Silica Measurement: Process Silica Analyzers
Dissolved silica measurement in feedwater, boiler water, steam and demineralized water: colorimetric (molybdate) online silica analyzers and SWAS systems.
Measurement methods: Colorimetry (Silica/Phosphate)
What is silica and why is it measured?
Silica (SiO₂) is a dissolved compound found naturally in raw water that can be carried over with steam in high-pressure boilers. It forms hard, poorly soluble silica deposits on turbine blades, reducing efficiency. Failure to retain enough silica in the water treatment system indicates that ion exchange resin is exhausted and that reverse osmosis or demineralization efficiency has fallen. In power plants, silica in boiler feedwater, boiler water, steam and demineralized water is therefore monitored continuously.
Where is silica measured?
- Demineralization unit outlet: Resin exhaustion control (silica slip).
- Feedwater and boiler water: Dosing and blowdown control.
- Main steam: In turbine protection.
- Reverse osmosis outlet: In monitoring membrane performance.
How is silica measured?
Online silica analyzers work by the colorimetric (molybdate) method. When acid and molybdate reagent are added to the sample, silica forms a yellow silicomolybdate complex; this complex is then turned blue by a reducing agent, and its color intensity is read photometrically. The color intensity is proportional to silica concentration. The analyzer automatically mixes the sample and reagents, makes the measurement and flushes the line; pumps, valves and reagent tubing need periodic maintenance.
Choosing a silica analyzer
- Single-stream or multi-stream (3-4 streams) sample system
- Reagent consumption and maintenance kits
- Sample cooling and pressure reduction
- ATEX-compliant enclosure if installed in a hazardous area
Silica measurement with Tetra
Tetra supplies Thermo Orion process analyzers in Türkiye and provides installation, supply of reagents and consumables, commissioning, calibration and service. For power and treatment plants, complete SWAS or panel systems with sample cooling and conditioning are built at its facility in İzmir.
Frequently asked questions
What unit is silica measured in?
In water and steam applications, silica is generally given as SiO₂ in µg/L (ppb), and in mg/L for raw water and treatment inlets. Target values are determined according to boiler pressure and the turbine manufacturer's requirements. When comparing results, check whether the value is given as SiO₂ or as Si.
Does a silica analyzer measure colloidal silica?
The molybdate method measures only reactive silica (which reacts with molybdate); colloidal silica is not seen by this method. Because colloidal silica can convert to the reactive form at high temperature in the boiler, it should be assessed separately in raw water and treatment units. For total silica, different pretreatment or analysis methods are used in the laboratory.
How is phosphate interference eliminated in silica measurement?
Phosphate also forms a colored complex with molybdate, so it can raise the silica measurement. In analyzers, this interference is eliminated by adding a reagent such as oxalic or citric acid that destroys the phosphomolybdate complex. This step is particularly important for boiler water samples conditioned with phosphate.
How often is an online silica analyzer calibrated?
Silica analyzers are generally calibrated with a zero (reagent blank) and a standard solution of known concentration; many instruments do this automatically at set intervals. Reagent shelf life, temperature-dependent reaction rate and pump tubing wear affect accuracy, so maintenance and reagent replacement should be carried out together with the calibration plan. The exact interval is set according to the manufacturer's recommendation and the plant's quality procedure.
Products measuring this parameter
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