08 Fuel Oil System About fuel system about the me engine the latest technical developments

About fuel system about the me engine the latest technical developments

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Fuel Oil Systems

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Fuel Oil Systems
•Heavy fuel oil is not suitable for burning directly in
the diesel engine because it has some solids and
water as impurity, which may cause damage to the
engine parts and also has a very high viscosity, which
makes it difficult for atomization of fuel in the
combustion process. To make this fuel suitable for
burning, it has to go through a conditioning process
consisting of settling, centrifuging, boosting of
pressure, filtering and heating.

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Fuel Oil Systems
Heavy residual fuel consists of residues left
after lighter and costlier distillates fuels
and gases are removed from petroleum
crude oil in an oil refinery. Marine diesel
engines are designed to burn heavy
residual fuel blended with distillate gasoil
to meet the specification of fuel oil
ordered, especially viscosity and density.
This is popularly known as “heavy fuel oil”.

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Fuel oil supply system of a ship

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Specification Data for Fuel Oil
• (a) Density: It is the relationship between mass and
volume at 15C and is measured by hydrometer. This
value changes with temperature, depending upon
the coefficient of expansion of the substance. For
marine fuels the values are 800-1010
kg/m3.Knowledge of density is needed for quantity
calculations and to select the optimum size of gravity
disc for purifiers. 991 kg/m3 is the accepted limit for
normal centrifugal purification and 1010 kg/m3 in
ALCAP purifier.

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Density
•As already mentioned density is the ratio of the mass
of a substance to its volume, but not its weight to
volume ratio and therefore, density by definition is in
vacuo. The term ‘density in air”, although often used,
is incorrect and should be referred to as “weight
factor”. This is due to the fact that a substance
weighed in air is supported, to a small extent, by the
buoyancy of the air acting on it. In effect therefore,
the weight of a liquid in air is slightly less than its
weight in vacuo.

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Viscosity
•Viscosity can be termed as resistance to flow.
Viscosity is measured in a viscosimeter. The
kinematic viscosity is obtained by dividing
dynamic viscosity by density of the fluid and
its unit of measurement is stoke or centistoke
and is quoted with a reference temperature.
For distillate fuels the reference temperature
is 40C and for residual fuels the reference
temperature is usually 50C or 100C.

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Viscosity
•Each fuel has its own temperature viscosity
relationship and although oil suppliers publish
temperature/viscosity charts, it should be
understood that these charts are based on average
data of large number of representative fuels. Precise
relationship would depend upon crude oil source and
refining process. In general, for lower viscosity fuels
the difference is small, but it becomes wider as
viscosity of the fuel increases. A knowledge of
viscosity is necessary for the determination of the
heating required for a fuel for transfer purpose and
the temperature range required for satisfactory
injection and combustion at the fuel atomiser.

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Viscosity
•In order to ensure efficient atomization of the
charge, when burning residual fuels it is
essential to inject the fuel at the most suitable
viscosity. Despite wide differences in engine
and fuel system designs there is considerable
agreement that the most suitable viscosity of
the fuel leaving the injector nozzle lies
between 12.5 - 18.0 cSt. In well designed
systems, the viscosity is controlled
automatically within fairly close limits by
means of viscosity controllers.

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Pressure/viscosity characteristics
•The viscosity of hydrocarbon oils
increases under pressure. The very high
fuel injection pressures now employed
will increase the fuel viscosity markedly.
This should be allowed for when
preheating the fuel.

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Cloud and pour points
•The cloud point of a distillate fuel is the
temperature at which wax starts to
crystallise out, and this is seen when the
clear fuel becomes opaque. For marine
fuels this characteristic is only applicable
to some light grades.

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Pour point
•The pour point of an oil is the lowest
temperature at which the oil remains fluid. It
is determined by cooling the oil in a test tube
having a diameter of approx 30 mm. The pour
point is 3C higher than the temperature at
which the glass can be held in the horizontal
position for 5 second without any visible signs
of movement of the oil surface. (Solidifying
temperature is 3C below pour point).

