Patent Description:
Recently, consumption of liquefied gas such as liquefied natural gas (LNG) has been rapidly increasing worldwide. Liquefied gas obtained by cooling natural gas to an extremely low temperature has a much smaller volume than natural gas and thus is much more suitable for storage and transportation. In addition, since air pollutants in natural gas can be reduced or removed during a liquefaction process, liquefied gas such as LNG is an eco-friendly fuel that has low air pollutant emissions upon combustion.

LNG is a colorless and transparent liquid obtained by cooling natural gas mainly composed of methane to about -<NUM> to liquefy the natural gas and has a volume of about <NUM>/<NUM> that of natural gas. Thus, liquefaction of natural gas enables very efficient transportation.

However, since natural gas is liquefied at an extremely low temperature of - <NUM> under normal pressure, LNG can easily evaporate by a small change in temperature. Although an LNG storage tank is insulated, external heat can be continuously transferred to the storage tank, causing LNG in transit to naturally evaporate, thereby generating boil-off gas (BOG).

Generation of BOG means a loss of LNG and thus has a great influence on transportation efficiency. In addition, when BOG accumulates in a storage tank, there is a risk of pressure inside the storage tank excessively increasing, causing damage to the tank. Various studies have been conducted to treat BOG generated in an LNG storage tank. Recently, for treatment of BOG, there has been proposed a method in which BOG is re-liquefied to be returned to an LNG storage tank, a method in which BOG is used as an energy source in a source of fuel consumption such as a marine engine, and the like.

Examples of a method for re-liquefaction of BOG include a method of using a refrigeration cycle with a separate refrigerant in which BOG is allowed to exchange heat with the refrigerant to be re-liquefied and a method of using BOG as a refrigerant to re-liquefy BOG without any separate refrigerant. Particularly, a system employing the latter method is called a partial re-liquefaction system (PRS).

Examples of a marine engine capable of being fueled by natural gas include gas engines such as a DFDE engine, an X-DF engine, and an ME-GI engine.

A DFDE engine has four strokes per cycle and uses the Otto cycle in which natural gas having a relatively low pressure of about <NUM> bar is injected into a combustion air inlet, followed by pushing a piston upward to compress the gas.

An X-DF engine has two strokes per cycle and uses the Otto cycle using natural gas having a pressure of about <NUM> bar as fuel.

An ME-GI engine has two strokes per cycle and uses a diesel cycle in which natural has having a high-pressure of about <NUM> bar is injected directly into a combustion chamber in the vicinity of the top dead center of a piston.

Korean Patent Application Publication No. <CIT> discloses an apparatus according to the preamble of claim <NUM> and claim <NUM> including a lubricant separator and a liquefied gas processing system including a separation tank, a partition member, and a discharge port.

As such, when boil-off gas (BOG) generated in a liquefied natural gas (LNG) storage tank is compressed and re-liquefied through heat exchange using the boil-off gas without a separate refrigerant, it is necessary to compress the BOG at high pressure for reliquefaction efficiency using an oil-lubrication type cylinder.

Boil-off gas compressed by the oil-lubrication type cylinder compressor contains lubricant oil. The inventors of the present invention found that the lubricant oil contained in the compressed BOG is condensed or solidified prior to the BOG and blocks a fluid channel of the heat exchanger during cooling of the compressed BOG in a heat exchanger. Particularly, a printed circuit heat exchanger (PCHE) having a narrow fluid channel (for example, micro-fluid channel type fluid channel) suffers from more frequent clogging of the fluid channel due to the condensed or solidified lubricant oil.

Accordingly, the inventors of the present invention have developed various techniques for separating the lubricant oil from the compressed BOG in order to prevent the condensed or solidified lubricant oil from clogging the fluid channel of the heat exchanger.

Embodiments of the present invention provide a method and system for relieving or preventing clogging of a fluid channel of a heat exchanger by condensed or solidified lubricant oil and capable of removing the condensed or solidified lubricant oil clogging the fluid channel of the heat exchanger through a simple and economical process.

In accordance with one aspect of the present invention, there is provided a BOG reliquefaction system according to claim <NUM> and claim <NUM>.

The second oil filter may separate lubricant oil having a solid phase.

The second oil filter may be an upward discharge type.

The liquefied gas separated by the gas/liquid separator and discharged along the fifth supply line may be sent to a storage tank.

The second oil filter may be a downward discharge type.

The compressor may compress the BOG to a pressure of <NUM> bar to <NUM> bar.

The heat exchanger may include a micro-channel type fluid channel.

The heat exchanger may be a printed circuit heat exchanger (PCHE).

The BOG reliquefaction system may further include a bypass line through which the BOG is supplied to the compressor after bypassing the heat exchanger.

The BOG reliquefaction system may further include a first valve disposed upstream of a cold fluid channel of the heat exchanger and regulating a flow rate of fluid and opening/closing of a corresponding supply line, wherein the bypass line may be branched from the corresponding supply line upstream of the first valve.

The BOG reliquefaction system may further include a second valve disposed downstream of the cold fluid channel of the heat exchanger and regulating a flow rate of fluid and opening/closing of a corresponding supply line, wherein the bypass line is joined to the corresponding supply line downstream of the second valve.

The BOG reliquefaction system may further include a first oil filter disposed downstream of the compressor and separating lubricant oil from the BOG.

The first oil filter may separate lubricant oil having a vapor phase or mist phase.

According to embodiments of the invention, it is possible to remove condensed or solidified lubricant oil inside a heat exchanger through a simple and economical process using existing equipment without installation of separate equipment or supply of a separate fluid for removing the lubricant oil.

According to the embodiments of the invention, it is possible to overhaul the heat exchanger while continuing operation of an engine by driving the engine during removal of the condensed or solidified lubricant oil. In addition, it is possible to remove the condensed or solidified lubricant oil using surplus BOG not used by the engine. Furthermore, it is possible to burn the lubricant oil mixed with the BOG using the engine.

According to the embodiments of the invention, it is possible to efficiently discharge the molten or viscosity-reduced lubricant oil using an improved gas/liquid separator if the lubricant oil is collected in the gas/liquid separator.

According to the embodiments of the invention, a cryogenic oil filter is disposed on at least one of a fifth supply line through which liquefied gas is discharged from the gas/liquid separator, and a sixth supply line through which the BOG is discharged from the gas/liquid separator, thereby achieving efficient removal of the lubricant oil mixed with the BOG.

According to the embodiments of the invention, it is possible to satisfy an intake pressure condition for a compressor and engine fuel requirement for an engine while maintaining reliquefaction performance through a simple and economical process even with existing equipment without separate equipment.

Hereinafter, examples not claimed per se and embodiments of the present invention will be described in detail with reference to the accompanying drawings. BOG reliquefaction systems according to the present invention may be applied to various vessels, such as vessels equipped with engines fueled by natural gas, vessels including liquefied gas storage tanks, marine structures, and the like. It should be understood that the following embodiments can be modified in various ways and do not limit the scope of the present invention.

Further, fluid in each fluid supply line of a system according to the present invention may have a liquid phase, a vapor-liquid mixed phase, a vapor phase, and a supercritical fluid phase depending upon operation conditions of the system.

<FIG> is a schematic diagram of a BOG reliquefaction system.

Referring to <FIG>, the BOG reliquefaction system according to this embodiment includes a compressor <NUM>, a heat exchanger <NUM>, a pressure reducer <NUM>, a bypass line BL, and a bypass valve <NUM>.

The compressor <NUM> compresses BOG discharged from a storage tank T and may include a plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and a plurality of coolers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The BOG compressed by the compressor <NUM> may have a pressure of about <NUM> bar to <NUM> bar.

Some BOG compressed by the compressor <NUM> may be supplied to a main engine of a vessel along a fuel supply line SL, and the other BOG not to be used by the main engine may be supplied to the heat exchanger <NUM> along a third supply line L3 so as to be subject to a reliquefaction process. The main engine may be an ME-GI engine that uses high pressure natural gas having a pressure of about <NUM> bar as fuel.

Some BOG having passed through some cylinders <NUM>, <NUM> among the cylinders of the compressor <NUM> is divided and supplied to a generator. The generator according to this embodiment may be a DF engine that uses low pressure natural gas having a pressure of about <NUM> bar as fuel.

The heat exchanger <NUM> cools the BOG compressed by the compressor <NUM> and supplied along the third supply line L3 through heat exchange using the BOG discharged from the storage tank T and supplied along a first supply line L1 as a refrigerant. The BOG used as the refrigerant in the heat exchanger <NUM> is sent to the compressor <NUM> along the second supply line L2, and the fluid cooled by the heat exchanger <NUM> is supplied to the pressure reducer <NUM> along a fourth supply line L4.

The pressure reducer <NUM> reduces the pressure of the BOG compressed by the compressor <NUM> and then cooled by the heat exchanger <NUM>. Part or all of the BOG is re-liquefied through compression by the compressor <NUM>, cooling by the heat exchanger <NUM>, and pressure reduction by the pressure reducer <NUM>. The pressure reducer <NUM> may be an expansion valve, such as a Joule-Thomson valve, or may be an inflator.

The BOG reliquefaction system according to this embodiment may further include a gas/liquid separator <NUM> disposed at the back of the pressure reducer <NUM> to separate the BOG remaining in a vapor phase from liquefied natural gas generated by reliquefaction of the BOG through the compressor <NUM>, the heat exchanger <NUM>, and the pressure reducer <NUM>.

The liquefied gas separated by the gas/liquid separator <NUM> is supplied to the storage tank T along a fifth supply line L5, and the BOG separated by the gas/liquid separator <NUM> may be combined with the BOG discharged from the storage tank T and be supplied to the heat exchanger <NUM>.

A ninth valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on a sixth supply line L6 through which the BOG having a vapor phase is discharged from the gas/liquid separator <NUM>.

If the heat exchanger <NUM> is not available, for example, upon overhaul or failure of the heat exchanger <NUM>, the BOG discharged from the storage tank T may be allowed to bypass the heat exchanger <NUM> through the bypass line BL. The bypass line BL is provided with the bypass valve <NUM> that opens and closes the bypass line BL.

Referring to <FIG>, the BOG reliquefaction system according to this embodiment includes a heat exchanger <NUM>, a first valve <NUM>, a second valve <NUM>, a first temperature sensor <NUM>, a second temperature sensor <NUM>, a compressor <NUM>, a third temperature sensor <NUM>, a fourth temperature sensor <NUM>, a first pressure sensor <NUM>, a second pressure sensor <NUM>, a pressure reducer <NUM>, a bypass line BL, and a bypass valve <NUM>.

