Gemilerde Artık Isı Sistemlerinin Optimum İşletilmesi

Abstract

Gemilerde büyük miktarlarda artık ısı enerjisi oluştuğundan modern gemilerde enerji optimizasyonunu amaçlayan sistemler çok yaygın olarak kullanılmaktadır. Kurulan artık ısı sistemleri egzost gazları ile yaklaşık %35'i ve silindir soğutma suyu ile yaklaşık %20'si atılan ısı enerjisinin %35-50 kadarı geri kazanılabilmektedir. Bu nedenle bu sistemlerin hedeflenen yakıt tasarrufunu sağlayabilmesi için işletme optimizasyonu büyük önem arzetmektedir. Gemilerde genellikle bir yada birkaçının birarada kullanıldığı bu sistemlerin yaygın ve enerjinin geri kazanılmasında büyük paya sahip olanları şu başlıklar altında toplanabilir: 1. Turboblöverli aşırıdoldurma sistemi 2. Evaporeyterli tatlısu üretme sistemi 3. Yardımcı kazan egzostundan üretilen inertgaz sistemi 4. Egzost gaz kazanlı buhar üretme sistemi işletme optimizasyonunun sağlanabilmesi için bu sistemlerin tasarlanan çalışma şartlarına yakın şartlarda tutulması, muayene, test, çalıştırma, kapatma, temizlik, bakım ve onarımlarının prosedürüne uygun ve bilinçli bir şekilde yapılması ile mümkün olur. Aksi taktirde, bu sistemler tasarlandığı gibi optimum kapasitede çalıştırılamadığı gibi hatta kapasitesinin çok altında çalıştırılırlar. Bu yüzden bu sistemlerin pratik olarak iyi tanınmış olması ve çalışma prensiplerinin teorik olarak iyi bilinmesi ve dolayısıyla işletme koşullarının bilinçli oluşturulması şarttır. Ancak bu şekilde sistemlerin tasarlanan optimizasyon beklentilerine yaklaşılabilir. Enerji optimizasyonu için kurulan sistemlerin, çalışma ömürlerine kıyasla ilk yatırım maliyetlerini karşıladıkları bilinmelidir. Geri ödeme süresi, yapılan iyi bir işletim ile her türlü maliyet düşürülerek, çalışma ömrü uzatılarak ve tasarlanan optimizasyona mümkün olduğu kadar yakın kalması sağlanarak kısa tutulabilir

Waste heat recovery systems are commonly used because of with marine engine the amount of the heat carried away by the exhaust gases varies between 30 to 35 per cent and by the jacket cooling water between 20 to 25 per cent of the total heat energy supplied to the engine. Recovery of some of this heat loss to the extent 35 to 55 is possible by means of waste heat recovery systems. These systems have many kind of damages in the long run because of they are exposed corrosion, dirty, mechanical difficulties, overheating, overpressure etc. Thats why performance of these systems decrease. In order to prevent to decline systems must be kept in good conditons and their inspection, testing, starting, closing, cleaning, maintenance and repair must be performed according to procedures on time. Thus the damages and the loss can be reduced to minimum. A few waste heat recovery systems which are commonly built up on board, as follows: 1 Turbocharging system, 2. Fresh water plant which is heated by main engine jacket water, 3. Inert gas sytem with auxiliary boilers, 4. Steam generated system with Exhaust gas boilers. TURBOCHARGING SYSTEM It is recommended that the compressor and turbine be cleaned with the turbocharger running. Periodic cleaning reduces or even prevents contamination, allowing significantly longer intervals between overhauls. Cleaning the Compressor The proposed cleaning method, carried out periodically, will prevent a thick layer of dirt from forming. A thick layer of dirt can cause a drop in efficiency and increase unbalance on the compressor side of the turbo- charger, which could influence the lifetime of the bearings. The cleaning interval will depend on the environmental condition and the installed air filter. The compressor whell of the turbocharger can be cleaned during operation by spraying water into the air inlet casing. The dirt layer is removed by the impact of the injected water. Since the liquid does not act as a solvent there is no need to add chemicals. The use of saltwater is not allowed, as this would cause corrosion of the aliminium compressor wheel and the engine. Water is injected from a water vessel that holds the required quantity of water. This water vessel can be either ordered together with the turbocharger or ordered separately. Procedure The best results are obtain by injecting water during full-load operation of the engine, i.e. when the turbocharger is running at fullspeed. The complete contents of the water vessel should be injected within 4 to 10 seconds. Succesful cleaning is indicated by a change in the charge air or scavenging pressure, and in most cases by a drop in the exhaust gas temperature. If cleaning has not produced the desired results, it can be repeated after 10 minutes. The interval between compressor cleanings will depend on the condition of the turbocharger suction air. It can vary from 1 to 3 days of operation. If a very thick layer has built up and it cannot be removed using the method described, it