Stability and Compatibility

Before taking on bunkers it is advisable to take a sample from the suppliers tanks and test it for stability and compatibility. If the fuel is unstable it is likely to stratify in the tanks and form asphaltenic sludges. If it is incompatible, then although it may be stable on its own, when mixed with existing bunkers in the ships tanks, it may form sludges. This is why fuel should be bunkered into empty tanks whenever possible.

A small sample of the fuel is heated to 50°C to encourage instability. A sample of the preheated mixture is placed on chromatographic paper and dried in a small oven. The resultant spot is compared against reference samples. In the case of a test for compatibility the two fuels are mixed in the same proportions as would be found in the bunker tanks and the procedure repeated.


The water test for fuel oil is similar to that for lubricating oil. A sample of the fuel (5ml) is diluted with kerosene or toluene (20ml) and placed in the test container. A satchet of a metal hydride is opened and placed upright in the container without allowing the chemical to come in contact with the diluted fuel. A lid containing a pressure gauge is screwed on carefully and the container inverted and shaken for 2 minutes. The water in the fuel reacts with the metal hydride producing hydrogen. The pressure in the container is proportional to the water content and can be read from the scale.

If water is present then a further test should be made for salt water content. This involves separating the water from the oil and testing for the presence of sodium chloride using a go/no go indicator.

The latest equipment has a pressure transducer fitted in the base of the container. After shaking, the container is placed on the test station module and the water content read off a digital display. ( http://www.kittiwake.com )


A sample of the fuel is heated to 50°C and a hydrometer lowered into the oil. When a steady condition has been reached, a reading is taken where the meniscus touches the hydrometer stem and the density corrected to that at 15°C in vacuo.


Although the laboratory test for viscosity of fuel involves timing a quantity of fuel passing through an orifice, the on board testing of viscosity usually involves rolling a ball down a tilted or inverted tube, which contains a sample of the fuel. Improved accuracy is obtained by heating the sample (50°C). The time the ball takes to roll down the tube is measured and the result read directly from the instrument or from a conversion chart.

Calculated Carbon Aromacity Index (CCAI)

This is calculated from an empirical formula using the density and viscosity. Modern sophisticated test kit computers will carry out the calculation for you. However the calculation is not difficult and can be carried out using a calculator with a logarithmic function. The value, which should lie between the limits of 800 and 870 give a guide to the burnability of the fuel and its ignition characteristics. The higher the number, the longer the ignition delay, and the hotter it will burn. This number is used as a guide to setting the fuel quality lever on the fuel pumps.

The formula is CCAI = (density in kg/m3 @ 15°C) - 81 - 141log[log(viscosity in cSt @ 50°C + 0.85)]

For example: We have bunkered a fuel and measured its density at 15°C as 989.9kg/m3, and its kinematic viscosity as 300cSt at 50°C.

CCAI = 989.9 - 81 - 141log[log(300 + 0.85)]

= 908.9 - 141log[2.47835]

= 908.9 - (141 × 0.39416)

= 908.9 - 55.6

= 853.4

Pour Point

If required, an on board estimation of the fuel's pour point can be made. It is not necessary to purchase special equipment for this purpose. The following on board method indicates if the fuel has a high or low pour point.

A sample of 200ml of fuel oil is placed in beaker of 250ml and heated by immersing the beaker in a water bath of boiling water. It may be necessary to hold the fuel oil beaker in the water while the fuel is heated to about 50°C. The beaker is then placed in a refrigerator and the temperature noted by means of either a digital or stem thermometer. At increments of 3°C the beaker should be removed from the refrigerator and the contents tilted to see if they flow. This should be repeated until no flow is observed.


Closed Flash Point

In the Pensky Martens apparatus approximately 75ml of sample is heated at a slow constant rate with continual stirring. A small flame is then directed through an opening at regular intervals with simultaneous interruption of the stirring. The flash point is the lowest temperature at which application of the test flame causes the vapour above the test portion to ignite. Alternatively an automatic go/no go flash point tester can be used. Minimum flash point is 60°C


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