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Pour point
•The pour point result will give guidance
regarding the lowest temperature at which a
fuel may be stored. If fuels are held at
temperatures below pour point, wax will
begin to separate out. This wax may cause
blocking of filters and can deposit on heat
exchangers. In severe cases the wax will build
up in storage tank bottom and on heating
coils, which can restrict the coils from heating
the fuel.

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Pour point
•When dealing with heavy marine fuels, both the pour
point and the viscosity of the fuel need to be
considered, if the fuel is to be maintained at a
temperature to prevent wax formation and allow
pumping. For efficient pumping the viscosity of the
fuel should not be above approximately 600 cSt. If
the suction line from the pump to the tank is very
long the viscosity should be lower.

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Flash Point
•The flash point of a fuel is the lowest temperature at
which sufficient vapour is given off to produce a flash
on application of flame under specified test
condition. The flash point may be measured as a
closed or open cup figure and for marine fuels the
closed cup figure is used. The test method uses the
Pensky-Marten apparatus. The minimum flash point
for fuel in the machinery space of a merchant ship is
60C.

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Flash Point
•For fuels used for emergency purposes,
external to the machinery space, for example
the lifeboats, the flash point must be greater
than 43C. The purpose of defining a minimum
flash point is to minimise fire risk during
normal storage and handling. The general rule
is that fuels should not be heated above 10C
below the flash point, unless specific
requirements are met. (Solas Chapter II-2,
Regulation 15)

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Fire Point
•It is the lowest temperature at which
vapour is generated at a rate sufficient to
sustain combustion for 5 second. The
same equipment which is used for
determining flash point is used for this
test also.

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Auto-ignition temperature or Self ignition
temperature
•It is the lowest temperature at which the
generated vapour will ignite
spontaneously without any source of
ignition.

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Calorific Value or Heat of Combustion or
Specific Energy :
•Heat of combustion of a fuel is the amount of
heat released during combustion of a unit
mass.

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Water
•Normally the water content in the fuel oil is
very low and 0.1-0.2% by volume is typical.
Ingress of water can come from tank
condensation, tank leakage and heating coil
leakage. Water is normally removed by
gravitational separation in fuel oil tanks and
centrifugal purification system.

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Ash : Nickel, Aluminium, Silicon, Sodium and
Vanadium
•The ash content is defined as the residue left
after all the combustible components of the
oil has been burnt. In distillate fuel this
quantity is negligible. The ash constituents are
concentrated in residual fuels. The ash
consists generally of oxides and/or sulphates
of nickel, aluminium, silicon, sodium and
vanadium.

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Source of Ash : Nickel, Aluminium, Silicon,
Sodium and Vanadium
•The sources of these are (a) inorganic
material naturally present in the crude oil, (b)
Catalytic fines picked during refining process
( Catalytic fines are particles arising from the
catalytic cracking process in the refinery and
are in the form of complex alumino-silicates)
(c) Contamination by sand, dirt, rust scale
and sea water subsequent to refining
process.

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Sodium and Vanadium
•Fuels leaving refinery have sodium level below 50
mg/kg. If contaminated with sea water
subsequently, sodium level will increase. A 1% sea
water contamination represents a potential 100
mg/kg increase. Normally sea water can be
removed by gravitation separation in settling tank
and centrifugal separation. Vanadium is present in
all crude oils in an oil soluble form and the levels
found in residual fuels depends mainly on the crude
oil source, with those from Venezuela and Mexico
having the highest levels.

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Sodium and Vanadium
•The actual level is also related to the concentrating
effect of the refining processes used in the
production of the residual fuel. There is no
economic process for removing vanadium from
either the crude oil or residue. During combustion
of the fuel, vanadium and sodium constituents form
a mixture of sodium sulphate and vanadium
pentoxide. This mixture has a low melting point
(approx 500-600C) corresponding to the
temperature of the exhaust valve seating.

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Sodium and Vanadium
•The semi-fluid particles of ash adhere firmly to the surfaces
they touch, gradually forming a very hard, thin layer of slag
which, after having reached a certain thickness, allows the
hot combustion gases to leak out, the result being that the
slag melts forming a narrow channel. If the layer of slag is of
sufficient thickness, the channel grows and the combustion
gases heat up the seating material, causing what is known as
high-temperature oxidation, which in turn results in the
seating material melting in the vicinity of the channel. The
most critical sodium to vanadium ratio is about 1 to 3.

