Fuel oil properties and effects – Part I

Into the following posts I will explain about fuel oil properties and their effects from operational point of view.  I will not write and show any mathematical formulas as these formulas can be found in dedicated manuals and anyone who is interested in depth theoretical knowledge can search for the same. The properties described in these posts are explained in order to gain a better understanding of the complexities of fuel oil behavior. Very good supervision, engine maintenance and fuel treatment equipment is necessary especially when the properties of the fuel used is near the permitted maximum and minimum limits. Poor quality fuels or insufficient or inadequate preparation can lead to problems in handling and/or combustion. This will itself lead to higher maintenance requirements, shorter service intervals and possibly shorter service life of various components of the equipment.


The term “density” is essentially synonymous with the older terms “specific gravity” or “relative density” numerically. Both of these terms are devoid of units because they refer to the mass of a given volume of oil in relation to the mass of the same volume of water at specified temperatures. Because density varies with temperature, the higher the temperature – the lower the density, densities must be compared at the same temperature, which is typically 15 degrees Celsius nowadays. Density is not a direct indicator of fuel quality, nor is it necessary to associate high density with high viscosity. A high density shows a high aromatic content. A precise understanding of fuel density (information is provided on the bunker receipt) is critical for several reasons:

  • It enables the verification of the actual weight of oil supplied;
  • the accurate determination of specific fuel consumption;
  • operationally, the optimal selection of gravity discs for purification equipment (although nowadays modern purification equipment doesn’t require a selection and replacement of gravity discs anymore).

Densities of the order of 0.991 and higher are expected, but with the new low sulphur fuels, the density is now lower than this value. As the density of the fuel approaches that of water, water removal by centrifugal separators becomes increasingly difficult. The 0.991 limit is imposed by the use of conventional separators used as purifiers with a water seal. At operating conditions, the density differential between the oil and the water must be 4 percent or greater, corresponding to an oil with an upper density of 0.991 kg/m3. However, improved separator design allows some modern vessels to operate with fuel oils with densities as high as 1010 Kg/m3. 


The viscosity of a fluid is a measure of its resistance to flow. Viscosity decreases as temperature rises and increases when pressure rises dramatically. While viscosity should not cause any operational issues as long as the proper temperatures for handling and combustion are maintained, it should be noted that the viscosity quoted on the fuel delivery note frequently only refers to the maximum viscosity ordered (e.g. IFO380 is 380 cSt @ 50 deg C) and the actual viscosity of the fuel delivered may be different. It was common practice in the past for the delivered fuel to be much lower viscosity than requested, making it easier to handle and burn. However, due to cost, the fuel has recently tended to be closer to the quoted viscosity, and in some cases even higher. It should be noted that the majority of heavy fuel supplied as ship bunker is a blend of extremely high viscosity residual oil (possibly in the region of 500-600 cST at 50 degrees Celsius) and a low viscosity “cutter stock” to achieve the desired viscosity. Certain fuel blends may have a tendency to “layer” in storage tanks, causing issues with bunker handling, treatment, and combustion. The marine industry has traditionally used viscosity to determine the price of fuel oils. Although higher viscosity fuels are less expensive, they are not always of lower quality than lower viscosity products. Viscosity is not a good indicator of fuel quality on its own.However, it should be noted that viscosity has a direct relevance on the ease with which a fuel can be burned (though it does not define the combustion properties of the fuel), as correct atomization of the fuel at the injectors in terms of drop size, penetration, and spray pattern requires that the viscosity at the injector be kept within the engine manufacturer’s recommended limits. In practice as some companies are using high viscosity fuels (RMK 750) due lower price, a recommended injection viscosity between 13 mm2/s cSt and 17 mm2/s (cSt) is impossible to achieve  taking into consideration the safe injection temperature of 150 deg. C.In my practice we use to run RT-Flex engines on RMK 750 cSt fuels at 148 deg.C with a viscosity of 22 cSt to 23 cSt, obviously with company’s approval. 
The viscosity index is defined as “an arbitrary number used to characterize the variation of the kinematic viscosity of a petroleum product with temperature” by standard test procedures (ASTM D2270/IP226). For oils with similar kinematic viscosity, the higher the viscosity index, the less effect temperature has on its kinematic viscosity. The vast majority of viscosity-temperature conversion diagrams are based on fuel with a viscosity index of approximately 70. Historically, this has been found to be sufficiently accurate for day-to-day conversions, but some of the more severely processed fuels now available have viscosity indices that differ significantly from the accepted norm.In the absence of any knowledge of the viscosity index, fuel of this type will cause viscotherm temperature readings to deviate from the “normal” expected value for a specific viscosity fuel. The set temperatures are maintained on thermostatically controlled heaters, but the actual viscosity of the fuel at these temperatures does not correspond to the charts – this may affect injection viscosity and purification conditions.Higher-than-expected viscosities at low temperatures should not be confused with the effects of wax crystallization, which causes the fuel to behave in a non-Newtonian manner, rendering standard measuring techniques invalid. 

Flash Point

Residual fuels are typically supplied with a “minimum flashpoint of 60°C,” with the actual value not usually quoted at delivery.The flashpoint of a liquid fuel is the temperature at which the fuel emits enough inflammable vapour to form an explosive mixture with air that ignites or “flashes” when it comes into contact with a small flame.The flashpoint temperature can be expressed as “closed” or “open,” depending on the type of test method used.For many years, a minimum flash point has been an international requirement. SOLAS (Safety of Life at Sea) regulations state that fuel with a flash point lower than 60°C renders the vessel not seaworthy. The flash point of the fuel is another criterion established by Maritime Insurers. As high temperatures are required for purification/clarification, in combination with the design of these systems, may result in high service tank temperatures. It is sometimes necessary to have an accurate knowledge of the flashpoint in order to comply with classification society and/or company regulations concerning upper allowable tank temperatures in relation to flashpoint temperatures. 

Pour point

The terms upper (or maximum) pour point and lower (or minimum) pour point refer to the ASTM D97 procedure for testing. Pour point is strictly defined as the lowest temperature (expressed in multiples of 3°C) at which the oil flows when cooled and examined under specified conditions.In practice, this means that the pour point temperature of a fuel is the temperature at which wax crystallization prevents the oil from flowing, to the nearest 3°C. It is thus useful as a guide to the lowest permitted bunker storage temperature in order to avoid handling difficulties. However, it should be noted that the lowest permissible pumping viscosity is in the region of 1000 cSt, but the temperature corresponding to this viscosity is unrelated to the pour point temperature.The pour point temperature is usually a good indicator of the amount of wax present.Pour points ranging from 30°C to 45°C are becoming available as a result of the discovery of high pour point African fuels, which have a higher wax content. Because wax has a low coefficient of heat transfer, allowing a high pour point fuel to cool and solidify in storage tanks makes re-liquefying by subsequent heating nearly impossible. As a result, double bottom and wing tanks containing high pour point fuel should be kept at a temperature above the oil’s pour point and the fuel used as soon as possible. Experience has shown that a small amount of wax is enough to cause a significant reduction in the pour point temperature of a fuel. In the absence of written confirmation of pour point temperature on the bunker receipt/delivery note, the delivery temperature may serve as a guide, as if bunkers are delivered at an unusually high temperature, there is a possibility that it has a high pour point. 

-End of Part I –

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