Water Quality (hardness)

Hardness

Water is referred to as being either 'hard' or 'soft'. Hard water contains scale-forming impurities while soft water contains little or none. The difference can easily be recognised by the effect of water on soap. Much more soap is required to make a lather with hard water than with soft water.

Hardness is caused by the presence of the mineral salts of calcium and magnesium and it is these same minerals that encourage the formation of scale.
There are two common classifications of hardness:

• Alkaline hardness (also known as temporary hardness) - Calcium and magnesium bicarbonates are responsible for alkaline hardness. The salts dissolve in water to form an alkaline solution. When heat is applied, they decompose to release carbon dioxide and soft scale or sludge.
  
  The term 'temporary hardness' is sometimes used, because the hardness is removed by boiling. This effect can often be seen as scale on the inside of an electric kettle.
  
  See Figures 3.9.3 and 3.9.4 - the latter representing the situation within the boiler.

Fig. 3.9.3
Alkaline or temporary hardness

Fig. 3.9.4
Non-alkaline or permanent hardness (scale + carbonic acid)

• Non-alkaline hardness and carbonates (also known as permanent hardness) - This is also due to the presence of the salts of calcium and magnesium but in the form of sulphates and chlorides. These precipitate out of solution, due to their reduced solubility as the temperature rises, and form hard scale, which is difficult to remove.
  
  In addition, the presence of silica in boiler water can also lead to hard scale, which can react with calcium and magnesium salts to form silicates which can severely inhibit heat transfer across the fire tubes and cause them to overheat.