Device and method for pressure control of cargo tank of liquefied natural gas carrier

1. An apparatus for controlling pressure in a cargo tank on a liquefied natural gas carrier, in which boil-off gas generated from liquefied natural gas stored in the cargo tank is compressed by a compressor, wherein a reliquefaction plant is disposed on the downstream side of said compressor and on the upstream side of said cargo tank so that said boil-off gas discharged from said compressor is liquefied by said reliquefaction plant and the liquefied fluid is returned again into said cargo tank, wherein a mist separator is disposed on the downstream side of said cargo tank to keep the supply temperature of boil-off gas supplied to said reliquefaction plant constant, and said mist separator is connected to an inlet portion and an outlet portion of said compression and also is connected to an outlet portion of said reliquefaction plant.

2. An apparatus for controlling pressure in a cargo tank on a liquefied natural gas carrier, in which boil-off gas generated from liquefied natural gas stored in the cargo tank is supplied to a burning system through a compressor, wherein first and second compressors are disposed in parallel on the downstream side of said cargo tank, and said burning system is disposed on the downstream side of said first compressor and a reliquefaction plant is disposed on the downstream side of said second compressor and on the upstream side of said cargo tank, by which said boil-off gas discharged from said second compressor is liquefied by said reliquefaction plant and the liquefied liquid is returned again into said cargo tank.

3. The apparatus for controlling pressure in a cargo tank on a liquefied natural gas carrier according to claim 2 , wherein a mist separator is disposed on the downstream side of said cargo tank to keep the supply temperature of boil-off gas supplied to said reliquefaction plant constant, and said mist separator is connected to inlet portions of said first and second compressors, connected to an outlet portion of said second compressor, and also connected to an outlet portion of said reliquefaction plant, and a return pump is provided between an inlet portion of said cargo tank and an outlet portion of said mist separator.

4. A method for controlling pressure in a cargo tank on a liquefied natural gas carrier, in which boil-off gas generated from liquefied natural gas stored in the cargo tank is compressed by a compressor, wherein by using a reliquefaction plant disposed on the downstream side of the compressor and on the upstream side of said cargo tank, and a mist separator disposed on the downstream side of said cargo tank, said boil-off gas passing through said mist separator and discharged from said compressor is liquefied by said reliquefaction plant, and the liquefied fluid is returned again into said cargo tank; some of said boil-off gas discharged from said compressor is supplied to said mist separator while being mixed with said boil-off gas evaporating from said cargo tank; and some of reliquefied fluid liquefied by said reliquefaction plant is sprayed into said mist separator so that the boil-off gas supplied from said compressor and said cargo tank is cooled by the sprayed reliquefied fluid, whereby the supply temperature of boil-off gas supplied to said reliquefaction plant is kept constant.

5. A method for controlling pressure in a cargo tank on a liquefied natural gas carrier, in which boil-off gas generated from liquefied natural gas stored in the cargo tank is supplied to a burning system through a compressor, wherein by using a second compressor disposed in parallel with a first compressor, connected with said burning system, on the downstream side of said cargo tank, a reliquefaction plant disposed on the downstream side of said second compressor and on the upstream side of said cargo tank, a mist separator disposed on the downstream side of said cargo tank, and a return pump provided between an inlet portion of said cargo tank and an outlet portion of said mist separator, said boil-off gas passing through said mist separator and discharged from said second compressor is liquefied by said reliquefaction plant, and the liquefied fluid is returned again into said cargo tank through said mist separator and said return pump; some of said boil-off gas discharged from said second compressor is supplied to said mist separator while being mixed with said boil-off gas evaporating from said cargo tank; and some of reliquefied fluid liquefied by said reliquefaction plant is sprayed into said mist separator so that the boil-off gas supplied from said second compressor and said cargo tank is cooled by the sprayed reliquefied fluid, whereby the supply temperature of boil-off gas supplied to said reliquefaction plant is kept constant.

6. An apparatus for controlling pressure in a cargo tank on a liquefied natural gas carrier in which boil-off gas generated from liquefied natural gas stored in the cargo tank is compressed by a compressor, wherein a reliquefaction plant is disposed on the downstream side of said compressor and on the upstream side of said cargo tank so that said boil-off gas discharged from said compressor is liquefied by said reliquefaction plant and the liquefied fluid is returned again into said cargo tank and wherein two compressors connected in series are disposed on the upstream side of said reliquefaction plant, and wherein a mist separator is disposed on the downstream side of said cargo tank to keep the supply temperature of boil-off gas supplied to said reliquefaction plant constant, and said mist separator is connected to an inlet portion and an outlet portion of said compressors and also is connected to an outlet portion of said reliquefaction plant.