Water Contents

  General
 
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.

Various problems caused by water
 
An uncontrolled water content can result in a number of operating difficulties as follows ;

    • To cause some retardation in the speed of combustion, resulting in still burning particles striking the cylinder wall and crown
    • To cause to corrode tanks and pipelines with a consequent rise in the iron content Steam formation, resulting in vapour locking in heaters and foaming in the mixing tanks
    • To erode injectors with steam, caused by cavitation
    • To disrupt the atomizing spray pattern
    • To incur sludge formation
    • To dilute the cylinder liner oil film
    • To foul turbo chargers
    • To corrode exhaust valves

Sea Water/ Fresh Water

    • Sea Water Contamination
      Sea water is sometimes delivered with fuels as a result of contamination by the delivery barge. This could be due to barge ballast water being mixed with the fuel during barge loading, barge voyage or during the discharge, being caused by structure defects on the barge or incorrect valve operations. Seawater can also enter the ship's storage tanks, again from admixture of ballast water or ingress of the sea by defective hatch covers and shell plate fractures. One fairly common cause of ballast water entering ship's fuel tanks is the corrosion of fuel tank sounding pipes which pass through ballast tanks.
    • Fresh Water Contamination
      Fresh water may be delivered with the fuel, or introduced on board by tank condensation, incorrect setting of purifiers, leakage of steam heating coils and ingress of rain water. Fresh water contamination is not as serious as sea water, however, emulsion forming tendencies and sludge problems may be experienced.