The vast majority of fuels contain less than 0.2% water; significantly below the ISO8217: 2010 limit of 0.5% for residual fuels in the RMB to RMK categories.  

Do not pay for any water in the fuel over and above the limit stated in the appropriate standard. Water can be introduced into the fuel supply chain from a number of sources including tank condensation, tank leakage or deliberate contamination. In practice, the nature of the water present may be fresh, brackish or salt. The worldwide salt content of seawater varies however. In first order terms, 100 mg/kg (PPM) is associated with 1% seawater contamination in the fuel.

Excessive water represents a triple loss:

Firstly, there is the loss of specific energy in the fuel, which will affect the fuel consumption
Secondly, there is the cost of disposal of the water removed by the treatment system. Such water is unlikely to pass through a 15 mg/kg (PPM) oily water separator, so it has to be retained for disposal later, with a cost to the ship operator
Thirdly, water will damage fuel injection equipment, cause corrosion and failure to exhaust valves and turbochargers. The settling tank will remove gross water as long as it is not emulsified. Warm fuel will shed water faster than cold fuel in the settling tanks. The drains should be checked regularly, especially on first use of a new fuel delivery

Comment

Water in fuel oil graph

A well-operated centrifuge should remove the vast majority of water. Operating temperatures in the centrifuge should be maintained as high as possible for efficient separation, usually around 95° - 98°C, but not above as this can result in steam bubbles which can cause erratic behaviour and loss of the water seal. It should also be remembered that water can be removed from high-density fuels of up to 1010 kg/m³ using modern purifiers without gravity disc, whereas traditional types are only suitable for fuels up to 991 kg/m³.

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