Recently CIMAC along with industry stalwarts such as shippers, refiners, and suppliers have issued guidance regarding 2020 fuel compliance. Part of this guidance concerns the fuel compatibility issues and the suggested good practices that needs to be followed while blending fuels.
Here we are highlighting that with this recommendation by CIMAC
Understanding Compatibility Risks
To understand the increased risk of incompatibility arising from the introduction of max.
0.50%-sulphur fuels, an explanation of the principles of asphaltene precipitation leading to
sludge formation is provided below.
- A residual fuel can be looked upon as a colloidal dispersion of asphaltenes uniformly distributed throughout an oily medium, often referred to as a ‘continuous phase’.
- For a stable residual fuel oil, a state of equilibrium exists between the asphaltenes and the continuous phase, and the asphaltenes will remain in stable dispersion.
- However, changes to the chemical characteristics of the asphaltenes (for example by exposure to high temperatures in some refining processes) or to the continuous oil phase (for example by blending two different fuel oils together or blending a cutter stock into the residual fuel) can upset the equilibrium.
- When this happens, the asphaltenes will no longer be held in stable dispersion, but will instead begin to agglomerate (or flocculate).
- The larger agglomerated particles may start to drop out of the fuel, producing what
is referred to as sludge. - Depending on the level of precipitation, the presence of sludge in bunker fuel has the potential to cause significant operational impacts through accumulation in tanks and fuel lines, choking of separators and filters, and sticking of fuel injection pumps.
- Instances of mild levels of sludge can generally be dealt with successfully by the ship’s crew, albeit requiring increased cleaning and maintenance of separators and filters.
- High levels of sludge can cause severe fouling which, if not dealt with, can result in the potential interruption of the fuel supply to the engine (fuel starvation) and, ultimately, loss of power and propulsion.
- Additionally, combustion can be compromised through excessive fouling of the cylinders, causing piston rings to stick and differential thermal loadings on the cylinders, which can potentially result in engine failure.
- A residual fuel is defined as stable if asphaltenic material is not precipitated during normal storage and use. The ‘Total Sediment Potential’ (TSP) test method (ISO 10307-2 Procedure A, thermal ageing) simulates normal storage by measuring the sediment which includes asphaltenic sludge, after heating it to 100°C for 24 hours.
- If the TSP does not exceed the specification limit of 0.10% m/m, it is normally assumed to be stable. However, if significant precipitation still occurs, the fuel is said to be unstable.
Mixing Up Compatibility and Stability
The term ‘compatibility’ is often confused with stability, but although the chemical and physical processes at work are the same, these terms have different meanings. Compatibility cannot be described as a characteristic of a single fuel, rather it is an indication of the suitability of commingling one fuel with another.
If two fuels are commingled together and the resulting blend remains stable (i.e. does not precipitate asphaltenic sludge), the fuels would be termed compatible.
On the other hand, if the resulting blend is unstable, then the component fuels are
said to be incompatible, even though each component is individually stable. Incompatibility
generally arises as a consequence of lack of stability reserve and changes to the solvency of the continuous phase for the asphaltenes.
Good Practices While Blending Fuels
The industry good practice is, in the first instance, to avoid commingling fuels from different sources in bunker tanks, because arbitrary commingling can lead to incompatibility and loss of stability of the resultant blend. For example, when a residual fuel oil is commingled with a distillate with a predominance of paraffinic hydrocarbons, the solvency reserve can be depleted and asphaltenes can flocculate and precipitate as sludge.
In such a case, there would be an increased risk of problems arising during fuel switching, such as when entering or leaving an ECA, when fuels become commingled in settling and service tanks. Precautionary measures to minimize or preferably eliminate commingling in the tanks will help to reduce this risk.
It is anticipated that issues of incompatibility could become more prevalent with max. 0.50%-sulphur content fuels.
geographical location problem?
Fuels meeting the new limit may be blended from a wider range of blending
components than currently utilized today. Depending on the manufacturing route and blending component availability, the blended fuels may be predominantly aromatic or paraffinic in nature, or somewhere in between. While these fuels will be stable in their own right, the variation in paraffinicity/aromaticity may lead to an increased risk of incompatibility when commingled.
It is recognized that fuels bunkered at different geographical locations (even fuels obtained from the same supplier) may not be compatible. It is therefore important that commingling of bunker fuels from different batches is avoided.
It should also be noted that commingling may not result in an homogeneous product; ships should continue to segregate bunker stems on board and minimize their commingling throughout the fuel system, in line with standard operating procedures. This includes potential commingling in bunker fuel tanks and settling/service tanks.
Distillates & Residuals Blending
In addition to potential incompatibility between residual fuels, there is also a higher risk when commingling distillate and residual fuels, as these may also be incompatible.
In practice, complete segregation will not be possible and even in ‘empty’ tanks there will be a degree of commingling due to the presence of existing tank products, e.g. in the heel of the tank or in the transfer system pipework.
One Hour Standardized Testing
Carrying out a compatibility test between the existing and proposed bunker fuel delivery is the only way to provide a realistic indication as to the potential issues that might result.
The most widely used on-board test method for compatibility assessment is ASTM D4740, the Standard Test Method for Cleanliness and Compatibility of Residual Fuels by Spot Test.
This one–hour test can be performed by the ship’s personnel using an on-board test kit (as well as in a laboratory); however, its efficacy and accuracy depends heavily on the proficiency and experience of the ships’ crew in the interpretation of the spots, and it will only be of benefit to the ship if its personnel can become proficient in carrying out this test.
How To Perform the Test?
The test method was initially developed to assess the degree to which asphaltenes were already precipitated in the as-received sample (the so-called ‘cleanliness procedure’).
- For compatibility assessment, samples of the two fuels which may be commingled are mixed in the expected ratio in which they will be used.
- The blend is then tested according to the cleanliness procedure.
- Users of this test method should be alerted to the fact that the test may be less
predictive/accurate for 0.10% sulphur fuels and the max. 0.50% sulphur fuels made available leading up to 2020 and beyond. - This is anticipated to be due to the variable characteristics of the fuel blending components that will be used in their manufacture, particularly those with a high wax content.
- Personnel involved in handling bunker fuels, whether shoreside or shipside, should
ensure that any recommendations or new methodologies are adopted for use, to minimize the risks associated with both stability and compatibility.
Did you subscribe to our daily newsletter?
It’s Free! Click here to Subscribe!
Source: CIMAC
