Compression train including two centrifugal compressors and LNG plant including two centrifugal compressors

Abstract

The compression train (200) comprises an engine (210), a first centrifugal compressor (220) driven by the engine (210) and a second centrifugal compressor (230) driven by the engine (210); the first centrifugal compressor (220) is housed inside one case; the second centrifugal compressor (230) is housed inside one case; the first centrifugal compressor (220) has a first inlet fluidly connected to a line of high molecular weight gas, in particular higher than 40; the second centrifugal compressor (230) has a second inlet fluidly connected to a line of low molecular weight gas, in particular between 20 and 30; the second centrifugal compressor (230) is arranged to provide a compression ratio higher than 10:1, preferably higher than 15:1.

Description

TECHNICAL FIELD

[0001] Embodiments of the subject matter disclosed herein correspond to compression trains including two centrifugal compressors and LNG [= Liquefied Natural Gas] plants including two centrifugal compressors.BACKGROUND ART

[0002] Fig. 1 shows a schematic diagram of an LNG plant 100 according to the prior art, in particular a plant implementing an APCI process, i.e. a well-known liquefaction technology with a first cycle using one pure-refrigerant and a second cycle using one mixed-refrigerant.

[0003] The plant 100 consists of a first compression train with a centrifugal compressor 130 and a centrifugal compressor 160, having a first common shaft, and a second compression train with a centrifugal compressor 140 and a centrifugal compressor 150, having a second common shaft. The compressor 130 is used for compressing propane; an inlet 131 of compressor 130 is fluidly connected to a line of propane; an outlet 132 of compressor 130 provides compressed propane. The compressors 140, 150 and 160 are used for compressing a mixed-refrigerant gas; an inlet 141 of compressor 140 is fluidly connected to a line of mixed refrigerant; an outlet 142 of compressor 140 is fluidly connected to an inlet 151 of compressor 150; an outlet 152 of compressor 150 is fluidly connected to an inlet 161 of compressor 160; an outlet 162 of compressor 160 provides compressed mixed refrigerant.

[0004] The first compression train is driven by a first engine 110, and the second compression train is driven by a second engine 120. The first engine 110 and the second engine 120 are low speed engines and may be for example an electric engine rotating at a speed of e.g. 1500 RPM or a gas turbine rotating at a speed of e.g. 3000 or 3600 RPM.

[0005] Each of the compressors 130, 140, 150 and 160 is housed inside a distinct case.

[0006] An LNG plant is known from WO 2008 / 015224 wherein there is a first compression arrangement for propane and a second compression arrangement for a so-called "mixed refrigerant" (i.e. a mixture of hydrocarbons having different molecular weights). According to the example process of figure 2, the mixed refrigerant is subject to a compression of 18.5. At the priority date of WO 2008 / 015224, compression of a mixed refrigerant was typically carried out through three compressors inside three distinct cases; this also applies to the solution of WO 2008 / 015224 that reflects the solution shown in figures 2 and 3 of the article by Perez entitled "The 4.5 MMTBA LNG Train - A Cost Effective Design" International Conference and Exhibition on Liquefied Natural Gas, 4 May 1998 (1998-05-04), pages 1-15, (cited by WO 2008 / 015224); therefore, it is to be noted that block 122 in figures 1 and 2 of WO 2008 / 015224 corresponds to three compressors in three cases. Furthermore, according to WO 2008 / 015224 the first compression arrangement and the second compression arrangement rotate at the same speed (i.e. there is no gearbox provided), while the power ratio of these compression arrangements can be freely chosen.SUMMARY

[0007] It would be desirable to provide an LNG plant with a reduced number of compressor cases with respect to the prior art solutions; this is also advantageous from the footprint point of view.

[0008] In general, it is advantageous to increase efficiency, availability and modularity of LNG plants and to reduce CAPEX for LNG plants.

[0009] The above-mentioned objects and advantages apply in particular to LNG plants implementing an APCI process.

[0010] First embodiments of the subject matter disclosed herein relate to compression trains as defined in claim 1.

[0011] Second embodiments of the subject matter disclosed herein relate to LNG plants as defined in claim 8.BRIEF DESCRIPTION OF DRAWINGS

[0012] The accompanying drawings, which are incorporated herein and constitute an integral part of the present specification, illustrate exemplary embodiments of the present invention and, together with the detailed description, explain these embodiments. In the drawings: Fig. 1 shows a schematic diagram of an LNG plant according to the prior art; Fig. 2 shows a schematic diagram of embodiments of a compression train; Fig. 3 shows a schematic diagram of an embodiment of a compressor that may be a component of the compression train of Fig. 2; and Fig. 4 shows a schematic diagram of an embodiment of a LNG plant. DETAILED DESCRIPTION

[0013] The following description of exemplary embodiments refers to the accompanying drawings.

[0014] The following description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

[0015] Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0016] In the following (and according to its mathematical meaning) the term "set" means a group of one or more items.

