The invention relates to a precooling method before unloading of an LNG (
Liquefied Natural Gas) tank wagon. The precooling method comprises the steps of opening a first liquid inlet control valve of a
storage tank, opening a self-pressurization gaseous phase and liquid inlet pipeline communicating valve of an unloading
deck, opening a second upper liquid inlet control valve of a liquid-unloading
storage tank to form an included angle of 25-35 degrees to precool a liquid inlet pipeline, opening the second upper liquid inlet control valve of the liquid-unloading
storage tank to form an included angle of 55-65 degrees after 4-6min, completely opening the second upper liquid inlet control valve of the liquid-unloading storage tank after 4-6min again, opening a gaseous
phase control valve of a self-pressurization
system, a self-pressurization gaseous
phase control valve of the tank wagon and a first gaseous
phase control valve of the tank wagon, opening a self-pressurization
liquid phase control valve of the unloading
deck, opening a self-pressurization
liquid phase control valve of the tank wagon to form an included angle of 10-20 degrees, pressurizing the tank wagon, opening a first self-pressurization gaseous phase and
BOG (Boil Off Gas)
system communicating valve of the unloading
deck after the pressure is increased to be 0.5-0.6MPa, opening the self-pressurization gaseous phase and liquid inlet pipeline communicating valve of the unloading deck, enlarging the opening degree of the self-pressurization
liquid phase control valve of the tank wagon to form an included angle of 60-90 degrees after the pressure of the tank wagon is reduced to be 0.3-0.4MPa, and completely opening after 4-5min. The
pipe network can be well precooled, the precooling speed is high, and the precooling effect is thorough without blind areas.