Blow-down method of reliquefaction system for ship

Abstract

A blow-down method is applied to a reliquefaction system in which boil-off is compressed by a compressor, cooled and reliquefied through a heat exchanger, and returned to the tank after passing through a gas-liquid separator. The reliquefaction system includes a pressure compensation line extending from a downstream side of the compressor to an upside of the gas-liquid separator without passing through the heat exchanger; and a nitrogen blanket line along which nitrogen is supplied to the pressure compensation line. In the event of a trip of the reliquefaction system, nitrogen is supplied to the pressure compensation line along the nitrogen blanket line and is delivered to the downstream side of the compressor to pass through the heat exchanger along the reliquefaction line such that the heat exchanger and the reliquefaction line are blown down to remove compressed gas and reliquefied gas therefrom while being subjected to nitrogen (N2) purging.

1 . A blow-down method of a reliquefaction system for ships,
 wherein boil-off generated from a liquefied gas stored in a storage tank of a ship is compressed by a compressor, cooled and reliquefied through a heat exchanger along a reliquefaction line, and returned to the storage tank after passing through a gas-liquid separator, and   wherein the reliquefaction system is provided with a pressure compensation line extending from a downstream side of the compressor to an upside of the gas-liquid separator without passing through the heat exchanger; and a nitrogen blanket line along which nitrogen is supplied to the pressure compensation line, and   wherein, in the event of a trip of the reliquefaction system, nitrogen is supplied to the pressure compensation line along the nitrogen blanket line and is delivered to the downstream side of the compressor to pass through the heat exchanger along the reliquefaction line such that the heat exchanger and the reliquefaction line are blown down to remove compressed gas, and reliquefied gas therefrom while being subjected to nitrogen (N 2 ) purging.   
     
     
         2 . The blow-down method according to  claim 1 , wherein the reliquefaction system comprises:
 a bypass line connecting a downstream side of the heat exchanger to the storage tank without passing through the gas-liquid separator;   a first valve disposed on the reliquefaction line downstream of a junction between the reliquefaction line and the bypass line; and   a second valve disposed on the bypass line, and   wherein the compressed gas and the reliquefied gas removed from the heat exchanger and the reliquefaction line and the nitrogen are discharged along the bypass line.   
     
     
         3 . The blow-down method according to  claim 2 , wherein the reliquefaction system further comprises:
 a third valve disposed on the pressure compensation line upstream of a junction between the pressure compensation line and the nitrogen blanket line;   a fourth valve disposed on the nitrogen blanket line; and   a fifth valve disposed on the pressure compensation line downstream of the junction between the pressure compensation line and the nitrogen blanket line.   
     
     
         4 . The blow-down method according to  claim 3 , wherein a refrigerant circulated along a refrigerant circulation line is supplied to the heat exchanger to cool the compressed gas, and uncompressed boil-off gas from the storage tank is supplied to the compressor after passing through the heat exchanger such that the compressed gas is cooled in the heat exchanger through heat exchange with the refrigerant and the uncompressed boil-off gas. 
     
     
         5 . The blow-down method according to  claim 4 , wherein the refrigerant circulated along the refrigerant circulation line is nitrogen and the heat exchanger is a brazed aluminum heat exchanger (BAHE). 
     
     
         6 . The blow-down method according to  claim 1 , wherein, upon delivery of the reliquefied gas from the gas-liquid separator to the storage tank, a pressure in the gas-liquid separator is maintained by flash gas generated from the reliquefied gas supplied to the gas-liquid separator, and, when the reliquefied gas supplied to the gas-liquid separator is in a subcooled state and fails to generate enough flash gas to maintain the pressure in the gas-liquid separator, boil-off gas or nitrogen is supplied to the gas-liquid separator along the pressure compensation line to maintain the pressure in the gas-liquid separator.