A method and a plant for preventing contamination of a fluid in a fluid storage tank with a liquid cooling or heating medium is provided. In a method of controlling a temperature of a fluid in a fluid storage tank under a predetermined pressure by allowing a liquid cooling or heating medium to flow in an enclosed pressure-resistant jacket provided around the outer wall of the fluid storage tank, said method comprises allowing the cooling or heating medium to flow in the enclosed pressure-resistant jacket at a pressure not higher than the pressure x (atm) applied within the fluid storage tank, preferably lower than x (atm), whereby preventing contamination of said fluid with said cooling or heating medium. A plant for the method is also provided.