What is Viscosity ?

 Among the various characteristics of fuel oils, the most important thing is viscosity. In the marine fuel market, for the classification of fuels, it is commonly referred to; for example as IF ***, or as IFO ***.
IF is the abbreviation for "Intermediate Fuel".
IFO for is the abbreviation for "Intermediate Fuel Oil". But terminologically speaking,  IFO is better than IF.
*** is a numerical value, which is the kinematic viscosity of the residual fuel in centistokes (cSt) at 50oC. For example IFO 180, IFO 380.
Kinematic viscosity is the ratio of the absolute viscosity to the density. 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.

 Knowledge of viscosity is necessary for the determination of heattemperatures required for a fuel for transfer purposes. It is also necessary for the estimation of the required temperatures for storage, pumping and injection. 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 minimum storage temperature for all fuels should be at least 5oC above the pour point. For fuels greater than IF 100 the actual temperature required for transfer will be higher than that of the pour point limit.

 The viscosity of all marine fuels varies with temperature, and reduces as the temperature increases. Knowledge of the viscosity/temperature characteristics enables the temperature for a required viscosity to be estimated. The viscosity/temperature relationship for any particular fuel depends on the crude oil source and the refinery processes employed during manufacture.

 Residual Fuels invariably have to be heated to reduce the viscosity to that required for atomization. For diesel engines the injection viscosity is usually in the range 8-27 cSt, while for a steam plant the atomization viscosity is usually in the range 15-65 cSt.


ISO 8217 max CST
at 50oC
RMA 10 50
RMB 10 50
RMC 10 50
RMD 15 100
RME 25 225
RMF 25 225
RMG 35 390
RMH 35 390
RMK 35 390
RMH 45 585
RMK 45 585
RMH 55 810
 If the measured viscosity is marginally above the ordered viscosity, various practical aspects have to be considered to determine the suitability of the fuel. These are the increased heating for correct atomization and the operation of the fuel treatment plant. In practical terms the required increase in temperature will only be a few degrees, which the existing heater should be able to accommodate. Should the actual viscosity be significantly in excess of that ordered the fuel may be unsuitable because of the constraints of the fuel treatment system. For example a fuel of 35 cSt at 100oC (ISO grade RM35) would be unsuitable for a system designed to handle a maximum of RM25 grades. Also in the case of smaller tonnage where no fuel heating exists, fuel in excess of 10 cSt at 100oC would also be unsuitable, because of the requirement for heat at a higher viscosity.