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Silicon and aluminium
•Silicon may be present in the fuel in form of sand and
aluminium may also be present in very small
quantities, having been picked up by the crude oil in
sub-surface rocks. However presence of aluminium
and silicon is mainly due to catalytic fines discussed
earlier. Catalyst is an expensive material for the oil
refiner and stringent methods are taken for its
retention but some still find their way in residual fuel.

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Silicon and aluminium
•Excessive catalytic fines can lead to high
wear of piston rings and liners, fuel pump
barrels and plungers, and fuel injector
nozzle needle and guide. The level of
catalytic fines in delivered fuels can be
significantly reduced by efficient
centrifugal purification prior to
combustion in the engine.

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Carbon Residue
•The carbon residue of a fuel is the tendency to form
carbon deposits under high temperature conditions
in an inert atmosphere, and may be expressed as
either Ramsbottom carbon residue, Conradson
carbon residue (CCR) or micro carbon residue (MCR).
This parameter is considered by some to give an
approximate indication of the combustibility/deposit
forming tendency of the fuel.

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Sulphur
•Sulphur is naturally occurring element in crude oil
which is concentrated in the residual component.
The amount of sulphur in fuel oil depends mainly on
the source of crude oil and to a lesser extent on the
refining process. Sulphur content is typically 1.5-4%
wt in residual fuel world wide. In the combustion
process in a diesel engine the presence of sulphur in
the fuel can give rise to corrosive wear.

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Sulphur
•This can be minimised by suitable
operating conditions, and suitable
lubrication of the cylinder liner with
alkaline lubricant. MARPOL Annex VI
limits the sulphur content of marine oil
to reduce atmospheric pollution, in the
form of sulphur dioxide, from
international shipping.

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Amendment to Marpol Annex VI
•Regulation 14 of MARPOL Annex VI has been
significantly revised. For the Global Cap, the
sulphur content limits are as follows:
•4.5% prior to 1 January 2012
•3.50% on and after 1 January 2012
•0.50% on and after 1 January 2020

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Special Emission Control Areas
•For the Special Emission Control Areas, the
sulphur content will be as follows:
•1.50% prior to 1 March 2010
•1.00% on and after 1 March 2010
•0.10% on and after 1 January 2015
•The existing Emission Control Areas (ECAs) are
the North Sea and the Baltic Sea.

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Ignition Quality
•Cetane number - The cetane number for any fuel is a
measure of the oil’s readiness to ignite, under
conditions prevailing in the diesel engine. This
number is determined by comparing the oil with a
mixture of cetane and heptamethylnanone. Cetane,
which has a very high spontaneous combustion
ability is rated at 100 and the corresponding cetane
number for heptamethylnonane is 15.

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Cetane number
•The oil for which cetane number is to be determined
is used as fuel in a so-called CFR (Co-operation Fuel
Research) engine, which is a single cylinder diesel
engine with a variable and controllable compression
ratio. Fuel injection and combustion timing are
controlled by electronic equipment. When these
have been determined then engine is run with
different mixtures of of cetane and
heptamethylnanone until a mixture gives same
results.

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Calculated Ignition Index (CII) and Calculated
Carbon Aromatic Index (CCAI) :
•These are calculated by empirical
equations ,where use is made of the
density and viscosity of the residual fuel.

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Standards of Fuels - Need for quality
control in bunker fuel
•The cost of bunker fuel is one of the most significant
components of a ships operating cost. Ship owners in
their effort to limit this cost have preferentially
turned to the use of heavier and thus less expensive
bunker fuels. Technology developments in petroleum
refining, such as in vacuum distillation, catalytic
cracking etc, often result in a deterioration of the
characteristics of heavy fuels as lesser volumes of
residues are left after petroleum refining.

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Standards of Fuels - Need for quality
control in bunker fuel
•These residuals may contain elevated levels of
undesirable constituents such as Aluminium and
Silicon, compounds that could result to significant
engine wear and damage. In addition to the above
the supply of marine bunker fuels is nowadays often
the result of a complex sequence of buying, selling
and mixing of fuels of different origins. The use of
poor quality fuel is known to result to the serious
damage of boilers, fuel pumps springs, pistons and
cylinders.

