Why is the main engine lubricating oil pressure low?

Low lubricating oil pressure on the main engine threatens every bearing in the machine, because the crankshaft, crosshead and camshaft bearings run on a hydrodynamic oil film that collapses without supply pressure, so a falling gauge or an alarm demands an immediate, ordered response. Large engines typically run their main lubricating system at a few bar, on the order of 2 to 3 bar at the engine inlet at around 40 to 50 degrees C, and the automation backs the alarm with a slowdown and, at the lowest limit, an automatic shutdown. Those protections must never be overridden.

The causes group naturally:

• Supply side: a worn or faulty pump, air drawn into the suction, or a low sump level from a leak or a purifier lined up wrongly.
• Restriction: dirty filters raising differential pressure, or a fouled oil cooler.
• Oil condition: overheated oil, or oil diluted by fuel or contaminated by water, loses viscosity and pressure falls with it.
• Escape paths: a cracked pipe or loose joint inside the crankcase, worn bearings with opened clearances, or a pressure regulating or relief valve stuck open.
• Instrumentation: a defective gauge or transmitter reading falsely low.

The response sequence is quick and disciplined. Confirm the reading on a local mechanical gauge; a genuine fall in supply pressure with rising bearing temperatures is an emergency, a transmitter fault is not. Check the sump level and the filter differential pressure, change over to the standby filter, and start the standby pump; if pressure recovers on the standby pump, the running pump was the problem. Check the cooler outlet temperature and the setting and behaviour of the regulating valve. Look and feel over the system for leaks. Draw an oil sample: smell for fuel, do a crackle test for water, and land a sample for analysis if contamination is suspected, since diluted oil must be dealt with at its source as well as its symptom.

If genuine low pressure cannot be restored promptly, reduce power and stop the engine in coordination with the bridge rather than run the bearings to destruction; minutes of running on marginal pressure can mean weeks of crankshaft repairs. After any significant low-pressure event, stop, allow the crankcase to cool and ventilate, and inspect: feel the bearings, look for white metal in the filters and crankcase, and check the bottom of the sump before starting again. A cheap inspection has saved many an engine; a hopeful restart has wrecked a few.