阅读说明：本技术 蒸发气体再液化系统 (Boil-off gas reliquefaction system ) 是由 张在亨 柳承恪 柳珍烈 崔员宰 于 2017-08-03 设计创作，主要内容包括：公开一种蒸发气体再液化系统。所述蒸发气体再液化系统包括：压缩机,用于压缩蒸发气体；热交换器,用于使用被所述压缩机压缩前的所述蒸发气体作为制冷剂来使被所述压缩机压缩的所述蒸发气体经历热交换,且因此冷却所述蒸发气体；减压装置,安装在所述热交换器的后端处,且对由所述热交换器冷却的所述流体进行减压；以及第二滤油器,安装在所述减压装置的所述后端处,其中所述压缩机包括至少一个油润滑型气缸,且所述第二滤油器用于极低温。(A boil-off gas reliquefaction system is disclosed. The boil-off gas reliquefaction system includes: a compressor for compressing the boil-off gas; a heat exchanger for subjecting the evaporation gas compressed by the compressor to heat exchange using the evaporation gas before being compressed by the compressor as a refrigerant, and thus cooling the evaporation gas; a decompression device installed at a rear end of the heat exchanger and decompressing the fluid cooled by the heat exchanger; and a second oil filter installed at the rear end of the decompression device, wherein the compressor includes at least one oil-lubricated cylinder, and the second oil filter is used for very low temperature.)

1. A boil-off gas (BOG) reliquefaction system comprising:

a compressor for compressing the boil-off gas;

a heat exchanger that cools the evaporation gas compressed by the compressor by heat exchange using the evaporation gas that is not compressed by the compressor as a refrigerant;

a pressure reducer that is provided downstream of the heat exchanger and reduces a pressure of the fluid cooled by the heat exchanger; and

a second oil filter disposed downstream of the pressure reducer,

wherein the compressor comprises at least one oil lubricated cylinder and the second oil filter is a low temperature oil filter.

2. A boil-off gas reliquefaction system comprising:

a compressor for compressing the boil-off gas;

a heat exchanger that cools the evaporation gas compressed by the compressor by heat exchange using the evaporation gas that is not compressed by the compressor as a refrigerant;

a pressure reducer that is provided downstream of the heat exchanger and reduces a pressure of the fluid cooled by the heat exchanger;

a gas/liquid separator that is provided downstream of the pressure reducer and separates the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas; and

a second oil filter provided on a fifth supply line through which the liquefied gas separated by the gas/liquid separator is discharged,

wherein the compressor comprises at least one oil lubricated cylinder and the second oil filter is a low temperature oil filter.

3. A boil-off gas reliquefaction system comprising:

a compressor for compressing the boil-off gas;

a heat exchanger that cools the evaporation gas compressed by the compressor by heat exchange using the evaporation gas that is not compressed by the compressor as a refrigerant;

a pressure reducer that is provided downstream of the heat exchanger and reduces a pressure of the fluid cooled by the heat exchanger;

a gas/liquid separator that is provided downstream of the pressure reducer and separates the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas; and

a second oil filter provided on a sixth supply line through which the gaseous boil-off gas separated by the gas/liquid separator is discharged,

wherein the compressor comprises at least one oil lubricated cylinder and the second oil filter is a low temperature oil filter.

4. The boil-off gas reliquefaction system according to any one of claims 1 to 3, wherein the second oil filter separates a lube oil having a solid phase.

5. The boil-off gas reliquefaction system of claim 1 further comprising:

a gas/liquid separator provided downstream of the pressure reducer and separating the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas,

wherein the second oil filter is provided between the pressure reducer and the gas/liquid separator.

6. The boil-off gas reliquefaction system according to any one of claims 1, 3 and 5 wherein the second oil filter is of the upflow type.

7. The boil-off gas reliquefaction system according to claim 2, wherein the liquefied gas separated by the gas/liquid separator and discharged along the fifth supply line is sent to a storage tank.

8. The boil-off gas reliquefaction system according to claim 2 or 7, wherein the second oil filter is of a downward discharge type.

9. The boil-off gas reliquefaction system according to any one of claims 1 to 3, wherein the compressor compresses the boil-off gas to a pressure of 150 to 350 bar.

10. The boil-off gas reliquefaction system according to any one of claims 1 to 3, wherein the compressor compresses the boil-off gas to a pressure of 80 to 250 bar.

11. The boil-off gas reliquefaction system according to any one of claims 1 to 3, wherein the heat exchanger includes microchannel-type fluid channels.

12. The boil-off gas reliquefaction system of claim 11 wherein the heat exchanger is a Printed Circuit Heat Exchanger (PCHE).

13. The boil-off gas reliquefaction system according to any one of claims 1 to 3, further comprising:

a bypass line through which the evaporation gas is supplied to the compressor after bypassing the heat exchanger.

14. The boil-off gas reliquefaction system of claim 13 further comprising:

a first valve provided upstream of the cold fluid channel of the heat exchanger and adjusting a fluid flow rate of a corresponding supply line and opening/closing the corresponding supply line,

wherein the bypass line branches from the corresponding supply line upstream of the first valve.

15. The boil-off gas reliquefaction system of claim 13 further comprising:

a second valve disposed downstream of the cold fluid passage of the heat exchanger and adjusting a fluid flow rate of a corresponding supply line and opening/closing the corresponding supply line,

wherein the bypass line joins to the corresponding supply line downstream of the second valve.

16. The boil-off gas reliquefaction system according to any one of claims 1 to 3, further comprising:

a first oil filter disposed downstream of the compressor and separating lubricating oil from the boil-off gas.

17. The boil-off gas reliquefaction system of claim 16 wherein the first oil filter separates lube oil having a gas phase or a mist phase.

Technical Field

The present invention relates to a method and system for reliquefying Boil-off Gas (BOG) generated by Natural vaporization of Liquefied Gas, and more particularly, to a Boil-off Gas reliquefaction system in which Boil-off Gas generated in a storage tank of a Liquefied Natural Gas (LNG) ship to be supplied as fuel to an engine is reliquefied using the Boil-off Gas as a refrigerant in excess of a fuel demand of the engine.

Background

In recent years, the consumption of Liquefied gases such as Liquefied Natural Gas (LNG) has been rapidly increasing on a global scale. The volume of liquefied gas obtained by cooling natural gas to very low temperatures is much smaller than that of natural gas and is therefore more suitable for storage and transportation. In addition, since air pollutants in natural gas may be reduced or removed during the liquefaction process, liquefied gases, such as liquefied natural gas, are environmentally friendly fuels with low air pollutant emissions when combusted.

The liquefied natural gas is a colorless and transparent liquid obtained by cooling natural gas mainly composed of methane (methane) to about-163 ℃ to liquefy the natural gas, and has a volume of about 1/600 of the volume of the natural gas. Thus, the liquefaction of natural gas enables very efficient transportation.

However, since natural gas is liquefied at an extremely low temperature of-163 ℃ under normal pressure, liquefied natural gas may be easily vaporized due to a small change in temperature. Although the lng storage tank is insulated, external heat may be continuously transferred to the storage tank to naturally vaporize the lng during transportation, thereby generating Boil-Off Gas (BOG).

The production of boil-off gas means loss of liquefied natural gas and therefore has a significant impact on transport efficiency. In addition, when boil-off gas accumulates in the storage tank, there is a risk that the pressure within the storage tank rises excessively to cause damage to the tank. Various studies have been conducted to process boil-off gas generated in a liquefied natural gas storage tank. In recent years, for handling boil-off gas, a method in which boil-off gas is reliquefied to be returned to a liquefied natural gas storage tank, a method in which boil-off gas is used as an energy source in a fuel consumption source such as a marine engine, and the like have been proposed.

Examples of methods of reliquefying boil-off gas include: a method of using a refrigeration cycle having a separate refrigerant to enable an evaporation gas to exchange heat with the refrigerant to be reliquefied and a method of reliquefying an evaporation gas using an evaporation gas as a refrigerant without using any separate refrigerant. Specifically, a System adopting the latter method is called a Partial Re-liquefaction System (PRS).

Examples of marine engines that can be fueled with natural Gas include Gas engines such as Dual Fuel Diesel Electric (DFDE) engines, X generation Dual fuel (X-DF) engines, and M-type electronically Controlled Gas Injection (ME-GI) engines.

The DFDE engine has four strokes per cycle and uses an otto cycle (Ottocycle) in which natural gas having a relatively low pressure of about 6.5 bar (bar) is injected into the combustion gas inlet and then the piston is pushed upwards to compress the gas.

The X-DF engine has two strokes per cycle and uses an otto cycle using natural gas as fuel, which has a pressure of about 16 bar.

The ME-GI engine has two strokes per cycle and uses a Diesel cycle (Diesel cycle) in which natural gas having a high pressure of about 300 bar is directly injected into a combustion chamber located near top dead center of the piston.

Disclosure of Invention

Technical problem

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

The boil-off gas compressed by the Oil Lubrication type cylinder compressor contains lubricating Oil (Lubrication Oil). The inventors of the present invention have found that the lubricating oil contained in the compressed boil-off gas condenses or solidifies before the boil-off gas and blocks the fluid passages of the heat exchanger during cooling of the compressed boil-off gas in the heat exchanger. In particular, Printed Circuit Heat Exchangers (PCHEs), which have narrow fluid channels (e.g., microfluidic channel Type (Microchannel Type) fluid channels), suffer from more frequent fluid channel blockage due to condensed or solidified lubricating oil.

Accordingly, the inventors of the present invention have developed various techniques for separating the lubricating oil from the compressed boil-off gas, thereby preventing the condensed or solidified lubricating oil from clogging the fluid passages of the heat exchanger.

Embodiments of the present invention provide a method and system for mitigating or preventing condensed or solidified lubricating oil from clogging fluid passages of a heat exchanger, and capable of removing condensed or solidified lubricating oil clogging fluid passages of the heat exchanger by a simple and economical method.

Technical solution

According to one aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; and a second oil filter provided downstream of the pressure reducer, wherein the compressor includes at least one oil-lubricated cylinder, and the second oil filter is a cryogenic oil filter.

According to another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; a gas/liquid separator provided downstream of the pressure reducer and separating the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas; and a second oil filter provided on the fifth supply line, the liquefied gas separated by the gas/liquid separator being discharged through the fifth supply line, wherein the compressor includes at least one oil-lubricated cylinder, and the second oil filter is a low-temperature oil filter.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; a gas/liquid separator provided downstream of the pressure reducer and separating the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas; and a second oil filter provided on the sixth supply line through which the gaseous boil-off gas separated by the gas/liquid separator is discharged, wherein the compressor includes at least one oil-lubricated cylinder, and the second oil filter is a low-temperature oil filter.

The second filter separates the lubricating oil having a solid phase.

The boil-off gas reliquefaction system may further include a gas/liquid separator that is disposed downstream of the pressure reducer and separates the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas, wherein the second oil filter is disposed between the pressure reducer and the gas/liquid separator.

The second oil filter may be of the "drain-up 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 of the "drain-down type".

The compressor may compress the boil-off gas to a pressure of 150 to 350 bar.

The compressor may compress the boil-off gas to a pressure of 80 bar to 250 bar.

The heat exchanger may include microchannel-type fluid channels.

The heat exchanger may be a Printed Circuit Heat Exchanger (PCHE).

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

The boil-off gas reliquefaction system may further include a first valve disposed upstream of the cold fluid passage of the heat exchanger and adjusting a fluid flow rate of the corresponding supply line and opening/closing, wherein the bypass line may branch from the corresponding supply line upstream of the first valve.

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

The boil-off gas reliquefaction system may further include a first oil filter disposed downstream of the compressor and separating the lube oil from the boil-off gas.

The first oil filter can separate lubricating oil having a gas phase or a mist phase.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: compressing an evaporation gas by a compressor, cooling the compressed evaporation gas by heat exchange with the uncompressed evaporation gas by means of a heat exchanger, and reducing the pressure of a fluid cooled by the heat exchange by means of a pressure reducer, wherein the evaporation gas which will be used as a refrigerant in the heat exchanger is supplied to the heat exchanger along a first supply line, the evaporation gas which will be used as a refrigerant in the heat exchanger is supplied to the compressor along a second supply line, and the evaporation gas which will not be used as a refrigerant in the heat exchanger is supplied to the compressor along a bypass line around the heat exchanger, and wherein a bypass valve for adjusting the fluid flow rate and opening/closing of the respective supply line is provided on the bypass line, a first valve for adjusting the fluid flow rate and opening/closing of the respective supply line is provided on the first supply line upstream of the heat exchanger, a second valve for adjusting the fluid flow rate and opening/closing of the respective supply line is provided downstream of the heat exchanger On the second supply line, and the compressor comprises at least one cylinder of the oil-lubricated type, the method of discharging the lubricating oil comprises: 2) opening the bypass valve while closing the first valve and the second valve; 3) passing the boil-off gas not used as refrigerant in the heat exchanger along a bypass line to the compressor, and then compressed by the compressor; and 4) sending a part or all of the boil-off gas compressed by the compressor to a heat exchanger, the condensed or solidified lubricating oil being discharged from the boil-off gas reliquefaction system after being melted or reduced in viscosity by the boil-off gas increased in temperature during compression by the compressor.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the evaporation gas compressed by the compressor by heat exchange using the evaporation gas discharged from the storage tank as a refrigerant; a first valve for adjusting a fluid flow rate and opening/closing of a corresponding supply line provided on the first supply line, the evaporation gas to be used as a refrigerant in the heat exchanger being supplied to the heat exchanger via the first supply line; a second valve for adjusting a fluid flow rate and opening/closing of a corresponding supply line provided on a second supply line through which the evaporation gas used as the refrigerant in the heat exchanger is supplied to the compressor; a bypass line through which the boil-off gas is supplied to the compressor after bypassing the heat exchanger; and a pressure reducer disposed downstream of the heat exchanger and reducing a pressure of the fluid cooled by the heat exchanger, wherein the compressor includes at least one cylinder of an oil lubrication type and compresses the boil-off gas to a pressure of 150 to 350 bar, and the bypass line branches from the first supply line upstream of the first valve and joins to the second supply line downstream of the second valve.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: the method includes compressing an evaporation gas by a compressor, cooling the compressed evaporation gas by heat exchange with an uncompressed evaporation gas using a heat exchanger, and reducing a fluid pressure cooled by the heat exchange by a pressure reducer, wherein the compressor includes at least one oil lubrication type cylinder, the evaporation gas bypasses the heat exchanger via a bypass line and is sent to the compressor and compressed by the compressor, the evaporation gas compressed by the compressor is supplied to an engine, and an excess evaporation gas that is not supplied to the engine is supplied to the heat exchanger to discharge condensed or solidified lubricating oil after melting the lubricating oil or reducing the viscosity of the lubricating oil using the evaporation gas whose temperature is increased during compression by the compressor.

