Fuels Technical Information

For practical operation viscosity is the most important fuel characteristic. It is used for the classification of residual fuels, as in the marine market they are commonly referred to by viscosity; for example as IF ***, or as IFO ***.

IF is the abbreviation for "Intermediate Fuel" and IFO for "Intermediate Fuel Oil". The *** is a numerical value, which is the kinematic viscosity of the residual fuel in centistokes (cSt) at 50^oC. For example IF 380.

Knowledge of viscosity is necessary for the estimation of the required temperatures for storage, pumping and injection. It is also necessary for the determination of heating required for a fuel for transfer purposes.

The minimum viscosity for fuel transfer is determined by the maximum viscosity acceptable by the transfer pump. If the actual viscosity at transfer is lower than the pump design, the rate of transfer will be faster than the rated capacity of the pump.

The viscosity of a fuel is its resistance to shear or flow, and is a measure of the fuel's adhesive/cohesive or frictional properties. This arises because of the internal molecular friction within the fuel producing the frictional drag effect. There are two related measures of fuel viscosity which are known as dynamic and kinematic viscosity.

Dynamic viscosity is also termed "absolute viscosity" and is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid.

In SI units the theoretical unit is the Poise. As these units are large, it is more usual to divide them by 100, to give a smaller unit called the centipose (cP).

Kinematic viscosity is the ratio of the absolute viscosity to the density. In SI units, the theoretical unit is the Stokes. As these units are large, it is usual to divide them by 100, to give a smaller unit called centistokes (cSt), and these are the units used for marine fuel.

For either dynamic or kinematic viscosity to be meaningful a reference temperature must be quoted. In ISO 8217 the reference temperature for residual fuels is 100^oC, whilst for distillate fuels it is 40^oC.