The heat exchanger <NUM> cools the BOG compressed by the compressor <NUM> through heat exchange using the BOG discharged from the storage tank T as a refrigerant. The BOG discharged from the storage tank T and used as the refrigerant in the heat exchanger <NUM> is sent to the compressor <NUM>, and the BOG compressed by the compressor <NUM> is cooled by the heat exchanger <NUM> using the BOG discharged from the storage tank T as the refrigerant.

The BOG discharged from the storage tank T is supplied to the heat exchanger <NUM> along a first supply line L1 and used as the refrigerant, and the BOG used as the refrigerant in the heat exchanger <NUM> is sent to the compressor <NUM> along a second supply line L2. Part or all of the BOG compressed by the compressor <NUM> is supplied to the heat exchanger <NUM> along a third supply line L3 so as to be cooled, and the fluid cooled by the heat exchanger <NUM> is supplied to the pressure reducer <NUM> along a fourth supply line L4.

The first valve <NUM> is disposed on the first supply line L1 to regulate the flow rate and opening/closing of the corresponding supply line, and the second valve <NUM> is disposed on the second supply line L2 to regulate the flow rate and opening/closing of the corresponding supply line.

The first temperature sensor <NUM> is disposed in front of the heat exchanger <NUM> on the first supply line L1 to measure the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger <NUM>. Preferably, the first temperature sensor <NUM> is disposed immediately in front of the heat exchanger <NUM> to measure the temperature of the BOG immediately before being supplied to the heat exchanger <NUM>.

Herein, the term "in front of" means upstream and the term "at the back of" means downstream.

The second temperature sensor <NUM> is disposed downstream of the heat exchanger <NUM> on the second supply line L2 to measure the temperature of the BOG used as the refrigerant in the heat exchanger <NUM> after being discharged from the storage tank T. Preferably, the second temperature sensor <NUM> is disposed immediately at the back of the heat exchanger <NUM> to measure the temperature of the BOG immediately after being used as the refrigerant in the heat exchanger <NUM>.

The compressor <NUM> compresses the BOG used as the refrigerant in the heat exchanger <NUM> after being discharged from the storage tank T. The BOG compressed by the compressor <NUM> may be supplied into a high-pressure engine to be used as fuel, and the remaining BOG after being supplied into the high-pressure engine may be supplied to the heat exchanger <NUM> for reliquefaction.

A sixth valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the fuel supply line SL through which the BOG compressed by the compressor <NUM> is supplied to the high-pressure engine.

The sixth valve <NUM> acts as a safety device to shut off supply of the BOG to the high-pressure engine upon interruption of a gas mode operation of the high-pressure engine. The gas mode means a mode in which the engine is operated using natural gas as fuel. When the BOG to be used as the fuel is insufficient, the engine is switched to a fuel oil mode to allow fuel oil to be used as fuel for the engine.

A seventh valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on a supply line through which the surplus BOG above fuel requirement of the high-pressure engine among the BOG compressed by the compressor <NUM> is supplied to the heat exchanger <NUM>.

When the BOG compressed by the compressor <NUM> is supplied to the high-pressure engine, the compressor <NUM> can compress the BOG to a pressure required by the high-pressure engine. The high-pressure engine may be an ME-GI engine that uses high pressure BOG as fuel.

The ME-GI engine is known to use, as fuel, natural gas having a pressure of about <NUM> bar to <NUM> bar, preferably about <NUM> bar to about <NUM> bar, more preferably about <NUM> bar. The compressor <NUM> can compress the BOG to a pressure of about <NUM> bar to about <NUM> bar in order to supply the compressed BOG to the ME-GI engine.

Instead of the ME-GI engine as the main engine, an X-DF engine or a DF engine using BOG as fuel at a pressure of about <NUM> bar to about <NUM> bar may be used. In this case, since the compressed BOG for supply to the main engine has a low pressure, the compressed BOG to be supplied to the main engine may be further compressed to reliquefy the BOG. The further compressed BOG for re-liquefaction may have a pressure of about <NUM> bar to <NUM> bar.

<FIG> and <FIG> are graphs depicting reliquefaction amounts depending upon BOG pressure in a partial reliquefaction system (PRS). A reliquefaction target BOG means BOG to be re-liquefied though cooling and is distinguished from BOG used as a refrigerant.

Referring to <FIG> and <FIG>, it can be seen that, when the pressure of the BOG is in the range of <NUM> bar to <NUM> bar, the reliquefaction amount reaches a maximum value, and when the pressure of the BOG is in the range of <NUM> bar to <NUM> bar, there is substantially no change in reliquefaction amount. Accordingly, as the high-pressure engine, the ME-GI engine using BOG having a pressure of about <NUM> bar to about <NUM> bar (mainly <NUM> bar) as fuel can easily control the reliquefaction system to supply fuel to the high-pressure engine while maintaining a high liquefaction amount.

The compressor <NUM> may include a plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and a plurality of coolers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> disposed downstream of the plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, respectively. The coolers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> cool BOG compressed by the cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and having high pressure and temperature.

In the structure wherein the compressor <NUM> includes the plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, the BOG sent to the compressor <NUM> is compressed through multiple stages by the plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. Each of the cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> can act as a compression terminal of each of the compressor <NUM>.

The compressor <NUM> may include a first recirculation line Rc1 through which part or all of the BOG having passed through a first cylinder <NUM> and a first cooler <NUM> is supplied to a front end of the first cylinder <NUM>; a second recirculation line Rc2 through which part or all of the BOG having passed through a second cylinder <NUM> and a second cooler <NUM> is supplied to a front end of the second cylinder <NUM>; a third recirculation line Rc3 through which part or all of the BOG having passed through a third cylinder <NUM> and a third cooler <NUM> is supplied to a front end of the third cylinder <NUM>; and a fourth recirculation line Rc4 through which part or all of the BOG having passed through a fourth cylinder <NUM>, a fourth cooler <NUM>, a fifth cylinder <NUM> and a fifth cooler <NUM> is supplied to a front end of the fourth cylinder <NUM>.

In addition, a first recirculation valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the first recirculation line Rc1, a second recirculation valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the second recirculation line Rc2, a third recirculation valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the third recirculation line Rc3, and a fourth recirculation valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the fourth recirculation line Rc4.

The recirculation lines Rc1, Rc2, Rc3, Rc4 protect the compressor <NUM> by recirculating part or all of the BOG when the storage tank T has a low pressure to satisfy an intake pressure condition required by the compressor <NUM>. When the recirculation lines Rc1, Rc2, Rc3, Rc4 are not used, the recirculation valves <NUM>, <NUM>, <NUM>, <NUM> are closed, and when the intake pressure condition required by the compressor <NUM> is not satisfied and the recirculation lines Rc1, Rc2, Rc3, Rc4 are required to be used, the recirculation valves <NUM>, <NUM>, <NUM>, <NUM> are opened.

Although <FIG> shows the structure wherein the BOG having passed through all of the plurality of cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the compressor <NUM> is supplied to the heat exchanger <NUM>, the BOG having passed through some of the cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be divided in the compressor <NUM> to be supplied to the heat exchanger <NUM>.

In addition, the BOG having passed through some of the cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be divided in the compressor <NUM> to be supplied to a low-pressure engine so as to be used as fuel, and the surplus may be supplied to a gas combustion unit (GCU) so as to be combusted.

The low-pressure engine may be a DF engine (for example, DFDE) using BOG having a pressure of about <NUM> bar to <NUM> bar as fuel.

Some of the cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM> included in the compressor <NUM> may operate in an oil-free lubrication manner and the other may operate in an oil lubrication manner. In particular, when the BOG is compressed to <NUM> bar or more, preferably <NUM> bar or more, in order to use the BOG compressed by the compressor <NUM> as fuel for a high-pressure engine or for reliquefaction efficiency, the compressor <NUM> includes an oil-lubrication type cylinder in order to compress the BOG to high pressure.

In the related art, lubricant oil for lubrication and cooling is supplied to the reciprocation type compressor <NUM>, for example, a piston sealing part thereof, in order to compress the BOG to <NUM> bar or more.

Since the lubricant oil is supplied to the oil-lubrication type cylinder, some lubricant oil is mixed with the BOG having passed through the oil-lubrication type cylinder in the related art. The inventors of the present invention found that that the lubricant oil mixed with the compressed BOG is condensed or solidified prior to the BOG in the heat exchanger <NUM> to clog the fluid channel of the heat exchanger <NUM>.

The BOG reliquefaction system according to this embodiment may further include an oil separator <NUM> and a first oil filter <NUM> disposed between the compressor <NUM> and the heat exchanger <NUM> to separate the oil from the BOG.

The oil separator <NUM> generally separates the lubricant oil in a liquid phase, and the first oil filter <NUM> separates the lubricant oil in a vapor phase or in a mist phase. Since the oil separator <NUM> separates the lubricant oil having a larger particle size than the lubricant oil separated by the first oil filter <NUM>, the oil separator <NUM> is disposed upstream of the first oil filter <NUM> such that the BOG compressed by the compressor <NUM> can be supplied to the heat exchanger <NUM> after sequentially passing through the oil separator <NUM> and the first oil filter <NUM>.

Although <FIG> shows the structure wherein the BOG reliquefaction system includes both the oil separator <NUM> and the first oil filter <NUM>, the BOG reliquefaction system according to this embodiment may include one of the oil separator <NUM> and the first oil filter <NUM>. Preferably, both the oil separator <NUM> and the first oil filter <NUM> are used.

In addition, although <FIG> shows the structure wherein the first oil filter <NUM> is provided to the second supply line L2 downstream of the compressor <NUM>, the first oil filter <NUM> may also be provided to the third supply line L3 upstream of the heat exchanger <NUM> and may be provided in plural so as to be arranged in parallel.

In the structure wherein the BOG reliquefaction system includes one of the oil separator <NUM> and the first oil filter <NUM> and the compressor <NUM> includes the oil-free lubrication type cylinder and the oil-lubrication type cylinder, the BOG having passed through the oil-lubrication type cylinder may be supplied to the oil separator <NUM> and/or the first oil filter <NUM>, and the BOG having passed only through the oil-free lubrication type cylinder may be directly supplied to the heat exchanger <NUM> without passing through the oil separator <NUM> or the first oil filter <NUM>.

