Manual Alfa Laval

DNVPS / Alfa LavalFuel Oil Treatment Printed Book No. Mar 2010 EPS002-E-1 V 3 Alfa Laval reserves the right to make changes at any time without prior notice. Any comments regarding possible errors and omissions or suggestions for improvement of this publication would be gratefully appreciated. Copies of this publication can be ordered from your local Alfa Laval company. Published by: Alfa Laval Tumba AB Competance Development SE - 147 80 Tumba Sweden © Copyright Alfa Laval Tumba AB 2010. Original instructions Contents 1 Installation....................................................................... .5 1.1 Settling Tanks............................................................ 5 1.1.1 Settling tank design......................................................... 6 1.1.2 Recommendations .......................................................... 7 1.2 Pumping system. ....................................................... 8 1.2.1 Built-on feed pumps. ....................................................... 9 1.2.2 Separate feed pumps...................................................... 9 1.2.3 Lubricity and viscosity................................................... 10 1.2.4 Strainer .......................................................................... 11 1.2.5 Recommendations ........................................................ 11 1.3 Flow control .............................................................. 12 1.3.1 The flow control system................................................. 13 1.4 Heating System....................................................... 15 1.4.1 Electric heaters.............................................................. 17 1.4.2 HEATPAC system .......................................................... 18 1.4.3 Steam installation .......................................................... 19 1.5 Service Tanks........................................................... 20 1.5.1 Service tank design....................................................... 21 1.6 Separation system.................................................. 23 1.6.1 Double fuel tanks........................................................... 24 1.6.2 ULSMDO/GO................................................................. 25 1.6.3 Primary steps................................................................. 27 1.6.4 Further steps ................................................................. 28 1.6.5 Conclusions................................................................... 29 2 Separation..................................................................... .31 2.1 Introduction .............................................................. 31 2.1.1 Separation by gravity .................................................... 31 2.2 Continuous separation .......................................... 32 2.2.1 How a disc stack centrifuge works ............................... 35 2.2.2 Disc stack centrifuge sections. ..................................... 36 2.3 Fuel and lubricating oils ....................................... 40 2.4 Running the separator........................................... 45 2.4.1 Conventional separators, Purifier/Clarifier. .................... 45 2.4.2 Separation results with purifiers .................................... 46 2.4.3 Cleaning intervals.......................................................... 49 2.5 Basic operation........................................................ 51 2.5.1 Purifier/ Clarifier limitations ............................................ 53 2.5.2 How to find the right Gravity disc.................................. 55 2.6 Operation on off-spec fuel. .................................. 56 2.6.1 Normal operation........................................................... 56 2.6.2 High density oil .............................................................. 57 2.6.3 High catfines oil ............................................................. 58 2.7 Alcap technology .................................................... 59 2.7.1 ALCAP separators ......................................................... 59 2.7.2 Operating principle........................................................ 60 2.7.3 Application..................................................................... 61 2.7.4 Design ........................................................................... 61 2.7.5 Working Principle........................................................... 62 2.7.6 Transducer WT 200 operating principle in EPC 400..... 63 2.7.7 Transducer MT 50 operating principle in EPC 50 ......... 73 2.7.8 Self adapting system. .................................................... 78 3 Booster system.......................................................... .79 3.1 Purpose of the system........................................... 79 3.2 Applications .............................................................. 79 3.3 Atmospheric and pressurized ............................. 80 3.3.1 Introduction.................................................................... 80 3.4 An atmospheric system........................................ 81 3.5 A pressurized booster system............................ 86 3.5.1 A standard booster module : ......................................... 90 3.6 Viscosity of heavy fuel oil .................................... 93 3.6.1 Viscosity and its effects on diesel engines.................... 94 5 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1 Installation 1.1 Settling Tanks Fig 1.1 Settling tank Suction to separator Oil return from separator Overflow from service tank High Level alarm switch Low level alarm switch Steam heating with P type controller Drain Venting from tank Start/ Stop transfer pump Transfer pump From bunker tanks I 0 0 1 0 0 7 2 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 6 1.1.1 Settling tank design The main purpose of the settling tank is: – To act as buffer tank – To provide a constant temperature to the separator plant – To settle and drain high-level water and solids contamination Heavy fuel oil heating is usually provided by steam, hot water or oil, or by electric coils running through the tanks.The heavy fuel oil temperature must be regulated by a temperature controller to minimize temperature fluctuations in the settling tank. This is important in order to maintain a constant separation temperature in the settling tank. The temperature should not be below 40-50°C, otherwise, the oil may not be pumpable. However, it must not be higher than 10 °C below the flash point, which is normally around 60 °C for HFO. Too high a temperature may lead to ageing (thickening during long term storage), carbon deposition on the heating surfaces, and excessive energy consumption. The settling tank should have a sloping bottom for the collection of water and heavy sludge. The fuel should be in a still condition and therefore the effect of turbulence in the supply and return pipes should be minimized by directing them against the upper part of the tank wall. Thermal insulation of the settling tanks is useful to avoid thermal losses and will contribute to the stillness by eliminating convection currents. A sensor may be fitted to give a signal when high water level is reached to secure proper draining of the settling tank. The separator suction duct should be located close to the bottom of the tank. However, when the ship starts to roll and sludge is stirred up from the bottom of the tank, the separator and the strainer installed in front of the pump can be overloaded. If the tank is well drained, there is less risk of overloading the separator and strainer. 7 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION Often the separator is connected to the fuel suction pipe for the main engine just adjacent to the tank. This pipe has two suction levels, a high and a low. The fuel feed pipe to the main engine from the settling tank is not used except in an emergency. The low suction valve should, however, be open so that the separator can keep the tank bottom clean. 1.1.2 Recommendations • Install level switches in the settling tank connected to bunker feed pump to avoid extensive temperature fluctuations. • Install a P and I temperature controller to minimize temperature fluctuations in the settling tank. • Install a sensor which gives an alarm when high water level is reached in the settling tank. • Clean the settling tank at least once a year. There is continuous separation in the settling tank and the heavy particles, such as cat-fines fall to the bottom. In bad weather these particles can be suspended in the oil again and overload the separator. • Have a sloping bottom in the tank. If there is no sloping bottom heavy particles can accumulate in the tank bottom furthest away from the drain and get mixed into the oil in bad weather. • If there has been an incident with high cat- fines on board we recommend cleaning the tank as soon as possible after the incident. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 8 1.2 Pumping system. A strainer should be incorporated in the suction line of the feed pump to remove coarse particles (0.5 to 0.8 mm mesh size). The drawing above has a separate positive displacement feed pump operating at a constant flow rate. The pumps should be installed close to the settling tanks. This is to avoid long suction pipes, which are often the cause of fluctuating flow. An important factor influencing the cleaning efficiency of a centrifugal separator is the feed pump arrangement which, in turn, is directly related to the need for a correct interface position in the purifier. The interface position in a conventional purifier is dependent on the following external factors: • constant flow rate • constant temperature • correct gravity disc. The pump arrangement has a direct influence on the first requirement - a constant flow rate. Fig 1.2 Pumping system PI PI S 0 0 1 0 1 2 A 9 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.2.1 Built-on feed pumps. In some of the older fuel cleaning systems, the feed pump is mounted on the separator. The separator and its pump are often located far away from the settling tank with the obvious risk of pump cavitation and flow rate fluctuations. A common fault is the built-on pumps usually have far too high capacity. The nominal size may in some cases be three times greater than the flow rate actually needed.The capacity of the pump should not significantly exceed the required flow rate of the cleaning plant. The pump capacity can normally be reduced by changing pump wheels. In some cases the pump itself must be changed. Built-on inlet pumps are always gear type. 1.2.2 Separate feed pumps In modern systems, the built-on pumps are replaced by separate positive displacement type pumps operating at a constant flow rate. Modern separate screw and gear pumps treat the oil gently, max pump speed 1800 rpm. The feed pump, which must always be placed as close to the oil tank as possible, should be fitted with a built-on relief valve, set for an opening pressure of 600 kPa (6 bar). The recommended flow to a separator varies with viscosity and temperature. As a rule, the flow should be as small as possible to allow the oil being treated to stay as long as possible in the separator bowl, so that maximum cleaning effect is achieved. Controlling the oil flow by throttling the valve before or after the pump is a poor technical solution. This method is unfortunately very common and results in a partly emulsified oil entering the separator and also cavitation problems in the pump. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 10 1.2.3 Lubricity and viscosity If we look at the built on pump it has a minimum viscosity limit of 3 cSt when new. Old worn pumps will have a higher viscosity, which means problems handling the new ULS oil. Another big issue is the lubricity. All the pressure in the pump is taken up by the sleeve bearings. The total bearing surface is small and the low lubricity issue will increase the veer on the sleeve bearings resulting in siezing and non functioning pump.  The solution for this is to increase the number of services and change the sleeve bearings more often, or replace the built-on gear pump with a stand alone screw pump, thus rebuilding the separator system. This will not apply to smaller separators with double gear pumps, pumping in to the separator and out from the separator, for example the MAB separator. The screw pump does not have the same problems with viscosity. The min. viscosity for the IMO pump is 1.4 cSt, which is well under the average viscosity of ULSMDO/GO oil. The lubricity issue will of course have an effect on the IMO screw pumps. IMO say that there can be some heightened veer in the pump and especially at the shaft seal that can result in leaking. This is of course not a situation we wish for with the circulation pumps in the booster system. One solution is to increase the frequency and change the shaft seal more often. Another solution is to change to the new “OptiLine”, magnetically coupled pumps. These operate without shaft seals and thus do not leak. I 0 0 1 1 1 0 B I 0 0 1 1 1 0 A 11 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.2.4 Strainer To protect the pump from coarse particles, there must be a strainer in front of the pump. The diameter of the holes in the screen should be 0.5-0.8 mm. The strainer free-flow area must be at least 35% of the total filter area. It is a good idea to check that a screen is actually installed. 1.2.5 Recommendations • Fit a correctly-sized pump. • Built-on pumps should be replaced by separate feed pumps installed close to the settling tank, if possible below the suction connection on the tank. • Use a positive displacement type for the separate feed pump. If there is a problem in keeping a constant level in the service tank, suspect leakage in the three-way valve in front of the separator instead of malfunction of the pump. • All the pumps should be able to deliver to all the separators in the system and a flow control system should be installed. in all fuel oil systems, see chapter 1.3 for more info. N O T E The return pipe from the three-way valve in front of the separator should lead back to the settling tank, and not to the suction side of the pump. The latter case will lead to operational problems. