Updated 11 Oct 2019
Unless a ship is purely battery-powered and recharged from shore, a storage and delivery system for fuel is essential. As things stand, all of the fuels currently used on ships also require some form of additional treatment before use.
Regardless of how clean an oil fuel may have been when it came on board, a few hours in the bunker tanks would most likely result in it becoming contaminated with water, dirt and corrosion scale from the tanks themselves. The heaviest fuels of all – often described as the rubbish from the refining process – bring with them other contaminants such as ash, heavy metals, sludge and cat fines. The latter are arguably the most damaging contaminants of all and have the potential to completely destroy an engine if they are not removed.
To remove these contaminants an onboard treatment plant is required that has the capacity to keep pace with the fuel demands of the engine. This will involve multiple tanks, heaters, pumps, filters and separators. Since NOx emission limits were made mandatory, further treatments such as emulsifiers and micronisers may well be incorporated into the final stages. To control and monitor the process, flow meters and temperature sensors are needed at various stages.
There is currently no fuel treatment process that can reduce SOx emissions so it is the exhaust stream that is treated. That may change in the future if a means can be found to scale down some of the sulphur removal systems that are used in refining. More information on exhaust gas cleaning systems can be found in the ShipInsight Guide to Environmental Technology.
Oil fuel treatment
A normal heavy or intermediate fuel oil treatment process begins with the oil loaded into the bunker tank. Under normal circumstances the fuel will not be used immediately and should be analysed to confirm its specifications match the order and that it is appropriate to burn in the engine.
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 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, high- and low-level alarms and a 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, the 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.
A vessel operating on HFO normally has two service tanks: one for high-sulphur HFO for use outside ECAs and one for low-sulphur HFO for use inside ECAs. When a service tank is full, there is generally a return line allowing overflow back to the settling tank.
Even before the 2020 date for reducing sulphur in fuels, there have been problems with miscibility of different fuels taken on board that ostensibly should be perfectly compatible. In future, when many more varieties of low sulphur fuel will be available, it may be necessary to segregate different fuels to avoid such problems. If compatibility proves to be a big problem after 2020, it is not inconceivable that owners will be obliged to partition tanks to allow each bunkering to be kept apart from other fuels.
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 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 80ppm levels in ISO 8217:2005. The level of 60ppm Al+Si is maintained in the latest published ISO 8217:2017 Fuel Standard.
However, engine manufacturers generally recommend a maximum of 15ppm level of cat fines in the fuel entering their 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 15ppm.
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. In the small four-stroke gensets, wear from cat fines is found primarily in the fuel system, where the fuel atomiser holes are worn out and have become 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.
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.
Additional system components
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 into smaller particles that combust more easily and reducing deposits.
In very heavy fuels, molecules of carbon often form long chains that will never combust entirely in the engine. The homogeniser overcomes this by grinding the long chains and cutting them into smaller particles. Such a device will greatly reduce the amount of sludge and improve the combustion characteristics of the fuel.
Some of these devices also have a modification that allows the introduction of controlled amounts of water to form an emulsion. Emulsified fuels are known to produce less NOx and are predicted to become highly beneficial in helping ship operators meet the most stringent NOx emission regulations.
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.
An area that may well see more additive product activity is in ensuring that stability of blended fuels is improved and sludge reduced.
Ensuring safe fuel storage and use
The design of the fuel system and choice of components is mostly in the control of the shipowner but there are safety elements to be considered and some aspects are regulated under SOLAS where requirements for fuel systems include:
- As far as practicable, parts of the oil fuel system containing heated oil under pressure exceeding 0.18N/mm² shall not be placed in a concealed position such that defects and leakage cannot readily be observed. The machinery spaces in way of such parts of the oil fuel system shall be adequately illuminated.
- Fuel oil, lubrication oil and other flammable oils shall not be carried in forepeak tanks.
- Safe and efficient means of ascertaining the amount of oil fuel contained in any oil fuel tank shall be provided.
