Boil-Off Gas (BOG) Compressor

What sets a BOG compressor apart from other cargo machinery

LNG and LPG cargo is never perfectly insulated; heat leaks into the tank continuously and boils a small fraction of the liquid into gas regardless of how well the containment system performs. Left alone, that boil-off gas raises tank pressure until the pressure relief valve lifts and vents cargo to atmosphere — an operational and commercial loss, and on LNG carriers a source of methane emissions now under direct regulatory attention. The BOG compressor is the machine that instead draws gas off the tank and pushes it somewhere useful: to the engine room as fuel gas, to a reliquefaction plant to be condensed back into the tank as liquid, or to a shore vapour return line during cargo operations. It is a low-duty, continuous-running compressor rather than a high-pressure unit, because tank boil-off arrives at low pressure and modest, fairly steady flow under normal conditions.

Main components

Compressor stage

Typically a reciprocating or centrifugal machine depending on ship size and BOG flow, built for cryogenic-temperature suction gas and continuous duty rather than intermittent peak loads.

Suction knock-out drum

Removes any liquid droplets carried with the boil-off gas before it reaches the compressor, since liquid ingestion into a compressor cylinder or impeller causes immediate mechanical damage.

Interstage and aftercoolers

Manage the temperature rise from compression so downstream piping, valves and any reliquefaction equipment stay within their design limits.

Gas seals and purge system

Keep cargo gas from leaking along the compressor shaft into the machinery space, backed by an inert gas or nitrogen purge on the seal itself.

Sizing and selection

• Expected boil-off rate for the specific cargo tank design and insulation performance, which sets the baseline flow the compressor must handle continuously.
• Destination of the compressed gas — fuel gas system, reliquefaction plant, or shore connection — since discharge pressure requirements differ significantly between these.
• Cargo type and composition, because LNG and the various LPG grades have different molecular weight, condensation behaviour and material compatibility requirements.
• Turndown capability, since boil-off rate is not constant across a voyage and the compressor has to handle low-flow periods without surging or cycling excessively.

Regulations and class

The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) governs the design of BOG handling systems, including material selection for cryogenic gas service, gas detection around the compressor, and ventilation of the compressor room. Class societies survey the compressor's gas seals, safety shutdown functions, and gas detection interlocks as part of the gas system survey, and the compressor room itself is treated as a hazardous area with its own ventilation and gas detection requirements under the IGC Code.

Typical faults

What to look for in a supplier

• Compressor materials and seals certified for the specific cargo gas composition the ship actually carries, not a generic hydrocarbon gas rating.
• Demonstrated turndown range covering the boil-off profile across a full voyage, not just the design maximum flow.
• Gas detection and shutdown interlock integration matching the ship's existing safety system architecture.
• Spare parts and service support for cryogenic-service seals and bearings, which are the components most likely to need attention between dry dockings.

Trend the compressor's suction and discharge temperatures against the tank pressure, not just the pressure alone — a compressor that looks like it is keeping up on pressure can still be losing efficiency from cooler fouling or seal purge issues well before an alarm triggers.