NO20191266A1

NO20191266A1 - Compression device

Info

Publication number: NO20191266A1
Application number: NO20191266A
Authority: NO
Inventors: Katsuhiro Seyama; Satoshi Tezuka
Original assignee: Kobe Steel Ltd
Priority date: 2018-10-30
Filing date: 2019-10-23
Publication date: 2020-05-01
Also published as: CN111120262B; JP2020070733A; KR102197463B1; CN111120262A; KR20200050372A; JP7036702B2

Description

COMPRESSION DEVICE

Technical Field

[0001]

The present invention relates to a compression device for compressing a boil-off gas. Background Art

[0002]

Conventionally, as disclosed in JP 2011-517749 A, for example, there is known a compression device for compressing a boil-off gas generated in a storage tank which stores a liquefied gas such as a liquefied natural gas (LNG). The compression device includes a multi-stage compressor. A boil-off gas which is compressed by the multi-stage compressor is supplied to an engine as a fuel, for example. A boil-off gas is generated irrespective of a gas demand such as an engine load and hence, a re-liquefying line is provided to the compression device. A heat exchanger or the like is provided to the re-liquefying line. In the re-liquefying line, a portion of the compressed boil-off gas is re-liquefied through cooling by a heat exchanger and expansion by an expansion valve.

[0003]

In a case of supplying a compressed boil-off gas to an engine of a ship or the like, it is necessary to compress a boil-off gas such that the boil-off gas has an extremely high pressure. In this case, in an oil-supply-type compressor, an amount of leakage of a boil-off gas from a compression chamber can be suppressed compared to an oil-free type compressor and hence, the oil-supply-type compressor is preferably used from a viewpoint of sealing property. However, when the oil-supply-type compressor is used, a lubricating oil contained in a boil-off gas discharged from the compressor may flow in a re-liquefying line, and the lubricating oil may adhere to the inside of a tube of the re-liquefying line.

Summary of Invention

[0004]

It is an object of the present invention to provide a compression device which can prevent adhesion of oil to a re-liquefying line while ensuring sealing property of the compression device.

[0005]

According to an aspect of the present invention, there is provided a compression device including: an oil-free-type pre-stage compression part which is formed of a reciprocating compression mechanism which sucks and compresses a boil-off gas; a post-stage compression part for compressing the boil-off gas compressed by the pre-stage compression part; and a re-liquefying line for re-liquefying at least a portion of the boil-off gas compressed by the post-stage compression part. The post-stage compression part includes a reciprocating compression mechanism in one or more stages. The compression mechanism in one or more stages includes a rod lubrication compression mechanism which includes: a piston; a piston rod having a distal end on which the piston is mounted; and rod packing which surrounds the piston rod and seals a periphery of the piston rod, and oil is supplied between the piston rod and the rod packing.

Brief Description of Drawings

[0006]

FIG. 1 is a view schematically showing the entire configuration of a compression device according to an embodiment of the present invention;

FIG. 2 is a view showing a configuration of a rod lubrication compression mechanism provided to the compression device;

FIG. 3 is a view for describing a configuration of rod packing provided to the rod lubrication compression mechanism; and

FIG. 4 is a view for describing a configuration of a modification of the rod packing;

Description of Embodiments

[0007]

Hereinafter, an embodiment is described in detail with reference to drawings.

[0008]

As shown in FIG.1, a compression device 10 according to this embodiment compresses a boil-off gas (BOG) generated from a liquefied gas stored in a tank 12 and supplies the compressed gas to predetermined gas utilizing appliances 14, 16. The compression device 10 is installed on a ship which transports a liquefied gas such as a liquefied natural gas (LNG).

[0009]

A liquefied natural gas is stored in the tank 12 in a state where a temperature of the liquefied natural gas is set to an approximately -160 ºC. In the tank 12, due to intrusion of heat from the outside, a portion of the liquefied gas evaporates so that a boil-off gas is generated. The tank 12 is not limited to a tank which stores a liquefied natural gas, and the tank 12 may be a tank which stores another kind of liquefied gas such as a liquefied petroleum gas, for example.

[0010]

The compression device 10 includes: a pre-stage compression part 21 which is formed of a single-stage compression mechanism; a post-stage compression part 23 which is formed of compression mechanisms 23a to 23d in a plurality of stages; and a downstream compression part 25 which is formed of a single-stage compression mechanism. The pre-stage compression part 21, the compression mechanisms 23a to 23d of the post-stage compression part 23, and the downstream compression part 25 are each formed of a single compression device which is driven by a common crankshaft 45 (see FIG.2) as described later.

[0011]

The compression device 10 includes: a gas line 27 through which a boil-off gas flows; and a re-liquefying line 29 which is branched from the gas line 27. The gas line 27 includes a suction line 27a, a first connection line 27b, a second connection line 27c, a third connection line 27d, a first supply line 27e, and a second supply line 27f. The suction line 27a connects the tank 12 and a suction part of the pre-stage compression part 21 to each other. The first connection line 27b connects the pre-stage compression part 21 and the post-stage compression part 23 to each other The second connection line 27c connects the compression mechanisms which form the post-stage compression part 23 to each other. The third connection line 27d connects the post-stage compression part 23 and the downstream compression part 25 to each other. The first supply line 27e is connected to a discharge portion of the downstream compression part 25. The second supply line 27f is branched from the second connection line 27c.