[0017] The compression train 200 of Fig. 2 comprises an engine 210, a first centrifugal (i.e. centrifugal flow) compressor 220 driven by the engine 210 and a second centrifugal (i.e. centrifugal flow) compressor 230 driven by the engine 210. The first centrifugal compressor 220 is housed inside one case; the second centrifugal compressor 230 is housed inside one case. The first centrifugal compressor 220 has a first inlet fluidly connected to a line of high molecular weight gas of a molecular weight higher than 40; the second centrifugal compressor 230 has a second inlet fluidly connected to a line of low molecular weight gas of a molecular weight between 20 and 30. Therefore, the gas processed by the compressor 220 and then provided at a first outlet 222 is different from the gas processed by the compressor 230 and then provided at a second outlet 232.

[0018] The second centrifugal compressor 230 is a high-compression-ratio compressor; which is arranged to provide a compression ratio higher than 10:1, preferably higher than 15:1.

[0019] A train identical or similar to the one shown in Fig. 2 is particularly advantageous when arranged to provide both compressed propane and compressed mixed refrigerant for implementing an APCI process. In this case, the high molecular weight gas mentioned above is propane, and the low molecular weight gas mentioned above is a mixed-refrigerant gas, in particular mixture of propane, ethylene or ethane, and methane.

[0020] The train of Fig. 2 comprises only two centrifugal compressors.

[0021] Fig. 2 shows two shafts and the second compressor 230 is indirectly mechanically connected to the first compressor 220 through a gear box 250. In Fig. 2, the gear box is drawn with dashed lines.

[0022] The compression train has two shafts.

[0023] The second centrifugal compressor 230 is mechanically connected to the first centrifugal compressor 220 through a gear box 250 having a transmission ratio higher than 2:1.

[0024] The engine 210 is an electric motor or a steam turbine or a gas turbine, in particular an aeroderivative gas turbine.

[0025] The engine 210 is a low speed engine having preferably a maximum rotation speed in the range of 1500-5000 RPM, more preferably a maximum rotation speed in the range of 1500-4000 RPM.

[0026] The train may comprise further an auxiliary engine, preferably electric motor, such as the engine 240 in Fig. 2. In Fig. 2, the engine 240 is directly connected, for example, to the second compressor 230.

[0027] It is to be noted that the auxiliary engine may be used at start-up of the train and / or to help the main engine when the power absorbed by the compressor or compressors exceeds certain thresholds; such auxiliary engine is sometimes called "helper".

[0028] According to the embodiment of Fig. 3, the high-compression-ratio compressor 230 is a high-compression-ratio centrifugal (i.e. centrifugal flow) compressor and comprises a first set of impellers (i.e. one or more impellers) and a second set of impellers (i.e. one or more impellers) arranged downstream or upstream (preferably downstream) the first set of impellers.

[0029] As shown in Fig. 3, the first set includes two impellers 311 and 312, but any number of impellers from 1 to e.g. 20 is suitable. According to this embodiment, the second set includes three impellers 321 and 322 and 323, but any number of impellers from 1 to e.g. 20 is suitable. The impellers 311 and 312 of the first set are centrifugal and unshrouded. As shown in Fig. 3, the impellers 321 and 322 and 323 of the second set are centrifugal and shrouded. At least impellers 311 and 312 and 321 and 322 and 323 of the first set and of the second set are housed inside one case 300. The impellers 311 and 312 and 321 and 322 and 323 of the first set and of the second set are coupled to each other through mechanical connections. According to an alternative embodiment, all the impellers are centrifugal and shrouded.

[0030] The sets of compression stages may be more than two, for example three or four.

[0031] There may be one or more auxiliary inlets.

[0032] There may be one or more auxiliary outlets.

[0033] Advantageously, as in the embodiment of Fig. 3, at least some of the impellers of said high-compression-ratio centrifugal compressor are stacked on each other and mechanically coupled by means Hirth joint. The stacked and coupled impellers are tightened together by means of a tie rod, in this way, a very stable and reliable mechanical connection is achieved. Each impeller has for example a passing hole at its rotational axis and is configured so that the tie rod can pass through it. A rotor is achieved when the impellers are stacked and tightened together.

[0034] In the embodiment of Fig. 3 all impellers 311, 312, 321, 322, 323 of the two sets are stacked, coupled by Hirth joints 340A, 340B, 340C, 340D, and tightened together by a tie rod 330.

[0035] Compressor 230 has a main inlet 301 (labelled 231 in Fig. 2), a main outlet 302 (labelled 232 in Fig. 2), and at least one auxiliary inlet and / or at least one auxiliary outlet at an intermediate position along the flow path from the main inlet 301 to the main outlet 302; Fig. 3 shows the general case of one intermediate tap 303, being in some embodiments an auxiliary inlet (see upward arrow) and being in some embodiments an auxiliary outlet (see downward arrow).

[0036] Advantageously, as in the embodiment of Fig. 3, the second set of impellers (321 and 322 and 323) are downstream the first set of impellers (311 and 312), and the impellers (321 and 322 and 323) of the second set may have a smaller diameter than the impellers (311 and 312) of the first set.

