The difference between TBN and TAN

TBN is total base number and TAN is total acid number. TBN is a measure of the reserve alkalinity or reserve acid neutralization remaining in the oil. TAN measure the increase of oil oxidation and build-up of corrosive acidic compounds. Engine manufacturers often recommend utilizing both tests to gain a more in depth understanding of oil condition and engine oil remaining protection. In utilizing both tests, the TBN will decrease over time and TAN will increase over time. The point where the two numbers meet or cross over can be considered the point where the oil can no longer provided adequate corrosive where protection.

TBN In Diesel Engine Oils

TBN is an important property of engine oils. The abstract definition is as follows;

“Total Base Number (TBN) is the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in 1 gram of sample (ASTM Designation D 974)”. But this tells us little about what TBN in an engine oil does, nor how much we need for effective engine oil performance and engine protection.

The detergent additive in an engine oil has two functions

• To control deposits in the hot parts of the engine such as the pistons and turbocharger bearings.

• To neutralize acidic products of combustion from the fuel that can cause corrosive wear.

Engine oil formulators have always matched the amount of TBN to the amount of sulfur in the fuel. Today Chevron manufactures engine oils with 70 TBN which are used in marine engines operating on 5% sulfur fuel. This is very high sulfur content, 50,000 parts per million. Diesel fuel in the US was approximately 2,500 to 3,000 ppm sulfur (the legal maximum for ASTM 2D fuel was 5000 ppm) until 1993, when EPA regulations required a reduction to a maximum limit of 500 ppm for on road use. Today all diesel fuel is limited to 15 ppm sulfur maximum (Ultra Low Sulfur Diesel, or ULSD).

With 3000 ppm sulfur diesel fuel, oil TBN in the range of 10 to 14 was common, with lower priced oils at approximately 8 TBN. Current engine oils for use with ULSD are around 8 to 9+. Clearly the need for high TBN does not exist with today’s ULSD fuels.

How is TBN measured? It is important to note there are several test methods for Total Base Number. The one used in product data sheets is generally ASTM D 2896. This method uses perchloric acid to neutralize the alkalinity in the oil and yields a slightly higher number than the test method used by the oil analysis labs. They generally use ASTM D 4739 and the acid used here is hydrochloric acid. This produces a number approximately 2 mg KOH/g LOWER than ASTM D 2896 for the same oil. Due to chemical interferences, this test method does not recognize all of the alkalinity that ASTM D 2796 sees.

Why are there two test methods? The oil manufacturers have typically used ASTM D 2896 and their labs are set up to handle perchloric acid, which is toxic and hard to handle. In addition ASTM D 2896 can measure both the “hard base” from metallic detergent as well as the “soft base” from organic, non- metallic ingredients. So it is a more accurate method. BUT, the production oil analysis labs prefer to use a safer and easier to use titration acid, namely hydrochloric acid. The tests can be run faster, more cost effectively and more safely.

How much TBN do we need to protect the engine? The old rule was to change the engine oil when 50% of the new oil TBN had been consumed. Because of the virtual absence of fuel sulfur today, much less is needed. Chevron now sets the TBN guidelines for all of its diesel engine oils as follows:

FOR ALL OILS when using ULSD

• Severity 1: 50%-44% of new oil TBN or 3.5 to 4

• Severity 2: 43%-36% of new oil TBN 3 48 to 2.9

• Severity 3: <35% of new oil TBN < 2.8 to 2

• Severity 4: less than 2 <2

Other parameters of engine oil are now more important to engine durability and extended service protection than TBN. These are parameters such as oxidation stability, wear control, effective soot dispersancy. A balanced oil has multiple performance abilities and TBN is only one of the performance measures that are important in today’s high performance engine oil.

Fuel Characteristic Definition as per ISO 8217:2010

ISO specification 8217 stipulates acceptable characteristics of marine fuel oil products. In order to understand the relative importance of each characteristic it is important to understand the definition. The following definitions are deemed useful to users of marine fuels products.

