Kazan besleme suyunun hazırlanması
Bolier water treatment
- Tez No: 39630
- Danışmanlar: PROF.DR. OSMAN F. GENCELİ
- Tez Türü: Yüksek Lisans
- Konular: Enerji, Makine Mühendisliği, Energy, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 71
Özet
ÖZET Buhar üretiminde kullanılacak olan bir suyun gerekli şekilde hazırlanmış, yani içinde bulunan yabancı maddeler den mümkün olduğunca arındırılmış olması gerekir. Su ha zırlama konusu buhar kazanlarında önemli bir yer tutar. Direkt olarak kazana verilmiş olan kaba su, bir ta kım arızaların oluşmasına neden olur. Çünkü bu su içinde bulunan gazlar kazan malzemesinin korozyona uğramasına organik maddeler, su yüzeyinde köpük ve kabarmalar oluşma sına; su içindeki çeşitli tuzlar ise, cidarların üzerini kaplayan kazan taşının veya kazanın alt kısımlarında top lanan kazan çamurunun oluşmasına neden olur. Bir beslenme suyu, kazana basılmadan önce C02 ' den tamamen, O^'den ise mümkün olduğunca temizlenmiş olmalıdır, Bu temizleme mekanik, termik veya kimyasal olarak yapıla bilir. Büyük buhar tesislerinde sistemde gaz alıcı olarak degazör kullanılması çok ekonomik bir yöntemdir. Degazör sürekli çalışan bir sistem olarak sürekli bir gazsız- landırma olayını gerçekleştirerek ekonomiklik sağlar. Degazör kapasitesinin belirlenmesi de sistemin ekonomik olması için önem taşır. Degazör artık günümüzde buhar tesislerinde vazgeçilmez bir eleman olarak yer almaktadır. v
Özet (Çeviri)
BOILER WATER TREATMENT SUMMARY Water is one of the mo,st complex substance known to man. At the safle time, uext to time air we breathe, water is the most important thing in our lives, Scientists call water the“Universal solvent”beca use it dissolves so many different compounds. Due to this fact, pure water is a rare substance and our natu ral waters always contain a great many impurities. The most common of which are compounds of sodium, cal cium, magnesium and iron. Water from the ground always contains some or all of these compounds and they give the water its hardness or mineral content. Groundwater, for example, picks up mineral matter as it seeps through sand and rock formations. While natural filtration proceses remove much of the suspended solids, dissolved solids, organics, gases and suspended matter are retained in varying degrees. Surface waters contain insoluble silt and sand as well as organic resi dues, industrial pollutants etc. Moreover, impurity concantrations are not constant being subject to rain fall, stream velocities and other variable of nature. initial treatment (pretreatment) is applied to inco ming water to screen out the bulk of impurities that create most boiler deposits and corrosion. How they are selected and arnanged dependo on many factors, par- ticulary the nature of the water and requirements of the boiler system. Suspended solids draw early consideration because they are readily identified and their removal simplifies techniques and protects downstream equipment used to remove dissolved impurities. Altough some plants limit soiidsseperation methods to direct filtration of large particulates, most opt first for clarification a more flexible step desig ned to deal with a broader range of suspended solids. vxCarbondioxide is alw.ays present in the atmosphere and in industrial areas there are sometimes sulphur dioxide and sulphur trioxide also; these gases are very readily absorbed by water, with which they form carbonic, sulphurous and sulphuric acid solutions respectively, The feed water alkalinity is maintained at a predetermined value by chemical treatmant, which changes the sulphur acids into benefical salts and converts the carbonic acid to sodium carbonate. The object of deaeration is to reduce the dissolved oxygen content of feed water to as low a value as possiple, Concurrently the dissolved carbon dioxide content is greatly reduced and the inert and innocuous dissolved nitrogen is entirely removed. The deaerator consists of deaerating chambers come or more depending upon the quantity of water to be dealt with, a vapour condenser and a deaerated-water storage tank with fioat control gear, all of which are combined to form a compact unit. The deaerating chamber consists of asteel shell, a galvanized-steel tray nest with perforated brass, tray plates, and a dished cast-iron cover containing a water distribution plate, the function of which is to distribute the incoming water evenly over the trays. The vapour condenser through which water passes befo re entering the deaerating chamber, is of the surface type and is mounted on the side of the storage tank. The storage