Marine Fuel oil

Marine Fuel oil

Marine fuel quality can significantly affect the performance, operation and maintenance of the diesel engine. A better understanding of Fuel properties and contaminants  can result in timely action , which reduces overall costs and downtime involved in corrective actions thereafter.

A fuel property is considered to be a characteristic occurring in the fuel carried over from its crude source and also a  result of the refining processes by which it was produced.

A contaminant is considered as foreign matter introduced into a fuel as a result of refining crude, blending , transport or storage.

Fuel Oil Characteristic Properties

Viscosity

Heavy fuel oils are normally purchased on the basis of a limiting viscosity due to storage, handling, or engine-related restrictions. Viscosity does not, however, carry a quality implication, as heavy fuel oils are produced in the future by more and more intensive secondary processing, the relationship between fuel oil viscosity and fuel oil quality becomes less and less meaningful.

Viscosity is used principally to give information about the handling, treatment and atomization of the fuel. However, it also is a rough indicator of its carbon and asphalt content. The lower the viscosity, the easier it is to settle or to separate entrained water and solid particles. Although high viscosity fuels require proper preheating for good separator operation and heating before injection for good atomization, this characteristic usually can be handled without any problems

Caution must be exercised when heating prior to injection to temperatures above 135 degrees Centigrade because cracking may occur, gases may be given off, and water may vaporize forming steam pockets in the fuel line. Insufficiently heated fuel, on the other hand, can result in poor atomization and delayed burning, which may lead to higher thermal loading, scuffing problems, possible piston and piston ring failure, and to an increase in fuel consumption.

In October, 1977, the world’s marine fuel suppliers agreed to replace the Redwood 1 (SR1) at 100 degrees Fahrenheit system with the Kinematic (cSt) system of viscosity at 50 degrees Centigrade. In future, European suppliers are expected to quote Kinematic viscosities at 8 degrees Centigrade (instead of 50 degrees Centigrade) in line with CIMAC and ESI recommendations.

Specific Gravity

Specific gravity is defined as the ratio of the weight of a given volume of the product at 15 degrees Centigrade to the weight of an equal volume of water at the same temperature.

Specific gravity is determined by floating a hydrometer in the liquid and noting the point at which the liquid level intersects the hydrometer scale. Corrections must then be made in accordance with the temperature of the sample at the time of test.

The importance of specific gravity relative to diesel engine operation lies in the fact that today’s standard fuel/water separating techniques are based upon the difference in density between the two substances. A maximum specific gravity of 0.991 (at 15 degrees Centigrade) can be handled satisfactorily.

Therefore, as the specific gravity of fuel approaches 1.0, centrifuging becomes less effective. Since diesel engine fuels should be free of water and the salts normally dissolved therein, extra centrifuging capacity will be required for high gravity fuel.

High specific gravity indicates a heavily cracked,( vis-breaking and thermal or catalytic Cracking) aromatic fuel oil with poor combustion qualities, which can cause abnormal liner wear. This effect is most pronounced in smaller, higher speed diesels. higher specific gravity  indicates a lower hydrogen-carbon ratio.

Carbon Residue/Asphaltenes

Conradson Carbon Residue (CCR) is a measure of the tendency of a fuel to form carbon deposits during combustion and indicates the relative coke forming tendencies of a heavy oil. Carbon-rich fuels are more difficult to burn and have combustion characteristics which lead to the formation of soot and carbon deposits. Since carbon deposits are a major source of abrasive wear, the CCR value is an important parameter for a diesel engine. The type of carbon also can affect abrasive wear.

Carbon residue is the percent of coked material remaining after a sample of fuel oil has been exposed to high temperatures under ASTM Method D-189 (Conradson) or D-524 (Ramsbottom).

Asphaltenes are those components of asphalt that are insoluble in petroleum naphtha and hot heptane but are soluble in carbon disulfide and hot benzene. They can be hard and brittle and made up of large macromolecules of high molecular weight, consisting of poly nuclear hydrocarbon derivatives containing carbon, hydrogen, sulfur, nitrogen, oxygen and, usually, the three heavy metals − nickel, iron and vanadium.