Viscosity A measure of fluid resistance to flow. Viscosity of fuel oil decreases with increasing temperature. The viscosity of the fuel oil at the point of injection into the engine is key to performance. Viscosity is used to classify residual fuel types but is not a key indicator of fuel quality. For example, all other characteristics being equal, a fuel of 360 cSt is of no better or worse quality than a fuel of 400 cSt, it is just less viscous.

Density Mass per unit volume of a product. It is used to convert the volume delivered into the quantity purchased. Density varies with temperature and is an important parameter in the onboard purification of the marine fuel product.

Calculated Carbon Aromaticity Index (CCAI) The most widely accepted empirical formula to estimate the ignition quality of fuel oil. CCAI uses the physical properties of density (d) and viscosity (V) in the following equation: CCAI = d - 81 –141*log [log (V+0.85)]

Sulfur Sulfur is the main inorganic component of fuel. It occurs naturally in crude oils and tends to concentrate in the heavier fractions. Sulfur concentration in fuel oil strongly influences the choice of lubricant. Energy content of fuel oil diminishes with increasing sulfur.

Flash Point Flash point is the minimum temperature at which vapours released from the fuel oil will ignite when exposed to an open flame. The flash point of a blended fuel oil is the same as that of the lightest component in the fuel oil product.

Acid Generally, marine fuel products should not contain inorganic acids, however ISO 8217 allows for minimal acceptable levels.

Sediment Sediment in distillates is composed mainly of rust, general dirt & scale. Marine fuel oil sediment can be both inorganic and organic in nature.

Carbon Residue Carbon residue is a measure of the carbonaceous material left after the volatile components of a fuel have been vaporized in the absence of air. It is used to estimate the potential of a fuel to create deposits in an engine upon combustion.

Pour Point The pour point of a fluid is the lowest temperature at which it ceases to flow. In fuels, the pour point is largely determined by the petroleum wax content in the oil. Pour point determines the minimum temperature required for storage and handling onboard of fuel oil products.

Ash Ash is the carbon free (inorganic) residue remaining after completely burning the fuel in air. It occurs naturally in crude oils and tends to concentrate in the heavier fractions. Ash can contain hard and erosive particles, some of which may also be corrosive.

Vanadium Vanadium is a metal occurring naturally in some crude oils and is concentrated in residual components during refining. In high concentration, it can form high melting point, corrosive deposits. In combination with sodium, it can form lower melting point, oxygen deficient deposits.

Sodium Sodium occurs naturally in crude oils and is concentrated in residual streams during refining. It can be introduced into fuel streams as a scavenger used to control the hydrogen sulfide content of fuel oil, via salt water contamination, or through sodium ingress into a marine diesel engine due to salt water saturated air.

Cat Fines Cat fines contamination in fuel oil is caused by carryover of catalytic material used in the refining process and evidenced by the presence of Alumina and Silica. Cat fines are hard and abrasive.

Used Lubricant (or Lube) Oil Some used lube oil may contain components harmful to an engine, but all used lube oils may not necessarily be unfit for purpose. Some additives used to identify used lube oil such as calcium are naturally occurring in crude oil and hence residual fuel. Test methods are designed to eliminate false positives.

Calcium A soft grey alkaline earth metal, the fifth most abundant element in the earth’s crust. Essential for living organisms, particularly in cell physiology, and is the most common metal in many animals. Calcium occurs naturally in crude oils. It is introduced into the combustion space via cylinder lubrication oil. The alkaline Total Base Number (TBN) additives of cylinder lube oil contain calcium. Calcium is concentrated in the residual part of the refinery process as lighter products are removed.

Compatibility Compatibility of a fuel is a function of the stability of the two individually stable oils used to blend marine fuel oil when they are co-mingled. Heavy marine fuels are complex mixtures of hydrocarbons. Some very large molecules called asphaltenes are held in suspension by maltenes. Mixing fuels can adversely affect this equilibrium.