tank to which the deaeated water passes on leaving the chamber is of fabricated steel. When an external float tank is provided it is bolted to supports on the storage tank. Two balance pipes connect the float tank to the storage tank, and isolating values are provided to facilitate inspection of the floats and chamber when the deaerator is in service. The capacity of a deaerating unit depends on the quantity of water requiring deaeration and on the ratio of the concentration of oxygen in the water of inlet to that at outlet. Thus the duty must be specified in terms of the final oxygen content required when the deaerator in supplied with a certain quantity of water at a definite degree of aeration. vııAlum, iron salts, and polyelectrolytes are the predominant coagulants. Alum, the most popular of all, reacts with soluble alkalis to form aluminium hydroxide a compound that is practically insoluble over the pH range from 5.7 to 7.5. Sodium aluminate, another form of alum, reacts with calsium bicarbonate, precipitating calcium carbonate from and producing an aluminium hydro xide floe. Depending on other chemicals present its action is induced either at low or high pH. It can also reaact with magnesium, precipitating magnesium aluminate and magnesium hydroxide. It can be seen that pH must be adjusted and cont rolled as required by the process selected. This is done by feeding an acid on an alkali such as lime, soda, ash or caustic soda. Softening results in removal of more than just hardness. It can also remove silica and reduce carbonate alkalinity. Silica with magnesium hydroxide to produce an insoluble silican magnesium complex. Cold process softening is effective for many large capacity low-pressure boilers that do not require comp lete demineralization of feed water. If much lower hardness is reguired, an alternative is hot-process soffening, using live or exhaust steam as the heat source, The dissolved oxygen content of water is the main factor causing corrosion of steel, but if dissolved carbon dioxide is also present it will enhance the carro- sion. The presence of dissolved carbon dioxide and ammonia in water can cause corrosion of metals other than steel. In modern steam power plant the operating conditions for the boilers are such as to demand not only the elimination of dissolved oxygen from the boiler feed water but also rigorous control of its alkalinity. The solubility of a gas in water is determined chiefly by the temperature of the water and the partial pressure of the gas in the surrounding atmosphere..In atmospheric air the partial pressures of oxygen and oitrogen are about one-fifth and four. Fifths respecti vely of the total pressure, but the greater solubility of oxygen means that water in contact. with atmospheric air dissolves oxygen in a relatively higher proportion, about one-thirdoxygen to two-thirds nitrogen by volume; this ratio is approximately constant over the temperature range 0-'100°c/. vxnThe standard connection for a Metrovick deaerator in a closed feed system were shown in fig. 1. The feed water passes through the ejectors and low pressure heaters before entry to the deaerator the deaerated water is feed into the boilers by way of the boiler feed pump and high pressure heaters. The system of water control ensures that, if the supply exceeds the demand, feed water at a low temperature passes to the reserve tank instead of entering the deaerating unit, and, in the event of the demand exceeding the supply, water is fed back into the system by way of the condenser.. The deaerator is supplied with bled steam from a low-pressure stage of the turbine. The steam and the incondensible gases leave the deaerating chamber and pass to the vapour condenser, where they are concentra ted and discharged to the atmosphere via the main conden ser air extraction plant. A deaerator installed in a transfer connection between a low pressure and a high pressure station is shown in fig. 2. The deaerator is supplied with live steam through a reducing valve. Water from the low pressure power plant is supplied to the deaerating unit, the quantity being controlled by a float-operated valve at the entry to the vapour condenser. The deaerated water is fed into the high-pressure boilers by means of a transfer pump and the boiler feed pumps. The quantity of the deaerated make-up water passing from the deaera ting plant of the high-pressure boilers is controlled by a valve, which is float-operated from a normally full tank in the high pressure plant feed system xx
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