A high CCR/asphaltene level denotes a high residue level after combustion and may lead to ignition delay caused by slow burning and  high boiling point constituents result  in higher thermal loading and changes in the rate of heat release (ROHR)in the cylinder as well as after-burning of carbon deposits leading to engine fouling and abrasive wear

The maximum permissible CCR value depends on engine speed. The higher the speed, the shorter the time for combustion and the more residue deposited; hence, acceptable CCR values should decrease as engine speed increases. The increased time needed for combustion exposes a greater area of cylinder liner to flame than would normally occur and subjects the cylinder lubricant to higher pressure and temperature stresses resulting in hot spots, severe radiation, and burning of the lube oil film. The latter leads to scuffing, cylinder wear and engine deposits.

It does contribute to increased fouling of gas ways and turbochargers, especially during low power operation or at idle.

 The combination of higher Conradson Carbon content and higher asphaltene content necessitates  frequent centrifuge desludging and filter element cleaning/replacement.

A standard heavy fuel oil contains up to 14% asphaltenes   and   a   good   quality   fuel   up   to   8% asphaltenes. Higher Conradson Carbon content/asphaltene content in heavy fuel oils is to be expected in the future.  The increased content will result from refining more viscous, heavier crude oils and from additional, secondary processing, such as catalytic cracking and vis-breaking. These conditions will further concentrate more carbon in the bottoms that are blended or vis-broken to produce marine heavy fuel oils

Ignition quality

Contaminants,  unstable  fuels  and  incorrect  injection (temperature  and  timing)  are  the  main  reasons  for incomplete or improper combustion. Some fuels cause more   combustion   problems   by  nature.  These  can possibly  be  detected  by  looking  at  the  unnatural  ratio between   viscosity   and   density   (CCAI),   and   with combustion  analyzing  equipment  like  FIA  tests.  Older medium  speed  engines  are  most  prone  to  problems with improper combustion

Sulphur

Sulphur derivatives tend to concentrate in the heavier fractions during crude distillation, leaving the lighter fractions with relatively low sulfur contents. The extent to which various concentrations of Sulphur can be tolerated depends on the type of engine and the operating power level of that engine.

The oxides in sulfur combine with condensing water vapor in the combustion chamber to form highly corrosive sulfuric acid. Some of this water is present in the fuel already, while another source of moisture may be in the intake and scavenging air.

Ignition lag and poor combustion quality of high sulfur fuels cause ignition to occur later than normal in the power stroke, requiring a large volume of fuel to be burned in a shorter time. The subsequent pressure rise is greater than that normally experienced and places a severe strain on the cylinder lubricating oil film. This oil film also is subjected to the combustion flame farther down the cylinder liner where the oil film may be thinner. Sulfur has a tendency to combine with the water mist precipitated by the lower inlet air temperatures, and the formation of sulfuric acid on cylinder liners takes place. Careful control of the cooling water temperature in the inlet air coolers and/or installation of a water mist separator after the air coolers should remedy the problem of condensing moisture in the intake air.

When operating an engine on a fuel with a high sulfur content, care must be taken to avoid reaching the acid dew point temperature within the cylinder. One way of controlling this is to adjust the cooling water temperature at the cylinder wall. As the M.E.P. of the diesel increases, the acid dew point temperature increases as well. Since sulfur is oil soluble, it cannot be removed from the fuel by centrifuging. It can, however, be neutralized by the use of proper alkaline additives in the cylinder and/or engine lubricating oils.

As the sulfur content rises above three percent (3%) by weight, the problem of condensation of corrosive acids becomes increasingly troublesome. This is especially important in trunk-type engines which are characteristic of 4-cycle diesel engines. In trunk-type engines the cylinder lube oil is scraped into the crankcase by oil control rings on the pistons. This oil has been contaminated by the sulfur in the fuel. Upon entering the crankcase, the sulfur is free to combine with moisture which may collect there. The TBN of the lubricating oil is eventually lowered to a point where it is rendered ineffective in controlling the sulfur content of the fuel.

Ash/Sediment

The ash contained in heavy fuel oil includes the (inorganic) metallic content, other non-combustibles and solid contamination. The ash content after combustion of a fuel oil takes into account solid foreign material (sand, rust, catalyst particles) and dispersed and dissolved inorganic materials, such as vanadium, nickel, iron, sodium, potassium or calcium.

Ash deposits can cause localized overheating of metal surfaces to which they adhere and lead to the corrosion of the exhaust valves. Excessive ash may also result in abrasive wear of cylinder liners, piston rings, valve seats and injection pumps, and deposits which can clog fuel nozzles and injectors. In heavy fuel oil, soluble and dispersed metal compounds cannot be removed by centrifuging. They can form hard deposits on piston crowns, cylinder heads around exhaust valves, valve faces and valve seats and in turbocharger gas sides.

