CH716089A2 - Compressor unit for boil-off gas from a ship's LNG storage tank and method for stopping the compressor unit. - Google Patents

Abstract

This invention discloses a compressor unit (100) installed in a ship and configured to collect a target gas that is a boil-off gas from an LNG storage tank (101) of the ship and at least a portion of the Delivering target gas to a destination that needs the target gas. The compressor unit (100) comprises: a plurality of dampers (263-272) which are provided between a plurality of compression stages (201-205) in order to prevent pressure fluctuations; a first sealing part configured to seal between a piston and a cylinder part; and a second seal part that surrounds a periphery of a piston rod and is configured to prevent the target gas sucked into the cylinder part from flowing toward the crank mechanism. All of the first and second seal parts are of the oil-free type.

Description

Field of technology

The present invention relates to a compressor unit that supplies a target gas, which is a boil-off gas, from an LNG storage tank of a ship to a destination requiring the target gas.

State of the art

[0002] Conventionally, as disclosed in JP-A-2011-517749, a compressor has been developed which increases the pressure of a boil-off gas generated from a liquefied natural gas (LNG) and the boil-off gas delivers to a destination that needs the target gas, e.g. an engine.

In this connection, in a compressor used in an LNG ship, a lubricating compressor is used (e.g., paragraph 0021 of JP-A-2018-128039, paragraph 0114 of JP-B-6371930).

Normally, an oil used in the compressor is discharged from the compressor in a state of being mixed with a boil-off gas, and then separated and collected from the boil-off gas by an oil separator. In recent years, however, the demand for a clean boil-off gas has increased, and developments are being made to collect an oil more reliably by using a carbon filter in addition to the oil separator. It should be noted that, as disclosed in paragraph 0024 of US 2018/0066802 A, an oil slinger or oil wiper seal may be provided to prevent oil from moving between a compression cylinder and a compression frame.

Meanwhile, as disclosed in paragraph 0019 of JP-A-2017-89595, a labyrinth reciprocating compressor that does not require lubricant has also been developed. However, since the labyrinth sealing method is generally non-contact between a piston and a cylinder, the problem is that a gas in the compression chamber is more likely to leak than in the case of the piston ring sealing method. This problem arises particularly when a high pressure gas is compressed.

Summary of the invention

The present invention is to improve the reliability of the compressor unit.

A compressor unit according to one aspect of the present invention is installed in a ship and configured to collect a target gas that is a boil-off gas from an LNG storage tank of the ship and at least a part of the target gas to one Destination that needs the target gas. The compressor unit includes: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each of the compression stages. Each of the plurality of compression stages includes: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and define a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas sucked into the cylinder part from flowing toward the crank mechanism; a scraper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second seal part and configured to further prevent the lubricant from entering the cylinder part. All of the first and second seal parts are of the oil-free type. In at least one final compression stage, the first sealing part comprises a piston ring group which is provided on an outer peripheral part of the piston and is configured to seal between the piston and the cylinder part, the second sealing part comprising: a plurality of housing parts interposed between the cylinder part and the Piston rod are arranged; and a plurality of ring parts held by the plurality of housing parts, wherein the first sealing part and the second sealing part of the at least final compression stage are of the contact type.

A compressor unit according to another aspect of the present invention is installed in a ship and configured to collect a target gas that is a boil-off gas from an LNG storage tank of the ship and at least a part of the target gas provides a destination that requires the target gas. The compressor unit includes: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each of the compression stages. Each of the plurality of compression stages from a first compression stage to an immediately preceding compression stage of a final compression stage comprises: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and define a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas sucked into the cylinder part from flowing toward the crank mechanism; a scraper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second seal part and configured to further prevent the lubricant from entering the cylinder part. The immediately preceding compression stage of the final compression stage and the final compression stage have a tandem structure in which the cylinder part of the final compression stage is arranged on the cylinder part of the immediately preceding compression stage of the final compression stage. The piston in the immediately preceding compression stage of the final compression stage and the piston in the final compression stage, which is smaller in diameter than the piston in the immediately preceding compression stage of the final compression stage, are unitarily configured. The final compression stage is shared by the piston rod, the second sealing part, the wiper part and the oil slinger part with the compression stage immediately preceding the final compression stage. At least in the final compression stage, the first seal part includes a piston ring group that is provided on an outer peripheral part of the piston and is configured to seal between the piston and the cylinder part and configured as a contact type. In at least the immediately preceding compression stage of the final compression stage, the second sealing part comprises: a plurality of housing parts which are arranged between the cylinder part and the piston rod; and a plurality of ring parts held by the plurality of housing parts and configured as a contact type. All of the first and second seal parts are of the oil-free type.

Regarding a method of stopping a compressor unit according to another aspect of the present invention, the compressor unit may include: a check valve provided in the discharge-side flow path of the final compression stage; a decompression line connected to the discharge side flow path further downstream of the check valve; and an on-off valve provided in the discharge-side flow path downstream of the decompression pipe. The method can include the following: closing the on-off valve and reducing the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill.

Regarding a method of stopping a compressor unit according to another aspect of the present invention, the compressor unit may include: a check valve provided in the discharge-side flow path of the final compression stage; and a decompression line connected to the discharge side flow path between the final compression stage and the check valve. The method can include the following: reducing the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill.

The technique described above can improve the reliability of the compressor unit.

The object, feature and advantage of the present invention will become more apparent from the following detailed description and accompanying drawings.

Brief description of the drawings

[0013] <tb> FIG. 1 <SEP> is a schematic flow chart of a compressor unit according to an embodiment of the present invention; <tb> FIG. 2 <SEP> is a schematic diagram of a compressor; <tb> FIG. 3 <SEP> is a schematic cross-sectional view of a second seal part of the compressor; <tb> FIG. 4 <SEP> is a schematic flow chart of part of another compressor unit; <tb> FIG. 5 <SEP> is a schematic flow chart of part of another compressor unit; <tb> FIG. 6 <SEP> is a schematic flow chart of another compressor unit; <tb> FIG. 7 <SEP> is a schematic flow chart of another compressor unit; <tb> FIG. 8 <SEP> is a schematic flow chart of another compressor unit; <tb> FIG. 9 <SEP> is a schematic cross-sectional view of the second sealing part; <tb> FIG. 10 <SEP> is a schematic plan view of a cylinder part of the compressor; <tb> FIG. 11 <SEP> is a schematic longitudinal sectional view of the cylinder part; <tb> FIG. Fig. 12 <SEP> is a schematic longitudinal sectional view of the cylinder part; <tb> FIG. 13 <SEP> is a schematic plan view of another cylinder part; <tb> FIG. 14 <SEP> is a schematic longitudinal sectional view of the cylinder part; <tb> FIG. 15 <SEP> is a schematic plan view of another cylinder part; <tb> FIG. 16 <SEP> is a schematic cross-sectional view of the second sealing part; <tb> FIG. 17 <SEP> is a schematic representation of a compression stage having a tandem structure; <tb> FIG. 18 <SEP> is a schematic representation of the compression stage with tandem structure; <tb> FIG. 19 <SEP> is a schematic representation of the compression stage with tandem structure; <tb> FIG. 20 <SEP> is a schematic representation of two compression stages with a double-acting structure; <tb> FIG. 21 <SEP> is a schematic flow chart of another compressor unit; <tb> FIG. 22 <SEP> is a schematic flow chart of another compressor unit; and <tb> FIG. 23 <SEP> is a schematic diagram of a horizontal compressor.

Description of the embodiments

FIG. 1 is a schematic flow diagram of a compressor unit 100 according to an embodiment of the present invention. FIG. 2 is a schematic illustration of a compressor 500 that forms the compressor unit 100. The compressor unit 100 is illustrated with reference to FIG. 1 and 2 described.

The compressor unit 100 is installed in a ship (not shown) with an LNG storage tank 101 in which a liquefied natural gas (LNG) is stored. The compressor unit 100 is configured to collect a target gas that is a boil-off gas generated in the LNG storage tank 101. The compressor unit 100 is configured to increase the pressure of the collected target gas to about 300 bar and deliver the pressurized target gas to a predetermined destination requiring the target gas (e.g., a ship engine). In the following description, the terms “upstream” and “downstream” are used depending on the direction of flow of the target gas.

The compressor unit 100 includes a flow path 110 through which the target gas flows to the destination requiring the target gas; the compressor 500; a bypass line 411 configured so that the target gas is returned to the upstream side; several dampers and several coolers (see FIG. 1). In FIG. 1, the compressor unit 100 is shown as a device with components which are shown in a double-dashed chain line from FIG. 1 (similar to FIG. 6 to 8). The compressor 500 includes multiple compression stages, a crank mechanism that serves as a common drive source for the multiple compression stages, a crankcase 301 in which the crank mechanism is housed, and six transverse guides 303 which are attached to the crankcase 301 (see FIG. 2). The multiple compression stages include a first compression stage 201, a second compression stage 202 which is a next stage of the first compression stage 201, a third compression stage 203 which is a next stage of the second compression stage 202, a fourth compression stage 204 which is a next stage of the third Compression stage 203 is, and a fifth compression stage 205, which is a next stage of the fourth compression stage 204. The pressure of the target gas flowing through the flow path 110 is sequentially increased through the multiple stages of compression. The plurality of dampers are provided before and after the compression stages in order to prevent pressure fluctuations in the target gas caused by intermittent suction and discharge associated with the reciprocation of the piston in the respective compression stages 201-205. Several coolers are provided for cooling the target gas compressed in the various compression stages.

An upstream end of the flow path 110 is connected to an upper part of the LNG storage tank 101 so that the boil-off gas generated in the LNG storage tank 101 flows in. A downstream end of the flow path 110 is connected to the destination requiring the target gas.

