Fuel polishing has become a subject of interest and discussion among the owners of diesel-powered boats, especially power boats. What is fuel polishing? How do you do it? Why might you want to polish your fuel? Is a dedicated fuel polishing system a necessity, a “nice to have” capability or is it something you can get along very well without?
First, lets consider the basic, factory-installed diesel fuel supply system used on most voyaging boats. Fuel, drawn from a point close to but not at the bottom of the tank flows into a primary (often a Racor Marine Turbo) filter. Three sequential operations within the filter unit are used to separate water from the fuel, remove and trap debris. A swirling motion is imparted to the fuel as it flows over a plastic spiral device at the lower end of the unit, creating a centrifugal force sufficient to move drops of liquid water to the wall of the bottom bowl where it collects for later removal.
The fuel then flows upward and is drawn through a cylindrical filter element whose hydrophobic surface forces emulsified water to coalesce and drop to the water collection bowl below. The microscopic pores in the filter element block the passage of particles larger than the “micron” rating of the filter, which in the Racor filters are available in three levels of porosity, 30 (red cap), 10 (blue cap) and 2 (brown cap). (It’s important to note that the filter unit contains a one-way valve that will allow fuel to flow only when a vacuum is applied to the outlet port. This provision will prevent fuel from flowing from the fuel tank in the event of a fuel line leak when the engine is not running.)
Filtered again in lift pump
The filtered fuel next flows to the engine where it passes through the engine’s lift pump and final fuel filter, which in most instances contains a 5µ filter element, although some of the common-rail diesels are using 2 final, on-engine filters. (Racor Marine FBO fuel filters for on-engine use are available with micron ratings of 25, 10, 5 and 1. The on-engine fuel filter is best sourced from the engine manufacturer to assure the use of the correct element.) The filtered fuel, in a quantity significantly greater than the amount needed by the engine (even when operating at full power), is delivered to the high-pressure injection system. The excess fuel, which is used to cool the high pressure pump, is returned to the fuel tank. The continuing reflow of fuel through the primary and secondary fuel filters creates a basic fuel polishing system.
How do you know if your present fuel filtration system is adequate? The answer is in your logbook. If it contains notations that you have had to change the primary filter only at the scheduled maintenance intervals of 200 hours or thereabouts and the secondary (on-engine) fuel filter at every second or third change of the primary filter, your existing fuel filtering or polishing system is serving you well. It is removing the bad stuff from the fuel and storing it in the filter elements.
If you wish, you can prolong the time between necessary filter changes by fitting a larger filter that can store more trash before the accumulation creates an unacceptable decrease in fuel flow or install a dual filter with a flow selector valve that will allow you to switch filters without having to shut-down the engine.
You can also install a vacuum gage on the fuel filter that will provide a visual (and optional remote gage) indication of the pressure drop across the filter, announcing the need for filter replacement. An electrical sensor can be added to the water collection bowl and wired to an annunciator at the helm to signal the need to drain the accumulated water.
Dealing with dirty fuel
A basic fuel filtering system works very well when the boat is fueled with the clean fuel we are accustomed to in North America and Europe. However, it may be less than adequate when power voyaging in places where you will have to accept less than pristine fuel. In some locations the only fuel available may be stored in dented drums that look rusty on the outside and whose filler cap was loose or missing when you started to pump fuel into your boat. In this extreme situation (or whenever you have any reason to doubt the cleanliness of the fuel) you will want to fill the fuel tank through a baja filter. Commercial versions of this originally home-made device are typically funnel-like and contain fine mesh metal filter screens that trap the major bits of dirt and a hydrophobic filter that prevents the liquid water (remember that loose cap on the drum?) from flowing into the fuel tank. Racor’s filter funnels, the RFF1C through RFF15C accept fuel at flow rates from 2.7 to 15 gallons per minute. While the use of the baja filter will help, it’s still likely that the fuel is still not as clean as we would like it to be.
If at all possible, the newly acquired and suspect fuel should be quarantined, kept entirely separate from whatever clean fuel is onboard. If the boat has two fuel tanks, plan refueling stops so that one tank is close to full when it’s time to fill the other tank.
Regardless of whether we are able to segregate the newly-loaded and possibly contaminated fuel from whatever clean fuel may be available, we need a way to remove the impurities in the new fuel. This is when a second fuel filtering (polishing) system is most needed. The second system must operate independently from the basic engine supply fuel filtering system. Its pump must be able to move fuel at a flow rate commensurate with that of the fuel filters. To be effective, the fuel will have to pass through the system many times and the more rapidly that can be accomplished the better. Be prepared to consume a large and surprising number of fuel filters.
The secondary filtering system will have a continuous-duty electric fuel pump, one or more filters and a fuel flow or pressure drop gage whose reading will indicate the need to replace the fuel filter elements. On a vessel with two fuel tanks, the secondary of the fuel polishing filter system should be plumbed so that it can be used to filter fuel from either tank and return processed fuel to either tank. The fuel return line from the final filter in the secondary filtration system should deposit the fuel near the bottom of the fuel tank so that the flow will create turbulence, mixing the bottom contents of the tank to maximize the scouring of the most contaminated fuel. Simply dropping the fuel into the headspace at the top of the tank will only aerate the fuel. A typical system might consist of two Racor Marine Turbine Series water separating/coalescing/filtration units with a red top, 30µ filter element in the first and a blue top, 10µ filter in the second. If desired, a third filter unit containing a brown top, 2µ filter might be included, although fuel that has been filtered down to 10µ will be clean enough for transfer to the main fuel tank or for direct use, once the filtered fuel has been checked.
Checking a sample
Once the new fuel is on board, the secondary system is turned on and remains running until you are confidant that the fuel is clean enough to use. A reasonable check of the cleanliness of the fuel can be done by drawing off a small sample (include a drain valve in the system plumbing for this purpose) onto a clean piece of white paper. Examine the color of the stain and sniff its odor. Good fuel will wet the paper just as a few drops of olive oil from the galley would and although it won’t smell like best quality extra virgin, it won’t smell burned or sour. Once the fuel is judged acceptable for use, set the secondary filtering system’s valves to route its fuel flow to the main tank or to the tank that was supplying the engine during the time when the new fuel was being cleaned.
Regardless of the details of a vessel’s fuel storage and delivery system or where the fuel is obtained, there are three routine operational steps that are necessary to ensure a reliable supply of clean fuel: a biocide must be added to fuel as it is taken aboard, the tank sump (the lowest part of the tank if a physical sump is not provided) should be drained at least once a year and the O-ring on the underside of the deck mounted fuel filler cap inspected and lubricated. The biocide will discourage the growth of bacteria and fungus. Removing the water that collects at the bottom of the tank will discourage the growth of the “bugs” since they exist primarily at the water/fuel interface and ensuring that the fuel filler cap O-ring is okay will prevent rain and seawater from flowing into the tank.
The bottom line regarding fuel polishing systems is that if you voyage in areas where clean diesel fuel is available, the vessel’s existing system is very likely doing all that needs to be done to provide the engine with a reliable supply of fuel. If you plan to cruise where the fuel will be of questionable quality, an additional fuel filtering/recirculation (polishing) system will be a welcome addition. Regardless of where you may voyage, always carry a supply of your favorite biocide and more spare filters than you think you will ever need.
Contributing editor Chuck Husick is a sailor, pilot, photographer and engineer who lives in Tierra Verde, Fla.