will be necessary to dismantle the turbocharger in order to clean the compressor side. Principle Since the dirt layer is removed by the kinetic energy of the water droplets, the engine has to be run at full load. Cleaning The Turbine The combustion of heavy fuel in diesel engines causes fouling of the turbine blades and nozzle ring. The result of this fouling is reduced turbine efficiency and engine performance as well as increase in the exhaust gas temperature. Experience has shown that the contamination on the turbine side can be reduced by regular cleaning in operation, and that such cleaning allows longer intervals between turbocharger overhauls. Some of the deposits have their origin in soot, molten ash, scale and unburned oil, partially burnt fuel and sodium vanadyl vanadat. Investigations has shown that most of the residues are caused by the calsium in the lube oil reacting with the sulphur from the fuel to form calsium sulphate during the combustion process. The quantity of the deposits depends on the quality of the combustion, the fuel used, and lube oil consumption. The frequency with which cleaning has to be carried out depends on the extent of the contamination on the turbine side. Cleaning Methods 1.Wet cleaning(water injection) The dirt layer on the turbine components is removed by thermal shock rather than the kinetic energy exerted by the water droplets. The exhaust gas temperature before the turbine should be in the range of 200 to 430°C. The boost pressure should be above 0.5 bar to prevent water entering the oil chamber on the turbine side. The quantity of injected water will depend on the exhaust gas temperature, water pressure, size of the turbocharger and number of gas inlets. Water should be injected for 5 to 10 minutes. Check if the water has enterad the turbine parts by opening the drain of the gas outlet casing. Water flowing out provides assurance that enough water has passed the nozzle ring and the tubine blades. The interval between turbine cleanings will depend on the combustion, the fuel used and the fuel oil consumption. It can vary from 1 to 20 days of operation. 2. Dry cleaning (solid particle injection) The layer of deposits on the turbine components is removed by the kinetic energy of the granulate causing it to act as an abrasive. Devices for both methods are usually supplied by the engine builder and are manufactured in accordance with turbocharger manufacturer's recommendations. Experience has shown a combination of the two to be very effective, especially in the case of 2-stroke engines. The exhaust gas temperature before the turbine should not exceed 580 °C. Dry cleaning has to be carried out more often than water cleaning as it is only possible to remove thin layers of deposits. A cleaning intervals of 1 to 2 days is recommended. To ensure effective mechanical cleaning, granulate dry cleaning media are best injected into the turbine at a high turbocharger speed. The quantity needed will vary from 0.21 to 31, depending on the size of the turbocharger. Experience has shown that the best results are achieved with crush nut shel or granulate. WASTE HEAT FRESH WATER PLANT Fresh water are provided with two methods at sea as follows: One of the methods is provided from shore connection. In another method, sea water are converted to fresh water by using a special arrangement. First methods is especially used on the ships in the short passage and can be used all of ships. Second method especially used on the ships in the long passages because of the water which is carried causes to reduces capacity of cargo and is economic due to be used main engine waste heat energy. Fresh water which generated is also used as feed water in boilers and others and theirs lifetime extend. Evaporators Evaporator essentially is equipment that liquids are decomposed by boiling. They are used to provide from sea water to fresh water. Two type of evaporators are used as follow: Boiling evaporator: This type generally are used as boiler for generating of steam. A boiler system was built under evaporator because of a heat source is needed for heating. Flash evaporator: System which is provided to generate the steam due to evaporation of sea water at low temperature and in a closed place and under the vacuum. Flash evaporators are generally used on board because of waste heat energy of main engine jacket water are optimized. Processing of Evaporation : In order to generate steam, sea water are boiled are under the vacuum and low temperature conditions and then it is distilled. Boiling sea water was decomposed from their salts and naturally flow into the condenser. Distilled water which was accumulated in a container and then is pumped from here to fresh water tank. In order to heat sea water under the vacuum (approimately 1 kg/cm2) in the evaporator by heat