By way of example, the compressor <NUM> according to this embodiment includes five cylinders <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, in which front three cylinders <NUM>, <NUM>, <NUM> may be oil-free lubrication type cylinders and rear two cylinders <NUM>, <NUM> may be oil-lubrication type cylinders. Here, in the BOG reliquefaction system according to this embodiment, the BOG may be directly supplied to the heat exchanger <NUM> without passing through the oil separator <NUM> or the first oil filter <NUM> upon division of the BOG in three stages or less and may be supplied to the heat exchanger <NUM> after passing through the oil separator <NUM> and/or the first oil filter <NUM> upon division of the BOG in four stages or more.

The first oil filter <NUM> may be a coalescer oil filter.

A check valve <NUM> may be disposed on the fuel supply line SL between the compressor <NUM> and the high-pressure engine. The check valve <NUM> serves to prevent the BOG from returning to and damaging the compressor if the high-pressure engine is stopped.

In the structure wherein the BOG reliquefaction system includes the oil separator <NUM> and/or the first oil filter <NUM>, the check valve <NUM> may be disposed downstream of the oil separator <NUM> and/or the first oil filter <NUM> in order to prevent the BOG from flowing back to the oil separator <NUM> and/or the first oil filter <NUM>.

In addition, since the BOG can flow back to the compressor <NUM> and damage the compressor <NUM> when an expansion valve <NUM> is suddenly closed, the check valve <NUM> may be disposed upstream of a branch point of the third supply line L3 branched from the fuel supply line SL.

The third temperature sensor <NUM> is disposed upstream of the heat exchanger <NUM> on the third supply line L3 to measure the temperature of the BOG compressed by the compressor <NUM> and then supplied to the heat exchanger <NUM>. Preferably, the third temperature sensor <NUM> is disposed immediately in front of the heat exchanger <NUM> to measure the temperature of the BOG immediately before being supplied to the heat exchanger <NUM>.

The fourth temperature sensor <NUM> is disposed downstream of the heat exchanger <NUM> on the fourth supply line L4 to measure the temperature of the BOG compressed by the compressor <NUM> and then cooled by the heat exchanger <NUM>. Preferably, the fourth temperature sensor <NUM> is disposed immediately at the back of the heat exchanger <NUM> to measure the temperature of the BOG immediately after being cooled by the heat exchanger <NUM>.

The first pressure sensor <NUM> is disposed upstream of the heat exchanger <NUM> on the third supply line L3 to measure the pressure of the BOG compressed by the compressor <NUM> and supplied to the heat exchanger <NUM>. Preferably, the first pressure sensor <NUM> is disposed immediately in front of the heat exchanger <NUM> to measure the pressure of the BOG immediately before being supplied to the heat exchanger <NUM>.

The second pressure sensor <NUM> is disposed downstream of the heat exchanger <NUM> on the fourth supply line L4 to measure the pressure of the BOG compressed by the compressor <NUM> and then cooled by the heat exchanger <NUM>. Preferably, the second pressure sensor <NUM> is disposed immediately at the back of the heat exchanger <NUM> to measure the pressure of the BOG immediately after being cooled by the heat exchanger <NUM>.

As shown in <FIG>, although it is desirable that all of the first to fourth temperature sensors <NUM> to <NUM>, the first pressure sensor <NUM>, and the second pressure sensor <NUM> be provided to the reliquefaction system, it should be understood that the present invention is not limited thereto. Alternatively, the reliquefaction system may be provided with only the first temperature sensor <NUM> and the fourth temperature sensor <NUM> ('first pair'), only the second temperature sensor <NUM> and the third temperature sensor <NUM> ('second pair'. ), only the first pressure sensor <NUM> and the second pressure sensor <NUM> ('third pair'. ), or two pairs among the first to third pairs.

The pressure reducer <NUM> is disposed downstream of the heat exchanger <NUM> to decompress the BOG compressed by the compressor <NUM> and then cooled by the heat exchanger <NUM>. Part or all of the BOG is re-liquefied through compression by the compressor <NUM>, cooling by the heat exchanger <NUM>, and pressure reduction by the pressure reducer <NUM>. The pressure reducer <NUM> may be an expansion valve, such as a Joule-Thomson valve, or may be an inflator.

The BOG reliquefaction system according to this embodiment may further include a gas/liquid separator <NUM> disposed downstream of the pressure reducer <NUM> to separate the BOG remaining in a vapor phase from liquefied natural gas generated by reliquefaction of the BOG through the compressor <NUM>, the heat exchanger <NUM>, and the pressure reducer <NUM>.

The liquefied gas separated by the gas/liquid separator <NUM> is supplied to the storage tank T along the fifth supply line L5, and the BOG separated by the gas/liquid separator <NUM> may be combined with the BOG discharged from the storage tank T along the sixth supply line L6 and be supplied to the heat exchanger <NUM>.

Although <FIG> shows the structure wherein the BOG separated by the gas/liquid separator <NUM> is combined with the BOG discharged from the storage tank T and then supplied to the heat exchanger <NUM>, it should be understood that the present invention is not limited thereto. By way of example, the heat exchanger <NUM> may be composed of three fluid channels and the BOG separated by the gas/liquid separator <NUM> may be supplied to the heat exchanger <NUM> along a separate fluid channel so as to be used as a refrigerant therein.

Alternatively, the gas/liquid separator <NUM> may be omitted and the BOG reliquefaction system may be configured to allow the fluid partially or totally re-liquefied through pressure reduction by the pressure reducer <NUM> to be directly supplied to the storage tank T.

An eighth valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the fifth supply line L5. A level of the liquefied gas in the gas/liquid separator <NUM> is regulated by the eighth valve <NUM>.

A ninth valve <NUM> for regulating the flow rate and opening/closing of the corresponding supply line may be disposed on the sixth supply line L6. Internal pressure of the gas/liquid separator <NUM> can be regulated by the ninth valve <NUM>.

<FIG> is an enlarged view of a gas/liquid separator according to one embodiment of the present invention. Referring to <FIG>, the gas/liquid separator <NUM> may be provided with a fluid level sensor <NUM> that measures the level of natural gas in the gas/liquid separator <NUM>.

The BOG reliquefaction system according to this embodiment may include a second oil filter <NUM> disposed between the pressure reduce <NUM> and the gas/liquid separator <NUM> to filter the lubricant oil mixed with the fluid subjected to pressure reduction by the pressure reducer <NUM>.

Referring to <FIG> and <FIG>, the second oil filter <NUM> may be disposed on the fourth supply line L4 between the pressure reducer <NUM> and the gas/liquid separator <NUM> (Position A in <FIG>), on the fifth supply line L5 through which the re-liquefied gas is discharged from the gas/liquid separator <NUM> (Position B in <FIG>), or on the sixth supply line L6 through which the gaseous BOG is discharged from the gas/liquid separator <NUM> (Position C in <FIG>). <FIG> shows the structure wherein the second oil filter <NUM> is disposed at Position A in <FIG>.

The BOG separated by the gas/liquid separator <NUM> may be combined with the BOG discharged from the storage tank T and be supplied to a cold fluid channel of the heat exchanger <NUM>. Here, since the lubricant oil is collected in the gas/liquid separator <NUM>, there is a possibility that even a small amount of the lubricant oil can be mixed with the gaseous BOG separated by the gas/liquid separator <NUM>.

The inventors of the present invention found that, when the gaseous BOG separated by the gas/liquid separator <NUM> is mixed with the lubricant oil and sent to the cold fluid channel of the heat exchanger <NUM>, more difficult circumstances can occur than the case where the lubricant oil mixed with the BOG compressed by the compressor <NUM> is supplied to a hot fluid channel of the heat exchanger <NUM>.

Since a fluid to be used as a refrigerant in the heat exchanger <NUM> is sent to the cold fluid channel of the heat exchanger <NUM>, cryogenic BOG is supplied throughout operation of the reliquefaction system and a fluid having a high enough temperature to melt the condensed or solidified oil is not supplied thereto. Therefore, it is very difficult to remove the condensed or solidified oil accumulated in the low-temperature fluid channel of the heat exchanger <NUM>.

In order to reduce the possibility of supplying the mixture of the lubricant oil and the gaseous BOG separated by the gas/liquid separator <NUM> to the cold fluid channel of the heat exchanger <NUM> as low as possible, the second oil filter <NUM> may be disposed at Position A or C in <FIG>.

In the structure wherein the second oil filter <NUM> is disposed at Position C in <FIG>, since most of the molten or viscosity-reduced lubricant oil is collected in a liquid phase in the gas/liquid separator <NUM> and the amount of gaseous lubricant oil discharged along the sixth supply line L6 is small, there are advantages in that the reliquefaction system has high filtering efficiency and does not require frequent replacement of the second oil filter <NUM>.

In the structure wherein the second oil filter <NUM> is disposed at Position B in <FIG>, since the lubricant oil can be prevented from flowing into the storage tank T, it is possible to prevent contamination of the liquefied gas stored in the storage tank T.

Since the first oil filter <NUM> is disposed downstream of the compressor <NUM> and the BOG compressed by the compressor <NUM> has a temperature of about <NUM> to about <NUM>, it is not necessary to use a cryogenic oil filter. However, since the fluid reduced in pressure by the pressure reducer <NUM> has a temperature of about -<NUM> to about -<NUM> to allow reliquefaction of at least part of the BOG, and since the liquefied gas and the BOG separated by the gas/liquid separator <NUM> have a temperature of about -<NUM> to about -<NUM>, the second oil filter <NUM> must be designed for cryogenic temperatures regardless of the position of the second oil filter <NUM> among the positions A, B, C and D in <FIG>.

In addition, since most lubricant oil mixed with the BOG compressed by the compressor <NUM> and having a temperature of about <NUM> to <NUM> has a liquid phase or a mist phase, the oil separator <NUM> is designed to be suitable for separation of the lubricant oil of the liquid phase and the first oil filter <NUM> is designed to be suitable for separating the lubricant oil of the mist phase, (which may include some lubricant oil in a vapor phase).

Conversely, the fluid, which is a cryogenic fluid and reduced in pressure by the pressure reducer <NUM>, the BOG separated by the gas/liquid separator <NUM>, and the lubricant oil mixed with the liquefied gas separated by the gas/liquid separator <NUM> in a solid phase (or in a solidified state) below a flow point, the second oil filter <NUM> is designed to be suitable for separation of the lubricant oil in the solid phase (or in the solidified state).

<FIG> is an enlarged view of a second oil filter according to one embodiment of the present invention and <FIG> is an enlarged view of a second oil filter according to another embodiment of the present invention.