The flow path 110 includes a storage tank connection flow path 111, a stage connection flow path 113, and a demand target connection flow path 114. The storage tank connection flow path 111 is connected to the LNG storage tank and directs the boil-off gas to the compressor unit 100. Da When there are two first compression stages 201, the storage tank connection flow path 111 branches into the branch parts 111A and 111B, these branch parts 111A and 111B being connected to the first compression stage 201, respectively. The branch parts 111A and 111B are provided with the dampers 261 and 262, respectively. The stage connection flow path 113 connects the compression stages 201 to 205. In the stage connection flow path 113, a connection part with the first compression stages 201 branches into two branch parts 113A and 113B. Other parts of the stage connection flow path 113 are provided with the second to fifth stages of compression 202 to 205, the dampers 263 to 268, 271, 272, and a plurality of coolers 281 to 284. The demand-target connection flow path 114 is a flow path that connects the fifth stage compression 205 and the target requesting the target gas, and a damper 273 and a cooler 285 are provided.

The two first compression stages 201 are provided in two branch parts 111A and 111B so that they are arranged parallel to one another. Second to fifth stages of compression 202 to 205 are provided in the stage connection flow path 113 at intervals.

The crank mechanism is configured so that rotation of a crankshaft is converted into linear reciprocating motion of a plurality of cross heads. The crankshaft is driven by a motor 302. The cross heads are used as connecting areas with piston rods 213 of the first to fifth compression stages 201 to 205.

The crankshaft is connected to the engine 302 through a through hole formed in the crankcase 301. The crankcase 301 is configured to prevent leakage of a lubricant used to lubricate the crank mechanism around the through hole, but does not have a sealed structure (airtight structure). Therefore, the pressure inside the crankcase 301 is substantially equal to atmospheric pressure.

The six transverse guides 303 are arranged at intervals in the horizontal direction and protrude in a direction substantially perpendicular to the horizontal direction (more precisely, in the present embodiment, upward in a gravitational direction). In the transverse guides 303, the traverses described above perform a back and forth movement.

A locking part 306 is provided in each transverse guide 303. In the center of the locking part 306 a through hole is formed through which the piston rod 213 connecting the piston, which is moved back and forth in each of the compression stages 201 to 205, and the corresponding cross head can be passed.

To increase the safety of the compressor unit 100, an inert gas (e.g. nitrogen) is fed to the interior of the transverse guide 303 above the blocking part 306. The supply pressure of the inert gas is substantially equal to the atmospheric pressure. Therefore, the pressure in the inner space of the cross guide 303 is substantially equal to the atmospheric pressure, similar to the pressure in the inner space of the crankcase 301.

The first to fifth compression stages 201 to 205 are constructed in accordance with the positions of the transverse guides 303 arranged in the horizontal direction. The first compression stage 201, the fourth compression stage 204, the fifth compression stage 205, the second compression stage 202, the third compression stage 203, and the first compression stage 201 are arranged in this order from the engine 302 side. The first to fifth compression stages 201 to 205 are connected via the flow path 110 to produce the steps shown in FIG. 1 pipe connection shown. Note that FIG. Fig. 2 schematically shows the arrangement of the first to fifth compression stages 201 to 205, and actually the first to fifth compression stages 201 to 205 are close to each other. In addition, the arrangement order of the respective compression stages 201 to 205 is not limited to this.

The first compression stage 201 comprises a cylinder part 211, a piston 212, the piston rod 213, a pair of suction valves 214, a pair of exhaust valves 215 and a cylinder liner (not shown).

The cylinder part 211 comprises a tube part 216 which is substantially coaxial with the cross guide 303, a rear head 217 which is attached to an opening end of the tube part 216 on one side of the crank mechanism, and a front head 218 which is the other opening end of the pipe part 216 closes. In the central position of the rear head 217, the through hole and a recessed part are formed substantially coaxially with the through hole. The recessed part of the rear head 217 is open to the crank mechanism side.

The piston 212 is accommodated in a housing space of the cylinder part 211 which is surrounded by the tube part 216, the rear head 217 and the front head 218. In the cylinder part 211, the compression chambers 221 and 222 for compressing the target gas are respectively formed between an end surface on the crank mechanism side of the piston 212 and the rear head 217 and between an end surface on the opposite side of the crank mechanism of the piston 212 and the front head 218. In this way, the first compression stage 201 has a double-acting structure in which the compression chambers 221 and 222 are formed on both sides of the piston 212.

The pair of suction valves 214 are attached to suction ports that are formed at positions corresponding to the compression chambers 221 and 222. When the pressure of the target gas in the compression chambers 221 and 222 becomes equal to or less than the pressure upstream of the suction valves 214, the suction valves 214 allow the target gas to flow into the compression chambers 221 and 222.

The pair of discharge valves 215 is attached to discharge ports that are formed at positions corresponding to the compression chambers 221 and 222. When the pressure of the target gas in the compression chambers 221 and 222 becomes equal to or greater than the pressure behind the exhaust valves 215, the exhaust valves 215 let the target gas flow out of the compression chambers 221 and 222.

The cylinder liner (not shown) is a cylindrical member which is fixed to the inner peripheral surface of the cylinder part 211 to prevent the cylinder part 211 from being worn, and is made of cast iron or alloy steel. The cylinder liner can be exchanged if it is worn through contact with a first sealing part described later. In the following description, the cylinder liner is described as part of the cylinder part 211.

The piston rod 213 is connected to the end face of the piston 212 on the side of the crank mechanism and the cross head of the crank mechanism. The piston rod 213 goes through the rear head 217, reaches to the crank mechanism in the transverse guide 303 and is inserted into the through hole of the locking part 306.

The first compression stage 201 includes a scraper part 231 and an oil slinger 232 to prevent the lubricant used to lubricate the crank mechanism from entering the compression spaces 221 and 222 through the outer peripheral part of the piston rod 213.

The scraper part 231 is an annular sealing element which surrounds a circumference of the piston rod 213. The wiper part 231 is attached to the locking part 306. The inner peripheral part of the wiper part 231 is in contact with the outer peripheral part of the piston rod 213.

The oil slinger 232 is an annular plate member. The oil slinger 232 is attached to the piston rod 213 between the scraper part 231 and the rear head 217.

The first compression stage 201 includes a first seal part 241 and a second seal part 242. The first seal part 241 is provided to prevent the circulation of the target gas between the compression chambers 221 and 222. The second seal part 242 is provided to prevent the target gas from leaking from the compression chamber 221 into the cross guide 303.

The first seal part 241 includes a plurality of piston rings 243 (piston ring group) attached to the outer peripheral part of the piston 212. That is, the first seal part 241 is a contact seal member that seals between the piston 212 and the inner surface of the cylinder part 211 by bringing the outer peripheral parts of the piston rings 243 into contact with the cylinder part 211 (more specifically, the cylinder liner is not shown ). Meanwhile, the first sealing part 241 is also an oil-free (i.e., non-lubricating) sealing element in which the lubricant is not directed to the piston rings 243. It should be noted that a follower ring that prevents contact between the piston 212 and the inner surface of the cylinder part 211 is not shown.

In the first compression stage 201, each piston ring 243 is molded from a material, the main component of which is polytetrafluoroethylene (PTFE) or modified PTFE. The situation is similar in the second to fourth compression stages 202 to 204.

FIG. 3 shows a schematic cross section of the second sealing part 242. As shown in FIG. 2 and 3, the second sealing part 242 is a so-called rod seal and comprises several housing parts 244, several ring parts 249 and a holding part 294. The housing parts 244 and the ring parts 249 surround a circumference of the piston rod 213, which is arranged in the rear head 217.

The plurality of housing parts 244 are housed in the recessed part between the rear head 217 and the piston rod 213.

Each housing part 244 includes a substantially circular bottom part 251 and a peripheral wall part 252 protruding from an outer edge of the bottom part 251 toward the side of the crank mechanism. A through hole into which the piston rod 213 is inserted is formed substantially in the center of the bottom part 251. The ring parts 249 are housed in each housing part 244.

The holding part 294 is closer to the crank mechanism than the housing parts 244. The holding part 294 is fastened with a bolt or the like. (not shown) attached to rear head 217.

The plurality of ring parts 249 are arranged along an axial direction of the piston rod 213. The inner peripheral parts of the ring parts 249 come into contact with the outer peripheral part of the piston rod 213. That is, the second sealing part 242 seals between the piston rod 213 and the rear head 217 as a contact sealing element. Meanwhile, the second sealing part 242 is also an oil-free (i.e. non-lubricating) sealing element in which the lubricant is not directed to the ring parts 249.

In the present embodiment, each ring member 249 is formed from a material whose main component is polytetrafluoroethylene (PTFE) or modified PTFE. The situation is similar in the second to fourth compression stages 202 to 204.

The second to fourth compression stages 202 to 204 are substantially identical to the first compression stage 201, except that a diameter of the piston 212 and an inner diameter of the cylinder part 211 are smaller than those of the first compression stage 201. That is, the first seal part 241 and the second seal part 242 of each of the second to fourth compression stages 202 to 204 are of the contact type and the oil-free type. The second to fourth compression stages 202 to 204 also have a double-acting structure.

The diameter of the piston 212 and the inner diameter of the cylinder part 211 are smaller in the fifth compression stage 205 than in the first to fourth compression stages 201 to 204. In the cylinder part 211 of the fifth compression stage 205 is formed in a space on the opposite side of the crank mechanism with the piston 212 arranged in between, the compression chamber 222, similar to the first compression stage 201.

In the meantime, a piece of pipe 119 is connected to the space on the side of the crank mechanism with the piston 212 without the suction valve interposed at a place where the suction valve is to be attached. The pipe piece 119 is connected to the stage connection flow path 113 on the suction side of the fifth stage compression 205. Thereby, the space on the crank mechanism side with the intervening piston 212 of the cylinder part 211 always communicates with the stage connection flow path 113. That is, the space is a non-compression chamber 223 which is not used for compressing the target gas. In this way, in contrast to the other compression stages 201 to 204, the fifth compression stage 205 has a single-acting structure in which only the space on one side of the piston 212 serves as the compression chamber 222. It should be noted that the pipe piece 119 on the discharge side of the fifth compression stage 205 may be connected to the on-demand target connection flow path 114.

