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Europe 992.2 5.0 17.5 0.11 88 0.09
In
order to fully define the grade of fuel loaded by the vessel, a fuel standard
or specification should always be used. The old method of using the criteria of
density and viscosity to specify the fuel required does not provide enough
detail of the limits that must be adhered to in order to prevent premature
damage to the main engine.
There
is only one standard which is ISO
8217 (2005). The attached fuel
standard indicates the full specification for residual fuels of varying grades.
To
specify which fuel should be ordered the engine makers books should be
consulted, and this will indicate their suggestion to the permissible grade of
fuel.
For
example:
MAN
B&W MC RMH 55,
with 1010 density if correct purifier units installed
B&W GF Reference in
manual only to limits of 600cSt at 50oC, and 0.990 density
SULZER
RTA RMH 55, with
additional limit on sodium at 100ppm
MITSUBISHI
UET45/80 RMD 15, with limits of 0.2%
water, 3.5% sulphur, 10% carbon residue, and max viscosity 650cSt at 50oC
WÄRTSILÄ VASA 32 RMH 55, with additional limits on
sodium of 100ppm, and asphaltenes at 14%. Allows 1010 density with correct
purifiers.
MaK
M35 & M551 RMH 55
Thus
we can see that the engine builders have constructed recent engines to consume
the average `worst-grade' fuels in order to remain competitive, although
continuously operating the engine at the fuel maximum limits will increase
engine fuel related problems and fuel preparation difficulties.
The
oil refiners also attempt to produce oil that is close to these standards,
depending ultimately on the crude oil base. However the pricing of fuels is
still governed by the final viscosity, with about a $4 discount per tonne for
380cSt oil against 180cSt. Both of these fuels is produced by `cutting' a high
viscosity fuel (over 600cSt) with a lighter fraction to produce the desired
viscosity, and hence the higher viscosity fuel used will often dictate the
impurities in both oils. Thus buying a lighter grade fuel does not imply a
better grade fuel, i.e. one with fewer impurities; in fact it may even produce
more problems than the original residual stock.
Publications
such as Lloyds List indicate the average grade of fuel in different areas of
the world, and Northern Europe and the US Gulf
have been chosen to show that bunker grades differ slightly around the world.
Density Water MCR Ash Alum/Si Sediment
US
Gulf 994.5 0.4 19.3 0.08 58 0.14
Hence
although you may specify the correct grade of fuel, the supplier may deliver
(sometimes unintentionally) fuel that can severely damage the engine. Fuel
could be classified as unsuitable if the damage occurs over
a small period of time, or if the fuel was obviously totally unusable,
such as too low a flash point. Poor quality fuel would indicate
that the fuel could damage the engine, but this would occur over a much longer
period.
The
only way of monitoring the delivered fuel quality would be to test the fuel
using a recognised fuel testing agency. The results obtained from this sampling
could be used in
1. Disputes over quality with the supplier
2. Highlights
areas where operational adjustments of the main engine or preparation equipment
may reduce damage
3. Reduce frequency of poor bunkers, (supplier
more likely to supply `other' vessel)
In
order to obtain effective results, the sampling of the fuel must follow accepted
guidelines to ensure that a representative
sample is obtained. This sample should be obtained at the point of
transfer, usually the ship’s rail or the closest point, (i.e. the ship’s
manifold). Sampling using either the “Continuous Drip System”, or a “Flow
Proportional Automatic Continuous Sampler” would be acceptable. This ensures
that the small sample and the subsequent fuel oil analysis results give a true
representation of the fuel oil quality that has been bunkered.
Once
the sample has been taken, the sample bottle is sealed and signed by the
engineer, before being sent to a recognised fuel testing agency. The test
results will normally be available within three days of the test sample
arriving at the testing agency, and will specify some of the following
results:
à
Viscosity at 1000 C. This
will allow the engineer to find the required heating temperature required for
transfer of the oil within the vessel, and the injection temperature.
Temperature ~ viscosity charts held on-board will be used to find these
temperatures.
à
Viscosity at 500 C. This is
an alternate temperature for measuring the viscosity. For residual fuel oils
the 1000 C measurement is considered more accurate. The 500 C
temperature is used as its results are still widely used within the marine
industry i.e . when 180cSt fuel is
referred to, it will be the viscosity of the fuel at 500 C; hence
the viscosity measurement should always be read in conjunction with the
measuring temperature.
à
Density at 150 C. This
measurement is required by the engineer to calculate the quantity of fuel in
tonnes within the vessel’s tanks from volume measurement calculated from the
tank gauging. There is a density limit for the fuel treatment plant i.e.
purifiers, with the modern purifiers capable of cleaning oils up to a density
of 1010 kg/m3. Older purifiers using a water seal can only accept
oils up to a density of 991 kg/m3, and still maintain effective
operation. Ignition and combustion characteristics of higher density fuels may
be inferior and be an indication of poor ignition quality.
à
Water. The water limit of 0.5% for
residual and less for distillate is defined in all fuel specifications.
Quantity shortfall, gassing up, poor lubrication and reduction of combustion
energy being some of the problems. Salt water being the main problem resulting
in deposits and corrosion. It is possible to homogenise after centrifuging to
emulsify (NOx reduction).
