Silica is sand dissolved in water. Silica tests are used to control boiler blowdown and cooling water bleed.

High concentrations of silica form difficult to remove deposits in cooling systems, boilers, and turbines.

Silica exists in a wide range of ions; from simple silicate to a complicated polymeric material. The size of the silica polymer varies over a wide range up to a colloidal state. Colloidal silica is rarely present in ground water. Colloidal silica is most commonly present in surface waters during periods of high runoff.

The polymeric form of silica does not produce color in the standard molybdate based colorimetric test for silica. This form of silica is termed “nonreactive”. The polymeric form of silica is not thermally stable and when heated in a boiler reverts to the basic silicate monomer. The basic silicate monomer is reactive with molybdate. As a result, molybdate testing of a boiler feedwater may reveal little or no silica, while boiler blowdown measurements show a level of silica that is above control limits. High boiler water silica and low feedwater values are often a first sign that polymeric or colloidal silica is present in makeup water.

Silica may be removed from water by:

• Reverse osmosis
• Demineralization
• Hot lime softening

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