DME-II_Thermal Engineering-II_Boiler-VI

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Supercritical Boiler

A large number of steam generating plants are designed between working ranges of 125 atm and 5100C to 300 atm and 6600C. These are basically characterized as subcritical and super critical boiler. Subcritical boilers consist of preheater, evaporator and superheater while super critical boiler requires only preheater and superheater.

  
  In supercritical pressure boiler, only one tube almost about 2 km long is used and there is no steam drum. It is the only type suited to supercritical pressure operation, i.e. above 221.2 bar, as there is no latent heat of vaporization involved and water on being heated at constant pressure suddenly flashes into vapour and steam is further heated to desired temperature in superheater.




Feedwater Treatment:

Feedwater treatment is needed for----
  • Prevention of hard scale formation on the heating surfaces.
  • Elimination of corrosion.
  • Prevention of silica deposition and corrosion damage to turbine blades.

Raw water is, therefore, first pre-treated and then demineralized. For once through boilers and boiling water nuclear reactors, which require high water purity, a condensate polishing system is used to further polish the water. Condensate polishing is achieved by passing the condensate through large demineralizing vessels characteristics of resins, they remove dissolved salt as well as act as filters for impurities. Raw water contains a variety of impurities such as:
  • Suspended solids and turbidity
  • Organics
  • Hardness (salts of calcium and magnesium)
  • Alkalinity (bicarbonates, carbonates, hydrates)
  • Other dissolved ions (sodium, sulphate, chloride etc.)
  • Silica
  • Dissolved gases such as Oxygen Carbondyoxide

Deaeration:

        Deaeration (degasification) is one of the most important steps in boiler water treatment. It depends on the decrease in solubility of dissolved gases notably Oxygen and Carbondyoxide as the water temperature is increased. It is carried out in a direct contact feed heater.

Internal Treatment:

The dissociation and recombination of water go on, till equilibrium is reached.

       The presence of excess hydrogen ions makes the water acidic and hence corrosive. Trisodium phosphate  is injected to the water in suitable doses to increase alkalinity. Monosodium phosphate is used to decrease alkalinity. A pH meter can be used to measure the pH of water. A PH 10.5 is usually maintained for boiler water in order to minimize corrosion but it will cause scale formation. Scale formation and corrosion are reciprocal phenomenon.  Two principle steps in scale prevention are: a) Periodic or continuous blowdown and b) external or internal treatment to eliminate the scale producer by chemical means.
        Internal treatment to prevent scale deposition is usually carried out with some form of sodium phosphate, once in the boiler; all these phosphates react with the calcium salts to form tricalcium phosphate which is rather a flocculent precipitate.

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