[0012]

The pre-stage compression part 21 compresses a boil-off gas generated in the tank 12.

The pre-stage compression part 21 is formed of a reciprocating compression mechanism which is also an oil-free type compression mechanism. The compression mechanism includes a piston (not illustrated) which compresses gas by being operated by the rotation of the crankshaft 45 described later. In this embodiment, although the pre-stage compression part 21 is formed of a single-stage compression mechanism, the pre-stage compression part 21 may be formed of compression mechanisms in a plurality of stages in place of the compression mechanism in a single stage.

[0013]

The post-stage compression part 23 is positioned downstream of the pre-stage compression part 21 in the gas line 27. The post-stage compression part 23 further compresses a boil-off gas compressed by the pre-stage compression part 21. The respective compression mechanisms 23a to 23d of the post-stage compression part 23 each include a piston (not illustrated) which compresses gas by being operated by the rotation of the crankshaft 45 described later. In the illustrated example, although the configuration in which the post-stage compression part 23 has the compression mechanisms 23a to 23d in four stages is shown, the present invention is not limited to such a configuration. For example, the post-stage compression part 23 may have the compression mechanism in one stage, or may have the compression mechanisms in two stages or three stages, or may have the compression mechanisms in five stages or more.

[0014]

The post-stage compression part 23 includes a rod lubrication compression mechanism 31. In this embodiment, out of the compression mechanisms 23a to 23d in four stages, the frontmost-stage compression mechanism 23a is formed as the rod lubrication compression mechanism 31. The specific configuration of the rod lubrication compression mechanism 31 is described later. Out of compression mechanisms 23a to 23d in four stages, the compression mechanisms 23b to 23d (three post-stage mechanisms) other than the frontmost-stage compression mechanism 23a are each formed of an oil-free-type reciprocating compression mechanism. That is, the oil-free-type compression mechanism is configured such that a lubricating oil is not supplied to a sliding portion between a cylinder (not illustrated) and a piston (not illustrated). When the post-stage compression part 23 is formed of a single-stage compression mechanism, the post-stage compression part 23 is formed of the rod lubrication compression mechanism 31.

[0015]

On a discharge-side portion of the rod lubrication compression mechanism 31 in the second connection line 27c, an oil remover 33 which is formed of a coalescer or an activated carbon is disposed.

[0016]

The second supply line 27f is connected to the second connection line 27c at the position located immediately downstream of the oil remover 33. That is, the second supply line 27f is branched from a portion of the second connection line 27c located between the oil remover 33 and the second-stage compression mechanism 23b of the post-stage compression part 23. The second supply line 27f is connected to a power generating engine (power generator) 14 which is a gas utilizing appliance.

[0017]

The downstream compression part 25 is disposed at a portion of the gas line 27 located downstream of the post-stage compression part 23. The downstream compression part 25 further compresses the boil-off gas compressed by the post-stage compression part 23. The downstream compression part 25 is formed of a reciprocating compression mechanism. The downstream compression part 25 is formed of an oil-supply-type compression mechanism. That is, the downstream compression part 25 is configured such that a lubricating oil is supplied to the sliding portion between a cylinder (not illustrated) and a piston (not illustrated).

[0018]

A boil-off gas discharged from the downstream compression part 25 is supplied to a propulsion engine 16 which is a gas utilizing appliance through the first supply line 27e. In this embodiment, although the downstream compression part 25 is formed of a compression mechanism in a single stage, the downstream compression part 25 may be formed of compression mechanisms in a plurality of stages in place of the compression mechanism in a single stage.

[0019]

The re-liquefying line 29 is branched from the third connection line 27d which connects the post-stage compression part 23 and the downstream compression part 25 to each other. The re-liquefying line 29 re-liquefies at least a portion of the gas compressed by the post-stage compression part 23. With respect to the gas compressed by the post-stage compression part 23, a degree of a ratio of gas which flows into the re-liquefying line is determined according to a load (gas demand) of the propulsion engine 16, for example. The heat exchanger 35 is provided to the re-liquefying line 29, and gas which flows into the re-liquefying line 29 is condensed by being cooled by the heat exchanger 35, and is expanded by an expansion valve (not illustrated). The condensed and expanded liquefied gas is returned to the tank 12.

[0020]

The configuration of the re-liquefying line 29 is not limited to the configuration in which the re-liquefying line 29 is connected to the third connection line 27d. For example, the re-liquefying line 29 may be connected to a portion of the second connection line 27c where the compression mechanism 23c in a third stage and the compression mechanism 23d in a fourth stage of the post-stage compression part 23 are connected to each other. The re-liquefying line 29 may be connected to a portion of the second connection line 27c where the compression mechanism 23b in a second stage and the compression mechanism 23c in a third stage in the post-stage compression part 23 are connected to each other.