FUNDAMENTALS OF REFINERY PROCESSING

The basic products from fractional distillation are:

Liquid petroleum gas (LPG) has carbon numbers of 1-5 and a boiling point up to 20 °C. Most of the LPGs are propane and butane, with carbon number 3 and 4 and boiling points -42 °C and -1 °C, respectively. Typical usage is domestic and camping gas, LPG vehicles and petrochemical feedstock.

Naphtha, or full range naphtha, is the fraction with boiling points between 30 °C and 200 °C and molecules generally having carbon numbers 5 to 12. The fraction is typically 15–30% of crude oil by weight. It is used mainly as a feedstock for other processes:

• In the refinery for producing additives for high octane gasoline

• A diluent for transporting very heavy crude

• Feedstock to the petrochemical olefins chain

• Feedstock for many other chemicals

• As a solvent in cleaning

Gasoline has carbon numbers mainly between 4 and 12 and boiling points up to 120 °C. Its main use is as fuel for internal combustion engines. Early on, this fraction could be sold directly as gasoline for cars, but today’s engines require more precisely formulated fuel, so less than 20% of gasoline at the pump is the raw gasoline fraction. Additional sources are needed to meet the demand, and additives are required to control such parameters as octane rating and volatility. Also, other sources such as bioethanol may be added, up to about 5%.

Kerosene has main carbon numbers 10 to 16 (range 6 to 16) boiling between 150 °C and 275 °C. Its main use is as aviation fuel, where the best known blend is Jet A-1. Kerosene is also used for lighting (paraffin lamps) and heating.

Diesel oil, or petrodiesel, is used for diesel engines in cars, trucks, ships, trains and utility machinery. It has a carbon number range of 8 to 21 (mainly 16-20) and is the fraction that boils between 200 °C and 350 °C.

White and black oils: The above products are often called white oils, and the fractions are generally available from the atmospheric distillation column. The remaining fraction below are the black oils, which must be further separated by vacuum distillation due to the temperature restriction of heating raw crude to no more than 370-380 °C. This allows the lighter fractions to boil off at a lower temperatures than with atmospheric distillation, avoiding overheating.

Lubricating oils, or mineral base lubricating oil (as opposed to synthetic lubricants), form the basis for lubricating waxes and polishes. These typically contain 90% raw material with carbon numbers from 20 to 50 and a fraction boiling at 300-600 °C. 10% additives are used to control lubricant properties, such as viscosity.

Fuel oils is a common term encompassing a wide range of fuels that also includes forms of kerosene and diesel, as well as the heavy fuel oil and bunker that is produced at the low end of the column before bitumen and coke residues. Fuel oil is graded on a scale of 1 to 6 where grade 1 and 2 is similar to kerosene and diesel, 3 is rarely used anymore. 4-6 are the heavy

fuels, also called Bunker A, B and C, where B and C are very high viscosity at normal ambient temperatures and requires preheating to about 100 °C and 120 °C respectively, before it flows enough to be used in an engine or burner. Fuel oil grade 4 does not require preheating and is sometimes mixed with off spec products, such as tank residue and interface liquid from multiphase pipelines or with grade 2 fuel oil to achieve low-enough viscosity at ambient temperatures. Fuel oil 6 is the lowest grade, its specification also allows 2% water and 0.5% mineral soil and is consumed almost exclusively by large ships in international waters, where pollutants such as sulfur is less regulated.

Bitumen and other residues like coke and tar has carbon numbers above 70 and boiling points above 525 °C. Low sulfur coke can be used for anodes in the metals industry (aluminum and steel) after processing (calcining). The remainder is a problem fuel, because of high sulfur content and even higher CO2 emissions than coal (typically 15% higher). Bitumen in the form of asphalt boiling above 525 °C is used for roofing and road paving. Asphalt concrete pavement material is commonly composed of 5% asphalt/bitumen and 95% stone, sand, and gravel (aggregates).