exchanger that main engine jacket water are passed through. The heat source is not affected except some fluctuation of temperature. Then vapour of sea water pass through seperator which decomposed salty water drops from vapour. Seperators consist of tubes which was drilled with distinctive bore and strung out. When vapour passed out from these orifices decomposed from salty water drops and then vapour passed into condenser which is cooled by sea water passed through. For scale fomation in long run, extreme quantity of sea water passed into the evaporator. Waste salty water (known "brain") are expelled by means of arrangement of ejector. The scale formation is very slow because of sea water boil at low temperature under the vacuum. Non-condensate gases are expelled by air ejector. Thus, vacuum arise by means of air ejector. During starting the operation all of valves must be closed and all of gauge's cocks must be opened. Sea water valves which are provided to distilled water condenser must be opened. Feed control valve is opened until the flowmeter shows required value. Air vent must be opened to expel air which remained in evaporator and then ejector pump starts. When vacuum gauge reachs to the required value, main engine jacket water are allowed to heat exchanger. In a few minutes, sea water in the evaporator start to boil. When distilled water arise to marked line on the inspection glass. Distilled water pump is started in order to pump to fresh water tank. Salinity of condensated water arise and initial water has high salinity after starting the distile pump because of remained salts on the surface of the evaporator mixe up to the vapour. Thats why the dump valve is kept in open position until fresh water get out of it. During this action, quantity of entering water is reduced. Controlling of water flow is provided by solenoid controlled two-way valve and salinometer. If water is salty, salinometer opens two-way valve for dumping to the bilge. In order to finish operation of evaporator, firstly feed water is stoped. When internal pressure of evaporator is atmospheric pressure, ejector pump is stoped. Main engine jacket water and sea water is ceased to evaporator. Finally vacuum breaker is closed. Maintenance and Protection The unity must be kept clean and dry conditions. In order to carry out enough efficiency, the following maintenances must be performed. All of pumps and motors mustn't be in overheat condition. The heat exchanger must be cleaned two or three times a year against scaling. Descaling may be performed by diluted acids, such as hydrochloride, special scale powder etc. The cell of salinometer must be dismounted one time a six mounths and cleaned inside of it. Scaling To prevent scale formation in evaporators is impossible and all of distilation units may have possible damages because of scaling. But operators may reduce damages and difficulties to minimum by applying descaling operation with regularly intervals in order to provide to operate under the most suitable conditions for special units. Normally if the sea water is heated at high temperature, scale formation is harder and less soluable. Scale formation at low temperatures is calcium carbonate and at high temperatures rate of magnesium hydroxide increases and cause deposits. If temperature is higher than 82°C, calcium sulphate scaling occurs and it cannot removed chemically. Scaling on the heat surfaces generally causes to reduce production of fresh water. Thats why scale must be remove three or four times a year. If the evaporator output reduces under 75% of initial output, descaling must be performed. Chemical Descaling Effectual scale formation is calcium carbonate or magnesium hydroxide. These can be cleaned chemically. Compounds for chemical descaling is available commercially. Most of descaling compounds are effective. If these are not available, 15-25% (by volume) of hydro cabonate solution can use. By using necessary equipments for desacling, a circulation arrangement is formed and then operation can be started. After the very short time circulation is started, effervescence starts. If operation take a long time, this shows that descaling is carried out. Effervescence starts to decrease when dissolution is very slowly or finish. In order to accelerate operation of descaling, main engine jacket water may be allowed to pass through the heat exchanger by heating the solution. Must be avoid from high temperature because of high temperature may causes damage. The most suitable temperature is between 48-60°C. INERT GAS SYSTEM WITH AUXILIARY BOILER inert gas systems are necessary to prevent flashing or fire may occur because of type of cargo which is carried on tankers and combined cargo ships, such as OBO, O/O etc. The term "inerting" is generally used for the replacement of inert gas air or