[0037] According to the embodiment of Fig. 3, the impellers of the first set of impellers (311 and 312) are unshrouded and with a larger diameter than those of the second set of impellers (321 and 322 and 323).

[0038] Unshrouded impellers can rotate faster than shrouded impellers, due to the absence of the shroud; in fact, when the impeller rotates the shroud is pull outwardly by the centrifugal force acting on it and over a certain rotary speed the shroud risks to pull out the impeller.

[0039] Thanks to the rotor configuration of the high-compression-ratio centrifugal compressor defined above, the compressor can rotate faster than traditional centrifugal compressors thus achieving a greater compression ratio.

[0040] It is to be noted that unshrouded impellers and shrouded impellers may alternate between each other; this happens, in particular, when there is one or more auxiliary inlets and / or outlets.

[0041] Centrifugal compressors identical or similar to the one shown in Fig. 3 may rotate very quickly and so they can reach a very high compression ratio. Therefore, a single innovative centrifugal compressor in a single (and small) case may replace two or three or more traditional centrifugal compressors in distinct cases.

[0042] Furthermore, thanks to high rotation speeds of the impellers, high flow coefficients may be obtained.

[0043] By using a train identical or similar to the one shown in Fig. 2 (in particular with a compressor identical or similar to the one shown in Fig. 3), a high LNG production may be obtained in a smaller space and / or in a smaller footprint and with a lesser number of machines.

[0044] It is to be noted that having only one case instead of two or more cases is advantageous from many points of view: it simplifies installation and maintenance, it reduces maintenance time, it increases reliability (less components and less likelihood of failure), it reduces footprint and weight of machines, it reduces leakages of gasses, it reduces the complexity and size of the lubricant oil system.

[0045] A train identical or similar to the one shown in Fig. 2 is mainly designed to be used in a LNG plant.

[0046] Fig. 4 shows a schematic diagram of an embodiment of a LNG plant comprising two such trains; gear boxes are not shown.

[0047] In such embodiment, both trains are advantageously identical.

[0048] In such embodiment, both trains implement an APCI process.

[0049] In such embodiment, both trains comprises a compressor identical or similar to the one shown in Fig. 3.

[0050] A plant such as the one shown in Fig. 4 may have a power substantially equal to the plant of Fig. 1. Anyway, one of the advantages of the plant of Fig. 4 with respect to the plant of Fig. 1 is that if one component of the plant breaks the plant of Fig. 1 is not able to produce any LNG while the plant of Fig. 4 will be able to produce 50% of the rated production.

Claims

1. A compression train (200) comprising an engine (210), a first centrifugal compressor (220) driven by the engine (210) and a second centrifugal compressor (230) driven by the engine (210); wherein the first centrifugal compressor (220) is housed inside one case; wherein the second centrifugal compressor (230) is housed inside one case; wherein the first centrifugal compressor (220) has a first inlet fluidly connected to a line of high molecular weight gas of a molecular weight higher than 40; wherein the second centrifugal compressor (230) has a second inlet fluidly connected to a line of low molecular weight gas of a molecular weight between 20 and 30; wherein the second centrifugal compressor (230) is arranged to provide a compression ratio higher than 10:1, preferably higher than 15:1; characterized in that the second centrifugal compressor (230) is mechanically connected to the first centrifugal compressor (220) through a gear box (250) having a transmission ratio higher than 2:1.

2. The compression train of claim 1, wherein the high molecular weight gas is propane, wherein the low molecular weight gas is a mixed-refrigerant gas, in particular mixture of propane, ethylene or ethane, and methane.

3. The compression train of claim 1 or 2, wherein the engine (210) is an electric motor or a steam turbine or a gas turbine, in particular an aeroderivative gas turbine.

4. The compression train of any of the preceding claims, comprising further an auxiliary engine (240).

5. The compression train of any of the preceding claims, wherein the second centrifugal compressor (230) comprises a first set of impellers (411, 412) and a second set of impellers (421, 422, 423); the impellers (411, 412) of the first set being centrifugal and unshrouded; the impellers (421, 422, 423) of the second set being centrifugal and shrouded.

6. The compression train of any of the preceding claims from 1 to 4, wherein the second centrifugal compressor (230) comprises a first set of impellers and a second set of impellers; the impellers of the first set being centrifugal and shrouded; the impellers of the second set being centrifugal and shrouded.

7. A LNG plant (100) comprising a compression train (200) according to any of the preceding claims from 1 to 6.

8. The LNG plant (100) of claim 7, comprising two compression trains (200, 300) according to any of the preceding claims from 1 to 6.

9. The LNG plant (100) of claim 7 or 8, wherein the or each first centrifugal compressor (220, 320) is arranged to compress a high molecular weight gas, wherein the or each second centrifugal compressor (230, 330) is arranged to compress a low molecular weight gas; the or each first centrifugal compressor (220, 320) and the or each second centrifugal compressor (230, 330) cooperating to liquefy a flow of natural gas.

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