Water impurities fall into three categories:
- Dissolved gases
A first type of water impurity is dissolved gases such as oxygen and carbon dioxide. In the hydrologic cycle, various gases become dissolved in rainwater as it passes through the atmosphere. In addition, any equipment which is open to the atmosphere will permit gases to enter and be absorbed by the water.
- Suspended solids
These are impurities which do not dissolve in water. Suspended solids such as sand, mud, oil and decaying vegetation tend to separate from standing water. Suspended insoluble matter is also referred to as turbidity.
- Dissolved solids
Commonly encountered dissolved solids include calcium, magnesium, iron, chloride and silica.
The type and amount of impurities present in your plant's make-up water will depend largely on the water's source.
Well water picks up dissolved minerals as it percolates down through the earth's crust. This process also has a filtering effect, usually keeping the water free of suspended matter.
Surface waters from lakes, rivers and streams, on the other hand, may contain high levels of suspended matter such as sand, silt and leaf mold. Surface waters vary greatly in composition depending on factors such as the amount of rainfall, time of year, and the discharge of industrial pollution.
The amount of an impurity in a water sample is generally expressed in parts per million or ppm. One ppm is equal to one part of an impurity in one million parts of solution. For example, if certain water contains 1 ppm of silica, there is one kilogram of silica present in one million kilogram of that water. The amount of impurities that may be considered acceptable depends on the water's intended use.
While relatively high concentrations of impurities can be tolerated in water used for open recirculating cooling systems, ultra high purity water is required for high pressure steam generating systems. The problem with even relatively small amounts of impurities in boiler feedwater is the fact that these impurities concentrate in the boiler.
Just as river water leaves impurities behind as it evaporates, boiler water leaves impurities behind in the boiler as it turns to steam. This is explained as follows, starting from a beaker of tap water containing 2 ppm of calcium, the amount of calcium left behind becomes twice as concentrated to 4 ppm. If half the water again evaporates, the calcium concentration again doubles to 8 ppm. Adding more tap water to make-up for the water lost by evaporation adds even more calcium.
The same thing happens in your boiler: As water turns to steam and escapes in the steam drum, the water left behind in the boiler becomes more concentrated with impurities.