cargo vapour by inert gas before loading or gas-freeing respectively, to prevent the formation of flammable mixtures, inert gas which is pumped to tank reduces concentration of hydrocarbon and oxygen in the tank. inert gas has the following general characteristics which provide the desired protection: oxygen content is less than 5% by volume, total content of S02 and S03 are less than 0.03% by volume, water vapour is less than 1% by volume, solid particles are less than 8 mg/m3. Proceeding The Gas After leaving the boiler uptake and passing through the boiler uptake valve. The hot gas before entering the base of the scrubber through a water seal. The flue gas must first be cooled and cleaned before distribution to the cargo tanks. This function is carried out by a scrubbing tower known commonly as a scrubber. This item of the system differs in construction from system to system, but in most cases the principle of operation remains the same. The function of scrubber is to cool and clean the flue gas by removing the solid particles and sulphur dioxode contents. Sea water are used for cooling. Then gas passed through demister pad. Distribution of inert gas really begins with the inert gas fans or blowers. These electric or steam turbin driven centrifugal blowers pressurise the gas before passing it through the deck water seal and into the the distribution piping (known as inert gas deck main) to the cargo tanks. Then inert gas are passed through deck water seal, non-return valve respectively. The purpose of the deck water seal is prevent feedback of hydrocarbon gases from the cargo tanks to the engine room and boiler uptake via the inert gas main. The inert gas deck main, in this case lying along the main deck, is connected by branch lines to the individual cargo tanks. In most case, each branch line is fitted with a branch line isolating valve and a pressure/vacuum valve which protects the tanks from abnormal pressure/vacuum conditions. Methods of Inerting and Purging The term "purging" is generally used for the replacement of inert gas of unaccetable quality or previous cargo vapour by nitrogen or suitable cargo vapour before loading, inerting and purging operations may take place at sea if the ship is suitable equipped, or in harbour. One of three basic methods may be adopted: 1. Displacement (stratification) : This makes use of difference in vapour densities between the gas in the tank and the inerting vapour. The lighter gas is passed into or vented from the top on the tank and havier gas is passed into or vented from the bottom. A fairly distinct layer is formed between the two gases because of density difference. Most hydrokarbons are havier than inert gas. 2. Mixing (Turbulance, or Dilution) : Large volumes of inerting gas are blown into the tanks and are mixed with the cargo vapour already there, inert gas should be blown in vigorously to reduce the possibility of isolated pockets. In this method, velocity of inert gas is higher. 3. Vacuum/pressure : A vacuum is created in the tanks using the ship's compressors. Inerting and purging gas is then admitted until at positive pressure. Maintenance and Testing of the System Safety arrangements are important components of inert gas system. Must be pay attention these arrangements during the inspection. Scrubber can be controlled by means of inspection glass. Controlling must be accomplished for rusting, corrosion, dirty, and damage on its site. Inspections are must be cotained the following parts of scrubber. Floor and wall of the scrubber, for dirty on the cooling water and spray nozzles, floats and temperature sensors and other internal components such as, demister pad and plate. If inert gas fans has damages, they are found by means of internal inspection. Inspection of the inert gas fans must be contained as follows: internal inspection of fan case for the corrossion marks and the soot deposits. Inspection of fix and mobile washing system. If it was fitted, inspection of functions of washing by fresh water. Inspection of drain lines from fan case. When fans is running, they must be oversaw for extreme vibration and non-balance. Deck water seal must be opened and internal inspection must be contained as follows: Choking of venturi tube in semi-dry type deck water seal. Corrossion of internal tube and hosepipe. Corrossion of heating coils. Rusting of the feed and drain valves and level floats. Non-return valves must be open for inspection of corrossion and condition of valve seat. During operation, testing that it accomplish its functions. Testing of other units and alarms contain as follows: Inspection and testing are cantained the following equipments and mountings. All of the safety functions and alarms. Testing of flue gas isolating valves and remote control and automatic equipments accomplish their functions, inspection