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Fuel Oil Density

From the commercial point of view, bunker fuel density is an essential parameter to measure because residual fuel is ordered by weight but supplied by volume. Mass (weight) is directly proportional to the power that can be gained from using the fuel.  If the actual value is less than that stated, there will be a proportional shortfall in the quantity of product supplied. In addition, the density gives an indication of other fuel characteristics, including specific energy and ignition quality (CCAI).

Density meter

Remember - Mass = Power so an accurate density calculation has a direct financial value.  

Bunker Fuel Density is extremely important;  For example:  

  • Cost/Tonne $500 
  • Average Stated Delivery 800 T, 12 Times/Year = 9600 T
  • Stated Density at 15°C 0.991
  • Volume of 800 T x 12 / 0.991 = 9687 m3 
  • Overstatement in Density 0.005
  • Actual Density at 15°C 0.986
  • Actual quantity received 9687 x 0.986 = 9551 T
  • Shortfall in supply = 49 T

 Cost/Year in Lost Energy = $24500!!

From the technical aspect, the incorrect gravity disc may be selected for setting the traditional purifier.  If the purifier is fitted with a gravity disc for a greater bunker fuel density than the actual value, the internal diameter will be too small. Reduced separation efficiency results because the interface between the oil phase and water seal is moved towards the centre of the disc pack. If the internal diameter is too large then oil can be lost with water to the sludge tank.
It should be noted that as bunker fuel density approaches the limit for traditional purifiers of 991 kg/m3, the difference in density between fuel and water becomes so small that the hydraulic equilibrium in the bowl becomes unstable. This means that any small changes in viscosity/temperature and flow will cause the interface to fluctuate with the potential for reduced separation efficiency or malfunction from a lost water seal. As density of fuels increased in the 1980’s it became common industry practice to position the interface too close to the centre of the purifier, which effectively blocked flow up through the disc stack. This quickly overloaded the lower part resulting in poor separation, which emphasised the importance of operating two traditional centrifuges in series with a purifier followed by a clarifier.
Modern centrifuges such as the Alfa Laval Alcap system effectively act as clarifiers without a gravity disc and have a density limit of 1010 kg/m3. Any water and solids separated from the fuel accumulate at the outside of the bowl. This is emptied on a timed basis. Alternatively when the sludge space fills, traces of water escape with the clean oil, a transducer then senses the water content, which triggers emptying of the bowl.


Bunker fuel density is an essential parameter to measure because fuel is delivered by volume but price and energy is measured by mass. The Parker Kittiwake Density Meter can be used for simple and accurate measurement of fuel density.

Off-line Bunker Fuel

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