According to still another aspect of the present invention, there is provided a method of discharging lubricating oil from an boil-off gas reliquefaction system configured to reliquefy boil-off gas using the boil-off gas as a refrigerant, wherein a heat exchanger cools the boil-off gas compressed by a compressor by heat exchange using the boil-off gas discharged from a storage tank as the refrigerant at the time of reliquefaction of the boil-off gas; the compressor comprises at least one cylinder of oil-lubricated type; and the condensed or solidified lubricating oil is discharged through a bypass line provided to bypass the heat exchanger and used for maintenance of the heat exchanger after melting or viscosity reduction.

According to still another aspect of the present invention, there is provided a fuel supply method for an engine, in which fuel is supplied to the engine by melting condensed or solidified lubricating oil or reducing the viscosity of the lubricating oil during the discharge of the condensed or solidified lubricating oil.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; and a gas/liquid separator that is provided downstream of the decompressor and separates the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas, wherein the compressor includes at least one oil lubrication type cylinder, and the gas/liquid separator is connected to a lubricating oil discharge line through which the lubricating oil collected in the gas/liquid separator is discharged.

According to still another aspect of the present invention, there is provided a method of discharging a lubricating oil from an boil-off gas reliquefaction system configured to reliquefy the boil-off gas using the boil-off gas as a refrigerant, wherein the lubricating oil collected in the gas/liquid separator is discharged from the gas/liquid separator via a lubricating oil discharge line separated from a fifth supply line, and a liquefied gas produced by reliquefaction of the boil-off gas is discharged from the gas/liquid separator via the fifth supply line.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; and at least one of a combination of a first temperature sensor disposed upstream of the cold fluid passage of the heat exchanger, a combination of a second temperature sensor disposed downstream of the hot fluid passage of the heat exchanger, and a combination of a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger, and a combination of a first pressure sensor disposed downstream of the hot fluid passage of the heat exchanger, wherein the compressor includes at least one oil lubricated cylinder.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; and at least one of a combination of a first temperature sensor and a fourth temperature sensor, a combination of a second temperature sensor and a third temperature sensor, and a differential pressure sensor, the first temperature sensor being disposed upstream of a cold fluid passage of the heat exchanger, the fourth temperature sensor being disposed downstream of a hot fluid passage of the heat exchanger, the second temperature sensor being disposed downstream of the cold fluid passage of the heat exchanger, the third temperature sensor being disposed upstream of the hot fluid passage of the heat exchanger, the differential pressure sensor measuring a differential pressure between the hot fluid passage upstream of the heat exchanger and the hot fluid passage downstream of the heat exchanger, wherein the compressor includes at least one oil lubricated cylinder.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system configured to reliquefy a boil-off gas by: compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by heat exchange with the non-compressed boil-off gas using a heat exchanger, and reducing a pressure of a fluid cooled by the heat exchange by a pressure reducer, wherein the compressor includes at least one oil-lubricated cylinder, and an alarm is generated when a malfunction of the heat exchanger is detected.

According to still another aspect of the present invention, there is provided a method of discharging lubricating oil from an evaporation gas reliquefaction system configured to reliquefy evaporation gas using the evaporation gas as a refrigerant, wherein the evaporation gas is cooled by a heat exchanger using the evaporation gas as the refrigerant at the time of the reliquefaction of the evaporation gas, and whether it is time to discharge condensed or solidified lubricating oil is determined based on: a lower value between a temperature difference between a temperature measured by a first temperature sensor disposed upstream of a cold fluid passage of the heat exchanger and a temperature measured by a fourth temperature sensor disposed downstream of a hot fluid passage of the heat exchanger and a temperature difference between a temperature measured by a second temperature sensor disposed downstream of the cold fluid passage of the heat exchanger and a temperature measured by a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger, or a pressure difference between a pressure measured by a first pressure sensor disposed upstream of the hot fluid passage of the heat exchanger and a pressure measured by a second pressure sensor disposed downstream of the hot fluid passage of the heat exchanger.

According to still another aspect of the present invention, there is provided a method of discharging lubricating oil from an evaporation gas reliquefaction system configured to reliquefy evaporation gas using the evaporation gas as a refrigerant, wherein the evaporation gas is cooled by a heat exchanger using the evaporation gas as the refrigerant at the time of the reliquefaction of the evaporation gas, and whether it is time to discharge condensed or solidified lubricating oil is determined based on: a lower value between a temperature difference between a temperature measured by a first temperature sensor disposed upstream of a cold fluid passage of the heat exchanger and a temperature measured by a fourth temperature sensor disposed downstream of a hot fluid passage of the heat exchanger and a temperature difference between a temperature measured by a second temperature sensor disposed downstream of the cold fluid passage of the heat exchanger and a temperature measured by a third temperature sensor disposed upstream of the hot fluid passage of the heat exchanger, or a pressure difference measured by a pressure difference sensor for measuring a pressure difference between upstream of the hot fluid passage of the heat exchanger and downstream of the hot fluid passage of the heat exchanger.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by heat exchange with the non-compressed boil-off gas using a heat exchanger, and reducing a pressure of a fluid cooled by the heat exchange by a pressure reducer, wherein the compressor includes at least one oil lubrication type cylinder, and it is determined that it is time to discharge the condensed or solidified lubricating oil if at least one of the following conditions is satisfied: a condition that a temperature difference between the evaporation gas upstream of the heat exchanger, which will be used as a refrigerant in the heat exchanger, and the evaporation gas compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "cold flow temperature difference") is a first preset value or higher and lasts for a predetermined time period or longer; a condition that a temperature difference between the evaporation gas used as the refrigerant in the heat exchanger and the evaporation gas compressed by the compressor and sent to the heat exchanger (hereinafter, referred to as "heat flow temperature difference") is a first preset value or more and is continued for a predetermined time period or more; and a condition that a pressure difference between the boil-off gas compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the boil-off gas cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter referred to as "hot fluid passage pressure difference") is a second preset value or more and is continued for a predetermined time period or more.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by heat exchange with the non-compressed boil-off gas using a heat exchanger, and reducing a pressure of a fluid cooled by the heat exchange by a pressure reducer, wherein the compressor includes at least one oil lubrication type cylinder, and it is determined when the condensed or solidified lubricating oil is discharged if: the lower value between the temperature difference between the evaporating gas upstream of the heat exchanger which will be used as the refrigerant in the heat exchanger and the evaporating gas compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as "cold flow temperature difference") and the temperature difference between the evaporating gas used as the refrigerant in the heat exchanger and the evaporating gas compressed by the compressor and sent to the heat exchanger (hereinafter referred to as "hot flow temperature difference") is a first preset value or more and is continued for a predetermined time period or more, or a pressure difference between the boil-off gas compressed by the compressor and sent to the heat exchanger at a position upstream of the heat exchanger and the boil-off gas cooled by the heat exchanger at a position downstream of the heat exchanger (hereinafter referred to as "hot fluid passage pressure difference") is a second preset value or more and is continued for a predetermined time period or more.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from an evaporation gas reliquefaction system configured to reliquefy evaporation gas using the evaporation gas as a refrigerant, wherein a point in time at which condensed or solidified lubricating oil is discharged is determined based on at least one of a temperature difference and a pressure difference of equipment, and an alarm is generated to indicate the point in time at which the condensed or solidified lubricating oil is discharged.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; and a pressure reducer that reduces a pressure of the fluid cooled by the heat exchanger, the boil-off gas reliquefaction system further including: a detection unit disposed upstream and/or downstream of the heat exchanger to detect whether the heat exchanger is clogged with the lubricating oil; and an alarm indicating that the heat exchanger is clogged with the lubricating oil based on a detection result of the detection unit.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the vapor compressed by the compressor by heat exchange using the vapor not compressed by the compressor as a refrigerant; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; a bypass line disposed upstream of the heat exchanger such that an evaporation gas to be used as a refrigerant in the heat exchanger is supplied to the compressor bypassing the heat exchanger along the bypass line; and a bypass valve provided on the bypass line and adjusting a fluid flow rate of the bypass line and opening/closing, wherein the bypass valve is partially or fully opened when a pressure of the evaporation gas supplied to the compressor is lower than an intake pressure condition of the compressor.

According to yet another aspect of the present invention, there is provided a method of supplying fuel to an engine of a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: the method includes compressing an evaporation gas by a compressor, cooling the compressed evaporation gas by heat-exchanging with the non-compressed evaporation gas using a heat exchanger, and reducing a pressure of a fluid cooled by the heat-exchanging by a pressure reducer, wherein when the pressure of the evaporation gas supplied to the compressor is lower than a suction pressure condition of the compressor, a part or all of the evaporation gas to be supplied to the compressor is supplied to the compressor after bypassing the heat exchanger.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the evaporation gas compressed by the compressor by heat exchange using the evaporation gas discharged from the storage tank as a refrigerant; a bypass line through which the boil-off gas is supplied to the compressor after bypassing the heat exchanger; a second valve provided on the second supply line through which the evaporation gas used as the refrigerant in the heat exchanger is supplied to the compressor, the second valve adjusting a fluid flow rate of the second supply line and opening/closing; and a pressure reducer disposed downstream of the heat exchanger and reducing a pressure of the fluid cooled by the heat exchanger, wherein the compressor includes at least one cylinder of an oil lubrication type, and the bypass line is joined to the second supply line downstream of the second valve.

According to yet another aspect of the present invention, there is provided a method of discharging lubricating oil from a boil-off gas reliquefaction system configured to reliquefy boil-off gas by: compressing the boil-off gas by a compressor, cooling the compressed boil-off gas by heat exchange with the uncompressed boil-off gas by a heat exchanger, and reducing the pressure of the fluid cooled by the heat exchange by a pressure reducer, wherein the compressor includes at least one cylinder of an oil lubrication type, and a second valve for adjusting a flow rate of fluid of a corresponding supply line and opening/closing is provided on the second supply line, the boil-off gas used as the refrigerant in the heat exchanger is supplied to the compressor via the second supply line, and wherein the boil-off gas is compressed by the compressor after bypassing the heat exchanger via the bypass line, excess boil-off gas in excess of the fuel demand of the engine being supplied to the heat exchanger, to discharge the condensed lubricating oil after melting the condensed lubricating oil with the boil-off gas whose temperature has risen during compression by the compressor, and the bypass line is joined to the second supply line downstream of the second valve.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the evaporation gas compressed by the compressor by heat exchange using the evaporation gas discharged from the storage tank as a refrigerant; a bypass line through which the boil-off gas is supplied to the compressor after bypassing the heat exchanger; a first valve provided on the first supply line, the boil-off gas to be used as a refrigerant in the heat exchanger being supplied to the heat exchanger via the first supply line, the first valve adjusting a fluid flow rate of the first supply line and opening/closing; and a pressure reducer disposed downstream of the heat exchanger and reducing a pressure of the fluid cooled by the heat exchanger, wherein the compressor includes at least one cylinder of an oil lubrication type, and the bypass line branches from the first supply line upstream of the first valve.

According to yet another aspect of the present invention, there is provided a boil-off gas reliquefaction system including: a compressor for compressing the evaporation gas; a heat exchanger for cooling the evaporation gas compressed by the compressor by heat exchange using the evaporation gas discharged from the storage tank as a refrigerant; a bypass line through which the evaporation gas is supplied to the compressor after bypassing the heat exchanger, the bypass line branching from the first supply line, the evaporation gas to be used as refrigerant in the heat exchanger being supplied to the heat exchanger through the first supply line; a pressure reducer that is provided downstream of the heat exchanger and reduces the pressure of the fluid cooled by the heat exchanger; and a gas/liquid separator that is provided downstream of the pressure reducer and separates the boil-off gas into a liquefied gas produced by reliquefaction and a gaseous boil-off gas, wherein the compressor includes at least one cylinder of an oil lubrication type, and the gaseous boil-off gas separated by the gas/liquid separator is discharged from the gas/liquid separator along a sixth supply line joined to the first supply line upstream of a branch point of the bypass line.

