Water is the most common fuel contaminant and the level of water present is very low. The standards allow water up to a maximum of 1% in residual fuels, however, the majority of fuel deliveries have water contents below 0.5%.
The ingress of water can come from a number of sources, which include tank condensation and tank leakage, and can generally be avoided by good management. Where steam is used for tank heating purposes the pressure in the coils is usually greater than the head pressure in the tank, hence any leakage will result in an increased water content in the fuel. A further potential source is the purifier if an incorrect gravity disc is used for the fuel being treated, or if the water control solenoid valves are leaking on the centrifuge. Sounding pipes should be securely capped at all times when not in use. Air pipes must remain open and consequently be arranged to terminate as high as possible, or be fitted with approved self closing devices to avoid breaking water. They should be in protected locations, safe from mechanical damage during cargo handling or from flooding when washing down decks.
Gross water contamination may be reduced by gravity separation in the settling tank. The practical effect of this depends on various parameters, which include the density and viscosity of the fuel and the height of the settling tank. Both the density and viscosity are affected by the fuel temperature and the rate of settling is governed by Stokes' law. Further reduction in the water content should take place in the purifier. Water can become intimately mixed with the fuel to form a stable emulsion, through churning in pumps or throttling through valves. This stable emulsion is very difficult to remove by conventional means.
caused by water
Sea Water/ Fresh Water