Fuel preparation — from settling tank to final conditioning

Updated 5 Sep 2019

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The treatment of fuel effectively breaks down into two separate and distinct parts; between the settling and the service tank and between the service tank through to injection. The settling tank itself is part of the first preparation process.

Assuming the fuel has passed any analysis tests, it will be pumped from the bunker tanks to the settling tank. In the case of HFO it will almost certainly contain some level of cat fines and all fuels will have some amount of water and sludge. In the settling tank, water and the heavier contaminants will migrate to the bottom but some will remain suspended in the fuel.

Settling tanks will have a sloping bottom with a drain valve at the lowest point by which sludge and water can be drained at regular intervals. The feed to the next stage in the process is above the drain cock. The other components include a heating coil so as to be able to control viscosity and high- and low-level alarms and sight glass or gauges.

Because a ship operates in a dynamic environment the settling tank is not entirely successful in keeping the fuel separate from heavy contaminants and water. In anything but a dead calm, the movements of the vessel will stir up the contents of the settling tank to a greater or lesser degree. The longer the fuel can remain in the settling tank, he more gravitational settlement there will be and the better the fuel will be prepared for the next stage.

From the settling tank the fuel is sent to the service tank. On its way it passes first through a filter to remove the larger contaminants and then to a preheater. After heating the fuel is sent to the first separator. In some ships there may be only a single separator but as the importance of proper fuel treatment for modern engines has become recognised, so more sophisticated treatment systems have been installed. A second route from the settling tank to the service tank not only allows for improved treatment but also provides redundancy and a chance to carry out necessary maintenance when necessary.

The separator is a centrifuge device for removing solids and water from the fuel. When operated, the spinning action causes denser substances and particles to move outward in the radial direction. At the same time, the lighter oil is separated and moves to the centre where it is drawn off and pumped to the service tank.

To be effective at separating fuel from water and contaminants, separators need to be able to cope with the flow rates needed to meet engine demands. A separator with a greater capacity will be most effective while those at the upper limits of their operating range will struggle to cope. Manufacturers have agreed on a voluntary standard laid down in the Separation Performance Standard of CE standard CWA15375:2005. The standard includes a certified flow rate (CFR) which if equal to or higher than that needed by the engine should ensure satisfactory separation under most circumstances.

Coping with cat fines

In the past, only sludge and water needed to be removed from the fuel but over the last ten years or so the bigger problem has been the removal of cat fines. This has come about because of the newer methods of refining needed to extract more lighter products from crude oil. The process uses an aluminium silicate catalyst and while most of it is recovered for reuse, small particles do become detached and find their way into the heavier types of fuel.

The problem of cat fines was first raised in the early years of the 21st century but despite it receiving a great deal of publicity and having been recognised by insurers, engine makers and the International Organization for Standardization (ISO) it seems to be a growing, rather than a receding problem.

Once only two-stroke main engines used HFO but today many four-strokes and gensets are also able to run on it as well. In some ships there is a single fuel strategy with only HFO being used.

Cat fines are extremely hard and damaging to machinery which is why their removal is imperative. Maximum levels in fuels are set down in the various versions of ISO 8217 but these are higher than engine makers’ recommendations. ISO Standard 8217:2012 introduced a maximum permissible 60ppm level of cat fines, expressed as Aluminium + Silicon, for marine residual fuels, a reduction from the 80 ppm levels in ISO 8217:2005. The level of 60 ppm Al+Si is maintained in the latest published ISO 8217:2017 Fuel Standard.

However, engine manufacturers generally recommend a maximum of 15 ppm level of cat fines in the fuel entering the engines. As this level is significantly lower than the levels specified in the ISO Standards, it is essential to ensure adequate fuel handling and purification equipment and procedures are in place onboard to effectively bring the levels of cat fines in the fuel below 15 ppm.

Wear from cat fines in two-stroke engines is found in the combustion chamber on cylinder liner, piston rings and piston ring grooves, resulting in high wear rates and possible scuffing. While the wear from cat fines in the small four-stroke gensets are found primarily in the fuel system, where the fuel atomiser holes are worn out, and becoming too large for making suitable atomisation of the fuel. This causes poor combustion and increased soot and deposits in turbochargers creating a vicious circle which eventually can lead to major engine damage.

Applying the separation standard to the fuel treatment plant is not mandatory, but the advice of independent experts is that it will definitely reduce the risk of damage posed by the significant quantity of cat fines found in some of the heaviest fuels.

After separation the fuel then moves on to the service tank in readiness to be pumped to the engine. More heaters are needed at this stage to ensure the viscosity at the engine is in accordance with engine maker’s operating standards and booster pumps to ensure fuel pressure is maintained. While it is possible to construct a fuel treatment system from individually sourced components, the leading specialists in the field produce modular units that can be customised to specific requirements.

Apart from the core components such as pumps, heat exchangers and separators, many fuel treatment systems will incorporate additional components. Counted among the most useful of these is a homogeniser that breaks down long chain molecules in smaller particles that combust more easily reducing deposits.

Water in fuels is one of the biggest problems on board ships which is why so much effort is taken in removing water. However, some water can be beneficial and combustion is often better under humid conditions or when emulsified fuels are used. In addition, water in the fuel can limit the production of NOx and thereby reducing emissions. There are good scientific reasons for this but the quantity of water and how it is mixed with the fuel must be carefully controlled if it is used.

Mechanical treatment of fuel can also be complemented by using additives to achieve improved fuel consumption and reduced emissions. The history of chemical additives is a chequered one, as not all products have been found to produce the claimed results. Nevertheless, some products do achieve improvements and it is almost certain that as emission reduction demands increase More products will be put forward as solutions.

After the fuel is moved to the service tank, it will be finally treated before being pumped to the engine. In this part of the preparation journey, there will be a booster system to meet the pressure, temperature, viscosity, and flowrate specified by diesel engine manufacturers. These parameters are vital for the engine’s combustion performance.

There will also be further filtration of the fuel at a much finer level than previously. The filtrate will be directed back to the settling tank or may be further treated using a device to separate usable fuel from any sludge. Further treatment of any sludge that can recover usable fuel adds to the overall efficiency of the vessel.

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