Cargo Compressor (Gas Carrier)

What defines a gas carrier cargo compressor

The cargo compressor handles vapour rather than liquid: boil-off gas generated by cargo tank heat leak, vapour displaced from the tank during loading, and vapour returned to shore or recirculated during discharge. It sits apart from the cargo pumps, which move the liquid, and from the reliquefaction plant, which it usually feeds. Reciprocating compressors dominate on LPG and ethylene carriers because they handle the wide range of suction pressures and cargo compositions typical of those trades, while centrifugal compressors are more common on larger, more homogeneous cargo services where continuous high throughput matters more than flexibility.

Main components

Compressor block

Reciprocating units use multiple cylinders with cargo-resistant valve plates and piston rings; centrifugal units use one or more impeller stages, favoured where the cargo composition and flow are relatively stable.

Shaft seal

Because the shaft passes from the cargo-side casing to the driving motor, a labyrinth or mechanical seal, often with a barrier gas purge, prevents cargo vapour from escaping along the shaft into the compressor room.

Suction and discharge separators

Knock-out drums ahead of the compressor remove any entrained liquid, since liquid carryover into a reciprocating compressor can damage valves or cause a hydraulic lock in the cylinder.

Cooling and lubrication

Cylinder cooling jackets and forced lubrication systems, sized for the specific cargo and its compatibility with standard lubricants, keep the compressor within safe operating temperature across a wide range of suction conditions.

Gas detection and ESD interface

Gas detectors in the compressor room and vibration and temperature sensors on the machine feed the emergency shutdown system, which stops the compressor automatically on a confirmed gas leak or a mechanical trip condition.

Selection and sizing

Capacity is set by the required loading and discharge rate together with the boil-off rate the ship's insulation and cargo tank design produce at the carried cargo's boiling point. Suction pressure range matters more here than for most compressor types, since a fully refrigerated cargo and a semi-refrigerated or pressurised cargo on the same ship demand very different operating envelopes. Materials in contact with cargo, seals, valve plates and cylinder liners, must be checked against the full range of cargoes the ship is intended to carry, not just the current one.

Regulations and class

The IMO IGC Code governs the design, gas detection, shutdown and material requirements for cargo compressors on gas carriers, with class societies applying it through the ship's specific gas certificate. Compressor rooms fall under strict area classification and ventilation requirements, and the compressor's emergency shutdown must be tested and logged as part of the ship's periodic gas system checks.

Typical faults

What to look for in a supplier

• Compressor material compatibility confirmed against the full cargo list the ship is designed to carry
• IGC Code type approval documentation specific to the compressor model and cargo range
• Realistic spare parts holding recommendations for valve plates, piston rings and seals given the trading pattern
• Vendor experience with the specific cargo compressor type, reciprocating versus centrifugal, matched to the ship's actual service

Check knock-out drum liquid level trip settings after any change in cargo grade; a setting tuned for one cargo's density can let liquid through on another.