Silica is an important parameter for measurement and control, particularly in systems where steam is sent to turbines. Silica monitoring is important because it is difficult to remove from water, volatilizes with steam, can rapidly foul turbines and the resulting deposits are difficult to remove. This paper will discuss the importance of silica measurement, where on-line measurement should be used and the basics of silica monitors. INTRODUCTION In systems that do not contain a turbine or other steam driven rotating equipment, the limitations on silica in boiler water are generally less restrictive but they are still very important because they serve to prevent the deposition of silica on boiler tube surfaces. Silica does have a limited solubility in boiler water but on-line monitoring of silica is not routinely practiced in these systems. In high pressure systems that do contain a turbine or other steam driven rotating equipment, the concern becomes silica transport due to the volatility of silica. The volatility of silica is a very complex phenomenon. It is primarily dependent on pressure and pH. Silica can volatilize at pressures above 400 psig (2.76 Mpa) but does not usually become significant until pressures exceed 600 psig (4.14 Mpa). Volatilization of silica decreases as boiler pH increases. In most industrial applications, boiler water silica concentrations are controlled so that the silica content of the steam does not exceed 0.02 mg/L (ppm). In these systems, on-line monitoring can play an important role in protecting steam driven equipment from silica deposition. APPLICATION On-line methods of silica monitoring are primarily applicable to those boilers operating at 600 psig or greater, utilizing a demineralized makeup and using the steam in equipment such as steam turbines. The on-line methods can be used to directly measure the silica content of steam or at points around the cycle that may add silica to the boiler and thence to the steam.