Vaporous Silica and Other Steam Volatiles

As a result of the volatility of silica and its ensuing risks, such as the deposition of hard, amorphous, or glassy silicates in superheaters and on turbine blades, various actions are commonly taken in higher pressure boiler plants to limit silica vapor in steam. The maximum permissible silica concentration in steam is generally accepted as 0.02 ppm 

Boiler water silicas may steam distill and may be present in various forms including silicic acid (H2Si03) and silicate ion (Si032-). They become more volatile at higher boiler pressure, although volatility is decreased slightly with an increase in pH. 

Volatile silica is seldom a problem at under 400 psig, where only 0.1 to 0.2% of total BW silica may be present as volatiles. It becomes significant at 900 to 950 psig and above, however, when silica volatiles rise sharply to 1.0 to 2.0% or more of total BW silica. 

Actions taken to minimize the risk of silica distilling over into the steam include  

NOTE Where coordinated phosphate programs are employed, the entire rationale is designed to inhibit the presence of free caustic. As a consequence, the alkalinity buffering capacity is severely reduced and the tolerance for silica is likewise diminished. 

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