- Oil fuel pipes and their valves and fittings shall be of steel or other approved material, except that restricted use of flexible pipes shall be permissible in positions where the Administration is satisfied that they are necessary. Such flexible pipes and end attachments shall be of approved fire-resisting materials of adequate strength and shall be constructed to the satisfaction of the Administration.
- External high-pressure fuel delivery lines between the high-pressure fuel pumps and fuel injectors shall be protected with a jacketed piping system capable of containing fuel from a high-pressure line failure. A jacketed pipe incorporates an outer pipe into which the high-pressure fuel pipe is placed, forming a permanent assembly. The jacketed piping system shall include a means for collection of leakages and arrangements shall be provided with an alarm in case of a fuel line failure.
- Oil fuel lines shall not be located immediately above or near units of high temperature, including boilers, steam pipelines, exhaust manifolds, silencers or other equipment required to be insulated.
- As far as practicable, oil fuel lines shall be arranged far apart from hot surfaces, electrical installations or other sources of ignition and shall be screened or otherwise suitably protected to avoid oil spray or oil leakage onto the sources of ignition. The number of joints in such piping systems shall be kept to a minimum.
- Components of a diesel engine fuel system shall be designed considering the maximum peak pressure which will be experienced in service, including any high-pressure pulses which are generated and transmitted back into the fuel supply and spill lines by the action of fuel injection pumps. Connections within the fuel supply and spill lines shall be constructed having regard to their ability to prevent pressurised oil fuel leaks while in service and after maintenance.
- In multi-engine installations which are supplied from the same fuel source,
- means of isolating the fuel supply and spill piping to individual engines, shall be provided. The means of isolation shall not affect the operation of the other engines and shall be operable from a position not rendered inaccessible by a fire on any of the engines.
- Surfaces with temperatures above 220°C which may be impinged as a result of a fuel system failure shall be properly insulated.
- Precautions shall be taken to prevent any oil that may escape under pressure from any pump, filter or heater from coming into contact with heated surfaces.
Tank heating & viscosity
Different oil fuel types need differing treatment mostly to control their viscosity.
Distillates are much lighter fuels and will usually flow quite well except under conditions of very low temperatures when they may wax. Heavy fuel oils on the other hand have a very high viscosity and normally require heating.
Fuel oil tanks on the most modern ships may need to be in protected locations but on some older vessels there may be just a few millimetres of steel between the fuel and the sea. Even when protected, the location of storage tanks means temperatures will only ever match the ambient outside temperature.
It would be impossible to pump heavy fuel oil at such temperatures because it becomes highly viscous, so heating is essential to bring the temperature above the pour point. For most heavy fuel oils this will be around 40ºC. As the fuel progresses through the treatment system, the temperature will be raised to aid purifying and injection. The usual means of heating are steam coils in large vessels and electric heating in smaller ships.
The steam coils will draw their heat either from boilers or via heat exchangers in the engine exhaust stream. Heat exchangers are also used further along the treatment system. In the settling tank, a temperature of 60ºC is required, thinning the oil sufficiently for heavy contaminants to gravitate to the bottom. The fuel will be heated more before entering the separator, but care needs to be taken to keep the temperature below the boiling point of water otherwise separation will not be carried out efficiently.
After passing through the separator, the fuel will be maintained at around 80ºC - 85ºC in the service tank. The temperatures required for treating heavy residual fuels exceed the flashpoint of the distillates and therefore a different treatment system is required.
There is still a need to remove impurities particularly scale and water so filtration and separation is needed. However, to achieve the viscosity requirements of some engines it may be necessary to cool rather than heat the fuel in the final stages of its journey to the combustion chamber.
There are moves to ban heavy fuel oil for ships operating in high latitudes and instead require them only to use lighter fuels. Because of the waxing problem, distillate fuels in those ships will also need heating in tanks if problems are to be avoided.