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DISEL FUELS(HSD/LDO)
VOLATILITY
As a rule, the
higher the viscosity of a liquid fuel, the lower its volatility. Therefore provided
the viscosity lies within specified limits, a satisfactory volatility is automatically
ensured. However, the percentage recovered at some particular temperature e.g. 366
deg C, is specified in the case of HSD mainly to control engine fouling due to incomplete
combustion of the higher boiling components.
TOTAL
SULPHUR
This is significant
because it governs the amount of sulphur oxides formed during combustion. Water
from combustion of fuel collects on the cylinder walls, whenever the engine operates
at low jacket temperatures. Under such conditions, sulphurous and sulphuric acids
are formed, which attack the cylinder walls and piston rings, promote corrosion,
and thus cause increased engine wear and deposits.
Total sulphur
is expressed as a percentage of the weight of the fuel sample.
CORROSIVE
SULPHUR
It is important
that diesel fuels shall be free of these sulphur compounds which in themselves attack
metal parts of the engine or the fuel system. This characteristic is tested by the
Copper Strip Corrosion Test, a severe discoloration or pitting of the polished strip
indicating the presence of corrosive sulphur compound in the fuel.
ACIDITY
This should
be low in order that corrosion of metals in contact with the fuel during storage
and distribution is minimised.
INORGANIC
OR MINERAL ACIDITY
Where diesel
fuels are treated with mineral acid as part of the refining procedure, traces of
mineral acid remaining in the final product would obviously be undesirable. Hence,
zero limit is usually specified for this property.
ORGANIC
ACIDITY
This is due
to the naphthenic type which are constituents of crude petroleum. Their presence
in small amounts is not necessarily an indication of improper refining or poor quality.
Although much weaker than mineral acids, they may attack galvanised metal and this
is why the use of galvanised containers for the storage of diesel fuels is not recommended.
ASH
CONTENT
Ash is a measure of the incombustible material present in a fuel and is expressed as a percentage
of the weight of the fuel sample. In the case of distillate fuels, it usually consists
of rust, tank scale or sand which settles out readily. Blends of distillate and
residual fuel, e.g. LDO may additionally contain metal oxide derived from oil soluble
and insoluble metallic compounds. Ash is significant because it can give
rise to
deposit problems such as abrasion, malfunctioning of injectors and high temperature
corrosion, particularly with residual fuels.
SEDIMENT
AND WATER
These are absolutely
undesirable contaminants and should be as low as possible. The higher the specific
gravity and viscosity of a fuel, the greater the quantities of water and sediment
it can hold in suspension. Large quantities of sediment can affect the combustion
of the fuel, and if abrasive, may cause excessive wear of closely fitting parts
of fuel pumps and injectors. It may also clog filters and build up deposits in tanks
and piping.