Silica present as an impurity in waters used for the production of steam have resulted in both water side deposition in steam generators and steam/condensate fouling within turbines. A brief review
of the history of silica deposition / fouling and its control will be provided. The suggested boiler water silica limits in the ASME booklet; “Consensus on Operating Practices for the Control of Feedwater and
Boiler Water Chemistry in Modern Industrial Boilers,” and their derivation will be discussed. The elements comprising total steam purity as well as a procedure for estimating the silica contribution will be disclosed. The effects of silica volatility and prediction methods will be provided. Thus, allowing one to determine pretreatment selection and system operational control range set points required to meet
the silica steam purity specification associated with a particular turbine and its duty.

Keywords: Silica, Silicic Acid, Amorphous Silica, Polymerized Silica, Quartz, deposition, steam
generators, steam turbines, internal treatment regimes, volatility, fugacity, partitioning, steam purity
limits, total steam purity, vaporous carryover, mechanical carryover, attemperation quality, pH,
hydroxyl ion, dissociation constant, phase transition zone.