What to check on a BOG Compressor.
LNG cargo is never perfectly insulated, so a small, continuous quantity boils off inside the cargo tanks regardless of how well the containment system is built. The BOG compressor draws that vapour off the top of the tanks and raises its pressure, either to feed it to gas-fuelled main engines and boilers as low-pressure fuel gas, to route it back to shore during loading through the vapour return line, or to send it to a reliquefaction plant. Without it, boil-off gas has nowhere to go except the vent mast, which…
What a BOG compressor does
LNG cargo is never perfectly insulated, so a small, continuous quantity boils off inside the cargo tanks regardless of how well the containment system is built. The BOG compressor draws that vapour off the top of the tanks and raises its pressure, either to feed it to gas-fuelled main engines and boilers as low-pressure fuel gas, to route it back to shore during loading through the vapour return line, or to send it to a reliquefaction plant. Without it, boil-off gas has nowhere to go except the vent mast, which wastes cargo and, above certain rates, is not permitted in port.
Main components
Compressor stages
Most BOG compressors are two-stage reciprocating or screw machines, oil-free or oil-injected depending on whether the downstream fuel gas system tolerates trace oil carryover. Reciprocating types dominate on LNG carriers because they handle the wide range of suction pressure and flow that boil-off produces between loaded passage, ballast passage and cargo operations.
Suction (knock-out) drum
Removes any liquid droplets carried with the vapour before it reaches the compressor cylinders. Liquid slugging a reciprocating compressor is one of the fastest ways to damage valves and rods, so the drum's level instrumentation and high-level trip are treated as safety-critical.
Seal gas and lubrication system
Cryogenic-service compressors use a dedicated seal gas supply, typically nitrogen, to keep hydrocarbon vapour away from the crankcase lubricating oil and prevent methane from migrating into spaces where it is not wanted.
Selection and sizing
| Parameter | Typical range |
|---|---|
| Suction pressure | Near atmospheric to ~150 mbar(g) tank pressure |
| Discharge pressure, low-pressure fuel gas | Up to ~10-16 bar(g) |
| Suction temperature | Around -160 degrees C |
| Capacity turndown | Wide, matched to voyage boil-off rate, typically 0.1-0.15% of cargo volume per day |
Sizing is driven by the ship's design boil-off rate, the number of compressors fitted for redundancy, usually two or three with one spare, and whether the propulsion plant needs high-pressure gas, which pushes toward a separate high-pressure booster compressor instead of a single machine doing both jobs.
Regulations and class
Design, materials and testing of BOG compressors fall under the IGC Code requirements for cargo machinery handling flammable, cryogenic cargo, including gas-tight bulkhead penetrations where the compressor room adjoins other spaces, and hazardous area electrical classification around the unit. Class surveys check gas detection and shutdown interlocks tied to the compressor, not just the machine itself, since an uncontrolled release in the compressor room is treated as a major hazard scenario.
Typical faults
| Fault | Consequence |
|---|---|
| Liquid carryover past the knock-out drum | Valve and piston damage, in severe cases a hydraulic lock on a reciprocating stage |
| Seal gas supply pressure drops below the process gas pressure | Hydrocarbon vapour migrates into the lube oil system, contaminating it |
| Suction valve leakage | Falling volumetric efficiency, rising discharge temperature on that stage |
| Gas detector fault in the compressor room | Automatic shutdown or trip-and-hold of the compressor even with no actual leak, stopping cargo or fuel gas operations |
What to look for in a supplier
- Documented experience with the specific cargo, LNG behaves differently from LPG or ethylene, and seal and metallurgy choices are not interchangeable
- Spare parts availability for valves and packing, the highest-wear items on a reciprocating machine in cryogenic service
- Compatibility with the ship's existing cargo control and gas detection interlocks, not a standalone control philosophy
- Reference installations of a comparable capacity and duty cycle, not just the same compressor family at a different size
Watch discharge temperature trend on each stage, not just the alarm setpoint; a slow creep upward over weeks is usually valve wear announcing itself long before it trips anything.