Referring to <FIG> and <FIG>, the second oil filter <NUM> may have a structure as shown in <FIG> (hereinafter, 'downward discharge type') or a structure as shown in <FIG> (hereinafter, 'upward discharge type'). In <FIG> and <FIG>, a dotted line indicates a fluid flow direction.

Referring to <FIG> and <FIG>, the second oil filter <NUM> includes a fixing plate <NUM> and a filter element <NUM> and is connected to an inflow pipe <NUM>, a discharge pipe <NUM> and an oil discharge pipe <NUM>.

The filter element <NUM> is provided to the fixing plate <NUM> to separate the lubricant oil from the fluid flowing through the inflow pipe <NUM>.

<FIG> is a plan view of the filter element <NUM> shown in <FIG> and <FIG>. Referring to <FIG>, the filter element <NUM> may have a hollow (Z space in <FIG>) cylindrical shape, in which multiple layers having different meshes are stacked one above another. The lubricant oil is filtered from the fluid while the fluid flowing into the second oil filter <NUM> through the inflow pipe <NUM> passes through the multiple layers of the filter element <NUM>. The filter element <NUM> may separate the lubricant oil by a physical adsorption method.

The fluid (BOG, liquefied gas, or fluid of a vapor-liquid mixture) filtered by the filter element <NUM> is discharged through the discharge pipe <NUM> and the lubricant oil filtered by the filter element <NUM> is discharged through the oil discharge pipe <NUM>.

The components of the second oil filter <NUM> are formed of materials capable of enduring cryogenic conditions in order to separate the lubricant oil from the fluid having an extremely low temperature. The filter element <NUM> may be formed of a metal capable of enduring cryogenic conditions, particularly, SUS.

Referring to <FIG>, in the downward discharge type oil filter, the fluid supplied through the inflow pipe <NUM> connected to an upper portion of the oil filter passes through the filter element <NUM> and a space (X in <FIG>) defined under the fixing plate <NUM>, and is then discharged through the discharge pipe <NUM> connected to a lower portion of the oil filter.

In the downward discharge type oil filter, the fixing plate <NUM> is connected to a lower portion of the oil filter, the filter element <NUM> is disposed on an upper surface of the fixing plate <NUM>, and the discharge pipe <NUM> is connected to a side of the oil filter opposite to the filter element <NUM> with respect to the fixing plate <NUM>.

Further, in the downward discharge type oil filter, the inflow pipe <NUM> is preferably connected to the oil filter to be disposed above an upper end of the filter element <NUM> in order to allow the fluid flowing into the oil filter through the inflow pipe <NUM> to be filtered even by an upper portion of the filter element <NUM> (that is, in order to use as much of the filter element as possible).

It is desirable that the inflow pipe <NUM> and the discharge pipe <NUM> be disposed on opposite sides (on the left and right sides with respect to the filter element <NUM> in <FIG>) in terms of fluid flow, and since the lubricant oil filtered by the filter element <NUM> is collected at a lower side of the oil filter, it is desirable that the oil discharge pipe <NUM> be connected to the lower portion of the filter element <NUM>.

In the downward discharge type oil filter, the oil discharge pipe <NUM> may be connected to the oil filter to be disposed immediately above the fixing plate <NUM>.

As shown in <FIG>, when a fluid mainly composed of a liquid component (for example, <NUM> vol% of liquid and <NUM> vol% of gas) is supplied to the downward discharge type oil filter, a downward flow of the fluid is generated due to a high density of the liquid component, thereby securing good filtering effects.

On the other hand, as shown in <FIG>, when a fluid composed of a gaseous component (for example, <NUM> vol% of liquid and <NUM> vol% of gas) is supplied to the downward discharge type oil filter, the gaseous component having a small density stays in the upper portion of the oil filter, thereby deteriorating the fluid flow and the filtering effects.

Referring to <FIG>, in the upward discharge type oil filter, the fluid supplied through the inflow pipe <NUM> connected to a lower portion of the oil filter passes through the filter element <NUM> and a space (Y in <FIG>) defined above the fixing plate <NUM>, and is then discharged through the discharge pipe <NUM> connected to an upper portion of the oil filter.

In the upward discharge type oil filter, the fixing plate <NUM> is connected to an upper portion of the oil filter, the filter element <NUM> is disposed on a lower surface of the fixing plate <NUM>, and the discharge pipe <NUM> is connected to a side of the oil filter opposite to the filter element <NUM> with respect to the fixing plate <NUM>.

Further, in the upward discharge type oil filter, the inflow pipe <NUM> is preferably connected to the oil filter to be disposed below a lower end of the filter element <NUM> in order to allow the fluid flowing into the oil filter through the inflow pipe <NUM> to be filtered even by a lower portion of the filter element <NUM> (that is, in order to use as much of the filter element as possible).

It is desirable that the inflow pipe <NUM> and the discharge pipe <NUM> be disposed on opposite sides (on the left and right sides with respect to the filter element <NUM> in <FIG>) in terms of fluid flow, and since the lubricant oil filtered by the filter element <NUM> is collected at the lower side of the oil filter, it is desirable that the oil discharge pipe <NUM> be connected to the lower portion of the filter element <NUM>.

Referring to <FIG>, in the upward discharge type oil filter, the fluid supplied to the oil filter through the inflow pipe <NUM> connected to the lower portion of the oil filter passes through the filter element <NUM> and is discharged through the discharged pipe <NUM> connected to the upper portion of the oil filter. The lubricant oil filtered by the filter element <NUM> is discharged through a separate pipe <NUM>.

As shown in <FIG>, when a fluid mainly composed of a gaseous component (for example, <NUM> vol% of liquid and <NUM> vol% of gas) is supplied to the upward discharge type oil filter, an upward flow of the fluid is generated due to a low density of the gaseous component, thereby providing a suitable upward flow while securing good filtering effects.

On the other hand, as shown in <FIG>, when a fluid composed of a liquid component (for example, <NUM> vol% of liquid and <NUM> vol% of gas) is supplied to the upward discharge type oil filter, the liquid component having a high density stays in the lower portion of the oil filter, thereby deteriorating fluid flow and filtering effects.

Accordingly, in the structure wherein the second oil filter <NUM> is disposed at Position B of <FIG>, it is desirable that the downward discharge type oil filter as shown in <FIG> be used as the second oil filter <NUM>, and when the second oil filter <NUM> is disposed at Position C of <FIG>, it is desirable that the upward discharge type oil filter as shown in <FIG> be used as the second oil filter <NUM>.

In the structure wherein the second oil filter <NUM> is disposed at Position A in <FIG>, the fluid reduced in pressure by the pressure reducer <NUM> is a vapor-liquid mixture (theoretically <NUM>% reliquefaction is possible) in which the volume ratio of the gas component is higher than the volume ratio of the liquid component. Thus, it is desirable that the upward discharge type oil filter as shown in FIG be used as the second oil filter <NUM>.

According to the embodiments, the bypass line BL is branched from the first supply line L1 upstream of the heat exchanger <NUM> to bypass the heat exchanger <NUM> and is joined to the second supply line L2 downstream of the heat exchanger <NUM>.

Typically, the bypass line bypassing the heat exchanger is disposed inside the heat exchanger to be integrated with the heat exchanger. In the structure wherein the bypass line is disposed inside the heat exchanger, the fluid cannot be supplied to the heat exchanger and the bypass line when the valves disposed upstream and/or downstream of the heat exchanger are closed.

In the embodiments of the invention, the bypass line BL is disposed outside the heat exchanger <NUM> to be separate from the heat exchanger <NUM> and is branched from the first supply line L1 upstream of the first valve <NUM> and joined to the second supply line L2 downstream of the second valve <NUM> such that the BOG can be sent to the bypass line BL even when the first valve <NUM> upstream of the heat exchanger <NUM> and/or the second valve <NUM> downstream of the heat exchanger <NUM> are closed.

The bypass valve <NUM> is disposed on the bypass line BL and is opened when there is a need for use of the bypass line BL.

Fundamentally, when the heat exchanger <NUM> cannot be used, for example, when the heat exchanger <NUM> fails or is overhauled, the bypass line BL will be used. For example, if the heat exchanger <NUM> cannot be used when the BOG reliquefaction system according to this embodiment sends part or all of the BOG compressed by the compressor <NUM> to the high-pressure engine, the BOG discharged from the storage tank T is directly sent to the compressor <NUM> along the bypass line BL bypassing the heat exchanger <NUM>, instead of reliquefying the surplus BOG not used by the high-pressure engine, and the BOG compressed by the compressor <NUM> is supplied to the high-pressure engine while sending the surplus BOG to the GCU to burn the surplus BOG.

In use of the bypass line BL for overhaul of the heat exchanger <NUM>, for example, when the fluid channel of the heat exchanger <NUM> is clogged by the condensed or solidified lubricant oil, the condensed or solidified lubricant oil can be removed through the bypass line BL.

In addition, if there is no need for reliquefaction of the BOG due to little surplus BOG as in a ballast condition of the vessel, all of the BOG discharged from the storage tank T may be sent to the bypass line BL so as to allow all of the BOG to be directly sent to the compressor <NUM> while bypassing the heat exchanger <NUM>. The BOG compressed by the compressor <NUM> is used as fuel for the high-pressure engine. If it is determined that there is no need for reliquefaction of the BOG due to little surplus BOG, the bypass valve <NUM> may be controlled to be automatically opened.

The inventors of the present invention found that, when the BOG is supplied to the engine through the heat exchanger having a narrow fluid channel according to the embodiments, the BOG suffers from a severe pressure drop due to the heat exchanger. If there is no need for reliquefaction of the BOG, fuel can be smoothly supplied to the engine by compressing the BOG while bypassing the heat exchanger, as described above.

In addition, the bypass line BL may also be used for reliquefaction of the BOG due to increase in the amount of BOG not re-liquefied.

When there is a need for reliquefaction of the BOG due to increase in the amount of BOG (that is, upon start or restart of BOG reliquefaction), all of the BOG discharged from the storage tank T may be sent to the bypass line BL so as to allow all of the BOG to be directly sent to the compressor <NUM> while bypassing the heat exchanger <NUM>, and the BOG compressed by the compressor <NUM> may be sent to the hot fluid channel of the heat exchanger <NUM>. Some of the BOG compressed by the compressor <NUM> may be supplied to the high-pressure engine.

When the temperature of the hot fluid channel of the heat exchanger <NUM> is increased through the aforementioned process upon start or restart of BOG reliquefaction, there is an advantage in that BOG reliquefaction can be started after removing any condensed or solidified lubricant oil, other residues or impurities that can remain in the heat exchanger <NUM>, other equipment, pipes, and the like in the previous BOG reliquefaction process.

