Requirement for liquid seal in petroleum tanks

[dsg1985]

Australian standard AS 1692 "Steel tanks for flammable and
combustible liquids" states -

"2.2.7 Liquid seal
Every fill pipe, suction pipe or dip pipe that enters through the top of the tank and that is likely to be opened to atmosphere at some time during normal filling, shall be provided with a liquid seal sufficient to ensure that the lower end of the pipe is submerged in at least 25 mm of liquid at all times after the initial filling."

Could anyone enlighten me as to why this cause might exist?

 

[BigInch]

To keep vapors from exiting through a pipe without such a liquid seal.

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