[0021]

A plurality of bypass lines are provided to the gas line 27. The bypass lines include a pre-stage bypass line 37, a first post-stage bypass line 38, a second post-stage bypass line 39, and a downstream bypass line 40. The pre-stage bypass line 37 bypasses the pre-stage compression part 21. The first post-stage bypass line 38 bypasses the compression mechanisms 23a, 23b in front two stages of the post-stage compression part 23. The second post-stage bypass line 39 bypasses the compression mechanisms 23c, 23d in two post stages of the post-stage compression part 23. The downstream bypass line 40 bypasses the downstream compression part 25.

[0022]

One end of the pre-stage bypass line 37 is connected to the suction line 27a, and the other end of the pre-stage bypass line 37 is connected to the first connection line 27b. A regulating valve 37a capable of regulating the degree of opening of the pre-stage bypass line 37 is provided to the pre-stage bypass line 37. When the regulating valve 37a is opened, a portion of gas discharged from the pre-stage compression part 21 is returned to a suction side of the pre-stage compression part 21. With such an operation, a pressure on a discharge side of the pre-stage compression part 21 can be regulated.

[0023]

On end of the first post-stage bypass line 38 is connected to the first connection line 27b. The other end of the first post-stage bypass line 38 is connected to a portion of the second connection line 27c where the compression mechanism 23b in the second stage and the compression mechanism 23c in the third stage in the post-stage compression part 23 are connected to each other. A regulating valve 38a capable of regulating the degree of opening of the first post-stage bypass line 38 is provided to the first post-stage bypass line 38. When the regulating valve 38a is opened, a portion of gas discharged from the second-stage compression mechanism 23b is returned to a suction side of the first-stage compression mechanism 23a. With such an operation, a pressure on a discharge side of the compression mechanism 23b in the second stage in the post-stage compression part 23 can be regulated.

[0024]

One end of the second post-stage bypass line 39 is connected to a portion of the second connection line 27c where the compression mechanisms 23b in the second stage and the compression mechanisms 23c in the third stage in the post-stage compression part 23 are connected to each other. The other end of the second post-stage bypass line 39 is connected to the third connection line 27d. A regulating valve 39a capable of regulating the degree of opening of the second post-stage bypass line 39 is provided to the second post-stage bypass line 39. When the regulating valve 39a is opened, a portion of gas discharged from the compression mechanism 23d in a fourth stage is returned to a suction side of the compression mechanism 23c in a third stage.

With such an operation, a pressure on a discharge side of the compression mechanism 23d on the fourth stage in the post-stage compression part 23 can be regulated.

[0025]

One end of the downstream bypass line 40 is connected to the third connection line 27d, and the other end of the downstream bypass line 40 is connected to the first supply line 27e. A regulating valve 40a capable of regulating the degree of opening of the downstream bypass line 40 is provided to the downstream bypass line 40. When the regulating valve 40a is opened, a portion of gas discharged from the downstream compression part 25 is returned to a suction side of the downstream compression part 25. With such an operation, a pressure on a discharge side of the downstream compression part 25 can be regulated.

[0026]

The configuration of the rod lubrication compression mechanism 31 is described below with reference to FIG.2. The rod lubrication compression mechanism 31 is formed of a reciprocating compression mechanism which compresses a boil-off gas. The rod lubrication compression mechanism 31 includes: an operating part 31b which is operated due to the rotation of the crankshaft 45; and a pressurizing part 31a which is a portion for compressing a boil-off gas by working of the operating part 31b. In the same manner as the rod lubrication compression mechanism 31, other compression mechanisms of the post-stage compression part 23 shown in FIG.

1 each also include: an operating part which is operated due to the rotation of the crankshaft 45; and a pressurizing part which is a portion for compressing a boil-off gas by working of the operating part. The same applies also for the compression mechanism which forms the pre-stage compression part 21 and the compression mechanisms which form the downstream compression part 25.

[0027]

The crankshaft 45 is housed in a shaft case 50. The crankshaft 45 is driven by a drive source (not illustrated), and rotates about an axis thereof. The crankshaft 45 is used in common by all of the compression mechanisms of the pre-stage compression part 21, the post-stage compression part 23 and the downstream compression part 25 and hence, when the crankshaft 45 rotates, operating parts (not illustrated) of the respective compression mechanisms are operated. In the respective compression mechanisms, gas is compressed by the pressurizing part due to an operation of the operating part. Accordingly, due to the rotation of the crankshaft 45, gas is compressed in all of the pre-stage compression part 21, the post-stage compression part 23 and the downstream compression part 25. The crankshaft 45 may be provided to only some compression mechanisms, and another crankshaft (not illustrated) used for remaining compression mechanisms may be further provided.

[0028]

The operating part 31b is a portion which is operated by being driven by the crankshaft 45. The operating part 31b includes a crosshead 46, a connecting rod 47 which converts a rotary motion of the crankshaft 45 into a reciprocating linear motion of the crosshead 46, and a piston rod 48 which is connected to the crosshead 46.