Since the fifth compression stage 205 receives the highest pressure of the second to fifth compression stages 202 to 205, the cylinder part 211 contains a forged material.

The fifth compression stage 205 comprises the first sealing part 241 and the second sealing part 242. The first sealing part 241 of the fifth compression stage 205 is a contact sealing element which, similar to the first compression stage 201, contains the plurality of piston rings 243 (piston ring group) and between the piston 212 and the inner surface of the cylinder part 211. In addition, the first seal part 241 is also oil-free (i.e., a structure in which no lubricant is supplied to the piston rings). Each piston ring 243 is formed from a material whose main component is at least one of polyimide (PI) and polyetheretherketone (PEEK), or a material whose main component is a mixture of either or both and PTFE or modified PTFE. By using such a main component material, the piston ring 243 has a higher flexural strength (modulus of elasticity) than a piston ring whose main component consists only of polytetrafluoroethylene (PTFE). Alternatively, the piston ring 243 can be formed from a material whose main component is a different engineering plastic (e.g. polyamide (PA)) with a higher flexural strength (modulus of elasticity) than a piston ring whose main component is only polytetrafluoroethylene (PTFE). Furthermore, the piston ring 243 may alternatively be formed by molding carbon fibers. These alternative materials also have high sealing performance and high durability performance, similarly to the piston ring 243 which is formed by using a material whose main component is at least one of polyimide (PI) and polyetheretherketone (PEEK), or a material whose main component is is a mixture of either or both and PTFE or modified PTFE. The situation is similar with the ring parts 249 described later.

The second seal part 242 of the fifth stage of compression 205 is a contact seal member in which the inner peripheral parts of the ring parts 249 come into contact with the outer peripheral part of the piston rod 213 in a manner similar to that of the first stage of compression 201. The second seal part 242 is also oil-free (i.e., of a structure in which the lubricant is not supplied to the ring parts 249).

Each ring member 249 is formed from a material whose main component is at least one of polyimide (PI) and polyetheretherketone (PEEK), similar to the piston rings 243. Alternatively, the ring member 249 may be formed from a material whose main component is another technical one Plastic (e.g. polyamide (PA)) with a higher flexural strength (modulus of elasticity) than polytetrafluoroethylene (PTFE). Further, alternatively, the ring member 249 may be formed by molding carbon fibers. These alternative materials also have high sealing performance and high durability performance, similar to the ring member 249 which is formed by using a material whose main component is at least one of polyimide (PI) and polyetheretherketone (PEEK).

The number of sets of the housing parts 244 and the ring parts 249 of the second sealing part 242 is greater in the fifth compression stage 205 than in the first compression stage 201. In this configuration, the length of the second sealing part 242 in the axial direction in the fifth compression stage 205 longer than in the first compression stage 201, with part of the second sealing part 242 protruding from the rear head 217 in the direction of the crank mechanism. A sealing area of the second sealing part 242 is larger in the fifth compression stage 205 than in the first compression stage 201, whereby it is possible to seal a higher-quality target gas. The fifth stage of compression 205 is similar to the first stage of compression 201 in a different structure.

In order to reduce the force imbalance, the total weight of the piston 212 and the piston rod 213 and the weight of the corresponding cross head between the first to fifth compression stages 201 to 205 are essentially the same. It should be noted that the weight of the cross head can be adjusted by adding a weight.

Each of the plurality of dampers is a pressure-resistant container provided on the flow path 110. The volume of these dampers is set large enough to reduce the pressure fluctuations of the inflowing target gas. The dampers 261 and 262 are provided in the two branch parts 111A and 111B, respectively, and are located in the vicinity of the first compression stages 201. Fluctuations in the suction pressure of the two first compression stages 201 are prevented.

Another damper 263 is provided at the downstream end of the branch parts 113A and 113B. The target gas compressed in the first two compression stages 201 flows into the damper 263. The damper 263 is located in the vicinity of the first compression stages 201 and prevents fluctuations in the outlet pressure of the first compression stages 201. The damper 263 can also be divided into two parts.

Another damper 264 is provided downstream of the damper 263. The damper 264 is located in the vicinity of the second compression stage 202 and prevents fluctuations in the suction pressure of the second compression stage 202. In this way, the two dampers 263 and 264 are in a flow path section between the first pressure stage 201 and the second pressure stage 202 in the stage connection flow path 113 intended. A distance between the dampers 263 and 264 (distance along the stage connection flow path 113, hereinafter the same) is greater than a distance between the first compression stage 201 and the damper 263 and a distance between the second compression stage 202 and the damper 264. Also between others , compression stages described below, two dampers are arranged to have a ratio similar to this distance ratio.

The dampers 265 and 266 are provided in a flow path section between the second compression stage 202 and the third compression stage 203. The damper 265 is located in the vicinity of the second stage compression 202, and fluctuations in the discharge pressure of the second stage compression 202 are prevented. The damper 266 is close to the third compression stage 203, and fluctuations in the suction pressure of the third compression stage 203 are prevented.

The dampers 267 and 268 near the third compression stage 203 and the fourth compression stage 204 are provided in a flow path section between the third compression stage 203 and the fourth compression stage 204. The dampers 267 and 268 prevent fluctuations in the final pressure of the third compression stage 203 and the suction pressure of the fourth compression stage 204. The dampers 271 and 272 near the compression stages 204 and 205 are provided in a flow path section between the fourth compression stage 204 and the fifth compression stage 205. Variations in the discharge pressure of the fourth compression stage 204 and the suction pressure of the fifth compression stage 205 are prevented.

The remaining one damper 273 is disposed near the fifth compression stage 205 in the on-demand target connection flow path 114. The damper 273 prevents fluctuations in the discharge pressure of the fifth compression stage 205.

The multiple coolers are provided in the stage connection flow path 113 and the on-demand target connection flow path 114. In particular, the cooler 281 is arranged in the flow path section between the dampers 263 and 264. Another cooler 282 is arranged in the flow path section between the dampers 265 and 266. Another cooler 283 is arranged in the flow path section between the dampers 267 and 268. Another cooler 284 is arranged in the flow path section between the dampers 271 and 272. The remaining cooler 285 is disposed downstream of the damper 273 in the on-demand target connection flow path 114. The coolers 281 to 285 are provided to cool the target gas compressed by the first to fifth compression stages 201 to 205.

The compressor unit 100 is configured to perform control to adjust the pressure and the flow rate of the target gas to be supplied to the destination requiring the target gas and control to decompress the flow path 110 when the compressor 500 is stopped . The control regions used for such controls are described below.

In order to adjust the pressure and the flow rate of the target gas to be supplied to the destination that needs the target gas, the compressor unit 100 comprises the bypass line 411, a control valve 412, a pressure sensor 413 and a control unit 414. The bypass line 411 branches off between the cooler 284 and the damper 272 on the suction side of the fifth stage compression 205 in the stage connection flow path 113 and is connected to the accumulator connection flow path 111. That is, the bypass line 411 bypasses the first to fourth compression stages 201 to 204 and the dampers 261 to 268 and 271 to return the target gas upstream of the first compression stage 201. The control valve 412 is provided in the bypass line 411. The pressure sensor 413 is arranged between the cooler 284 and the damper 272 and detects the pressure of the target gas on the suction side of the fifth compression stage 205.

The pressure sensor 413 and the control valve 412 are electrically connected to the control unit 414. The control unit 414 controls the opening degree of the control valve 412 based on the pressure detected by the pressure sensor 413. It should be noted that the control unit 414 can be designed as software or as a dedicated circuit.

The compressor unit 100 comprises a decompression line 415, two on-off valves 416 and 417 and a check valve 418 for decompression control. The check valve 418 is located in a discharge-side flow path of the fifth compression stage 205, i. the final compression stage (i.e., on-demand target connection flow path 114). The on-off valve 416 is provided downstream of the check valve 418. The opening degree of the on-off valve 416 is controlled in response to receipt of a command signal from the control unit 414. The decompression line 415 branches off the demand target connection flow path 114 downstream of the check valve 418 and upstream of the on-off valve 416. The tip of the decompression line 415 can be open to the atmosphere or connected to a flare system that burns the target gas released from the compressor unit 100 through the decompression line 415. The on-off valve 417 is provided in the decompression pipe 415. The opening degree of the on-off valve 417 is controlled in response to receiving a command signal from the control unit 414. When the compressor unit 100 is driven, the on-off valve 417 is normally closed.

The operation of the compressor unit 100 and the passage of the target gas are described below.

When the motor 302 operates, the cross heads in the crank mechanism perform a linear reciprocating motion. The force of the cross heads is transmitted via the piston rods 213 of the first to fifth compression stages 201 to 205 to the pistons 212 of the first to fifth compression stages 201 to 205. As a result, these pistons 212 also reciprocate linearly.

At this point in time, in each of the compression stages 201 to 205, the lubricant used in the crank drive is to move along the outer peripheral part of the piston rod 213 to the cylinder part 211. However, since the inner peripheral part of the wiper part 231 is in contact with the outer peripheral part of the piston rod 213, most of the lubricant that has flowed out of the crankcase 301 is wiped off by the wiper part 231. With this configuration, the penetration of the lubricant into the cylinder part 211 is prevented.

In addition, the oil slinger 232 is provided closer to the cylinder part 211 than the scraper part 231 is provided on the piston rod 213. In this configuration, the oil slinger 232 prevents the lubricant from penetrating even if only a very small amount of the lubricant passes through the wiper part 231.