for vibration level of inert gas fans, inspection for gas leakage in ships which are older than four years old. Testing of fixed and mobile oxygenmeter accomplish their functions correctly. EXHAUST GAS BOILERS Waste Heat Boilers With Diesel machinery the amount of heat carried away by the exhaust gases varies between 25 to 35 per cent of the total heat energy supplied to the engine. Recovery of some of this heat loss to the extent of 30 to 50 per cent is possible by means of an exhaust gas boiler or heater. These consist of some form of exhaust gas heat exchanger mounted in the main engine uptake. The various steam raising can be tank or water tube design, using natural or forced circulation. The latter provides many advantages in waste heat installations, one of these being that the various units can be positioned to best suit the prevailing engineroom layout without having to provision for natural circulation. Many arrangements are used, the most simple consisting of separate exhaust gas and oil fired boilers, each having its own feed connection, but each discharging into a common auxiliary steam main. This type of installation often make use of tank type boilers, and are suitable for providing fairly small amounts of low pressure saturated steam. However the output of these units is directly dependant upon the main engine output and so it is necessary to provide an additional oil fired boiler to supplement or surplant the steam produced by the exhaust gas unit when the main engine is operating at low load conditions, or when it is stopped. In many case a convenient arrangement is to use the drum of the oil fired boiler as a steam receiver for the exhaust gas exchanger. This gives the advantages that only a single steam drum with its associated mountings is required, and that the oil fired is kept in stand-by condition ready for immediate oil firing to support or replace the heat from the main engine exhaust gases. Composite boilers are often used in conjunction with Diesel machinery, since if the exhaust gas from the engine is low in temperature due to slow running of the engine and reduced power output, the pressure of the steam can be maintained by means of an oil fired furnace. Steam supply can also be maintained with this type of boiler when the engines are not in operation. The Cochran boiler whose working pressure is normally of the order of 7 bar (0.7 MN/m2) is available in various types and arrangements, some of which are: Single pass composite, i.e. one pass of the exhaust gases and two uptakes. One for the oil fired system and one for exhaust system. Double pass composite, i.e. two passes for the exhaust gases and two uptakes, one for the oil fired system and one for the exhaust system. Double pass exhaust gas, no oil fired furnace and a single uptake. Double pass alternatively fired, i.e. two passes from the furnace for either exhaust gases or oil fired system with one common uptake. Composite boilers form a simple waste heat system for the continuous generation of steam for auxiliary purposes, with the advantages of the boiler being kept in operation both at sea and in port, so avoiding long periods of shut down which can often result in corrosion problems. Some of these waste heat recovery systems provide superheated steam for use in turbo alternators which are capable of supplying the ship's electrical power requirements while at sea and proceeding at or near her normal speed. XIII Problems in the Operating of Boilers Boilers damage can be considered under five main headings: Corrosion: There are two principle forms of corrosion. One is direct chemical attack and mainly occurs in boilers due to the high metal temperature involved. It can result pitting or cracking in the tube bores. Or in scaling or flaking on the gas side of tubes. This form of corrosion also occurs when loss of water circulation causes the metal to overheat in the presence of steam. This more common form of corrosion found in boilers is the result of electro-chemical attack usually involving acidic water conditions in the presence of dissolved oxygen. General wastage of the boiler metal due to this form of attack has been virtually eliminated by the use of chemical feedwater treatment, but isolated pitting can still occur if the treatment is not operated within the correct limits. Erosion: This is mechanical wearing away of the boiler metal due to water, steam or gas flowing over the metal surface. Thus tubes can wear thin in the region of bends, due to water impingement, or wear externally as the stand in the flow of hot abrasive gases leaving the furnaces. Overheating: In-service boiler metal subjected to the heat of combustion must be continually cooled by water or steam. If for any reason this cooling affect is lost, or greatly reduced, the boiler metal overheats, loses