All fuels contain some water in suspension. But unlike gasoline, diesel fuel is less refined and will hold a much larger amount. This water can cause severe problems with water separators on the equipment. It can also cause the fuel injector tips to explode, resulting in expensive repairs. In fact, slugs of water in the fuel can cause sudden cooling in the engine and may result in shortened engine life.
Diesel fuel can contain two types of water: water in solution and “free water.”
“Diesel fuel can contain low levels of water that may be dissolved in the fuel, thus the term ‘water in solution’,” says David Harvey, manager of product quality and technology, CITGO Petroleum Corp. “These very low concentrations of water are reported in parts per million (ppm). There are various reasons that a diesel fuel may contain dissolved water. Among them are condensation of water in a fuel tank, components in the diesel fuel which help to retain the water in solution and fuel temperature.”
Only low concentrations of water can remain as water in solution. “Water does not normally exist in a standard diesel fuel in high concentrations due to the significant differences in the chemical properties of diesel and water,” says Harvey. “The two liquids will typically separate, with water dropping out of the diesel fuel and accumulating at the bottom of a fuel tank.”
Water in the diesel fuel that is not a dissolved component is considered free water. “This would include water suspended in diesel as the result of agitation or which is phase separated from the diesel fuel,” Harvey explains. “The most common cause of free water in diesel fuel is poor housekeeping of storage tanks. The most common cause of dissolved water is the composition of the fuel itself, or additives which help to dissolve the water into a solution.”
Water in suspension in burning fuel reduces the amount of energy available and may result in lower horsepower output. Water in fuel tanks, lines, injectors, filters, etc., will freeze more readily than fuel. Most fuels freeze at less than -20° F; water freezes at 32° F. Note that the cloud point of diesel is around 10° F to 15° F. The pour point is typically around 0° F.
It is important to understand that low levels of water dissolved in the fuel are not necessarily a problem. “Diesel fuel containing low levels of dissolved water, in the ppm concentration range, will typically provide satisfactory service,” says Harvey. “However, free water in diesel fuel could result in excessive injector wear, filter plugging, power loss and corrosion of engine fuel system parts.”
You can often tell if there is a problem by a simple visual inspection. “If the fuel is hazy, or there is evidence of free water, then there is too much water in the fuel system,” says Harvey. “Hazy fuel would suggest that sufficient water is being retained in the fuel, typically by a co-solvent or additive, which is holding the water in suspension.”
Practice good housekeeping
Water in diesel fuel becomes a problem during transport, storage and use. Newly refined fuel is normally clean and free of excessive moisture. Refiners and pipeline operators adhere to strict fuel storage tank maintenance procedures with regular removal of water bottoms and occasional chemical treatment to assure ASTM specifications are met. Unfortunately, after fuel leaves these facilities, water bottoms removal is often neglected.
Several factors contribute to moisture accumulation, including climate, storage tank installation and fuel handling procedures. Temperature changes can cause suspended water in the fuel to settle out.
For instance, any time warm fuel is placed in a cooler tank, whether for storage or transport, suspended water or moisture will drop out of the fuel as it cools. This makes it necessary to periodically drain off the water, which is a simple task. Water is more dense than fuel, so it always settles out to the bottom of the tanks.