Advantageous effects

According to the embodiments of the present invention, condensed or solidified lubricating oil inside the heat exchanger can be removed by a simple and economical process using existing equipment without installing separate equipment or supplying a separate fluid for removing lubricating oil.

According to an embodiment of the present invention, by driving the engine during removal of the condensed or solidified lubricating oil, the heat exchanger can be serviced while the engine is continuously running. In addition, the use of excess boil-off gas not used by the engine removes condensed or solidified lubricating oil. In addition, the engine may be used to burn lubricating oil mixed with boil-off gas.

According to an embodiment of the present invention, if the lubricating oil is collected in an improved gas/liquid separator, the melted or viscosity-reduced lubricating oil can be discharged efficiently using the gas/liquid separator.

According to an embodiment of the present invention, the low-temperature oil filter is provided at least one of a position downstream of the pressure reducer, a fifth supply line through which the liquid gas is discharged from the gas/liquid separator, and a sixth supply line through which the boil-off gas is discharged from the gas/liquid separator, thereby achieving efficient removal of the lubricating oil mixed with the boil-off gas.

According to the embodiment of the present invention, it is possible to satisfy the intake pressure condition of the compressor and the engine fuel demand of the engine while maintaining the reliquefaction performance through a simple and economical process even if the existing equipment is used instead of a separate equipment.

Drawings

Fig. 1 is a schematic diagram of a boil-off gas reliquefaction system according to a first embodiment of the present invention.

Fig. 2 is a schematic diagram of a boil-off gas reliquefaction system according to a second embodiment of the present invention.

Fig. 3 is a schematic diagram of a boil-off gas reliquefaction system according to a third embodiment of the present invention.

FIG. 4 is an enlarged view of a gas/liquid separator according to one embodiment of the invention.

FIG. 5 is an enlarged view of the second oil filter according to one embodiment of the present invention.

Fig. 6 is an enlarged view of a second oil filter according to another embodiment of the present invention.

Fig. 7 is a schematic diagram of a boil-off gas reliquefaction system according to a fourth embodiment of the present invention.

Fig. 8 is an enlarged view of a stress-reducer according to an embodiment of the present invention.

Fig. 9 is an enlarged view of a pressure reducer according to another embodiment of the present invention.

FIG. 10 is an enlarged view of a heat exchanger and gas/liquid separator according to one embodiment of the invention.

Fig. 11 and 12 are graphs showing changes in reliquefaction amount according to the pressure of the boil-off gas in a Partial Re-liquefaction System (PRS).

Fig. 13 is a plan view of the filter element shown in fig. 5 and 6.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The boil-off gas reliquefaction system according to the present invention is applicable to various vessels such as a vessel equipped with an engine using natural gas as fuel, a vessel including a liquefied gas storage tank, a marine structure (marine structure), and the like. It is to be understood that the following examples may be modified in various ways, and do not limit the scope of the present invention.

Further, the fluid in each fluid supply line of the system according to the present invention may have a liquid phase, a gas-liquid mixed phase, a gas phase, and a supercritical fluid phase (supercritical fluid phase) depending on the operating conditions of the system.

Fig. 1 is a schematic diagram of a boil-off gas reliquefaction system according to a first embodiment of the present invention.

Referring to fig. 1, the boil-off gas reliquefaction system according to this embodiment includes a compressor (200), a heat exchanger (100), a pressure reducer (600), a Bypass Line (BL), and a bypass valve (590).

The compressor (200) compresses the boil-off gas discharged from the storage tank (T), and may include a plurality of cylinders (210, 220, 230, 240, 250) and a plurality of coolers (211, 221, 231, 241, 251). The boil-off gas compressed by the compressor (200) may have a pressure of about 150 to 350 bar.

Some of the boil-off gas compressed by the compressor (200) may be supplied to the main engine of the ship along the fuel Supply Line (SL), and another boil-off gas not used by the main engine may be supplied to the heat exchanger (100) along the third supply line (L3) so as to be subjected to the reliquefaction process. The primary engine may be a ME-GI engine using high pressure natural gas having a pressure of about 300 bar as fuel.

Some of the boil-off gas that has passed through some of the cylinders (210, 220) of the compressor (200) is divided and supplied to the generator. The generator according to this embodiment may be a dual fuel (duel fuel DF) engine using low pressure natural gas having a pressure of about 6.5 bar as fuel.

The heat exchanger (100) cools the evaporation gas compressed by the compressor (200) and supplied along the third supply line (L3) by heat exchange using the evaporation gas discharged from the storage tank (T) and supplied along the first supply line (L1) as a refrigerant. The evaporation gas used as refrigerant in the heat exchanger (100) is sent to the compressor (200) along the second supply line (L2), and the fluid cooled by the heat exchanger (100) is supplied to the pressure reducer (600) along the fourth supply line (L4).

The decompressor (600) lowers the pressure of the boil-off gas compressed by the compressor (200) and then cooled by the heat exchanger (100). Part or all of the boil-off gas is reliquefied by compression by the compressor (200), cooling by the heat exchanger (100), and pressure reduction by the pressure reducer (600). The pressure reducer (600) may be an expansion valve, such as a Joule-Thomson valve, or may be an inflator.

The boil-off gas reliquefaction system according to this embodiment may further include a gas/liquid separator (700), the gas/liquid separator (700) being disposed after the decompressor (600) to separate the boil-off gas still in a gas phase from liquefied natural gas produced by reliquefaction of the boil-off gas using the compressor (200), the heat exchanger (100) and the decompressor (600).

The liquefied gas separated by the gas/liquid separator (700) is supplied to the storage tank (T) along a fifth supply line (L5), and the boil-off gas separated by the gas/liquid separator (700) may be combined with the boil-off gas discharged from the storage tank (T) and supplied to the heat exchanger (100).

A ninth valve (582) for adjusting the flow rate and opening/closing of the corresponding supply line may be provided on the sixth supply line (L6), and the evaporation gas having a gas phase is discharged from the gas/liquid separator (700) via the sixth supply line (L6).

If the heat exchanger (100) is not available, for example, during service or failure of the heat exchanger (100), boil-off gas discharged from the storage tank (T) may be allowed to bypass the heat exchanger (100) via the Bypass Line (BL). The Bypass Line (BL) is provided with a bypass valve (590) that opens and closes the Bypass Line (BL).

Fig. 2 is a schematic diagram of a boil-off gas reliquefaction system according to a second embodiment of the present invention.

Referring to fig. 2, the boil-off gas reliquefaction system according to this embodiment includes a heat exchanger (100), a first valve (510), a second valve (520), a first temperature sensor (810), a second temperature sensor (820), a compressor (200), a third temperature sensor (830), a fourth temperature sensor (840), a first pressure sensor (910), a second pressure sensor (920), a pressure reducer (600), a Bypass Line (BL), and a bypass valve (590).

The heat exchanger (100) cools the boil-off gas compressed by the compressor (200) by heat exchange using the boil-off gas discharged from the storage tank (T) as a refrigerant. The evaporation gas discharged from the storage tank (T) and used as the refrigerant in the heat exchanger (100) is sent to the compressor (200), and the evaporation gas compressed by the compressor (200) is cooled by the heat exchanger (100) using the evaporation gas discharged from the storage tank (T) as the refrigerant.

The evaporation gas discharged from the storage tank (T) is supplied to the heat exchanger (100) along the first supply line (L1) and used as a refrigerant, and the evaporation gas used as a refrigerant in the heat exchanger (100) is sent to the compressor (200) along the second supply line (L2). Part or all of the boil-off gas compressed by the compressor (200) is supplied to the heat exchanger (100) along the third supply line (L3) to be cooled, and the fluid cooled by the heat exchanger (100) is supplied to the pressure reducer (600) along the fourth supply line (L4).

A first valve (510) is provided on the first supply line (L1) to adjust the flow rate and open/close of the corresponding supply line, and a second valve (520) is provided on the second supply line (L2) to adjust the flow rate and open/close of the corresponding supply line.

A first temperature sensor (810) is provided on the first supply line (L1) in front of the heat exchanger (100) to measure the temperature of the boil-off gas discharged from the storage tank (T) and supplied to the heat exchanger (100). Preferably, a first temperature sensor (810) is provided immediately before the heat exchanger (100) to measure the temperature of the boil-off gas immediately before being supplied to the heat exchanger (100).

Herein, the term "forward" refers to upstream and the term "aft" refers to downstream.

A second temperature sensor (820) is provided on the second supply line (L2) downstream of the heat exchanger (100) to measure the temperature of the boil-off gas used as refrigerant in the heat exchanger (100) after discharge from the storage tank (T). Preferably, a second temperature sensor (820) is arranged immediately after the heat exchanger (100) to measure the temperature of the boil-off gas immediately after being used as refrigerant in the heat exchanger (100).

The compressor (200) compresses the boil-off gas that is used as a refrigerant in the heat exchanger (100) after being discharged from the storage tank (T). The boil-off gas compressed by the compressor (200) may be supplied to the high-pressure engine to be used as fuel, and the boil-off gas remaining after being supplied to the high-pressure engine may be supplied to the heat exchanger (100) for re-liquefaction.

A sixth valve (560) for adjusting the flow rate and opening/closing of the corresponding supply line may be provided on the fuel Supply Line (SL) through which the evaporation gas compressed by the compressor (200) is supplied to the high-pressure engine.

The sixth valve (560) serves as a safety device to cut off the supply of boil-off gas to the high-pressure engine when the gas mode operation of the high-pressure engine is interrupted. Gas mode means a mode in which the engine is operated using natural gas as fuel. When there is a shortage of boil-off gas that will be used as fuel, the engine is switched to a fuel mode to allow the fuel to be used as fuel for the engine.

A seventh valve (570) for adjusting the flow rate and opening/closing of the corresponding supply line may be provided on the supply line through which surplus boil-off gas higher than the fuel demand of the high-pressure engine among the boil-off gas compressed by the compressor (200) is supplied to the heat exchanger (100).

When the evaporation gas compressed by the compressor (200) is supplied to the high-pressure engine, the compressor (200) may compress the evaporation gas to a pressure required by the high-pressure engine. The high pressure engine may be a ME-GI engine that uses high pressure boil-off gas as fuel.

ME-GI engines are known to use natural gas as fuel having a pressure of about 150 to 400 bar, preferably about 150 to about 350 bar, more preferably about 300 bar. The compressor (200) may compress the boil-off gas to a pressure of about 150 bar to about 350 bar to supply the compressed boil-off gas to the ME-GI engine.

An X-DF engine or a DF engine using boil-off gas as fuel at a pressure of about 6 bar to about 20 bar may be used instead of the ME-GI engine as the main engine. In this case, since the compressed boil-off gas for supply to the main engine has a low pressure, the compressed boil-off gas to be supplied to the main engine may be further compressed to re-liquefy the boil-off gas. The further compressed boil-off gas used for reliquefaction may have a pressure of about 80 bar to 250 bar.

Fig. 11 and 12 are graphs showing changes in reliquefaction amount depending on the boil-off gas pressure in the Partial Reliquefaction System (PRS). The re-liquefaction target boil-off gas means a boil-off gas that will be re-liquefied by cooling, and is different from the boil-off gas used as a refrigerant.

Referring to fig. 11 and 12, it can be seen that the reliquefaction amount reaches a maximum value when the pressure of the evaporation gas is in the range of 150 to 170 bar, and the reliquefaction amount is substantially unchanged when the pressure of the evaporation gas is in the range of 150 to 300 bar. Therefore, as a high-pressure engine, an ME-GI engine using as fuel boil-off gas having a pressure of about 150 bar to about 350 bar (mainly 300 bar) can easily control the reliquefaction system to supply fuel to the high-pressure engine while maintaining a high liquefaction amount.

The compressor (200) may include a plurality of cylinders (210, 220, 230, 240, 250) and a plurality of coolers (211, 221, 231, 241, 251) respectively disposed downstream of the plurality of cylinders (210, 220, 230, 240, 250). The cooler (211, 221, 231, 241, 251) cools the evaporation gas compressed by the cylinder (210, 220, 230, 240, 250) and having high pressure and high temperature.

In a structure in which the compressor (200) includes the plurality of cylinders (210, 220, 230, 240, 250), the evaporation gas sent to the compressor (200) is compressed through a plurality of stages using the plurality of cylinders (210, 220, 230, 240, 250). Each of the cylinders (210, 220, 230, 240, 250) may serve as a compression terminal for each of the compressors (200).

The compressor (200) may include a first recirculation line (RC1) through which part or all of the evaporation gas having passed through the first cylinder (210) and the first cooler (211) is supplied to the front end of the first cylinder (210) via a first recirculation line (RC 1); a second recirculation line (RC2) through which part or all of the evaporation gas having passed through the second cylinder (220) and the second cooler (221) is supplied to the front end of the second cylinder (220) via a second recirculation line (RC 2); a third recirculation line (RC3) through which part or all of the evaporation gas having passed through the third cylinder (230) and the third cooler (231) is supplied to the front end of the third cylinder (230) via the third recirculation line (RC 3); and a fourth recirculation line (244) through which part or all of the evaporation gas having passed through the fourth cylinder (240), the fourth cooler (241), the fifth cylinder (250), and the fifth cooler (251) is supplied to the front end of the fourth cylinder (240) via the fourth recirculation line (244).

