BoiloffGas

BoiloffGas refers to the gas produced when cryogenic liquids absorb heat and boil at or near atmospheric pressure, causing the liquid to vaporize and escape from a containment system. The term is commonly used in the handling of cryogenic fluids such as liquid nitrogen, liquid oxygen, liquid hydrogen, and liquid methane, as well as in spacecraft propellant management where cryogenic propellants are stored and used.

In storage and handling, heat input from the surrounding environment causes a portion of the liquid to warm and boil. The resulting gas increases the ullage pressure, which is typically managed by venting through relief or vent lines. In some facilities, boil-off gas is captured and either reliquefied or routed to energy recovery systems; in others, it is simply vented to maintain safe pressure levels.

In spaceflight, boil-off gas is the controlled venting of cryogenic propellants to prevent overpressure and maintain tank temperature. Long-duration missions and upper-stage operations must account for propellant loss due to boil-off, which can influence mission planning and propulsion scheduling. Some systems explore methods to utilize boil-off gas, such as pressurization or recondensation, but venting remains a common practice to ensure safety and stability.

Factors affecting boil-off rate include the heat leak into the cryogenic storage, insulation quality, tank design and geometry, ullage volume, and the presence of active cooling or recondensation hardware. Boil-off rate is typically expressed as mass flow rate (kg/s) or volume per time (L/day).

Safety considerations center on the flammability or reactivity of certain boil-off compositions, as well as potential asphyxiation hazards in confined spaces. Proper leak detection, venting design, and containment are essential in both industrial and aerospace contexts.