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 12 1.3 Flow control Conventional separators; For optimum separation results, it is very important that the feed flow to the separator is kept constant. Controlling the flow is very important when treating “bad” fuel with lots of contamination such as cat-fines. To maintain the highest possible separation efficiency, the flow through the separator then has to be reduced and we also have to make sure the separator bowl is clean. The interface between separated oil and water in the bowl must be kept in the correct position. It should be well outside the disc stack, but inside the edge of the top disc to preserve the water seal and prevent oil escaping through the water outlet. The correct interface position is obtained by fitting the correct size gravity disc matching the density, viscosity and flow rate of the oil under treatment. Pump capacity varies constantly depending on temperature, viscosity and pump net suction height. Maintaining a constant flow rate is the key to consistent optimum separation efficiency and the prevention of oil losses. Fig 1.3 Constant flow system PI PS PI PI Mt4 S 0 0 1 0 1 0 C 13 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION ALCAP separators; The same principle applies for the ALCAP separators as for the conventional separators i.e. the more time the oil has in the separator, the higher the separation efficiency. With HFO containing high cat fines, or unstable oil, the recommendation is to lower the flow to increase the efficiency. 1.3.1 The flow control system Alfa Laval has developed the constant flow regulation system as a complement to the feed pump to ensure a constant feed to mineral oil separators. This equipment ensures the desired constant flow rate by maintaining a constant pressure in the oil feed line. The desired flow rate is set manually by means of a flow regulating valve. Besides helping to maintain the correct interface position, the constant flow regulation system also enables the feed pump capacity to be matched to the desired separator throughput In principle, there are four ways to control the flow: • Adjustment of the built-in safety valve on the pump. This method is NOT recommended since the built-on valve is nothing but a safety valve. The opening pressure is often too high and its characteristic far from linear. In addition, circulation in the pump may result in oil emulsions and cavitation in the pump. • In our S separator systems we have developed a flow control with a regulating valve and a spring loaded valve set at 2 bars. When the regulating valve is throttled and the pressure overcomes 2 bar the valve opens and oil is then sent back to the settling tank. This is a very cheap and easy way to make a good flow control system. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 14 • A flow control valve arrangement on the pressure side of the pump, such as the Alfa Laval MCFR system. In an alarm condition, the oil should be recirculated back to the settling tank and not fed directly back to the suction side of the pump. • Speed control of the motor with a frequency converter. This is a relatively cheap solution today and can be a good alternative for flow control. 1.4 Heating System A large proportion of the smaller tonnage does however use electric heaters. It is essential to keep the incoming oil temperature to the separator steady with only a small variation in temperature allowed (maximum ±2 °C). The position of the interface between oil and water in the separator bowl is a result of the density and the viscosity of the oil which in turn depends on the temperature. A control circuit including a temperature transmitter, and a PI-type controller with accuracy of ±2 °C should be installed. The heating system could preferably be controlled from the EPC. One of the most common sources of problems with fuel or lube oil cleaning systems is undersized heaters or heaters that have very poor temperature control. If steam heated, a correctly-sized steam valve should be fitted with the right KvS value. The steam trap should be a mechanical float type. The most common heaters on board are steam heaters. This is due to the fact that steam in most cases is available at low cost. Most ships are equipped with an exhaust boiler utilizing the exhaust gases to generate steam. Fig 1.4 Heating system PI PI I 0 0 1 0 6 9 15 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION When the optimum gravity disc has been selected corresponding to a given oil, temperature, and flow, any change in the flow and temperature parameters will move the interface. A temperature increase will move the interface into the disc stack resulting in a very poor cleaning efficiency. The bowl discs will be contaminated and clogged and will have to be cleaned very often. On the other hand, a decrease in temperature will cause a broken water seal. The latter is most common, and to avoid broken water seals the operator often changes to a gravity disc that is too small. This affects the cleaning efficiency of the separator and it will act more as a pump than a separator. There are several reasons why the temperature does not remain steady after being set correctly: • With the 3-way valve between the heater and separator circulating the oil back to the pump suction side instead of back to the tank, the temperature will fluctuate often more than 10 degrees, thus requiring a smaller gravity disc than the optimum choice. This will happen at each discharge (except separators with “feed on” during discharge). At each start-up, however, the problem will remain. • A temperature controller with only (P) proportional function cannot cope with temperature changes in the incoming oil. Changes can occur when filling up the settling tank with oil which has a lower temperature or stopping a diesel engine in port. • An undersized or dirty heater is also a problem as the temperature cannot be maintained under all conditions thus requiring frequent changes of gravity discs. The following requirements must be considered for all types of heaters and are especially important for heaters with high surface loads such as electric heaters. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 16 • A turbulent oil flow is required for optimum thermal efficiency and to avoid coking. • No dead zones present where hot spots can occur. • The heater should be designed to prevent “short circuiting” of the oil flow. • Short retention time in the heater. • To prevent blockages caused by coking, the heater must always be started after the feed pump, and switched off prior to the pump being stopped. The heating media which can be used in the different basic types of heaters are: Electric power; HEATPAC EHM Steam; HEATPAC SHM, CBM Hot water; Thermal oil; CBM 17 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.4.1 Electric heaters Conventional electric heaters for separator preheating are mostly of the same basic design: • a tube stack of heating elements in a pressure vessel together with • an electro-mechanical stepwise on-off regulation, often with manual control. Pressure vessels are normally quite bulky resulting in a large oil volume in the vessel. A large volume in combination with stepwise regulation of heating power results in either too much or too little power. Outlet oil temperature variations of up to ±10 °C are not unusual. Another factor that limits the suitability of conventional electric heaters is the uncontrolled laminar flow distribution of oil inside the heater. This results in “dead pockets” and the consequent obvious risk of fouling and coking of oil on the elements. The above mentioned temperature fluctuation of ±10 °C means that operators may have to decrease the temperature setpoint to approximately 90 °C in order to avoid the boiling of water in the untreated oil before separation. As the optimum separation temperature is 98 °C (±2 °C), this lower setpoint has a direct negative effect on separation efficiency. It was to overcome and eliminate these weak points that Alfa Laval developed the HEATPAC heating system. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 18 1.4.2 HEATPAC system . The HEATPAC EHS 62 system is developed to work with Alfa Laval separation systems. The HEATPAC EHS 62 system consists of a heater vessel, and a heater cabinet controlled by the EPC control system. The heater regulation is stepless working with Triacks controlling two of the three phases in the system. This secures a very exact temperature control and is perfect for separation systems. The HEATPC system offers the following advantages: • Compact and robust heater with minimal hold-up volume, with few and reliable seals • Specially designed heating elements and shell for high heat transfer • Infinitely variable electronic temperature control • Temperature accuracy ± 1°C • Fully automatic operation • Easy to retrofit in existing systems • Resistance to pressure pulsation The HEATPAC electric heater comprises a heating element bundle inserted in a pressure vessel. Mineral oil is fed continuously to the heater. A baffle plate inside the vessel divides the flow into two passes. Fig 1.6 EHM electric heater oil flow A INFO OP ENTER HEATER SEPARATION STOP DISCHARGE OPACTIVE ALARM EPC-50 TT 16 MT PT PT PT 15 10 19 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.4.3 Steam installation The steam installation plays a large part in securing trouble-free operation of the steam heater. Operation of the steam heater can be very difficult if the installation is wrong. Steam trap Installation One of the most important parts of the steam installation is the steam trap. We recommend a mechanical steam trap due to the fact that this opens when there is condensate present. Other solutions such as thermo-dynamic steam traps react to temperature and are not suitable for marine installation. The steam trap should be mounted underneath the heater to make sure there is no condensate accumulation in the heater. Another important aspect is the height from the steam trap to the collecting tank. If this height is over 10m, it can have an effect on the transportation of the condensate. After the heater the pressure on the condensate can be as low as 0,1 bar and if the lifting height is too high, the heater can be filled with condensate.This can result in thermal cracking and internal leakage in the area inside the heater where there is condensate and hot steam.The solution is to mount a condensate pump in the system after the last steam trap. P 0 0 1 1 5 0 B INFO OP ENTER HEATER SEPARATION STOP DISCHARGE OP ACTIVE ALARM EPC-50 TT 16 MT PT PT PT 15 10 I 0 0 1 0 3 8 H 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 20 1.5 Service Tanks Fig 1.8 Service tank Oil from separator Overflow to settling tank High Level alarm switch Low level alarm switch Steam controller with P type controller Suction for Booster pumps Suction for Boliler Drain Venting from tank I 0 0 1 0 0 7 1 21 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.5.1 Service tank design An overflow pipe should be installed from the service tank to the settling tank. The overflow pipe should be connected to the lower part of the service tank to recirculate any water that may find its way into the oil after the separators, due to condensation, coil leakage, etc. It will also remove any heavy particles collecting in the lower part of the tank. If the overflow is mounted in the top of the tank, all the benevolent lighter fractions in the oil, will return to the settling tank and not the heavy fractions that might need more separation.This feature also helps fulfil the design requirement to have constant flow through the cleaning system at all times. It is not necessary to reduce the flow because of high level in the clean oil tank. Heating of heavy fuel is usually provided by steam, hot water or thermal oil running through the tanks. The heavy fuel oil temperature must be regulated by a temperature controller to minimize temperature fluctuations in the day tank. Too high a temperature may lead to ageing (thickening during long term storage), carbon deposits on the heating surfaces, and excessive energy consumption. The fuel should be in a static condition in the service tank and therefore the effect of the supply pipe should be minimized by directing the flow against the upper part of the tank wall. Thermal insulation of the day tank is useful to avoid thermal losses and can contribute to a static condition by eliminating convection currents.The day tank should have a sloping bottom to collect the water and sludge. A sensor should be installed to give a signal when a high water level is reached, to secure proper drainage of the day tank. It is recommended that the clean oil day tank should also be equipped with a draining arrangement at the lowest point. In a modern ship, regular manual draining of the day tank is considered to be a burden. It is also difficult to judge how much to drain off. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 22 Some operators use a system where the oil in the service tank is separated when the ship is at birth, to make sure the service tank is clean. This is not what Alfa Laval recommends as this will only clean the service tank. For all installations with settling and service tanks this can be solved by the overflow pipe to the settling tank leading from the bottom of the service tank. This makes sure that the most contaminated products are sent back to the settling tank and once more through the separator. A system like this secures that the oil is cleaner in the service tank and in the settling tank, increasing the efficiency of the whole separation system. N O T E Our recommendation is to clean the service tank at least once a year, or every time the ship docks. It is also important to clean the tanks if they have contained oil with a high cat fines content. 