In addition, a first recirculation valve (541) for adjusting the flow rate and opening/closing of the corresponding supply line may be disposed on the first recirculation line (RC1), a second recirculation valve (542) for adjusting the flow rate and opening/closing of the corresponding supply line may be disposed on the second recirculation line (RC2), a third recirculation valve (543) for adjusting 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 (543) for adjusting the flow rate and opening/closing of the corresponding supply line may be disposed on the fourth recirculation line (RC 4).

When the storage tank (T) has a low pressure to meet the intake pressure condition required by the compressor (200), the recirculation line (RC1, RC2, RC3, RC4) protects the compressor (200) by recirculating part or all of the boil-off gas. When the recirculation line (RC1, RC2, RC3, RC4) is not used, the recirculation valve (541, 542, 543, 544) is closed, and when the intake pressure conditions required by the compressor (200) are not met and the recirculation line (RC1, RC2, RC3, RC4) needs to be used, the recirculation valve (541, 542, 543, 544) is opened.

Although fig. 2 shows a structure in which the evaporation gas having passed through all of the plurality of cylinders (210, 220, 230, 240, 250) of the compressor (200) is supplied to the heat exchanger (100), the evaporation gas having passed through some of the cylinders (210, 220, 230, 240, 250) may be divided in the compressor (200) to be supplied to the heat exchanger (100).

In addition, boil-off Gas that has passed through some of the cylinders (210, 220, 230, 240, 250) may be divided in the compressor (200) to be supplied to a low pressure engine for use as fuel, and the excess may be supplied to a Gas Combustion Unit (GCU) for Combustion.

The low pressure engine may be a DF engine (e.g., DFDE) using as fuel boil-off gas having a pressure of about 6 to 10 bar.

Some of the cylinders (210, 220, 230, 240, 250) included in the compressor (200) may be operated in an oil-free lubricated manner, and other cylinders may be operated in an oil lubricated manner. Specifically, when the boil-off gas is compressed to 80 bar or more than 80 bar, preferably (100) bar or more than (100) bar, in order to use the boil-off gas compressed by the compressor (200) as fuel for a high-pressure engine or in order to reliquefy efficiency, the compressor (200) includes an oil lubrication type cylinder so as to compress the boil-off gas to a high pressure.

In the related art, lubricating oil for lubrication and cooling is supplied to a reciprocating type compressor (200) (e.g., a piston seal portion thereof) so as to compress boil-off gas to (100) bar or more.

Since the lubricating oil is supplied to the oil lubrication type cylinder, some of the lubricating oil is mixed with the boil-off gas that has passed through the oil lubrication type cylinder in the related art. The inventors of the present invention found that the lubricating oil mixed with the compressed boil-off gas condenses or solidifies before the boil-off gas in the heat exchanger (100) to block the fluid passages of the heat exchanger (100).

The boil-off gas reliquefaction system according to this embodiment may further include an oil separator (300) and a first oil filter (410) disposed between the compressor (200) and the heat exchanger (100) to separate oil from the boil-off gas.

The oil separator (300) typically separates the lube oil in the liquid phase, and the first oil filter (410) separates the lube oil in the gas (Vapor) phase or in the Mist (Mist) phase. Since the oil separator (300) separates the lubricating oil having a larger particle diameter than the lubricating oil separated by the first oil filter (410), the oil separator (300) is disposed upstream of the first oil filter (410) so that the boil-off gas compressed by the compressor (200) can be supplied to the heat exchanger (100) after passing through the oil separator (300) and the first oil filter (410) in this order.

Although fig. 2 shows a structure in which the boil-off gas reliquefaction system includes both the oil separator (300) and the first oil filter (410), the boil-off gas reliquefaction system according to this embodiment may include one of the oil separator (300) and the first oil filter (410). Preferably, both the oil separator (300) and the first oil filter (410) are used.

In addition, although fig. 2 shows a structure in which the first oil filter (410) is provided to the second supply line (L2) downstream of the compressor (200), the first oil filter (410) may be provided to the third supply line (L3) upstream of the heat exchanger (100), and may be provided in plurality so as to be arranged in parallel.

In a structure in which the boil-off gas reliquefaction system includes one of the oil separator (300) and the first oil filter (410) and the compressor (200) includes the oil-lubricated cylinder and the oil-lubricated cylinder, the boil-off gas having passed through the oil-lubricated cylinder may be supplied to the oil separator (300) and/or the first oil filter (410), and the boil-off gas having passed through only the oil-lubricated cylinder may be directly supplied to the heat exchanger (100) without passing through the oil separator (300) or the oil filter (410).

For example, the compressor (200) according to this embodiment includes five cylinders (210, 220, 230, 240, 250), wherein the first three cylinders (210, 220, 230) may be oil-lubricated type cylinders, and the last two cylinders (240, 250) may be oil-lubricated type cylinders. Here, in the boil-off gas reliquefaction system according to this embodiment, when the boil-off gas is divided in three stages or less, the boil-off gas may be directly supplied to the heat exchanger (100) without passing through the oil separator (300) or the first oil filter (410), and when the boil-off gas is divided in four stages or more, the boil-off gas may be supplied to the first heat exchanger (100) after passing through the oil separator (300) and/or the first oil filter (410).

The first oil filter (410) may be a coalescing (Coalescer Type) oil filter.

A check valve (550) may be disposed on the fuel Supply Line (SL) between the compressor (200) and the high pressure engine. The check valve (550) is used to prevent the boil-off gas from returning and damaging the compressor when the high pressure engine is stopped.

In configurations where the boil-off gas reliquefaction system includes an oil separator (300) and/or first oil filter (410), a check valve (550) may be disposed downstream of the oil separator (300) and/or first oil filter (410) to prevent boil-off gas from flowing back into the oil separator (300) and/or first oil filter (410).

In addition, since the evaporation gas may flow back to the compressor (200) and damage the compressor (200) when the expansion valve (600) is abruptly closed, the check valve (550) may be disposed upstream of the branch point where the third supply line (L3) branches from the fuel Supply Line (SL).

A third temperature sensor (830) is provided on the third supply line (L3) upstream of the heat exchanger (100) to measure the temperature of the boil-off gas compressed by the compressor (200) and then supplied to the heat exchanger (100). Preferably, a third temperature sensor (830) is provided immediately before the heat exchanger (100) to measure the temperature of the boil-off gas immediately before being supplied to the heat exchanger (100).

A fourth temperature sensor (840) is disposed on the fourth supply line (L4) downstream of the heat exchanger (100) to measure the temperature of the boil-off gas compressed by the compressor (200) and then cooled by the heat exchanger (100). Preferably, a fourth temperature sensor (840) is arranged immediately after the heat exchanger (100) to measure the temperature of the boil-off gas immediately after cooling by the heat exchanger (100).

A first pressure sensor (910) is provided on the third supply line (L3) upstream of the heat exchanger (100) to measure the pressure of the boil-off gas compressed by the compressor (200) and supplied to the heat exchanger (100). Preferably, a first pressure sensor (910) is arranged immediately before the heat exchanger (100) to measure the pressure of the boil-off gas immediately before being supplied to the heat exchanger (100).

A second pressure sensor (920) is provided on the fourth supply line (L4) downstream of the heat exchanger (100) to measure the pressure of the boil-off gas compressed by the compressor (200) and then cooled by the heat exchanger (100). Preferably, a second pressure sensor (920) is arranged immediately after the heat exchanger (100) to measure the pressure of the boil-off gas immediately after cooling by the heat exchanger (100).

As shown in fig. 2, while it is desirable to provide all of the first through fourth temperature sensors (810-840), the first pressure sensor (910), and the second pressure sensor (920) to the reliquefaction system, it is to be understood that the present invention is not so limited. Alternatively, the reliquefaction system may be provided with only the first temperature sensor (810) and the fourth temperature sensor (840) ('the first pair (pair)'), with only the second temperature sensor (820) and the third temperature sensor (830) ('the second pair'), with only the first pressure sensor (910) and the second pressure sensor (920) ('the third pair'), or with two of the first to third pairs.

A decompressor (600) is provided downstream of the heat exchanger (100) to decompress the boil-off gas compressed by the compressor (200) and then cooled by the heat exchanger (100). Part or all of the boil-off gas is reliquefied by compression by the compressor (200), cooling by the heat exchanger (100), and pressure reduction by the pressure reducer (600). The pressure reducer (600) may be an expansion valve, such as a joule-thomson valve, or may be an inflator.

The boil-off gas reliquefaction system according to this embodiment may further include a gas/liquid separator (700) disposed downstream of the decompressor (600) to separate the boil-off gas still in a gas phase from the liquefied natural gas produced by reliquefying the boil-off gas using the compressor (200), the heat exchanger (100), and the decompressor (600).

The liquefied gas separated by the gas/liquid separator (700) is supplied to the storage tank (T) along the fifth supply line (L5), and the boil-off gas separated by the gas/liquid separator (700) may be combined with the boil-off gas discharged from the storage tank (T) along the sixth supply line (L6) and supplied to the heat exchanger (100).

Although fig. 2 shows a structure in which the boil-off gas separated by the gas/liquid separator (700) is combined with the boil-off gas discharged from the storage tank (T) and then supplied to the heat exchanger (100), it is to be understood that the present invention is not limited thereto. For example, the heat exchanger (100) may be constituted by three fluid passages, and the evaporation gas separated by the gas/liquid separator (700) may be supplied to the heat exchanger (100) along a separate fluid passage so as to be used as a refrigerant in the heat exchanger (100).

Alternatively, the gas/liquid separator (700) may be omitted, and the boil-off gas reliquefaction system may be configured to allow the fluid to be partially or fully reliquefied by decompression performed by the decompressor (600) to be directly supplied to the storage tank (T).

An eighth valve (581) for adjusting the flow rate and opening/closing of the corresponding supply line may be provided on the fifth supply line (L5). The level (level) of the liquefied gas in the gas/liquid separator (700) is regulated by an eighth valve (581).

A ninth valve (582) for adjusting the flow rate and opening/closing of the corresponding supply line may be provided on the sixth supply line (L6). The internal pressure of the gas/liquid separator (700) is adjustable by a ninth valve (582).

FIG. 4 is an enlarged view of a gas/liquid separator according to one embodiment of the invention. Referring to fig. 4, the gas/liquid separator (700) may be provided with a fluid level sensor (940) that measures the level of natural gas in the gas/liquid separator (700).

The boil-off gas reliquefaction system according to this embodiment may include a second oil filter (420) provided between the pressure reducer (600) and the gas/liquid separator (700) to filter the lubricating oil mixed with the fluid subjected to the pressure reduction by the pressure reducer (600).

Referring to fig. 2 and 4, the second oil filter (420) may be disposed on the fourth supply line (L4) (position a in fig. 4), on the fifth supply line (L5) through which the reliquefied gas is discharged from the gas/liquid separator (700) (position B in fig. 4), or on the sixth supply line (L6) through which the gaseous boil-off gas is discharged from the gas/liquid separator (700) (position C in fig. 4) between the pressure reducer (600) and the gas/liquid separator (700). Fig. 2 shows a structure in which a second oil filter (420) is provided at a position a in fig. 4.

The boil-off gas separated by the gas/liquid separator (700) may be combined with the boil-off gas discharged from the storage tank (T) and supplied to the cold fluid passage of the heat exchanger (100). Here, since the lubricating oil is collected in the gas/liquid separator (700), it is possible that even a small amount of lubricating oil can be mixed with the gaseous boil-off gas separated by the gas/liquid separator (700).

The inventors of the present invention have found that, when the gaseous boil-off gas separated by the gas/liquid separator (700) is mixed with lubricating oil and fed to the cold fluid passage of the heat exchanger (100), a more difficult situation may occur than in the case where lubricating oil mixed with the boil-off gas compressed by the compressor (200) is supplied to the hot fluid passage of the heat exchanger (100).

Since the fluid that would be used as refrigerant in the heat exchanger (100) is sent to the cold fluid channel of the heat exchanger (100), the low temperature boil-off gas is supplied during the entire operation of the reliquefaction system, while fluid having a temperature high enough to melt the condensed or solidified oil is not supplied to the reliquefaction system. Therefore, it is extremely difficult to remove the condensed or solidified oil accumulated in the cryogenic fluid passage of the heat exchanger (100).

In order to reduce the possibility that the mixture of the lubricating oil and the gaseous evaporated gas separated by the gas/liquid separator (700) is supplied to the cold fluid passage of the heat exchanger (100) as low as possible, a second oil filter (420) may be provided at position a or position C in fig. 4.

In the structure in which the second oil filter (420) is disposed at the position C in fig. 4, since most of the melted or viscosity-reduced lubricating oil is collected in the gas/liquid separator (700) in the liquid phase, and the amount of gaseous lubricating oil discharged along the sixth supply line (L6) is small, there is an advantage in that the reliquefaction system has high filtration efficiency and frequent replacement of the second oil filter (420) is not required.

In the structure in which the second oil filter (420) is provided at the position B in fig. 4, since the lubricating oil can be prevented from flowing into the storage tank (T), it is possible to prevent the liquefied gas stored in the storage tank (T) from being contaminated.

Since the first oil filter (410) is disposed downstream of the compressor (200) and the boil-off gas compressed by the compressor (200) has a temperature of about 40 ℃ to about 45 ℃, it is not necessary to use a low-temperature oil filter. However, since the fluid whose pressure is reduced by the pressure reducer (600) has a temperature of about-160 ℃ to about-150 ℃ to allow at least partial reliquefaction of the boil-off gas, and since the liquefied gas and the boil-off gas separated by the gas/liquid separator (700) have a temperature of about-160 ℃ to about-150 ℃, the second oil filter (420) must be designed for low temperatures regardless of the position of the second oil filter (420) in the positions A, B, C and D in fig. 4.

