Water condensation in diesel fuel storage tanks is a common problem. The longer the fuel is stored, the more pronounced the problem. If water is allowed to remain in the diesel while it is stored, microorganisms or bacteria can form that feed on the hydrocarbons in the fuel. This results in slime, which can clog filters, cause a foul odor and result in discoloration of the fuel.

“Good housekeeping practices are the most important means of minimizing water in diesel fuel systems,” Harvey emphasizes. “Such practices include periodic draining of water accumulated in fuel tanks, maintaining the seal integrity of fuel storage tanks and allowing settling time of the fuel after a delivery into a storage tank. (This affords water the opportunity to separate out from the diesel prior to distribution.) You must also adhere to a maintenance program which includes eliminating or preventing microbial contamination of the tank contents.”

There is no set schedule for draining water from storage and equipment tanks. “Water should be drained from a fuel/water separator whenever free water is detected or visible in the system,” says Harvey. “Tank size does not necessarily influence the maintenance interval unless the fuel system is not properly sealed. Climate can have an impact.”

Above-ground tanks are more prone to significant daytime to nighttime temperature swing, which generates water. The fuel temperature drops at night, lowering the water solubility limit, and moisture drops out of the fuel. Unless the fuel is agitated, this water does not return to the fuel.

When the tank warms, humid air above the fuel cools and water condenses from the air. Underground tanks are often cooler than the air above ground. As fuel is dispensed, warm, humid air takes its place. As the air cools, water condensation forms.

No matter what types of tanks are used, make sure they are properly sealed to prevent rainwater contamination.

Effects of Humidity and Temperature fluctuations

“Areas of high humidity accompanied by low temperature would be expected to experience greater incidences of water accumulation from condensation,” says Harvey. “Diesel fuel can hold some water in solution. But as the ambient temperature decreases, the water has a higher potential of separating from the diesel and accumulating in the bottom of the tank.”

Warm, humid climates can promote microbial growth.“Warmer temperatures tend to be more susceptible to microbial contamination with water contamination of the fuel phase,” says Harvey.

Sometimes temperature fluctuations are caused by the operation of the equipment itself. While the equipment is operating during the day, warm air is drawn into the fuel tank. As the air above the fuel cools, water condenses. If the tank is left partially filled overnight, it is a prime candidate for moisture accumulation. Topping off the tanks at the end of the day gets rid of the humid, warm air in the tank and helps prevent condensation.

What about the use of desiccant filters? “Utilizing desiccant filters could provide extra protection,” says Harvey. “In low humidity environments, such filters may be unnecessary. But under high humidity conditions, these filters would become saturated relatively quickly, resulting in increased operational expense. These filters perform no function if they are saturated and not promptly replaced. Conducting periodic checks for water in the fuel tanks, when performed properly, can eliminate the need for desiccant filters.”

Chemical treatment

There are chemical treatments available to help with the effects of moisture contamination. “Glycol ethers commonly used for diesel fuels are typically applied to suppress the freeze point of water, which may be in a diesel system, in order to prevent filter plugging from ice crystals,” says Harvey. “They are also applied to ‘dry out’ a fuel system.”

Yet, there are drawbacks to glycol ethers in some cases. “These chemicals increase water contamination by attracting water into the diesel fuel as dissolved water,” Harvey notes. “The glycol ether holds the water in the fuel and thereby delivers more water to the fuel filter, injectors and combustion chamber. When properly applied, however, these chemicals can be a satisfactory part of a good housekeeping practice.”