Residues may include the BOG, which is compressed by the compressor <NUM> and then supplied to the heat exchanger in the previous BOG liquefaction, and the lubricant oil mixed with the BOG compressed by the compressor <NUM>.

If the cold BOG discharged from the storage tank T is directly supplied to the heat exchanger <NUM> without increasing the temperature of the heat exchanger <NUM> through the bypass line BL upon start or restart of BOG reliquefaction, the cold BOG discharged from the storage tank T is sent to the cold fluid channel of the heat exchanger <NUM> in a state that the hot BOG is not sent to the hot fluid channel of the heat exchanger <NUM>. As a result, the lubricant oil remaining in a non-condensed or non-solidified state in the heat exchanger <NUM> can also be condensed or solidified as the temperature of the heat exchanger <NUM> decreases.

When the bypass line BL is used to increase the temperature of the heat exchanger <NUM> for a certain period of time (if it is determined that the condensed or solidified lubricant oil or other impurities are almost completely removed, the certain period of time can be determined by those skilled in the art and may be about <NUM> minute to about <NUM> minutes, preferably about <NUM> minutes to about <NUM> minutes, and more preferably about <NUM> minutes to about <NUM> minutes), BOG reliquefaction is started by slowly opening the first valve <NUM> and the second valve <NUM> while slowly closing the bypass valve <NUM>. As the time further elapses, the first valve <NUM> and the second valve <NUM> are completely opened and the bypass valve <NUM> is completely closed to allow all of the BOG discharged from the storage tank T to be used as a refrigerant for reliquefaction of the BOG in the heat exchanger <NUM>.

In addition, the bypass line BL may be used to satisfy the intake pressure condition of the compressor <NUM> when the internal pressure of the storage tank T is low.

Furthermore, if the internal pressure of the storage tank T is required to be controlled to a low pressure, the bypass line BL may be used to satisfy the intake pressure condition of the compressor <NUM> even if the internal pressure of the storage tank T is decreased.

The following description will focus on the case where the bypass line BL is used to remove the condensed or solidified lubricant oil and the case where the bypass line BL is used to satisfy the intake pressure condition of the compressor <NUM> when the internal pressure of the storage tank T is low.

The inventors of the present invention found that, since a certain amount of lubricant oil is mixed with the BOG having passed through the oil-lubrication type cylinder of the compressor <NUM> and the lubricant oil contained in the BOG is condensed or solidified prior to the BOG in the heat exchanger <NUM> and accumulated in the heat exchanger <NUM>, there is a need for removal of the condensed or solidified lubricant oil from the heat exchanger <NUM> after a predetermined period of time due to increase in amount of the condensed or solidified lubricant oil accumulated in the heat exchanger <NUM> over time.

Particularly, although it is desirable that the heat exchanger <NUM> according to this embodiment be a printed circuit heat exchanger (PCHE, also referred to as DCHE) in consideration of pressure and/or flow rate of BOG to be re-liquefied, reliquefaction efficiency, and the like, the PCHE has a narrow serpentine fluid channel (micro-channel type fluid channel) and thus has a problem such as easy clogging of the fluid channel by the condensed or solidified lubricant oil, easy accumulation of the condensed or solidified lubricant oil at a serpentine portion of the fluid channel, and the like. The PCHE (DCHE) is manufactured by Kobelko Co. , Alfalaval Co. , and the like.

The condensed or solidified lubricant oil can be removed through the steps of:.

When the fluid channel of the heat exchanger <NUM> is clogged by the condensed or solidified lubricant oil, cooling efficiency of the heat exchanger <NUM> can be reduced. Therefore, if performance of the heat exchanger <NUM> falls below a preset value of normal performance, it can be estimated that the condensed or solidified lubricant oil is accumulated in a certain amount or more in the heat exchanger <NUM>. By way of example, it can be determined that it is time to remove the condensed or solidified lubricant oil from the heat exchanger <NUM> if the performance of the heat exchanger <NUM> falls to about <NUM>% to about <NUM>% of normal performance, preferably about <NUM>% to about <NUM>%, more preferably about <NUM>% or less.

Herein, the range of "about <NUM>% to about <NUM>%" of normal performance includes all of values of about <NUM>% or less, about <NUM>% or less, about <NUM>% or less, about <NUM>% or less, and about <NUM>% or less, and the range of "about <NUM>% to about <NUM>%" of normal performance include all of values of about <NUM>% or less, about <NUM>% or less, and about <NUM>% or less.

When the performance of the heat exchanger <NUM> deteriorates, the temperature difference between cold BOG (L1) supplied to the heat exchanger <NUM> and cold BOG (L4) discharged from the heat exchanger <NUM> increases, and the temperature difference between hot BOG (L2) discharged from the heat exchanger <NUM> and hot BOG (L3) supplied to the heat exchanger <NUM> also increases. In addition, when the fluid channel of the heat exchanger <NUM> is clogged by the condensed or solidified lubricant oil, the fluid channel of the heat exchanger <NUM> becomes narrow, thereby increasing the pressure difference between a front end (L3) and a rear end (L4) of the heat exchanger <NUM>.

Accordingly, it is possible to determine whether it is time to remove the condensed or solidified lubricant oil, based on the temperature difference <NUM>, <NUM> of the cold fluid supplied to the heat exchanger <NUM> or discharged from the heat exchanger <NUM>, the temperature difference <NUM>, <NUM> of the hot fluid supplied to the heat exchanger <NUM> or discharged from the heat exchanger <NUM>, and the pressure difference <NUM>, <NUM> of the hot fluid channel of the heat exchanger <NUM>.

Specifically, if the temperature difference (meaning an absolute value, hereinafter referred to as "temperature difference of the cold flow") between the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger <NUM>, as measured by the first temperature sensor <NUM>, and the temperature of the BOG compressed by the compressor <NUM> and cooled by the heat exchanger <NUM>, as measured by the fourth temperature sensor <NUM>, is higher than a normal temperature difference and continues for a certain period of time or more, it can be determined that heat exchange is normally performed in the heat exchanger <NUM>.

By way of example, when the state wherein the temperature difference of the cold flow is <NUM> to <NUM> or higher, preferably <NUM> to <NUM> or higher, more preferably about <NUM> or higher, continues for <NUM> hour or more, it can be determined that it is time to discharge the condensed or solidified lubricant oil.

When the heat exchanger <NUM> is normally operated, the BOG compressed to about <NUM> bar by the compressor <NUM> has a temperature of about <NUM> to about <NUM>, and the BOG discharged from the storage tank T and having a temperature of about - <NUM> to about -<NUM> is supplied to the heat exchanger <NUM>. Here, the temperature of the BOG discharged from the storage tank T is increased to about -<NUM> to about -<NUM>, preferably about -<NUM>, during delivery to the heat exchanger <NUM>.

In the BOG reliquefaction system according to this embodiment that includes the gas/liquid separator <NUM>, when gaseous BOG separated by the gas/liquid separator <NUM> is combined with the BOG discharged from the storage tank T and is then supplied to the heat exchanger <NUM>, the temperature of the BOG finally supplied to the heat exchanger <NUM> is lower than that of the BOG discharged from the storage tank T to the heat exchanger <NUM>, and the temperature of the BOG supplied to the heat exchanger <NUM> can be further lowered with increasing amount of the gaseous BOG separated by the gas/liquid separator <NUM>.

The BOG supplied to the heat exchanger <NUM> along the third supply line L3 and having a temperature of about <NUM> to <NUM> is cooled to about -<NUM> to about - <NUM> by the heat exchanger <NUM>, and the temperature difference of the cold flow is preferably about <NUM> to about <NUM> in a normal state.

In addition, if the temperature difference (meaning an absolute value, hereinafter referred to as "temperature difference of the hot flow") between the temperature of the BOG discharged from the storage tank T and used as a refrigerant by the heat exchanger <NUM>, as measured by the second temperature sensor <NUM>, and the temperature of the BOG compressed by the compressor <NUM> and supplied to the heat exchanger <NUM>, as measured by the third temperature sensor <NUM>, is higher than a normal temperature difference and continues for a certain period of time or more, it can be determined that heat exchange is normally performed in the heat exchanger <NUM>.

When the state wherein the temperature difference of the hot flow is <NUM> to <NUM> or higher, preferably <NUM> to <NUM> or higher, more preferably about <NUM> or higher, continues for <NUM> hour or more, it can be determined that it is time to discharge the condensed or solidified lubricant oil.

When the heat exchanger <NUM> is normally operated, the BOG discharged from the storage tank T and having a slightly increased temperature of about -<NUM> to about -<NUM> (preferably about -<NUM>) during delivery to the heat exchanger <NUM> may have a temperature of about -<NUM> to <NUM> depending upon the speed of the vessel after being used as the refrigerant in the heat exchanger <NUM>, and the BOG used as the refrigerant in the heat exchanger <NUM> and having a temperature of about - <NUM> to <NUM> is compressed by the compressor <NUM> to have a temperature of about <NUM> to about <NUM>.

Furthermore, if the pressure difference (hereinafter referred to as "pressure difference of the hot fluid channel") between the pressure of the BOG compressed by the compressor <NUM> and supplied to the heat exchanger <NUM>, as measured by the first pressure sensor <NUM>, and the pressureof the BOG cooled by the heat exchanger <NUM>, as measured by the second pressure sensor <NUM>, is higher than a normal pressure difference and continues for a certain period of time or more, it can be determined that the heat exchanger <NUM> is abnormally operated.

Since the BOG discharged from the storage tank T is not mixed with oil or has a trace amount of oil and a time point at which the lubricant oil is mixed with the BOG is when the BOG is compressed by the compressor <NUM>, the condensed or solidified lubricant oil is not substantially accumulated in the cold fluid channel of the heat exchanger <NUM>, which uses the BOG discharged from storage tank T as the refrigerant and then supplies the BOG to the compressor <NUM>, and is accumulated in the hot fluid channel of the heat exchanger <NUM>, in which the BOG compressed by the compressor <NUM> is cooled and supplied to the pressure reducer <NUM>.

Accordingly, since the pressure difference between the front end and the rear end of the heat exchanger <NUM> due to blocking of the fluid channel by the condensed or solidified lubricant oil rapidly increases in the hot fluid channel, it is determined whether it is time to remove the condensed or solidified lubricant oil by measuring the pressure of the hot fluid channel of the heat exchanger <NUM>.

Considering that the PCHE having a narrow and serpentine fluid channel can be used as the heat exchanger according to this embodiment, determination as to whether it is time to remove the condensed or solidified lubricant oil based on the pressure difference between the front end and the rear end of the heat exchanger <NUM> can be advantageously used.

