Internal Boiler Water Treatment
In internal treatment, chemicals are added in the boiler water. Internal treatment to prevent scale deposition is usually carried out with some form of sodium phosphate (trisodium, disodium or monosodium). These chemicals react with dissolved salts to convert them into less harmful sludge which can stay in the boiler water without creating any harm to the boiler tube. Later on, this sludge can be removed from the boiler water by blowdown. The main aim of internal treatment is to precipitate the impurities present in the boiler water to get harmless salts or sludge.
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Soda Ash (Sodium Carbonate or Na₂CO₃) Treatment
Soda ash (Na₂CO₃) treatment is done in smaller size boiler.
In this process, soda ash is added into the boiler water which reacts with chlorides and sulphates of magnesium and calcium to form insoluble sludge.
CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl
MgSO₄ + Na₂CO₃ → MgCO₃ + Na₂SO₄
CaSO₄ + Na₂CO₃ → CaCO₃ + Na₂SO₄
Phosphate Treatment or High Pressure (HP) Dosing
In high pressure boiler, soda ash treatment cannot be used, as it forms caustic soda and increases with the increase in temperature. Hence, it is difficult to maintain pH of the boiler water. Also, at higher pH, solubility of calcium carbonate increases. So, in high pressure boiler, phosphate treatment is preferred as internal treatment.
Phosphate treatment is preferred in drum type boiler. Phosphate is added to the boiler water at the boiler drum.
If it is dosed at feed pipe, it may react with impurities and sludge may be deposited at the feed line. Phosphate dosing is done at boiler drum which is at higher pressure. So, this dosing is called as high pressure (HP) dosing.
Phosphate dosing is done in only drum type boiler. In once through boiler, phosphate treatment is not done.
Trisodium phosphate (Na3PO4), disodium phosphate (Na2HPO4) and monosodium phosphate (NaH2PO4) are used in boiler for dosing. These chemicals are also called as orthophosphates.
Trisodium phosphate (TSP) is highly alkaline. Disodium phosphate is less alkaline and monosodium phosphate is slightly acidic.
Blowdown
Concentrated solid precipitates at drum bottom (by internal treatment) are removed through blowdown, which may be either intermit tent or continuous, the amount of blowdown being governed by the allowable solids concentration and the purity of feedwater. It is usually expressed as a percentage. Thus,
% blowdown = (quantity of water blow down/quantity of feed water) x 100
For instance, 3% blowdown means that 3 per cent of the total amount of feedwater supplied is removed as liquid by blowdown of the drum. The continuous blowdown has the advantage of closer control of concentration and the recovery of part of the energy of the blowdown.
Since water purity is controlled by blowdown in drum type boilers, condensate polishing is not required in such boilers.
Example
Calculate the blow down rate for a boiler with an evaporation rate of 5 tons/hr, if the maximum permissible TDS in boiler water is 3000 ppm and with 18 % make up water addition. The feed water TDS is around 400 ppm.
Solution
Blow down in percentage
= (feed water TDS x % makeup)/(permissible TDS - feed water TDS)
= (400 x 18)/(3000 - 400) = 2.77%
If boiler evaporation rate is 5000 kg/hr then required blow down rate is
= 5000 x 2.77/100 = 138.5 kg /hr
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