In each of the first to fourth compression stages 201 to 204, the suction and discharge of the target gas in the two compression chambers 221 and 222 in conjunction with the reciprocating movement of the piston 212 are alternately repeated. In the fifth compression stage 205, the target gas is sucked and discharged in the compression chamber 222. The target gas discharged from the compression stages 201 to 205 is cooled by the coolers 281 to 285.

While the compressor 500 is in operation, the pressure sensor 413 detects the suction pressure of the fifth compression stage 205. The detected pressure is output to the control unit 414. Based on the detected pressure, the control unit 414 controls the opening degree of the control valve 412 so that the suction pressure of the fifth compression stage 205 becomes substantially constant. In the fifth compression stage 205, the pressure of the target gas is increased from approx. 100 bar to 150 bar by the first to fourth compression stages 201 to 204 to approx. 300 bar. Therefore, wear of the first seal part 241 is likely to occur, and pressure fluctuation is likely to occur due to a decrease in the processing amount. Meanwhile, stable operation can be continued in the compressor unit 100 because the suction pressure of the fifth compression stage 205 is controlled to be substantially constant using the bypass line 411.

When the compressor 500 stops, an external signal requesting decompression processing for the compressor unit 100 is input to the control unit 414. The external signal may be generated in response to an operation by an operator or when a sensor that monitors the status of the compressor unit 100 detects an abnormality of the compressor unit 100. In response to receiving the external signal from an instrument downstream of the compressor unit 100, the control unit 414 generates a command signal to close the on / off valve 416 and a command signal to open the on / off valve 417. These command signals are on the on-off valves 416 and 417 are output. The on-off valve 416 closes in response to the command signal, while the on-off valve 417 opens in response to the command signal.

As soon as the on-off valve 417 is opened, the target gas is expelled in the fifth compression stage 205 through the decompression line 415. By arranging the check valve 418 between the fifth compression stage 205 and the decompression line 415, a backflow from the decompression line 415 to the fifth compression stage 205 is prevented. Also, once the on-off valve 416 is closed, backflow of the target gas from the destination requiring the target gas is prevented. The target gas that has flowed into the decompression line 415 is released into the atmosphere or burned in the flare. It should be noted that, in the compressor unit 100, the target gas can also be decompressed through the decompression pipe 415 in the first to fourth compression stages 201 to 204. In addition, another decompression line can be provided in the first to fourth compression stages 201 to 204.

The compressor unit 100 according to the present embodiment has been described above. Conventionally, as a compressor that supplies a boil-off gas to a destination requiring the target gas such as an engine in a ship as shown in JP-A-2018-128039 uses a lubricating compressor, and the lubricant contained in the boil-off gas leaking from the compressor was collected by a separator or the like. In contrast to this, in the case of the compressor 500, since the first and second seal parts 241 and 242 are free of oil in all the compression stages 201 to 205, contamination of oil in the target gas is prevented from the outset. In addition, the scraper part 231 and the oil slinger 232 prevent the lubricant used to lubricate the crank mechanism from entering the cylinder part 211, whereby the target gas can be kept clean with higher reliability.

Since the first and second sealing members 241 and 242 are of the contact type, the sealing properties can be improved. In particular, since the fifth compression stage 205, which is the final compression stage, is in a high pressure environment in which the pressure of the target gas is increased from 100 bar to 150 bar to 300 bar (or more), the first and second sealing parts 241 and 241 242 of the fifth compression stage 205 preferably of the contact type and not of the non-contact type such as a labyrinth seal.

In this way, the reliability of the compressor 500 can be improved by using oil-free and contact sealing members for the first and second sealing parts 241 and 242.

In the compressor 500, the single-acting structure of the fifth compression stage 205, which is driven under the highest pressure environment, can reduce the load on the second seal part 242, and the double-acting structure of the other compression stages 201-204 can reduce the processing amount of the target gas can secure.

In the fifth compression stage 205, the load on the second seal part 242 can be further reduced since the non-compression chamber 223 is provided between the compression chamber 222 and the second seal part 242. By compressing the target gas by the two first compression stages 201 arranged in parallel, the processing amount of the target gas can be further secured.

In contrast to conventional techniques, the compressor 500 does not require lubrication of the sealing part, so that no additional lubrication devices are required. Consequently, the arrangement in the compressor unit 100 can be simpler than that in the lubrication compressor.

The internal pressure of the crankcase 301, to which the wiper part 231 is attached, is substantially equal to the atmospheric pressure. An inert gas having a pressure substantially equal to atmospheric pressure is supplied to the space on the side of the piston 212 of the scraper part 231 (i.e., the inside of the cross guide 303). Therefore, the pressure difference in the intermediate area in front of and behind the wiper part 231 is essentially zero. With this configuration, the deformation of the wiper part 231 due to the pressure difference can be prevented, and hence the sealing performance of the wiper part 231 can be achieved for a long period of time. Furthermore, in all of the first to fifth stages of compression 201 to 205, since the pressure difference around the wiper part 231 is substantially zero, the wiper part 231 of the first to fifth stages of compression 201 to 205 can be formed using a common member.

In the fifth compression stage 205, the piston ring 243 and the ring part 249, which are used for the first sealing part 241 and the second sealing part 242, are formed from a material whose main component is at least one of polyimide (PI) and polyetheretherketone (PEEK) or of a material whose major component is a mixture of either or both and PTFE or modified PTFE. Since these materials are harder than materials whose main component is only PTFE even in a high pressure environment, the first sealing part 241 and the second sealing part 242 are not easily deformable and can exhibit excellent sealing performance over a long period of time.

By arranging the dampers 261 to 268 and 271 to 273 in the vicinity of the suction and discharge sides of the first to fifth compression stages 201 to 205, the pressure fluctuation of the target gas is effectively suppressed. With this configuration, the vibration of the compressor unit 100 due to the pressure fluctuation is prevented.

A connection position of an upstream end of the bypass line 411 in the stage connection flow path 113 (upstream end in a flow direction in the bypass line 411) is between the dampers 271 and 272. In this configuration, the bypass line 411 is from the fluctuation in the final pressure of the fourth compression stage 204 and the suction pressure of the fifth stage compression 205 are less influenced than in a case where the connection position of the upstream end of the bypass line 411 is between the fourth stage compression 204 and the damper 271 or between the fifth stage 205 and the damper 272.

Meanwhile, there is a junction of a downstream end of the bypass line 411 in the storage tank connection flow path 111 (downstream end in the flow direction in the bypass line 411) in front of the dampers 261 and 262 on the suction side of the first compression stage 201. The bypass line 411 is affected by the suction pressure fluctuation of the first stage compression 201 is less likely to be influenced than a case where the connection position of the downstream end of the bypass pipe 411 is between the first stage compression 201 and the damper 261.

In the in FIG. 1, the decompression control can be performed by a control unit other than the control unit 414. As another method of decompression processing, the on-off valve 416 may be kept open to reduce the pressure of the destination requiring the target gas. Note that, since the check valve 418 is provided in front of the branch part of the decompression pipe 415 from the flow path 110, the flow of the target gas from the destination requiring the target gas toward the compressor unit 100 is prevented.

FIG. 4 is a diagram showing another example of the bypass line. The upstream end of the bypass line 411 in the stage connection flow path 113 may branch off from the flow path 110 between the damper 271 on the discharge side of the fourth stage compression 204 and the cooler 284.

FIG. Fig. 5 is a diagram showing still another example of the bypass line. The upstream end of the bypass line 411 can be directly connected to the damper 271 on the discharge side of the fourth compression stage 204.

FIG. 6 is a diagram showing still another example of the bypass line. In FIG. 6, two bypass lines 421 and 422 are used to control the pressure and flow rate of the target gas to be delivered to the destination requiring the target gas. Other configurations of a compressor unit 100A are similar to the configuration of the compressor unit 100.

A connection position of the upstream end of the bypass line 421 in the stage connection flow path 113 (upstream end in the flow direction in the bypass line 421) is between the damper 272 on the suction side of the fifth compression stage 205 and the cooler 284. The connection position of the downstream end of the Bypass line 421 in the stage connection flow path 113 (downstream end in the flow direction in the bypass line 421) is located between the damper 266 on the suction side of the third compression stage 203 and the cooler 282. The pressure sensor 413 is between the bypass line 421 and the damper 271 on the suction side the fifth compression stage 205 is provided.

A connection position of the upstream end of the bypass line 422 in the stage connection flow path 113 is between the damper 266 on the suction side of the third compression stage 203 and the cooler 282. Meanwhile, the connection position of the downstream end of the bypass line 422 in the storage tank connection flow path 111 is present the dampers 261 and 262 on the suction side of the first compression stage 201. A pressure sensor 419 is provided between the bypass line 422 and the damper 265 on the discharge side of the second compression stage 202.

A control valve 423 is attached to the bypass line 421. A control valve 424 is attached to the bypass line 422.

The degree of opening of the control valve 423 is controlled by the control unit 414 on the basis of the pressure detected by the pressure sensor 413 so that the suction pressure of the fifth compression stage 205 is substantially constant. Meanwhile, the opening degree of the control valve 424 is controlled based on a detection value detected by the pressure sensor 419 so that the suction pressure of the third compression stage 203 is substantially constant.

In the in FIG. 6, a very large pressure difference (approx. 300 bar) is generated between the suction side of the first compression stage 201 and the discharge side of the fifth compression stage 205. By using the two bypass lines 421 and 422, the pressure can be regulated in two stages, whereby the pressure fluctuation can be prevented more effectively.

As described above, in the compressor units 100 and 100A, the first seal part 241 and the second seal part 242 of the entire first compression stage 201 to the fifth compression stage 205 are of the oil-free type. Therefore, there is no possibility that the lubricant is mixed into the target gas flowing through the bypass pipe. Therefore, the connecting positions of the upstream and downstream ends of the bypass lines and the number of the bypass lines can be arbitrarily set.