[0029]

The crosshead 46 is disposed in a cylindrical rod case 52 which is connected to the shaft case 50. The piston rod 48 extends from the inside of the rod case 52 to the inside of a cylinder 55 described later. The crosshead 46 moves along an inner peripheral surface of the rod case 52 in a reciprocating manner due to the rotation of the crankshaft 45. Due to the rotation of the crankshaft 45, the piston rod 48 also moves along an axial direction of the rod case 52 in a reciprocating manner.

[0030]

The pressurizing part 31a includes the cylinder 55, a first cylinder head 56, a second cylinder head 57, and a piston 58.

[0031]

The piston rod 48 is connected to the crosshead 46. The piston 58 is joined to one end (distal end) of the piston rod 48, and is housed in the cylinder 55. An outer diameter of the piston rod 48 is smaller than an outer diameter of the piston 58. The piston 58 is disposed between the first cylinder head 56 and the second cylinder head 57. The piston 58 makes a reciprocating linear motion integrally with the piston rod 48 while being brought into slide contact with an inner peripheral surface of the cylinder 55. That is, the piston 58 moves in a reciprocating manner due to the operation of the operating part 31b. A space between the piston 58 and the second cylinder head 57 functions as a compression chamber 60, and a space between the piston 58 and the first cylinder head 56 also functions as a compression chamber 61. That is, the compression mechanism is formed of a double acting type compression mechanism. A plurality of piston rings 59 are mounted on an outer peripheral surface of the piston 58. The rod lubrication compression mechanism 31 may not be formed of the double acting type compression mechanism, and may be formed of a single acting type compression mechanism.

[0032]

The cylinder 55 is connected to the rod case 52 in a state where a center axis of the cylinder 55 is aligned with a center axis of the rod case 52.

[0033]

A suction pipe 55a and a discharge pipe 55b are connected to the cylinder 55. The suction pipe 55a is a pipe which forms the first connection line 27b, and a boil-off gas discharged from the pre-stage compression part 21 flows through the suction pipe 55a. The boil-off gas which flows through the suction pipe 55a is introduced into the compression chamber 60 and the compression chamber 61 through communication paths (not illustrated). The discharge pipe 55b is a pipe which forms a portion of the second connection line 27c where the compression mechanism 23a in the first stage and the compression mechanism 23b in the second stage in the post-stage compression part 23 are connected to each other. A boil-off gas compressed by the compression chamber 60 and a boil-off gas compressed by the compression chamber 61 are discharged to the discharge pipe 55b through the communication paths (not illustrated). A boil-off gas which flows through the discharge pipe 55b is introduced into the compression mechanism 23b in the second stage in the post-stage compression part 23.

[0034]

The first cylinder head 56 is disposed on a crankshaft 45 side with respect to the piston 58. A flange portion 56a is formed on the first cylinder head 56, and the flange portion 56a is sandwiched between the rod case 52 and the cylinder 55. With such a configuration, an opening of the cylinder 55 on one end in an axial direction (an opening on a crankshaft 45 side) is closed.

The second cylinder head 57 is disposed on a side opposite to the crankshaft 45 with respect to the piston 58. A flange portion 57a is formed on the second cylinder head 57, and the flange portion 57a is joined to an end surface of the cylinder 55. With such a configuration, an opening of the cylinder 55 on the other end in an axial direction (an opening on a side opposite to the crankshaft 45) is closed.

[0036]

A through hole 56b through which the piston rod 48 passes is formed in the first cylinder head 56. Further, a recessed portion 56c which is recessed from an inner peripheral surface of the through hole 56b is formed in the first cylinder head 56, and rod packing 62 is accommodated in the recessed portion 56c. The rod packing 62 surrounds a portion of the piston rod 48 over the whole circumference. The rod packing 62 is provided for sealing the periphery of the piston rod 48. That is, the rod packing 62 is provided for preventing a boil-off gas from being leaked to a rod case 52 side from the inside of the cylinder 55 through between the first cylinder head 56 and the piston rod 48.

[0037]

The rod packing 62 is disposed in the recessed portion 56c of the first cylinder head 56. As shown in FIG.3, the rod packing 62 includes: a plurality of ring elements 63 which are arranged in an axial direction of the piston rod 48; and a pressing part 64 which presses these ring elements 63. The pressing part 64 has a disc shape in which a through hole for allowing the piston rod 48 to pass therethrough is formed. The pressing part 64 is formed in a disc shape having a diameter larger than a diameter of the ring element 63. The plurality of ring elements 63 are held between the pressing part 64 which is fixed to the first cylinder head 56 and the first cylinder head 56.

