When it comes to internal oil transfer or bunkering of fresh fuel, it is inadvertent that some mixing of two different fuels will happen. If the vessel’s endurance is not adequate, at times ship’s staffs are forced to co-mingle two different fuels in order to maximize bunker in-take.
Thus, this technical write-up would help to understand critical points to consider before co-mingling two different fuels.
- Stability is the propensity of the fuel to stay stable under long storage conditions. Typically fuel oils can be stored for at least a year without significant changes.
- ISO 8217 specifically states that “The fuel shall be a homogeneous blend of hydrocarbons derived from petroleum refining.” This means that if the fuel is not stable, it is not meeting the specification.
- A simple ASTM D4740 test gives a good idea both for compatibility between two fuels to be blended and also the stability of an already blended fuel. These tests can be carried out onboard. You can also get a compatibility test kit to keep onboard. This costs around €600. Ship staff can use this to assess the compatibility.
- Lack of stability occurs when there is not enough aromatics to support the asphaltenes in the fuel. This throws the asphaltenes out of solution and the precipitation of this substance leads to filter and purifier choking with possible risk of machinery coming to a halt.
- From the supplier’s side, the co-mingling or blending of bunker fuels is a must. The problem occurs if he uses a low quality and low cost cutter stock. Problems also occur when the supplier tries to do line blending when delivering the fuel to the ship. This can lead to wrong sampling, stratification and other problems.
- From the ship’s point of view, it is ideal if the storage tank is empty to receive fresh bunkers. Often the necessity to blend with an existing fuel in the tank leads to problems particularly if the two fuels are not compatible.
- Blending onboard is not advisable. If it is unavoidable, first check for compatibility. Secondly, the blending has to be carried out in a certain order such as a lower density fuel should be pumped on top of a higher density fuel, circulate the fuel as much as possible, heat the fuel to achieve mingling through thermal convection currents. Keep in mind that two perfectly stable fuels can become unstable when blended.
- The stability of the fuel can be categorized into blending stability (which is already covered above) and secondly, storage stability. When the fuel is heated for a long time, degradation of stability may take place with thickening and asphaltene precipitation.
- Please keep in mind that waste lube oil, contaminants in the fuel, excessive quantities of low quality cutter stock such as shale oil etc. will contribute to instability in the fuel.
- Two other tests that can be employed to assess the potential of the fuel to become unstable are Xylene Equivalent and Reserve Stability Number. It is possible to identify fuels which are fully stable or which are on the verge of becoming unstable.
- Adding good fuel to an unstable fuel which has already precipitated asphaltene may not help. It is possible that some additives may help.
- When switching from HFO to distillate fuel while approaching port or similar conditions, there is a risk of incompatibility between the two fuels. The asphaltenes could precipitate leading to filter clogging.
- In addition to compatibility test which can be carried out onboard, a mini blending test can be carried out with half to one liter of fuel samples. In an emergency, the best way is to take the settling tank level to the bottom and then put the second fuel into the settling tank. A certain amount of incompatibility may occur but it will be restricted to the small quantity in the settling tank.
- Please keep in mind that all stability/compatibility tests are carried out in a 50:50 ratio of the two fuels. This is the worst condition. If they are compatible at this ratio, they will be compatible at all ratios such as 80:20, 65:35 etc.
If you have any further queries or questions regarding Compatibility/stability or co-mingling two different fuels, please comment below.