The expansion ratio of a liquefied and cryogenic substance is the volume of a given amount of that substance in liquid form compared to the volume of the same amount of substance in gaseous form, at room temperature and normal atmospheric pressure.
If a sufficient amount of liquid is vaporized within a closed container, it produces pressures that can rupture the pressure vessel. Hence the use of pressure relief valves and vent valves.
The expansion ratio of liquefied and cryogenic from the boiling point to ambient is:
References
Expansion ratio Wikipedia(Text) CC BY-SA