23 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.6 Separation system The low grade of today’s HFO makes it necessary to have a pre-treatment system and a separation system in good working in order to obtain an optimum separation result. It also sets high demands on the operators of the separation system. They have to know how to set up the separators according to the fuel quality. The complexity of a system with multiple fuel grades increases the possibility of contamination of high grade fuel with low grade fuel. Strict guidelines for how to handle the different grades of fuel must be made. Mixing a high sulphur fuel with low sulphur fuel can have devastating consequences. This is a schematic drawing of a common separation system, one settling tank, the separators with the ancillaries and two service tanks. A straightforward system that is fairly easy to operate and in which not much can go wrong. I 0 0 1 0 1 0 A 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 24 1.6.1 Double fuel tanks This is a schematic drawing of a more complex separation system mainly used for operating on multiple fuel grades.There are two settling tanks, the separator with the ancillary’s and two service tanks. When operating on fuels with different sulphur content, it is imperative that the fuels are not mixed. Low sulphur fuel mixed into the high sulphur fuel results in higher cost since the low sulphur fuel is more expensive. High sulphur oil mixed into low sulphur oil is more serious. Very little high sulphur oil is needed to contaminate low sulphur oil since the low sulphur oil is already on the limit from the supplier and the oil is then off spec. Off spec. oil can have severe consequences for the operators, with different penalties in the different ports. Some involve jail for the Chief engineer and captain, others involve high fines (millions of euro). Some even involve both. Make sure to give strict and rigorous instructions on the operation of the system and make sure the operators are educated and able to understand the system and instructions. I 0 0 1 0 1 0 E 25 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.6.2 ULSMDO/GO We now have to handle another type of fuel, ULSMDO/GO and the low sulphur cap and implications if the fuel becomes off spec. demands this is done carefully. Since this type of fuel has to be used for the main engine entering California and at birth in ECA, to date, the small separators usually handling the MGO and MDO fuel are too small and we have to use separators with higher capacities. We can use one of the existing HFO separators for cleaning the ULSMDO. This creates some handling problems! When changing from HFO, even LSHFO the piping, pumps and heaters are filled with the higher sulphur fuel. Getting this higher sulphur fuel into the ULSMDO, even in small quantities can make the fuel off spec. and cause a lot of problems. This means we have to clean the system by running the ULSMDO into the HFO tank until the system is clean of any higher sulphur oil. The ULSMDO is more unstable and can create compability problems in the HFO! We also introduce more valves and consequently have a more complex system, increasing the possibility for all kinds of handling problems. I 0 0 1 0 1 0 E 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 26 If we going to use an existing separator for ULSMDO separation we recommend to use the standby separator for this purpose. This separator must then be dedicated to the ULSMDO treatment only so as not to cause any problems with the ULSMDO spec. Make sure to give strict and rigorous instructions on the operation of the system and make sure the operators are educated and able to understand the system and instructions. Recommendation Invest in a higher capacity separator to handle the ULSMDO. This will remove all the problems associated with using one of the HFO separators for cleaning. There will be no danger of mixing fuels and ending up with an off spec. fuel. The separator used for MDO/MGO can be a purifier type separator since the density is more stable and we do not need to change the gravity disc that often. I 0 0 1 0 1 0 H 27 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.6.3 Primary steps Settling tank: • Install level switches. • Install temperature control. • Install a sensor for high water level Pumps: • Install a suitable flow control device (see chapter 1.2.5). • Check that the return pipe from the three- way valve is led back to the settling tank and is not in front of the pump. If it is in the front of the pump, change it. • Remove the built-on feed pump and install separate feed pump(s). • Never use the internal safety valve on the pump for flow control - it will create lots of problems. Steam heating system: • Install a P/I temperature control for the steam heater. Preferably use the built in P/I controller in the Alfa Laval EPC control equipment in the separator system. • Check that the steam valve is neither over dimensioned or under dimensioned. • Check that a mechanical float-type steam trap is fitted and in the right position. • Set the temperature set point at 98 °C and check that this temperature is reached and remains constant. Electric heating system: • Install a stepless electronic temperature control system, Preferably the Alfa Laval EPC system. • Establish the temperature set point at 98 °C and check that this temperature is reached. Separator inlet temperature should not deviate by more than ±2 °C. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 28 Separators • Use self-cleaning type separators only. • Use the stand-by separator for parallel operation. Install a separate small separator for DO. • Preferably use ALCAP separators in parallel for best results and lower maintenance time. Operating water system: Check the operating water system. If it does not meet the recommendations for the separator, it is strongly suggested that action be taken to meet these recommendations. This is one of the major causes of malfunction of self-cleaning separators. Be aware of growth in old pipes. After a number of years all water pipes have mineral growth inside reducing the pipe diameter and thereby the flow of water. Always check the piping during upgrading to new equipment. 1.6.4 Further steps • Replace the regular electric or steam heater with a complete HEATPAC System. • Set the discharge frequency according to the recommendations and fuel quality. • Check that sludge pipes are vertical, or max. 30° from the vertical without sharp bends, etc. • Check that the sludge tank ventilation is correctly dimensioned and installed, see installation instructions in the service manual for the specific separator. • Be aware that the proper equipment, wrongly installed, not will do the required job! 29 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 1 INSTALLATION 1.6.5 Conclusions Experience and future trends indicate that there is an increasing need for efficient and reliable fuel and lube oil treatment on board ships - whether existing operational ships or projected new buildings. There are numerous factors that must be considered and various actions that can be taken to upgrade existing installations or designs. Our basic recommendations can be summarized as follows: • Install the best equipment to ensure reliable, efficient operation. • Consider life-cycle costs when specifying new equipment and modifications. • Apply a “systems” approach to the selection and installation of the equipment. Alfa Laval Separation, with its worldwide marketing and service organization, is at the customer’s disposal to assist with any level of upgrading of an existing separator installation. 1 INSTALLATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 30 31 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2 Separation 2.1 Introduction The purpose of separation can be: • To free a liquid from solid particles, e.g. sludge in lube oil. • To separate two mutually insoluble liquids with different densities while removing any solids present at the same time, e.g. bunker oil. • To separate and concentrate solid particles from a liquid, e.g. the content in a sludge tank. In Marine installations, water and solids are removed from fuel oil and lube oil. In fuel oil, the percentage of water and solids can vary from batch to batch. Whenever you change to another bunker tank, the physical character of the oil may differ. It may be necessary to make some changes to reach optimum separation depending on the separator installation. 2.1.1 Separation by gravity A liquid mixture in a stationary tank will clear slowly as the heavy particles in the liquid mixture sink to the bottom under the influence of gravity, 1 g. A lighter liquid (e.g. oil) rises while a heavier liquid (e.g. water) sinks. Continuous separation of sludge can be achieved in a settling tank with outlets arranged according to the difference in density of the liquids. Heavier particles in the liquid mixture will settle and form a layer on the tank bottom.To increase the efficiency in the tank we add buffer plates to reduce the distance for the particles to travel and thereby increasing the efficiency. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 32 2.2 Continuous separation Accelerating the process Virtually all branches of industry need to separate different liquids and solids at some point in their manufacturing processes. Alfa Laval has well over a century of experience in meeting these requirements using different kinds of centrifuge technology. The basic centrifuge idea is based on what happens in a settling tank, in which particles, sediment and solids gradually fall to the bottom, and the liquid phases of different density separate due to the force of gravity. However, such clarification is an extremely slow process and is unable to meet industry's needs for rapid, controllable results. The general idea behind centrifuges is therefore to ensure that the mechanical separation of different liquid phases and solids can be carried out on a rapid, continuous basis in order to meet the demands associated with modern industrial processes. Fig 2.1 Tank with oil, water and solids In essence, a centrifuge is a settling tank whose base is wrapped around a centre line. Rotating this entire unit rapidly means that the effect of gravity is replaced by a controllable centrifugal force that can have an effect up to 10,000 times greater. This force is then used to separate liquids from other liquids and solids efficiently and with great accuracy, and in a manner that is easy to control Fig 2.2 Tank with buffer plates I 0 0 1 0 6 8 A I 0 0 1 0 6 8 B 33 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Centrifugal separation In fuel oil, the percentage of water and solids can vary from batch to batch. Whenever you change to another bunker tank the physical character of the oil may differ. It may be necessary to make some changes to reach optimum separation depending on the separator installation. See chapter 2.4 Running the separator. The discs must be kept clean. If the small distances between the discs are reduced due to dirt, the flow speed between the discs will increase and the separation effect will be reduced. Types of centrifuge There are several different basic types of centrifuge normally used in industrial separation. Disc stack centrifuges are ideal for a wide range of separation tasks that involve lower concentrations of solids and smaller particle and droplet sizes. This applies to both liquid-liquid and liquid-solid separation. The most difficult separation tasks can often involve three phases, where there is hardly any difference in the densities of the separate liquid phases and where the particles to be separated are very small in size. In such applications, no other technology can compete with disc stack centrifuge technology. In a rapidly rotating bowl, the force of gravity is replaced by centrifugal force, which can be thousands of times greater. Separation of sludge is continuous and fast. The centrifugal force in the separator bowl can in a few seconds achieve what takes many hours in a tank under the influence of gravity. In the new S-type separators with up to 13000 rpm, the G-force in the bowl can reach up to 7000 G. In the marine installations water and solids are removed from fuel oil and lube oil. Fig 2.3 Rotating bowl 0 6 8 C 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 34 Typical marine separators are; • MAPX; Older type of conventional separator with an MRC (Maximum rated capacity) varying from 900 to 7300 l/h depending on the installation and application. Water outlet is open. Can be used for both lube oil and fuel oil. • MOPX; same as the MAPX with an MRC from 6900 to 21500 l/h. • WHPX: Same as the two earlier mentioned with an MRC from 4700 to 29400 l/h. The WHPX separator can also be rebuilt to an ALCAP machine with a conversion kit. • MMPX; Conventional separator with an MRC varying from 1700 to 2900 l/h depending on the installation and application. Water outlet is open. • FOPX and MFPX ALCAP HFO separators with the water outlet closed. The capacity varies from 850 to 15200 l/h. These separators are only for fuel oil and cannot be used in a lube oil system. • LOPX: ALCAP separators with no water outlet. This means that the water handling capacity is very low and can thereby only be utilised in lube oil systems. The capacity varies from 1800 to 9600 l/h. • MAB; small size separator ideal for diesel oil and smaller fishing vessels • S separators, the newest ALCAP separators in Alfa Laval portfolio. Ideal for HFO, Lube oil and diesel oil cleaning. Applicable for all flow requirements. 35 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.2.1 How a disc stack centrifuge works A disc stack centrifuge separates solids and one or two liquid phases from each other in one single continuous process, using extremely high centrifugal forces. When the denser solids are subjected to such forces, they are forced outwards against the rotating bowl wall, while the less dense liquid phases form concentric inner layers. The area where these two different liquid phases meet is called the interface position. This can be easily varied in order to ensure that the separation takes place with maximum efficiency. Inserting special plates (the "disc stack") provides additional surface settling area, which contributes to speeding up the separation process dramatically. It is the particular configuration, shape and design of these plates that make it possible for a disc stack centrifuge to undertake the continuous separation of a wide range of different solids from either one or two liquids. The concentrated solids phase formed by the particles can be removed continuously, intermittently or manually, depending on centrifuge type and the amount of solids involved in the specific application. The clarified liquid phases overflow close to the rotating axis, in the outlet area on top of the bowl. The liquids then flow into separate chambers. Each separated liquid phase then leaves the bowl due to the force of gravity or by means of a paring disc, which is a special pumping device. The chambers can be sealed off from each other to prevent any risk of cross- contamination. