Kinematic Viscosity
Kinematic viscosity of an oil is its
resistance to flow at a specific temperature. The viscosity of a fuel decreases
with increasing temperature. The viscosity of the fuel at the injectors has to
be within the limits prescribed by the engine manufacturers. Incorrect viscosity
at the injectors may lead to poor combustion, deposit formation and energy
loss.
Density
Density is the weight of one litre of
the fuel, at 15oC, expressed in kg. Density is used in the
calculation of the quantity of the fuel delivered and invoiced to the customer.
From a more technical viewpoint the density gives an indication of the heating
value of a fuel within a certain product class.
Cetane Number, Cetane Index
Cetane number and the cetane index are
applicable primarily to gasoil and distillate fuels. The cetane number is a
measure of the ignition/combustion quality of the fuel in a diesel engine. The
higher the rpm of the engine, the higher the required cetane number.
The cetane number of a fuel is
determined in an engine test procedure. The cetane index is a calculated value,
based on the density and the distillation of the fuel.
Microcarbon Residue, Ramsbottom and Conradson Carbon Residue
Microcarbon residue, ramsbottom and
conradson carbon residue are three different test methods to check the same
characteristic of a diesel fuel and a heavy fuel: the residue formed when the
combustion takes place under reduced air supply. This residue contains
incompletely burned fuel particles and also the ash formed by the fuel upon
combustion.
Flash Point
Flash point is the temperature at
which the vapours of a fuel ignite, under the specific conditions of the test,
when an external ignition device (flame) is applied.
For safety reasons, the minimum flash
point for all fuels (with the exception of DMX) to be used onboard ships has
been set at 60oC.
Water Content
A small amount of water in fuel is a
common contaminant and has to be removed by onboard purification of the fuel. An
excessively high water content of the fuel after purification may lead to
erratic engine operation.
Ash
Ash content of a fuel is a measure of
the metal content in the fuel.
Sulphur
Sulphur is an inherent element of
certain fuel molecules. Depending on the crude oil origin, the sulphur content
of an IFO can easily vary from below 1.0% to above 4.0%.
Sulphur is oxidized during combustion
and produces oxides of sulphur which may lead to corrosive wear in the engine if
the proper lubricants are not used.
Pour Point
Pour point is the lowest temperature
at which the fuel will pour or flow when chilled under prescribed test
conditions. Bunker fuels originating from a complex refinery generally have
pour points below 5 °C. This is reflected in the fact that bunker fuel is
generally not completely heated anymore but only before the fuel transfer pump.
If a vessel receives high-pour straight-run bunker fuel, heating of the fuel
above the pour point temperature is required. Cold-temperature behaviour
also can be important for marine distillate fuels.
Vanadium
Vanadium is one of the metals found in
most crudes and fuel oils from these crude types. Some vanadium oxides formed
during combustion, particularly in the presence of sodium, have critical melting
temperatures that may lead to deposit formation in diesel engines turbochargers
and boilers of steam turbine ships.
Sediment by Extraction
Sediment determination gives the amount of inorganic sediment (rust, sand)
in the fuel (applicable only to DMB type marine diesel).
Total Potential Sediment
Total potential sediment gives the
total amount of sediment that can be formed under normal storage conditions,
excluding external influences. Fuel oil stability is guaranteed if the total
potential sediment meets the specification of 0.10 % max.
Silicon and Aluminum
During one of the refining processes for the production of gasoline fractions, an aluminum
silicate-type catalyst is used. Catalytic fines are often present in the heavy
process streams from this operation, which are then used in blending IFOs. Fuel
purification onboard vessels is an effective means of removing most of the
silicon and aluminum particles from the IFOs.
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