Silica Measurement: Monitoring in Boiler Feedwater, Demineralized Water and EDI Outlet
Why silica is monitored, which method operates in which range, and where limit analyzers and handheld colorimeters are used.
Three Questions in Silica Monitoring
- In what range? Low mg/L and below at the DM or EDI outlet; several to tens of mg/L in boiler water depending on boiler pressure. Low range is covered by Pyxis SP-800 SiLR (0.02–5.0 ppm), high range by SiHR (1.0–75.0 ppm), and the limit window by Testomat 808 SiO2 (0.3–1.2 mg/L).
- Limit monitoring or spot measurement? Testomat 808 SiO2 (green/red LED, relay) for alarm and regeneration triggering; SP-800 for routine readings and verification.
- Which form of silica? Molybdenum blue methods measure reactive silica; colloidal silica is evaluated separately.
Scope note: This page does not present a continuous silica analyzer with an analog trend output; the solutions covered are limit-value monitoring and handheld colorimeter methods.
What Silica Does on the Steam and Turbine Side
Salts and metal oxides in boiler water become more soluble in steam as pressure and temperature increase. Because the solubility of silica in steam is higher than that of other salts and metal oxides, silica carryover into steam is a primary concern, especially in subcritical-pressure boilers. When superheated steam reaches the turbine, silica deposits on turbine blades and nozzles and reduces turbine efficiency; sodium salts precipitate in the high-pressure stages, whereas silica precipitates predominantly after the intermediate-pressure stages.
The Handbook of Water Treatment states that silica in steam must be kept below 0.02 mg SiO2/L to prevent this deposition. Silica in steam depends on boiler pressure and silica in the boiler water: the relationship is log A = 0.0135 × P + log B − 3.6 (where A is silica in steam, mg SiO2/L; B is silica in boiler water, mg SiO2/L; P is boiler pressure, kgf/cm²). According to this relationship, as pressure increases, the permissible boiler water silica decreases in order to keep steam silica within the same limit. Your plant's boiler specification and operating standard govern.
Typical silica in demineralized water is given in the same reference as 0.05–0.3 mg SiO2/L; in high-pressure boiler feedwater, a mixed-bed polisher is used in addition. Consequently, silica at the DM and EDI outlet serves as an indirect indicator both of the silica load going to the boiler water and of resin or membrane performance.
Silica Monitoring Methods and Ranges
| Method / instrument | Range | Operating principle | Output | When to use |
|---|---|---|---|---|
| Testomat 808 SiO2 | 0.3 – 1.2 mg/L (0.1 mg/L step) | Colorimetric titration and photometric evaluation by the molybdate method; limit-value monitoring | Green/red LED; 3 potential-free relays and 4-20 mA as series features | Silica breakthrough alarm at EDI/DM outlet, steam generator feedwater, sterilization water (DIN 285) |
| Pyxis SP-800 SiLR | 0.02 – 5.0 ppm SiO2 | Heteropoly blue method, APHA 4500-SiO2 D; 10 mL sample; 4 + 1 + 2 min (20°C) | Concentration on display | DM, EDI, RO permeate; feedwater verification |
| Pyxis SP-800 SiHR | 1.0 – 75.0 ppm SiO2 | Silicomolybdate method, APHA 4500-SiO2 C; 10 mL sample; 10 + 2 min | Concentration on display | Boiler water, raw water, high-silica lines |
Which method to use depends on the expected silica level. In steam samples, the target is around 0.02 mg/L, which lies at the lower end of the SiLR range; at that level, sampling error and the lower detection limit of the method become decisive.
How Molybdate Methods Work
In the silicomolybdate method, silica and phosphate react with molybdate ions in an acidic medium to form yellow complexes. When citric, oxalic or tartaric acid is added, the phosphomolybdate complex is masked and only silicomolybdate remains. If silica is high, the yellow color is read directly. At low concentrations, silicomolybdic acid is reduced to a deep blue molybdenum blue complex; the color intensity is proportional to the silica concentration. The SP-800 implements these two paths as SiHR (yellow, silicomolybdate) and SiLR (blue, heteropoly blue).
These methods detect only reactive silica. Trimers, higher polymers and larger colloidal particles are not detected by the molybdenum blue method. The Handbook on Power Plant Chemistry notes that while colloidal silica is unlikely in the thermal power plant feedwater-steam cycle itself, it cannot be ruled out from raw water through to demineralized water. Ultrafiltration units are cited as the treatment stage that removes colloidal silica.
In SP-800 measurements, the sample temperature must be 15–25°C for SiHR; SiLR reaction times are specified for 20°C (at 10°C the 1st period becomes 8 min and the 2nd period 2 min; at 30°C the 1st period drops to 2 min and the 2nd period to 30 s). In both methods, glassware is rinsed with 1:1 hydrochloric acid solution and deionized water. Reagents are used in accordance with the sets specified by the manufacturer of the method.