High temperature corrosion caused by the metallic ash content can be minimized by taking these engine design factors into consideration;

(1) hardened atomizers to minimize erosion and corrosion

 (2) reduction of valve seat temperatures by better cooling.

3 .By the application of ash modifiers.

Vanadium

Vanadium is a metallic element that chemically combines with sodium to produce very aggressive low melting point compounds responsible for accelerated deposit formation and high temperature corrosion of engine components. ratios of vanadium   to   sodium   (3:1),   a   mixed   salt   (oxide, sulphate) is formed with a minimum melting point; this should  be  avoided.  High  sodium  levels  at  engine  inlet can  cause  fouling  of  turbocharger  componentsVanadium itself is responsible for forming slag on exhaust valves and seats on 4-cycle engines, and piston crowns on both 2- and 4-cycle engines, causing localized hot spots leading eventually to burning away of exhaust valves, seats and piston crowns. When combined with sodium, this occurs at lower temperatures and reduces exhaust valve life. As the vanadium content (ppm) increases, so does the relative corrosion rate.

Vanadium is oil soluble. It can be neutralized during combustion by the use of chemical inhibitors (such as magnesium or silicon). Cooling exhaust valves and/or exhaust valve seats will extend valve and seat life.

Raising fuel/air ratios also prolongs component life. Other measures which can be used to extend component life are the use of heat resistant material, rotating exhaust valves, and the provisions of sufficient cooling for the high temperature parts.

Compatibility

Residual fuel can be considered to be a colloidal dispersion of high molecular weight substances held in chemical and/or physical equilibrium in heavy fuel oil. When the equilibrium forces are disturbed, the high molecular weight components (typically asphaltenes) are thrown out of solution or precipitated to form a sludge or sediment.

Under normal conditions, asphaltenes are held in dispersed colloidal suspension within the oil. If the oil is stable then the asphaltenes will remain in this suspension for an indefinite period of time and survive normal operating heating

 Compatibility problems occur when heavy fuel oils with a high asphaltene content are mixed with lighter fractions with a predominance of aliphatic hydrocarbons. The mixing can cause precipitation of the asphaltenes.

It occurs when fuel oil suppliers blend in order to reduce final fuel oil viscosity, specific gravity, or other fuel property.

Incompatible fuel oils result in rapid strainer and separator plugging with excessive sludge. In the diesel engine, incompatible fuel oils can cause injection pump sticking, injector deposits, exhaust valve deposits, and turbocharger turbine deposits. Once an incompatible fuel oil is lifted or blended onboard, little can be done to undo the resultant problems except to continue to desludge, clean out, and maintain the engine(s) round-the-clock until the incompatible fuel oil is consumed. If another bunker supply is available onboard, switching to a compatible fuel oil will eliminate the handling and engine problems, but will probably result in a significant build-up of sludge in the bottom of all tanks containing the incompatible oil. Removal of the incompatible oil from affected tanks can be a very time consuming operation.

Frequently, compatibility problems are caused onboard by the indiscriminate mixing of different bunkers. Bunker segregation practices should be applied to storage, settling, and service tanks. As crude oils are subjected to more intense refining, compatibility problems can be expected to become more common because the secondary processes used tend to produce more unstable and incompatible marine fuel oils.

Cetane

Ignition quality is indicated by cetane number. The lower the cetane number of a fuel, the greater the ignition delay, and the longer the period of time between fuel injection and the beginning of the rapid pressure rise associated with fuel ignition and combustion. As crude oils are refined more intensely, the fuel oils possess a greater aromaticity, which can increase the ignition delay, and can result in hard knocking or noisy engine running, which is undesirable over long periods of time.

ignition delay and burning duration increase with increased viscosity and aromatic content of the fuel and such fuels contain a higher percentage of high boiling point hydrocarbons. Both factors tend to lower the cetane number. Physical factors which influence ignition

Experience also has indicated that raising inlet air temperature can reduce the cetane sensitivity of higher speed diesel engines.

Cetane number is normally quoted for distillate fuels only.

