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Stability and Compatibility of VLSFOs

Read time: 7 minutes

In 2020, the first year of the enforcement of the Global Sulphur Cap, Van Ameyde Marine dealt with over sixty quality disputes related to Very Low Sulphur Fuel Oil (VLSFO). Stability and compatibility issues represented over 50% of the cases investigated last year. What’s causing the problem and how can potential issues be identified?

Over the past year, we identified the following main causes of bunker quality issues with very low sulphur fuel oils:

  • stability and compatibility problems leading to sediment formation
  • sulphur content exceeding the 0.50% limit
  • fuels with a high acid number, indicating contamination
  • high cat fine content, which may cause abrasive wear to engine components
  • the presence of waxy components, affecting cold flow properties
  • poor ignition due to significant change in viscosity and density

The high frequency of fuel stability and compatibility issues has triggered us to study this  in greater detail.

Blending and mixing: maintaining the balance

The introduction of VLSFOs has seen a significant change in blending practices. Viscosity and density control were the main targets for blending traditional fuel oils. However, VLSFO has shifted the target to sulphur content, which is why refineries and bunker suppliers have introduced new types of blended fuels.

The composition of these blends varies widely. Most VLSFO available is blended from residual and distillate components. To understand the cause of sediment formation, we need to understand the composition of residual fuel oil. The so-called actual oil is composed of:

  • saturated hydrocarbons – paraffins and naphthenes – which contain only single bonds between carbon atoms
  • aromatic hydrocarbons, which are organic components with one or more benzene rings

The other two main components of residual fuel oil are:

  • asphaltenes, which cover a wide range of high molecular weight hydrocarbon structures
  • resins, the structure of which is similar to asphaltenes, but which have a lower molecular weight

A stable fuel oil is a homogeneous blend, in which the paraffinic content is in balance with the aromatics, asphaltenes and resins. All components remain in suspension, reducing the risk of sediment or sludge formation.

Even stable blends can, however, cause problems when mixed with other fuels, if the fuels aren’t compatible. Mixing may disturb the balance of the four main components, which then no longer remain in suspension and form sludge, also called ‘asphaltene precipitation’ or ‘flocculation’. Yet mixed fuels may remain stable when mixed in the correct ratios. The fuel becomes unstable if the balance is broken.

The stability of a fuel oil is a characteristic of a single batch and is defined as follows:

The stability of a fuel oil is defined by its resistance to precipitate asphaltenic sludge despite being subjected to forces, such as thermal and ageing stresses, while handled and stored under normal operating conditions.

Suffice to say that it’s the supplier’s responsibility to deliver a stable product. However, mixing different batches is very much the responsibility of the on-board engineers, who need to secure best fuel management practice. It’s their duty to mitigate the risks associated with mixing potentially incompatible fuels, as per the CIMAC (the International Council on Combustion Engines) guidelines.

Testing stability and compatibility

Will blends or mixed fuel batches pose problems? Testing is the obvious way to find out. There are three categories of tests available:

  • sediment test methods (TSE, TSP and TSA as per ISO 10307-1 and ISO 10307-2)
  • spot tests (cleanliness/compatibility as per ASTM D4740)
  • advanced test methods (ASTM D7157, D7112, D7060)

Sediment test methods

These tests apply hot filtration using a double filter paper to determine the quantity of sediments in a fuel. After filtration of a known quantity of fuel oil, the filters are washed to remove all heptane soluble components, leaving the heptane insolubles, which are basically precipitated asphaltenes and inorganic dirt. This sediment is measured and reported as the percentage of sediment present in the fuel.

  • TSE (Total Sediment Existent) – hot filtration without ageing the sample – determines the amount of dirt/sediment that is likely to be separated by an on-board centrifuge.
  • TSP (Total Sediment Potential) determines the amount of sediment after thermal ageing. For this purpose, samples are heated to and stored at 100 °C for 24 hours: the ageing process is supposed to simulate on-board storage at 40 – 50 °C for a number of weeks.
  • TSA (Total Sediment Accelerated), which determines sediment after dilution with a paraffinic solvent, followed by storage at 100 °C for one hour, simulating both chemical and thermal ageing. This shows how adding a paraffinic component may affect the balance of the four main components.

In accordance with the latest ISO 8217 standard, the fuel is considered stable if TSP and TSA are below 0.10% m/m. It should be noted, though, that the method is rather inaccurate. Even on-spec, but elevated TSP results may indicate that the fuel could clog within weeks. However, in most of the cases identified, TSP exceeded 0.20%.

Spot test

The spot test serves two purposes:

  • determining the cleanliness of fuel oils containing asphaltenes, to verify whether the single fuel batch is stable
  • verifying whether two or more fuel oils are compatible when mixing in a specific proportion

A drop of fuel or a mix of fuels on test paper is heated to 100 °C. After one hour, the spot is examined for evidence of precipitation. The stability/compatibility is rated against the ASTM D4740 reference spots (five-level scale). Ratings one and two, the latter with a poorly defined inner ring, are considered stable. Levels three to five are unstable.

The spot test is particularly practical for on-board testing of fuel compatibility. However, it should be noted that fuels with more paraffinic content may result in false negatives. They may in fact be compatible. It requires experience to interpret the results correctly.

Advanced methods

Refiners and blenders in particular use more advanced test methods to assess the intrinsic stability of blended asphaltene-containing fuels. The tests are, however, expensive and most laboratories conducting routine testing don’t have the equipment.

Prevention

Prevention starts with early detection of off-spec bunkers. Once on board, best fuel management practice kicks in. Ranging from storage in separate tanks and storage temperatures to cleaning. If you’d like to learn more, we recommend you read ‘Bunker disputes and shipboard fuel treatment’.

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