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Introduction
Engine type: 5DK-20
Maker: Anqing-Daihatsu
This model lacks an attached F.O supply pump, relying entirely on the fuel supply unit to maintain pressure.
When operating on HFO, the auxiliary engines share a common fuel supply pipeline with the main engine, with a pump discharge pressure of 0.8 MPa.
When operating on MDO, an emergency pump supplies independently, maintaining the same discharge pressure of 0.8 MPa.
A sudden drop in fuel pressure during operation can trigger load surges and blackouts, posing significant operational risks.
I. Fault Symptoms
During onboard operations, No. 3 auxiliary engine exhibited a gradual decline in fuel pressure, fluctuating between 0.38 MPa and a minimum of 0.25 MPa. Pressure slightly increased when switching to MDO.
In contrast, No. 1 and No. 2 auxiliary engines maintained normal fuel pressure at 0.6 MPa, consistent with the manufacturer’s specification of 0.5–0.6 MPa.
II. Fault Analysis
Since the low fuel pressure issue was isolated to No. 3 auxiliary engine, the fuel supply unit fault was ruled out. Focus shifted to the engine’s fuel pipeline and associated components.
1. Fuel Pipeline Inspection
Pressure gauge calibration and inspection of the connecting copper tube revealed no blockages.
2. Fuel Filter Cleaning
Maintenance records confirmed regular filter servicing.
Cleaning the filter showed minimal contamination. Swapping filters with No. 1 engine yielded negligible pressure improvement (0.01 MPa increase), eliminating the filter as the root cause.
3. Pressure Measurement
Installing pressure gauges upstream and downstream of the filter confirmed low upstream pressure with minimal differential, indicating no filter or valve malfunction.
4. Return Pressure Valve Check
Adjusting the return pressure valve (backpressure valve) on No. 3 engine had no effect. Disassembly revealed worn valve seats and a weakened spring.
Remedial actions (reseating the valve and adjusting spring tension) temporarily restored pressure to 0.62 MPa.
III. Fault Resolution
After switching to HFO during voyage, No. 3 engine’s pressure dropped to 0.3 MPa. Reverting to MDO did not resolve the issue.
Further Investigation:
Closing the return oil valve caused pressure to plummet to 0.20 MPa, confirming insufficient fuel flow.
Disassembly of the fuel inlet pipeline revealed a carbonized ball trapped at the junction of two pipes .
Root Cause:
A metal screen between Pipe 1 (34 mm OD) and Pipe 2 (16 mm OD) accumulated carbonized debris, forming a hardened ball that obstructed fuel flow.
Diameter mismatch between pipes exacerbated the blockage.
Corrective Action:
Removal of the metal screen and thorough cleaning restored normal fuel pressure.
IV. Conclusion
(1) Prioritize Simplicity in Troubleshooting
Assuming pipeline blockage without systematic checks would prolong repairs. Accessing pipelines under insulation and heat tracing requires significant effort.
(2) Combine Field Observations with Hypothesis Testing
The undocumented metal screen (absent in manuals) was critical to identifying the blockage.
(3) Address Design Flaws
The pipeline diameter discrepancy (16 mm vs. 34 mm as per drawings) contributed to the issue. Standardizing pipe dimensions could prevent recurrence.
(4) Enhance Fuel Purification and System Cleanliness
Strict management of fuel quality and system cleanliness is essential to avoid carbon buildup.