[0038]

The respective ring elements 63 have the same configuration. Each ring element 63 includes: a housing 63a provided with the recessed portion 63c; and ring-shaped sealing members 63b which are disposed in a space formed in the recessed portion 63c. The housing 63a is formed in a disc shape where a through hole 63d is formed at a center portion of the housing 63a. Further, the recessed portion 63c which is formed concentrically with the through hole 63d is formed on the housing 63a in a state where the recessed portion 63c is recessed in an axial direction from one main surface of the housing 63a (a main surface facing a side opposite to the piston 58). That is, on the housing 63a, a bottom surface and a peripheral surface are formed. The bottom surface orthogonally intersects with an axial direction of the piston rod 48 (that is, a direction in which the piston rod 48 extends), and forms an end surface of the recessed portion 63c in an axial direction. The peripheral surface is connected with an outer periphery of the bottom surface, extends parallel to the axial direction of the piston rod 48, and forms an outer peripheral surface of the recessed portion 63c.

[0039]

The sealing members 63b are arranged in the axial direction of the piston rod 48, and are disposed so as to surround the piston rod 48. The sealing members 63b are deformed by a high-pressure gas so as to be brought into close contact with the outer peripheral surface of the piston rod 48. The sealing member 63b may be formed in a size which allows the sealing member 63b to come into close contact with the outer peripheral surface of the piston rod 48 even in a state where a pressure of a high-pressure gas is not applied to the sealing member 63b.

[0040]

In the illustrated example, each ring element 63 includes three sealing members 63b.

However, the present invention is not limited to such a configuration. For example, a configuration in which each ring element 63 includes one sealing member 63b may also be adopted. By disposing the plurality of sealing members 63b on each ring element 63, it is possible to further enhance the sealing property, and such a configuration is more suitably applicable to a

high-pressure compression mechanism.

[0041]

A poly- α-olefin (PAO)-based lubricating oil is supplied to the rod packing 62. The lubricating oil is supplied from an oil filling system 66 (FIG.2), and is supplied to the ring element 63 which is farthest from the piston 58 out of the plurality of ring elements 63. A flow path 67 is formed in the housing 63a of the ring element 63, and oil can be supplied to the outer peripheral surface of the piston rod 48 through the flow path 67. Further, a bent line 69 for discharging a lubricating oil to the outside is formed so as to extend from the ring element 63 which is farthest from the piston 58 out of the plurality of ring elements 63 and the first cylinder head 56. A flow path (not illustrated) for introducing a lubricating oil to the outside of the cylinder 55 is connected to the bent line 69.

[0042]

In the rod lubrication compression mechanism 31, although a lubricating oil is supplied to the rod packing 62, a lubricating oil is not supplied to the sliding portion between the cylinder 55 and the piston rings 59.

[0043]

The ring element 63 to which a lubricating oil is supplied is not limited to the ring element 63 which is farthest from the piston 58. It is sufficient that the ring element 63 to which oil is supplied is a ring element 63 other than the ring element 63 closest to the piston 58, that is, a ring element 63 which is positioned on a side opposite to the piston 58. In other words, it is sufficient that another ring element 63 is present more on a piston 58 side than the ring element 63 to which a lubricating oil is supplied. With such a configuration, it is possible to suppress occurrence of a phenomenon that the supplied lubricating oil flows toward the piston side. Further, in the case where three or more ring elements 63 are provided, when oil is supplied to the ring element 63 farthest from the piston 58, it is possible to suppress flowing of oil toward the piston 58 side more effectively.

[0044]

The poly- α-olefin-based lubricating oil has the narrower molecular weight distribution and has the extremely small vapor pressure compared to a mineral oil-based lubricating oil which is usually used in the reciprocating compressor. That is, the poly- α-olefin-based lubricating oil has an extremely small vapor component compared to a mineral oil-based lubricating oil. A lubricating oil which is used in the rod lubrication compression mechanism 31 may be mixed into a boil-off gas which is discharged from the compression mechanism 31. However, by using the poly- α-olefin-based lubricating oil having the small amount of vapor components, an amount of vaporous oil component contained in the boil-off gas discharged from the compression mechanism 31 can be greatly reduced.

[0045]

The poly- α-olefin-based lubricating oil contains a base oil made of poly- α-olefin or a hydride of poly- α-olefin, and various additives. Poly- α-olefin is an oligomer or a polymer which can be obtained by performing polymerization using straight chain α-olefin having a double bond at a terminal ( α position) as a raw material. Poly- α-olefin is a synthetic lubricating oil which is characterized in high viscosity index and a low flow point.

[0046]

As a monomer used for polymerization of poly- α-olefin, for example, α-olefin where the number of carbons is set to three to twenty can be used, and it is preferable to use α-olefin where the number of carbons is set to eight to twelve. Specifically, examples of α-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nanodecene, and 1-eicosene. Particularly, from a viewpoint of balance of the viscosity index, the low-temperature fluidity, and a low amount of evaporation, it is preferable to use α-olefin selected from a group consisting of 1-octene, 1-decene, and 1-dodecene, and it is more preferable to use 1-decene.

