What's In A Fuel/Water Separator

Regardless of your boat or fuel type, a good quality fuel/water separator is your first line of defense against water ingress that can ruin your day in the best-case scenario and cause costly damage in the worst.

There are a variety of styles, from simple spin-on filters to modern turbine systems that spiral fuel in a centrifuge to separate its contents, but your engine requirements and style of boating will determine the best choice for your boat. Most filters are designed specifically for gas or diesel, so keep that in mind when selecting the right filter for your boat. But how do they work?

The Science Of Separation

The most basic type of fuel/water separator consists of a metal canister that threads onto a mounting bracket with inlet and outlet ports to attach fuel lines. The canister houses a filter element with a spring beneath it to hold the element against the top of the canister, allowing space for water to accumulate in the bottom of the housing. These are similar in appearance and function to an oil filter.

As incoming fuel flows into the inlet port on the bracket, it enters through the small holes lining the outside perimeter of the filter. Next, the laws of physics take over. Water is 2.5 pounds heavier per gallon than gasoline (1.5 pounds heavier for diesel), and because fuel and water do not mix, large droplets of water immediately fall to the bottom of the filter canister.

Fuel is then pulled through the outside of the filter media, which surrounds a perforated metal center that keeps the media from collapsing under the vacuum of the engine’s fuel pump(s). The media is pleated to increase its surface area and is usually made of cellulose-based material derived from wood pulp, with a paper-like composition. Most filters use media treated with a hydrophobic substance, which blocks water by causing smaller drops to coalesce, forming larger droplets that settle to the bottom of the canister. This part of the filter also traps any particles, like dirt, metal shavings, or errant rubber from the fuel system.

An example of specialized treated media is RACOR’s Aquabloc type, which incorporates pleat-spacing corrugations and a graduated pore structure to increase dirt-holding capacity and extend filter life. Aquabloc media is waterproof and rustproof, capturing contaminants, while the specially treated surface separates water from the fuel.

With water at the bottom of the canister and any debris trapped by the filter media, clean fuel moves up through the center portion of the filter and exits the bracket via suction from the engine fuel pump(s).

Another popular style also uses a metal canister that contains filter media but employs a separate collection bowl that threads onto the bottom of the filter element to collect separated water, usually with a small petcock at the lowest point to easily drain any accumulated water. With this style, there are two common types of bowls: clear plastic and metal. The clear design is beneficial because it provides an easy way to visually identify any water in the system, and it is the favored type by many boaters. However, this type can’t be installed just anywhere.

A clear bowl is acceptable if installed in an open-air location, or an enclosed location if a metal “fire bowl” is used, which is installed underneath the collection bowl and shields it from flames and heat. The Code of Federal Regulations, or CFR, dictates a filter bowl must withstand 2.5 minutes of fire-induced heat without leaking excessively. The metal fire bowl shields the collection bowl to accomplish this. The alternative is a solid metal collection bowl, which is acceptable in most enclosed locations.

More To Consider

For boats with large engines and high fuel consumption rates, turbine filter systems offer an added degree of filtration with higher flow rates. In addition to the filter element and collection bowl, these filters use a centrifugal fin design to rapidly “spin” incoming fuel, forcing large particles and water droplets to the outer edges of the assembly, where they can fall into the collection bowl before the fuel passes through traditional filter media and out to the engine(s). These filters are remarkably effective at polishing large amounts of fuel and are ideal for long-range vessels.

Regardless of design, the two main criteria for choosing a fuel/water separator are fuel-flow rate and micron level. Flow rate is measured in gallons-per-hour (gph) and engine manufacturers will almost always specify the required filter size. But generally speaking, the larger the engine, the larger the fuel/water separator should be. The common 60 gph and 30 gph sizes are adequate for most recreational boats, but filters with higher flow rates exist for those that need them.

Micron level is the size of the pores in the filter media, which indicates the size of particles it will catch. For example, a 10-micron filter will catch particles 10 microns and larger (for comparison, the average human hair is around 100 microns in diameter). Most marine water separators are available in 2, 10, or 30 microns, however, your engine manufacturer will typically specify the recommended micron level, despite opinions from folks around the docks.

Maintenance

Whatever size and style of filters suit your boat, check them often for water accumulation, because if the assembly becomes full of water, the filter will no longer be able to do its job, and water will flow out and to your engine(s). These filters are designed to catch any water or debris before it makes its way into your engine(s) because, by the time you receive a water-in-fuel alarm (if one exists), you will be forced to immediately shut down or risk an intrusion that can quickly lead to severe – and costly – mechanical damage.

In addition to regular inspections, filter elements or canisters should be replaced periodically based on both the boat/engine and filter manufacturers’ recommendations. Changing them at yearly engine maintenance intervals is a good minimum, but for boaters using fuel of questionable quality or those putting above-average engine hours on, more frequent changes may be necessary. Dirty filters will impede the flow rate and can adversely affect performance. Some filter manufacturers, such as RACOR, offer an optional fuel vacuum gauge that indicates an increasing vacuum value as the filters become clogged. When changing filters or elements, be sure to fill the new filter with clean fuel and take the necessary steps to purge any air from the system before starting the engine(s).

For cruisers fueling abroad or from otherwise unreliable fuel sources, water-separating funnels are also a great tool to have aboard, as they can be used to catch water during fill-ups and prevent any contaminated fuel from making it to your tank in the first place. While effective, many of these funnels have a low flow rate, which makes fueling very slow. For example, one of the most common models, the RACOR 50-micron RFF1C, has a flow rate of just 2.5 gpm, which can make fueling a major inconvenience for boats with large tanks, but it can also save the day (or a trip) in a pinch. Another popular model, the Mr. Funnel F15C has a flow rate of up to 15 gpm with a 140-micron level, making it a good option with a higher flow rate despite slightly less particle filtration ability.

We have discussed different types of filters, but there are also others. The bottom line is to investigate what’s best for your engine and system and spend the necessary money. It will probably save you more in the near future.