According to the embodiment described above, the target gas is supplied to a single destination that needs the target gas. However, the target gas can be delivered to multiple destinations that require the target gas. FIG. 7 shows a compressor unit 100B configured to deliver the target gas to three destinations requiring the target gas. The compressor unit 100B is illustrated with reference to FIG. 1 and 7.

“Demand target 1” corresponds to the one shown in FIG. 7 on the discharge side of the fifth stage compression 205 (on-demand target connection flow path 114). Demand target 1 is a ship's engine. “Demand target 2” is connected to a supply line 431 that extends from the flow path section between the fourth compression stage 204 and the fifth compression stage 205 in the stage connection flow path 113. Demand target 2 is a liquefaction device that liquefies the target gas again. The liquefaction device is connected to the LNG storage tank 101 via a pipe section (not shown) in such a way that the reliquefied target gas flows back into the LNG storage tank 101. “Demand target 3” is connected to a supply line 432 that extends from the flow path section between the second compression stage 202 and the third compression stage 203 in the stage connection flow path 113. Demand target 3 is a ship-mounted power generator.

The structure used for processing the target gas supplied from the LNG storage tank 101 to the demand target 1 is the same as the structure of the compressor unit 100 shown with reference to FIG. 1 is described.

The compressor unit 100B comprises the bypass lines 433, 434 and 435 instead of the bypass line 411 of the compressor unit 100.

The bypass line 433 bypasses the fifth compression stage 205 and the dampers 272 and 273 before and after the fifth compression stage 205. The bypass line 434 bypasses the third and fourth compression stages 203 and 204 and the dampers 266 to 268 and 271 before and after the third and fourth compression stage 203 and 204. The bypass line 435 bypasses the first and second compression stages 201 and 202 as well as the dampers 261 to 265.

The control valves 436, 437 and 438 are connected to the bypass lines 433, 434 and 435, respectively. The control valves 436, 437 and 438 are connected to the control unit 414.

The opening degree of the control valve 436 is controlled by the control unit 414 based on the detection value of the pressure sensor 413 so that the discharge pressure of the fifth compression stage 205 is constant. Similarly, the opening degree of the control valve 437 is controlled based on the detection value of the pressure sensor 441 so that the suction pressure of the fifth compression stage 205 is constant. The opening degree of the control valve 438 is controlled based on the detection value of the pressure sensor 442 so that the suction pressure of the third compression stage 203 is constant.

The compressor unit 100B includes the three bypass lines 433, 434 and 435 and the control valves 436, 437 and 438 provided in the bypass lines 433, 434 and 435 to adjust the pressure of the target gas flowing into the three demand destinations 1 to 3 to be able to and thus to achieve the flow rate and / or pressure suitable for the requirements.

FIG. 8 is a diagram showing another example of a compressor unit. A compressor unit 100C may include a damper when the pulsation of the flow path portion between the respective compression stages 201 to 205 in the stage connection flow path 113 can be ignored. Thereby, the compressor unit 100C can be manufactured at a low price.

FIG. 9 shows another example of the second sealing part 242 of the fifth compression stage 205. A through hole 295 is formed in the holding part 294, through which a cooling liquid for cooling the ring part 249 and the like is supplied. In the present embodiment, the cooling liquid is water. The coolant can be an anti-freeze solution. The through hole 295 is formed at a position which is offset in the radial direction from the through hole through which the piston rod 213 is inserted.

With the exception of the uppermost housing part 244, a housing cooling flow path 290 through which the cooling liquid flows is formed in the housing parts 244.

The housing cooling flow path 290 includes annular grooves 291 formed on the surfaces of the housing parts 244 facing the compression chamber 221 side, and through holes 292 penetrating the housing parts 244 in the axial direction so as to be connected to the annular grooves 291 are connected. The formation positions of the through holes 292 in the radial direction correspond to a formation position of the through hole 295 in the holding part 294.

The annular groove 291 in the lowermost housing part 244 communicates with a discharge path (shown in FIG. 9 by a dashed line).

When supplied to the through hole 295 in the holding part 294, the cooling liquid flows into the annular grooves 291 to cool the housing parts 244 and is discharged through the discharge path. With this configuration, the frictional heat generated between the ring parts 249 and the piston rod 213 is dissipated. As a result, the second seal member 242 can maintain excellent sealing performance for a long period of time even when the lubricant is not supplied.

The structure of the second seal part 242 can be used for the first to fourth compression stages 201 to 204. It should be noted that in the second sealing part 242 of FIG. 9 the annular groove 291 can be formed in the uppermost housing part 244.

FIG. 10 to 12 are views showing another example of the cylinder part 211 of the fifth compression stage 205. FIG. 10 is a schematic plan view of the cylinder portion 211. FIG. 11 is a schematic cross-sectional view of cylinder portion 211 taken along line A-A of FIG. 10. FIG. 12 is a schematic cross-sectional view of the cylinder part 211 taken along the line B-B orthogonal to the line A-A on an axis of the cylinder part 211. The cylinder part 211 is illustrated with reference to FIG. 2 and FIG. 10 to 12.

The cylinder part 211 consists of the front head 218, the pipe part 216 in which the piston 212 is housed, two shells 526 which are attached to the outer surface of the pipe part 216, and the rear head 217 as shown in FIG. 3. As shown in FIG. 10, the tube part 216 has a substantially rectangular, planar shape in plan view. The peripheral surfaces of the front head 218 and tubular member 216 include a pair of first surfaces 523 (left and right surfaces of FIG. 10) and a pair of second surfaces 524 (top and bottom surfaces of FIG. 10) that are substantially orthogonal to the first Areas 523 run.

A plurality of first through holes 541 and a plurality of second through holes 542 penetrating the pair of first surfaces 523 are formed in the pipe part 216. Both ends of the first through holes 541 and the second through holes 542 are opened on the pair of first surfaces 523, respectively. The first through holes 541 run between the housing space in which the piston 212 is accommodated and one of the second surfaces 524 (upper side in FIG. 10). The second through holes 542 are located on the opposite side of the first through holes 541 with the piston 212 in between and run between the housing space in which the piston 212 is accommodated and one of the second surfaces 524 (bottom in FIG. 10).

As shown in FIG. 11, an existence area of the plurality of first through holes 541 and the plurality of second through holes 542 overlaps with part of an existence area of the first seal part 241 (i.e., the plurality of piston rings 243) in the radial direction.

As shown in FIG. As shown in FIG. 10, the cylinder portion 211 includes the pair of casings 526 attached to the pair of first surfaces 523. Each of the shells 526 includes a lower wall portion 527 disposed at a location spaced from the corresponding first surface 523 and peripheral wall portions 528 protruding from the outer peripheral edge of the lower wall portion 527 toward the corresponding first surface 523. The distal edge surfaces of the peripheral wall parts 528 are in contact with the corresponding first surface 523. The contact areas between the peripheral wall parts 528 and the first surface 523 are sealed with a sealing material.

A flow path 529 surrounded by the first surface 523, the peripheral wall parts 528, and the lower wall part 527 is formed in the cylinder part 211. The flow path 529 communicates with the first through hole 541 and the second through hole 542.

In the compressor 500, a cylinder cooling flow path part 540 that surrounds the first seal part 241 (and the piston 212) in a circumferential direction is formed by using the flow path 529, the plurality of first through holes 541, and the plurality of second through holes 542. A supply path (not shown) for supplying the coolant to the flow path 529 is formed in one of the two casings 526. An exit path (not shown) for the cooling liquid to exit after the first sealing part 241 has cooled is formed in the other casing 526. When the compressor 500 is driven, the cooling liquid is supplied to the flow path 529 of the one jacket 526 through the supply path, passes through the first through hole 541 and the second through hole 542, and then flows into the flow path 529 of the other jacket 526 and is discharged from the discharge path.

The cylinder cooling flow path part 540 cools the first seal part 241 over the entire circumference, whereby the heat generated in the first seal part 241 can be efficiently dissipated. As a result, the first seal member 241 can maintain excellent sealing performance for a long period of time even when the lubricant is not supplied.

With the configuration described above, by providing the first through holes 541 and the second through holes 542 directly in the pipe part 216, the cooling liquid can be supplied to a location near the piston 212, whereby the cooling efficiency can be further improved.

FIG. 13 is a schematic plan view showing another example of the cylinder cooling flow path part 540 after the fifth compression stage 205. FIG. 14 is a schematic longitudinal section of the cylinder part 211. The cylinder cooling flow path part 540 can be formed without using the jacket 526.

As shown in FIG. 13, the cylinder cooling flow path part 540 includes the plurality of first through holes 541, the plurality of second through holes 542, a plurality of third through holes 543, a plurality of fourth through holes 544, and a plurality of flow path parts 532 in the axial direction. The plurality of first through holes 541 are formed to penetrate the pair of first surfaces 523. The plurality of second through holes 542 are located on the opposite side of the first through holes 541 with the piston 212 therebetween, and penetrate the pair of first surfaces 523. The plurality of third through holes 543 are formed to penetrate the pair of second surfaces 524. The plurality of fourth through holes 544 are located on the opposite side of the third through holes 543 with the piston 212 therebetween, and penetrate the pair of the second surfaces 524. Openings of the first to fourth through holes 541 to 544 are blocked by seal members 533. In the cylinder cooling flow path part 540, a flow path surrounding the first seal part 241 (and the piston 212) is formed by the set of the first to fourth through holes 541 to 544. As shown in FIG. 14, the flow path in the axial direction is connected to another flow path in the axial direction via the flow path parts 532. The cooling liquid flows through the entire cylinder cooling flow path from a supply path (not shown) and is discharged from a discharge path (not shown). One end or both ends of the flow path part 532 in the axial direction penetrate an upper or lower surface of the cylinder part 211 and are sealed.

