flow through one or more compressor modules of the compressor arrangement, and each of the different gas flows of different pressure levels is conducted through one of different consumer branch lines to the respective different pressure level consumer.

The term "vaporized gas is supplied from the source of liquefied gas" is primarily to be understood as withdrawing evaporated gas from the ullage space of the container/source of liquefied gas where the stored liquefied gas changes its stage from liquid to vapor lt is, however, also possible to withdraw a flow of liquefied gas and to vaporize the liquefied gas in order to supply such vaporized gas to the compressor arrangement.

The term "compressor module" is to be understood as a compressor skid including one or a plurality of compressor stages mounted on one or a plurality of mechanical shafts. The present invention can be applied to different types of compressor technology including integrally geared centrifugal compressors, piston or screw compressors or magnetic bearing type compressors lt can be envisaged to equip each or all of the centrifugal compressor stages with variable diffusor vanes (VDV) to cope with the range of suction conditions at the inlet of each compressor stage lnter-stage or after coolers can be implemented either inside a compressor module or outside a compressor module. Several independently operable modules can be installed in series and/or in parallel.

The term "a plurality of compressor modules connected in series and/or in parallel" is to be understood that compressor modules can either be connected in series or can be connected in parallel or one or more can be connected in series while other ones can be connected in parallel. Also parallel trains of one or more compressor modules can be advantageous.

The proposed approach according to the present invention is to provide a modular compressor train philosophy with a limited foot print. This allows the adaption of the fuel gas supply system according to the number and the type of FG consumers. Optimization of (fuel) gas compressor efficiency is achieved by selecting the compressor modules put in operation according to the required pressure level and conducting the (fuel) gas through the selected compressor modules into an individual consumer branch line assigned to the respective pressure level consumer.

The proposed method allows bypassing one or more of the compressor modules of the compressor arrangement, namely bypassing all downstream compressor modules which are not included into the respective consumer branch line which allows for a flexible operation depending on the required gas pressure level. ln a preferred embodiment, if a specific consumer is not operating, only the compressor modules needed for the other consumers are operated, while the remaining ones are deactivated. Therefore it is no longer necessary to run all compressor stages just to feed a LP consumer. Hitherto, the approach has been to maintain in operation a fuel gas (FG) compressor designed to provide fuel gas to the consumers requiring the highest pressure level, even if only the low pressure (LP) consumers are in operation. The LP consumers have either been fed by a FG compressor side-stream or by a pressure let down of medium pressure (MP) or high pressure (HP) fuel gas flow. Thanks to the modular approach according to the present invention, the fuel gas supply system power efficiency is enhanced over a wide range of operations. Compressor modules selected for a required pressure level can be regarded a Fuel Gas Supply System (FGSS) module dedicated to a specific pressure. For example, the low pressure supply compressor modules can be maintained in operation while the MP and HP ones can be shut down. ln a preferred embodiment, at least two compressor modules are connected in series, and gas flows of different pressure levels are branched off into the respective consumer branch lines at the outlets of two or more of the at least two compressor modules connected in series ln case of fuel gas (FG), for example, each module supplies FG at one pressure level to the respective FG consumer but also to the next compressor module dedicated to the supply of fuel gas to the next pressure level consumer. Most of the time, the LP consumers are always in operation since they are dedicated to on-board power generation ln case the higher pressure consumers are not operated, the respective compressor modules can be deactivated.

A gas flow can be cooled by conducting the gas flow through a compressor module comprising an integrated first cooling unit at the outlet of the compressor module, which outlet feeds the inlet of a subsequent compressor module and/or the respective consumer branch line. lt is also possible to cool a gas flow by conducting the gas flow through the respective consumer branch line which comprises a second cooling unit.

Thanks to the after coolers (first and/or second cooling unit) the fuel gas supplied by each compressor module can be delivered at a temperature of e. g. about 40 °C. ln another embodiment of the present invention, at least two compressor modules are arranged in parallel and gas flows of different pressure levels are fed into the respective consumer branch lines at the outlets of two or more of the at least two compressor modules arranged in parallel. This embodiment includes the possibility of parallel trains of essentially identical compressor modules, each train comprising one or more compressor modules connected in series, for example, a low pressure level can be reached by having only one compressor module in one of the parallel trains, and a higher pressure level can be reached by having two or more compressor modules connected in series in another train of the parallel trains lt is, however, also possible to have a single higher pressure compressor module (instead of two or more compressor modules) in such a train ln such a case, a first compressor module (e.g. MP) compressor module is designed to provide a first gas flow at a first (MP) pressure level different to a second pressure level (HP) of a second gas flow provided by a second (HP) compressor module, the two compressor modules being arranged in parallel ln this case, if one of the MP and HP compressor modules is shut down, the other compressor module is not impacted. ln another preferred embodiment of the present invention, before being fed to a compressor module and/or into a consumer branch line, the respective gas flow is mixed at a mixing point with forced vaporized gas, forced vaporized gas being generated by conducting liquefied gas from the source of liquefied gas through a second input line to a vaporizer to generate a second flow of vaporized gas and conducting at least a part of the second flow of vaporized gas as said forced vaporized gas from the vaporizer to the mixing point ln this embodiment, a vaporizer and its associated equipment (spray coolers and demister) is implemented into the compressor arrangement such that vaporized gas (LNG) is mixed with the gas flow (BOG) at (fuel) gas compressor module discharge. Thus, forced vaporized gas can simultaneously be conducted to the gas consumer and to the gas compressor module of the next pressure level. ln this embodiment, different flows of forced vaporized gas from different vaporizers can be fed to different mixing points where different gas flows of different pressure levels are mixed with the respective flows of forced vaporized gas. lt is, however, also possible to split one flow of vaporized gas from a (single) vaporizer into different flows of forced vaporized gas which are then each fed to different mixing points where different gas flows of different pressure levels are mixed with the respective flows of forced vaporized gas. ln another embodiment, the gas flow fed into the respective consumer branch line is heated by conducting the gas flow through a heating unit arranged in the consumer branch line. Such additional heating may be required on a fuel gas consumer line to reach a standard temperature level of 20 °C to 40 °C.