Calculation of Fuel Quantity & Density-Volume Correction Factor

To calculate the weight of the fuel we need to find out the volume and temperature. Having Density and temperature enter Table 54B to obtain Volume Correction Factor.

Mass = Density x Volume

         = VCF x WCF x Actual Sounded Volume 

Where: 

         Density = Temperature Corrected Density = VCF x WCF 
         Volume = Actual Sounded Volume 
         VCF = 1- {(T-15) * 0.00064} 
        WCF = Density @ 15 deg.C - 0.0011

Download Table 54B

CCAI RELATED BUNKER PROBLEMS

In these days of burning residual fuels in our ships, various types of fuel related problems occur. These can, while being evident, be a considerable 'pain in the neck' for the engine crew and for the operator of the vessel. Hence, the measures to be taken from the owner's side to avoid these hick-ups are to specify as appropriate as possible the grade of fuel required for 'his' engine. And in case of a dispute, to be a subscriber to a recognized fuel analyzing scheme. Sampling procedures for receiving fuel should be accepted by all involved parties.

Let us dwell for a moment on the issue of ordering bunkers and the specification that normally is submitted to ensure the correct grade is received on board. A number of parameters are normally mentioned, such as; density max, viscosity max, sulphur max, poue point etc. There is however a parameter rarely being mentioned in these specifications and that is the CCAI, Calculated Carbon Aromaticity Index, which gives a value on the Ignition Quality for residual fuels, since these grades cannot be verified by methods used for distillates, i.e. Diesel Index, Cetane Index and Cetane Number.

Accepted method for determination of the ignition quality of residual fuels is currently not available. It has, however, been empirically established that there is a relationship between the density, the viscosity and the ignition performance and the Shell-developed CCAI is the one presently most accepted for indicating ignition delay, although there is also a BP- developed Calculated Ignition Index (CII). CCAI gives an idea of how much the ignition is delayed, the higher the index, the longer the delay. The CCAI can be determined, with limited accuracy, by the enclosed nomogram AAAA

The combustion starts with a short delay already when a small amount of the fuel has been injected and therefore the remaining quantity injected burns in a controlled manner. If,however, the delay is long, a large amount is injected before the combustion starts,producing a quick and violent raise of pressure. This produces the characteristic"diesel knock". The problem is generally related to medium speed diesel engines when burning blended fuels less than 220 cSt. and problems seems to appear in the CCAI-span 850-890. See enclosed diagram BBBB If it is required (necessary) to operate the engine within this span the stresses on the engine components might increase considerably and special attention should be paid to:

0  Connecting rod big-end and bearing shells.

0   Main bearing shells

0  Pistons(particularly composite pistons)

0  Piston rings and liners

0  Cylinder head with studs and gaskets

0  Tie bolts

0  Intake and exhaust valves

To alleviate the effect of the ignition delay, the ambition should be to keep the engine load within 50 - 85 % and to maintain the inlet air temperature  as high  as practically possible and through pre-heating prior start-up(the CCAI problems  are accentuated on a cooler engine, hence a known  scenario is the vessel makes it to port but the engine can not be restarted upon departure due to fouled/clogged piston rings, poppet valves and turbocharger).With the violent increase of combustion pressure, when operating on fuels delaying the ignition, the rate of blow-by will increase and it goes without saying that the lub.oil quality must be optimal to cope with the additional load imposed on the bearings.

So, by way of conclusion, if the shipowner is operating engines which  are sensitive to ignition quality he would  be wise to order fuels with a CCAI limit or to set density and viscosity limits which will control the CCAI.

Special care to CCAI is needed when a ship is forced to use low viscosity fuels (below

180 cSt), due to heating limitations. If the density of these fuels is high the CCAI will be too high and ignition problems may be encountered.