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 36 2.2.2 Disc stack centrifuge sections. Inlet zone The inlet zone accelerates the process liquid up to the speed of the rotating bowl. A properly designed inlet zone makes sure that the feed solids and liquids are not degraded or affected in any other way. Good inlet design also prevents foaming, reduces the sheer forces in the product, minimizes temperature increases and avoids disturbance of the separation processes taking place in the bowl. A number of different inlet configurations are available for Alfa Laval disc stack centrifuges, each designed to ensure maximum performance in conjunction with a specific process. Disc stack area The key to good separation performance lies in the efficiency of the disc stack, which is the heart of the centrifuge. The design of the disc is therefore crucial. Alfa Laval has the expertise needed to match the demands associated with specific industrial processes by providing particular disc stack configurations that ensure a flow evenly spread among the discs, along with an optimized flow pattern in the disc stack itself. The layout and design of the distribution holes also have a crucial influence on good performance. These ensure that the process flow is evenly spread among all the discs, for the most efficient results. Inside the disc stack. Inside the disc stack there is two forces working on the particle, flow and G force. The flow is decided by the size of the pump. In a HFO system there can also be a constant flow system; see chapter 1.3. The other force working is G force. The G force inside the bowl is decided by the bowl size and RPM and can reach forces up to 7000 G. 37 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION . It is important that all separator installations are designed for to the oil type used. An installation running on IF 60 needs a different installation than an installation on HFO 380. If we look inside the disc stack, between the discs, there is a distance of between 0.5 and 0.8 mm, and it is here forces come into play. In this area between the discs we have a flow that creates a “parabolic velocity” profile i.e. the forces of the flow are higher in the middle between the discs than on the disc surface. If we compare this with a river, you often see the rapid flow in the middle whereas near the banks the water is almost standing still. We utilize this in the separation process. We try to move the particles we need to remove into the area where the flow is at its lowest in the shortest distance possible. Fig 2.4 Flow between discs This means that the faster we can move the particles up underneath the disc the easier it is to remove them. When a particle is moved underneath the disc, the G-force vector is higher than the flow vector and the particle will move outwards to the periphery of the bowl (see drawing), collecting in the sludge area. If you have ever cleaned a disc stack, you will have noticed that there is more dirt under the discs than on top of the discs. This is the reason. If the flow is too high, however, the particles will travel further inside the disc stack and possibly through the disc stack. The oil is then not cleaned properly Fig 2.5 Particle movement 0,5 - 0,8 mm I 0 0 1 0 5 9 I 0 0 1 0 6 7 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 38 Liquid discharge section Once separated, the liquid must often be conveyed out of the centrifuge as delicately as possible. In some applications, it is important that oxygen pick-up is kept to a minimum, and that temperature increases in the liquid must be avoided in order to prevent problems later in the process. Alfa Laval has designed solutions to these and many other detailed requirements, in order to provide our customers with the best possible process conditions for their operations. The most straightforward way of discharging the liquid phases is the use of open outlets. In most applications, however, a positive pressure is needed. This is created by a stationary paring disc with specially designed channels. This disc decelerates the rotating liquid and transforms the kinetic energy into pressure, thus pushing the liquid out of the centrifuge via the channels in the disc. The pressure needed for the particular process is normally regulated by a valve on the outlet. Solids discharge section There are three basic ways of removing the solids from disc stack centrifuges • continuous solids discharge, in which solids and liquids exit via nozzles in the periphery • intermittent solids discharge, in which a carefully designed system opens ports in the bowl periphery at controlled intervals in order to remove the collected solids • manual removal, in which the machine is stopped and the bowl is opened so that the collected solids can be removed manually. The solution most appropriate for a particular application depends on a combination of factors. The most important of these are the amount of solids in the liquid, the nature of the particular application, and the consistency of the solids once they have been separated. 39 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Separator system Naturally, the overall efficiency of a disc stack centrifuge as part of a production set-up is heavily dependent on many other ancillary systems and equipment. Unparalleled experience means that Alfa Laval has a unique capability for providing all the necessary equipment to achieve maximum efficiency in the continuous separation of different liquid phases and solids in countless industrial processes. This can be done on the basis of highly efficient, standardised equipment packages and fully tested modular units, or specially customized disc stack centrifuge installations to meet individual liquid-liquid and liquid-solid separation requirements. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 40 2.3 Fuel and lubricating oils To understand the challenges of cleaning fuel and lubricating oil, one must put into context the different types of oil, their characteristics and contaminants that must be separated from the oils to ensure high performance and long service intervals of marine and diesel engines. The quality of the fuel and lubricating oils varies widely, depending on the grade and processing of fuel and lubricating oils. Some may contain higher levels of contaminants, such as water and abrasive solids, while others contain lower levels. Efficient cleaning of all fuel and lubricating oils is essential to achieve reliable and economical operation of diesel engines and other equipment. Fuel oil Diesel engines generally burn residual, or heavy fuel oils. For marine installations, fuel is purchased in different locations as the ship sails from port to port. Mixing heavy fuels with highly diverse compositions can lead to incompatibility problems, causing instability. Such instability problems are particularly severe for heavy fuel oil because of the diverse refining processes used to produce it. Heavy fuel oil is essentially a refinery by- product. After the most valuable fractions of crude oil have been extracted, the remains are processed further to recover what is known as heavy fuel oil, a cheap source of energy - and one that is not manufactured according to specifications. The ISO 8217 2005 Fuel Standard specifies a number of physical and chemical limitations for marine fuels, but does not define several critical characteristics that are essential for separation. Density, or more specifically the difference in density between the water and oil to be separated, is a critical parameter for effective cleaning of fuel oil. 41 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Because it is a residue, heavy fuel oil typically is a high density fraction that contains the heaviest components. In the heaviest types of residual oil, the difference in density is so great that it is impossible to clean the oil using a gravity disc type purifier. This was one of the driving forces behind the development of the ALCAP system. Catalytic fines; Catalytic fines are the most harmful of all substances in heavy fuel oil. These are fragments of a catalyst added to the oil to optimize the refining process. Composed of solid particles of aluminium and silicon compounds, catalytic fines are almost as hard as diamonds and vary in size from sub-micron to approximately 50 µ. If allowed to enter the engine, catalytic fines wear down engine components - sometimes causing considerable damage within a few hours. For more information about catalytic fines and how to separate them from fuel oil, ask your Alfa Laval representative for a copy of "Marine diesel engines, catalytic fines and a new standard to ensure safe operation," (Ref. No. EMD00078EN) written by Alfa Laval, BP Marine and MAN B&W Diesel. There is also a copy of this in the thumb drive presented on this course. Low sulphur fuel Environmental considerations have led to the increased use of fuels with low sulphur content. In certain regions known as sulphur emission control areas or SECA the maximum allowable content of sulphur in the fuel is 1.5 percent in HFO. Few heavy fuel oils satisfy this stringent limit without further mixing or processing to remove sulphur, which causes the oil characteristics to change. This processing of oil increases the risk of making the heavy fuel oil incompatible with the heavy fuel oil that has not been de-sulphurize and, for economical reasons, may be burned outside SECA. In addition, low sulphur fuel tends to contain more catalytic fines than ordinary heavy fuel oil, even though low sulphur fuel may have a lower viscosity. See DNVPS presentation for more information 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 42 Distillate and marine diesel oil Distillate and marine diesel oils generally are more uniform in nature than heavy fuel oil. The main differences can be attributed to the origin of the oils. Refining processes have less of an impact on the characteristics of marine diesel oil and distillates than on those of heavy fuel oils. In addition, marine diesel oils and distillates typically do not contain any emulsifying compounds, which can make the separation of heavy fuel oil difficult. Thanks to lower and more consistent densities, distillate and marine diesel oils are considered to be far easier to clean using centrifugal separation than heavy fuel oils. Lubricating oil Lubricating oils for diesel engines in ships and power plants are essential for operation. Keeping lubricating oil clean by means of separation helps prevent the accumulation of substances that increase viscosity as well as any solid particles that can cause engine wear and pose the risk of very expensive replacement costs. In general, there are two basic types of diesel engines that require lubricating oil: the trunk engine and the cross-head engine. Trunk engines There are two types of trunk engines, which are also called four-stroke engines: 1. The medium speed engine that runs at between roughly 300 and 600 rpm. 2. The high speed engine that operates at between 600 and 2000 rpm or higher. The lubricating oil used in trunk engines is prone to contamination from blow-by, which is a leaking gas stream from the combustion chamber to the oil sump that contains remainders of burned fuel and lubricating oil. 43 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Contaminants that result from blow-by, such as particles of soot and other partly burned hydrocarbons, must therefore be removed from lubricating oil. In addition, calcium sulphate or gypsum forms as a result of the reaction between sulphur in the fuel and the neutralizing calcium- based base number (BN) additives in the lubricating oil, and comprises a major portion of the resulting sludge. The BN additives are important for preventing the formation of sulphuric acid and thus protect against acidic, or cold corrosion. Cross-head engines Called long-stroke or two-stroke engines, crosshead engines operate at lower speeds than trunk engines, generally between 90 and 200 rpm. Larger than trunk engines, crosshead engines burn more fuel per stroke, and are capable of producing more power. System oils in crosshead engines are, however, less susceptible to contamination than those in trunk engines due to the crosshead engine's stuffing box. The stuffing box is a seal that surrounds the piston rod and protects the oil sump from contamination due to leaks from the combustion chamber. Contaminants in the lubricating oil of crosshead engines are therefore less advanced in composition than those found in the lubricating oil of trunk engines. Box oil is highly contaminated oil, which is emptied through the drain in the stuffing box. This ensures that box oil does not mix with system oil. Highly alkaline lubricants used between the cylinders and cylinder liners, called cylinder oil, are added separately. Any leaking oil together with remainders of oil burned during the combustion process is drained as box oil. Water is another contaminant found in engine lubricants. Water may result from condensate that forms when the engine is not in operation or originate from accidental leaks. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 44 Because lubricating oil contains dispersants and detergents, it easily emulsifies water. More importantly, some BN additives may deteriorate in the presence of water. It is therefore important to keep the water content in the lubricating oil system at the lowest possible level. An essential application Separating lubricating oil is a relatively straightforward but essential application, thanks to defined oil density and constant system conditions. Separation is recommended during engine operation. It is also recommended for short periods while the engine is not in operation in order to prevent condensate build- up and the formation of water, which can compromise operation when the engine is put into service. 45 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.4 Running the separator. 2.4.1 Conventional separators, Purifier/Clarifier. Conventional cleaning plants are based on purifier/clarifier type separators. Practical operation has confirmed that the maximum density limit for fuel oil is 991 kg/m3 at 15 °C. If this limit is exceeded at bunkering, operational difficulties with the cleaning plant will arise along with the obvious risks for unreliable engine operation or excessive engine wear. Consequently, the density of available fuel oils can restrict the use of purifier type separators. However, for lighter marine diesel oils and lubricating oils, conventional purifiers are suitable. The use of conventional purifiers is limited using the gravity disc: • It restricts the use of diesel engine fuels to those with a maximum density of 991 kg/m3 at 15 °C. • Optimum separation depends on selecting the correct gravity disc, which corresponds to the prevailing density, viscosity, feed flow rate and temperature. • To ensure satisfactory cleaning a second separator may be required in series. This means a separation system consisting of a purifier followed by a clarifier. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 46 2.4.2 Separation results with purifiers To achieve optimum separation results using conventional purifiers/clarifiers, the interface between the oil and water in the bowl must be located outside the disc stack but inside the top disc. The position of the interface is adjusted by means of a gravity disc. To get the correct interface position the purifier must be fitted with a correctly sized gravity disc. With higher fuel densities, maintaining optimum separation results by means of gravity discs becomes increasingly difficult. Factors that affect the interface position are - changes in oil density, viscosity, feed flow rate, and temperature. With increasing fuel density, the interface position becomes progressively more sensitive to these factors. This easily leads to a situation where the interface is not in the correct position: Fig 2.6 Interface in normal position I 0 0 1 0 6 3 A 47 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION If the interface moves too far away from the centre, the result is a broken water seal. This means that oil escapes via the water outlet and is lost. This scenario gives a “low pressure in oil outlet” alarm. This is caused by one or several of the following: • Gravity disc is too large • Density of the oil increases • Viscosity of the oil increases • Flow rate increases • Temperature of the oil decreases • Dirty disc stack This scenario is not dangerous since there is an alarm indication that something is wrong in the bowl. The only way to avoid this is to always have the right gravity disc in the bowl. This means a lot of work for the operator, and that he must have good knowledge of the separator system. Fig 2.7 Interface outside the top disc I 0 0 1 0 6 3 C 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 48 If the oil density is changed and there are no alarms, or if there are no alarms at all, this is a certain indication that something is wrong. If you suspect that there is something wrong, carry out the test in chapter 2.5 “Basic operation”. This is not accurate, but will give an indication. We strongly recommend that the disc size is checked to make sure you have the right gravity disc, follow the description in chapter 2.5.2 “How to find the right gravity disc”. If the interface moves too close to the centre, water blocks the upper part of the disc stack, and the lower part of the disc stack become overloaded. This results in poor separation efficiency. This is caused by one or several of the following: • Gravity disc is too small • Density of the oil decreases • Viscosity of the oil decreases • Flow rate decreases • Temperature of the oil increase This scenario is the most dangerous since there is no alarm to indicate that something is wrong in the bowl. The separator will keep on going but the oil will not be cleaned. The only way to avoid this is to always have the right gravity disc in the bowl. This means a lot of work for the operator, and that he must have good knowledge of the separator system. Fig 2.8 Interface into the disc stack I 0 0 1 0 6 3 B 49 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.4.3 Cleaning intervals Cleaning the separator at certain intervals is imperative for the high efficiency of the separator. Cleaning intervals are NOT the same as service intervals. Cleaning intervals in the separator depend on the oil passing through the separator. If separating light HFO or gas/diesel oil less cleaning is needed than for HFO. Determining the right cleaning interval is done by experience; know the system and the contamination in the oil you are separating. We recommend using the CIP unit for the cleaning. This will reduce the veer on the bowl parts and seal rings. It will also reduce the down time on the separator and work force needed so that they can do other work. See the manual for correct procedure to find right cleaning intervals. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 50 Limitations common to conventional clarifiers. Oil losses and limited water handling capability are the basic problems encountered when treating fuel oil of any density in a clarifier. Clarifiers are not to be installed for single stage operation but should always be preceded by a purifier. Oil losses When operating a separator in clarifier mode, no displacement water is added prior to sludge discharge. Therefore, not only sludge and separated water are discharged, but a certain volume of oil is also discharged. Limited water handling capability For optimum separation efficiency, separated water must not enter the disc stack. The separated water can only be discharged with the sludge through the sludge ports at the bowl periphery since the water outlet is closed in a conventional clarifier. 51 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.5 Basic operation The conventional purifier is much more labour intense than an ALCAP separator. It is important that the gravity disc on a purifier is the right size all the time. When bunker tanks are changed, the gravity disc has to be controlled. The gravity disc decides the interface between water and oil and has to be in a certain position, see chapter 2.4.2 for more information. One of the most common mistakes made is to fit the clarifier disc in the separator because then there are no alarms for broken water seal. This will only change the separator into a pump with no separation effect on the oil, and the operator will have no control over the process. To check if the interface is in the right position on a purifier, slowly close the counter pressure valve until water and eventually oil come out of the water outlet. If the oil is coming out very fast in the water outlet the interface is most likely in the right position. If the oil comes after a high pressure increase or not at all, the gravity disc is most likely too small. This is not an exact test method. The only way to be sure if the gravity disc is correct is to increase the size until the water seal breaks, and then reduce size by one, see chapter 2.5.2 for more information. The most important issue is to keep the bowl clean at all times, especially the disc stack. The intervals between cleaning can vary from fuel batch to fuel batch. There is great variation in the oil and some fuels can be almost impossible to treat. See DNVPS documentation for further information. In a purifier system the temperature of the oil is very important. If the temperature changes the interface will also change, and if the interface is moved too much, the water seal will break or be moved into the disc stack. The temperature is also important for the efficiency, so securing a stable temperature is one of the most important parameters in the separation process. Make sure that the heater system is in perfect operating order and use a P/ I control type, preferably the EPC controller in the separator system, to prevent alarms from temperature variations. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 52 If the separator is in clarifier mode, operated in series with a purifier, the same rules for cleanliness apply. However there is no water seal and temperature is not that important since there is no interface to consider. It is still important that conditioning water is added after a discharge to keep the sludge in a liquid state. 53 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.5.1 Purifier/ Clarifier limitations All conventional separators after 1985 have a density limit at 991 kg/m³. In lubricating oil cleaning systems a clarifier type separator may not be used for the following reason: Calcium sulphate is a product of neutralizing sulphuric acid when it forms in the engine. The acid, of course, arises through the burning of fuel that contains sulphur. Some of the sulphur is converted to sulphuric acid. Most marine engine lubricants are over-based, meaning that they contain calcium carbonate, which is a base. The reaction of calcium carbonate with sulphuric acid results in calcium sulphate, water and carbon dioxide. Gypsum is a hydrated form of calcium sulphate, CaSO4*2H2O, and there is usually a fair amount of water in the oil. This gypsum will dry out due to the lack of water in a clarifier (no water seal, therefore no water is added into the bowl at start-up) and will subsequently create a risk of only partial removal of sludge during a sludge discharge sequence. If only a part of the sludge cake is discharged, the remainder of the sludge in the bowl may be distributed unevenly and the result will be a severe unbalance of the bowl - a heavy side unbalance. This can lead to severe damage to the separator and to injury of personnel operating the separator. As for the LOPX or S type separators, which are basically clarifiers, this risk is eliminated by the introduction of conditioning water into the bowl. N O T E Separators from before 1985 have a density limit at 985 kg/m³. This is important to know when you order fuel as otherwise the separator might not be able to handle the oil. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 54 Conditioning water is fed into the bowl prior to the opening of the oil feed valves. This water will create a thin layer in the bowl periphery and all the sludge removed from the oil will have to pass through this water layer, where it will absorb a little water and thus obtain a “soft “consistency. It will not dry out and there will be no risk of gypsum formation and heavy side unbalance. 55 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.5.2 How to find the right Gravity disc As an aid, use the nomogram to find the correct gravity disc. It can be used when the density of the oil at a temperature of 15 °C is known. However, note that the nomogram is purely theoretical. In practical operation, practice the following general rule: • Fit a gravity disc one size larger than the recommended in the nomogram. • Run the separator. • Observe if oil flows through the water outlet. • If Yes, stop the separator and fit the gravity disc next size down. • If No, stop the separator and fit the gravity disc next size up. Repeat steps above until you find the gravity disc with the largest hole diameter without breaking the water seal. Now you have the right gravity disc for this oil. Fig 2.9 Nomogram 0,75 0,80 0,85 0,90 0,95 1,00 0 1 2 3 4 5 6 Max. 45,8 48,7 47,3 50,1 51,5 53,3 55,5 58,2 61,6 65,8 70,9 77 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 212 C o o F 800 850 900 950 991 1000 I II kg/m 3 20 30 40 50 60 70 80 90 100 Q m 3 /h O (mm) T D I 0 0 1 0 6 5 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 56 2.6 Operation on off-spec fuel. 2.6.1 Normal operation The illustration shows the normal way to run conventional separators. If we follow the oil flow in this system (in blue) we can see that the oil first enters the purifier, then the clarifier, then is led to the service tank. Operating in this way means the same oil is going through two separators before going to the service tank. Both separators are discharging meaning we have higher sludge production per litre oil cleaned through separators working in series than working in parallel. We will also consume more electricity and thereby more fuel via aux engines hence more expensive operation. It also means more labour and more down time on the system, since you cannot run one of the separators and stop the other. 57 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.6.2 High density oil If we have to treat oil with higher density than the separator limit, we can use the separators as clarifiers in parallel. This means changing the gravity disc to clarifier disc and splitting the flow through two separators. This can only be done if the water level in the oil is very low, since the clarifier has low water handling capacity and only removes water through discharge. The discharge time has to be set to 10 min to make sure that any water in the bowl is removed. CLARIFIER CLARIFIER 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 58 2.6.3 High catfines oil If we have HFO with high contamination of cat fines we can set up the separators to maximise the efficiency of the separation. We split the flow through two separators to reduce the flow and increase the time for the oil in the separators. We should reduce the flow to consumption and if the oil is heavily contaminated maybe less. The first thing we should do is to make sure that the separator bowl is clean so that the separating conditions are as good as possible. Make sure that the gravity disc installed is the correct one for the oil so that the interface is as far away from the disc stack as possible. PURIFIER PURIFIER 59 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.7 Alcap technology 2.7.1 ALCAP separators The ALCAP separator is much less labour intense than the conventional separators. There is no gravity disc and thereby no water seal to consider. This makes the separator much less dependent on oil and temperature changes. There is no need to check the separator after changing bunker tanks in regards to oil density. The temperature is still important due to the impact on the separation efficiency. In an ALCAP system one of the benefits is the continuous monitoring of the cleaned oil. This means that changes in the oil/water content are detected in the system, and when the alarm is given there is most likely something wrong in the system. We have found many times that the alarms are reset without any action from the operator and the same alarm repeats itself. When there is an alarm, make sure to check what the alarm means and act accordingly. It is important that the operators are educated to a level where they are familiar with the separator and system functions and limitations. The most important issue is still to keep the bowl clean at all times, especially the disc stack. The intervals between cleaning can vary from fuel batch to fuel batch and will vary from installation to installation, see recommendations in the service manual. There is great variation in the oil and some fuels can be almost impossible to treat, see DNVPS documentation. On the LOPX and FOPX separators we have what is known as partial discharge. We only discharge a small part of the bowl content - the sludge area, with the flow on. On the S-type separators we discharge the whole bowl content with the flow off, after replacing the oil with water. This keeps the disc stack cleaner. There are many advantages with the ALCAP system and our recommendation is to upgrade to the ALCAP system to secure the best possible oil quality and the lowest possible labour. