Lately, a concerning trend has emerged in the maritime industry: ship crews operating continuously on distillate fuels (MGO/LSMGO — Marine Gas Oil / Low Sulfur Marine Gas Oil) are completely halting the use of centrifugal fuel purifiers.
Furthermore, cases arise where even company superintendents support this approach, arguing that fresh laboratory fuel analysis reports look perfect, and since the diesel oil is visually clean, it requires no separation.
Often, the real motive stems from the crew’s reluctance to handle purifier maintenance, poor performance of shipboard incinerators, or a misguided desire to minimize sludge accumulation. However, machinery breakdown histories and official company safety alerts prove otherwise: continuous purification of distillate fuel is not just beneficial — it is vital for the safety of the power plant.
Why “Perfect” Laboratory Fuel Analyses Are Deceptive
Hiding behind “clean” bunker test results, many overlook the fact that a fuel sample is drawn during bunkering, whereas real-life operations occur under dynamic shipboard conditions. Two key factors turn on-spec distillate fuel into a hazardous mixture directly inside the vessel’s tanks:
1. Co-mingling Effect and Accumulation of Unpumpable Fuel
Even when a vessel operates solely on MGO/LSMGO, a small amount of unpumpable fuel always remains at the bottom of the storage tanks.
- Receiving a new batch of fuel—even one of pristine quality—leads to physical and chemical co-mingling with this older residual stock.
- As a result of this interaction, fine particulate sludge inevitably precipitates out of the fuel.
- Over time, this sludge builds up, gets stirred up from the tank top during heavy rolling, and flows straight into the fuel injection system, bypassing any treatment if the purifier is turned off.
2. Moisture Absorption and Water Ingress During Storage
Distillate fuel is highly hygroscopic. During its storage in shipboard tanks, preventing water contamination is virtually impossible. Water ingress typically occurs due to:
- Intense condensation forming on tank bulkheads during drastic ambient and seawater temperature changes.
- Atmospheric moisture, rain, or sea spray entering through the fuel tank air vents on deck.
Without continuous separation, free water reaches the Fuel Injection Pumps, causing corrosion of precision components, disrupting hydrodynamic lubrication, and ultimately leading to sudden plunger seizure.
The Only Correct Fuel Conditioning Sequence
Regardless of how “spotless” the distillate fuel appears on the bunker delivery note (BDN), the engine department must strictly adhere to the classic fuel treatment cycle:
Fuel MUST be continuously transferred from the Settling Tank to the Service Tank via an operational fuel purifier. This is the only way to guarantee that any accidental sludge surge or accumulated condensation water is removed before it can reach the main or auxiliary engines.
Conclusion for Shipowners and Top-Four Officers
Attempting to save purifier running hours or reduce sludge generation while operating continuously on distillate fuel introduces a hidden threat to the vessel’s safety. A flawless lab analysis of the bunker cannot protect against the physical and chemical realities occurring inside the ship’s tanks. The engineering department must always remember: marine gas oil demands the exact same centrifugal purification as heavy fuel oil.
For practical machinery failure analyses and technical discussions on fuel system operations, visit the marine engineers hub: dm.seamen.es.
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