ISO specification 8217 stipulates acceptable characteristics of marine fuel oil products. In order to
understand the relative importance of each characteristic it is important to understand the definition.
The following definitions are deemed useful to users of marine fuels products.
Viscosity
A measure of fluid resistance to flow. Viscosity
of fuel oil decreases with increasing temperature.
The viscosity of the fuel oil at the point of injection
into the engine is key to performance. Viscosity
is used to classify residual fuel types but is not a
key indicator of fuel quality. For example, all other
characteristics being equal, a fuel of 360 cSt is of
no better or worse quality than a fuel of 400 cSt,
it is just less viscous.
Density
Mass per unit volume of a product. It is used to
convert the volume delivered into the quantity
purchased. Density varies with temperature and
is an important parameter in the onboard purification
of the marine fuel product.
Calculated Carbon Aromaticity Index (CCAI)
The most widely accepted empirical formula to
estimate the ignition quality of fuel oil. CCAI uses
the physical properties of density (d) and viscosity
(V) in the following equation:
CCAI = d - 81 –141*log [log (V+0.85)]
Sulfur
Sulfur is the main inorganic component of fuel.
It occurs naturally in crude oils and tends to
concentrate in the heavier fractions. Sulfur concentration
in fuel oil strongly influences the choice
of lubricant. Energy content of fuel oil diminishes
with increasing sulfur.
Flash Point
Flash point is the minimum temperature at which
vapours released from the fuel oil will ignite when
exposed to an open flame. The flash point of a
blended fuel oil is the same as that of the lightest
component in the fuel oil product.
Acid
Generally, marine fuel products should not contain
inorganic acids, however ISO 8217 allows for
minimal acceptable levels.
Sediment
Sediment in distillates is composed mainly of rust,
general dirt & scale. Marine fuel oil sediment can
be both inorganic and organic in nature.
Carbon Residue
Carbon residue is a measure of the carbonaceous
material left after the volatile components of a fuel
have been vaporized in the absence of air. It is
used to estimate the potential of a fuel to create
deposits in an engine upon combustion.
Pour Point
The pour point of a fluid is the lowest temperature
at which it ceases to flow. In fuels, the pour
point is largely determined by the petroleum wax
content in the oil. Pour point determines the
minimum temperature required for storage and
handling onboard of fuel oil products.
Ash
Ash is the carbon free (inorganic) residue remaining
after completely burning the fuel in air. It
occurs naturally in crude oils and tends to concentrate
in the heavier fractions. Ash can contain
hard and erosive particles, some of which may
also be corrosive.
Vanadium
Vanadium is a metal occurring naturally in some
crude oils and is concentrated in residual components
during refining. In high concentration, it can
form high melting point, corrosive deposits. In
combination with sodium, it can form lower melting
point, oxygen deficient deposits.
Sodium
Sodium occurs naturally in crude oils and is concentrated
in residual streams during refining. It
can be introduced into fuel streams as a scavenger
used to control the hydrogen sulfide content
of fuel oil, via salt water contamination, or through
sodium ingress into a marine diesel engine due to
salt water saturated air.
Cat Fines
Cat fines contamination in fuel oil is caused by
carryover of catalytic material used in the refining
process and evidenced by the presence of Alumina
and Silica. Cat fines are hard and abrasive.
Used Lubricant (or Lube) Oil
Some used lube oil may contain components
harmful to an engine, but all used lube oils may
not necessarily be unfit for purpose. Some additives
used to identify used lube oil such as
calcium are naturally occurring in crude oil and
hence residual fuel. Test methods are designed
to eliminate false positives.
Calcium
A soft grey alkaline earth metal, the fifth most
abundant element in the earth’s crust. Essential
for living organisms, particularly in cell physiology,
and is the most common metal in many animals.
Calcium occurs naturally in crude oils. It is introduced
into the combustion space via cylinder
lubrication oil. The alkaline Total Base Number
(TBN) additives of cylinder lube oil contain calcium.
Calcium is concentrated in the residual
part of the refinery process as lighter products are
removed.
Compatibility
Compatibility of a fuel is a function of the stability
of the two individually stable oils used to blend
marine fuel oil when they are co-mingled. Heavy
marine fuels are complex mixtures of hydrocarbons.
Some very large molecules called asphaltenes
are held in suspension by maltenes.
Mixing fuels can adversely affect this equilibrium.