By way of example, when the pressure difference of the hot fluid channel is two or more times a normal pressure difference thereof and continues for <NUM> hour or more, it can be determined that it is time to discharge the condensed or solidified lubricant oil.

When the heat exchanger <NUM> is normally operated, the BOG compressed by the compressor <NUM> undergoes a pressure drop of about <NUM> bar to about <NUM> bar, preferably about <NUM> bar to about <NUM> bar, more preferably about <NUM> bar, without suffering a significant pressure drop even when the BOG is cooled while passing through the heat exchanger <NUM>. When the state wherein the pressure difference of the hot fluid channel is at least a predetermined pressure or more, for example, <NUM> bar to <NUM> bar or more, preferably <NUM> bar to <NUM> bar or more, more preferably about <NUM> bar (<NUM> kPa) or more, it can be determined that it is time to discharge the condensed or solidified lubricant oil.

Although the time point for removal of the condensed or solidified lubricant oil can be determined based on any one of the temperature difference of the cold flow, the temperature difference of the hot flow, and the pressure difference of the hot fluid channel as described above, the time point for removal of the condensed or solidified lubricant oil can be determined based on at least two among the temperature difference of the cold flow, the temperature difference of the hot flow, and the pressure difference of the hot fluid channel in order to improve reliability.

By way of example, when a lower value between the temperature difference of the cold flow and the temperature difference of the hot flow is maintained at <NUM> or more for <NUM> hour or more of when the pressure difference of the hot fluid channel is two or more times the normal pressure difference thereof or <NUM> kPa or more and continues for <NUM> hour or more, it can be determined that it is time to remove the condensed or solidified lubricant oil.

The first temperature sensor <NUM>, the second temperature sensor <NUM>, the third temperature sensor <NUM>, the fourth temperature sensor <NUM>, the first pressure sensor <NUM>, and the second pressure sensor <NUM> can be considered as a detection means for detecting whether the heat exchanger <NUM> is clogged by the lubricant oil.

In addition, the BOG reliquefaction system according to embodiments of the present invention may further include a controller (not shown) to determine whether the heat exchanger <NUM> is clogged by the lubricant oil based on a detection result obtained by at least one of the first temperature sensor <NUM>, the second temperature sensor <NUM>, the third temperature sensor <NUM>, the fourth temperature sensor <NUM>, the first pressure sensor <NUM>, and the second pressure sensor <NUM>. The controller can be considered as a determination means for determining whether the heat exchanger <NUM> is clogged by the lubricant oil.

If it is determined in Step <NUM> that it is time to remove the condensed or solidified lubricant oil from the heat exchanger <NUM>, the bypass valve <NUM> disposed on the bypass line BL is opened, and the first valve <NUM> disposed on the first supply line L1 and the second valve <NUM> disposed on the second supply line L2 are closed.

When the bypass valve <NUM> is opened while closing the first valve <NUM> and the second valve <NUM>, the BOG discharged from the storage tank T is sent to the compressor <NUM> through the bypass line BL and is prevented from being supplied to the heat exchanger <NUM>. Therefore, a refrigerant is not supplied to the heat exchanger <NUM>.

The BOG discharged from the storage tank T bypasses the heat exchanger <NUM> through the bypass line BL and is then sent to the compressor <NUM>. The BOG sent to the compressor <NUM> undergoes increase in temperature and pressure while being compressed by the compressor <NUM>. The BOG compressed to about <NUM> bar by the compressor <NUM> has a temperature of about <NUM> to about <NUM>.

When the BOG compressed by the compressor <NUM> is continuously supplied to the heat exchanger <NUM>, the cold BOG used as a refrigerant in the heat exchanger <NUM> and discharged from the storage tank T is not supplied to the heat exchanger <NUM> and the hot BOG is continuously supplied to the heat exchanger <NUM>, thereby gradually increasing the temperature of the hot fluid channel of the heat exchanger <NUM>, through which the BOG compressed by the compressor <NUM> passes.

When the temperature of the hot fluid channel of the heat exchanger <NUM> exceeds a condensation or solidification point of the lubricant oil, the condensed or solidified lubricant oil accumulated in the heat exchanger <NUM> gradually melts or decreases in viscosity, and then the lubricant oil melt or having low viscosity is mixed with the BOG and exits the heat exchanger <NUM>.

When the condensed or solidified lubricant oil is removed using the bypass line BL, the BOG is circulated through the bypass line BL, the compressor <NUM>, the hot fluid channel of the heat exchanger <NUM>, the pressure reducer <NUM>, and the gas/liquid separator <NUM> until the heat exchanger <NUM> is normalized.

In addition, when the condensated or solidified lubricant oil is removed using the bypass line BL, the BOG discharged from the storage tank T and passed through the bypass line BL, the compressor <NUM>, the hot fluid channel of the heat exchanger <NUM>, and the pressure reducer <NUM> may be sent to a separate tank or another collection facility separate from the storage tank T, with the BOG mixed with the molten or viscosity-reduced lubricant oil. The BOG stored in the separate tank or another collection facility is sent to the bypass line BL to continue the process of removing the condensed or solidified lubricant oil.

Even in the structure wherein the gas/liquid separator <NUM> is disposed downstream of the pressure reducer <NUM>, when the fluid composed of the BOG mixed with the molten or viscosity-reduced lubricant oil is sent to the separate tank or other collection facility, the gas/liquid separator <NUM> provides the same function as that of a typical BOG reliquefaction system and the molten or viscosity-reduced lubricant oil is not collected in the gas/liquid separator <NUM> (the molten or viscosity-reduced lubricant oil is collected by the separate tank or other collection facility separate from the storage tank T). Thus, the BOG reliquefaction system according to this embodiment can omit a gas/liquid separator configured to discharge the lubricant oil, thereby enabling cost reduction.

As the temperature of the hot fluid channel of the heat exchanger <NUM> increases, the condensed or solidified lubricant oil accumulated in the heat exchanger <NUM> gradually melts or decreases in viscosity and is then sent to the gas/liquid separator <NUM> after being mixed with the BOG. In the process of removing the condensed or solidified lubricant oil in the heat exchanger <NUM> through the bypass line BL, since the BOG is not re-liquefied, the re-liquefied gas is not collected in the gas/liquid separator <NUM>, and the BOG and the melted or low viscosity lubricant oil are collected.

The gaseous BOG collected in the gas/liquid separator <NUM> is discharged from the gas/liquid separator <NUM> along the sixth supply line L6 and sent to the compressor <NUM> along the bypass line BL. Since the first valve <NUM> is closed in Step <NUM>, the gaseous BOG separated by the gas/liquid separator <NUM> is combined with the BOG discharged from the storage tank T and sent to the compressor <NUM> along the bypass line BL without being sent to the cold fluid channel of the heat exchanger <NUM>.

Supplying the gaseous BOG separated by the gas/liquid separator <NUM> to the bypass line BL with the first valve <NUM> in the closed state can prevent the lubricant oil contained in the BOG from being supplied to the heat exchanger <NUM>, thereby preventing the cold fluid channel of the heat exchanger <NUM> from being blocked.

The circulation process in which the gaseous BOG collected in the gas/liquid separator <NUM> is discharged from the gas/liquid separator <NUM> along the sixth feed line L6 and then sent back to the compressor <NUM> along the bypass line BL continues until it is determined that the temperature of the hot fluid channel of the heat exchanger <NUM> is increased to the temperature of the BOG compressed by the compressor <NUM> and sent to the hot fluid channel of the heat exchanger <NUM>. However, the circulation process may be continued until it is empirically determined that a sufficient time has passed.

During removal of the condensed or solidified lubricant oil from the heat exchanger <NUM> using the bypass line BL, the eighth valve <NUM> is closed to prevent the lubricant oil collected in the gas/liquid separator <NUM> from flowing to storage tank T along the fifth supply line L5. If the lubricant oil is introduced into the storage tank T, the liquefied gas stored in the storage tank T can be deteriorated in purity, thereby deteriorating the value of the liquefied gas.

The molten or viscosity-reduced lubricant oil discharged from the heat exchanger <NUM> is collected in the gas/liquid separator <NUM>. For treatment of the lubricant oil collected in the gas/liquid separator <NUM>, the BOG reliquefaction system according to this embodiment may employ the gas/liquid separator <NUM> obtained by improving a typical gas/liquid separator.

<FIG> is an enlarged view of a heat exchanger and a gas/liquid separator according to one embodiment of the present invention. In <FIG>, some components are omitted for convenience of description.

Referring to <FIG>, the gas/liquid separator <NUM> is provided with a lubricant oil discharge line OL through which the lubricant oil collected in the gas/liquid separator <NUM> is discharged, as well as the fifth supply line L5 through which the liquefied gas separated by the gas/liquid separator <NUM> is sent to the storage tank T. In order to allow the lubricant oil collected at a lower portion of the gas/liquid separator <NUM> to be efficiently discharged, the lubricant oil discharge line OL is connected to a lower end of the gas/liquid separator <NUM> and one end of the fifth supply line L5 is disposed above the lower end of the gas/liquid separator <NUM> in the gas/liquid separator <NUM> connected to the lubricant oil discharge line OL. In order to prevent the fifth supply line L5 from being clogged by the lubricant oil, it is desirable that the end of the fifth supply line L5 be disposed above the level of the lubricant oil when the amount of the lubricant oil collected in the gas/liquid separator <NUM> reaches the maximum value.

A third valve <NUM> for regulating the flow rate of fluid and opening/closing of the corresponding line may be disposed on the lubricant oil discharge line OL and may be provided in plural.

Since the lubricant oil collected in the gas/liquid separator <NUM> can be naturally discharged or can require a long time for discharge, the lubricant oil in the gas/liquid separator <NUM> may be discharged through nitrogen purging. When nitrogen is supplied at a pressure of about <NUM> bar to <NUM> bar to the gas/liquid separator <NUM>, the internal pressure of the gas/liquid separator <NUM> increases and allows rapid discharge of the lubricant oil.

In order to discharge the lubricant oil from the gas/liquid separator <NUM> through nitrogen purging, a nitrogen supply line NL may be disposed so as to be joined to the third supply line L3 upstream of the heat exchanger <NUM>. A number of nitrogen supply lines may be disposed at different locations as needed.

