Mythbusters: Fuel Filters
Understanding how fuel filters work will help guarantee a safer engine room:
Every cruising powerboat has a primary fuel filter, and Parker Hannifin’s Racor line dominates the market. At a recent TrawlerFest diesel engine class, a number of questions came up regarding these filters. Having heard these questions before, I realized that many of us do not have a clear understanding about this critical piece of equipment. Lack of clarity can lead to poor decisions. Before we look at specific misconceptions, it will be helpful to look at the overall design. The image (of the internals, see below) illustrates the typical fuel-flow pattern. Diesel fuel enters through the side port and is directed down where it moves a ball out of the way and passes through a centrifuge. The fuel then flows into the upper chamber. The filter element fits onto a tube and the tube provides suction, drawing the fuel through the outside of the element into the center. This tube then draws the diesel down again, allowing it to exit directly opposite from the entry port.
The first misunderstanding about fuel filters revolves around the ball. Many believe the ball serves
as a check valve to keep water from going up into the filter element. As the theory goes, the ball is plastic and floats, blocking water. In fact, the ball is aluminum, doesn’t float, and has nothing to do with water.
The engineers put it there to prevent fuel from draining out of the filter and back into the supply line when the engine is shut off. The flow of fuel back toward the tank pushes the ball into a seat, acting as a check valve to stop the flow. In some applications, such as when the filter has been installed below the tank pick up, this back flow would not be a concern. But in many applications the filter can be at the high point, and fuel running back to the tank can easily lead to a loss of prime.
After the fuel flows past the ball it enters the centrifuge at the base of the assembly. A centrifuge works this way: By spinning a mixture, the denser material accelerates and moves to the outside where it falls to the bottom and can be collected and removed. Less dense material stays closer to the center and remains in the normal flow. When filtering diesel fuel, denser material consists of two compounds: water and asphaltene. Water weighs more than diesel and consequently the centrifuge separates the water out of the flow and it falls to the bottom of the bowl where it is drained off. All diesel fuel contains asphaltenes– molecular compounds found in crude oil. Asphaltenes can join together forming particles and sludge, and the process is compounded in ULSD fuel and in fuel that reaches higher temperatures (such as the fuel returned to your tank by your engine).
The term “centrifuge” leads to the second misunderstanding about filter performance. Most of us think of a centrifuge as a spinning device, and in the absence of any spinning parts, you might wonder if the filter is working properly. If you look at the centrifuge area visible through the clear plastic base while the engine is running, you won’t see any moving parts, and that leads some to wonder if it is working. The Racor’s design creates a centrifugal effect by guiding the fuel into an accelerated circular flow pattern. The simplicity of no moving parts results in a more reliable device.
On most marine filters, the clear plastic base is surrounded by a metal bowl. Contrary to many opinions, that metal bowl is not designed to capture fluid spills. In order to comply with ABYC safety standards, the filter must be able to withstand 2½ minutes of exposure to burning diesel fuel, without leaking. The metal provides a critical shield between the fire and the plastic base, hopefully buying enough time for the fire suppression system to take over.
(See picture with burn damage - Engine room fires do happen, and having a fuel filter that meets the ABYC safety standards might be a life saver.)
Keep in mind that this requirement applies to the filter housing, as well as any fittings attached to it. Plastic nipples, for example, will melt, allowing fuel to drain out of the filter and potentially exacerbating the fire. If your filter has the designation “MA,” then it meets ABYC safety standards.
After leaving the centrifuge, the fuel flows into the upper chamber where suction pulls it through the outside folds of the filter element into the center. These filter elements have been carefully engineered to filter particulates to a prescribed standard (30 micron, 10 micron, 2 micron), while allowing adequate fuel flow to the engine. Keep in mind that between the filter and the engine there is a pump which draws fuel from the filter to the engine’s fuel system. Restrictions will limit the fuel flow and the pump will create