3. PITFALLS IN FUEL OIL TREATMENT
The levels of contaminants given in fuels standards such as ISO and CIMAC are selected in such a way that a well functioning treatment system should be able to reduce them down to what is regarded as acceptable levels before the engine injection. There are however a number of potential pitfalls that can cause problems. These can be due to the fuel to be treated, the treatment system itself or the way in which the system is operated. The most important aspects of fuel oil treatment and how to avoid the potential pitfalls are given below.
3.1 Use a Correctly Sized System
A typical treatment system onboard a ship consists of a settling tank, the separator system, day tank and filter(s) in connection with the booster system. This system should be sized to be able to handle the worst case or at least the maximum allowed contamination levels according to the agreed bunker standard. The effects of a high level of cat fines in the treated fuel will soon be seen on the cylinder liners and piston rings.
3.2 Use the Settling Tank
The settling tank is a simple, well-proven part of the treatment train. Provided that it is kept well drained and heated correctly then it will give a good initial reduction of the contaminants. Failure to use the settling tank properly will result in an increased load on the separation system.
3.3 Ensure Stable Operating Conditions.
This statement is especially valid for conventional types of separators with gravity discs. All variations in flow, temperature etc. will affect the interface between oil and water in the separator. If the variations are too large then the water seal will break and this has to be counteracted by installing a smaller gravity disc. As the variations continue the new gravity disc will then be too small for a large part of the operation, giving an incorrect interface level which will result in a decrease in separation efficiency.
3.4 Avoid All Kinds of Emulsification/Mechanical Shear Forces.
All kinds of mechanical energy added to the oil will break up the water droplets and particle agglomerates into smaller units. The smaller the water droplets or particles the harder it will be to separate them from the oil. In the worst case, emulsions, that are not possible to separate, will be formed. This will not only lead to higher water contents but also to higher sodium levels since sodium is soluble in the water in the emulsion. Furthermore, removal of cat fines will be impaired as cat fines are hydrophilic and prefer to stay in the water phase forming a water/cat fine agglomerate. These have a critical diameter/weight ratio that makes it impossible to remove them by centrifugal force [2].
In practice this means that only positive displacement pumps should be used, and all kinds of throttling in the pipes and valves must be avoided. Throttled valves should not be used to control the pump flow and the return of oil to the suction side of the pump must also be avoided.
The most extreme case of malpractice in fuel treatment is the use of homogenisers installed upstream of the separator or on a by pass of the settling tank. Figures 2 and 3 are from a paper published in Motor Ship [3] and show the negative effect of homogenisers on the removal of sodium and cat fines. Figure 3 also shows the positive effect of an undisturbed settling tank.
3.5 Ensure Correct and Constant Separation Temperature.
When separating HFO the oil temperature should be 98℃ and this should be kept constant. This is because separation efficiency is directly linked to temperature as it affects the viscosity of the oil.
Figure 1. A catalyst fine particle inside an 8 micron water droplet.
