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Parameters of Boiler Feedwater

pH of water

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

HOH = H⁺ + (OH)⁻

The presence of excess hydrogen ions makes the water acidic and hence corrosive. The pH is the logarithm of the reciprocal of the hydrogen ion concentration. If the hydrogen ion concentration in water is 10⁻⁷, the pH value of water would be log (1/10⁻⁷ ) or 7.

The pH of the water varies with temperature . At 15 °C, the pH of neutral water is 7. If pH > 7, water is alkaline and if pH < 7, it is acidic.

Trisodium phosphate, Na₃PO₄, is injected to water in suitable doses to increase alkalinity. Monosodium phosphate, NaH₂PO₄, is used to decrease alkalinity. A pH meter can be used to measure the pH of water.

The essential factors required for corrosion are hydrogen ions (low pH) and dissolved oxygen. Scale formation and corrosion are reciprocal phenomena: conditions for one preclude the other, and the pH value of water determines it. A pH 10.5 is usually maintained for boiler water in order to minimize corrosion. It will, however, cause scale formation.

Turbidity

Pure water free from suspended solids is colourless. Cleanliness of water decreases when water is contaminated with any suspended insoluble particle like mud, sand, sediment, etc. Cleanliness of water is measured as turbidity. Less turbidity means clear water and high turbidity indicates that water is contaminated. The unit of turbidity is normal turbidity unit (NTU). Turbidity decreases when filtration is perfect.

Hardness

The water contains dissolved salts and minerals. Presence of these salts and minerals is responsible for hardness of water. More the salts and minerals, more is the hardness.

The hardness is classified as permanent hardness and temporary hardness. Bicarbonates of calcium and magnesium are responsible for the temporary hardness of water. Temporary hardness can be avoided by boiling the water.

Permanent hardness is due to the presence of chlorides, sulphates and nitrates of sodium, calcium and magnesium. Small quantity of iron, aluminium and manganese salts are also present in water which contribute a little to the water hardness.

Sometimes, the presence of aluminium and manganese contributes to the hardness. These salts are deposited in boiler tube as hard scale which disturbs the heat transfer and leads to tube failure. Hardness is of two types, i.e., carbonate and non-carbonate hardness. The unit of hardness is milligram per litre (mg/L) or parts per million (ppm).

Silica

Silica can exist in following three physical/chemical forms:

  • Dissolved silica (monomer, soluble/reactive)
  • Colloidal silica (polymeric, unreactive)
  • Particulate silica (granular, suspended)

Silica in dissolved state is called as reactive silica. This is volatile and easily carried over by steam and deposited at turbine blade where temperature and pressure of the steam drop.

This silica is not desirable in boiler water. Silica concentration in boiler water can be controlled by giving blowdown. At DM plant, only reactive silica is removed.

Colloidal silica is called as non reactive silica. At RO plant, colloidal silica along with reactive silica can be removed.

Dissolved Oxygen

If oxygen is present in feedwater, then it accelerates corrosion of the boiler tube. So, to make the feedwater free from dissolved oxygen, oxygen scavenger is used. Earlier, sodium sulphite was used for this purpose.

2Na₂SO₂ + 2O₂ → 2Na₂SO₄

But nowadays, hydrazine is widely used for oxygen scavenging.

N₂H₄ + O₂ → 2H₂O + N₂

It is a common practice to have traceable quantity of hydrazine in feedwater to eliminate the chances of presence of oxygen in the feedwater.

Sugar

Sugar is an organic contaminant of boiler water, particularly found in the boiler used in sugar plant. Sugar test of boiler feedwater is done in this case, where steam is used at evaporator and condensate is reused in boiler. There is possibility of contamination of condensate with sugar. In this case, sugar test is required to be done frequently.

Sugar breaks down at high temperature into organic acids, which lower the pH and increase conductivity of boiler water. This results in corrosion of boiler tube. In addition, the boiler water has a tendency to foam, resulting in carryover of salts. In this case, to control sharp drop of pH, caustic soda is to be dosed into the feedwater and fresh make up water is to be used. 

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