Engineer’s Angle: Centrifuge fuel purifiers avert catastrophic engine damage

Sep 23, 2019 by JD Anson

Engineer’s Angle: by JD Anson

There are two main ways dirt gets inside an engine. Each is bad. One is through the air intakes, hence the big filters before the turbos. The other is a little more sneaky: carried on to the boat like a virus through dirty fuel. 

All fuel has some contaminants in it, but these are usually minimal in mainland Europe or North America, where standards and monitoring are high. When bunkering in less-civilized places, cleanliness in storage and transfer are frequently afterthoughts. Questionable fuel is sold regardless of quality. Some naïve sellers equate large yachts to commercial ships that can burn dirty heavy bunker fuel without issue. When a boat calls at a remote island, it is unlikely to return, thus the lower grade fuel will be pawned off to unsuspecting vessels.

Passing this fuel through engines is a sure-fire way of causing major damage. Dirt clogs injection pumps and injectors. Water contamination can cause bent connecting rods and destroy bearings. Water comes into fuel by poor storage standards, but more frequently through cross-contamination. This is because fuel-hauling tanker trucks participate in something called switch loading. A truck could be hauling ethanol-based gasoline one day and diesel the next. Today’s Ultra Low Sulphur Diesel has a high affinity for water. When biofuels such as ethanol, even in small quantities, are mixed with ULSD, the reaction can cause accelerated corrosion of tanker tanks, as well as our onboard tanks. 

The main cause of this corrosion is microbial growth. Microbes that proliferate in diesel fuel love areas where water and fuel meet. High-sulfur fuel was a natural biocide, keeping microbes in check. Since water is denser than fuel, the bottoms of the tanks are most affected. The microbes reproduce at an accelerated rate. These microbes and their byproducts then attack the metals. Over time, this can cause excessive pitting in the tanks, leading to expensive replating and unfortunate downtime.

The bottom of the tank is where the fuel pickups are located. This means that the first liquid pulled from the tank will be the most water-laden. But this fact can be used to an advantage. While pulling this fuel directly into an engine would be catastrophic, the ability to strip the water first allows us to clean the fuel to a high degree if the boat is fitted with a fuel purifier. Many companies have designed various systems over the years. 

A centrifuge purifier is a time-proven method of cleaning fuel. Anyone who has used one can attest to the amount of water that ends up in the catch sump, and the layers of sludge and microbes that get pinned to the spinning bowl sides at several thousands of RPMs.

In personal experience, I spent 12 years on a yacht from the day it was launched. I never put a single gallon of fuel in the daytank that was not passed through the centrifuge. At each inspection, this tank was pristine with less than a cup of sludge from a tank that held well over a thousand gallons and had passed through a quarter of a million gallons in that time. When the boat was running, the purifier was running, either constantly polishing the same fuel or transferring fuel in from a holding tank. 

I was fortunate that the throughput rate of the purifier in four hours matched almost exactly with the engine consumption over 12 hours at cruising speed. This meant that, if at the beginning of each watch the suction valves were switched from daytank to storage tank, by the end of a four-hour watch the daytank would be at 90% full. Then returning the suction to the daytank would allow the engines to use down to a 50% level in the daytank. This could go on for days.

A bonus of the constant cleaning was that fuel filters would last for extended periods of time past what was normal before the vacuum gauges on the housings showed any restriction.  Of course, all that gook had to go somewhere, and that was into the purifier bowl. Disassembling the unit to the point of removing the bowl was usually sufficient to keep sludge in check. The discs would be dirty, but still clear and allowing flow. This allowed plenty of use until there was time to do a more thorough disassembly and cleaning.

A purifier may be a large up-front expense, but can easily pay for itself in fewer filters used coupled with invaluable damage protection.

JD Anson, who has more than 20 years of experience as a chief engineer on mega-yachts, is project manager at Fine Line Marine Electric ( in Fort Lauderdale. Comments are welcome below.