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ISO 8217
•To obviate dispute between ship owners and bunker suppliers
and also to meet MARPOL Annex VI requirements,
International Organisation for Standards published the first
edition of International fuel specification ISO 8217 known as
“Petroleum products - Fuels (class F) - Specifications of marine
fuels” in 1987. It was revised in 1996 and again in 2005. ISO
8217-2005 defines four distillate grades (DMX, DMA, DMB,
DMC) and ten residual grades.

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CHARACTERISTIC LIMIT CATEGORY ISO - F
DMX DMA DMB DMC(a)
Density at 15°C (Kg/m3)max. --- 890,0 900,0920,0
Viscosity at 40°C (mm2/s
b)
min.
max.
1,40
5,50
1,50
6,00
---
11,0
---
14,0
Flash Point (°C) min. 43 60 60 60
Sulfur (% m/m)max. 1,001,50 2,00
(e)
2,00
 (e)
Cetane index  min. 45 40 35 ---
Carbon residue (%m/m)max. --- --- 0,30 2,50
Carbon res. on 10% (V/V) distillation
bottoms
(% m/m)max. 0,300,30 --- ---
Ash (% m/m)max. 0,010,01 0,01 0,05
Appearance (f)   Clear and bright(f) ---
Total Sediment Existent (% m/m)max. ------ 0,10
(f)
0,10
Water (% V/V)max. ------ 0,3 (f)0,3
Vanadium (mg/kg)max. ------ --- 100
Aluminum plus silicon (mg/kg)max. ------ --- 25
Aluminum plus silicon (mg/kg)max. ------ --- 25
Used lubricating oil (ULO)
•Zinc
•Phosphorus
•Calcium
mg/kg max. __
__
_
_____ _____The fuel shall be free of
ULO (g) 15
15
30

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Fuel Testing
•Analysis of particular characteristics of the fuel
delivered may be carried out by some independent
shore based laboratory or by tests carried out on
board. Testing of fuel on board may range from one
or two tests to fully automated online monitors
where direct read out of viscosity, density and
elemental analysis (e.g. sulphur, silicon, vanadium) as
well as derived parameters such as ‘ignition index’
expressed as CII or CCAI are available.

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Storage and Transfer
•The pump for fuel transfer is of the positive displacement
type and are usually of screw or gear design. The temperature
of fuel in the storage should be maintained 5C above its pour
point otherwise there is a possibility of wax formation and in
case of high wax content, if left to cool, it may be difficult to
reheat the fuel to a temperature above the pour point. Also
the temperature has to be raised for higher viscosity fuel to
45C to bring it below 500 cSt for pumping it. Fuel oil is heated
in storage tanks by low pressure steam, but in some ships
thermal fluid heating is used.

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STORAGE
•Marine heavy fuel oils are blends of viscous
residues from various refinery operations, cut back
with distillate cutter stock. The growing trend is
towards cracked residues of a highly
aromatic/asphaltic nature to be cut back to an
acceptable viscosity with cracked aromatic
distillates. Both components have a high
carbon/hydrogen ratio, cracked distillates having
good solvency properties for large-asphaltene
hydrocarbons. In a stable fuel the asphaltenes are
carried in a colloidal dispersion in the lighter phase.

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STORAGE
•If the equilibrium between the two phases is disturbed the
asphaltene will agglomerate to a size which can no longer be
maintained in suspension, and they will tend to separate out
as ‘sludge’. If sludge deposition does occur this is made
worse, not better, by the addition of more distillate. This is
particularly true if a high-quality straight-run paraffinic
distillate is added to a cracked, high asphaltenic content,
residual fuel. It is possible that two residual fuels, each
stable by themselves, when mixed together can prove to be
incompatible and throw down objectionable sludge or
sediment.

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Compatibility tests
•If compatibility tests have not been carried out
beforehand, when bunkering, every effort should be
made to segregate bunkers from different source in
different tanks to avoid potential problems of
incompatibility. In such a case an unstable blend
may occur in the ship’s tanks, which could result in
precipitation of asphaltenic deposits as sludge in
the tanks, pipes, filters and centrifuges.