The cylinder part 211, which does not contain the casing 526, is smaller by the size of the casing 526 than the cylinder part 211 which, with reference to FIG. 10 will be described.

The one shown in FIG. Cylinder cooling flow path portion 540 shown in FIGS. 10-14 includes an annular, continuous flow path that surrounds piston 212. However, the piston 212 need not necessarily be surrounded by the annular continuous flow path, and the piston 212 may be surrounded by multiple independent flow paths. That is, an independent flow path corresponding to each of the four outer surfaces (surfaces corresponding to the first surfaces 523 and the second surfaces 524) of the substantially rectangular cylinder part can be formed. For example, as shown in FIG. 15, the cylinder cooling flow path part 540 is formed by the two flow paths 529 formed by the two casings 526, and the plurality of first through holes 541 and the plurality of second through holes 542 independently of the two flow paths 529.

The structure of the cylinder part 211 described with reference to FIG. 10 to 15 can be applied to the compression stages 201 to 204 with the exception of the fifth compression stage 205. In addition, in the cylinder part 211, the number of the first to fourth through holes 541 to 544 is allowed to be one as long as the first seal part 241 can be cooled sufficiently.

FIG. 16 is a view showing another structure of the cylinder part 211 of the fifth compression stage 205. In the cylinder part 211, the rear head 217 can be omitted and the second sealing part 242 can block the open end of the tube part 216 (i.e. the second sealing part 242 can also serve as a rear head 217). The cylinder part 211 of the other compression stages 201 to 204 may have a structure similar to the structure of FIG. 16 have.

FIG. 17 is a view showing another example of the compressor 500. In the case of the compressor 500, a fifth compression stage 205E (final compression stage) and a fourth compression stage 204E (immediately preceding compression stage) can have a tandem structure.

The fourth compression stage 204E is formed closer to the crank mechanism than the fifth compression stage 205E. The cylinder part 211 of the fourth compression stage 204E comprises a pipe part 511 which extends in the axial direction of the piston rod 213, and an upper part 512 which closes an open end of the pipe part 511 on the opposite side of the crank mechanism. In the upper part 512, a through hole is formed essentially coaxially to the tube part 511. An opening end of the pipe part 511 on the side of the crank mechanism is closed by the rear head 217. The second seal part 242 is attached to the rear head 217.

[0126] A piston 513 of the fourth compression stage 204E is connected to the piston rod 213. The plurality of piston rings 243 are fixed to the outer peripheral part of the piston 513, and the piston rings 243 constitute the first seal part 241 of the fourth compression stage 204E.

In the cylinder part 211, the space on the opposite side of the crank mechanism with the piston 513 interposed therebetween is used as the compression chamber 224a of the fourth compression stage 204E. The space on the side of the crank mechanism with the piston 513 interposed therebetween is a non-compression chamber 224b with a pipe connected to the non-compression chamber 224b to be opened to the suction side flow path of the fourth compression stage 204E. Note that the non-compression chamber can be connected to the discharge side.

The cylinder part 211 of the fifth compression stage 205E comprises a pipe part 514 and a front head 515. The pipe part 514 is provided on the upper part 512 of the fourth compression stage 204E. The inner diameter of the pipe part 514 of the fifth compression stage 205E is smaller than the inner diameter of the pipe part 511 of the fourth compression stage 204E.

A piston 516 of the fifth compression stage 205E is formed in one piece with the piston 513 of the fourth compression stage 204E. The diameter of the piston 516 of the fifth compression stage 205E is smaller than the diameter of the piston 513 of the fourth compression stage 204E. The plurality of piston rings 243 are attached to the outer peripheral part of the piston 516, the piston rings 243 forming the first seal part 241 of the fifth compression stage 205E.

In the cylinder part 211, the space on the opposite side of the crank mechanism with the piston 516 interposed therebetween is used as the compression chamber 225 of the fifth compression stage 205E.

Since the fourth compression stage 204E and the fifth compression stage 205E have a tandem structure, the fifth compression stage 205E shares the piston rod 213, the second seal part 242, the scraper part 231 and the oil slinger 232 with the fourth compression stage 204E. In other words, the piston rod 213, the second seal part 242, the wiper part 231 and the oil slinger 232 of the fourth compression stage 204E are used in common with the fifth compression stage 205E. This means that the piston rod 213 of the fourth compression stage 204E is used to drive the piston of the fifth compression stage 205E. The second sealing part 242 of the fourth stage compression 204E prevents the target gas in the cylinder part 211 of the fifth stage compression 205E from leaking to the crank mechanism side through the cylinder part 211 of the fourth stage compression 204E. The wiper part 231 and the oil slinger 232 of the fourth compression stage 204E not only prevent the lubricant from entering the cylinder part 211 of the fourth compression stage 204E, but also prevent the lubricant from entering the cylinder part 211 of the fifth compression stage 205E.

As described above, since the space between the piston 513 and the second seal part 242 of the fourth compression stage 204E is the non-compression chamber 224b, the stress applied to the second seal part 242 is reduced.

FIG. 18 is a view showing another example of the tandem structure of the fourth and fifth compression stages 204E and 205E. In the fourth compression stage 204E, the space on the opposite side of the crank mechanism with the piston 513 interposed is the non-compression chamber 224b, while the space on the crank mechanism side with the piston 513 interposed is used as the compression chamber 224c. In addition, as shown in FIG. 19, in the fourth compression stage 204E, the spaces on both sides of the piston 513 be the compression chambers 224d and 224e.

FIG. 20 is a view showing another example of the compressor 500. The fourth compression stage 204 and the fifth compression stage 205 can be implemented by a cylinder part 211. In the cylinder part 211, the suction valve 214 and the outlet valve 215 are provided in front of and behind the piston 212, respectively. In the cylinder part 211, the space on the opposite side of the crank mechanism with the piston 212 interposed therebetween with the flow path on the discharge side of the third compression stage 203 in FIG. 1 and functions as a compression chamber 224f of the fourth compression stage 204.

Meanwhile, the space on the side of the crank mechanism in the cylinder part 211 with the piston 212 interposed therebetween is connected to the compression chamber 224f and functions as the compression chamber 225a of the fifth compression stage 205 Dampers 271 and 272 are provided on the flow path connecting the compression chambers 224f and 225a.

In the compressor 500, the target gas is compressed and discharged from the compression chamber 224f of the fourth compression stage 204, while the target gas is simultaneously sucked into the compression chamber 225a of the fifth compression stage 205. The target gas is sucked into the compression chamber 224f of the fourth compression stage 204 while at the same time the target gas is compressed in and expelled from the compression chamber 225a of the fifth compression stage 205. In the example shown in FIG. 20, the number of parts is reduced.

FIG. 21 is a diagram showing still another example of the compressor unit 100. As shown in FIG. 21, the on-off valve 416 of FIG. 1 can be omitted. In this case, the decompression line 415 is located in the demand target connection flow path 214 before the check valve 418. During the decompression processing, the check valve 418 prevents the target gas from flowing back at the destination which needs the target gas (flowing the target gas toward the compressor unit 100). The in FIG. 21 is more simplified than that with reference to FIG. 1 because the on-off valve 416 is not provided.

The embodiment disclosed this time is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and scope of the claims and equivalents are intended to be embraced therein.

In the fifth compression stage 205 described above, when the main sealing functions of the first sealing part 241 are realized by the contact between the piston rings 243 and the cylinder part 211, a non-contact sealing structure such as e.g. a labyrinth can be partially used as the first sealing part 241. The situation is similar with the second sealing part 242. The situation is similar with the first sealing part 241 and the second sealing part 242 of the first to fourth compression stages 201 to 204. If the sealing functions can be reliably implemented, the first and second sealing parts 241 may and 242 in all or some of the compression stages 201 to 204 with the exception of the fifth compression stage 205 have only one contactless sealing structure (for example a labyrinth seal).

The piston rings 243 of the first to fourth compression stages 201 to 204 can contain the same material as the piston rings 243 of the fifth compression stage 205. The situation is similar with the ring part 249 of the second sealing part 242.

In the example shown in FIG. 2, the space between the front head 218 and the piston 212 is used as the compression chamber 222, but the space between the rear head 217 and the piston 212 can be used as the compression chamber of the fifth compression stage 205.

According to the embodiment described above, the compressor unit 100 may contain the single first compression stage 201, as shown in FIG. 22 shown.

The structure in which the first two, with reference to FIG. 1 are connected in parallel compression stages 201 can be used for the second to fifth compression stages 202 to 205.

According to the embodiment described above, a stepless capacity adjustment mechanism can be provided in the final compression stage instead of the bypass line. The capacity adjustment mechanism can be a suction valve relief system, a spacer pocket system, or a speed control system. The capacity adjusting mechanism is controlled by the control unit 414 so that the pressure detected by the pressure sensor 413 falls within a predetermined control target range.

According to the embodiment described above, in the case of the compressor units 100 and 100A, the number of compression stages can be set to either 3, 4 or 6, depending on the pressure to be delivered by the final compression stage.

According to the embodiment described above, the structure similar to that of the compressor 500 can be used for a horizontal compressor in which the piston 212 reciprocates in the horizontal direction (see FIG. 23).

An evaporator described according to the various embodiments described above mainly has the following characteristics.

A compressor unit according to one aspect of the embodiment described above is installed in a ship and configured to collect a target gas that is a boil-off gas from an LNG storage tank of the ship and at least a part of the target gas provides a destination that requires the target gas. The compressor unit includes: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each of the compression stages. Each of the plurality of compression stages includes: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and define a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas sucked into the cylinder part from flowing toward the crank mechanism; a scraper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second seal part and configured to further prevent the lubricant from entering the cylinder part. All of the first and second seal parts are of the oil-free type. In at least one final compression stage, the first sealing part comprises a piston ring group which is provided on an outer peripheral part of the piston and is configured to seal between the piston and the cylinder part, the second sealing part comprising: a plurality of housing parts interposed between the cylinder part and the Piston rod are arranged; and a plurality of ring parts held by the plurality of housing parts, wherein the first sealing part and the second sealing part of the at least final compression stage are of the contact type.

