Fuel/Water Separators

5 ways to combat water in fuel from the Technology & Maintenance Council


Water in fuel can cause serious maintenance issues, such as enhancing corrosion of fuel system components and accelerating wear on fuel components by reducing lubrication. Additionally, water in fuel promotes microbiological growth, which may plug filters prematurely and adhere to fuel system components.

Water can be found in fuel in three different forms: dissolved, free or course and emulsified, says Robert Braswell, technical director of Technology & Maintenance Council (TMC). A technical council of American Trucking Associations (ATA), TMC is North America’s premier technical society for truck equipment technology and maintenance professionals. ATA is the largest national trade association for the trucking industry.

Dissolved water - The presence of dissolved water in petroleum-based fuels usually has an upper limit of 100 to 200 part per million (ppm), Braswell says, but biodiesel can contain up to 1,500 to 3,000 ppm of dissolved water. Dissolved water is not known to have any damaging effect on fuel system components.

Free or coarse water - Free or coarse water does not have a universal definition, he observes. However, these terms are typically used to describe water that is not fully dispersed throughout the fuel.

SAE (Society of Automotive Engineers) J1839 test standard, “Coarse Droplet Water/Fuel Separation Test Procedure,” defines coarse water as having a mean droplet size of 180 to 260 microns. J1839 calls for a test apparatus that generates this range of water droplets to approximate field conditions, which is then used to determine the undissolved water removal performance of a fuel/water separator.

Emulsified water - Emulsified water does not have a universally accepted definition either. Nevertheless, says Braswell, this term is typically used to describe when water and fuel form an emulsion in which the water droplets are so tiny that they remain in suspension for a very long time. In high enough concentrations, it can make the fuel appear milky.

Test standards SAE J1488, “Emulsified Water/Fuel Separation Test Procedure,” and ISO (International Organization for Standardization) 4020, “Road Vehicles - Fuel Filters for Diesel Engines - Test Methods,” are used for quantifying filtration performance with emulsified water.

“Fuel/water separators are designed to remove free/coarse and emulsified water,” says Braswell. “They typically employ one or more methods to remove these types of water. Gravity is commonly used to aid water separation and precipitate water removed by the filter into a settling chamber where it can be drained during maintenance intervals.”

Water removal methods

The various methods of water removal are briefly described below:

Coalescer - A device or medium which unites relatively small water droplets to form larger droplets which are more easily removed from fuel.

Stripper - A device with a filter medium that allows fuel to pass through, but traps water on its surface. As more droplets are removed they coalesce to form larger droplets until they are large enough for gravity to transport them to a settling chamber.

Water stripping can be accomplished by using filter media containing certain fiber material or by applying a chemical treatment to the filter media.

Change in flow direction - A method in which a sharp change in fuel flow direction causes the water to separate due to inertia. This is commonly combined with gravity to aid in water separation.

This method is not effective for very small droplets. Heavier droplets respond better to inertia and gravity effects.

Centrifuge - A device that separates water from fuel due to the fact that water has a higher density than fuel.

Decrease in velocity - A method in which decreasing the velocity of the fuel flow allows the water to more readily separate from the fuel.

Settling tank - That portion of the water separator device where there is no or very little fuel velocity; usually at the bottom of the device. This “quiet” area allows water droplets to settle out and remain at the bottom and not be re-entrained in the fuel.

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