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 60 2.7.2 Operating principle In 1983, Alfa Laval introduced the ALCAP technology in response to the need for a reliable method to separate impurities from the heaviest fuel oils with densities above the maximum density limit for fuel oil handled by conventional purifiers. Conventional purifiers handle fuel oil densities of up to 991 kg/m 3 at 15 °C and require manual adjustment of the gravity disc to achieve optimum separation results. Computer-driven Alcap separators handle fuel oil densities of up to 1010 kg/m3 at 15 °C, automatically adjusting operation to the nature of the oil. This section describes the fuel and lubricating oil cleaning process using ALCAP separators. Dirty, pre-heated oil is continuously fed to the ALCAP separator, which essentially operates as a clarifier, with the ability to remove water. Clean oil is continuously discharged from the clean oil outlet. Separated sludge and water accumulate at the periphery of the bowl. When separated water approaches the disc stack, traces of water start to escape with the cleaned oil. This minor increase in water content of the cleaned oil is detected by the transducer WT 200, which is installed in the clean oil outlet. Increased water content in the cleaned oil is a sign of reduced separation efficiency not only of water, but of solid particles too. The transducer continuously measures changes in water content. No absolute values of water content or volume are involved. The transducer measures the deviation from a non-calibrated reference value and transmits a signal to the EPC process controller for interpretation. Measurements that fall within the permissible deviation values are known as the trigger range. The EPC process controller stores a new reference value after the transducer stabilizing time that follows every sludge discharge sequence has elapsed. 61 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION During the reference time the best possible separation result is obtained. This gives the separator time to establish the interface between oil and water. At the trigger point, which is when the water content in cleaned oil reaches its maximum allowable deviation of approximately 0.2 percent in water content, the EPC process controller initiates an automatic discharge of the water that has accumulated in the separator bowl. Depending on the amount of water in the oil, water is discharged either through the water drain valve, or with the sludge through the sludge ports at the periphery of the bowl. 2.7.3 Application The WT 200/MT 50 water transducer is used to monitor the water content in the processed oil leaving a separator. The signal from the water transducer is processed in the EPC program unit, and appropriate action is initiated, depending on the status in the separation system. 2.7.4 Design The water transducer consists of a housing (2), a concentric electrode (4), and an electrical connection box fitted to the housing. The box contains a test circuit board (1) and connections. The electrode is insulated (3) from the housing and forms a circular capacitor. The transducer is mounted in the oil pipe by flanges on the outer pipe, and the full oil flow passes through the capacitor. Fig 2.10 Water transducer 1 Test circuit board 2 Transducer housing 3 Insulators 4 Electrode I 0 0 1 0 5 0 D 1 4 3 3 2 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 62 2.7.5 Working Principle The EPC supplies direct current (DC) to the transducer. The oscillator converts the DC to a high frequency alternating current (AC) which is fed to the capacitor. Changes of capacitance are detected and continuously transmitted to and interpreted by the EPC. The capacitance varies with the dielectric constant of the liquid flowing through it. As the water content in the oil increases, so does the dielectric constant, and consequently its capacitance. There is a large difference between the dielectric constant of water and oil. Hence fluctuations in dielectric constant is a very sensitive measure of changes in water content. Both free and emulsified water contamination are measured. Dielectric constant (Approximate values) Mineral oil: 2 – 6 Water: 90 – 95 63 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.7.6 Transducer WT 200 operating principle in EPC 400 Fuel oil system. Dirty oil is continuously fed to the FOPX/MFPX separator. The flow of oil is not interrupted when sludge and/or water is discharged. The FOPX/MFPX separator basically operates as a clarifier. Clean oil is continuously discharged from the clean oil outlet. Separated sludge and water accumulate at the periphery of the bowl. When separated water approaches the disc stack, some droplets of water start to escape with the cleaned oil. The small increase of the water content in the cleaned oil is then immediately sensed by the water transducer, installed in the clean oil outlet. Increased water content in the cleaned oil is the significant sign of reduced separation efficiency of not only water but solid particles too. The signal from the water transducer is continuously transmitted to and interpreted by the EPC-400 control unit. Note that only changes in water content are measured, no absolute values of water content are involved. It is the deviation of the water transducer signal from a non-calibrated reference value that is measured. The allowed deviation range is the trigger range. A new reference value is stored by the EPC-400 control unit during the reference time that follows every sludge discharge sequence, see Fig. 2.11. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 64 During the reference time the best possible separation result is obtained. The trigger point is reached when the water transducer signal has reached its maximum allowable deviation. This is when the water content deviation equals about 0.2%. When the water content in the cleaned oil reaches the “trigger point”, 100% on the Trigger range, the EPC-400 unit will initiate an automatic discharge of the water that has accumulated in the FOPX/MFPX bowl. The water is discharged in one of two ways: • with the sludge through the sludge ports at the periphery of the bowl • or through the water drain valve When the separated water approaches the disc stack within a preset minimum time between sludge discharges set to 10 minutes, the water transducer signal triggers the EPC-400 unit to open the water drain valve, V5, to drain accumulated water from the FOPX/MFPX bowl, see Fig.2.12. Fig 2.11 EPC 400 trigger range 100% Separation period Ref. time Ref. Time D i s c h a r g e s e q u e n c e . F e e d O n Transducer value 0,2% increace in dialectic value Trigger range M i n S e p a r a t i o n p e r i o d . M i n S e p a r a t i o n p e r i o d . Separation period I 0 0 1 0 6 1 65 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION This may occur several times within the minimum time between sludge discharges, see Fig. 2.3 When the separated water reaches the disc stack after this minimum time has elapsed, the water transducer signal triggers the EPC-400 unit to initiate a sludge discharge sequence, thus discharging the accumulated water together with the sludge and solid particles. See Fig. 2.14. Fig 2.12 Trigger during pre-set separation period Fig 2.13 Several triggering during pre-set separation period V5 closing 100% Ref. time Separation period Min Sep. period Stored reference Trigger point V5 opening P 0 0 1 0 6 0 A V5 closing 100% Ref. time Separation period Min Sep. period Trigger point V5 opening Stored reference I 0 0 1 0 6 0 B 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 66 If the water content of the dirty oil is extremely low, the separated water will not reach the disc stack within the preset maximum time between sludge discharges. A sludge discharge sequence will therefore be initiated by a timer in the EPC- 400 unit to prevent excessive sludge build-up in the FOPX/MFPX bowl, see Fig. 2.11. In the case of very low free water content in the dirty oil, water addition to the FOPX/MFPX bowl takes place automatically. Bunkered oils normally contain sufficient free water to displace the oil prior to sludge discharge. Water addition is therefore only necessary when treating dry oils to minimize oil losses and benefit sludge condition. The EPC-400 software decides whether or not the oil feed contains sufficient separable water to fill the sludge space in the separator bowl. When insufficient separable water is present, water addition is performed. In the case of Fig. 2.11, the trigger point is never reached within the preset maximum time between sludge discharges. This tells the EPC-400 that the oil does not contain any separable water. Fig 2.14 Trigger in adjustable period 100% Ref. time Trigger point Separation period Min Separation Sludge discharge P 0 0 1 0 6 0 C 67 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION At the time when the sludge discharge sequence starts, some displacement water is therefore added to bring the transducer value up to the trigger point, thus triggering a sludge discharge. Conditioning water is then added to condition the sludge that will accumulate. With oils containing more water, as in Figs. 2.12 and 2.13, the fact that the water drain valve has operated at least once within the minimum time between sludge discharges is stored in the EPC-400 memory. Therefore no conditioning water need be added after the next sludge discharge as there is enough water in the oil for the purpose. Also, when a sludge discharge occurs initiated by the water transducer value reaching the trigger point, no displacement water is needed. The dielectric constant of oil contaminated with water increases when the water content of the oil increases and vice versa. Thus, change in dielectric constant of the cleaned oil is a very sensitive and convenient measure of change in water content of the oil. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 68 Lube oil system. Dirty oil is continuously fed to the LOPX separator. The flow of oil is not interrupted when sludge and/or water is discharged. Clean oil is continuously discharged from the clean oil outlet. Separated sludge and water accumulate at the periphery of the bowl. When the separated water approaches the disc stack, some droplets of water start to escape with the cleaned oil. This small increase of water content in the cleaned oil is immediately sensed by the water transducer, installed in the clean oil outlet. Increased water content in the cleaned oil is the most significant sign of reduced separation efficiency and that insufficiently cleaned oil leaves the separator. The signal from the water transducer is continuously transmitted to the EPC-400 control unit and interpreted as an amount of water in the lube oil sludge. A reference value from this signal is stored in the EPC-400 after each discharge. Normal condition, low water content in the oil A new reference value is stored by the EPC-400 control unit during the reference time that follows every sludge discharge sequence, see fig. 2.11. During this time the best possible separation result is obtained, because the interface is correctly placed outside the disc stack. The signal condition from the water transducer illustrated below reflects a normal condition, i.e. no water contamination is present in the oil. The max. time between sludge discharges is set in the EPC-400 control unit as a timer function. 69 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Once conditioning water is added to the separator bowl the reference value is stored. Conditioning/ displacement water is added in a number of pulses. There will always be water in the bowl to soften the sludge in order to accomplish a good discharge. When the max. time between sludge discharges has elapsed, displacement water is added to the separator bowl prior to the sludge discharge. Initially, water is continuously admitted. Then, a number of short pulses add a minor amount of water, to eliminate the risk of water escaping with the cleaned oil. The interface is pushed towards but never into the disc stack. This means that there is no risk that water contaminated oil is leaving the separator. The sludge discharge is initiated after a predetermined number of pulses have elapsed. After the sludge discharge a new cycle starts with addition of conditioning water and storing of a new reference value. Fig 2.15 Displacement water 100% Separation period Ref. time Ref. time Min separation period Transducer value 0,2% increace in dialectic Trigger range M i n S e p a r a t i o n p e r i o d . Discharge 30% increace in trigger range D i s c h a r g e s e q u e n c e . F e e d O n I 0 0 1 0 6 4 A 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 70 High water content in the oil When a certain amount of water is present in the lube oil (~0.5%) the separated water will fill out the sludge space of the bowl before the “max time” has elapsed, see fig. 2.16. Water escaping with the cleaned oil is immediately detected by the water transducer, and a sludge discharge is initiated. No displacement water is added in this case. After the sludge discharge a new cycle starts with addition of conditioning water and storing of a new reference value. Should five (5) consecutive sludge discharges be initiated before the “max. time” has elapsed, the alarm function in the EPC-400 for high water content is activated. The alarm indicates that a leakage has occurred somewhere in the lube oil system. Fig 2.16 Conditioning water 100% Ref. time Trigger point Separation period Min Sep Period Cond. Water Ref. time D i s c h a r g e s e q u e n c e . F e e d O n Cond. Water I 0 0 1 0 6 4 B 71 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Setting for separation of high water content in oil When the water content is high, the separator system must be set for maximum water separation. It is possible to change a parameter to set the separator in emergency mode. There will be an indication on the control unit showing that the system is set for separation of high water content in the oil without the water transducer operating. Sludge discharges are then set to 10 or 15 min, depending on EPC version. No displacement water is added which can result in oil losses. Emergency mode is only recommended if really necessary and the system should be turned back to normal operation as soon as possible. Adaptive addition of displacement water The LOPX separation system continuously monitors the separator efficiency, the cleaned lube oil, and the oil system with respect to water content. The objectives are no oil losses and no water in the cleaned oil. An optimum separation efficiency is achieved by positioning the interface outside the disc stack. To avoid oil losses, the interface should be pushed close to but never into the disc stack by the displacement water before a sludge discharge is initiated. If the interface is pushed into the disc stack, water will escape with the cleaned oil. The adaptive function makes it possible for the system to adapt itself to changing working conditions, such as increased content of water in the system, scaling in the displacement water line etc. