Hello colleagues, I’m Shannon Peters, Second Engineer on an LNG carrier. After several years in charge of our boil-off gas (BOG) compressor room, I’ve learned some vital lessons I want every LNG engineer to know. Keeping these compressors in top shape isn’t just routine—it’s the linchpin of cargo safety, fuel efficiency, and even your own life on board.

1. Watch Your Seal Gas System Like a Hawk

BOG compressors run nonstop to keep tank pressures under control. Their mechanical seals rely on a steady supply of clean, dry seal gas—usually dry nitrogen. If that seal-gas pressure drops or picks up contaminants, you risk a catastrophic seal failure, which can spray flammable gas into the engine room. I always check seal-gas filters and pressure gauges at every round. A clogged filter or a leaking gauge might seem trivial, but it’s often the first sign of trouble.

2. Prioritize Bearing Inspections and Lubrication

These machines operate at high speeds under cryogenic conditions. Even a slight metal-to-metal contact can generate heat that melts lubrication, leading quickly to bearing failure. I’ve found that adding vibration sensors on each compressor bearing housing and analyzing trends weekly lets you spot creeping wear before it becomes a fire hazard. Also, follow the lubricant manufacturer’s low-temperature guidelines—some oils thicken dangerously at –160 °C.

3. Maintain Heat-Tracing Cables Religiously

Your BOG piping and compressor inlets often sit in unheated spaces. If the trace-heating cables fail, moist air condenses, freezes, and blocks the flow. I once had an inlet frost-block during a North Atlantic crossing—it took hours to thaw out and nearly cost us a cargo vent diversion. Now I log heating-cable amp draw and temp-sensor readings every watch, and any dip in power or rise in temperature is a red flag.

4. Regularly Test Turbine or Electric-Motor Drives

Whether your compressors are turbine-driven or electric, the drive system deserves proactive attention. For turbines, check fuel-gas regulators, turbine exhaust temperatures (EGTs), and differential pressures across the turbine inlet filter. For electric drives, monitor motor current and bearing temperatures. A creeping EGT or rising motor-current trend often foreshadows an actuator fault or lubrication issue.

5. Stay Sharp on Refrigeration Plant Interface

Many carriers use a combined reliquefaction-plus-compression setup. The way the refrigeration plant interacts with the BOG compressors can make or break system stability. I always verify that the plant’s controller seamlessly transitions from full reliquefaction mode to BOG-only backup without interrupting compressor suction pressure. A single hiccup there can force an emergency venting.

6. Embrace Digital Monitoring, But Keep Your Senses

We now have SCADA dashboards, alarms, and remote data logging—wonderful tools. Yet I still make a weekly “hands-on” walk through the compressor room: feeling for unusual vibrations, listening for odd hisses, and smelling for ammonia or gas traces. Digital alarms may alert you after a parameter crosses a threshold; your eyes, ears, and nose catch anomalies the moment they begin.

Keeping BOG compressors healthy is about layering these practices. Seal-gas vigilance, bearing care, heat-trace maintenance, drive-system checks, refrigeration-interface testing, and human senses combined—this comprehensive approach prevents unplanned stops, spares cargo venting, and shields both your ship and crew from needless risk.

Stay proactive, stay safe, and keep those compressors humming.