In this article we will discuss about feed water and its heating in a steam power plant.
The steam coming out of turbine is condensed and the condensate is fed back to the boiler as feed water. Some water may be lost due to blow-down, leakage etc. and to make up these losses additional water, called the make-up water, is required to be fed to the boiler. The make-up water in a modern thermal plant is about 1-4%. In a large size plant this quantity may be around a few hundred tonnes per hour.
The source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities, dissolved gases etc. The suspended impurities include mud, silt, clay and silica either in suspension or in colloidal form. The dissolved impurities include carbonates, bicarbonates, sulphates and chlorides of calcium, magnesium and sodium, iron oxide, silica.
Dissolved gases include oxygen and carbon dioxide. These impurities in feed water may lead to scale formation, corrosion, carry over and embrittlement in boiler and other apparatus. Scale formation reduces the heat transmission through the heating surfaces and causes the overheating of the boiler tubes and the shell plates. Corrosion produces pits, grooves and cracks or a general wastage of material.
Embrittlement leads to accelerated cracks in the edges of riveted boiler plates, rivets and the metal around opening which has been highly stressed during its fabrication and operation. Foaming and priming of the boiler water, forms the carry over. It causes filling up of steam passages with a mass of frothy bubbles. It is necessary to heat and purify the water before feeding to the boiler.
(i) Improves the overall efficiency of the plant,
(ii) Removes dissolved oxygen and carbon dioxide,
(iii) Causes precipitation of other impurities carried by steam and condensate outside the boiler and
(iv) Avoids thermal stresses owing to entry of cold water into the boiler.
Two types of heaters are used, namely open or contact type heaters and closed or surface type heaters for heating of feed water. Open or contact type heaters are used in small plants. Such heaters receive steam from back pressure turbine or engine driving auxiliaries. In large modern plants heaters bleed steam from the main turbine and use it for heating of feed water. The feed water is heated, put under pressure and then further heated so that its temperature approaches and pressure exceeds that of water in the boiler. The water is treated for removal of suspended and soluble solids and removal of gases.
The various methods used for water treatment are:
(i) Mechanical (sedimentation and filtration),
(ii) Thermal (distillation and deaerative heating), and
(iii) Chemical (lime treatment, soda treatment, lime soda treatment, zeolite treatment and demineralisation).
Sedimentation involves the water to standstill or flow with a very low velocity and causes the solid matter to settle down which can be removed periodically or continually. Clear water drains from the surface of the settling chamber. In filtration the dirty water is allowed to flow downward through the beds of material like graded sand or anthrafilt. The suspended matter adheres to the filter material and the clear water drains from the bottom.
In distillation, the raw water is evaporated with the help of steam vapours are collected and condensed to give supply of pure feed water. In dearerative heating the water is broken up into fine droplets and heated to produce vapour within the deaerator and in the process air, CO2 and other gases are separated from the vapour as it condenses. Steam is used for heating in deaerators.
Lime treatment is suitable for the treatment of carbonate hardness, carbon dioxide in the water, either in a free state or in bicarbonate combination. In the process lime is taken up in hydrated form and relatively insoluble precipitate of calcium carbonate and magnesium hydroxide are formed. The process is best carried on in large tanks from which the softened water may be taken out and the sludge removed from the treating plant.
Soda treatment process is suitable for water containing non-carbonate hardness and soda ash or sodium carbonates are usually employed in the process. The decomposition of the sulphate occurs together with the precipitation of insoluble calcium carbonates and sodium sulphate. Lime soda treatment is used when both carbonate and non-carbonate hardness exists. The lime takes up the carbon dioxide not affected by the soda used in the treatment of non-carbonate hardness.
Zeolite treatment is one of the ion- exchanging processes wherein cations of two or more positive charges are exchanged for sodium or hydrogen and zeolite is either a natural or an artificial material that has the property of being able to exchange its base. Natural zeolites have a sodium base that can be exchanged for calcium or magnesium in raw water and the water will be softened.
The action of sodium zeolite, Na2Z on water are:
CaSO4 + Na2Z = Na2SO4 + CaZ
Mg(HCO3)2 + Na2Z = 2NaHCO3 + MgZ
Ca(HCO3)2 + Na2Z = 2NaHCO3 + CaZ
Thus as the raw water flows into a zeolite bed and emerges as soft water, there is loss of sodium and gain of calcium or magnesium by zeolite. Therefore the bed will become exhausted and fail to soften the water. However, zeolite has the property of regeneration of its strength through back washing with a sodium solution. The advantage of this process is that hardness is disposed off as a solution instead of solid requiring filtration.
In demineralisation water may be freed of its mineral content by evaporation or by a series of cation and anion exchanges to produce essentially distilled water. It is usually the most economical method to produce make-up water for high pressure boilers.