A nitrogen valve <NUM> for regulating the flow rate of fluid and opening/closing of the corresponding line may be disposed on the nitrogen supply line NL and is normally kept in a closed state when the nitrogen supply line NL is not used. Then, when there is a need for use of the nitrogen supply line NL to supply nitrogen to the gas/liquid separator <NUM> for nitrogen purging, the nitrogen valve <NUM> is opened. The nitrogen valve <NUM> may be provided in plural.

Although discharge of the lubricant oil can be performed through nitrogen purging by directly injecting nitrogen into the gas/liquid separator <NUM>, if the nitrogen supply line for other purposes is already installed, it is desirable that the lubricant oil be discharged from the gas/liquid separator <NUM> using another installed nitrogen supply line which may be previously disposed for other purposes.

After the processes of sending the entirety of the BOG discharged from the storage tank T to the bypass line BL to be compressed by the compressor <NUM>, sending the BOG compressed by the compressor <NUM> to the hot fluid channel of the heat exchanger <NUM>, sending the BOG passed through the heat exchanger <NUM> and reduced in pressure in the pressure reducer <NUM> to the gas/liquid separator <NUM>, and sending the BOG discharged from the gas/liquid separator <NUM> to the bypass line BL, if it is determined that most of the condensed or solidified lubricant oil in the heat exchanger <NUM> is collected in the gas/liquid separator <NUM> (that is, if it is determined that the heat exchanger <NUM> is normalized), nitrogen purging is performed by blocking of the BOG compressed by the compressor <NUM> from flowing into the heat exchanger <NUM> and opening the nitrogen valve <NUM>.

If it is determined that the heat exchanger <NUM> is normalized again through discharge of the condensed or solidified lubricant oil from the heat exchanger <NUM> and when the process of discharging the lubricant oil from the gas/liquid separator <NUM> is completed, the BOG reliquefaction system is normally operated again by opening the first valve <NUM> and the second valve <NUM> while closing the bypass valve <NUM>. When the BOG reliquefaction system is normally operated, the BOG discharged from the storage tank T is used as a refrigerant in the heat exchanger <NUM> and part or all of the BOG used as the refrigerant in the heat exchanger <NUM> is re-liquefied through compression by the compressor <NUM>, cooling by the heat exchanger <NUM>, and pressure reduction by the pressure reducer <NUM>.

As in determination as to whether it is time to remove the condensed or solidified lubricant oil, determination as to whether the heat exchanger <NUM> is normalized again is based on at least one of the temperature difference of the cold flow, the temperature difference of the hot flow, and the pressure difference of the hot fluid channel.

In addition to the condensed or solidified lubricant oil inside the heat exchanger <NUM>, the condensed or solidified lubricant oils accumulated in pipes, valves, instruments, and other equipment can also be removed through the aforementioned processes.

Conventionally, during the step of removing the condensed or solidified lubricant oil inside the heat exchanger <NUM> using the bypass line BL, the high-pressure engine and/or the low-pressure engine (hereinafter referred to as 'engine') may be driven. Upon overhaul of part of equipment included in the fuel supply system or the reliquefaction system, since fuel cannot be supplied to the engine or surplus BOG cannot be re-liquefied, the engine is generally in a non-driven state.

Conversely, if the engine can be driven during removal of the condensed or solidified lubricant oil from the heat exchanger <NUM> as in the present invention, since it is possible to overhaul the heat exchanger <NUM> during operation of the engine, there are advantages in that it is possible to propel the vessel and generate power and to remove the condensed or solidified lubricant oil using surplus BOG during overhaul of the heat exchanger <NUM>.

Furthermore, when the engine is driven during removal of the condensed or solidified lubricant oil from the heat exchanger <NUM>, there is an advantage in that it is possible to burn the lubricant oil mixed with the BOG during compression by the compressor <NUM>. That is, the engine is used not only for the purpose of propelling the vessel or power generation, but also for removing the oil mixed with the BOG.

On the other hand, the process of determining based on an alarm whether it is time to remove the condensed or solidified lubricant oil may include ① alarm activation, and/or ② alarm generation.

<FIG> is a schematic diagram of a BOG reliquefaction system according to a fourth embodiment of the present invention, <FIG> is an enlarged view of a pressure reducer according to one embodiment of the present invention, and <FIG> is an enlarged view of a pressure reducer according to another embodiment of the present invention.

Referring to <FIG>, two compressors <NUM>, <NUM> may be arranged in parallel in the present invention. The two compressors <NUM>, <NUM> may have the same specifications and can act as redundancy for preparation against malfunction of any one of the compressors. Illustration of other devices is omitted for convenience of description.

Referring to <FIG>, in the structure wherein the compressors <NUM>, <NUM> are arranged in parallel, the BOG discharged from the storage tank T is sent to the second compressor <NUM> through the seventh supply line L22 and the BOG compressed by the second compressor <NUM> is partially discharged to the high-pressure engine through the fuel supply line SL while surplus BOG is sent to the heat exchanger <NUM> through the eighth supply line L33 to undergo the reliquefaction process. A tenth valve <NUM> for regulating the flow rate and opening/closing of the corresponding line may be disposed on the eighth supply line L33.

In other embodiments, two pressure reducers <NUM>, <NUM> may be arranged in parallel as shown in <FIG> and two pairs of pressure reducers <NUM>, <NUM> arranged in series may be arranged in parallel as shown in <FIG>.

Referring to <FIG>, both pressure reducers <NUM>, <NUM> arranged in parallel can act as redundancy for preparation against malfunction of any one of the compressors, and each of the pressure reducers <NUM>, <NUM> may be provided at front rear ends thereof with isolation valves <NUM>.

Referring to <FIG>, two pairs of pressure reducers <NUM>, <NUM> connected in series are arranged in parallel. Depending upon manufacturer, two pressure reducers <NUM> are connected in series for pressure reduction stability. The two pairs of pressure reducers <NUM>, <NUM> connected in parallel can act as redundancy for preparation against malfunction of any pair of pressure reducers.

Each of the pressure reducers <NUM>, <NUM> connected in parallel may be provided at front rear ends thereof with isolation valves <NUM>. The isolation valves <NUM> shown in <FIG> isolate the pressure reducers <NUM> upon maintenance or overhaul of the pressure reducers <NUM> due to malfunction of the pressure reducers <NUM>, <NUM> and the like.

In the structure wherein the BOG reliquefaction system includes one compressor <NUM> and one pressure reducer <NUM> as shown in <FIG>, an alarm is activated under conditions that the degree of opening of the pressure reducer <NUM> is a preset value or more, the seventh valve <NUM> and the second valve <NUM> are opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the structure wherein the BOG reliquefaction system includes one compressor <NUM> as shown in <FIG> and two pressure reducers <NUM>, <NUM> connected in parallel as shown in <FIG>, an alarm is activated under conditions (hereinafter referred to as 'first alarm activation condition') that the degree of opening of a first pressure reducer <NUM> or a second pressure reducer <NUM> is a preset value or more, the seventh valve <NUM> and the second valve <NUM> are opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the structure wherein the BOG reliquefaction system includes one compressor <NUM> as shown in <FIG> and two pairs of pressure reducers <NUM>, <NUM> connected in parallel as shown in <FIG>, an alarm is activated under conditions (hereinafter referred to as 'second alarm activation condition') that the degree of opening of one of two first pressure reducers <NUM> arranged in series or one of two second pressure reducers <NUM> connected in series is a preset value or more, the seventh valve <NUM> and the second valve <NUM> are opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the structure wherein the BOG reliquefaction system includes two compressors <NUM>, <NUM> connected in parallel as shown in <FIG> and one pressure reducer <NUM> as shown in <FIG>, an alarm is activated under conditions (hereinafter referred to as 'third alarm activation condition') that the degree of opening of the pressure reducer <NUM> is a preset value or more, the seventh valve <NUM> or the tenth valve <NUM> is opened, the second valve <NUM> is opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the structure wherein the BOG reliquefaction system includes two compressors <NUM>, <NUM> connected in parallel as shown in <FIG> and two pressure reducers <NUM>, <NUM> connected in parallel as shown in <FIG>, an alarm is activated under conditions (hereinafter referred to as 'fourth alarm activation condition') that the degree of opening of the first pressure reducer <NUM> or the second pressure reducer <NUM> is a preset value or more, the seventh valve <NUM> or the tenth valve <NUM> is opened, the second valve <NUM> is opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the structure wherein the BOG reliquefaction system includes two compressors <NUM>, <NUM> connected in parallel as shown in <FIG> and two pairs of pressure reducers <NUM>, <NUM> connected in parallel as shown in <FIG>, an alarm is activated under conditions (hereinafter referred to as 'fifth alarm activation condition') that the degree of opening of one of two first pressure reducer <NUM> arranged in series or one of two second pressure reducer <NUM> connected in series is a preset value or more, the seventh valve <NUM> or the tenth valve <NUM> is opened, the second valve <NUM> is opened, and the level of liquefied gas in the gas/liquid separator <NUM> is a normal level.

In the first to fifth alarm activation conditions described above, the predetermined degree of opening of the first pressure reducer <NUM> or the second pressure reducer <NUM> may be <NUM>%, and the normal level of the liquefied gas in the gas/liquid separator <NUM> means the case where it can be determined that the reliquefaction process is normally carried out by confirming the re-liquefied gas in the gas/liquid separator <NUM>.

An alarm may be generated to indicate a time point for removal of the condensed or solidified lubricant oil, if any one of the following conditions is satisfied: the condition that the temperature difference of the cold flow is a preset value or more and continues for a predetermined period of time, the condition that the temperature difference of the hot flow is a preset value or more and continues for a predetermined period of time, and the condition that the pressure difference of the hot fluid channel is a preset value or more and continues for a predetermined period of time.

In order to improve reliability, an alarm may be generated to indicate a time point for removal of the condensed or solidified lubricant oil, if at least two of the following conditions are satisfied: the condition that the temperature difference of the cold flow is a preset value or more and continues for a predetermined period of time, the condition that the temperature difference of the hot flow is a preset value or more and continues for a predetermined period of time, and the condition that the pressure difference of the hot fluid channel is a preset value or more and continues for a predetermined period of time.

Furthermore, an alarm may be generated to indicate a time point for removal of the condensed or solidified lubricant oil, if a lower value of the temperature difference of the cold flow and the temperature difference of the hot flow is a preset value or more and continues for a predetermined period of time (or condition), or if the pressure difference of the hot fluid channel is a preset value or more and continues for a predetermined period of time.