With the configuration described above, the reliability of the compressor unit can be improved. That is, since the scraper part and the oil slinger are provided, the lubricant in the crank mechanism is prevented from entering the cylinder part and mixing with the target gas. In addition, since all of the first seal part and the second seal part are of the oil-free type, the lubricant is prevented from mixing with the target gas. If only the oil-free type sealing parts are used, the stresses exerted on the sealing parts become excessive. However, since a plurality of dampers are provided to prevent pressure fluctuations between the various compression stages, the sealing parts are not subjected to large pressure fluctuations. Since the seal parts can maintain a shape that performs sealing functions even when the lubricant is not supplied, the compressor unit can seal the target gas in the compression chamber. Therefore, the target gas can be compressed in the compression chamber with high reliability. Since the first and second sealing parts of the at least final compression stage are of the contact type, the sealing properties in the compression stage including the first and second sealing parts of the contact type are maintained even in high pressure environments. Therefore, gas leakage through the sealing parts is prevented.

In view of the configuration described above, at least in the final compression stage, a case cooling flow path can be formed in the plurality of case parts. Water or an anti-freeze solution can be used as the cooling liquid to be supplied to the case cooling flow path.

In the configuration described above, the oil-free type seal parts are used in an environment where the heat generation is more likely than the lubrication type seal parts. With the configuration described above, the second seal part can be cooled efficiently by circulating the cooling liquid through the case cooling flow path.

In view of the configuration described above, the compression chamber in the final compression stage may only be a space on a first side of the cylinder part with the piston interposed therebetween. A space on a second side of the cylinder part can be open to one of the suction-side flow paths of the final compression stage, i. to a stage connection flow path connecting the final compression stage and the immediately preceding compression stage of the final compression stage, and to an on-demand target connection flow path.

In the configuration described above, the compression chamber in the final compression stage is only the space on the first side of the cylinder part with the piston interposed therebetween, the space on the second side of the cylinder part toward one of the suction-side flow paths of the final compression stage is open, ie the stage connection flow path connecting the final compression stage and the immediately preceding compression stage of the final compression stage, and the on-demand target connection flow path. That is, the final compression stage has a simple-looking structure. Compared to the double action, there is the effect that the number of high pressure parts can be reduced, e.g. A suction and discharge valve arranged in the cylinder, which control the gas flow in and out of the compression chamber, can be reduced to one chamber.

In view of the configuration described above, the space on the first side may be a space on an opposite side of the crank mechanism with the piston interposed. The space on the second side may be a space on one side of the crank mechanism.

With the configuration described above, the load on the second seal part (rod seal) can be reduced by using the space of the crank mechanism side as a pressureless space.

In view of the configuration described above, a tandem structure in which the cylinder part of the final compression stage is disposed on the cylinder part of an immediately preceding compression stage of the final compression stage can be used. The piston in the immediately preceding compression stage of the final compression stage and the piston in the final compression stage, which is smaller in diameter than the piston in the immediately preceding compression stage, can be configured in one piece. In the last compression stage, only one space on the opposite side of the crank mechanism with the piston arranged in between may be the compression chamber.

With the configuration described above, the load on the second seal part (rod seal) can be reduced by using the space of the crank mechanism side as a pressureless space.

In view of the configuration described above, in a cylinder part, a space on a side of the crank mechanism with the piston interposed may be the compression space of the final compression stage. A space on the opposite side of the crank mechanism with the piston arranged in between can be the compression chamber of an immediately preceding compression stage of the final compression stage.

With the configuration described above, it is possible to reduce the number of parts of the first and second seal parts and reduce the possibility of target gas leakage by forming the compression chambers of the last and immediately preceding compression stages in the spaces on both sides of a piston , compared to a case where the compression chambers are provided in separate cylinder parts.

In view of the configuration described above, the first seal part in all stages of compression may include the piston ring group that is provided on the outer peripheral part of the piston and configured to seal between the piston and the cylinder part. The second seal part may include: the plurality of housing parts disposed between the cylinder part and the piston rod; and the plurality of ring parts held by the plurality of housing parts. The first and second sealing parts may be of the contact type.

With the above-described configuration, the sealing properties can be improved more than the non-contact seal (labyrinth seal).

With regard to the configuration described above, a main component of a ring material of the first seal part and / or the second seal part of the final compression stage may contain one or both of PEEK and PI, or one or both of PEEK and PI may be mixed with PTFE.

With the configuration described above, the compressive strength of the piston rings in the final compression stage can be improved.

A compressor unit according to another aspect of the embodiment described above is installed in a ship and configured to collect a target gas that is a boil-off gas from an LNG storage tank of the ship and at least a part of the target gas delivers to a destination that requires the target gas. The compressor unit includes: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each of the compression stages. Each of the plurality of compression stages from a first compression stage to an immediately preceding compression stage of a final compression stage comprises: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and define a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas sucked into the cylinder part from flowing toward the crank mechanism; a scraper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second seal part and configured to further prevent the lubricant from entering the cylinder part. The immediately preceding compression stage of the final compression stage and the final compression stage have a tandem structure in which the cylinder part of the final compression stage is arranged on the cylinder part of the immediately preceding compression stage of the final compression stage. The piston in the immediately preceding compression stage of the final compression stage and the piston in the final compression stage, which is smaller in diameter than the piston in the immediately preceding compression stage of the final compression stage, are unitarily configured. The final compression stage is shared by the piston rod, the second sealing part, the wiper part and the oil slinger part with the compression stage immediately preceding the final compression stage. At least in the final compression stage, the first seal part includes a piston ring group that is provided on an outer peripheral part of the piston and is configured to seal between the piston and the cylinder part and configured as a contact type. In at least the immediately preceding compression stage of the final compression stage, the second sealing part comprises: a plurality of housing parts which are arranged between the cylinder part and the piston rod; and a plurality of ring parts held by the plurality of housing parts and configured as a contact type. All of the first and second seal parts are of the oil-free type.

With the configuration described above, the reliability of the compressor unit can be improved. That is, since the scraper part and the oil slinger are provided, the lubricant in the crank mechanism is prevented from entering the cylinder part and mixing with the target gas. In addition, since all of the first seal part and the second seal part are of the oil-free type, the lubricant is prevented from mixing with the target gas. If only the oil-free type sealing parts are used, the stresses exerted on the sealing parts become excessive. However, since a plurality of dampers are provided to prevent pressure fluctuations between the various compression stages, the sealing parts are not subjected to large pressure fluctuations. Since the seal parts can maintain a shape that performs sealing functions even when the lubricant is not supplied, the compressor unit can seal the target gas in the compression chamber. Therefore, the target gas can be compressed in the compression chamber with high reliability. Since the first sealing part of the at least last compression stage and the second sealing part of the immediately preceding compression stage of the at least last compression stage are of the contact type, in the compression stage including the first sealing part of the contact type and the second sealing part, the sealing properties are maintained even in high pressure environments and gas leaks through the sealing parts prevented.

In the configuration described above, in the final compression stage, only a space on the opposite side of the crank mechanism with the piston interposed can be the compression chamber.

Regarding the configuration described above, in the immediately preceding compression stage of the final compression stage, a space on an opposite side of the crank mechanism with the piston interposed may be a non-compression chamber, while a space on a side of the crank mechanism with the piston interposed therebetween can be used as a compression chamber.

In view of the configuration described above, at least in the final compression stage, the cylinder part may include a cylinder cooling flow path part through which a cooling liquid flows to surround the piston. The cylinder cooling flow path part may have a through hole formed in the cylinder part.

The oil-free type seal members are used in an environment where the heat generation is more likely than the lubrication type seal members. With the configuration described above, the first seal part can be efficiently cooled by supplying the cooling liquid to the cooling flow path part surrounding the cylinder part.

In view of the configuration described above, in the at least immediately preceding compression stage of the final compression stage, a case cooling flow path can be formed in the plurality of case parts. Water or an anti-freeze solution can be used as the cooling liquid to be supplied to the case cooling flow path.

In the configuration described above, the oil-free type seal parts are used in an environment where the heat generation is more likely than the lubrication type seal parts. With the configuration described above, the second seal part can be cooled efficiently by circulating the cooling liquid through the case cooling flow path.

In view of the configuration described above, the first seal part in all stages of compression may include the piston ring group provided on the outer peripheral part of the piston and configured to seal between the piston and the cylinder part, and may be of the contact type. In each of the compression stages from the first compression stage to the immediately preceding compression stage of the final compression stage, the second sealing part may comprise: the plurality of housing parts disposed between the cylinder part and the piston rod; and the plurality of ring parts held by the plurality of housing parts and may be of the contact type.

With the above-described configuration, the sealing properties can be improved more than the non-contact seal (labyrinth seal).

In view of the configuration described above, a main component of a ring material of the first sealing part of the final compression stage and / or the second sealing part of the immediately preceding compression stage of the final compression stage may include one or both of PEEK and PI, or one or both of PEEK and PI can be mixed with PTFE.

With the configuration described above, the compressive strength of the piston rings in the final compression stage can be improved.

With respect to the configuration described above, a pressure difference between the spaces in front of and behind the scraper part in the crank mechanism can be substantially zero.

With the configuration described above, a load on the wiper part can be reduced.

In view of the configuration described above, the pressure in the spaces may be substantially equal to atmospheric pressure.

With the configuration described above, in order to make the pressure in the spaces higher than the atmospheric pressure, a sealed structure is required in the crankcase. However, the structure is not required for the atmospheric pressure, and the cost can be reduced.

In view of the above-described configuration, the compressor unit may further include a bypass line that bypasses the compression stages to return the target gas to an upstream side.

