Boil-off gas, usually shortened to BOG, is the vapor that continuously forms when a cryogenic liquid like LNG absorbs heat and evaporates in storage. No insulation is perfect, so a small heat leak into a cold tank is unavoidable, and that heat has to go somewhere: it boils a little of the liquid back into gas. This guide explains where boil-off gas comes from, how facilities capture and use it, and how tank pressure is managed so the vapor is an asset rather than a hazard.
Boil-Off Gas in one line: Boil-off gas is the vaporized product that forms when heat leaks into a cryogenic storage tank or transfer line and evaporates part of the stored liquid, most commonly with LNG but also with cold NGL and other liquefied gases. Because the liquid is stored at its boiling point, any heat input generates vapor and raises tank pressure. Facilities manage BOG by collecting it and either using it as fuel, recompressing it back into the process, or reliquefying it back into the tank.
A cryogenic liquid is stored at, or very near, its boiling point at tank pressure. LNG, for example, sits at around minus 160 degrees Celsius. At that temperature the surroundings are enormously warmer, so heat is always trying to flow in through the tank walls, roof, and connected piping, no matter how good the insulation is. That inward heat is the heat leak, and the liquid absorbs it by boiling. The result is a steady generation of vapor above the liquid.
Heat leak is not the only source. Pumping adds heat, so running transfer pumps produces extra boil-off. Flashing during loading and unloading, warm ship or truck lines cooling down, and the displacement of vapor as liquid moves in and out all add to the BOG load. During a ship loading, the arriving liquid pushes tank vapor out and the operation generates a surge of gas that has to be handled in real time.
The rate is often described as a small percentage of tank contents per day, but the exact figure depends on tank design, fill level, ambient conditions, and activity. What matters operationally is that BOG is continuous and unavoidable, so every cryogenic storage facility is built around managing it rather than eliminating it.
The first job is pressure control. As vapor forms, tank pressure rises, and if nothing removes the gas the pressure would climb toward the relief setpoint and eventually vent. So BOG is drawn off the vapor space by a BOG compressor to keep tank pressure inside its target band. The compressed gas then has somewhere to go, and the destination defines the strategy.
There are three broad routes. The gas can be used as fuel, feeding boilers, engines, or gas turbines on site or on a ship, which turns unavoidable boil-off into useful energy. It can be recompressed and sent back into the process, for example returned to a pipeline or blended into the send-out stream at a regasification terminal. Or it can be reliquefied, cooled back down and condensed into liquid that is returned to the tank, which conserves product on facilities where flaring or fuel use would be wasteful, such as large storage tanks and modern LNG carriers.
Which route a facility uses depends on economics and location. A peaking or import terminal with pipeline access often recompresses BOG into send-out. A remote or marine facility may reliquefy to avoid losing cargo. The last resort, flaring or venting, is minimized because it wastes product and creates emissions, so a well-run site sizes its BOG handling to avoid ever reaching relief.
Managing boil-off is fundamentally a monitoring and control problem, which is where a SCADA system earns its place. The core signals are tank pressure, tank level, tank temperature, and the status and load of the BOG compressors. Holding tank pressure inside a narrow band, tight enough to avoid relief but not so low that the tank is over-pumped, is a continuous balancing act between the boil-off rate and the removal rate.
A cloud platform such as Merobix lets operators trend those variables together and see the balance in real time. Pressure creeping up while compressors are already loaded warns that heat leak or activity is outrunning the handling capacity, well before a relief valve lifts. Correlating BOG generation with ambient temperature, fill level, and transfer operations helps crews anticipate surges, for example the extra vapor produced during a loading, and stage compressors ahead of it.
Because cryogenic storage carries both a safety and a product-loss consequence, alarming on rising pressure and on compressor availability is central. Remote visibility means an operator does not have to be standing at the tank to know the BOG system is keeping up, and the historized trends make it possible to tune the strategy and quantify how much product is being conserved by reliquefaction versus sent to fuel or flare.
Heat leaking into a cryogenic tank. The liquid is stored at its boiling point, so any heat input, through the tank walls and piping, from pumping, or from warm lines cooling down, boils a little of the liquid into vapor. Because no insulation is perfect, boil-off is continuous and cannot be eliminated, only managed.
It is drawn off to control tank pressure and then used one of three ways: burned as fuel for on-site or shipboard engines and boilers, recompressed and sent back into the process or pipeline, or reliquefied and returned to the tank as liquid. Flaring or venting is a last resort because it wastes product and creates emissions.
Because vapor forming in a closed tank takes up far more volume than the liquid it came from. As boil-off accumulates in the vapor space, pressure climbs. If it is not removed, pressure would eventually reach the relief setpoint and vent, so a boil-off gas compressor continuously draws vapor off to hold pressure in its target band.
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