Bulk Carrier Oil Mist Detector Emergency Troubleshooting at Zhoushan Shipyard —— Guard the Last Critical Safety Barrier in the Engine Room
Every onboard device serves its unique function. Together, they ensure safe vessel navigation — this is their core purpose, and why they are installed.
All equipment will experience wear and potential failure after long-term operation. Timely and proper maintenance is therefore essential. Reliable operation of these systems guarantees vessel safety and crew safety, and supports safe and sustainable maritime operations.
Recently, we received an urgent client request for emergency troubleshooting and repair of the main engine oil mist detector.
As the last critical safety defense line in the engine room, the oil mist detector prevents catastrophic crankcase fire and explosion risks, with consequences no one can afford. We always prioritize these high-risk, life-critical marine equipment repairs above all other routine work, to restore the unit to fully normal and reliable working condition.
PART I: Preparation before work
1. Arranged an electrical technician on board, then held a toolbox meeting with the ship’s engineers to confirm the troubleshooting plan based on their detailed fault description.
2. The oil mist detector model is DAIHATSU MD-9M, the original crankcase oil mist concentration monitoring device for marine diesel engines supplied by DAIHATSU. It is essential safety equipment for the engine room of merchant ships.
3. The core function is to extract gas from each crankcase of the ship’s main and auxiliary diesel engines in real time and detect the oil mist concentration.
3.1 Once bearing overheating, piston scuffing, or abnormal component friction occurs, the oil mist concentration in the crankcase will rise rapidly
3.2 The system will trigger audible and visual alarms in advance, plus an interlock for main engine slowdown and shutdown.
3.3 This prevents catastrophic accidents such as crankcase explosion and engine room fire.
4. Existing faults:
4.1 Test function failure – No audible and visual alarm The ship’s staff suspected main board failure.
4.2 Rotary Valve Indicator (Crankcase No.) – Not indicating correct crankcase number, even stuck at one crankcase for a long time.
PART II: Troubleshooting based on current fault description
2.1 Troubleshooting Steps & Expected Results for 4.1
(1) Press the TEST button
Result: No response at all — no display change, no relay action, no alarm output.
Strongly suggests the main board is not functioning.
(2) Check power input to main board
Power light on panel is on, but the main board shows no internal activity.
Result: Power is supplied to the unit, but the board fails to process signals. Further doubts on main board malfunction.
(3) Check alarm output relay
When TEST is pressed, the relay does not actuate or switch.
Result: The main board is not sending control voltage to the relay.
(4) Check buzzer & lamp power terminals
Use multimeter to measure voltage when TEST is activated.
Result: No voltage output from main board terminals.
(5) Visually inspect main board
Check for burnt components, swollen capacitors, burned circuits, or burnt smell.
Result: Obvious burn marks and damaged ICs observed.
Main board failure is confirmed; replacement with a new one is recommended.
(6) Replace main board and test again
After replacement, the TEST function is restored to normal:
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Buzzer sounds
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Indicator lights flash
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Relay operates properly
(7) After main board replacement, alarm and test functions are fully restored.
The remaining issue of incorrect rotary valve indication / stuck crankcase is confirmed as an independent mechanical and sensor fault, not related to the new main control board. With the main board repaired in good order, the cause of the fault can be easily identified and rectified.
2.2 Troubleshooting Steps & Expected Results for 4.2
(1) Mechanical Stuck & Jam Check
Power off first, then gently rotate the right dial manually to check for jamming or excessive resistance; no excessive resistance found.
Dismantle the rotary valve chamber and clean internal oil sludge, dirt and foreign debris.
Apply high-temperature resistant special grease to rotary bearings and drive gears.
(2) Sampling Scanning System Inspection
Inspect all multi-path sampling pipes for blockage, bending or air leakage.
Verify suction performance of the sampling pump; an aging pump may cause positioning failure during scanning.
Purge all cylinder sampling lines to ensure unobstructed air path.
(3) Position Indication & Sensor Troubleshooting
Clean the optical/magnetic position detection sensor on the dial.
After cleaning the sensor surface and inducer, faded and darkened lens, as well as hardened and discolored plastic housing caused by long-term high temperature exposure are found, showing obvious signs of aging.
Replacement with a new sensor is recommended.
After installing the new sensor, adjust the sensing distance to ensure accurate recognition of each crankcase position.
(4) Drive & Control Logic Check
Perform full power cycle restart and system factory reset.
Check the stepping drive motor; step loss and motor jamming are common causes of continuous sticking.
Confirm that the scanning control logic of the main board is normal, without program disorder or parameter loss.
(5) Final Confirmation (Mandatory After Full Rotary Valve Adjustment & Repair)
After main board replacement, basic alarm and test functions had already been verified normal.
After rotary valve troubleshooting and adjustment:
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Re-performed complete zero point calibration and alarm threshold resetting.
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Full system function test: Normal scanning rotation, accurate crankcase indication, audible and visual alarm, and main engine shutdown interlock under simulated oil mist overload condition.
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30-minute continuous stability running test to confirm all previous faults are fully and permanently eliminated.
PART III: PSC & Classification Society Inspection Reminders
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The unit shall be kept in automatic operation at all times. Long-term manual shielding or shutdown is strictly prohibited.
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The latest calibration and maintenance records must be properly kept.
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Self-test, alarm and engine shutdown interlock functions must be 100% functional.
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Sampling pipelines shall be free of damage, and no unauthorized bypass modification is allowed.
PART IV: Routine Maintenance Tips
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Do not arbitrarily increase the alarm threshold to avoid alarms, as this will completely lose protection against crankcase explosion.
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A full set of recalibration and function test must be performed after each major engine overhaul or cylinder removal.
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For older MD-9M models, replacement of the core optical sensor every 2 years is recommended to ensure detection accuracy.
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Recommended spare parts: sealing O-rings, sampling filters, calibration kits.
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