[0047]

Although the rod packing 62 shown in FIG.3 has the configuration in which the respective ring elements 63 are arranged adjacently to each other with no gap therebetween, the present invention is not limited to such a configuration. For example, as shown in FIG.4, a spacer 68 may be disposed between one ring element 63 to which oil is supplied and another ring element 63 which is positioned on a piston 58 side of the one ring element 63. In this case, since the ring elements 63 are spaced apart from each other due to the presence of the spacer 68 between the ring elements 63, it is possible to suppress occurrence of a phenomenon that a lubricating oil flows out toward the piston 58 side. The spacer 68 can be formed of a member having the same shape as the housing 63a, for example. In this case, on an outer peripheral surface of the rod 48, a portion which is not brought into contact with the ring elements 63 is present between one ring element 63 to which oil is supplied and the another ring element 63 positioned on a piston 58 side of the one ring element 63. In this case, the bent line 69 may be formed not on the ring element 63 but on the spacer 68.

[0048]

As described above, in this embodiment, the pre-stage compression part 21 which sucks and compresses a boil-off gas is formed of an oil-free-type compression mechanism. Accordingly, even in the case of compressing a low-temperature boil-off gas, it is possible to avoid the solidification of the oil. Further, the pre-stage compression part 21 is not designed to compress a boil-off gas to an extremely high pressure and hence, even when the oil-free-type compression mechanism is used as the pre-stage compression part 21, the lifetime of the pre-stage compression part 21 is not greatly affected. The post-stage compression part 23 includes the rod lubrication compression mechanism 31 which includes the piston 58 and the piston rod 48, and in which oil is supplied between the piston rod 48 and the rod packing 62. Accordingly, it is possible to prevent a compressed gas from leaking out toward the crankshaft 45 side through between the piston rod 48 and the rod packing 62, that is, it is possible to ensure the sealing property between the piston rod 48 and the rod packing 62. Further, in the rod lubrication compression mechanism 31, oil is not supplied between the piston 58 and the cylinder 55. Accordingly, it is possible to suppress mixing of oil into gas which is compressed by the rod lubrication compression mechanism 31.

Accordingly, even when a portion of the boil-off gas compressed by the post-stage compression part 23 is introduced into the re-liquefying line 29, it is possible to prevent oil from adhering to the re-liquefying line 29.

[0049]

Further, in this embodiment, the frontmost-stage compression mechanism 23a out of the plurality of stages of the compression mechanisms 23a to 23d which form the post-stage compression part 23 is used as the rod lubrication compression mechanism 31. In other words, in the post-stage compression part 23, while oil is supplied to the sliding portion of the piston rod 48 in the frontmost-stage compression mechanism 23a, the compression mechanisms 23b to 23d which are provided in the post stage of the compression mechanism 23a are each formed into an oil-free-type compression mechanism. Accordingly, even when oil supplied to the sliding portion of the piston rod 48 of the frontmost-stage compression mechanism 23a in the post-stage compression part 23 leaks out to the post-stage-side compression mechanisms 23b to 23d, it is possible to suppress the intrusion of the oil to the re-liquefying line 29 from such a state.

[0050]

In this embodiment, by providing the downstream compression part 25, a boil-off gas can be compressed to a higher pressure. Further, the downstream compression part 25 is formed of an oil-supply-type compression mechanism and hence, it is possible to prevent shortening of the lifetime of the downstream compression part 25 which is designed to realize a high pressure of a boil-off gas. The downstream compression part 25 is arranged downstream of the branch portion of the re-liquefying line 29 and hence, even when the downstream compression part 25 is formed of an oil-supply-type compression mechanism, it is possible to prevent intrusion of oil in the re-liquefying line 29.

[0051]

In this embodiment, oil which is supplied between the piston rod 48 and the rod packing 62 is a poly- α-olefin-based lubricating oil. The poly- α-olefin-based lubricating oil exhibits an extremely small vapor pressure compared to a mineral oil-based lubricating oil which is usually used in a reciprocating compressor. Accordingly, in the configuration in which a

poly- α-olefin-based lubricating oil is used, compared to a compression device which uses a mineral oil-based lubricating oil, it is possible to greatly reduce an amount of vaporous oil component contained in a boil-off gas discharged from the post-stage compression part 23. Accordingly, it is possible to suppress the precipitation of an oil component in the re-liquefying line 29.

[0052]

In this embodiment, the oil remover 33 is arranged at a portion of the gas line 27 between the rod lubrication compression mechanism 31 and the branch portion of the re-liquefying line 29. Accordingly, even when oil leaks out from the rod lubrication compression mechanism 31, the oil is removed by the oil remover 33 which is made of a coalescer or an activated carbon and hence, a possibility that oil intrudes into the re-liquefying line 29 can be further reduced.

[0053]

In this embodiment, another ring element 63 is disposed on a piston 58 side of the ring element 63 to which oil is supplied. Accordingly, it is possible to suppress occurrence of a state where oil supplied to the ring element 63 disposed on a side opposite to the piston 58 flows toward a piston 58 side. Accordingly, it is possible to suppress mixing of oil into a boil-off gas discharged from the rod lubrication compression mechanism 31.

[0054]

The present invention is not limited to the above-mentioned embodiment, and various modifications, improvements and the like are conceivable without departing from the gist of the present invention. For example, although the rod packing 62 includes the plurality of ring elements 63, the present invention is not limited to such a configuration. For example, the rod packing 62 may be formed of one ring element 63.