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 72 Decreasing the amount of displacement water Water to the bowl is added in pulses intermittently. The time between pulses corresponds to the response time of the system. Displacement water is added to the bowl in one continuous long pulse and then followed by a few shorter pulses. The amount of water added is stored in the memory of the EPC-400. Should a sludge discharge be triggered by the transducer value during addition of displacement water, too much water has been added. The amount of displacement water will then be reduced at the next discharge. Increasing the amount of displacement water Every fourth (4th) discharge the amount of displacement water is checked to ensure that an accurate amount of water is added. A minor amount of additional water is then added with the displacement water. If the transducer responds by increasing the transducer value it is confirmed that the amount of displacement water is sufficient. Should the transducer not respond to the additional amount of water the displacement water is insufficient and increased at the following discharge. This procedure is repeated a number of times and if no response is obtained the alarm function for insufficient displacement water is activated. This alarm is an indication of blocked or stopped water supply, blocked bowl etc. Reduced sludge production/water consumption In the LOPX separation system only the exact amount of displacement water is added. In a traditional purifier an excess must be used because there is no way of controlling the exact interface position. This means that a certain safety margin has to be used. The water consumption for the LOPX separator is therefore 20 – 30% less than for a traditional purifier. 73 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION 2.7.7 Transducer MT 50 operating principle in EPC 50 The operating principle for the newer MT 50 water transducer is the same as for the WT 200. We get the same function and signals from the transducer, see chapter 2.6.6. The EPC 50 will react in a different way to the signals from the transducer and that is what we will look into in this chapter. There are some major changes in the programme for the EPC 50. There is now only one separation period, (not a min. separation, 10 minutes, and an adjustable separation period as we had on the old type), see fig 2.17. We can also see that the feed to the separator is off during discharge on the new S separator. See separator manual for more info regarding discharge sequence. The trigger value is the increase of the stored transducer value that is needed for the EPC to initiate an action - draining or discharge. This increase is measured in % from 0 to 100. Fig 2.17 EPC 50 separation period 70% 100% Separation period Reference time Reference time D i s c h a r g e s e q u e n c e . F e e d o f f Separation period Transducer value 3pF increase in HFO mode 0,6 pF increase in LO mode Trigger range I 0 0 1 0 6 6 H 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 74 The EPC stores a transducer value after the reference time. The reference time allows the oil and water to settle down and establish an interface after a discharge. If the oil contains very little water there will be little or no change in the trigger value during the separation period, and no action initiated from the EPC 50 until the end of the separation period, when it will initiate a discharge. 75 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION Medium free water content in feed When the interface between oil and water approaches the disc stack, some small drops of water will escape with the clean oil, (see illustration Fig 2.19), and the trigger value will increase. Fig 2.18 Draining under 70% When the trigger value has reached the 100% limit, the EPC 50 signals the drain valve V5 in the water outlet to open. In the EPC 50 system we have two limits controlling the V5. The 100% limit that triggers the action, and a 70% limit that is used during draining. If the trigger value decreases from 100% to under 70% the valve closes again and the process continues as normal, see fig 2.18. When the trigger value is under 70% after a drain the process will continue and there will be no alarm from the control unit. Fig 2.19 water drops in clean oil V5 closing 100% 70% Separation period Ref. time D i s c h a r g e s e q u e n c e . F e e d o f f V5 opening Reference value 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 76 High free water content in feed When excessive amounts of water are present in the feed and if the water drain valve activation does not provide sufficient drainage, the EPC 50 process controller automatically initiates a sludge discharge. If there is much water in the oil, the draining action may not bring the value down under 70%. In this case the valve will stay open for a preset time. If there are 5 consecutive drainings without reaching the 70% limit, the EPC will initiate a discharge. If there are 2 x 5 drainings without reaching under the 70% limit, the EPC will initiate a discharge and give an alarm "Water drain insufficient". If the separator is part of a lubricating oil system, the alarm "LO draining frequently" will arise after only two drainings without reaching the 70% limit. This is to prevent oil losses and to inform there is water in the oil. When the alarm “Water drain insufficient" shows, some additional steps have to be made in order to reduce the amount of water. Fig 2.20 Five consecutive drains 100% 70% Separation period Ref. time D i s c h a r g e s e q u e n c e . F e e d o f f V5 opening Reference value 77 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 2 SEPARATION If the separator is part of a fuel system, the settling tank should be checked to see if there is accumulated free water in the tank. If not check if there is emulation in the tank. Check if there is a leaking valve on the water valve block, on the separator, or if there is a leaking heater in the steam system. If the separator is a part of a LO system it is important to find the leak. It could be a leaking valve on the water valve block, on the separator, a leaking heater in the steam system, or a leak in the engine. If there is much water in the fuel oil it is possible to run the separator in emergency mode, but then the ALCAP system is by-passed. This means that the discharge time is reduced to 15 min. and there will be an alarm every 24 hours signalling that the ALCAP system is disabled. This is intended for Emergency operation only and water is not added before discharge. This means that the risk of oil losses is increased. See separator manual for further information. 2 SEPARATION DNVPS / ALFA LAVAL FUEL OIL TREATMENT 78 2.7.8 Self adapting system. The EPC 50 is continuously monitoring the transducer value and compares it to the stored reference value. If the amount of water is reduced during the separation period and becomes lower than the stored value, the EPC stores the new lower value and uses that as the reference value, see fig 2.21. It is only lower values that will be stored, not higher values than the stored value. This means that the system operates with the most favourable value at all times and secures a high quality final product. Fig 2.21 Self adapting reference value 70% 100% Separation period Ref. time D i s c h a r g e s e q u e n c e . F e e d Reference value New lower ref. value 79 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM 3 Booster system 3.1 Purpose of the system To provide the diesel engine with fuel with the correct viscosity and flow stipulated by the engine manufacturer. All low speed engines and most modern medium speed engines operate today on heavy fuel oil. To ensure proper treatment and a good combustion, a booster system is needed. The principal purpose of these systems is to ensure proper conditioning of the heavy fuel oil fed from the daily service tank to the diesel engines. The system ensures that the correct flow, pressure and viscosity of the heavy fuel oil is maintained. The booster system is situated between the daily service tanks and the diesel engines 3.2 Applications Booster systems are primarily suitable for ships or power stations using HFO diesel engines. There are generally two separate conditioning systems for shipboard applications where HFO engines are in use, as today, heavy fuel is commonly used for the auxiliaries (Unifuel system) as well as for the main engines. For safety reasons, there is usually one independent booster system for the main engines and one for the auxiliaries. 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 80 3.3 Atmospheric and pressurized 3.3.1 Introduction Most of the booster systems built before 1984 are atmospheric fuel systems. An atmospheric fuel system is a system where the hot excess fuel flow from the engine is led back to a mixing pipe (stand-pipe) which has an atmospheric pressure. This system can in some cases cause operating problems in situations where the engines can accept high injection temperatures, i.e. high viscosity fuels. The system is not recommended for fuels above 120 cSt/50 °C (injection temperatures above +110 °C). The reason for this is that in the atmospheric system, the pressure drop over the pressure regulating valve will be too high. The pressure and temperature needed for HFO over 120 cSt can be up to 150 °C and the pressure in the circulating side will be over 4 bar, while the pressure after the regulating valve will be atmospheric. The result is that all the water and light products in the HFO will instantly boil and turn to gas. This could make the oil start to foam and the mixing pipe and the vent pipe can fill up with this foam. The danger is that the pumps start cavitating and increase the danger of emulsified oil. The foam could also escape through the venting pipe and cause contamination of the vessel and water. All ships designed to operate on HFO above 120 cSt/50 °C should have pressurized systems (a fuel system specified by all engine builders). Modern high viscosity fuels require high injection temperatures. To prevent excessive and harmful vaporization of the light fractions and possible remaining water in the fuel, sufficient pressure has to be maintained. This is done by installing a pressurized booster system. 81 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM 3.4 An atmospheric system System description with possible problems and solutions. The atmospheric booster system is usually not built in a compact unit like the cleaning system. The components are spread around the engine room. In many cases, it can be difficult to get a good overall understanding of the complete system. Service tank The clean heavy fuel oil is pumped into the daily service tank. The temperature in the tank is between 70 - 90 °C. The daily service tank is usually situated high up in the engine room. From the daily service tank the oil flows by gravity into the mixing pipe. See chapter 1.4 for more info. Fig. 3.1 Atmospheric booster system I 0 0 1 0 0 5 8 B 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 82 Mixing pipe The purpose of the mixing pipe (called deaerator in a pressurized system) is to mix the “cold” fuel from the daily service tank with the excess hot fuel which comes from the diesel engines. The pipe is manufactured and installed by the shipyard. The mixing pipe is made of steel, insulated and often heat traced with a diameter of 300 - 400 mm and a height of 10 - 15 meter. The mixing pipe should be installed as high up as possible near the daily service tank to ensure a static positive suction head for the circulating pumps (booster pumps) to prevent cavitation and vibration. In certain vessels such as ferries, supply boats etc. the mixing pipe cannot be installed in a high position in the engine room. This is to prevent vaporizing and foaming of the hot fuel. Flow meter A flow meter is installed in the system to measure the fuel consumption of the engines. The flow meter is normally installed between the daily service tank and the mixing pipe. Low mixing pipe height can cause functional problems of the flow meter. Circulating pumps (booster pumps) Circulating pumps feed the engine with HFO at the required flow rate and pressure. Pump capacities are always multiples of the recommended maximum fuel consumption rates to ensure ample filling of the injection pumps. The pressure required in the circulating system is specified by the engine builders. This pressure is controlled by a regulating valve mounted on the engine, or directly after the engine in the return line to the mixing pipe. In atmospheric booster systems, the pumps are located on the first level in the engine room to ensure a high static pressure. 