424 Diesel fuel quality issues Contaminants

Diesel fuel contaminants can be divided into three general categories: (1) those originating as foreign materials and introduced into the fuel such as dust, dirt, rust, and water; (2) Those produced in the fuel through biological activity of bacteria, yeast, and fungi; and (3) those produced by fuel degradation such as gums and sediments. This latter category relates to fuel stability and is discussed as a separate section.

Dust, dirt, and rust can be controlled by rigorously enforced standards for cleanliness in fuel transport, storage, and utilization. Regular cleaning of fuel tanks and supply lines can minimize the potential for damage from these sources.

Water can cause corrosion of fuel storage tanks and fuel injection system components. It contributes to fuel degradation and microbial growth. Water most often enters the fuel tank as condensate from moist air entering through vents. Many fuel storage tanks have condensed water at the bottom, and this water should be removed regularly. When the water cannot be removed, the fuel should be treated with a biocide and fuel should not be drawn from the bottom of the tank.

Diesel fuel can be contaminated with microbial growth. Species such as cladosporium resinae and Candida tropicalis have been frequently observed in diesel fuel samples. These microbes, consisting of both aerobic and anaerobic bacteria as well as yeast and fungi, require water and are generally found at the fuel-water interface. They are usually controllable by eliminating any free water. Microbes can also be controlled by adding biocides to the fuel or to the water at the bottom of the tank, but good housekeeping practices can minimize water contamination. This latter approach is preferred. Fuel stability

When diesel fuel is subjected to air at elevated temperatures for extended times, it undergoes chemical changes that alter its colour from light yellow to dark brown or black. It also forms gum and other sediments. Gum may be either soluble gum, which is indicated by darkening of the fuel, or insoluble gum, which precipitates from the fuel and deposits on exposed surfaces. The processes that form gum are not well understood but may involve the formation of radicals by heating or catalytic action of transition metals such as Fe, Cu, and Mn. These radicals react with oxygen to form peroxides that combine to form gum particles that can plug fuel filters and cause sticking of fuel pumps, injectors, and governor components34. The presence of water can aggravate gum plugging of fuel filters by converting a reduction in fuel flow into a completely plugged filter34.

Peroxides and hydroperoxides can form in diesel fuel and can actively attack elastomers that are normally compatible with diesel fuel. Antioxidants such as hindered phenols and amines effectively prevent peroxide buildup although they may produce colorbodies as they consume peroxides. Metal deactivators are also sometimes added to diesel fuel. They function by converting any trace metal salts, which could catalyse fuel oxidation, into chelates that have no catalytic effect.

ASTM Test Methods D3241, Thermal Oxidation Stability of Aviation Turbine Fuels (JFTOT Procedure), and D2274, Oxidation Stability of Distillate Fuel Oil, can be used to characterize a fuel's stability and the effectiveness of additives.

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