strenght and distorts. This can result in expand tubes pulling out of tube plates, local bulging of tube surfaces with eventual rupture and sagging. A build up of deposits on the water side acts as an insulating layer, reducing the rate of heat transfer through the metal so causing it to overheat and leading to eventual distortion. Oil entering the boiler only forms a thin, but efficient, insulating layer upon heated surfaces, but also encourages a further built up of scale deposits. Cracking: Welded boilers are especially vulnerably to fatigue cracking resulting from bad design, poor workmanship or both. Cracks of this nature, even if starting in a minor weld, can continue to propagate even into the main shell plate. Mechanical Damage: This can result from poor workmanship, such as damage to tube plates by over-expanding during tube attachment, scoring of joint faces, distortion of doors by overtightening. Cleaning the Boilers The frequency of boiler cleaning depends upon various factors such as the nature of the service in which the vessel has been engaged, the quality of feed water and fuel with which the main engine has been supplied. In general, every reasonable oppurtunity should be taken, whenever the boiler is shut down, to examine and internal surfaces. Where possible the boiler should be clean. When boiler pressure has fallen, Blow down valves on drums and headers open to remove sludge deposits. Finally empty the boiler by running down through suitable drains etc. The boiler must not be attempted to cool forcibly as this can lead to thermal shock. All feed and steam line must be isolated, and appropriate valves locked. Boiler Water Tests Boiler water should be regularly tested and the treatment the boiler water should be conducted according to the results obtained from the tests. For exhaust gas boilers such as vertical Cochran, salinometer and xiv litmus papers are still fequently used as testing equipment. Therefore alkalinity test, chloride test, sulphite test, phosphate test, hardness test, dissolved oxygen test, total dissolved solids test and hydrazine test are supplied. In salinometer test the range of scale is normally from 0 to 32 and when the salinometer is floating in pure water 93°C the salinometer scale shows zero and in salt water 93CC it shows 1/32(32000ppm). Litmus papers are used to ascertain the degree of acidity or alkalinity of the water. A litmus paper when inserted into a sample of boiler water may change color, turning blue if the water is alkaline, or red if the water is acidic. Tests for alkalinity are applied three types, Alkalinity to phenolphthalein, total alkalinity and caustic alkalinity. Other tests of boiler water are as follows: Chloride test, sulphit test, phosphate test, hardness test, dissolved oxygen test, total dissolved solids, hydrazine test and pH value. A boiler water's pH value can be obtain by three basic methods: Litmus papers, colourimetrically, and electrolytically. Boiler Water Threatment The principal objects of boiler feed water treatment should be: i) Prevention of scale formation in the boiler and feed system by (a) using distilled water or (b) precipitating all scale forming salts into the form of a non-adherent sludge. ii) Prevention of corrosion in the boiler and feed system by maintaining the boiler water in an alkaline condition and free from dissolved gases. iii) Control of the sludge formation and prevention of carry over with the steam. iv) Prevention of entry into the boiler of foreign matter such as oil, waste, mill-scale, iron oxides, copper particles, sand, weld spatter, etc. By careful use of oil heating arrangements, effective pre-commission cleaning and maintaining the steam or condensate systems in a non-corrosive condition. RESULTS AND PROPOSALS 1. Waste heat energy recovery system must be known practically and theoretically by operators. 2. Operation conditions of these systems must be kept in opreration limits. They musn't be operated under or above their operation limits. 3. Their inspection, testing, starting, closing, cleaning, maintenance and repair must be performed according to procedures on time. Thus the damages and the loss can be reduced to minimum. 4. Quantity of sample for testing or dosing of treatments musn't be used under or above the necessary limits. On the contrary they may be without avail. 5. Automatic control loops do not think for themselves, and subjected to external irregulaties will still try to perform as normal. This can result in their final control action being incorrect, or to some other piece of equipment being overworked in attempt to compensate. In situations where the automatic control of critical parameters is not dependable, or where it becomes necessary to use manual control, reduce operating condition so as to increase in acceptable margins of error