[0055]

In the embodiment, although the oil remover 33 is disposed in the gas line 27 between the rod lubrication compression mechanism 31 and the branch portion of the re-liquefying line 29, the oil remover 33 may be omitted. In FIG.3, an opening portion of the flow path 67 (a portion through which oil is discharged to the outside of the flow path 67) may be disposed on the outer peripheral surface of the recessed portion 63c.

[0056]

In the embodiment, the description has been made of an example in which a lubricating oil supplied to the rod packing 62 is a poly- α-olefin-based lubricating oil. However, the lubricating oil is not limited to such oil. A generally-used mineral oil-based lubricating oil may be used as the lubricating oil.

[0057]

In the embodiment, the downstream compression part 25 is provided. However, the downstream compression part 25 may be omitted. In the configuration shown in FIG.4, the spacer 68 may be omitted, and a portion where the spacer 68 is provided may be formed as a gap.

The above-mentioned embodiment is briefly described below.

[0058]

(1) The compression device according to the embodiment includes: the oil-free-type pre-stage compression part which is formed of a reciprocating compression mechanism which sucks and compresses a boil-off gas; the post-stage compression part for compressing the boil-off gas compressed by the pre-stage compression part; and the re-liquefying line for re-liquefying at least a portion of the boil-off gas compressed by the post-stage compression part. The post-stage compression part includes a reciprocating compression mechanism in one or more stages. The compression mechanism in one or more stages includes a rod lubrication compression mechanism which includes: a piston; a piston rod having a distal end on which the piston is mounted; and rod packing which surrounds the piston rod and seals a periphery of the piston rod, and oil is supplied between the piston rod and the rod packing.

[0059]

In the compression device, the pre-stage compression part which sucks and compresses a boil-off gas is formed of the oil-free-type compression mechanism. Accordingly, even in the case of compressing a low-temperature boil-off gas, it is possible to avoid the solidification of the oil. Further, the pre-stage compression part is not designed to compress a boil-off gas to an extremely high pressure and hence, even when the oil-free-type compression mechanism is used as the pre-stage compression part, the lifetime of the pre-stage compression part is not greatly affected. The post-stage compression part includes the rod lubrication compression mechanism which includes the piston and the piston rod, and in which oil is supplied between the piston rod and the rod packing. Accordingly, it is possible to prevent a compressed gas from leaking out through between the piston rod and the rod packing. That is, it is possible to ensure the sealing property between the piston rod and the rod packing. Further, in the rod lubrication compression mechanism, oil is not supplied between the piston and the cylinder. Accordingly, it is possible to suppress mixing of oil into gas which is compressed by the rod lubrication compression mechanism. Accordingly, even when a portion of the boil-off gas compressed by the post-stage compression part is introduced in the re-liquefying line, it is possible to prevent oil from adhering to the re-liquefying line.

[0060]

(2) The post-stage compression part may include compression mechanisms in a plurality of stages. In this case, the rod lubrication compression mechanism may be the compression mechanism in a frontmost stage out of the compression mechanisms in a plurality of stages, and the compression mechanisms in a post stage of the compression mechanisms including the rod lubrication compression mechanism may be each formed of an oil-free-type compression mechanism.

[0061]

In this mode, in the post-stage compression part, while oil is supplied to the sliding portion of the piston rod in the compression mechanism in a frontmost stage, the compression mechanisms in the compression mechanisms subsequent to the compression mechanism in the frontmost stage are each formed of an oil-free-type compression mechanism. Accordingly, even when oil supplied to the sliding portion of the piston rod of the compression mechanism in the frontmost stage in the post-stage compression part leaks out to the compression mechanisms in the post stages, it is possible to suppress the intrusion of the oil to the re-liquefying line from such a state.

[0062]

(3) The post-stage compression part may include compression mechanisms in four stages. In this case, the rod lubrication compression mechanism may be the compression mechanism in a frontmost stage in the post-stage compression part, and the compression mechanisms in three stages other than the rod lubrication compression mechanism may be each formed of an oil-free-type compression mechanism.

[0063]

In this mode, in the post-stage compression part, while oil is supplied to the sliding portion of the piston rod in the compression mechanism in a frontmost stage, the compression mechanisms in three stages subsequent to the compression mechanism are each formed of an oil-free-type compression mechanism. Accordingly, even when oil supplied to the sliding portion of the piston rod of the compression mechanism in the frontmost stage in the post-stage compression part leaks out to the compression mechanisms in the post stages, it is possible to suppress the intrusion of the oil to the re-liquefying line from such a state.

[0064]

(4) The compression device may further include a downstream compression part which is disposed downstream of a branch portion of the re-liquefying line, and is formed of a reciprocating compression mechanism for further compressing the boil-off gas compressed by the post-stage compression part. In this case, the downstream compression part may be formed of an

oil-supply-type compression mechanism.