83 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM Prior to injection into the diesel engine, the temperature is increased to reach the correct injection viscosity. The recommended injection viscosity is 14 cSt for trunk piston engines and 18 cSt for crosshead engines. For heavy fuel oil, 700 cSt/50 °C, this viscosity corresponds to a temperature of about 140 to 155 °C. The limit of FO viscosity to be used in an atmospheric system is max 120 cSt/50°C. Since booster heating is recognized as an essential service, a 100-percent stand-by heater is normally installed. The two heaters could be steam heaters, electric heaters, or a combination of both. The steam heater, most common in larger vessels, used to be a shell and tube type heater. A new type of steam heater, Alfa Laval HEATPAC system is now available for booster heating. If the heater does not have the right capacity, the following can be the cause: • Some of the pipes inside the heater are plugged (tube heaters). • The steam has a lower pressure/temperature than calculated (7 bar saturated +164 °C is a common value). One solution is to operate both heaters in series to reach the correct injection temperature. For smaller vessels that cannot produce sufficient steam, one possible solution is to operate in series a steam heater which takes the basic load and an electric heater which can be used during peak loads. In ships with rapid load variations on the engines, with subsequent fluctuations of the injection temperature, it is very important to have heaters with a sensitive and fast control system, e.g. the Alfa Laval HEATPAC system. 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 84 Viscosity sensor One of the important components in the temperature control system is the viscosity sensor which measures and controls the injection viscosity of the fuel oil. The viscosity must be held at a specific level by means of the steam and/or electrical heaters. There are many different viscosity sensor suppliers on the market. One of the most common measuring principles of the various sensor types is to measure the differential pressure across a capillary tube. Others are of shear force measurement type or have different vibrating sensors in the fuel. The most modern systems are electronic with few mechanical parts. An example of such a system is the Alfa Laval designed advanced viscosity control system under the name VISCOCHIEF. The system is designed to ensure accurate automatic monitoring and control of the injection viscosity of fuel oils. To check if a viscosity sensor is working properly, read the actual temperature and compare the value with the bunker specification to get the right injection viscosity. 85 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM Automatic filter An automatic filter and a manual by-pass filter are positioned at the end of the system to protect the engine’s injection equipment from possible harmful particles. The filter screen mesh size is determined by the requirements of the diesel engine builders. Some also recommend installing an automatic filter on the “cold side” of the booster system, i.e. before the mixing pipe. Possible problems with conventional types of automatic filters are: • Pressure drop in the fuel supply when the filter is flushed. • Broken candles (particles pass into the fuel injection equipment). • Manual cleaning due to particles sticking on the screen (intervals between flushing too long). • Problems with the filter ancillary equipment. (Air pipes, electric motor etc.). A good way of preventing the above problems is to install an Alfa Laval Protector filter, Moatti. 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 86 3.5 A pressurized booster system Introduction As described earlier in this chapter, a pressurized booster system is essential if the injection temperature is above approx. 110 °C. The difference between an atmospheric and a pressurized system is that the mixing pipe (normally called a deaerator in a pressurized system), is much smaller (approx. 65 litres) and it is kept under a pressure of about 4 bar by two supply pumps. A pressurized system is required to eliminate gasification and vibration problems caused by high injection temperatures (110 - 160 °C). The booster module is normally a complete module for supplying HFO to the engine. The module consists of the main components in fig. 6.3.11 plus pipes, valves, electric cabling and accessories, all mounted on a common frame. All oil pipes are insulated and heat traced. Fig. 3.2 Pressurized booster module I 0 0 1 0 0 5 8 A 87 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM Alfa Laval standard booster modules are identified by their pump capacities. IMO pumps are used, and where size ACE 038K pumps are used as circulating pumps and ACE 032N as supply pumps the denomination will be SBM 38K-32N SS where SS stands for steam heaters. To size a new booster module, the following data must be available: - Main engine type - MCR - Maximum viscosity of fuel oil - Required injection viscosity - Fuel consumption - Fuel flow at engine - Fuel pressure at engine - Filter finess absolute required - Electrical power - Steam supply pressure and temperature If you are going to upgrade part of the system, the following points must be taken into account: Filter upgrading With a given circulation pump capacity, the flow to the engine will be reduced with the filter back- flush, normally 15% of the nominal flow. Alfa Laval ‘Protector’ automatic filters are delivered with electric motor. An electric motor is recommended at temperatures above 120 °C. A duplex filter is used, type auto/manual but also auto/auto is sometimes preferred. To design the filter you must know the pump flow and pressure, minimum flow to engine required, pressure in return line from engine or in deaerator tank (filter back-flush connection) and required filter mesh. The filter can be installed in different places. Hot side: after the heaters and viscosity control system. Cold side: after the supply pumps but before the flow meter and deaerator tank 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 88 Heater The capacity of the heater shall normally have a margin of 15%. The end heaters must be calculated on the average temperature in the deaerator pipe which normally will be around 100 - 115 °C and at an end temperature of 140 °C. A system that allows both parallel and series operation of the heaters is recommended. We always recommend a steam trap of float type for best results. Viscosity control A modern diesel engine is designed for efficient combustion and optimum power output when burning fuels ranging from 30 cSt/50 °C to 700 cSt/50 °C, provided that the fuel is properly treated and conditioned. The fuel injected into the engine must meet specific pressure, temperature, flow and viscosity requirements. Efficient fuel combustion can only be achieved if fuel temperature and viscosity can be controlled within recommended limits Injection of fuel into a diesel engine at the incorrect viscosity can have a number of adverse effects on the engine and its performance. A high injection viscosity fuel, in excess of 20 cSt, can cause poor combustion in the cylinder of the engine, contributing to a build up of deposits on exhaust valves and piston heads and an increase in lube-oil contamination. Another problem that can have very serious consequences is the effect of large and rapid changes in fuel oil temperature on the injection pumps. Without accurate viscosity measurement linked to temperature control, the heaters may fail to react properly to changes in fuel properties and engine load. This could result in a seizure of the pump cylinder liner and the plunger due to the varying thermal expansion properties of the liner and the plunger. Engine manufacturers have improved engine designs to cope with this problem by, for example, developing new fuel injection pumps and nozzles to compensate for viscosity fluctuations. However, accurate viscosity control is the recommended solution. 89 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM Viscosity sensor On a modern FCM system the EPC will control the viscosity with an EVT-20 system consisting of a viscosity censor, 2 x Pt 100 temperature sensor and a heater board to control the temperature. This compact, lightweight sensor accurately measures actual fuel oil viscosity within ±0.5 cSt to ensure that the correct injection viscosity is obtained. The sensor does not have any moving parts. This provides maintenance-free operation, which guarantees long life, maximum up-time, minimal, if any, servicing costs and efficient engine operation. Using the torsion vibration measuring principal, this factory calibrated sensor is not affected by variations in flow or pressure. A vibrating pendulum with rotational movement collects data and the sensor sends the data to the process controller. If required, the process controller then raises or lowers fuel oil temperature in the heaters to obtain the required viscosity. This ensures efficient fuel combustion and optimum power output. Controlling viscosity is critical to engine performance. Automation ensures that the actual fuel oil viscosity meets the set point specified by the engine manufacturer. Deviations are corrected by the process controller, which automatically raises or lowers temperature in the heater. This ensures efficient fuel combustion and optimum power output. Operation in diesel oil mode or heavy fuel oil mode. The process controller has two modes of operation, DO mode or HFO mode, with two sets of parameter settings, alarm limits, and other values that are suitable for each mode. The process controller handles the transition between the two modes via temperature ramps to control the change in temperature and to prevent temperature shock from damaging the engine’s fuel injection pumps. 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 90 Automation The new, fully automated Alfa Laval Fuel Conditioning Module (FCM) brings modern state-of-the-art information technology to the fuel conditioning process. Computer-based automation helps monitor and control FCM functions with high precision, reliability and performance. 3.5.1 A standard booster module : • 3-way change-over valve DO/HFO with electric actuator and limit switches for remote operation. • Two supply pumps with shut off valves and non return valves at each pump. • 500 micron suction strainer. • A pressure switch connected for stand-by pump control. • A pressure control valve for adjusting supply pump pressure. • Positive displacement van flow meter equipped with totalizer for local monitoring of fuel oil consumption. For automatic by- passing of the flow meter, there is a spring loaded non return valve which opens automatically should the differential pressure across the flow meter be too high. • A deaerator arrangement including a pipe of approx. 65 litres with level switch, safety valve, automatic deaeration system and a manual dearation valve. • Two circulation pumps with shut off valves and non return valves at each pump. Pump flow is normally 3 - 4 times the maximum fuel consumption and the pressure could be up to 10 - 12 bar. This is to make sure that no air and water vapour are present in the fuel and to guarantee that the fuel injection pumps are supplied with enough fuel. 91 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM • A pressure switch is connected for stand-by pump control. • Two heaters, steam or electric. Steam heaters come with safety valves, steam trap and drain valve. • One steam control valve type SRV with electric actuator connected to the viscosity control panel if steam/ thermal oil is requested. • Viscosity control system type Viscochief on old systems and EPC monitor. • One automatic/manual duplex filter type. Continuous backflushing and included differential pressure alarm. Control panel if equipped with electric motor. Filter finess must be decided. • A pneumatic operated fuel supply pump may be required for emergency use, by-passing the whole system. • Alarm panel with one or two outgoing common alarms available for ECR central alarm panel. 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 92 The following alarms are included: Process alarms: – High temperature – Low temperature – High viscosity – Low viscosity System alarms: – power alarm – communication failure – input signal failure – temperature/flow switch – computer failure – steam valve failure Following alarms are given to the alarm panel: – Low pump pressure, stand-by pump started – Low dearator tank level – High difference in pressure over automatic filter – Starter alarm for all pumps (and filter if applicable). Each pump has an independent starter activated by low fuel oil pressure. All modules are supplied insulated and heat traced as well as tested and certified to required class. 93 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM 3.6 Viscosity of heavy fuel oil A modern diesel engine is designed for efficient combustion and optimum power output when burning fuels ranging from 30 cSt/50 °C to 700 cSt/ 50 °C, provided that the fuel is properly treated and conditioned. The fuel injected into the engine must meet specific pressure, temperature, flow and viscosity requirements. Efficient fuel combustion can only be achieved if fuel temperature and viscosity can be controlled within recommended limits. Fig.3.3 Viscosity chart 3 BOOSTER SYSTEM DNVPS / ALFA LAVAL FUEL OIL TREATMENT 94 3.6.1 Viscosity and its effects on diesel engines Injection of fuel into a diesel engine at the incorrect viscosity can have a number of adverse effects on the engine and its performance. A high injection viscosity fuel, in excess of 20 cSt, can cause poor combustion in the cylinder of the engine, contributing to a build up of deposits on exhaust valves and piston heads and an increase in lube-oil contamination. If the viscosity gets too high it can cause serious damage to the cam shaft. Another problem that can have very serious consequences on the injection pumps is the effect of large and rapid changes in fuel oil temperature. If the viscosity gets too low it can cause coke build-up on fuel injectors and bad lubrication of the injectors, with risk for clogging of the injectors. Without accurate viscosity measurement linked to temperature control, the heaters may fail to react properly to changes in fuel properties and engine load. This could result in a seizure of the pump cylinder liner and the plunger due to the varying thermal expansion properties of the liner and the plunger. Engine manufacturers have improved engine designs to cope with this problem by, for example, developing new fuel injection pumps and nozzles to compensate for viscosity fluctuations. Accurate viscosity control is however the recommended solution.The only way to be sure to achieve this is to always use viscosity control and not temperature control. 95 DNVPS / ALFA LAVAL FUEL OIL TREATMENT 3 BOOSTER SYSTEM