Development of a Bunker Norm for Ships

6. Fuel Oil Parameters

A conventional analysis of fuel oil includes the determination of values for a number of parameters. Combined, these parameters are used to provide a description of:

• pretreatment methods and procedures to ensure that, prior to being fed to the engine, the oil satisfies the requirements which the engine manufacturer has specified for the fuel oil;
• the prospects of using the fuel oil in the relevant engine type without risking damage and operational snags; and
• as the case may be, altering the mode of operation for the engine as necessary to minimize the probability of damage and operational snags.

Parameters for pretreatment

The parameters that provide direct information as to the pretreatment include:

Density
The density determines the ability to separate water by extraction.

Viscosity
The viscosity determines the ability to preheat to the correct temperature at the fuel pumps. Typical viscosity values are in the region of 10 to 15 cSt in connection with injection.

Point of congelation/ pour point
The point of congelation determines the temperature to which the fuel must be preheated to ensure that it can be pumped. At temperatures below the point of congelation a precipitation of wax will prevent the oil from being pumped. The upper limit according to ISO 8217 is 30⁰C., but for most oil qualities the point of congelation will be lower. A few distillate fractions from thermal fission can have a point of congelation which approaches or even exceeds the threshold value.

Water
Water can be present in fuel in the form of fresh water or salt water. Contamination with water will normally occur in transit from the oil refinery to the bunker port, or from the bunker boat. The maximum water content according to ISO 8217 is 1%. Any content of water in excess of 1% may cause the bunker to be returned or the price to be reduced. Obviously, many shipowners are not interested in paying for and receiving heavy fuel oil with about 1% content of water; therefore, as already mentioned, the shipowners will specify additional demands or limitations relative to the ISO 8217 standard. Generally speaking, they are not interested in first paying for the water, then having to separate it from the oil, and then paying to get rid of this water/oil mixture.

Water accompanying the fuel oil into the injection system and the cylinder may cause coatings to be formed and give rise to corrosion, erosion and unstable and incomplete combustion – as well as damage to fuel nozzles and fuel pumps.

Ashes
The ash content is an indication of the quantity of non-combustible material in the fuel oil. The ashes also contain all sorts of contaminating solids. It is assumed that these are removed, as far as possible, during the pretreatment consisting of settling and extraction – and in filter systems. The upper limit for the ash content according to the ISO 8217 standard is 0.2% by weight for heavy fuel oil, but an ash content of more than 0.1% is considered to be high. A high ash content is an indication – particularly in those cases where the contaminating solids can consist of catalyst residue (so-called cat-fines) – that efficient pretreatment is of crucial importance to avoid excessive abrasion.

Mixability
The mixability describes the tendency to precipitate sludge (asphalthenes) when mixed with another type of oil. Basically, heavy fuel oils delivered to a ship must be stable, and this implies that the heavy hydrocarbon molecules are kept suspended in the oil. If such a fuel oil is mixed with another type of fuel oil which destroys the ability to keep the heavy molecules suspended in the oil, there will be a precipitation of asphalthenes in the form of sludge. The sludge represents combustible material and therefore involves a loss of bunkers. Any precipitation of sludge may cause choking of the oil treatment system and, in serious cases, stoppage of the engine.

Mixability can be determined in various ways, but none of the methods is standardised, and that must be the reason why the ISO 8217 standard contains no norms for mixability. Generally speaking, the following are facts:

• aromatic fuel oils are mixable with other aromatic fuel oils;
• non-mixability occurs especially when aromatic fuel oil is mixed with paraffin-based fuel oil; and
• aromatic fuel oil is more unstable than paraffin-based fuel oil, i.e., they will have a greater tendency to precipitate sludge at high temperatures.

Parameters relevant to the operation of diesel engines

The fuel oil parameters described above are primarily relevant to the treatment of the fuel oil before it reaches the injection system on the engine. A few of these parameters can provide some information as to the fuel oil properties that are relevant for the running of the engine.

Density and viscosity aspects for heavy fuel oil

The density and viscosity of a heavy fuel oil, when seen in combination, may give an indication of the composition of the fuel oil.

High density and high viscosity

A typical density of 0.99 g/cm³ or higher and a viscosity in excess of 400 to 500 cSt at 50⁰C indicates a fuel oil with a high content of thermally cracked residual oil.

High density and low viscosity

A typical density of 0.99 g/cm³ or higher and a viscosity below 200 cSt at 50⁰C indicates a fuel oil with a high content of catalytically cracked distillates.

Low density and high viscosity

A typical density between 0.96 and 0.98 g/cm³ and a viscosity in excess of 400 to 500 cSt at 50⁰C indicates a fuel oil with a high content of atmospheric residual oil.

Low density and low viscosity

A typical density around 0.96 g/cm³ and a viscosity below 200 cSt at 50⁰C indicates a fuel oil with a high content of atmospheric residual oil and atmospheric distillates. This correlation is very uncertain, however, because it is possible to compose a fuel oil with a high content of catalytically cracked distillates – which will give a comparatively low density. In these cases the viscosity will often be considerably below 100 cSt at 50⁰C.