[0065]

In this mode, by providing the downstream compression part, a boil-off gas can be compressed to a higher pressure. Further, the downstream compression part is formed of an oil-supply-type compression mechanism and hence, it is possible to prevent shortening of the lifetime of the downstream compression part which is designed to realize a high pressure of a boil-off gas. The downstream compression part is disposed downstream of the branch portion of the re-liquefying line and hence, even when the downstream compression part is formed of the oil-supply-type compression mechanism, it is possible to prevent intrusion of oil in the

re-liquefying line.

[0066]

(5) Oil which is supplied between the piston rod and the rod packing may be a

poly- α-olefin-based lubricating oil.

[0067]

The poly- α-olefin-based lubricating oil exhibits an extremely small vapor pressure compared to a mineral oil-based lubricating oil which is usually used in a reciprocating compressor. Accordingly, in the configuration in which the poly- α-olefin-based lubricating oil is used, compared to the compression device which uses a mineral oil-based lubricating oil, it is possible to greatly reduce an amount of vaporous oil component contained in the boil-off gas discharged from the post-stage compression part. Accordingly, it is possible to suppress the precipitation of the oil component in the re-liquefying line.

[0068]

(6) An oil remover made of a coalescer or an activated carbon may be disposed between the rod lubrication compression mechanism and the branch portion of the re-liquefying line.

[0069]

In this mode, even when oil leaks out from the rod lubrication compression mechanism, oil is removed by the oil remover which is made of a coalescer or an activated carbon and hence, a possibility that oil intrudes in the re-liquefying line can be further decreased.

[0070]

(7) The rod packing may include ring elements which are arranged in an axial direction of the piston rod. In this case, oil may be supplied to a ring element positioned on a side opposite to the piston out of the ring elements.

[0071]

In this mode, the ring element is disposed also on a piston side of the ring element to which oil is supplied. Accordingly, it is possible to suppress occurrence of a phenomenon that oil supplied to the ring element disposed on a side opposite to the piston flows toward the piston side. Accordingly, it is possible to suppress the mixing of oil into a boil-off gas discharged from the rod lubrication compression mechanism.

[0072]

(8) A spacer or a gap may be provided between the one ring element to which oil is supplied and a ring element which is positioned on a piston side of the one ring element.

[0073]

In this mode, it is possible to suppress occurrence of a phenomenon that oil supplied to the ring element disposed on a side opposite to the piston flows to the ring element on a piston side. Accordingly, it is possible to suppress the mixing of oil into a boil-off gas discharged from the rod lubrication compression mechanism.

[0074]

As described above, according to the present invention, it is possible to prevent adhesion of oil to the re-liquefying line while ensuring the sealing property of the compression device which compresses a boil-off gas.

This application is based on Japanese Patent Application No.2018-203782 filed on October 30, 2018, the contents of which are hereby incorporated by reference.

Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.

Claims

1. A compression device comprising:

an oil-free-type pre-stage compression part which is formed of a reciprocating compression mechanism which sucks and compresses a boil-off gas;

a post-stage compression part for compressing the boil-off gas compressed by the pre-stage compression part; and

a re-liquefying line for liquefying at least a portion of the boil-off gas compressed by the post-stage compression part,

wherein the post-stage compression part includes a reciprocating compression mechanism in one or more stages,

the compression mechanism in one or more stages includes a rod lubrication compression mechanism which includes: a piston; a piston rod having a distal end on which the piston is mounted; and rod packing which surrounds the piston rod and seals a periphery of the piston rod, and oil is supplied between the piston rod and the rod packing.

2. The compression device according to claim 1, wherein

the post-stage compression part includes compression mechanisms in a plurality of stages, the rod lubrication compression mechanism is the compression mechanism in a frontmost stage out of the compression mechanisms in a plurality of stages, and

the compression mechanisms in a post stage of the rod lubrication compression mechanism are each formed of an oil-free-type compression mechanism.

3. The compression device according to claim 1, wherein

the post-stage compression part includes compression mechanisms in four stages, the rod lubrication compression mechanism is the compression mechanism in a frontmost stage in the post-stage compression part, and

the compression mechanisms in three stages other than the rod lubrication compression mechanism are each formed of an oil-free-type compression mechanism.

4. The compression device according to any one of claims 1 to 3, wherein

the compression device includes a downstream compression part which is disposed downstream of a branch portion of the re-liquefying line, and is formed of a reciprocating compression mechanism for further compressing the boil-off gas compressed by the post-stage compression part, and

the downstream compression part is formed of an oil-supply-type compression mechanism.

5. The compression device according to any one of claims 1 to 3, wherein oil which is supplied between the piston rod and the rod packing is a poly- α-olefin-based lubricating oil.

6. The compression device according to any one of claims 1 to 3, further comprising an oil remover made of a coalescer or an activated carbon disposed between the rod lubrication compression mechanism and the branch portion of the re-liquefying line.

7. The compression device according to any one of claims 1 to 3, wherein

the rod packing includes ring elements which are arranged in an axial direction of the piston rod, and

oil is supplied to a ring element which is positioned on a side opposite to the piston out of the ring elements.

8. The compression device according to claim 7, wherein a spacer or a gap is provided between said one ring element to which oil is supplied and a ring element positioned on a piston side of said one ring element.