In addition, since most of the lubricating oil mixed with the boil-off gas compressed by the compressor (200) and having a temperature of about 40 ℃ to 45 ℃ has a liquid phase or Mist (Mist) phase, the oil separator (300) is designed to be suitable for the lubricating oil separated into the liquid phase, and the first oil filter (410) is designed to be suitable for the lubricating oil separated into the Mist phase (the lubricating oil in the Mist phase may include some of the lubricating oil in the gas phase).

In contrast, the second oil filter (420) is designed to be suitable for separating lubricating oil in a solid phase (or solidified state) as a cryogenic fluid and a fluid whose pressure is reduced by the pressure reducer (600), the boil-off gas separated by the gas/liquid separator (700), and lubricating oil mixed with the liquefied gas separated by the gas/liquid separator (700) in a solid phase (or solidified state) lower than the flow point.

Fig. 5 is an enlarged view of a second oil filter according to one embodiment of the present invention, and fig. 6 is an enlarged view of a second oil filter according to another embodiment of the present invention.

Referring to fig. 5 and 6, the second oil filter (420) may have a structure as shown in fig. 5 (hereinafter, referred to as 'drain-down type') or a structure as shown in fig. 6 (hereinafter, referred to as 'drain-up type'). In fig. 5 and 6, the dotted line indicates the fluid flow direction.

Referring to fig. 5 and 6, the second oil filter (420) includes a fixing plate (425) and a filter element (421), and is connected to an inflow pipe (422), an exhaust pipe (423), and an oil discharge pipe (424).

A filter element (421) is provided to the fixing plate (425) to separate lubricating oil from the fluid flowing through the inflow pipe (422).

Fig. 13 is a plan view of the filter element (421) shown in fig. 5 and 6. Referring to fig. 13, the filter element (421) may have a hollow (Z space in fig. 13) cylindrical shape in which a plurality of layers having different meshes are stacked on each other. The lubricating oil is filtered out of the fluid flowing into the second oil filter (420) via the inflow pipe (422) while the fluid passes through the layers of the filter element (421). The filter element (421) may separate the lube oil by physical adsorption methods.

The fluid (evaporated gas, liquefied gas, or fluid that is a vapor-liquid mixture) filtered by the filter element (421) is discharged through the discharge pipe (423), and the lubricating oil filtered by the filter element (421) is discharged through the oil discharge pipe (424).

The components of the second oil filter (420) are formed of a material capable of withstanding cryogenic conditions in order to separate lubricating oil from fluids having very low temperatures. The filter element (421) may be formed of Metal (Metal) capable of withstanding low temperature conditions, particularly stainless steel (SUS).

Referring to fig. 5, in the downward discharge type oil filter, fluid supplied through an inflow pipe (422) connected to an upper portion of the oil filter passes through a filter element (421) and a space (X in fig. 5) defined below a fixing plate (425), and then is discharged through a discharge pipe (423) connected to a lower portion of the oil filter.

In a downward discharge type oil filter, a fixing plate (425) is connected to a lower portion of the oil filter, a filter element (421) is disposed on an upper surface of the fixing plate (425), and a discharge pipe (423) is connected to a side of the oil filter opposite to the filter element (421) with respect to the fixing plate (425).

Further, in the drain-down type oil filter, the inflow pipe (422) is preferably connected to an oil filter that will be disposed above the upper end of the filter element (421), so as to allow the fluid that flows into the oil filter via the inflow pipe (422) to be filtered even by the upper portion of the filter element (421) (i.e., so as to use the filter element as much as possible).

It is desirable that the inflow pipe (422) and the discharge pipe (423) are provided on opposite sides (on the left and right sides with respect to the filter element (421) in fig. 5) in terms of fluid flow, and it is desirable that the discharge pipe (424) is connected to the lower portion of the filter element (421) because the lubricating oil filtered by the filter element (421) is collected at the lower side of the oil filter.

In a drain-down type oil filter, an oil drain pipe (424) may be connected to the oil filter to be disposed immediately above a fixing plate (425).

As shown in fig. 5(a), when a fluid mainly composed of liquid components (for example, 90 vol% of liquid and 10 vol% of gas) is supplied to the drain-down oil filter, the fluid flows downward due to the high density of the liquid components, thereby ensuring a good filtering effect.

On the other hand, as shown in fig. 5(b), when a fluid composed of gaseous components (for example, 10% by volume of liquid and 90% by volume of gas) is supplied to the downward discharge type oil filter, the gaseous components having a small density stay in an upper portion of the oil filter, thereby deteriorating the fluid flow and the filtering effect.

Referring to fig. 6, in the upward drain type oil filter, fluid supplied through an inflow pipe (422) connected to a lower portion of the oil filter passes through a filter element (421) and a space (Y in fig. 6) defined above a fixing plate (425), and then is discharged through a discharge pipe (423) connected to an upper portion of the oil filter.

In the oil filter of the upward discharge type, a fixing plate (425) is connected to an upper portion of the oil filter, a filter element (421) is provided on a lower surface of the fixing plate (425), and a discharge pipe (423) is connected to a side of the oil filter opposite to the filter element (421) with respect to the fixing plate (425).

Further, in the oil filter of the upward drain type, the inflow pipe (422) is preferably connected to an oil filter to be disposed below a lower end of the filter element (421) in order to allow the fluid flowing into the oil filter via the inflow pipe (422) to be filtered even by a lower portion of the filter element (421) (i.e., in order to use the filter element as much as possible).

It is desirable that the inflow pipe (422) and the discharge pipe (423) are provided on opposite sides (on the left and right sides with respect to the filter element (421) in fig. 6) in terms of fluid flow, and it is desirable that the discharge pipe (424) is connected to the lower portion of the filter element (421) because the lubricating oil filtered by the filter element (421) is collected at the lower side of the oil filter.

Referring to fig. 6, in the upward drainage type oil filter, fluid supplied to the oil filter via an inflow pipe (422) connected to a lower portion of the oil filter passes through a filter element (421), and is drained via a drainage pipe (423) connected to an upper portion of the oil filter. The lubricating oil filtered by the filter element (421) is discharged via a separate pipe (424).

As shown in fig. 6(a), when a fluid mainly composed of gaseous components (for example, 10% by volume of liquid and 90% by volume of gas) is supplied to the upward-discharge type oil filter, the fluid generates an upward flow due to the low density of the gaseous components, thereby providing a suitable upward flow while ensuring a good filtering effect.

On the other hand, as shown in fig. 6(b), when a fluid composed of liquid components (e.g., 90% by volume of liquid and 10% by volume of gas) is supplied to the upward-discharge type oil filter, the liquid components having a high density stay in the lower portion of the oil filter, thereby deteriorating the fluid flow and the filtering effect.

Therefore, in a structure in which the second oil filter (420) is provided at the position B shown in fig. 4, it is desirable that the drain-down oil filter as shown in fig. 5 is used as the second oil filter (420), and when the second oil filter (420) is provided at the position C shown in fig. 4, it is desirable that the drain-up oil filter as shown in fig. 6 is used as the second oil filter (420).

In the structure in which the second oil filter (420) is provided at the position a in fig. 4, the fluid whose pressure is reduced by the pressure reducer (600) is a gas-liquid mixture (theoretically (100)% re-liquefaction is possible) in which the volume ratio of the gas component is higher than that of the liquid component. Therefore, it is desirable that the upward draining type oil filter as shown in the drawing is used as the second oil filter (420).

According to an embodiment, the Bypass Line (BL) branches from the first supply line (L1) upstream of the heat exchanger (100) to bypass the heat exchanger (100) and joins to the second supply line (L2) downstream of the heat exchanger (100).

Typically, a bypass line bypassing the heat exchanger is provided inside the heat exchanger to be integrated with the heat exchanger. In a structure in which the bypass line is provided inside the heat exchanger, fluid may not be supplied to the heat exchanger and the bypass line when a valve provided upstream and/or downstream of the heat exchanger is closed.

In an embodiment of the present invention, the Bypass Line (BL) is provided outside the heat exchanger (100) to be separated from the heat exchanger (100) and branched from the first supply line (L1) upstream of the first valve (510) and joined to the second supply line (L2) downstream of the second valve (520), so that the boil-off gas can be sent to the Bypass Line (BL) even when the first valve (510) upstream of the heat exchanger (100) and/or the second valve (520) downstream of the heat exchanger (100) is closed.

A bypass valve (590) is disposed on the Bypass Line (BL) and is opened when use of the Bypass Line (BL) is desired.

Fundamentally, the Bypass Line (BL) will be used when the heat exchanger (100) may be out of service, for example, when the heat exchanger (100) is malfunctioning or being repaired. For example, if the heat exchanger (100) may not be used when the boil-off gas reliquefaction system according to this embodiment sends part or all of the boil-off gas compressed by the compressor (200) to the high-pressure engine, the boil-off gas discharged from the storage tank (T) is directly sent to the compressor (200) bypassing the heat exchanger (100) along the Bypass Line (BL) without reliquefying the excess boil-off gas that is not used by the high-pressure engine, and the boil-off gas compressed by the compressor (200) is supplied to the high-pressure engine while sending the excess boil-off gas to the GCU to burn the excess boil-off gas.

When the Bypass Line (BL) is used to service the heat exchanger (100), for example, when the fluid passage of the heat exchanger (100) is blocked by condensed or solidified lubricating oil, the condensed or solidified lubricating oil may be removed via the Bypass Line (BL).

In addition, if there is no need to re-liquefy boil-off gas due to the presence of a small amount of excess boil-off gas in the ballasted condition (ballast condition) of the ship, all of the boil-off gas in the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL) to allow all of the boil-off gas to be directly sent to the compressor (200) while bypassing the heat exchanger (100). The boil-off gas compressed by the compressor (200) is used as fuel for a high-pressure engine. If it is determined that there is no need to re-liquefy boil-off gas due to the presence of a small amount of excess boil-off gas, the bypass valve (590) may be controlled to automatically open.

The inventors of the present invention have found that, when boil-off gas is supplied to an engine via a heat exchanger having narrow fluid passages according to an embodiment, the boil-off gas is subjected to a severe pressure drop (pressure drop) due to the heat exchanger. As described above, if it is not necessary to re-liquefy the boil-off gas, the fuel can be smoothly supplied to the engine by compressing the boil-off gas while bypassing the heat exchanger.

In addition, since the amount of boil-off gas that is not reliquefied increases, the Bypass Line (BL) can also be used to reliquefy the boil-off gas.

When it is necessary to reliquefy the boil-off gas due to an increase in the amount of the boil-off gas (i.e., at the start or restart of the reliquefaction of the boil-off gas), the owner of the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL) to allow all of the boil-off gas in the boil-off gas to be directly sent to the compressor (200) while bypassing the heat exchanger (100), and the boil-off gas compressed by the compressor (200) may be sent to the hot fluid passage of the heat exchanger (100). Some of the boil-off gas compressed by the compressor (200) may be supplied to the high-pressure engine.

When the temperature of the hot fluid path of the heat exchanger (100) is increased by the aforementioned process at the beginning or at the beginning of the boil-off gas reliquefaction, an advantage is that the boil-off gas reliquefaction may begin after removing any condensed or solidified lube oil, other residue or impurities that may remain in the heat exchanger (100), other equipment, pipes, etc. in a previous boil-off gas reliquefaction process.

The residue may include boil-off gas compressed by the compressor (200) and then supplied to the heat exchanger in a previous boil-off gas liquefaction and lubricating oil mixed with the boil-off gas compressed by the compressor (200).

If the cold evaporation gas discharged from the storage tank (T) is directly supplied to the heat exchanger (100) without increasing the temperature of the heat exchanger (100) through the Bypass Line (BL) at the start or restart of the re-liquefaction of the evaporation gas, the cold evaporation gas discharged from the storage tank (T) is sent to the cold fluid passage of the heat exchanger (100) in a state where the hot evaporation gas is not sent to the hot fluid passage of the heat exchanger (100). Therefore, when the temperature of the heat exchanger (100) is lowered, the lubricating oil in the heat exchanger (100) which is kept in a non-condensed or non-solidified state can also be condensed or solidified.

When the Bypass Line (BL) is used to increase the temperature of the heat exchanger (100) over a period of time (which may be determined by one skilled in the art if it is determined that the condensed or solidified lube oil or other impurities are almost completely removed, and may be from about 1 minute to about 30 minutes, preferably from about 3 minutes to about 10 minutes, and more preferably from about 2 minutes to about 5 minutes), boil-off gas reliquefaction begins by slowly opening the first valve (510) and the second valve (520), while slowly closing the bypass valve (590). Over time, the first valve (510) and the second valve (520) are fully opened and the bypass valve (590) is fully closed to allow all of the boil-off gas discharged from the storage tank (T) to be used as refrigerant to re-liquefy the boil-off gas in the heat exchanger (100).

In addition, the Bypass Line (BL) may be used to satisfy an intake pressure condition of the compressor (200) when the internal pressure of the storage tank (T) is low.

Further, if it is necessary to control the internal pressure of the tank (T) to a low pressure, the Bypass Line (BL) can be used to satisfy the intake pressure condition of the compressor (200) even if the internal pressure of the tank (T) is reduced.