With the configuration described above, by providing the bypass line, an operation can be performed under optimal operating conditions.

Regarding a method of stopping a compressor unit according to another aspect of the above-described embodiment, the compressor unit may further include: a check valve provided in the discharge-side flow path of the final compression stage; a decompression line connected to the discharge side flow path further downstream of the check valve; and an on-off valve provided in the discharge-side flow path downstream of the decompression pipe. The method can include the following: closing the on-off valve and reducing the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill.

With the above-described configuration, by closing the on-off valve, backflow of the gas from the destination requiring the target gas can be prevented during decompression, and by providing the check valve, backflow of the gas to the side of the Compressor unit can be prevented. In addition, it is possible to perform decompression on a side of the destination that needs the target gas by opening the on-off valve as required at the time of discharge through the decompression pipe.

Regarding a method of stopping a compressor unit according to another aspect of the embodiment described above, the compressor unit may further include: a check valve provided in the discharge-side flow path of the final compression stage; and a decompression line connected to the discharge-side flow path between the final compression stage and the check valve. The method can include the following: reducing the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill.

With the configuration described above, backflow of the gas from a destination that needs the target gas during decompression can be prevented with a simple configuration.

In the compressor unit, the plurality of compression stages according to another aspect of the embodiment described above are used.

The technique of the embodiment described above is suitably used for a compressor unit mounted on a ship.

Claims (22)

1. Compressor unit installed on a ship and configured to collect a target gas, which is a boil-off gas from an LNG storage tank of the ship, and to deliver at least a portion of the target gas to a destination; that requires the target gas, wherein the compressor unit comprises: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each compression stage, wherein each of the plurality of compression levels comprises: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and form a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas drawn into the cylinder part from flowing to the crank mechanism; a wiper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second sealing part and configured to additionally prevent the lubricant from entering the cylinder part, wherein all of the first and second sealing parts are oil-free type., being in at least one final compression stage the first sealing part includes a piston ring group which is provided on an outer peripheral part of the piston and is configured to seal between the piston and the cylinder part, wherein the second sealing part comprises: a plurality of housing parts disposed between the cylinder part and the piston rod; and a plurality of ring parts held by the plurality of housing parts, and wherein the first sealing part and the second sealing part of the at least last compression stage are of the contact type. 2. Compressor unit according to claim 1, wherein a housing cooling flow path is formed in the plurality of housing parts in the at least last compression stage, and Water or an anti-freeze solution is used as the cooling liquid, which is fed to the flow path of the housing cooling. 3. Compressor unit according to claim 1, wherein in the final compression stage the compression chamber is only a space on a first side of the cylinder part with the piston interposed therebetween, and a space on a second side of the cylinder part is open to one of a suction side flow path of the final compression stage that is a stage connection flow path connecting the final compression stage and an immediately preceding compression stage of the final compression stage, and a demand target connection flow path. 4. The compressor unit according to claim 3, wherein the space on the first side is a space on an opposite side of the crank mechanism with the piston interposed therebetween, and wherein the space on the second side is a space on a side of the crank mechanism. 5. Compressor unit according to claim 1, wherein a tandem structure is used in which the cylinder part of the last compression stage is provided on the cylinder part of an immediately preceding compression stage of the final compression stage, the piston in the immediately preceding compression stage of the final compression stage and the piston in the final compression stage are configured smaller in diameter than the piston in the immediately preceding compression stage, and in the final compression stage only a space on an opposite side of the crank mechanism with the piston in between is the compression chamber. 6. Compressor unit according to claim 1, wherein in a cylinder part a space on one side of the crank mechanism with the piston interposed is the compression chamber of the final compression stage, and wherein a space on an opposite side of the crank mechanism with the piston interposed is the compression chamber of a directly previous compression stage is the final compression stage. 7. Compressor unit according to one of claims 1 to 6, wherein in all compression levels the first sealing part comprises the piston ring group that is provided on the outer peripheral part of the piston and is configured to seal between the piston and the cylinder part, the second seal part includes: the plurality of housing parts disposed between the cylinder part and the piston rod; and the plurality of ring parts held by the plurality of housing parts, and wherein the first sealing part and the second sealing part are of the contact type. 8. Compressor unit according to one of claims 1 to 6, wherein a main component of a ring material of the first sealing part and / or the second sealing part of the last compression stage comprises one or both of PEEK and PI, or wherein one or both of PEEK and PI are mixed with PTFE . 9. Compressor unit installed on a ship and configured to collect a target gas, which is a boil-off gas from an LNG storage tank of the ship, and to deliver at least a portion of the target gas to a destination, that requires the target gas, wherein the compressor unit comprises: a plurality of compression stages configured to sequentially increase the pressure of the target gas; a plurality of dampers provided between the plurality of compression stages to prevent pressure fluctuations; and a crank mechanism configured to drive a piston of each compression stage, wherein each of the plurality of compression stages from a first compression stage to an immediately preceding compression stage of a final compression stage comprises: the piston; a piston rod connected to the piston and configured to transmit the force of the crank mechanism to the piston; a cylinder portion configured to receive the piston and form a compression chamber; a first sealing part configured to seal between the piston and the cylinder part; a second seal part that surrounds a periphery of the piston rod and is configured to prevent the target gas drawn into the cylinder part from flowing to the crank mechanism; a wiper part that surrounds the periphery of the piston rod at a position closer to the crank mechanism than the second seal part and that is configured to prevent a lubricant in the crank mechanism from entering the cylinder part; and an oil slinger attached to the piston rod between the wiper part and the second sealing part and configured to additionally prevent the lubricant from entering the cylinder part, wherein the immediately preceding compression stage of the final compression stage and the final compression stage have a tandem structure in which the cylinder part of the final compression stage is arranged on the cylinder part of the immediately preceding compression stage of the final compression stage, wherein the piston in the immediately preceding compression stage of the final compression stage and the piston in the final compression stage, which is smaller in diameter than the piston in the immediately preceding compression stage of the final compression stage, are configured uniformly, wherein the final compression stage shares the piston rod, the second sealing part, the wiper part and the oil slinger part with the immediately preceding compression stage of the final compression stage, being in at least the last compression stage the first seal part includes a piston ring group provided on an outer peripheral part of the piston and configured to seal between the piston and the cylinder part, and wherein the first seal part is configured as a contact type, where in the immediately preceding compression stage the at least last compression stage wherein the second sealing part comprises: a plurality of housing parts disposed between the cylinder part and the piston rod; and a plurality of ring parts held by the plurality of housing parts, and wherein the second seal part is configured as a contact type, and all of the first and second sealing parts being of the oil-free type. 10. Compressor unit according to claim 9, wherein in the final compression stage only a space on an opposite side of the crank mechanism with the piston arranged therebetween is the compression chamber. 11. The compressor unit of claim 10, wherein in the immediately preceding compression stage of the final compression stage, the space on the opposite side of the crank mechanism with the piston interposed is a non-compression chamber, and wherein a space on one side of the crank mechanism with the piston interposed therebetween is used as a compression chamber. 12. The compressor unit according to claim 10, wherein in the immediately preceding compression stage of the final compression stage, the space on the opposite side of the crank mechanism with the piston interposed therebetween is the compression chamber, and a space on one side of the crank mechanism with the piston interposed as a non- Compression chamber is used. 13. Compressor unit according to claim 1 or 9, wherein the cylinder part in the at least compression stage has a cylinder cooling flow path part through which a cooling liquid flows to surround the piston, and the cylinder cooling flow path part has a through hole formed in the cylinder part. 14. Compressor unit according to one of claims 9 to 12, wherein a housing cooling flow path is formed in the plurality of housing parts in the immediately preceding compression stage of the at least last compression stage, and Water or an anti-freeze solution is used as the cooling liquid, which is fed to the flow path of the housing cooling. 15. Compressor unit according to one of claims 9 to 12, wherein in all compression levels the first seal part includes the piston ring group provided on the outer peripheral part of the piston and configured to seal between the piston and the cylinder part, and wherein the first seal part is of a contact type, and wherein in each of the compression stages from the first compression stage to the immediately preceding compression stage of the final compression stage the second seal part includes: the plurality of housing parts disposed between the cylinder part and the piston rod; and the plurality of ring parts held by the plurality of housing parts, and wherein the second seal part is of a contact type. 16. Compressor unit according to one of claims 9 to 12, wherein a main component of a ring material of the first sealing part of the final compression stage and / or the second sealing part of the immediately preceding compression stage of the final compression stage contains one or both of PEEK and PI, or one or both of PEEK and PI are mixed with PTFE. 17. Compressor unit according to one of claims 1 to 6 and 9 to 12, wherein the pressure difference between the spaces in front of and behind the scraper part in the crank mechanism is substantially zero. 18. The compressor unit of claim 17, wherein the pressure in the spaces is substantially equal to atmospheric pressure. 19. A compressor unit according to any one of claims 1 to 6 and 9 to 12, further comprising a bypass line that bypasses the compression stages to return the target gas to an upstream side. 20. Procedure for stopping a compressor unit, wherein the compressor unit according to any one of claims 1 to 6 and 9 to 12 comprises: a check valve provided in a discharge-side flow path of the final compression stage; a decompression line connected to the discharge side flow path further downstream of the check valve; and an on-off valve provided in the discharge-side flow path downstream of the decompression pipe; the method comprising: Closing the on-off valve; and reducing the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill. 21. Procedure for stopping a compressor unit, wherein the compressor unit according to any one of claims 1 to 6 and 9 to 12 comprises: a check valve provided in a discharge-side flow path of the final compression stage; and a decompression line connected to the discharge side flow path between the final compression stage and the check valve; the method comprising: Reduce the pressure in the cylinder part of the final compression stage by opening the decompression line when the compressor unit is at a standstill. 22. The multiple compression stages used in the compressor unit according to any one of claims 1-6 and 9-12.