Low Sulphur Fuel Oil

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Introduction

In order to reduce the environmental impact MARPOL regulations have gradually lowered the sulfur content in the fuels used in marine diesel engines in recent years. However the use of low-sulfur fuels has introduced new challenges for ship operators. Today we have a better understanding of the consequences of using low-sulfur fuel compared to when it was first introduced. Believing that understanding is crucial for better operations I wanted to address this topic today and contribute to this path.

"The International Convention for the Prevention of Pollution from Ships (MARPOL) is the main international convention covering prevention of pollution of the marine environment by ships from operational or accidental causes". [1]

Lubricating oils used in diesel engines were previously optimized for higher sulfur content fuels. During combustion the sulfur in these fuels is converted to Sulfur Trioxide SO3, which then interacts with the water produced during combustion to form sulfuric acid (H2SO4). The high Total Base Number (TBN) characteristic of the oils, typically in the range of 70-100 BN, helped neutralize the acid and protect piston rings and liners. [2]

Lube Oil [3]

With the implementation of MARPOL regulations and the reduction in sulfur content in fuel the traditional lubrication strategy (High Sulfur High TBN) became less effective. This is because low-sulfur fuel produces a significantly lower amount of sulfuric acid during combustion which in turn leads to excess amount of the additives in the oil and results these excess additives to form deposit. The excessive presence of neutralizing elements disrupts the graphite structure in the liner microstructure and causing the phenomenon of polishing to occur. This causes the liner surface unconvenient to maintaining an oil film leading to issues like scuffing and micro-seizures in piston rings. As a consequence the liner surfaces get damaged. [2],[4]

Various strategies were explored to address these emerging problems. Among these significant changes the use of low BN (15-40 BN) lube oils, ceramic-coated piston rings or piston rings with ceramic-metal (cermet) coatings can be highlighted.
 
 

Cermet Coated Piston Rings  [5]


Main Issues arising from low sulfur content

In addition to lubrication-related issues low-sulfur fuels brought about various other problems. Low-sulfur fuels are not traditional distillate fuels but rather blends composed of additives. This situation has made obtaining standard products during bunkering more challenging. Bunkering issues have led to increased problems in machinery when the fuels' compatibility is not suitable causing compatibility-related problems. [2]

Particularly the viscosity difference between fuels obtained from different suppliers was a significant issue [1]. If viscosity drops excessively it can disrupt the hydrodynamic oil film in the fuel pump leading to scuffing of plunger and barrels. Additionally due to lower pressure during injection in fuel injectors, it can disturb fuel atomization and result in decreased efficiency of fuel index margins. A fuel index value 5-10 points higher than the specified index stated sea trial records can indicate the need for replacement of the pump element. Especially worn fuel pumps with low viscosity fuels may encounter problems in achieving the required injection pressure during initial start-up [2,6].


Increased variation of fuel batch to batch [6]

Another issue is the presence of Al+Si known as Cat fines in the fuel. These very hard and fine particles formed during the refining process can accelerate the wear rate of equipment such as liners and piston rings. It is recommended to reduce the amount of Cat fines to below 10 ppm at the engine inlet through fuel separation. Fuel cleaning systems (separators) on most ships are designed for traditional fuels which might lead to compatibility issues when using new fuels. To ensure safe operation it's important to consult separator user manuals and analyze fuel specifications. While distillate fuels are usually separated at temperatures around 40-50°C HFO is separated at temperatures above 98°C. For low-sulfur fuels separation at 98°C and low flow rates is generally recommended for fuel cleaning purposes.

A micrograph showing catalyst fines (round white particles). It is also possible to see vertical abrasive wear traces.[4]

A micrograph showing catalyst fines (round white particles) embedded in the graphite structure.It is also possible to see closed/partly-closed graphite structure encircled in red ovals and abrasive wear traces [4]

Another important aspect to consider is fuel temperature. In blended fuels it's essential to maintain the fuel temperature above a certain threshold. Cloud point and pour point are important parameters in this regard. The cloud point is the temperature at which the first visible wax crystal appears when the fuel is cooled. The pour point temperature is the lowest temperature at which fuel can flow. If the fuel temperature falls below the cloud point wax-like residues can cause filters or other equipment to become clogged. If it falls below the pour point it can lead to a complete stop in the fuel flow.

Viscosity[7]

Care should be taken regarding the compatibility of different fuels. Fuels do not mix well with each other. The potential mixing of newly acquired fuel with the fuel already present in the tanks should be examined beforehand. Otherwise, the mixing of fuels can lead to sludge formation.

Conclusion

The use of low-sulfur fuel has revealed the need for certain operations outside of traditional maritime practices. This process, starting from the moment the fuel enters the vessel, requires the fuel to be known by the responsible engineers on board. It is essential for the crew to understand the mechanisms associated with low-sulfur fuel in order to enhance operations. This necessitates appropriate training and company instructions. The proper selection and feed rate of the lubricating oil for cylinder lubrication is crucial for extending the engine's lifespan and ensuring safe operation. When using low-sulfur fuel, oils with low BN values should be used, and when using high-sulfur fuel, high-BN oils should be employed. The condition of fuel pumps should be checked and worn-out ones should be replaced. The fuel system along with its heat transfer components may need to be updated to be compatible with new fuels. Fuel storage should be conducted carefully taking compatibility issues into consideration. Prior to fuel entering the engine it's essential that especially cat fines are properly cleaned. During fuel changeovers manufacturer instructions should be followed and if possible change-over should occur at suitable locations before maneuvers.

 Thank you for your valuable time. 

Sources 

[1] - https://www.rina.org/en/marpol-introduction

[2] – MAN Service Letter SL2014-593/DOJA

[3]- https://www.machinerylubrication.com/Read/31107/oil-lubricant-additives

[4]- MAN Diesel MATERIAL TECHNOLOGY & RESEARCH

[5]- MAN Service Letter SL2018-659/JAPMAN Service letter SL2019-670/DOJA

[6] – MAN Service letter SL2019-670/DOJA

[7]- https://www.withamgroup.co.uk/blog/fuel-efficient-heavy-duty-diesel-engine-oils/





















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