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TREATMENT OF FUEL OIL
•Before the fuel is burnt in diesel engine or a boiler, a
shipboard treatment takes place. Distillate fuels are
generally filtered through a coalescer type filter to
remove water and solid impurities. For boilers
burning residual fuels, in addition to settling tanks,
cold and hot filters are installed in the system prior
to boiler. In case of diesel engines burning residual
fuel, in addition to settling tanks and filters,
centrifuges are installed to clean the fuel to take
account of the fine clearances which exist in fuel
system of diesel engine.

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Treatment of High Density Fuel
•As the density of fuel oil increases and exceeds 991kg/m3, the
density difference between the fuel oil and fresh water is so
small that any change in oil temperature, viscosity or flow rate
will cause the oil/water interface to fluctuate leading to a
potential failure of water seal. For residual fuel having density
above 991 kg/m3, alternative arrangements to traditional
purifier are used. One such arrangement called ALCAP system
is used, where fuels of density upto 1010 kg/m3 can be
treated. The centrifuge operates as a clarifier and clean oil is
continuously discharged from clean oil outlet, and any free
water and separated sludge accumulate at the periphery of
the bowl.

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Fuel Heating
•Residual fuels have to be heated to reduce the
viscosity to that required for atomisation. In case of
boilers this is in the range of 15-65 cSt, whilst for
diesel engines the injection viscosity is usually 12.5 -
18 cSt. Fuel heaters may be operated by low
pressure saturated steam, a thermal fluid or
electrical elements. It is important to maintain
correct viscosity range under all conditions. Local
overheating may cause cracking of fuel, which may
lay down deposits on the heating surface, impairing
efficient operation of the heater.

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Viscosity Controller
•A viscosity controller is often installed
downstream of a fuel oil heater so that a
constant injection viscosity can be maintained.
There are various types of these. One of these
measures the differential pressure resulting
from laminar flow through a capillary tube and
compares this value to a set point, generating
a signal to control the temperature of fuel oil
heater.

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Combustion In Diesel Engine
•For combustion of fuel in a diesel engine, the
air charge is highly compressed to a
temperature well above the spontaneous
ignition temperature (SIT) of the fuel. As the
piston approaches TDC fuel is injected at high
pressure and suitable viscosity. This continues
for 14-28 degrees of crankshaft rotation,
depending upon engine speed and design.

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Combustion In Diesel Engine
•The fuel passes through the following phases :
1.A delay period between the commencement of injection of
the very finely divided fuel droplets and the commencement
of ignition.
2.Rapid combustion of the fuel accumulated in the cylinder
during the initial delay period, accompanied by a rise in
pressure.
3.Steady combustion of the remainder of the fuel charge as it is
mixed.
4.An after burning period during which remaining unburnt fuel
finds oxygen and combustion is completed.

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Factors Influencing ignition
1.Exactly when ignition commences is dependent upon
several factors, the most important being:
2.The size of the droplets injected into the cylinder;
3.The pressure of the fuel at the injector tip;
4.The velocity of the droplets entering the dense air mass;
5.The air pressure and temperature in the cylinder;
6.The air turbulence in the cylinder;
7.The ignition delay properties of the fuel;
8.The surface tension of the fuel;
9.The chemical composition of the fuel;
10.The engine design.

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Droplet formation and size
•The size of the droplets in the injected fuel spray is controlled
primarily by the size, shape and number of holes in the
injector tip, their position and the fuel injection pressure and
the viscosity of the fuel leaving the injector. The higher the
viscosity, the larger will be the droplet size. As the fuel leaves
the small injector orifices at pressures in modern pressure-
charged engines of upto 1500 bar the pressure falls sharply as
it enters the cylinder, in which the charge-air pressure is
much lower. The pressure energy is converted into kinetic
energy, so that there is a sharp rise in velocity. Both the fall in
pressure and the shearing action as the fuel passes through
the dense air charge at high velocity break up the liquid
stream, while its viscosity and surface tension form the
mechanically disrupted liquid into small droplets.

