Answer EKG Question 4

Q4. With respect to properties of fuel oil, explain the significance of the following terms; (A) Calculated Carbon Aromaticity Inbox (CCAI).

(B) Open flash point and Closed flash point.

(C) The importance of Sodium to Vanadium Ratio.

(D) Octane Number.

Answer: (A) The calculated carbon Aromaticity index (CCAI) is an index of the ignition quality of residual fuel oil.
Formula for CCAI: $\displaystyle\mathrm{CCAI=D - 140.7log(log(V + 0.85)) - 80.6 - 483.5log (\frac{t + 273}{332})}$ .
Where:
D= density at 15°C (kg/m3)
V= viscosity (cST)
t = temperature (°C)
This will normally give a value somewhere between 800 and 880.
The lower the value is the better the ignition quality.
Fuels with a CCAI higher than 880 are often problematic or even unusable in a diesel engine.
CCAI are often calculated under testing of marine fuel, the values are dependent on density, viscosity and temperature of the fuel. In case of high CCAI, the manufacturers recommendations and guidance limits should be consulted to ensure that the fuel falls within the permissible range for the engine type.
Attention should be given to the combustion profile, peak pressures and exhaust temperatures on the Engine.

(B) Open flash point and Closed flash point:-The flash point of a volatile material is the lowest temperature at which vapors of the material will ignite, when given an ignition source. 
Flash point tests are used to show the lowest temperature that a volatile substance is vaporized into a flammable gas. This is done by introducing a source of ignition, then waiting for the “flash” where the substance is ignited.  There are a wide range of methods but most are available in open or closed cup formats.
open flash point:-
It is conducted in a vessel which is exposed to the air outside.
a key variable is the height of the ignition source over the cup.
this test simulates the potential ignition of the liquid spillage in the non-contained condition e.g pool of fuel on floor.
the results are likely to be interfered by the out side elements.
fire point, combustibility and sustained burning test use open cup instruments.
it is quoted in fire fighting techniques for oil spill conditions.

Close flash point:-
it is conducted in a closed vessel which is not opened to the outside atmosphere. the lid is sealed and the ignition source is introduced into vessel itself.
accumulation of vapour is well contained.
aim to simulate a fuel inside tank and ignition source is introduced.
the results are not interfered by outside elements.
test is highly significant for knowing flash point of a fuel.
quoted for carriage, transport and bunkering.

(C) The importance of Sodium to Vanadium Ratio:-
Running with a non - recommended sodium to vanadium ratio extended periods causes Vanadium, Sodium and Ash fouling in the Turbocharger.  
A sodium (Na) to vanadium (V) ratio of 1:3 causes the formation of low melting point vanadium and sodium oxide salts i.e vanadium pentoxide and sodium sulphate.
A ratio of 3:1 is less likely to cause deposition.  
Most residual fuels have vanadium levels of less than 150 mg/kg. Some fuels however, have a vanadium level greater than 400 mg/kg
In general, fuel when delivered contains a small amount of sodium, typically below 50 mg/kg. The presence of seawater increases this value by approximately 100 mg/kg for each percent of seawater. If not removed in the fuel treatment process, a high level of sodium will give rise to post-combustion deposits in the turbocharger. Although potentially harmful, these can normally be removed by water washing. 
High temperature corrosion and fouling can be attributed to vanadium and sodium in the fuel. During combustion, these elements oxidise and form semi-liquid and low melting salts that adhere to exhaust valves and turbochargers. In practice, the extent of hot corrosion and fouling are generally maintained at an acceptable level by employing the correct design and operation of the diesel engine. Temperature control and material selection are the principal means of minimizing hot corrosion. It is essential to ensure exhaust valve temperatures are maintained below the temperatures at which liquid sodium and vanadium complexes are formed and for this reason valve face and seat temperatures are usually limited to below 450°C.
When a fuel is bunkered with a vanadium level greater than that recommended by the engine designer, fuel additive, and numerous ash-modifying compounds can be used. They should be used with care as situations can arise where the effect of the ash-modifier, by incorrect application, can cause further problems in the downstream post-combustion phase.

(D) Octane Number:
It is a figure indicating the anti-knock properties of a fuel, based on a comparison with a mixture of iso-octane and heptane
The higher the octane number, the more compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higher octane rating are used in high-performance gasoline engine that require higher compression ratio.
Octanes are a family of hydrocarbons that are typical components of gasoline. They are colorless liquids that boil around 125 °C (260 °F). One member of the octane family, iso-octane, is used as a reference standard to benchmark the tendency of gasoline fuels to resist self-ignition.
The octane rating of gasoline is measured in a test engine and is defined by comparison with the mixture of iso-octane and heptane that would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of  iso octane (2,2,4-trimethylpentane, an organic compound with the formula (CH3)3CCH2CH(CH3)2  or C8H18) in that mixture is the octane number of the fuel.
Heptane is the straight-chain alkane (also called paraffin) H3C(CH2)5CH3or C7H16.