A cryogenic tank has two main walls that inner vessel holds the cryogenic liquid and outer vessel protects and supports the inner vessel. Between the two walls is a vacuum space, which acts like a thermal barrier. Additional insulation (like perlite or multi-layer reflective foil) is often added to block radiant heat. As some of the liquid naturally evaporates (called boil-off), pressure builds inside the tank. In order to keep the tank operating safely within design pressure limits, the tank includes pressure relief valves to safely vent excess gas and pressure-building coil to maintain constant pressure by converting some liquid to gas if needed.
