some definitions
Viscosity: It is a measurement of the resistance to flow of a liquid, at a reference temperature. Kinematic Viscosity in cSt [1 Centistokes = 1 mm2/s]
Viscosity of the Fuel Oil is Reduced by heating
- (i) To get good Atomisation
- (ii) To improve separation of sludge and water (purifier)
- (iii) To ensure ease of pumping
Density = Mass kg/m3 {Density at 150C is called standard Density}
Volume
Density of HFO = 920 kg/m3 to 1010 kg/m3 at 150C
Relative Density = Specific Gravity (it is a ratio; so has no units)
RD /SG of HFO= 0.92 -1.01 (no unit)
Purifiers/Separators – remove water & sludge (solid contaminants) from the oil by centrifugal force
Conventional purifiers remove water & sludge from fuel oil up to a max density of 991 kg/m3 at 150C (For removing water- Fuel oil density should be less than water – or else only sludge/Solids can be removed)
Modern Purifiers remove water & sludge from fuel oil up to a max density of 1010 kg/m3 at 150C (Water can be removed even if Fuel Oil density is more than water)
Flash Point It is the lowest temp at which sufficient vapour is given to produce a flash on application of a flame under specified test conditions
[min 600C – due to fire risk during normal storage and handling]
Pour Point: It is the lowest temp at which fuel oil can be handled/pumpable (free flow) [Below this temp fuel will form wax/gel preventing flow]
Fuel in the Tank must be 50C above the pour point to prevent solidification
If Fuel gets Solidified, it is impossible to re-liquefy in the tank due to Low heat transfer Coefficient of the Fuels
CCAI –Calculated Carbon Aromaticity Index for Residual Fuel Oils
CII – Calculated Ignition Index for Residual Fuel Oils
These are reference indices which gives the ignition quality of residual fuel oil
They are used to estimate how long the residual fuel will take from injection to ignition
Calculated Cetane Index: Reference index for Distillate Fuel Oils
Compatibility of residual fuel allows it to be blended with other fuels to give a stable mix. If two incompatible fuels are mixed they become unstable and causes precipitation of heavy sludge which blocks the fuel system
Micro Carbon Residue (MCR)/Asphaltenes: Also called Conradson Carbon Residue (CCR)
It is a measure of tendency of a fuel to form soot and carbon deposits (coke) during combustion/high temp [max limit is 22% m/m].
Water content – Fuel contaminated with sea water contains sodium.
Sodium burns to form with vanadium an ash which is very harmful to engine exhaust valves
Ash: It is the amount of inorganic material in the fuel. It is the incombustible material which remains after combustion
Ash is formed due to like Sodium, Vanadium, Sulphur, Catalytic fines (Silicon / Aluminum)
Ash causes Corrosion, Abrasive Wear, Fouling, Overheating of metal surfaces etc
Sulphur burns to sulphur dioxide and forms sulphuric acid which is corrosive
Calorific Value: It is the heat energy given out during combustion [calorimeter]
HFO has CV of 38.9 to 41.2 MJ/litre
Fuel Oil Handling (Supply & Storage) & Treatment
Supplying of Fuel oil from Shore Terminals / Barge to Ship Storage (Bunker) Tanks is called Bunkering
Fuel oil in Storage tank temp is maintained at 400C [50C above the pour point temp to avoid solidification [wax/gel formation]
Fuel oil is transferred via positive displacement pumps [through suction strainer & shell/tube heaters] to settling tanks through duplex fine mesh magnetic filters (ferrous metals are removed)
Settling tank Temp is maintained at 900C. Sludge [solid contaminants / sediments] and water are separated by gravity (diff in densities) & drained off
[large quantities of water or sludge can be drained effectively than separation by a centrifuge]
Fuel oil is passed through the centrifugal separators (purifiers separates Sludge & Water from fuel oil]
Sodium (dissolved in water) is removed along with water by Purifier
Vanadium is dissolved in Fuel Oil
Sticking of sodium and vanadium salts on exhaust valves is reduced by keeping the exhaust temp below 5300C
Silicon and aluminum (catalytic fines) are removed in a purifier or clarifier
(slow through put is essential – self driven purifier delivery pump is replaced with independently driven pumps )
Sulphuric acid is neutralized by alkali additives in cylinder L.O.
Also the Condensation of Sulphuric acid is reduced by keeping liner temp high
Fuel after passing through the purifier reaches the Service tank / Day tank Service Tank Temp is maintained at 900C
Supply pump pumps the oil from service tank to engine through heaters [1200C] and through very fine mesh strainers.
Temp and viscosity are maintained by automatic controllers fitted to the heaters.
Jerk type fuel pump (1 pump per cylinder) is fitted on the engine
It pumps fuel to the Injectors (1 or 2 per cylinder)