Patent Description:
<FIG> shows a schematic diagram of an LNG plant <NUM> 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.

The plant <NUM> consists of a first compression train with a centrifugal compressor <NUM> and a centrifugal compressor <NUM>, having a first common shaft, and a second compression train with a centrifugal compressor <NUM> and a centrifugal compressor <NUM>, having a second common shaft. The compressor <NUM> is used for compressing propane; an inlet <NUM> of compressor <NUM> is fluidly connected to a line of propane; an outlet <NUM> of compressor <NUM> provides compressed propane. The compressors <NUM>, <NUM> and <NUM> are used for compressing a mixed-refrigerant gas; an inlet <NUM> of compressor <NUM> is fluidly connected to a line of mixed refrigerant; an outlet <NUM> of compressor <NUM> is fluidly connected to an inlet <NUM> of compressor <NUM>; an outlet <NUM> of compressor <NUM> is fluidly connected to an inlet <NUM> of compressor <NUM>; an outlet <NUM> of compressor <NUM> provides compressed mixed refrigerant.

The first compression train is driven by a first engine <NUM>, and the second compression train is driven by a second engine <NUM>. The first engine <NUM> and the second engine <NUM> are low speed engines and may be for example an electric engine rotating at a speed of e.g. <NUM> RPM or a gas turbine rotating at a speed of e.g. <NUM> or <NUM> RPM.

Each of the compressors <NUM>, <NUM>, <NUM> and <NUM> is housed inside a distinct case.

An LNG plant is known from <CIT> 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 <FIG>, the mixed refrigerant is subject to a compression of <NUM>. At the priority date of <CIT>, compression of a mixed refrigerant was typically carried out through three compressors inside three distinct cases; this also applies to the solution of <CIT> that reflects the solution shown in <FIG> and <FIG> of the article by <NPL>, (cited by <CIT>); therefore, it is to be noted that block <NUM> in <FIG> and <FIG> of <CIT> corresponds to three compressors in three cases. Furthermore, according to <CIT> 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.

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.

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

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

First embodiments of the subject matter disclosed herein relate to compression trains as defined in claim <NUM>.

Second embodiments of the subject matter disclosed herein relate to LNG plants as defined in claim <NUM>.

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:.

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

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

Reference throughout the specification to "one embodiment" or "an embodiment" 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.

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

The compression train <NUM> of <FIG> comprises an engine <NUM>, a first centrifugal (i.e. centrifugal flow) compressor <NUM> driven by the engine <NUM> and a second centrifugal (i.e. centrifugal flow) compressor <NUM> driven by the engine <NUM>. The first centrifugal compressor <NUM> is housed inside one case; the second centrifugal compressor <NUM> is housed inside one case. The first centrifugal compressor <NUM> has a first inlet fluidly connected to a line of high molecular weight gas of a molecular weight higher than <NUM>; the second centrifugal compressor <NUM> has a second inlet fluidly connected to a line of low molecular weight gas of a molecular weight between <NUM> and <NUM>. Therefore, the gas processed by the compressor <NUM> and then provided at a first outlet <NUM> is different from the gas processed by the compressor <NUM> and then provided at a second outlet <NUM>.

The second centrifugal compressor <NUM> is a high-compression-ratio compressor; which is arranged to provide a compression ratio higher than <NUM>:<NUM>, preferably higher than <NUM>:<NUM>.

A train identical or similar to the one shown in <FIG> is particularly advantageous when arranged to provide both compressed propane and compressed mixed refrigerant for implementing an APCI process. In this case,.

The train of <FIG> comprises only two centrifugal compressors.

<FIG> shows two shafts and the second compressor <NUM> is indirectly mechanically connected to the first compressor <NUM> through a gear box <NUM>. In <FIG>, the gear box is drawn with dashed lines.

The second centrifugal compressor <NUM> is mechanically connected to the first centrifugal compressor <NUM> through a gear box <NUM> having a transmission ratio higher than <NUM>:<NUM>.

The engine <NUM> is an electric motor or a steam turbine or a gas turbine, in particular an aeroderivative gas turbine.

The engine <NUM> is a low speed engine having preferably a maximum rotation speed in the range of <NUM>-<NUM> RPM, more preferably a maximum rotation speed in the range of <NUM>-<NUM> RPM.