Flash Point

The flash point of a fuel is the temperature at which fuel vapors can be ignited when exposed to a flame. All petroleum products will burn. However, in order for this to occur, the ratio of fuel vapor to air must be within certain limits. The primary purpose of reporting flash point is for safety during storage, heating and handling of liquid marine fuels. A high viscosity fuel with high specific gravity and a low flashpoint would be difficult to handle, because preheating above its flash point might be required for pumping and storage. Care must be taken not to heat any fuel oil tank to the flash point of the fuel, oil. It is recommended to heat to within 10 degrees Centigrade of the flash point only

Flash point is important from a handling and storage consideration, The accepted, safe, minimum flash point for fuel oils established by most regulatory bodies is 60degrees Centigrade..

Pour Point

For pumping and handling purposes, it is often necessary to know the minimum temperature at which

a particular fuel oil loses its fluid characteristics.  This is important for storage and handling.

Paraffinic based oils as a whole is solidified – flow simply is prevented by the crystalline wax structure. This structure can be ruptured by agitation and the oil will proceed to flow, even though its temperature remains somewhat below the pour point.

Naphthenic predominant oils, on the other hand with a comparatively low wax content, oil as a whole thickens more than a paraffinic oil of comparable viscosity when it is cooled. For this reason, at its pour point the entire body of oil is congealed. Agitation has little effect upon fluidity, unless it raises the temperature.

Sufficient tank heating is required to maintain fuel temperature at least 12-28 degrees Centigrade above its pour point for satisfactory pumping.

Calorific Value

The specific gravity of a fuel oil is a reflection of its heating value. The heating value is determined primarily by the carbon/hydrogen ratio; as the carbon/hydrogen ratio increases, the specific gravity will increase and the heating value will decrease. The heating value is also decreased by the presence of sulfur. The result is that there is less hydrogen with its high heating value available per pound, and a consequent decrease in

Marine Fuel Oil Contaminants

The  water  content  of  the  fuel  oil  must  be  reduced  by centrifuging   and   by   the   use   of   proper   draining arrangements  on  the  settling  and  service  tanks. 

A thorough  removal  of  water  is  strongly  recommended, to  reduce  the  content  of  cat  fines  and  sodium  in  the fuel   oil.   Cat   fines potentially   reside   in   the   water droplets and marine fuel oil is often contaminated with  sea  water  containing  sodium,  1.0%  sea  water  in  the fuel  oil  corresponds  to  100ppm  sodium.

 To achieve a good separation, the throughput and the temperature of the fuel must be adjusted in relation to the viscosity. With  high  viscosity  fuels,  the  separating  temperature must  be  increased  whereas  the  throughput  must  be decreased  in  relation  to  the  nominal  capacity  of  the separator.  Salt water can also cause excessive separator sludge volume as a result of water/sludge emulsification during centrifuging.

Water can provide the beginning for microbial growth in heavy fuels. These very simple life forms live in the water and feed on the heavy fuel at the water-fuel interface. The result of microbial matter in the fuel can be slime, which is sometime corrosive that will foul strainers, filters and separators. The short range solution would be to add a “biocide” chemical additive to the fuel to kill the growth. The much preferred, long range solution would be to regularly drain the tank bottoms to eliminate the water, without which this growth cannot exist

Sodium (Na)

Salt water contamination in barges used to transport the fuel is not uncommon. To some extent, even salt air condensation in fuel tanks contributes to the overall sodium content.

sodium in fuel is usually water soluble and can, therefore,be removed with the centrifugal separator.

In the gas phase oxidation, the high temperature oxygen-containing exhaust gases react with steel to form oxides. Liquid phase oxidation (corrosion) takes place when molten sulfates and pyro sulfates in the exhaust gases deposit on valve surfaces

Sodium acts as a paste (flux) for vanadium slag. When unfavorable quantities of vanadium and sodium are present in a fuel they react at combustion temperatures to form (eutectic) compounds with ash melting points within operating temperatures. In molten form sodium/vanadium ash can corrode alloy steels.

 In their molten states, the vanadium-sodium-sulfur compounds also act to dissolve the exhaust valve surface ferric oxide (Fe₂03) layer, thus exposing the underlying steel surface to further oxidation attack and subsequent erosion.

In extreme situations, similar sodium/vanadium ash corrosion attack can also occur downstream of the exhaust valves in the turbocharger exhaust gas turbine and blading.

Sediment

Sediment is another contaminant Rust, scale, weld slag, dirt and other debris can be introduced in storage or in pipeline or barge transport. The majority of this sediment can be removed by settling, straining or filtration, or centrifuging in the shipboard fuel oil system.