According to a second aspect, the present invention relates to a compressor arrangement for providing pressurized gas at different pressure levels from a source of liquefied gas to different pressure level consumers. The compressor arrangement according to this second aspect comprises a plurality of compressor modules connected in series and/or in parallel, each compressor module being able to operate independently from any other compressor module of the compressor arrangement, and wherein different consumer branch lines are provided, a consumer branch line connecting the outlet of one of the compressor modules with one of said different pressure level consumers.

According to a preferred embodiment, the compressor arrangement comprises at least two compressor modules connected in series , and the outlet lines of two or more of the at least two compressor modules are branched off into the respective consumer branch lines. ln another preferred embodiment, the compressor arrangement additionally or alternatively comprises at least two compressor modules arranged in parallel and the outlet lines of two or more of the at least two compressor modules arranged in parallel are forming the respective consumer branch lines. lt should be noted that the above embodiments can be combined according to the consumer pressure requirements.

Regarding further explanations as to the advantages of the compressor arrangement and its embodiments reference is explicitly made to the statements in connection with the method according to the present invention above.

Further advantages and preferred embodiments of the invention are disclosed in the following description and figures. lt is understood by a person skilled in the art that the preceding and the following features are not only disclosed in the detailed combinations as discussed or showed in a figure, but that also other combinations of the features can be used without exceeding the scope of the present invention.

The invention will now be further described with reference to the accompanying drawings showing preferred embodiments.

Brief description of the drawings

Figure la schematically shows a first embodiment of a compressor

arrangement for implementing the method according to the present invention,

Figure lb schematically shows a second embodiment of a compressor

arrangement for implementing the method according to the present invention, Figure 2 schematically shows a third embodiment of a compressor

arrangement for implementing the method according to the present invention,

Figure 3a schematically shows a fourth embodiment of a compressor

arrangement for implementing the method according to the present invention, and

Figure 3b schematically shows a fifth embodiment of a compressor

arrangement for implementing the method according to the present invention.

Detailed description of the drawings ln the following, the different embodiments according to the Figures are discussed comprehensively, same reference signs indicating same or essentially same units lt is appreciated that a person skilled in the art may combine certain components like one or more compressor modules, a valve, a cooling unit, certain lines etc. of an embodiment shown in a figure with the features of the present invention as defined in the appended claims without the need to include more than this certain component or even all other components of this embodiment shown in said figure ln other words, the following figures show different preferable aspects of the present invention, which can be combined to other embodiments. The embodiments shown in the figures all relate to the application of supplying fuel gas from an LNG source, but it is appreciated that a person skilled in the art can easily transfer the embodiments to applications involving other cryogenic gases or gas mixtures.

Figure la schematically shows a compressor arrangement 3 comprising three independent compressor modules 30, 31, 32 connected in series. The source of liquefied gas is an LNG tank 1, from the ullage space of which a main input line 2 extends to the first compressor module 30. Boil off gas (BOG) can be pressurized by the first compressor module 30 to a first pressure level. The outlet of the first compressor module 30 is connected to the inlet of the second compressor module 31 and to a consumer branch line 20. Thus, BOG of a first pressure level can be supplied through consumer branch line 20 to one or more low pressure (LP) consumers 10. The outlet of the second compressor module 31 is connected with the inlet of the next compressor module and with a second consumer branch line 21 for supplying pressurized gas to one or more medium pressure (MP)

consumers 11. The outlet of the third compressor module 32 is directly connected to a third consumer branch line 22 for supplying fuel gas at a third pressure level to one or more high pressure (HP) consumers 12. As indicated in Figure la, more than three compressor modules can be provided.