According to the present invention, abnormality of the heat exchanger, alarm generation, and the like may be determined by a suitable controller. As a controller for determining abnormality of the heat exchanger, alarm generation, and the like, a controller used by the BOG reliquefaction system according to the present invention, preferably a controller used by a vessel or an offshore structure to which the BOG reliquefaction system according to the present invention is applied, may be used, and a separate controller for determining abnormality of the heat exchanger, occurrence of an alarm, and the like may also be used.

In addition, use of the bypass line, discharge of lubricant oil, fuel supply to the engine, start or restart of the BOG reliquefaction system, and opening or closing of various valves for these components may be automatically or manually controlled by the controller.

The case where the bypass line BL is used to satisfy an intake pressure condition of the compressor <NUM> when the internal pressure of the storage tank T is low.

The compressor <NUM> often does not satisfy the intake pressure condition upstream of the compressor <NUM> in the case where the storage tank T has a low internal pressure, such as when the amount of generated BOG is small due to a small amount of liquefied gas in the storage tank T or if the amount of BOG supplied to the engine for propulsion of the vessel is large due to high speed of the vessel.

Particularly, in a PCHE (DCHE) used as the heat exchanger <NUM>, the pressure drop is large due to a narrow fluid channel thereof when the BOG discharged from the storage tank T passes through the PCHE.

Conventionally, when the compressor <NUM> fails to satisfy the intake pressure condition, the recirculation valves <NUM>, <NUM>, <NUM>, <NUM> are opened to protect the compressor <NUM> by recycling part or all of the BOG through the recirculation lines Rc1, Rc2, Rc3, Rc4.

However, if the intake pressure condition of the compressor <NUM> is satisfied by recirculating the BOG, the amount of BOG compressed by the compressor <NUM> is decreased, thereby causing deterioration in reliquefaction performance and failing to satisfy fuel consumption requirement for an engine. Particularly, if the engine does not satisfy the fuel consumption requirements, operation of the vessel can be significantly disturbed. Therefore, there is a need for a BOG reliquefaction method capable of satisfying the intake pressure condition for the compressor and fuel consumption requirement for the engine even when the internal pressure of the storage tank T is low.

According to the present invention, instead of providing additional equipment, the bypass line BL provided for maintenance and overhaul of the heat exchanger <NUM> may be used to satisfy the intake pressure condition for the compressor <NUM> without decreasing the amount of the BOG compressed by the compressor <NUM> even when the internal pressure of the storage tank T is low. It is possible to satisfy the suction pressure condition required by the compressor <NUM> without reducing the amount of the BOG.

According to the present invention, when the internal pressure of the storage tank T is decreased to a preset value or less, the bypass valve <NUM> is opened to allow part or all of the BOG discharged from the storage tank T to be directly sent to the compressor <NUM> through the bypass line BL bypassing the heat exchanger <NUM>.

The amount of BOG sent to the bypass line BL can be adjusted depending upon the pressure of the storage tank T compared with the intake pressure condition required by the compressor <NUM>. That is, all of the BOG discharged from the storage tank T may be sent to the bypass line BL by opening the bypass valve <NUM> while closing the first valve <NUM> and the second valve <NUM>, or only some of the BOG discharged from the storage tank T may be sent to the bypass line BL and the remaining BOG may be sent to the heat exchanger <NUM> by partially opening the bypass valve <NUM>, the first valve <NUM>, and the second valve <NUM>. That is, all of the BOG discharged from the storage tank T may be sent to the bypass line BL by opening the bypass valve <NUM> while closing the first valve <NUM> and the second valve <NUM>, or only some of the BOG discharged from the storage tank T may be sent to the bypass line BL and the remaining BOG may be sent to the heat exchanger <NUM> by partially opening the bypass valve <NUM>, the first valve <NUM>, and the second valve <NUM>. Pressure drop of the BOG decreases with increasing amount of the BOG bypassing the heat exchanger <NUM> through the bypass line BL.

Although there is an advantage of minimizing the pressure drop when the BOG discharged from the storage tank T bypasses the heat exchanger <NUM> and is directly sent to the compressor <NUM>, cold heat of the BOG cannot be used for reliquefaction of the BOG. Thus, use of the bypass line BL to reduce the pressure drop and the amount of the BOG to be sent to the bypass line BL among the amount of the BOG discharged from the storage tank T are determined based on the internal pressure of the storage tank T, fuel consumption requirement for the engine, the amount of the BOG to be re-liquefied, and the like.

By way of example, it can be determined that it is advantageous to reduce the pressure drop using the bypass line BL when the internal pressure of the storage tank T is a preset value or less and the vessel is operated at a predetermined speed or more. Specifically, it can be determined that it is advantageous to reduce the pressure drop using the bypass line BL when the internal pressure of the storage tank T is <NUM> bar or less and the speed of the vessel is <NUM> knots or more.

In addition, the intake pressure condition of the compressor <NUM> is not often satisfied even when all of the BOG discharged from the storage tank T is sent to the compressor <NUM> through the bypass line BL. In this case, the intake pressure condition is satisfied using the recirculation lines Rc1, Rc2, Rc3, Rc4.

That is, when the intake pressure condition of the compressor <NUM> cannot be satisfied due to reduction in pressure of the storage tank T, the compressor <NUM> is protected using the recirculation lines Rc1, Rc2, Rc3, Rc4 in the related art, whereas, according to the present invention, the bypass line BL is primarily used in order to satisfy the intake pressure condition of the compressor <NUM>, and the recirculation lines Rc1, Rc2, Rc3, Rc4 are secondarily used when the intake pressure condition of the compressor <NUM> cannot be satisfied even by sending all of the BOG discharged from the storage tank T to the compressor through the bypass line BL.

In order to satisfy the intake pressure condition of the compressor <NUM> through primary use of the bypass line BL and secondary use of the recirculation lines Rc1, Rc2, Rc3, Rc4, a pressure condition under which the bypass valve <NUM> is opened is set to a higher value than a pressure condition under which the recirculation valves <NUM>, <NUM>, <NUM>, <NUM> are opened.

The condition under which the recirculation valves <NUM>, <NUM>, <NUM>, <NUM> are opened and the condition under which the bypass valve <NUM> is open are preferably determined based on pressure upstream of the compressor <NUM>. Alternatively, these conditions may be determined based on the internal pressure of the storage tank T.

The pressure upstream of the compressor <NUM> may be measured by a third pressure sensor (not shown) disposed upstream of the compressor <NUM> and the internal pressure of the storage tank T may be measured by a fourth pressure sensor (not shown).

On the other hand, in the structure wherein the sixth supply line L6 for discharging the gaseous BOG separated by the gas/liquid separator <NUM> is joined to the first supply line L1 at a location downstream of a branch point of the bypass line BL branched from the first supply line L1, some of the BOG discharged from the storage tank T while preventing the pressure drop may be used as a refrigerant in the heat exchanger <NUM> by directly sending the gaseous BOG separated by the gas/liquid separator <NUM> to the bypass line BL, with all of the bypass valve <NUM>, the first valve <NUM>, and the second valve <NUM> open in operation of the system.

Since the temperature of the gaseous BOG separated by the gas/liquid separator <NUM> is lower than the temperature of the BOG discharged from the storage tank T and supplied to the heat exchanger <NUM>, and cooling efficiency of the heat exchanger <NUM> can be deteriorated when the gaseous BOG separated by the gas/liquid separator <NUM> is directly sent to the bypass line BL, it is desirable that at least some of the gaseous BOG separated by the gas/liquid separator <NUM> be sent to the heat exchanger <NUM>.

Here, if the amount of the BOG generated in the storage tank T is less than the amount of the BOG required by the engine as fuel, it may not be necessary to re-liquefy the BOG. However, when there is no need for reliquefaction of the BOG, all of the gaseous BOGs separated by the gas/liquid separator <NUM> may be sent to the bypass line BL, since it is not necessary to supply the refrigerant to the heat exchanger <NUM>.

Accordingly, in the present invention, the sixth supply line L6 is joined to the first supply line L1 at a location upstream of the branch point of the bypass line BL branched from the first supply line L1. In the structure wherein the sixth supply line L6 is joined to the first supply line L1 upstream of the branch point of the bypass line, the BOG discharged from the storage tank T and the gaseous BOG separated by the gas/liquid separator <NUM> are combined with each other at a location upstream of the branch point of the bypass line BL, and then the amount of the BOG to be sent to the bypass line BL and the heat exchanger <NUM> are determined depending upon the degrees of opening of the bypass valve <NUM> and the first valve <NUM>, thereby enabling easy control of the system and preventing the gaseous BOG separated by the gas/liquid separator <NUM> from being directly sent to the bypass line BL.

Preferably, the bypass valve <NUM> is a valve providing a higher response than a typical valve in order to allow rapid regulation of the degree of opening depending upon the pressure change of the storage tank T.

Referring to <FIG>, the BOG reliquefaction system is different from the BOG reliquefaction system according to the first embodiment shown in <FIG> in that the BOG reliquefaction system according to the third embodiment includes a pressure difference sensor <NUM> instead of the first pressure sensor <NUM> and the second pressure sensor <NUM>, and the following description will focus on the different features of the BOG reliquefaction system according to the third embodiment. Descriptions of the same components as the BOG reliquefaction system according to the first embodiment will be omitted.

Unlike the first embodiment, the BOG reliquefaction system according to the third embodiment includes the pressure difference sensor <NUM> that measures a pressure difference between the third supply line L3 upstream of the heat exchanger <NUM> and the fourth supply line L4 downstream of the heat exchanger <NUM> instead of the first pressure sensor <NUM> and the second pressure sensor <NUM>.

The pressure difference of the hot fluid channel can be obtained by the pressure difference sensor <NUM>, and, as in the first embodiment, it can be determined that it is time to remove the condensed or solidified lubricant oil, based on at least one of the pressure difference of the hot fluid channel, the temperature difference of the cold flow and the temperature difference of the hot flow.

Claim 1:
A BOG reliquefaction system comprising:
a compressor (<NUM>) compressing BOG, the compressor comprising at least one oil-lubrication type cylinder (<NUM>);
a heat exchanger (<NUM>) cooling the BOG compressed by the compressor through heat exchange using BOG not compressed by the compressor as a refrigerant;
a pressure reducer (<NUM>) disposed downstream of the heat exchanger and reducing a pressure of fluid cooled by the heat exchanger;
a gas/liquid separator (<NUM>) disposed downstream of the pressure reducer and separating the BOG into liquefied gas generated through reliquefaction and gaseous BOG; and
characterized by a second oil filter (<NUM>) disposed on a fifth supply line (L5) through which the liquefied gas separated by the gas/liquid separator is discharged,
wherein the second oil filter is a cryogenic oil filter.