Boil-off gas reliquefication system, method for discharging lubricating oil in boil-off gas reliquefication system, and engine fuel supply method

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

a compressor compressing BOG;

a heat exchanger cooling the BOG compressed by the compressor through heat exchange using BOG discharged from a storage tank as a refrigerant;

a bypass line through which the BOG is supplied to the compressor after bypassing the heat exchanger;

a first valve disposed on a first supply line through which the BOG to be used as a refrigerant in the heat exchanger is supplied to the heat exchanger, the first valve regulating a flow rate of fluid and opening/closing of the first supply line;

a second valve disposed on a second supply line through which the BOG used as the refrigerant in the heat exchanger is supplied to the compressor, the second valve regulating a flow rate of fluid and opening/closing of the second supply line;

a bypass valve disposed on the bypass line and regulating a flow rate of fluid and opening/closing of the bypass line; and

a pressure reducer disposed downstream of the heat exchanger and reducing a pressure of fluid cooled by the heat exchanger,

a gas/liquid separator disposed downstream of the pressure reducer and separating the BOG into liquefied gas generated through reliquefaction and gaseous BOG,

wherein the compressor comprises at least one oil-lubrication type cylinder,

wherein the bypass line is branched from the first supply line upstream of the first valve and joined to the second supply line downstream of the second valve,

wherein the bypass valve is partially or totally opened when a pressure of the BOG supplied to the compressor is lower than an intake pressure condition for the compressor,

wherein the gaseous BOG separated by the gas/liquid separator is discharged from the gas/liquid separator along a sixth supply line, the sixth supply line being joined to the first supply line upstream of a branched point of the bypass line.

2. The BOG reliquefaction system according to claim 1 , wherein the sixth supply line is joined to the first supply line upstream of the first valve.

3. The BOG reliquefaction system according to claim 1 , wherein the first valve is open when the bypass valve is closed;

a degree of opening of the first valve decreases with increasing degree of opening of the bypass valve; and

the first valve is completely closed when the bypass valve is completely open.

4. The BOG reliquefaction system according to claim 3 , wherein a degree of opening of the bypass valve increases when a pressure sensor senses the pressure of the BOG supplied to the compressor has decreased below the intake pressure condition for the compressor.

5. The BOG reliquefaction system according to claim 4 , wherein the compressor comprises at least one recirculation line and at least one recirculation valve disposed on the at least one recirculation line,

the recirculation valve being open to allow the BOG to be recirculated through the recirculation line if the intake pressure condition for the compressor is not satisfied even when the bypass valve is completely open.

6. The BOG reliquefaction system according to claim 5 , wherein a pressure condition allowing the bypass valve to be open is set to a higher value than a pressure condition allowing the recirculation valve to be open.

7. The BOG reliquefaction system according to claim 6 , wherein the BOG used as the refrigerant by the heat exchanger and sent to the compressor is the BOG discharged from the storage tank, and the pressure condition allowing the recirculation valve to be open and the pressure condition allowing the bypass valve to be open are determined based on a pressure upstream of the compressor or an internal pressure of the storage tank.

8. A method for discharging lubricating oil in a BOG reliquefaction system configured to reliquefy BOG by compressing the BOG by a compressor, cooling the compressed BOG through heat exchange with non-compressed BOG by a heat exchanger, and reducing a pressure of the compressed BOG, passed through heat exchange by a pressure reducer,

wherein BOG to be used as a refrigerant in the heat exchanger is supplied to the heat exchanger along a first supply line,

the BOG used as the refrigerant in the heat exchanger is supplied to the compressor along a second supply line, and

BOG not used as the refrigerant in the heat exchanger is supplied to the compressor along a bypass line bypassing the heat exchanger, and

wherein a bypass valve for regulating a flow rate of fluid and opening/closing of a corresponding supply line is disposed on the bypass line,

a first valve for regulating a flow rate of fluid and opening/closing of a corresponding supply line is disposed on the first supply line upstream of the heat exchanger,

a second valve for regulating a flow rate of fluid and opening/closing of a corresponding supply line is disposed on the second supply line downstream of the heat exchanger, and

the compressor comprises at least one oil-lubrication type cylinder

the method comprising:

1) determining, using a controller, to remove condensed or solidified lubricant oil;

2) opening the bypass valve while closing the first valve and the second valve;

3) sending the BOG not used as the refrigerant in the heat exchanger to the compressor along the bypass line, and compressing the BOG not used as the refrigerant by the compressor; and

4) sending part or all of the BOG compressed by the compressor to the heat exchanger,

discharging the condensed or solidified lubricant oil from the BOG reliquefaction system after being melted or reduced in viscosity by the part or all of BOG not used as the refrigerant which has been compressed by the compressor,

wherein, in Step 4), the BOG compressed by the compressor is used as fuel by an engine, and surplus BOG not used by the engine is sent to the heat exchanger.

9. The method for discharging lubricating oil according to claim 8 ,

wherein, upon reliquefaction of the BOG, liquefied gas generated by reliquefaction and gaseous BOG are separated from each other by a gas/liquid separator, the liquefied gas separated by the gas/liquid separator is discharged from the gas/liquid separator along a fifth supply line, and the gaseous BOG separated by the gas/liquid separator is discharged from the gas/liquid separator along a sixth supply line,

the method further comprising:

5) sending the BOG having passed through the heat exchanger to the gas/liquid separator; and

6) discharging the lubricant oil accumulated in the gas/liquid separator.

10. The method for discharging lubricating oil according to claim 9 , wherein Step 3) to Step 5) are repeated until a temperature of a hot fluid channel of the heat exchanger increases to that of the BOG compressed by the compressor and sent to the heat exchanger.

11. The method for discharging lubricating oil according to claim 8 , wherein, in Step 1), it is determined that it is time to discharge the condensed or solidified lubricant oil, if performance of the heat exchanger is decreased to 60% to 80% of normal performance thereof.

12. The method for discharging lubricating oil according to claim 8 , further comprising:

determining, in Step 1), to discharge the condensed or solidified lubricant oil, if at least one of the following conditions is satisfied;

a condition that a temperature difference between the BOG upstream of the heat exchanger to be used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and cooled by the heat exchanger (hereinafter referred to as “temperature difference of a cold flow”) is a first preset value or more and continues for a predetermined period of time or more;

a condition that a temperature difference between the BOG used as the refrigerant in the heat exchanger and the BOG compressed by the compressor and sent to the heat exchanger (hereinafter referred to as “temperature difference of a hot flow”) is the first preset value or more and continues for a predetermined period of time or more; and

a condition that a pressure difference between the BOG compressed by the compressor and sent to the heat exchanger at a location upstream of the heat exchanger and the BOG cooled by the heat exchanger at a location downstream of the heat exchanger thereinafter referred to as “pressure difference of a hot fluid channel”) is a second preset value or more and continues for a predetermined period of time or more.

13. The method for discharging lubricating oil according to claim 8 , further comprising:

responsive to determining, based on at least one of the temperature difference of the cold flow, the temperature difference of the hot flow, and the pressure difference of the hot fluid channel, that the removal of the condensed or solidified lubricant oil is complete, reliquefaction of the BOG is performed after opening the first valve and the second valve while closing the bypass valve.