The following description will be focused on the case of removing the condensed or solidified lubricating oil using the Bypass Line (BL) and the case of satisfying the intake pressure condition of the compressor (200) using the Bypass Line (BL) when the internal pressure of the storage tank (T) is low.

1. Removing condensed or solidified lube oil using a Bypass Line (BL)

The inventors of the present invention found that, since a certain amount of lubricating oil is mixed with the evaporation gas having passed through the oil-lubricated cylinder of the compressor (200), and the lubricating oil contained in the evaporation gas is condensed or solidified in the heat exchanger (100) before the evaporation gas and accumulated in the heat exchanger (100), the condensed or solidified lubricating oil needs to be removed from the heat exchanger (100) after a predetermined period of time because the amount of condensed or solidified lubricating oil accumulated in the heat exchanger (100) increases.

In particular, although it is desirable that the Heat Exchanger (100) according to this embodiment is a Printed Circuit Heat Exchanger (PCHE), also called a diffusion bonded compact Heat Exchanger (DCHE), in consideration of the pressure and/or flow rate of the boil-off gas to be reliquefied, reliquefaction efficiency, and the like, the PCHE has narrow serpentine fluid channels (microchannel-type fluid channels), and thus has problems such as the fluid channels being easily clogged with condensed or solidified lubricating oil, the condensed or solidified lubricating oil being easily accumulated at the serpentine portions of the fluid channels, and the like. Pche (dche) is manufactured by Kobelko co., Ltd., afalaval co., LTd, etc., by kakko steel manufacturing co.

The condensed or solidified lubricating oil can be removed by:

1) determining whether it is time to remove the condensed or solidified lubricant;

2) opening the bypass valve (590) while closing the first valve (510) and the second valve (520);

3) compressing the boil-off gas discharged from the storage tank (T) and having passed through the Bypass Line (BL) by a compressor (200);

4) sending part or all of the hot boil-off gas compressed by the compressor (200) to the heat exchanger (100);

5) sending the boil-off gas having passed through the heat exchanger (100) to a gas/liquid separator (700);

6) discharging the lubricating oil from the gas/liquid separator (700); and

7) it is judged whether or not the heat exchanger (100) is normalized.

1) Determining whether it is time to remove the condensed or solidified lubricant

When the fluid passage of the heat exchanger (100) is clogged with the condensed or solidified lubricating oil, the cooling efficiency of the heat exchanger (100) may be reduced. Therefore, if the performance of the heat exchanger (100) drops below a preset value of normal performance, it can be estimated that condensed or solidified lubricating oil accumulates in the heat exchanger (100) in a certain amount or more. For example, if the performance of the heat exchanger (100) drops to about 50% to about 90%, preferably about 60% to about 80%, more preferably about 70% or less than 70% of normal performance, it may be determined when to remove condensed or solidified lubricating oil from the heat exchanger (100).

Herein, the range of "about 50% to about 90%" of normal performance includes all of the values of about 50% or less than 50%, about 60% or less than 60%, about 70% or less than 70%, about 80% or less than 80%, and about 90% or less than 90%, and the range of "about 60% to about 80%" of normal performance includes all of the values of about 60% or less than 60%, about 70% or less than 70%, and about 80% or less than 80%.

When the performance of the heat exchanger (100) is deteriorated, the temperature difference between the cold evaporation gas (L1) supplied to the heat exchanger (100) and the cold evaporation gas (L4) discharged from the heat exchanger (100) is increased, and the temperature difference between the hot evaporation gas (L2) discharged from the heat exchanger (100) and the hot evaporation gas (L3) supplied to the heat exchanger (100) is also increased. In addition, when the fluid passage of the heat exchanger (100) is clogged with the condensed or solidified lubricating oil, the fluid passage of the heat exchanger (100) is narrowed, so that the pressure difference between the front end (L3) and the rear end (L4) of the heat exchanger (100) is increased.

Therefore, based on the temperature difference (810, 840) of the cold fluid supplied to or discharged from the heat exchanger (100), the temperature difference (820, 830) of the hot fluid supplied to or discharged from the heat exchanger (100), and the pressure difference (910, 920) of the hot fluid passage of the heat exchanger (100), it can be judged whether it is time to remove the condensed or solidified lubricating oil.

Specifically, if the temperature difference (meaning the absolute value, hereinafter referred to as "cold flow temperature difference") between the temperature of the evaporation gas discharged from the storage tank (T) and supplied to the heat exchanger (100) as measured by the first temperature sensor (810) and the temperature of the evaporation gas compressed by the compressor (200) and cooled by the heat exchanger (100) as measured by the fourth temperature sensor (840) is higher than the normal temperature difference for a certain period of time or longer, it may be determined that the heat exchange is normally performed in the heat exchanger (100).

For example, when a state in which the cold flow temperature difference is 20 ℃ to 50 ℃ or more, preferably 30 ℃ to 40 ℃ or more, more preferably about 35 ℃ or more than 35 ℃ lasts for 1 hour or more, it can be determined that it is time to discharge the condensed or solidified lubricating oil.

When the heat exchanger (100) is normally operated, the boil-off gas compressed to about 300 bar by the compressor (200) has a temperature of about 40 ℃ to about 45 ℃, and the boil-off gas discharged from the storage tank (T) and having a temperature of about-160 ℃ to about-140 ℃ is supplied to the heat exchanger (100). Here, the temperature of the boil-off gas discharged from the storage tank (T) is increased to about-150 ℃ to about-110 ℃, preferably about-120 ℃ during the delivery to the heat exchanger (100).

In the boil-off gas reliquefaction system including the gas/liquid separator (700) according to this embodiment, when the gaseous boil-off gas separated by the gas/liquid separator (700) is combined with the boil-off gas discharged from the storage tank (T) and then supplied to the heat exchanger (100), the temperature of the boil-off gas finally supplied to the heat exchanger (100) is lower than the temperature of the boil-off gas discharged from the storage tank (T) to the heat exchanger (100), and the temperature of the boil-off gas supplied to the heat exchanger (100) may further decrease as the amount of the gaseous boil-off gas separated by the gas/liquid separator (700) increases.

The boil-off gas supplied to the heat exchanger (100) along the third supply line (L3) and having a temperature of about 40 ℃ to 45 ℃ is cooled by the heat exchanger (100) to about-130 ℃ to about-110 ℃, and the cold flow temperature difference is preferably about 2 ℃ to about 3 ℃ in a normal state.

In addition, if the temperature difference (meaning an absolute value, hereinafter referred to as "heat flow temperature difference") between the temperature of the evaporation gas discharged from the storage tank (T) and used as the refrigerant by the heat exchanger (100) as measured by the second temperature sensor (820) and the temperature of the evaporation gas compressed by the compressor (200) and supplied to the heat exchanger (100) as measured by the third temperature sensor (830) is higher than a normal temperature difference for a certain period of time or longer, it may be determined that the heat exchange is normally performed in the heat exchanger (100).

It can be determined when the condensed or solidified lubricating oil is discharged when a state in which the temperature difference of the heat flow is 20 ℃ to 50 ℃ or more, preferably 30 ℃ to 40 ℃ or more, more preferably about 35 ℃ or more, 40 ℃ or more lasts for 1 hour or more than 1 hour.

When the heat exchanger (100) is normally operated, the evaporation gas discharged from the storage tank (T) and having a slightly increased temperature of about-150 ℃ to about-110 ℃ (preferably about-120 ℃) during delivery to the heat exchanger (100) may have a temperature of about-80 ℃ to 40 ℃ after being used as a refrigerant in the heat exchanger (100), depending on the speed of the ship, while the evaporation gas used as a refrigerant in the heat exchanger (100) and having a temperature of about-80 ℃ to 40 ℃ is compressed to have a temperature of about 40 ℃ to about 45 ℃ by the compressor (200).

Further, if a pressure difference between the pressure of the evaporation gas compressed by the compressor (200) and supplied to the heat exchanger (100) as measured by the first pressure sensor (910) and the temperature of the evaporation gas cooled by the heat exchanger (100) as measured by the second pressure sensor (920) (hereinafter, referred to as "hot fluid channel pressure difference") is higher than a normal pressure difference for a certain period of time or longer, it may be determined that the heat exchanger (100) is abnormally operated.

Since the evaporation gas discharged from the storage tank (T) is not mixed with oil or has a trace amount of oil, and the point of time when the lubrication oil is mixed with the evaporation gas is when the evaporation gas is compressed by the compressor (200), the condensed or solidified lubrication oil is not substantially accumulated in the cold fluid passage of the heat exchanger (100) using the evaporation gas discharged from the storage tank (T) as a refrigerant, and then the evaporation gas is supplied to the compressor (200) and accumulated in the hot fluid passage of the heat exchanger (100), in which the evaporation gas compressed by the compressor (200) is cooled and supplied to the pressure reducer (600).

Therefore, since the pressure difference between the front and rear ends of the heat exchanger (100) rapidly increases in the hot fluid passage due to the condensed or solidified lubricating oil blocking the fluid passage, it is judged whether it is time to remove the condensed or solidified lubricating oil by measuring the pressure of the hot fluid passage of the heat exchanger (100).

Considering that a PCHE having a narrow and serpentine fluid passage may be used as a heat exchanger according to this embodiment, a determination of whether it is time to remove condensed or solidified lubricant based on a pressure difference between the front and rear ends of the heat exchanger (100) may be advantageously used.

For example, when the pressure difference of the hot fluid passage is two or more times its normal pressure difference for 1 hour or more than 1 hour, it can be determined when the condensed or solidified lubricating oil is discharged.

When the heat exchanger (100) is operating normally, the boil-off gas compressed by the compressor (200) experiences a pressure drop of about 0.5 bar to about 2.5 bar, preferably about 0.7 bar to about 1.5 bar, more preferably about 1 bar, even when the boil-off gas is cooled while passing through the heat exchanger (100), the boil-off gas does not suffer a significant pressure drop. In the case where the pressure difference of the hot fluid passage is at least a predetermined pressure or more, for example, 1 to 5 bar or more, preferably 1.5 to 3 bar or more, more preferably about 2 bar (200 kilopascal (kPa)) or more than 2 bar, the state where the pressure difference of the hot fluid passage is at least a predetermined pressure or more, it is determined that it is time to discharge the condensed or solidified lubricating oil.

Although the time point at which the condensed or solidified lubricating oil is removed may be determined based on any one of the cold flow temperature difference, the hot flow temperature difference, and the hot fluid channel pressure difference, as described above, to improve reliability, the time point at which the condensed or solidified lubricating oil is removed may be determined based on at least two of the cold flow temperature difference, the hot flow temperature difference, and the hot fluid channel pressure difference.

For example, when the lower value between the cold flow temperature differential and the hot flow temperature differential is maintained at 35 ℃ or greater than 35 ℃ for 1 hour or greater than 1 hour when the pressure differential of the hot fluid passage is two or more times its normal pressure differential or is 200kPa or greater than 200kPa and lasts for 1 hour or greater than 1 hour, it can be determined when to remove condensed or solidified lube oil.

The first temperature sensor (810), the second temperature sensor (820), the third temperature sensor (830), the fourth temperature sensor (840), the first pressure sensor (910), and the second pressure sensor (920) may be regarded as detection means for detecting whether or not the heat exchanger (100) is clogged with the lubricating oil.

In addition, the boil-off gas reliquefaction system according to the embodiment of the present invention may further include a controller (not shown) to determine whether the heat exchanger (100) is clogged with the lubricating oil based on a detection result obtained by at least one of the first temperature sensor (810), the second temperature sensor (820), the third temperature sensor (830), the fourth temperature sensor (840), the first pressure sensor (910), and the second pressure sensor (920). The controller may be regarded as a judgment means for judging whether or not the heat exchanger (100) is clogged with the lubricating oil.

2) Opening the bypass valve (590) while closing the first valve (510) and the second valve (520)

If it is determined in step 1 that it is time to remove the condensed or solidified lubricating oil from the heat exchanger (100), the bypass valve (590) provided on the Bypass Line (BL) is opened, and the first valve (510) provided on the first supply line (L1) and the second valve (520) provided on the second supply line (L2) are closed.

When the bypass valve (590) is opened while the first valve (510) and the second valve (520) are closed, the boil-off gas discharged from the storage tank (T) is sent to the compressor (200) via the Bypass Line (BL), and is prevented from being supplied to the heat exchanger (100). Therefore, the refrigerant is not supplied to the heat exchanger (100).

3) A step of compressing the boil-off gas discharged from the storage tank (T) and having passed through the Bypass Line (BL) by a compressor (200)

The boil-off gas discharged from the storage tank (T) bypasses the heat exchanger (100) via a Bypass Line (BL) and is then sent to the compressor (200). The boil-off gas sent to the compressor (200) undergoes an increase in temperature and pressure while being compressed by the compressor (200). The boil-off gas compressed to about 300 bar by the compressor (200) has a temperature of about 40 ℃ to about 45 ℃.

4) A step of sending part or all of the hot boil-off gas compressed by the compressor (200) to the heat exchanger (100)

When the evaporation gas compressed by the compressor (200) is continuously supplied to the heat exchanger (100), the cold evaporation gas used as a refrigerant in the heat exchanger (100) and discharged from the storage tank (T) is not supplied to the heat exchanger (100), and the hot evaporation gas is continuously supplied to the heat exchanger (100), thereby gradually increasing the temperature of the hot fluid path of the heat exchanger (100) through which the evaporation gas compressed by the compressor (200) passes.

