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 new 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. This then allows one to determine the steam generation
system operational control range set points required to meet the silica steam purity specification
associated with a particular turbine and its duty. Emphasis on the methodology used to avoid
silica deposits within steam generators and fouling of steam turbines will be outlined. To
illustrate the process; an example calculation is provided in the paper.

KEY WORDS: Steam silica, boiler water silica, steam total dissolved solids, boiler water total
dissolved solids, boiler water alkalinity, hydroxide alkalinity, pH, internal chemical treatment
regimes, steam purity, internal corrosion, internal deposition, operation pressure ranges, total
boiler water carryover, mechanical carryover, vaporous carryover, total steam purity,
attemperation, American Boiler Manufacturers Association (ABMA), fractional carryover
(FCO), quartz, reactive silica, non-reactive silica, amorphous silica, colloidal silica,
hydroxyapatite, serpentine, brucite, thermophoresis, silica-silicate deposition, complex metal
silicates, gamma iron oxide.