Lube-oil dilution on an auxiliary generator
The 1,500 kW auxiliary diesel generator No.2 on a berthed vessel is running at 800 kW; two generators are required for ship load. Over four hours its lube oil pressure has fallen from 3.0 to 2.3 bar and the lube oil filter differential pressure has risen to 1.1 bar, with the alarm set at 1.0 bar. The sump level is 18 mm above the previous mark. Bearing temperatures are 78°C compared with 70°C on No.1. A laboratory check shows viscosity at 40°C of 118 cSt against 175 cSt for new oil, and flash point 168°C against 220°C. No external oil leak is visible. The fuel injection pumps are mounted above the camshaft casing, and No.2 unit has had a slight fuel weep from its leak-off connection for two weeks. Cargo operations need two generators; the standby unit is available. The second engineer proposes simply changing the oil and filter and continuing.
1. Interpret the trends and state the immediate actions you would take.
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The trends are diagnostic, not just alarms: falling oil pressure, rising filter differential, rising sump level, lower viscosity and lower flash point together indicate fuel dilution. Bearing temperature rising to 78°C shows the diluted oil is already reducing load-carrying capacity. Changing oil alone would hide the symptom temporarily; the fuel source would re-contaminate the sump and could lead to bearing damage or a crankcase mist explosion. Immediate actions:
- Start the standby generator, transfer load smoothly, and stop No.2 under controlled conditions.
- Isolate the fuel system, lock off the starting system, and post a danger tag. Do not bar the engine until fuel pressure is released.
- Take a sump sample and a filter-casing sample; retain them for comparison.
- Inform the bridge and chief engineer; confirm cargo load can be carried by the remaining generators.
- Check crankcase breather for mist and inspect filters for metal. Do not open hot crankcase covers. The first judgement is to remove the engine from service before the bearing temperature rises further, while keeping the ship’s power secure.
2. How would you locate the source of dilution and decide the repair scope?
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Locate the source before deciding repair scope:
- Inspect the No.2 fuel-pump leak-off connection first; a two-week weep is a credible path into the camshaft casing.
- Remove the camshaft inspection covers after isolation and look for fuel washing, dye stains or strong fuel smell. Check high-pressure pipes, delivery valves and pump elements.
- Pressure-test the fuel system with the engine stopped and the injectors isolated, following the manufacturer’s procedure. Do not use compressed air on a fuel system unless the procedure allows it.
- If the leak is at a fitting, renew the olive/seal and re-torque. If a pump element or delivery valve is leaking into the camshaft, remove the pump for workshop test or renewal.
- Inspect bearings and journals if dilution is heavy or bearing temperature was high; at minimum, cut open the filter and check for bearing material. Repair scope should match the cause: a fitting repair is minor; internal pump leakage requires pump overhaul and a full oil flush. Do not return to service until the source is proven repaired.
3. What return-to-service criteria and records would you require before accepting the generator back on load?
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Return-to-service criteria:
- No fuel leakage at the pump, pipes or leak-off after a test run.
- Sump drained and flushed; new oil and filters; oil viscosity and flash point within specification after at least several hours of running.
- Oil pressure about 3.0 bar at normal load, filter differential below alarm, bearing temperatures stable and comparable with No.1.
- Oil sample free of fuel above the action limit and free of metal.
- Alarm and trip tests satisfactory. Records: maintenance log, oil analysis, repair report, spare parts used, and a note in the engine room log of the power transfer. If bearing damage is found, notify the technical manager and class as required. The key lesson is that oil dilution is a fuel-system fault until proven otherwise.