Understanding fuel treatment systems

Malcolm Latarche
Malcolm Latarche

08 April 2017

All types of fuel, regardless of how they are delivered to bunker tanks arrive in a very different condition from the state in which it is finally injected into the combustion chamber of the engine. Were it not, the likelihood of the engine surviving unscathed after even a short period of operation would be slim.

However 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 shale from the tanks themselves. The heaviest fuels of all – often described as the rubbish from the refining process – bring with them other contaminants.

These can take the form of 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. The term cat-fines is a contraction of catalytic fines and refers to small particles of the catalyst used in refining crude oil.

The catalysts are usually aluminum and silicon compounds which are very hard and abrasive. They can destroy seals and injectors causing leaks and excessive fuel consumption and also excessive wear in piston rings and cylinder liners. According to MAN Diesel, cat-fines are implicated in around 80% of engine failures. 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, 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 its is the exhaust stream that is treated. More information on exhaust gas cleaning systems can be found in the ShipInsight Guide to Environmental Technology.

A normal fuel treatment process begins with the oil loaded into the bunker tank heated and pumped into a settling tank. Here sludge migrates to the bottom and air that has been trapped is allowed to escape. Once this initial treatment is complete, the fuel will pass through a filter to remove large-size contaminants and be heated again before being routed to the first stage (and in some cases the only) separator where more water, sludge and the inevitable cat-fines will be removed.

Leading separator manufacturers, such as Alfa Laval and GEA Westfalia, have developed a separation standard with the aid of class societies and engine-makers. Many ships are fitted with an under-size separator which, although nominally able to cope with the flow rate demanded by the engine, will struggle to keep pace – meaning that separation may have only been partial. 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. 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.

From the service tank the fuel is pumped to the engine via a series of filters. 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, such as the Lemag Slashpol, or some similar device. 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.

Another potentially very useful device was first presented at SMM in 2012 by Chris Marine. The Fuel Analyzer monitors the cat fine level continuously in the fuel treatment system at different sample points. If a pre-set alarm level is reached at the engine inlet, it is possible to switch to cleaner fuel from the second day tank as an emergency action.

The device also permits an analysis of the cause of the rise in cat-fines and implements counter-measures preventing similar events. With the typical configuration, it is possible to trace a problem to the settling tank, the purifier or the service tank by comparing the measurements from the different sampling locations. A similar device called the Catguard was launched at the following SMM in 2014 by NanoNord.

Cat-fines have a higher density than fuel oil and therefore tend to settle at the bottom of the tanks. In rough seas, accumulated cat fines could stir up and suddenly generate a high concentration at the tank’s outlet. This may be circumvented by cleaning the tank as soon as the measurement device picks up an elevated cat fine level at the tank’s outlet (purifier inlet or engine inlet). The device monitors up to seven automatic sampling points and one off-line point at 1-10bar and 25-140°C.

The off-line point may be used for analysing manual samples taken during bunkering or samples from settling and storage tanks during trouble-shooting. 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.

Recovering waste

Waste fuel oil from the settling and service tank drainages, leakages, filters and purifiers is usually collected in the waste oil tank along with waste lubricating oil and subsequently landed or incinerated. Disposal can be expensive and so adds to the original cost of the fuel. Alfa Laval has developed a device called PureDry which is in effect an advanced separator that can recover a high percentage of usable fuel oil from the waste oil tank.

Because the waste lubricating oil is undesirable in fuel oil, there is a need to have separate waste tanks for fuel and lubricating oils. Although the waste fuel oil tank appears to contain just black oil, it is actually oil-polluted water containing 20 – 30 % energy in the form of recoverable fuel oil. The remainder is oil polluted water 70 - 80% and, accumulating at the bottom, suspended solids which make up approximately 1%.

The PureDry separator recovers the fuel oil from the oily water in the waste fuel oil tank and it is returned to the fuel oil bunker tank for re-use after normal treatment. For the ship owner, the result is a reduction of up to 2% in the total volume of fuel oil consumed and a corresponding reduction in the ship’s fuel bill.

The process reduces the volume of waste oil by 99%, producing typically 5-15 kg per day of non-pumpable “super-dry” solids that can be landed as dry waste and disposed of in the same way as oily rags and used filter cartridges. There are no oil losses and no additional wastes are generated. The separated water, now with an oil content of less than 1,000 ppm, is pumped to the bilge water system. The PureDry can also be used to treat waste lubricating oils but there will only be a reduction in the waste product to be disposed of and no usable product.

Tank Heating

All diesel oil fuels need heating to some degree but heavy fuel oils are the most demanding in this respect. 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 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.

LNG systems

Fuel systems on gas only or dual-fuel vessels that are not LNG carriers are something of a novelty and outside the experience of most operators. This is likely to be the case for many years to come as most industry observers now believe that it will be 2020 or even later before the number of LNG powered vessels other than LNG carriers reaches 1,000. A thousand vessels may sound a large number but it is less than 2% of all ships. Currently less than 200 vessels with gas fuel systems are in operation.

The IMO in adopting the IGF Code has requested the ISO to develop a standard for quick disconnect bunkering connections and work on this is progressing. A standard LNG bunkering checklist was not included as part of the request but the MSC has since been requested for this to be included as part of the task. The biggest problem of LNG is that the fuel must be stored at extremely low temperatures and under pressure.

In addition, it has a lower flash point than oil fuels and in the event of a leak in the storage or fuel delivery system any escape of fuel would be far more difficult to contain and to recover than is the case with oil. However, the system between tank and engine is less complex and there are virtually no waste products to be stored and disposed of ashore.