As can be seen from Figure la, only compressor module 30 can be operated, or compressor modules 30 and 31 can be operated, or all the three compressor modules 30, 31, 32 can be operated lf only one or two compressor modules are operated, the remaining ones can be deactivated. The same is true in case of more than three compressor modules ln general, most of the time the LP consumers 10 are always in operation since they are in charge of on-board power generation. Hence, in case the higher pressure consumers 11 and 12 or only 12are not operated, the corresponding compressor modules can be stopped. ln this embodiment, each of the compressor modules 30, 31, 32 comprises a first cooling unit 40, 41, 42, respectively. The first cooling units are integrated into the compressor modules at their outlet lines.

Figure lb shows a second embodiment of a compressor arrangement 3 which is very similar to the one shown in Figure la. For sake of conciseness, it is referred to the explanations given in connection with Figure la. ln contrast to the embodiment of Figure la, the compressor modules 30, 31, 32 of Figure lb each comprise a second cooling unit 50, 51, 52, respectively. The second cooling units are installed in the respective consumer branch lines extending from the outlets of each compressor module lt is, however, also possible to install the second cooling units 50, 51, 52 outside of the compressor modules 30, 31, 32. This form of after cooling of the BOG ensures that the fuel gas supplied by each compressor module can be delivered at an essentially constant temperature of e. g. about 40 °C.

Figure 2 shows a third embodiment of a compressor arrangement 3 comprising two compressor modules 61, 62 connected in parallel, the two compressor modules being connected in series with compressor module 60. Again, pressurized fuel gas at low pressure level is generated by the first compressor module 60 and supplied to the LP consumer 10 via consumer branch line 20. The outlet line of the first compressor module 60 is connected to a medium pressure (MP) compressor 61 and to a high pressure (HP) compressor 62. While the MP compressor 61 generates fuel gas at a medium pressure level, the HP compressor 62 generates fuel gas at a high pressure level ln this embodiment, both MP and HP compressors 61, 62 are operating in parallel such that, if one compressor is shut-down, the other compressor is not impacted, for example, if MP compressor 61 is deactivated, it is still possible to deliver fuel gas to HP consumers 12 via HP compressor 62 and the consumer branch line 22. lt should be noted that other combinations of compressor modules shown in Figures la, lb, 2 are possible, for example, the parallel trains shown in Figure 2 can also comprise two or more identical or different compressor modules depending on the consumer pressure requirements lt is also possible to have more than one compressor module connected in series instead of compressor module 60 shown in Figure 2 depending on the consumer pressure requirements.

Figure 3a shows a fourth embodiment of a compressor arrangement 3 which is similar to the one shown in Figure la. However, with the compressor arrangement 3 of Figure 3a it is possible to supplement the amount of BOG fed to the consumers by vaporizing LNG from the tank 1. To this end a pump in the tank 1 is connected to a second input line 4 which is connected to the inlet of a first vaporizer 80. ln fact, the vaporizer 80 forms an LNG vaporizer set including corresponding valves, spray coolers and demister. The outlet of the vaporizer 80 is connected to a mixing point 70 in the outlet line of the first compressor module 30. Thus, vaporized LNG can be fed as forced vaporized gas simultaneously to the LP consumers 10 and to the next compressor module 31. ln this embodiment, another vaporizer 81 is connected to the second input line 4, the exit line of vaporizer 81 being connected to a second mixing point 71. Thus it is possible to feed forced vaporized gas into the consumer branch line 21 leading to the MP consumers 11 and, at the same time, to the next compressor module. Finally, a third vaporizer 82 with an upstream pump is connected to the second input line 4, the exit line of vaporizer 82 being connected to a third mixing point 72. Thus it is possible to feed forced vaporized gas into consumer branch line 22 directly leading to HP consumers 12.

Optionally, additional heating by heating units 90, 91 installed in consumer branch lines 20, 21 can be performed to reach a standard temperature level of 20° - 40°C. lt goes without saying that such heating units can also be implemented in the other embodiments shown in the figures.

Figure 3b shows a fifth embodiment of a compressor arrangement 3 which is very similar to the one shown in Figure 3a. Therefore, reference is explicitly made to the embodiment discussed in connection with Figure 3a. ln contrast to the

embodiment of Figure 3a, in this embodiment only one single vaporizer 83 is provided and connected to the second input line 4. The flow of forced vaporized gas exiting the vaporizer 83 is split into different flows of forced vaporized gas which are each fed to mixing points 70, 71, 72 as shown in Figure 3b. Thus, the different gas flows of different pressure levels from the compressor modules 30,

31, 32 are mixed with the respective flows of forced vaporized gas and then - at least partially - fed into the consumer branch lines 20, 21, 22. The operating pressure of vaporizer 83 is defined by the highest pressure consumer and the other consumers are fed at the correct pressure thanks to dedicated pressure reduction valves as shown in Figure 3b. ln terms of power consumption, it is more advantageous to install pressure reduction devices on the liquid side than on the gas side. List of reference signs

1 tank, source of liquefied gas

2 main input line

3 compressor arrangement

4 second input line

10, 11, 12 different pressure level consumer

20,21,22 consumer branch line

30,31,32 compressor module

40,41, 42 first cooling unit

50,51,52 second cooling unit

60,61,62 compressor module

70, 71, 72 mixing point

80,81, 82,83 vaporizer

90,91 heating unit