The train may comprise further an auxiliary engine, preferably electric motor, such as the engine <NUM> in <FIG>. In <FIG>, the engine <NUM> is directly connected, for example, to the second compressor <NUM>.

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".

According to the embodiment of <FIG>, the high-compression-ratio compressor <NUM> 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.

As shown in <FIG>, the first set includes two impellers <NUM> and <NUM>, but any number of impellers from <NUM> to e.g. <NUM> is suitable. According to this embodiment, the second set includes three impellers <NUM> and <NUM> and <NUM>, but any number of impellers from <NUM> to e.g. <NUM> is suitable. The impellers <NUM> and <NUM> of the first set are centrifugal and unshrouded. As shown in <FIG>, the impellers <NUM> and <NUM> and <NUM> of the second set are centrifugal and shrouded. At least impellers <NUM> and <NUM> and <NUM> and <NUM> and <NUM> of the first set and of the second set are housed inside one case <NUM>. The impellers <NUM> and <NUM> and <NUM> and <NUM> and <NUM> 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.

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

There may be one or more auxiliary inlets.

There may be one or more auxiliary outlets.

Advantageously, as in the embodiment of <FIG>, 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.

In the embodiment of <FIG> all impellers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the two sets are stacked, coupled by Hirth joints 340A, 340B, 340C, 340D, and tightened together by a tie rod <NUM>.

Compressor <NUM> has a main inlet <NUM> (labelled <NUM> in <FIG>), a main outlet <NUM> (labelled <NUM> in <FIG>), 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 <NUM> to the main outlet <NUM>; <FIG> shows the general case of one intermediate tap <NUM>, being in some embodiments an auxiliary inlet (see upward arrow) and being in some embodiments an auxiliary outlet (see downward arrow).

Advantageously, as in the embodiment of <FIG>, the second set of impellers (<NUM> and <NUM> and <NUM>) are downstream the first set of impellers (<NUM> and <NUM>), and the impellers (<NUM> and <NUM> and <NUM>) of the second set may have a smaller diameter than the impellers (<NUM> and <NUM>) of the first set.

According to the embodiment of <FIG>, the impellers of the first set of impellers (<NUM> and <NUM>) are unshrouded and with a larger diameter than those of the second set of impellers (<NUM> and <NUM> and <NUM>).

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.

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.

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.

Centrifugal compressors identical or similar to the one shown in <FIG> 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.

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

By using a train identical or similar to the one shown in <FIG> (in particular with a compressor identical or similar to the one shown in <FIG>), a high LNG production may be obtained in a smaller space and/or in a smaller footprint and with a lesser number of machines.

It is to be noted that having only one case instead of two or more cases is advantageous from many points of view:.

A train identical or similar to the one shown in <FIG> is mainly designed to be used in a LNG plant.

<FIG> shows a schematic diagram of an embodiment of a LNG plant comprising two such trains; gear boxes are not shown.

In such embodiment, both trains are advantageously identical.

In such embodiment, both trains implement an APCI process.

In such embodiment, both trains comprises a compressor identical or similar to the one shown in <FIG>.

Claim 1:
A compression train (<NUM>) comprising an engine (<NUM>), a first centrifugal compressor (<NUM>) driven by the engine (<NUM>) and a second centrifugal compressor (<NUM>) driven by the engine (<NUM>);
wherein the first centrifugal compressor (<NUM>) is housed inside one case;
wherein the second centrifugal compressor (<NUM>) is housed inside one case;
wherein the first centrifugal compressor (<NUM>) has a first inlet fluidly connected to a line of high molecular weight gas of a molecular weight higher than <NUM>;
wherein the second centrifugal compressor (<NUM>) has a second inlet fluidly connected to a line of low molecular weight gas of a molecular weight between <NUM> and <NUM>;
wherein the second centrifugal compressor (<NUM>) is arranged to provide a compression ratio higher than <NUM>:<NUM>, preferably higher than <NUM>:<NUM>;
characterized in that the second centrifugal compressor (<NUM>) is mechanically connected to the first centrifugal compressor (<NUM>) through a gear box (<NUM>) having a transmission ratio higher than <NUM>:<NUM>.