Until the adoption of the IMO’s IGF Code, approval of LNG fuel storage and delivery systems was done by flag states on a more or less case by case basis. This has been a brake on the take up of LNG as a fuel because owners could have no guarantee that in the event of a change of flag following sale of a vessel, the new flag state would approve the ship’s systems. There are a small number of systems so far developed for LNG although the number
must be expected to increase.

Several shipbuilders either have or are developing system and while they will have much in common there will be variations. Few engineers apart from those who have been employed on gas carriers or the mall number of other ship types with dual-fuel or pure gas engines will have much knowledge of the fuel systems for gas fuelled ships.

The following is a description of the Wärtsilä LNGPac system that has been installed on some vessels. Other makers’ systems will be essentially similar. The first component is the bunkering station that provides the connection to the LNG bunkering barge. Each station includes one bunkering line (LNG line), one return line, and one nitrogen purging line with respective control/thermal relief valves (pressure safety valves) and flanges.

The return line is used in case the bunkering operation takes place with two hoses connected, and the evaporated gas is returned to the bunkering terminal or barge. During bunkering operations, LNG could evaporate due to heat leakages in the piping and/or due to the higher temperature in the storage tank onboard compared to the refilling tank.

From the bunkering station, LNG is led to the storage tank via insulated pipes. Vacuum insulation is selected for its excellent insulation properties, and to minimise LNG evaporation during bunkering. The pressurised storage tank is cylindrically shaped with dished ends and is designed in accordance with the IMO IGC Code and EN 13458-2 – Cryogenic vessels. Static vacuum insulated vessels. LNGPac tanks are insulated with perlite/vacuum. The tank consists of a stainless steel inner vessel, which is designed for an internal pressure, and an outer vessel that acts as a secondary barrier. The outer vessel can be made of either stainless steel or carbon steel.

According to the current IMO Guidelines, the LNG fuel tanks have to be selected from among the “Independent Types A, B, or C”. The LNGPac is designed according to Type C requirements. The pressure vessel allows easy handling of the evaporated gas (boil-off), since the tank is designed to withstand a significant pressure increase and the pressure relief valves are set at 9 bar(g). In practice, dual-fuel vessels can operate for a long time in liquid fuel mode (HFO or MDO) before having to take care of the pressure increase in the tank. The handling of the boil-off is done very simply by a temporary switch over of the engines to gas mode, and the gas is taken from the vapour phase in the upper part of the tank.

As an indication, a 200 m3 pressurized type C tank, filled at 50% could hold LNG for about 25 days, even without any gas consumption from the tank. A Wärtsilä dual-fuel engine requires approximately 4 – 5 bar(g) at the inlet of the gas valve unit. In case LNG is stored at atmospheric pressure (Type A and Type B tanks), the fuel system should include either compressors or cryogenic pumps to deliver the fuel at the correct pressure.

The process skid includes all the connections and valves between the tank and the Pressure Build-Up Unit (PBU) and the Product Evaporator, together with the evaporators themselves. The PBU consists of an insulated pipe, an evaporator, valves, a single wall pipe and sensors. The mission of the PBU is to build up the pressure in the tank after bunkering LNG and to maintain the required pressure in the tank (around 5 bar(g)), during normal operation.

Maintaining the correct pressure in the tank ensures that the Wärtsilä dual-fuel engines are able to meet the maximum power (100% MCR) at any time. Since the LNGPac system has no cryogenic pump or compressor, the engine gas inlet pressure requirements are met by achieving the correct storage pressure inside the LNG tank.
The circulation of LNG to the PBU evaporator is achieved by the hydrostatic pressure difference between the top and bottom of the tank, with LNG from the bottom of the tank being fed to the evaporator. The evaporated gas is then returned to the top of the tank. The natural circulation through the PBU continues until the required pressure in the tank is achieved.

The Product Evaporator circuit consists of an insulated pipe, an evaporator, valves, a single wall pipe, and sensors. The task of the product evaporator is to evaporate the LNG into gas and heat it to a minimum of 0°C as per engine specifications. The gas is then fed to the gas valve unit before the engines. Both the PBU and Product Evaporator are heated by a water/glycol mixture, which is re-circulated to an external cooler. Here, the mixture is heated by the waste heat from the low temperature engine cooling water circuit.

The process skid has a modular design, making it easy to be selected and assembled for the entire LNGPac product range. The key parameters influencing the modularisation of the process skid are the sizes and volume of the tank, and the engines (model and number) connected to the tank.

The tank room is a stainless steel barrier welded to the outer vessel of the tank. The structure contains the process skid and all the pipe penetrations to the tank. In the unlikely event of an LNG leakage, the tank room acts as a barrier that avoids damage to the external compartments, and facilitates the quick ventilation of the evaporated gas. The tank room and ventilation system are to be fire protected to A-60/A-0 insulation class, depending on the safety designation of the adjacent space.

The LNGPac control system is based on Wärtsilä’s vessel automation system platform. When combined with the Wärtsilä Integrated Automation System (IAS), the same hardware and Human Machine Interface can be used throughout the vessel to operate the LNGPac, the dual-fuel (DF) engines, and the propulsion system. In addition, separately delivered features typical of DF-engine applications, have been incorporated into the IAS. These include the Wärtsilä Operator’s Interface System, Condition Based Maintenance, and monitoring of IMO Tier III compliance in Emission Control Areas.

The core of the control system is a PLC cabinet placed in a safe area near the tank room. All LNGPac transmitters and intrinsically safe sensors, as well as all interfaces to external systems such as fire & alarm, gas detection, etc., originate from the cabinet.

The pneumatic valves are controlled by solenoids placed in a safe area adjacent to the tank room and bunkering station(s). In the case of ships retrofitted with LNGPac, the control system is able to operate as a separate system with monitors on the bridge and Engine Control Room, or by being integrated into the existing system