Operating system of liquefied natural gas ship for sub-cooling and liquefying boil-off gas

Abstract

An operating system of a liquefied natural gas ship for performing sub-cooled liquefaction of boil-off gas includes a boil-off gas compressor, a cryogenic heat exchanger connected to a refrigerator system, and a first check valve and a first pressure control valve, installed in a pipe between a liquefied natural gas phase separator and a gas combustion unit, and a second check valve and a second pressure control valve, installed in a parallel pipe connected to the pipe in parallel. The parallel pipe is connected to a pipe between the boil-off gas compressor and the cryogenic heat exchanger such that boil-off gas is supplied to an upper vapor region of the liquefied natural gas phase separator. Thus, pressure and level of the liquefied natural gas phase separator are stably controlled. Power consumption is effectively reduced, and economical efficiency is achieved by stable operating pressure and level of the liquefied natural gas phase separator.

1 . An operating system of a liquefied natural gas ship for performing sub-cooled liquefaction of boil-off gas for re-liquefaction comprising a boil-off gas compressor, a cryogenic heat exchanger connected to a refrigerator system, and a first check valve and a first pressure control valve, installed in a pipe connecting a liquefied natural gas phase separator with a gas combustion unit, the operating system further comprising: 
 a parallel pipe connected to the pipe for connecting the liquefied natural gas phase separator with the first check valve in parallel and having the same structure as that of the pipe in which the first check valve and the first pressure control valve are installed;    a second check valve installed in the parallel pipe and preventing reverse flow of gas generated when pressure of the pipe is abnormally increased; and    a second pressure control valve installed in the parallel pipe and maintaining a predetermined pressure of the liquefied natural gas phase separator by adjusting a quantity of boil-off gas generated by the boil-off gas compressor such that the liquefied natural gas phase separator, operated tinder the sub-cooling condition, is stably operated;    wherein an end of the parallel pipe is connected to a pipe between the boil-off gas compressor and the cryogenic heat exchanger such that boil-off gas, discharged from the boil-off gas compressor, is supplied to an upper vapor region of the liquefied natural gas phase separator for the purpose of blanket during sub-cooling liquefying operation, so that pressure and level of liquefied natural gas of the liquefied natural gas phase separator are stably controlled.    
     
     
         2 . An operating system of a liquefied natural gas ship for performing sub-cooled liquefaction of boil-off gas for re-liquefaction comprising a boil-off gas compressor, a cryogenic heat exchanger connected to a refrigerator system, and a first check valve and a first pressure control valve, installed in a pipe connecting a liquefied natural gas phase separator with a gas combustion unit, the operating system further comprising: 
 a parallel pipe connected to the pipe for connecting the liquefied natural gas phase separator with the first check valve in parallel and having the same structure as that of the pipe in which the first check valve and the first pressure control valve are installed;    a nitrogen generator connected to the parallel pipe and installed in a machinery space of the liquefied natural gas ship and to supply nitrogen gas to the liquefied natural gas phase separator, operated under the sub-cooling condition, for the purpose of maintaining a predetermined pressure of the liquefied natural gas phase separator;    a second check valve for preventing reverse flow generated when pressure of the pipe is abnormally increased; and    a second pressure control valve for maintaining a predetermined pressure of the liquefied natural gas phase separator by adjusting a quantity of nitrogen gas for the purpose of stable operation of the liquefied natural gas phase separator operated under the sub-cooling condition;    wherein nitrogen gas is supplied from the nitrogen generator to a vapor region of the liquefied natural gas phase separator for the purpose of blanket during the sub-cooled liquefaction, so that pressure and level of liquefied natural gas of the liquefied natural gas phase separator are stably controlled.