Water Treatment, Storage and Blowdown for Steam Boilers

Before boiler blowdown can be discussed and understood it is necessary to establish a definition of water along with its impurities and associated terms such as hardness, pH etc.

Water is the most important raw material on earth. It is essential to life, it is used for transportation, and it stores energy. It is also called the 'universal solvent'.

Pure water (H₂0) is tasteless, odourless, and colourless in its pure state; however, pure water is very uncommon. All natural waters contain various types and amounts of impurities.
Good drinking water does not necessarily make good boiler feedwater. The minerals in drinking water are readily absorbed by the human body, and essential to our well being. Boilers, however, are less able to cope, and these same minerals will cause damage in a steam boiler if allowed to remain.

Of the world's water stock, 97% is found in the oceans, and a significant part of that is trapped in the polar glaciers - only 0.65% is available for domestic and industrial use.

This small proportion would soon be consumed if it were not for the water cycle (see Figure 3.9.1). After evaporation, the water turns into clouds, which are partly condensed during their journey and then fall to earth as rain. However, it is wrong to assume that rainwater is pure; during its fall to earth it will pick up impurities such as carbonic acid, nitrogen and, in industrial areas, sulphur dioxide.

Charged with these ingredients, the water percolates through the upper layers of the earth to the water table, or flows over the surface of the earth dissolving and collecting additional impurities.

These impurities may form deposits on heat transfer surfaces that may:

• Cause metal corrosion.
• Reduce heat transfer rates, leading to overheating and loss of mechanical strength.

Table 3.9.1 shows the technical and commonly used names of the impurities, their chemical symbols, and their effects.

Fig. 3.9.1
Typical water cycle

Table 3.9.1
Impurities in water

The common impurities in raw water can be classified as follows:

• Dissolved solids - These are substances that will dissolve in water.
  
  The principal ones are the carbonates and sulphates of calcium and magnesium, which are scale-forming when heated.
  
  There are other dissolved solids, which are non-scale forming.
  
  In practice, any salts forming scale within the boiler should be chemically altered so that they produce suspended solids, or sludge rather than scale.
• Suspended solids - These are substances that exist in water as suspended particles.
  
  They are usually mineral, or organic in origin.
  
  These substances are not generally a problem as they can be filtered out.
• Dissolved gases - Oxygen and carbon dioxide can be readily dissolved by water.
  
  These gases are aggressive instigators of corrosion.
• Scum forming substances - These are mineral impurities that foam or scum.
  
  One example is soda in the form of a carbonate, chloride, or sulphate.

The amount of impurities present is extremely small and they are usually expressed in any water analysis in the form of parts per million (ppm), by weight or alternatively in milligrams per litre (mg/l).

The following sections within this Tutorial describe the characteristics of water.