à
Ash. This is a measure of all the
incombustible material free of carbonaceous matter contained within the fuel.
High ash content fuels will lead to abrasion of the fuel injection equipment,
and fouling of the exhaust gas path. Ash levels can contain -
·
natural organic compounds of
the oil such as silicon, sodium, vanadium etc
·
Contamination from the refining
process, such as aluminium and silicon which are present in the catalytic fines
used in some refineries
·
Contamination from the storage
of the fuel, such as sand, scale, rust, etc
à
Micro Carbon Residue. This test replaces
the Conradson Carbon Residue test (USA )
and Ramsbottom (Europe ), but gives the same
readings. The carbon residue indicates the difficult to burn carbon and fouling
nature of the fuel, and residual fuels always have higher carbon residue levels
than marine diesel oils. As the combustion of the residual fuel always produces
heavier exhaust fouling, then more frequent in service cleaning of the T/C and
EGB and out of service maintenance will be required for these fuels.
à
Sulphur
à
Total Sediment – Accelerated. This test is primarily for the residual fuels, and
gauges the stability of the asphaltene phase of the fuel. When a fuel becomes
unstable in storage, the various components of the fuel break-down, and the
resultant sludge formed at the base of the tank is high in asphaltenes, and
produces heavy filter blockage and fouling/damage to the components in the fuel
injection system. The accelerated nature of the test attempts to replicate the
long term instability of a fuel using a test, which only lasts one hour.
à
Pour
Point. This is a measurement of the
temperature at which the fuel oil ceases to flow. The pour point indicates the
minimum temperature that a fuel has to be held at in order to avoid filter plugging
from cold oil. Once a fuel starts to solidify, due to the formation of waxes
within the fuel structure, reheating will not totally convert the fuel back to
its original state (re-dissolve). Hence it is imperative that sufficient heat
is given to the fuel in storage to maintain the oil temperature at 5-70 C
above the quoted pour point (transfer pumps designed with a max 800-1000cst).
Residual and diesel fuels can have pour points up to 300 C and 150
C respectively.
à
Nett Calorific Value. This measures the
energy of the fuel as determined by actual combustion of the test sample.
Although it is desirable to have a high energy fuel, the chemical structure of
the hydrocarbon dictates the energy available, with the residual fuels having a
lower calorific value than the distillate fuels.
à
Flash Point. The international accepted
minimum temperature for the flash point of a marine fuel is 600 C. This limit is stipulated for safe storage of the fuel, store 100 C below, as most fuels are
heated in the tanks. Generally fuels have flash points in excess of 1000 C,
thus it is only when the fuels have been mixed with volatile substances such as
kerosene, crude oil, etc that the flash point will be low when tested.
à
Silicon. This element can naturally
occur in crude oils but usually at low levels. When testing indicates a high
level of silicon, then the presence of highly abrasive catalytic fines should
be suspected within the fuel.
à
Aluminium. This element can naturally
occur in crude oils but usually at low levels. When testing indicates a high
level of aluminium, then the presence of highly abrasive catalytic fines should
be suspected within the fuel. Note the catalyst would normally increase both
aluminium and silicon levels.
à
Iron. This can include both the naturally
occurring irons within the oil, but also the abrasive rust and scale particles
present from the transportation of the oil.
à
Vanadium. Another natural occurring
element, but this will cause corrosive deposits to be formed on the high
temperature components of the engine, such as the exhaust vale and turbocharger
exhaust gas side.
à
Sodium. High levels of this element
usually indicate salt water contamination, which should be reduced by an
effective purifier stage. It is important that the sodium level is reduced
before the fuel is injected into the cylinder, as sodium will combine with any
vanadium present to form highly corrosive deposits, as well as heavy fouling of
the turbocharger unit.
à
Phosphorus (P), Lead (Pb), Calcium (Ca), zinc (Zn), iron (Fe). All of these elements are measured to test for the presence of
waste lubricating oils within the fuel oil. Some suppliers add waste automotive
lube oil for easy disposal and cost gains. The presence of such waste oil is
highly undesirable, as the additives present in the lubricating oil are soluble
and will prevent the dirt and abrasive particles being removed by the purifier
unit. Hence abrasive damage of the fuel injection equipment, rings/liner and
formation of deposits on components such as turbochargers will take place.
Environmental issues involve the increase of particulates in the exhaust gas.
Note
that none of the specifications include any limits on mixing compatibility, or
sodium content, and only the DO grades have a reference to ignition quality.
For
compatibility, the aromaticity reserve in the fuel is the important factor. The
ashphaltene phase is kept in suspension by the aromatic of the fuel, and
extreme care should be taken when mixing residuals with fuels of low
aromaticity levels such as distillates. If this were to occur then the
ashphaltene phase would precipitate causing filter blockage, and purifier
difficulties. The biggest culprits of poor compatibility fuels were residuals
from vis-breaker refineries, but as these units are being used less frequently
this problem may not increase. Straight run residuals are fuels, which are
highly paraffinic oils with the correct viscosity without light fuel dilution,
and these tend to be the most stable fuels.
Stability is the resistance to chemical breakdown on its own
due to heating /moving which can form sludges.
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