Testomat 808 SiO2 Installation Conditions
| Item | Value (product page, 808 series) |
|---|---|
| Measuring range (SiO2) | 0.3–1.2 mg/L, in 0.1 mg/L steps |
| Operating pressure | 1–4 bar (0.3–1 bar in some versions) |
| Water temperature | 10–40°C |
| Power supply | 230 V, 50–60 Hz (24 V / 50–60 Hz version also available) |
| Outputs | 4-20 mA analog, 3 potential-free relays, RS232 (firmware update) |
| Unattended operation | Up to 72 hours with manual safety inspection |
| Flushing | Internal and external flushing options |
The same series is also used for hardness breakthrough; the hardness version covers 0.02–5 °dH. For hardness and alkalinity monitoring, see our online analyzers page.
Where Silica Comes From and How It Is Reduced
Silica monitoring is most often performed at the outlet of a removal stage. Silica rejection of EDI modules depends on feed conditions: Ionpure MX and LX series specify 90–99% silica removal, VNX-Max and VNX-Mini specify over 95%, and the VNX55 series for power generation specifies over 98% silica removal. Reverse osmosis and electrodeionization selection for boiler feedwater are covered in our RO membrane selection and EDI system selection guides, as well as the water grade guide. For boiler house parameters, see boiler water conductivity, and for trace oxygen in feedwater, see the boiler water dissolved oxygen measurement guide.
Brand and product pages: Ionpure MX, Ionpure VNX, Ionpure LX.
Frequently Asked Questions About Silica Measurement
Why it is monitored, Testomat 808 SiO2, SP-800 methods, reactive and colloidal silica.
Why is silica monitored in boiler feedwater?
In medium- and high-pressure boilers, silica is the main salt carried over into steam because its solubility in steam is higher than that of other salts and metal oxides. Silica carried with steam deposits on turbine blades and nozzles, reducing turbine efficiency. The Handbook of Water Treatment states that silica in steam must be kept below 0.02 mg SiO2/L to prevent silica deposition on turbine blades; this means limiting boiler water silica according to boiler pressure.
Do you offer a continuously operating silica analyzer?
The solutions described on this page fall into two categories: limit-value monitoring with the Testomat 808 SiO2 (0.3–1.2 mg/L window) and spot measurement with the Pyxis SP-800 handheld colorimeter. A continuous silica analyzer with an analog trend output is outside the scope of this page. If you have such a requirement, send your measuring range and sample conditions to [email protected] so we can clarify together whether it falls within scope.
In what range does the Testomat 808 SiO2 operate?
It operates in the 0.3–1.2 mg/L silicate range in 0.1 mg/L steps, using colorimetric titration and photometric evaluation according to the molybdate method. Operating pressure is 1–4 bar (0.3–1 bar in some versions), water temperature is 10–40°C, and power supply is 230 V at 50–60 Hz; green/red LEDs indicate the limit status, and 3 potential-free relays plus a 4-20 mA output are included. With manual safety inspection, it can operate unattended for up to 72 hours. These series specifications are taken from the product page; request confirmation for the SiO2 version.
How is silica measured with the SP-800?
The SP-800 includes two silica methods: high-range SiHR (1.0–75.0 ppm SiO2, silicomolybdate method, APHA 4500-SiO2 C) and low-range SiLR (0.02–5.0 ppm SiO2, heteropoly blue method, APHA 4500-SiO2 D). In both methods, two cuvettes are filled, one serves as the blank, reagents are added, and zeroing and reading are performed when the reaction time expires. Reaction times are 10 minutes for SiHR and 4+1+2 minutes at 20°C for SiLR.
What is the difference between reactive silica and colloidal silica?
The molybdenum blue method detects only dissolved monomeric and dimeric silica (reactive silica); trimers, higher polymers, and larger particles retained by fine filters (colloidal silica) are not detected. The Handbook on Power Plant Chemistry notes that in thermal power plant applications, determining both reactive and colloidal forms may be necessary, and that the possibility of colloidal silica cannot be excluded from raw water through to demineralized water. Ultrafiltration removes colloidal silica.
Does phosphate interfere with silica measurement?
In the silicomolybdate method, phosphate also produces a yellow color. In the SP-800 method, citric acid is added to destroy the phosphate complex: in SiHR, citric acid is added after the 10-minute yellow color development, and in SiLR before the heteropoly blue step. In both methods, the glassware used is rinsed with a 1:1 hydrochloric acid solution followed by deionized water.
How is silica breakthrough interpreted?
An increase in silica at the outlet of a deionization or RO/EDI system is an indirect indicator of declining resin or membrane performance. When demineralized water quality deteriorates, the Handbook of Water Treatment lists typical causes as raw water quality degradation, draw exceeding demineralizer capacity, insufficient regeneration, resin degradation or fouling, and mechanical malfunction.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.