When the temperature of the hot fluid passage of the heat exchanger (100) exceeds the condensation or solidification point of the lubricating oil, the condensed or solidified lubricating oil accumulated in the heat exchanger (100) gradually melts or decreases in viscosity, and then the melted or low-viscosity lubricating oil is mixed with the evaporation gas and exits the heat exchanger (100).

When the condensed or solidified lubricating oil is removed using the Bypass Line (BL), the boil-off gas circulates through the Bypass Line (BL), the compressor (200), the hot fluid passage of the heat exchanger (100), the pressure reducer (600), and the gas/liquid separator (700) until the heat exchanger (100) is normalized.

In addition, when condensate or solidified lubricating oil is removed using the Bypass Line (BL), boil-off gas discharged from the storage tank (T) and passing through the Bypass Line (BL), the compressor (200), the hot fluid passage of the heat exchanger (100), and the pressure reducer (600) may be sent to a separate tank or another collection facility separate from the storage tank (T) so that the boil-off gas is mixed with the melted or reduced-viscosity lubricating oil. Boil-off gas stored in a separate tank or another collection facility is sent to a Bypass Line (BL) to continue the process of removing condensed or solidified lube oil.

Even in a structure in which the gas/liquid separator (700) is disposed downstream of the pressure reducer (600), when a fluid composed of boil-off gas mixed with melted or viscosity-reduced lubricating oil is sent to a separate tank or other collection facility, the gas/liquid separator (700) provides the same function as that of a typical boil-off gas reliquefaction system, and the melted or viscosity-reduced lubricating oil is not collected in the gas/liquid separator (700) (the melted or viscosity-reduced lubricating oil is collected by the separate tank or other collection facility separate from the storage tank (T)). Therefore, the boil-off gas reliquefaction system according to this embodiment can omit the gas/liquid separator configured to discharge the lubricating oil, thereby enabling cost reduction.

5) A step of sending the boil-off gas having passed through the heat exchanger (100) to a gas/liquid separator (700)

As the temperature of the hot fluid passage of the heat exchanger (100) increases, the condensed or solidified lube oil accumulated in the heat exchanger (100) is gradually melted or reduced in viscosity, and then sent to the gas/liquid separator (700) after being mixed with the boil-off gas. In the process of removing the condensed or solidified lubricating oil in the heat exchanger (100) via the Bypass Line (BL), since the boil-off gas is not re-liquefied, the gas that is not re-liquefied is not collected in the gas/liquid separator (700), but the boil-off gas and the lubricating oil that is melted or has low viscosity are collected.

The gaseous boil-off gas collected in the gas/liquid separator (700) is discharged from the gas/liquid separator (700) along the sixth feed line (L6) and sent to the compressor (200) along the Bypass Line (BL). Since the first valve (510) is closed in step 2, the gaseous boil-off gas separated by the gas/liquid separator (700) is combined with the boil-off gas discharged from the storage tank (T) and sent to the compressor (200) along the Bypass Line (BL) without being sent to the cold fluid passage of the heat exchanger (100).

Supplying the gaseous boil-off gas separated by the gas/liquid separator (700) to the Bypass Line (BL) with the first valve (510) in the closed state prevents lubricating oil contained in the boil-off gas from being supplied to the heat exchanger (100), thereby preventing the cold fluid passage of the heat exchanger (100) from being blocked.

The cycle in which the gaseous boil-off gas collected in the gas/liquid separator (700) is discharged from the gas/liquid separator (700) along the sixth feed line (L6) and then returned to the compressor (200) along the Bypass Line (BL) continues until it is determined that the temperature of the hot fluid passage of the heat exchanger (100) is increased to the temperature of the boil-off gas compressed by the compressor (200) and sent to the hot fluid passage of the heat exchanger (100). However, the looping process may continue until it is empirically determined that sufficient time has elapsed.

During the removal of condensed or solidified lubricating oil from the heat exchanger (100) using the Bypass Line (BL), the eighth valve (581) is closed to prevent the lubricating oil collected in the gas/liquid separator (700) from flowing along the fifth supply line (L5) to the storage tank (T). If the lubricating oil is introduced into the storage tank (T), the purity of the liquefied gas stored in the storage tank (T) may be deteriorated, thereby deteriorating the value of the liquefied gas.

6) Step of discharging lubricating oil from gas/liquid separator (700)

The melted or viscosity-reduced lubricating oil discharged from the heat exchanger (100) is collected in a gas/liquid separator (700). To process the lubricating oil collected in the gas/liquid separator (700), the boil-off gas reliquefaction system according to this embodiment may employ the gas/liquid separator (700) obtained by modifying a typical gas/liquid separator.

FIG. 10 is an enlarged view of a heat exchanger and gas/liquid separator according to one embodiment of the invention. In fig. 10, some components are omitted for convenience of explanation.

Referring to fig. 10, the gas/liquid separator (700) is provided with a lube oil discharge line (OL) through which lube oil collected in the gas/liquid separator (700) is discharged, and a fifth supply line (L5), and the liquefied gas separated by the gas/liquid separator (700) is sent to the storage tank (T) through the fifth supply line (L5). To enable the lube oil collected at the lower portion of the gas/liquid separator (700) to be discharged efficiently, a lube oil discharge line (OL) is connected to the lower end of the gas/liquid separator (700), and in the gas/liquid separator (700) connected to the lube oil discharge line (OL), one end of a fifth supply line (L5) is disposed above the lower end of the gas/liquid separator (700). To prevent the fifth supply line (L5) from being clogged with the lubricating oil, it is desirable that the end of the fifth supply line (L5) be disposed above the level of the lubricating oil when the amount of lubricating oil collected in the gas/liquid separator (700) reaches a maximum.

A third valve (530) for adjusting the flow rate of fluid of the corresponding line and opening/closing may be provided on the lube oil drain line (OL), and may be provided in plurality.

Since the lubricating oil collected in the gas/liquid separator (700) may be naturally discharged or may take a long time to be discharged, the lubricating oil in the gas/liquid separator (700) may be discharged by nitrogen purging (nitrogen purging). When nitrogen is supplied to the gas/liquid separator (700) at a pressure of about 5 to 7 bar, the internal pressure of the gas/liquid separator (700) increases, and the lubricating oil is allowed to be quickly discharged.

To discharge the lubricating oil from the gas/liquid separator (700) by nitrogen purging, a nitrogen supply line (NL) may be provided joined to the third supply line (L3) upstream of the heat exchanger (100). A number of nitrogen feed lines may be provided at different locations as desired.

A nitrogen gas valve (583) for adjusting the flow rate of fluid of the corresponding line and opening/closing may be provided on the nitrogen gas supply line (NL), and the nitrogen gas valve (583) is normally maintained in a closed state when the nitrogen gas supply line (NL) is not used. Next, when it is necessary to supply nitrogen to the gas/liquid separator (700) using the Nitrogen Line (NL) for nitrogen purge, the nitrogen valve (583) is opened. The nitrogen gas valve (583) may be provided in plurality.

Although the discharge of the lube oil may be performed by nitrogen purge directly injecting nitrogen into the gas/liquid separator (700), if a nitrogen supply line for other purposes is already installed, it is desirable to discharge the lube oil from the gas/liquid separator (700) using another installed nitrogen supply line, which may be previously provided for other purposes.

After a process of sending all of the evaporation gas discharged from the storage tank (T) to the Bypass Line (BL) to be compressed by the compressor (200), sending the evaporation gas compressed by the compressor (200) to the hot fluid passage of the heat exchanger (100), sending the evaporation gas passing through the heat exchanger (100) and reduced in pressure in the pressure reducer (600) to the gas/liquid separator (700), and sending the evaporation gas discharged from the gas/liquid separator (700) to the Bypass Line (BL), if it is determined that most of the condensed or solidified lubricating oil in the heat exchanger (100) is collected in the gas/liquid separator (700) (i.e., if it is determined that the heat exchanger (100) is normalized), nitrogen purging is performed by blocking the evaporation gas compressed by the compressor (200) from flowing into the heat exchanger (100) and opening the nitrogen valve (583).

7) Step for judging whether the heat exchanger (100) is normalized

If it is determined that the heat exchanger (100) is normalized again by discharging the condenser or solidified lubricating oil from the heat exchanger (100), and when the process of discharging the lubricating oil from the gas/liquid separator (700) is completed, the boil-off gas reliquefaction system is normally operated again by opening the first valve (510) and the second valve (520) while closing the bypass valve (590). When the boil-off gas reliquefaction system is normally operated, the boil-off gas discharged from the storage tank (T) is used as a refrigerant in the heat exchanger (100), and part or all of the boil-off gas used as the refrigerant in the heat exchanger (100) is reliquefied by compression by the compressor (200), cooling by the heat exchanger (100), and decompression by the decompressor (600).

The determination of whether the heat exchanger (100) is again normalized is based on at least one of a cold flow temperature differential, a hot flow temperature differential, and a hot fluid passage pressure differential, as is the determination made when it is time to remove condensed or solidified lubricating oil.

Condensed or solidified lubricating oil accumulated in pipes, valves, instruments and other equipment, in addition to condensed or solidified lubricating oil inside the heat exchanger (100), can also be removed by the above-described process.

Conventionally, during the step of removing the condensed or solidified lubricating oil inside the heat exchanger (100) using the Bypass Line (BL), a high-pressure engine and/or a low-pressure engine (hereinafter referred to as 'engine') may be driven. When servicing portions of equipment included in the fuel supply system or the reliquefaction system, the engine is generally in a non-driving state because fuel may not be supplied to the engine or excess boil-off gas may not be reliquefied.

In contrast, if the engine can be driven during the removal of the condensed or solidified lubricating oil from the heat exchanger (100) as in the present invention, since the heat exchanger (100) can be serviced during the operation of the engine, there is an advantage in that the ship can be propelled and power can be generated during the servicing of the heat exchanger (100) and the condensed or solidified lubricating oil can be removed using surplus boil-off gas.

Furthermore, when the engine is driven during removal of condensed or solidified lubricating oil from the heat exchanger (100), there is an advantage in that lubricating oil mixed with the evaporation gas can be burned during compression by the compressor (200). That is, the engine is used not only for the purpose of propelling a ship or generating electricity, but also for removing oil mixed with boil-off gas.

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

Fig. 7 is a schematic view of a boil-off gas reliquefaction system according to a fourth embodiment of the present invention, fig. 8 is an enlarged view of a pressure reducer according to one embodiment of the present invention, and fig. 9 is an enlarged view of a pressure reducer according to another embodiment of the present invention.

Referring to fig. 7, in the present invention, two compressors (200, 210) may be arranged in parallel. The two compressors (200, 210) may have the same specifications and may act as a Redundancy scheme (Redundancy) in case of failure of either of the compressors. For convenience of explanation, illustration of other devices is omitted.

Referring to fig. 7, in the structure in which the compressors (200, 210) are arranged in parallel, the evaporation gas discharged from the storage tank (T) is sent to the second compressor (210) via the seventh supply line (L22), and the evaporation gas compressed by the second compressor (210) is partially discharged to the high-pressure engine via the fuel Supply Line (SL), while the surplus evaporation gas is sent to the heat exchanger (100) via the eighth supply line (L33) to undergo the reliquefaction process. A tenth valve (571) for adjusting the flow rate and opening/closing of the corresponding line may be provided on the eighth supply line (L33).

In other embodiments, the two stress-reducers (600, 610) may be arranged in parallel as shown in fig. 8, and the two pairs of stress-reducers (600, 610) arranged in series may be arranged in parallel as shown in fig. 9.

Referring to fig. 8, two pressure reducers (600, 610) arranged in parallel may be used as a redundancy scheme for preparing for failure of any one of the compressors, and each of the pressure reducers (600, 610) may be provided at front and rear ends thereof with an Isolation (Isolation) valve (620).

Referring to fig. 9, two pairs of reducers (600, 610) connected in series are arranged in parallel. Depending on the manufacturer, two pressure reducers (600) are connected in series to achieve pressure reduction stability. The two pairs of reducers (600, 610) connected in parallel may act as a Redundancy scheme (Redundancy) against failure of either of the reducers.

Each of the parallel-connected pressure reducers (600, 610) may be provided with an isolation valve (620) at front and rear ends thereof. When the pressure reducer (600) is maintained or repaired due to a failure of the pressure reducer (600, 610) or the like, the Isolation valve (620) shown in fig. 8 and 9 isolates (Isolation) the pressure reducer (600).

① alarm activation

In the structure in which the boil-off gas reliquefaction system includes one compressor (200) and one pressure reducer (600) as shown in fig. 2, an alarm is activated under the condition that the degree of opening of the pressure reducer (600) is a preset value or more, the seventh valve (570) and the second valve (520) are opened, and the liquid level of the liquefied gas in the gas/liquid separator (700) is a normal liquid level.

In the structure in which the boil-off gas reliquefaction system includes one compressor (200) as shown in fig. 2 and two pressure reducers (600, 610) connected in parallel as shown in fig. 8 to fig. 8, an alarm is activated under the conditions that the degree of opening of the first pressure reducer (600) or the second pressure reducer (610) is a preset value or more, the seventh valve (570) and the second valve (520) are opened, and the liquid level of the liquefied gas in the gas/liquid separator (700) is a normal level (hereinafter referred to as 'first alarm activation condition').

