When storing, bunkering and/or transferring LNG, boil-off gas (BOG) is formed which increases the temperature/pressure in the storage tank. This pressure build up needs to be managed to prevent venting of the boil off gas or, worse case scenario, tank failure.
Managing (re-liquefying) the BOG and decreasing the pressure build up within the tank is the best way to prevent this. There are two principal ways to manage BOG safely (on shore and off shore):
Direct BOG re-liquefaction
Boil-off gas is taken from a storage tank, re-liquefied and transferred back into the storage tank. This can be done by gravity or by using a separate transfer vessel.
Liquid subcooling and spraying
LNG is pumped from the bottom of an LNG storage tank to the StirLNG, subcooled to a temperature below the liquefaction temperature and sprayed back in the top of the storage tank thus re-liquefying the BOG.
The choice between BOG re-liquefaction or subcooling depends on for example the operational set up of the storage tank, the physical location and the engineering preferences.
Stirling Cryogenics has developed cryocoolers specifically suitable for handling LNG BOG both for land based and maritime storage tanks. In both cases the StirLNG units will provide the same cooling capacity based on the composition, temperature and pressure of the gas in the storage tank.
For more information contact us.