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.
|
Viscosity
|
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. |
|