VLSFO From Unknown Fuels Blend Can Cause Operational Issues


0.5% marine fuel oil can cause some operational issues caused by fuel supplier who does blending without knowledge of what happens when unknown fuels are blended to make VLSFO. Here we present Viswa Lab’s latest technical update that discusses about such issues.

Stability issues 

 Stability of residual fuel is defined by its resistance to breakdown and precipitate asphaltenic sludge, despite being subjected to forces, such as thermal, physical or aging stresses, while handled and stored under normal operating conditions. 


Instability can happen if the fuel is blended such that the asphaltenes are not well suspended and if the rest of the fuel does not have enough aromatic components to keep asphaltenes in suspension. 

Asphaltenes need other aromatic components in order to stay in suspension, whereas if they are blended with high amounts of paraffinic cutter stocks, the risk of instability is large. 

In case of asphaltene precipitation:

  • The result of an unstable fuel might be excessive sludge formation in tanks or further down in the fuel system, which then can block filters and separators. 
  • Once the asphaltenes have precipitated and formed sludge, there is a very low possibility that the process can be reversed and the asphaltenes can be re-suspended in the fuel again, either by physical or by chemical means. 

As a result 

  • The sludge accumulation in the storage tank, piping systems or centrifuges and filters which causes purifier and filter choking. 

Make sure that: 

  • The fuel must be stable and homogenous at delivery. 
  • The fuel shall also be stable in typical ship fuel systems and in the engine high-pressure fuel injection system. 
  • It is the responsibility of the fuel blenders and suppliers to ensure fuel stability. 

Compatibility issues 

Compatibility is defined as the ability of two or more fuels to be mixed at a defined ratio without evidence of material separation, resulting in the formation of multiple phases e.g. flocculation or separation of asphaltenes, precipitating sludge in the fuel system. 

  • The process of the asphaltenes dropping-out can be very quick and is IRREVERSIBLE. 

The new 0.5 wt. % S marine fuels are expected to be mostly paraffinic. There is always a risk of incompatibility issues after mixing paraffinic fuel with aromatic fuels which depends on the fuel characteristics. 

The resulting operational issues are;

  • Problems in the fuel cleaning system with sludge in tanks and further in the fuel system and thereby blocked filters and separators. 

Cold flow properties issues 

Three properties are used when evaluating the flow properties of marine fuels:

  • Cloud Point (CP) (ISO 3015) 
  • Cold Filter Plugging Point (CFPP) (ASTM D6371) 
  • Pour Point (PP) (ISO 3016) 

Special care needs to be taken in case of using VLSFO 

  • Some of VLSFOs are predominantly paraffinic base (Pour point~30°C) therefore one of the main issues related to these fuels is a wax formation which is because of high paraffin content. 
  • As the temperature goes below the wax appearance temperature, the wax crystal grows to form and deposit on-board the ship during handling. 

Wax formation causes:

  • Blocking the filters 
  • Depositing wax in the storage tank 
  • Reducing the fuel flow rate to the machinery plants 

Recommendation: Keep the temperature at least 10 ̊C above the PP. 

Corrosion issues 

The acid number (TAN) of the fuels need to be tested to ensure that the fuel is free of strong and inorganic acids. Please also have the pH of the fuel measured. Acid Number (AN) is an indicator of acid concentration but not acid strength. 

As such, it cannot be relied upon to provide a dependable indication of the corrosion potential. On the other hand, pH which stands for potentiometric hydrogen ion concentration represents how corrosive the oil is but it does not indicate the concentration of acidic constituents. 

The high acid number can result from Naphthenic acid in fuel from naphthenic source crude or be a reflection of contamination from the acidic source.

Fuels with high acid numbers or low pH arising from acidic compounds occasionally cause accelerated damage to marine diesel engines. 

Recommendation: Care needs to be taken when the acid number is greater than 0.6. 

Ignition and combustion properties 

Ignition and combustion performance is dependent on the Characteristics of the fuel, engine design, condition, settings, applied load, ambient conditions and fuel pre-treatment.

Most of the paraffinic fuels have good ignition-combustion properties while aromatic fuels have poor ignition combustion properties. 

Fuels with poor ignition and combustion properties can cause engine damage and even total breakdown. The operational difficulties affect more – high and medium-speed engines than low-speed two-stroke types. 

VLSFO samples can be paraffinic, aromatic or mixture of paraffin/aromatic so ignition-combustion properties of fuel need to be evaluated in case of aromatic fuels. 

Catalytic fines 

Cat fines in fuels should not exceed 15 mg/kg (ppm). There is a possibility of increasing cat fine content in blended low sulfur fuels. 

Cat fine content in VLSFO samples in some regions (Africa, Asia) is higher than 35-40 ppm while for most of the HFO samples the average content was 20-23 ppm. 

Engine makers typically permit No more than 15 ppm or, some, 10 ppm, for a wear-and-tear free engine 

Effect of high cat-fine content

  • High cat fine levels can cause damage to engine fuel injection equipment and pistons/liners. 


Effect of low viscosity 

The lower viscosity will reduce the film thickness between the fuel pump plunger and casing. 

As a result, it causes 

  • Excessive wear, possible sticking, and failure of the fuel pump. 
  • Loss of capacity in fuel supply and circulation pumps 
  • Fuel leaking around the pump plungers and rotors for boiler fuel pump which prevents the ship from achieving full power. 

Unusual constituents 

There is a possibility of contamination by Polymers, Polymethacrylates, Phenols, Tall oils, Chlorinated hydrocarbons, Estonian shale oil and Organic acids which can all be related to blending. 

These components can cause the following problems: 

  • Filter blocking, pump sticking, fuel pump seizures, and excessive separator sludging. 

Flash point 

Flashpoint is one of the valid indicators of fire and explosion hazard posed by the fuel. As required by SOLAS, the minimum flash point of any fuel used by the vessel or carried in its tanks should be not less than 60.00°C. 

A fuel with flashpoint less than 60°C will be outside the Safety of Life at Sea (SOLAS) requirement and the ISO 8217 specifications. Even a 0.1°C less than 60°C is not acceptable. 

There is a possibility of decreasing flash point in low sulfur fuels because of different characteristics of the blending component feedstocks, method of production and type of cutter stocks used may be different from those in use today. 

A low flash point will increase the risk of fire and explosion. 

You can find a summary of operational issues related to fuel properties in the below table. 



  • Guidance for onboard use of Compliant Fuel Oil with SOx regulation from 2020, ClassNK, March 2019. 
  • Detailed information on preparation and operation on fuels with maximum 0.50% sulphur, MAN Energy Solutions, 2019. 
  • The supply and use of 0.50%-sulphur marine fuel, Joint Industry Guidance, 2019. 
  • Development of draft guidelines for consistent implementation of regulation 14.1.3 OF MARPOL ANNEX VI Safety implications associated with 2020 fuels and their respective challenges, IMO, 2018.

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