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Droplet formation and size
•The droplets sprayed into the cylinder are of varying sizes; the
higher the injection pressure, the higher the percentage of
small droplets. With current trend towards much higher
injection pressures fuel droplet sizes will be reduced
correspondingly. The droplet size decreases as the
compression pressure increases. The increased density of the
air charge helps to break up the spray into smaller droplets.
This is beneficial, as the smaller the droplets, the quicker they
will vaporize as there is a greater overall area of the oil charge
exposed to the hot compressed air. This reduces the ignition
delay period, measured either in milliseconds or degrees of
crank angle.

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Importance of high fuel pressure
•If the droplets leaving the injector have a diameter of about
20-40m, there is minimum delay in combustion. Conversely,
if the droplet diameter exceeds some 100-120m, the
combustion period is so long that even a slow-speed, two-
stroke engine runs the risk of some particles remaining
unburnt when the exhaust ports or valves open. Below 20m
droplet size there is insufficient kinetic energy in the tiny
droplet to penetrate the dense air mass in the cylinder,
resulting in poor fuel/air admixture. In order to ensure the
required fine droplet size, an injection pressure exceeding
1200 bar is being used by some engine manufacturers.

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The effect of air temperature
•The temperature of the air compressed in the
cylinder has a major effect upon ignition delay. The
higher the temperature, the shorter the delay
period, everything else being equal. Several factors
determine the air compression temperature, the
main ones being the engine compression pressure,
which, in turn, is determined by the charge air
pressure, the compression ratio and the volumetric
clearance, the temperature of the induction air
entering the cylinder, and the temperature of the
cylinder head, liner and piston crown.

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The effect of air temperature
•In turn, the combustion chamber and piston
temperatures are controlled by the
temperature of the cooling water, or oil, and
by the design of the combustion chamber and
piston components. Compression
temperatures in normally aspirated engines
are in the order of 500-600C, but in modern
highly pressure-charged medium and large
output engines, they may be as high as 700C.

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Compression Pressure
•Increased compression pressure (or densities),
which are now as high as 90-110 bar in
modern crosshead and trunk piston engines,
not only promote the formation of more,
smaller fuel droplets but, equally important,
reduce the spontaneous ignition temperature
of the fuel appreciably.

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Air Turbulence
•Turbulence or swirl, in the compressed air
charge promotes efficient distribution of the
fuel spray droplets throughout the
combustion chamber, ensuring thorough
mixing of the fuel and clean air (increasing the
rate of heating and vaporization) thus tending
to reduce ignition delay and assisting in
efficient burning of the fuel charge.

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Droplet combustion process
•If an individual fuel droplet is considered it will
be found to be very small, the size depending
upon factors discussed previously but, as
compared with the physical size of individual
hydrocarbon molecules which form the
droplet, they are relatively large. Even the
smallest droplets in the fuel spray contain
thousands of hydrocarbon molecules having
widely different chemical structures. This is
particularly true of heavy residual fuels with
high carbon-numbers.

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Droplet combustion process
•The molecules vary appreciably in their volatility,
ignition temperature, rate of burning, the
completeness of burning and their tendency to
release carbon and associated organometallic
compounds. The heating of the spherical droplet
occurs from the outer surface inwards to the centre,
so that evaporation and subsequent ignition
commences at the surface. The more volatile
constituents with the lowest ignition temperatures
burn first, leaving the less combustible hydrocarbon
constituents to find clean air and burn slowly.

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Advanced injection timing
•During a long ignition delay, injection of fuel
into the cylinder continues, so that the longer
the delay, the greater is the amount injected
before ignition commences. When ignition
finally occurs, the accumulated fuel ignites
violently with a very rapid, high-pressure rise.
The resultant high pressure causes shock
loading on the piston and running-gear
bearings. With a poor equivalent Cetane
Number residual fuel, within fairly narrow
limits one way of reducing this harmful effect
is to advance the ignition timing.

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Advanced injection timing
•In case of low-speed crosshead engines, upto 2
degrees crank angle may be adequate, with a
somewhat greater advance for medium-speed
engines - possibly 3 to 6 degrees, depending upon
engine design and, in particular, engine speed.
Advancing the injection timing enables ignition to
occur at maximum compression pressure and
temperature and smooth combustion to be
completed earlier in the stroke. The manufacturer's
maximum firing pressure, related to load conditions,
should be maintained.

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From DO
Tank
Supply
Pumps
Injector recirculation