MARINE FUELS
COMPATIBILITY TESTING BY AUTOMATED
INSTRUMENTATION
ACCORDING TO ASTM D4740
Since 2016, when the International Maritime Organisation (IMO) confi rmed a global cap to 0.5% sulfur level in fuel oil used on board of ships, the marine fuel market has undergone complex and far-reaching changes. Issues surrounding fuel stability and compatibility have never been more important.
Stability relates primarily to the fuel’s capacity to retain asphaltenes in solution preventing its precipitation and the formation of sludge. However, a bulk fuel, stable and homogeneous at the point of delivery, can become unstable when stored for long periods. Once a fuel has chemically broken down there is no way to satisfactorily reverse the process. Precipitated asphaltene cannot be re-dissolved.
Talking about the compatibility, it is important to remember that a stable fuel oil can become unstable when mixed with another stable fuel, although generally fuels of the same viscosity grade and similar densities will be compatible.
Depending on the manufacturing route and blending components availability, the blended fuels can be predominantly aromatic or paraffi nic in nature, or somewhere in between. While these fuels will be stable at their own, the variation in solvency when blended may lead to increased risk of incompatibility creating serious problems such as fi lters plugging and fuel supply systems affected by fl occulated asphaltenes.
In the case of bunkering, a complete segregation of fuels is not possible. There is no guarantee that a vessel bunkering a 0.50%S fuel at Rotterdam will then be able to pick up a compatible fuel in Fujairah.
Carrying out of compatibility tests between the existing and proposed bunker fuel delivery is the only way to provide a realistic evaluation of the potential issues that might result.
A simple and effi cient test that can be carried out on board a vessel is the ASTM D4740 spot test.
This quick test helps the ship’s crew during subsequent operations such as fuel switching, or when there is a need to bulk fuels for new bunker preparation and/or tank transfers for ship stability purposes.
A drop of the heated and homogenized sample is put on a test paper and heated to 100°C. After 1 hour, the test paper is removed from the oven and the resultant spot is examined for evidence of precipitation and rated for compatibility against ASTM D4740 Adjunct Reference Spots (see Figure 1).
The test contains two procedures: cleanliness and compatibility.
• The cleanliness test is applied to the sample as received to assess the degree to which asphaltenes are already precipitated.
• For the compatibility assessment, a cleanliness test
is performed with a blend composed of new bunker and the bunkers remaining on board. It is recommended to test blends at different mixture ratios because some fuel blends may be stable at a certain ratio but unstable at another. Typically, tests are performed at the expected ratios at which fuels will be used or transferred, 10/90; 90/10 and the 50/50 as the worst-case scenario.
The effi cacity and accuracy of this simple and affordable test depend heavily on the profi ciency and experience of the ship’s crew in the interpretation of the spots.
To overcome these limitations, AD Systems, France has developed a portable, fully
automated instrument for cleanliness and compatibility testing of
Figure 1. D4740 Adjunct Reference Spots (source ASTM [1])
heavy fuel oils on-site or on-board. The Model ST10 completely automates both procedures. All accessories and devices required to perform a test are grouped in a robust case. It consists of an oven for sample conditioning, and preparation/drying of the spot, as well as a digital camera and associated software used for automatic spot rating. The only phases carried out by the operator are the sampling and the deposition of a drop of fuel on to a fi lter paper using a micropipette provided with the equipment. The visual, and therefore subjective, interpretation of the fuel spot has been replaced by a powerful image analysis algorithm.
Figure 2. Schematics of the automated apparatus (Source : ASTM [1] )
Since 2019 the automated procedure has been part of the ASTM D4740 test method.
10ml of fuel (cleanliness) or a mix of fuels (compatibility) is poured into a disposable vial and then placed into the built-in oven for 15 minutes. The fi lter paper is positioned on the conveyor. The ST10 automatically places the fi lter paper in the oven for drying. In 15 minutes, when the sample is hot, the operator is prompted and the fi lter paper is pre-positioned. With the micropipette, the operator pours one drop of fuel on the fi lter paper. The filter paper is automatically moved back to the 100°C oven. After 60 min drying time, the spot is automatically moved under the camera to be photographed.
The picture is binarised, analysed, and the ST10 reports a 1 to 5 rating depending on the cleanliness / compatibility of the fuel. The real image of the spot is displayed, stamped with date/time, and memorised in built-in database with its rating. The ST10 ensures perfect traceability of the test. The standard model of the ST10 includes an Ethernet connection: with central data management, the result can be immediately transmitted to the Cloud via a satellite connection.
Figure 3. Example of ST10 report D4740 compatibility mode
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