In the structure in which the boil-off gas reliquefaction system includes one compressor (200) as shown in fig. 2 and two pairs of pressure reducers (600, 610) connected in parallel as shown in fig. 9, an alarm is activated under the condition that the degree of opening of one of the two first pressure reducers (600) arranged in series or one of the two second pressure reducers (610) connected in series is a preset value or more, the seventh valve (570) and the second valve (520) are opened, and the liquid level of the liquefied gas in the gas/liquid separator (700) is a normal liquid level (hereinafter referred to as 'second alarm activation condition').

In the structure in which the boil-off gas reliquefaction system includes two compressors (200, 210) connected in parallel as shown in fig. 7 and one pressure reducer (600) as shown in fig. 2, an alarm is activated under the condition that the degree of opening of the pressure reducer (600) is a preset value or more, the seventh valve (570) or the tenth valve (571) is opened, the second valve (520) is opened, and the level of the liquefied gas in the gas/liquid separator (700) is a normal level (hereinafter referred to as 'third alarm activation condition').

In the structure in which the boil-off gas reliquefaction system includes two compressors (200, 210) connected in parallel as shown in fig. 7 and two pressure reducers (600, 610) connected in parallel as shown in fig. 8, an alarm is activated under the condition that the degree of opening of the first pressure reducer (600) or the second pressure reducer (610) is a preset value or more, the seventh valve (570) or the tenth valve (571) is opened, the second valve (520) is opened, and the liquid level of the liquefied gas in the gas/liquid separator (700) is a normal liquid level (hereinafter referred to as 'fourth alarm activation condition').

In a structure in which the boil-off gas reliquefaction system includes two compressors (200, 210) connected in parallel as shown in fig. 7 and two pairs of pressure reducers (600, 610) connected in parallel as shown in fig. 9, an alarm is activated under a condition that an opening degree of one of two first reduced pressures (600) arranged in series or one of two second reduced pressures (610) connected in series is a preset value or more, a seventh valve (570) or a tenth valve (571) is opened, a second valve (520) is opened, and a liquid level of liquefied gas in the gas/liquid separator (700) is a normal liquid level (hereinafter referred to as 'fifth alarm activation condition').

In the above-described first to fifth alarm activation conditions, the predetermined opening degree of the first pressure reducer (600) or the second pressure reducer (610) may be 2%, and the normal level of the liquefied gas in the gas/liquid separator (700) means a situation in which the normal performance of the reliquefaction process can be determined by confirming the reliquefied gas in the gas/liquid separator (700).

② alarm generation

An alarm may be generated to indicate a point in time for removing condensed or solidified lube oil if any of the following conditions are met: the cold flow temperature difference is a preset value or more and lasts for a preset time period, the hot flow temperature difference is a preset value or more and lasts for a preset time period, and the hot flow channel pressure difference is a preset value or more and lasts for a preset time period.

To improve reliability, an alarm may be generated to indicate the point in time when condensed or solidified lubricant is removed if at least two of the following conditions are met: the cold flow temperature difference is a preset value or more and lasts for a preset time period, the hot flow temperature difference is a preset value or more and lasts for a preset time period, and the hot flow channel pressure difference is a preset value or more and lasts for a preset time period.

Further, if a lower value of the cold and hot flow temperature differences is a preset value or more and continues for a predetermined time period (or condition), or if a pressure difference of the hot fluid passage is a preset value or more and continues for a predetermined time period, an alarm may be generated to indicate a point of time at which condensed or solidified lubricating oil is removed.

According to the present invention, abnormality of the heat exchanger, alarm generation, and the like can be determined by an appropriate controller. As the controller for determining abnormality of the heat exchanger, generation of an alarm, or the like, a controller used by the boil-off gas reliquefaction system according to the present invention, preferably a controller used by a ship or an offshore structure to which the boil-off gas reliquefaction system according to the present invention is applied, may be used, and a separate controller for determining abnormality of the heat exchanger, generation of an alarm, or the like may also be used.

In addition, the use of bypass lines, the draining of lubricating oil, the fueling of the engine, the starting or restarting of the boil-off gas reliquefaction system, and the opening or closing of various valves for these components may be automatically or manually controlled by the controller.

2. A case where the Bypass Line (BL) is used to satisfy the intake pressure condition of the compressor (200) when the internal pressure of the storage tank (T) is low

In a case where the storage tank (T) has a low internal pressure, for example, when the amount of boil-off gas generated is small due to a small amount of liquefied gas in the storage tank (T), or if the amount of boil-off gas supplied to the engine for propelling the ship is large due to a high speed of the ship, the compressor (200) does not normally satisfy the intake pressure condition upstream of the compressor (200).

Specifically, in a PCHE (dche) used as the heat exchanger (100), when the evaporation gas discharged from the storage tank (T) passes through the PCHE, the pressure drop is large because the flow passage of the PCHE is narrow.

Conventionally, when the compressor (200) fails to satisfy the intake pressure condition, the recirculation valves (541, 542, 543, 544) are opened to protect the compressor (200) by recirculating part or all of the boil-off gas via the recirculation lines (RC1, RC2, RC3, RC 4).

However, if the intake pressure condition of the compressor (200) is satisfied by recirculating the boil-off gas, the amount of the boil-off gas compressed by the compressor (200) is reduced, resulting in deterioration of reliquefaction performance and failure to satisfy the fuel consumption demand of the engine. In particular, the operation of the ship may be significantly disturbed if the engine does not meet the fuel consumption requirements. Therefore, there is a need for a boil-off gas reliquefaction method that can satisfy the intake pressure condition of the compressor and the fuel consumption demand of the engine even when the internal pressure of the storage tank (T) is low.

According to the invention, no additional equipment is provided, but the intake pressure condition of the compressor (200) can be satisfied using a Bypass Line (BL) provided for maintenance and repair of the heat exchanger (100) without reducing the amount of boil-off gas compressed by the compressor (100) even when the internal pressure of the storage tank (T) is low. The suction pressure condition required for the compressor (200) can be satisfied without reducing the amount of the evaporated gas.

According to the invention, when the internal pressure of the storage tank (T) drops to a preset value or below, the bypass valve (590) is opened so that part or all of the boil-off gas discharged from the storage tank (T) is sent directly to the compressor (200) bypassing the heat exchanger (100) via the Bypass Line (BL).

The amount of boil-off gas sent to the Bypass Line (BL) can be adjusted depending on the pressure in the storage tank (T) compared to the intake pressure conditions required by the compressor (200). That is, by opening the bypass valve (590) while closing the first valve (510) and the second valve (520), all of the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL), or by partially opening the bypass valve (590), the first valve (510), and the second valve (520), only some of the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL), and the remaining boil-off gas may be sent to the heat exchanger (100). That is, by opening the bypass valve (590) while closing the first valve (510) and the second valve (520), all of the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL), or by partially opening the bypass valve (590), the first valve (510), and the second valve (520), only some of the boil-off gas discharged from the storage tank (T) may be sent to the Bypass Line (BL), and the remaining boil-off gas may be sent to the heat exchanger (100). The pressure drop of the boil-off gas decreases as the amount of the boil-off gas bypassing the heat exchanger (100) via the Bypass Line (BL) increases.

Although there is an advantage in that the pressure drop is minimized when the boil-off gas discharged from the storage tank (T) bypasses the heat exchanger (100) and is directly sent to the compressor (200), the cold heat of the boil-off gas may not be used for the reliquefaction of the boil-off gas. Therefore, the use of the Bypass Line (BL) to reduce the pressure drop and the amount of boil-off gas to be sent to the Bypass Line (BL) out of the amount of boil-off gas discharged from the storage tank (T) are determined based on the internal pressure of the storage tank (T), the fuel consumption demand of the engine, the amount of boil-off gas to be reliquefied, and the like.

For example, it may be determined that it is advantageous to use the Bypass Line (BL) to reduce the pressure drop when the internal pressure of the storage tank (T) is at or below a preset value and the vessel is operating at or above a predetermined speed. In particular, it may be determined that it is advantageous to use the Bypass Line (BL) to reduce the pressure drop when the internal pressure of the storage tank (T) is 1.09 bar or less than 1.09 bar and the speed of the vessel is 17 knots or more than 17 knots.

In addition, even when all of the boil-off gas discharged from the storage tank (T) is sent to the compressor (200) via the Bypass Line (BL), the intake pressure condition of the compressor (200) is not normally satisfied. In this case, the intake pressure condition is satisfied using the recycle line (RC1, RC2, RC3, RC 4).

That is, when the intake pressure condition of the compressor (200) may not be satisfied due to the pressure reduction of the storage tank (T), the compressor (200) is protected using the recirculation line (RC1, RC2, RC3, RC4) in the related art, however, according to the present invention, the Bypass Line (BL) is mainly used to satisfy the intake pressure condition of the compressor (200), and even by sending all of the evaporation gas discharged from the storage tank (T) to the compressor via the Bypass Line (BL), the recirculation line (RC1, RC2, RC3, RC4) is secondarily used when the intake pressure condition of the compressor (200) may not be satisfied.

In order to satisfy the intake pressure condition of the compressor (200) by using the Bypass Line (BL) mainly and the recirculation lines (RC1, RC2, RC3, RC4) secondarily, the pressure condition for opening the bypass valve (590) is set to a value higher than the pressure condition for opening the recirculation valves (541, 542, 543, 544).

The conditions for opening the recirculation valves (541, 542, 543, 544) and the conditions for opening the bypass valve (590) are preferably determined based on the pressure upstream of the compressor (200). Alternatively, these conditions may be determined based on the internal pressure of the storage tank (T).

The pressure upstream of the compressor (200) may be measured by a third pressure sensor (not shown) disposed upstream of the compressor (200), and the internal pressure of the storage tank (T) may be measured by a fourth pressure sensor (not shown).

On the other hand, in a structure in which a sixth supply line (L6) for discharging the gaseous evaporation gas separated by the gas/liquid separator (700) is joined to the first supply line (L1) at a position downstream of a branch point of a Bypass Line (BL) branching from the first supply line (L1), some of the evaporation gas discharged from the storage tank (T) may be used as refrigerant in the heat exchanger (100) while preventing a pressure drop, by directly sending the gaseous evaporation gas separated by the gas/liquid separator (700) to the Bypass Line (BL) with all of the bypass valve (590), the first valve (510), and the second valve (520) opened in system operation.

Since the temperature of the gaseous boil-off gas separated by the gas/liquid separator (700) is lower than the temperature of the boil-off gas discharged from the storage tank (T) and supplied to the heat exchanger (100), and when the gaseous boil-off gas separated by the gas/liquid separator (700) is directly sent to the Bypass Line (BL), the cooling efficiency of the heat exchanger (100) may deteriorate, it is desirable that at least some of the gaseous boil-off gas separated by the gas/liquid separator (700) is sent to the heat exchanger (100).

Here, if the amount of boil-off gas generated in the storage tank (T) is smaller than the amount of boil-off gas required as fuel by the engine, it may not be necessary to re-liquefy the boil-off gas. However, when it is not necessary to re-liquefy the evaporation gas, since it is unnecessary to supply the refrigerant to the heat exchanger (100), all of the gaseous evaporation gas separated by the gas/liquid separator (700) may be sent to the Bypass Line (BL).

Therefore, in the present invention, the sixth supply line (L6) is joined to the first supply line (L1) at a position upstream of the branch point of the Bypass Line (BL) branching from the first supply line (L1). In the structure in which the sixth supply line (L6) is joined to the first supply line (L1) upstream of the branch point of the bypass line, the boil-off gas discharged from the storage tank (T) and the gaseous boil-off gas separated by the gas/liquid separator (700) are combined with each other at a position upstream of the branch point of the Bypass Line (BL), and then the amount of the boil-off gas to be sent to the Bypass Line (BL) and the heat exchanger (100) is determined depending on the degree of opening of the bypass valve (590) and the first valve (510), thereby making it possible to easily control the system and prevent the gaseous boil-off gas separated by the gas/liquid separator (700) from being sent directly to the Bypass Line (BL).

Preferably, the bypass valve (590) is a valve providing a higher response than a typical valve, so that the degree of opening can be quickly adjusted depending on the pressure change of the storage tank (T).

Fig. 3 is a schematic diagram of a boil-off gas reliquefaction system according to a third embodiment of the present invention.

Referring to fig. 3, the boil-off gas reliquefaction system according to the third embodiment of the present invention is different from the boil-off gas reliquefaction system according to the first embodiment shown in fig. 1 in that the boil-off gas reliquefaction system according to the third embodiment includes a differential pressure sensor (930) instead of the first pressure sensor (910) and the second pressure sensor (920), and the following description will focus on different features of the boil-off gas reliquefaction system according to the third embodiment. The description of the same components as those of the boil-off gas reliquefaction system according to the first embodiment will be omitted.

Unlike the first embodiment, the boil-off gas reliquefaction system according to the third embodiment includes a differential pressure sensor (930), and the differential pressure sensor (930) measures a differential pressure between the third supply line (L3) upstream of the heat exchanger (100) and the fourth supply line (L4) downstream of the heat exchanger (100), instead of the differential pressure between the first pressure sensor (910) and the second pressure sensor (920).

The pressure differential of the hot fluid channel may be obtained by a differential pressure sensor (930), and as in the first embodiment, it may be determined when to remove condensed or solidified lube oil based on at least one of the hot fluid channel pressure differential, cold flow temperature differential, and hot flow temperature differential.

It will be apparent to those skilled in the art that the present invention is not limited to the above-described embodiments, and various modifications, changes, variations, and equivalent embodiments can be made in the art without departing from the spirit and scope of the present invention.