Alumina/Silica

Aluminium and silicon in the fuel oil are regarded as an indication  of  the  presence  of  so-called  catalytic  fines (cat  fines).  These  are  porcelain-like  particles  used  as catalyst in petroleum refining. The particles cause high abrasive wear to piston rings and cylinder liners. In the extreme case, they will become embedded in the ring and  liner  surface,  resulting  in  increased  wear  over  a prolonged  time  period.  The  most  dangerous  cat  fines are  of  the  size  10  to  20  microns.  They  tend  to  be  attracted  to  water  droplets  and  are  very  difficult to remove  from  the  fuel  oil,  even  more  so  when  used lube  oil  is  present.

  Practical  experience  has  shown that  with  proper  treatment  in  the  fuel  oil  separator  an aluminium and silicon content of 80 mg/kg, determined  using a suitable analysis method (ISO 10478), can be  reduced  to  15  mg/kg,  which  is  considered  as  just tolerable. Proper treatment means reduced throughput in the separator and a fuel temperature of as close as possible to 98°C.  The  problems  which  arise  due  to  cat  fines  in  low speed  diesel  engines  are  often  unexpected.  Cat  fines are  not  always  evenly  distributed  in  the  fuel  and  are sometimes  present  even  when  they  do  not  appear  in the   analysis   results.  

They   can   accumulate   in   the sediment  of  the  fuel  tank  from  previous  bunkers  and be  mixed  into  the  fuel  when  the  sediment  is  churned up  in  bad  weather.  All fuels  should  be  assumed  to contain  cat fines, even if this is not apparent from the fuel oil analysis.

Sludge

Sludge is a contaminant that results from the handling, mixing, blending, and pumping of heavy fuel while stored at and after it leaves the refinery. Storage tanks, heavy fuel pipe lines, and barging can all contribute to the sludge. Water contamination of a high asphaltene fuel oil can produce an emulsion during fuel handling which can contain more than fifty percent (50%) water.

Shipboard transfer pumps frequently can provide the necessary energy to produce emulsified sludges during normal fuel transfers. These emulsified sludges can cause rapid fouling and shutdown of  centrifugal purifiers and clogging of strainers and filters in the fuel oil system, and rapid fouling if burned in the engine.

Fibers

Fiber contamination can cause significant problems in fuel handling aboard ship. This type of  contamination usually occurs during transport and storage. Fibers can plug suction strainers protecting pumps within minutes of initial operation. A centrifuge normally is ineffective in removing oil soaked fibers because they have the same density as the oil being purified. Hence, downstream manual or auto-strainers and fine filters can be expected to clog in a short period of time and continue to clog frequently until the entire amount of a fiber contaminated fuel has been consumed or removed.

Oxidation Products

This form of contamination is the result of the marine residual fuel aging, either before or after it is bunkered. Residual fuels are not stable for long periods of time at elevated storage temperatures. The fuel ideally should be consumed in  less than three (3) months from the date of bunkering

Heated heavy fuels, stored in uncoated steel tanks and exposed to air (oxygen) oxidize and polymerize with time. The resultant sludge, gums and resins will initially form in solution and then agglomerate and settle or adhere to the tank’s surfaces. Also, as heavy fuels age, their shipboard conditioning and treatment becomes  more difficult. In the extreme, the diesel engine’s combustion process can  deteriorate causing increased fouling, deposits and corrosion as a result of burning such partially oxidized older fuel oils.
Chemical contaminants
It has been observed of late that there has been a severe contamination of marine fuels observed due to chemical waste such as Styrene , Chlorinated compounds, Solvents and many Volatile organic thinners and binders from  paint and gum industry being part of base stock for reasons unknown .
This has resulted in damages such as fuel pump seizures, glazing of liners and other damages.

Trichloromethane and carbon tetrachloride adulterants in marine fuels can lead to seal shrinkage and expensive fuel pump seizures, and badly effect overall ship engine lubrication.

Trichloromethane, or chloroform, is used as a manufacturing solvent and in the production of dyes and pesticides. Carbon tetrachloride was also widely used as a dry cleaning agent, as well as a refrigerant and an aerosol propellant, before its ban from use in consumer products in 2002 under EC regulation No. 2037/2000.

Bunker fuels containing these chemicals are in breach of section 5.1 of ISO 8217:2005 specification, and section 5.5 of ISO 8217:2010.

These can only be determined by GCMS and standards are being worked out to fix limits in case this contamination is inevitable , though standard prohibits usage.