Density and viscosity aspects for MGO and MDO

Roughly speaking, the density and viscosity aspects can also be used as an indication of the composition of the distillates MGO and MDO. On the other hand, these distillates will be specified with an upper density and viscosity limit implying that the relationship can be divided into two groups:

High density and high viscosity

Typical density and viscosity values corresponding to the upper limit for the distillate in question indicate a high content of thermally or catalytically cracked distillates.

Low density and low viscosity

Typical values that are considerably lower than the upper limit indicate a high content of atmospheric distillates.

Sulphur
As the demand for reduction of the sulphur content in fuel oils will be given great attention in the future bunker norm, this subject will be dealt with in more detail later on in this report.

Vanadium and sodium
Vanadium follows the crude oil and is present in residual oil after refining, whereas sodium is in most cases the result of contamination with salt water. Vanadium cannot be removed during pretreatment, but the sodium content can be reduced by thorough elimination of sea water from the fuel oil. Both types lead to coating and corrosion at high metal temperatures – also called high-temperature corrosion. The content of vanadium in fuel oil is normally comparatively low, but values up to 600 ppm have been seen.

Operating experience has shown that the content of sodium must not exceed 30% of the content of vanadium, if coatings on and corrosion of the machinery components that are associated with the combustion chamber – such as cylinder liners, pistons, piston rings, exhaust valves, cylinder tops, and turbo-charger – are to be avoided.

The content of sodium is usually a function of the content of water (salt water) in the fuel oil, 1% sea water giving ab. 150 ppm.

Aluminium and silicon
Aluminium and silicon are contaminating solids in the fuel oil and produce ashes. They are present as particles with typical dimensions of less than 50 m and stem from the catalytically cracked residual oil as an admixed component in the fuel oil. This residual oil will always contain a certain quantity of contamination with catalyst particles from the cracking process. For heavy fuel oil the ISO 8217 standard allows up to a maximum of 80 mg/kg.

The horror of every shipowner and chief engineer is to receive a quantity of bunker oil with a high content of these so-called "cat-fines", in that there are examples where engines have had their fuel pumps, fuel valves, piston rings, and liners worn out in a few days.

Conradson Carbon
Residue (CCR) It has been the traditional opinion that CCR and the content of asphalthenes, if any, were indicators of the ignition and combustion properties of the oil. However, experience shows that CCR and the content of asphalthenes in fuel oils are irrelevant to the suitability of an oil for diesel engines.

CCAI (Calculated Carbon Aromatic Index)

The CCAI is a concept introduced by Shell in order to utilise the viscosity and density values for fuel oils to express the ignition properties of the oil.

"Ignition properties" means the ability of the fuel oil to self-ignite; it is expressed as the ignition delay, that is, the time from the start of injection until ignition occurs – in degrees of crankshaft angle, or expressed in milliseconds.

The CCAI value for a fuel oil can be calculated according to the following formula:

CCAI = D – 140.7 loglog(V + 0.85) – 80.6

where:

D = density in g/cm³ at 15⁰C.
V = viscosity in cSt at 50⁰C.

The formula is strictly an indication of the ignition properties and is not part of the ISO 8217 standard.

However, the interest in this value is increasing, as it is a highly important factor. Fuel oil analysis laboratories calculate the value in connection with their fuel oil analyses.

The following table shows some guidelines for CCAI values and their meaning:

The effect of running on an oil with poor ignition properties is illustrated by the following factors:

• Long ignition delay;
• high pressure gradient in bar per degree of crankshaft angle after ignition; and
• great pressure increase in the cylinder with high pressure gradient.

The variations are particularly pronounced when running at partial load.

Possible damage to the engine:

• Functionality of piston rings destroyed;
• piston rings broken;
• gas leakages, high abrasion, and high thermal load
• cracks in pistons and liners; and
• damage to bearings.

Poor ignition properties can be effectively countered by maintaining a high temperature in the cylinder at high scavenging air temperature.

Bunkers with high density combined with low viscosity, e.g., a viscosity of less than about 180 cSt at 50⁰C., will often result in long ignition delay – particularly at partial or low engine load.

A high content of catalyst particles such as aluminium and silicon is an indication that fractions from the catalytic cracking have been added to the fuel oil. These fraction will cause the oil’s ignition properties to deteriorate.

There are examples where bunkers with "normal" density but extremely low viscosity (perhaps as low as 20 to 30 cSt at 50⁰C.) can be totally unsuitable as fuel in diesel engines.

Cetane number

The cetane number for fuel oil is an indication of the oil’s ability to self-ignite (willingness to ignite) under the conditions prevailing in the diesel engine. The cetane number and the CCAI value both express the ignition properties of the oil. However, in the case of heavy fuel oil, it is normal to use the CCAI value, whereas for diesel oil and gas oil the cetane number is used.

In many cases another figure is substituted for the cetane number of an oil; this figure is called the diesel index for the oil and is calculated on the basis of an analysis of the oil; these values are inserted in a formula to calculate the diesel index.

An approximated relationship between the cetane number and the diesel index appears from the following table: