Boiler water silicate analyzers primarily operate based on the molybdenum blue spectrophotometric principle: silicate in water reacts with ammonium molybdate reagent to form yellow silicon molybdenum yellow, which is then reduced by a reducing agent to stable silicon molybdenum blue. By measuring its absorbance value, the silicate content can be accurately calculated. This is a key detection method for ensuring boiler safe operation.
For industries relying on high-pressure boilers, such as power plants and chemical plants, silicate (SiO₃²⁻) is an indicator that requires strict monitoring. Its solubility increases in high-temperature, high-pressure boiler water, allowing it to volatilize with steam and ultimately deposit on turbine blades.
Data shows: If the silicon content in steam exceeds 20 μg/kg, it can form hard silicate scale on turbine flow passages, reducing unit efficiency. According to the 《GB/T 12145-2016 Water and steam quality for thermal power generating units and steam power equipment》 standard, a silica scale thickness of 0.1 mm can decrease turbine efficiency by 2%-5%. This could mean increased fuel costs amounting to millions of RMB annually for a 300MW unit.
Its core hazards include:
Reduced thermal efficiency: The thermal conductivity of silica scale is only 1/10 to 1/100 that of steel, severely impeding heat transfer.
Induced corrosion: Concentrated impurities under the deposit can cause under-deposit corrosion, leading to tube rupture risks.
Damage to turbines: Silica deposits on blades alter the aerodynamic profile, reducing output and efficiency.
Therefore, accurate and rapid detection of silicate is the cornerstone for preventing the above issues and achieving "zero scaling" operation.
Currently, the molybdenum blue spectrophotometric method recommended by the national standard 《GB/T 12149-2017 Determination of silica in industrial circulating cooling water and boiler water》 is the most widely used and mature technology. Its principle can be divided into two key chemical reactions:
Step 1: Formation of Silicon Molybdenum Yellow (Yellow Complex)
Under acidic conditions (pH ~1.2), reactive silica (soluble silicate) in the water sample reacts with the ammonium molybdate reagent to form yellow silicomolybdic acid, commonly known as "silicon molybdenum yellow".
SiO₃²⁻ + 12MoO₄²⁻ + 22H⁺ → H₄[Si(Mo₃O₁₀)₄] (Silicon Molybdenum Yellow) + 9H₂O
Step 2: Reduction to Silicon Molybdenum Blue (Blue Complex)
Subsequently, a reducing agent like ascorbic acid (Vitamin C) is added to reduce the yellow silicon molybdenum yellow into a very stable blue complex – silicon molybdenum blue.
H₄[Si(Mo₃O₁₀)₄] + 2C₆H₈O₆ → H₄[Si(Mo₃O₁₀)₃] (Silicon Molybdenum Blue)
Detection Key: The resulting silicon molybdenum blue has a maximum absorption peak near 815 nm. The depth of its color (absorbance) is proportional to the silicate concentration. The optical system inside the analyzer measures the absorbance and compares it with a pre-calibrated standard curve to directly display the silicate concentration.
Following the national standard method, manual laboratory detection typically includes the following steps. Understanding these steps helps in comprehending the logic of automated instruments:
Sampling & Dilution: Precisely measure a certain volume of water sample using a polyethylene bottle (pre-dilute if silicon content is high).
Reagent Addition & Reaction: Sequentially add acidified ammonium molybdate solution, mix well, and let stand for 5-10 minutes to ensure complete formation of silicon molybdenum yellow.
Reduction & Color Development: Add tartaric acid solution (to mask phosphate interference) and ascorbic acid solution, mix thoroughly.
Standing & Measurement: Let stand for 5-10 minutes for full color development, then pour into a cuvette and measure absorbance at 815 nm wavelength using a spectrophotometer.
Result Calculation: Calculate the silicate content in the sample based on the standard curve.
Accuracy Range: This method is suitable for silicon content detection from 0.01 mg/L to 5 mg/L, with a detection limit as low as 0.01 mg/L, meeting the monitoring needs of most boiler water quality applications.
Boiler Pressure Grade (MPa) | Boiler Water Silicate (mg/L) | Saturated Steam Silicon Content (μg/kg) | Reference Standard |
≤5.9 | ≤2.0 | ≤20 | GB/T 12145-2016 |
5.9~12.0 | ≤0.45 | ≤15 | GB/T 12145-2016 |
12.0~18.0 | ≤0.20 | ≤10 | DL/T 805.1-2011 |
Note: Specific control indicators need dynamic adjustment based on boiler design, feedwater quality, and actual operating conditions.
From traditional laboratory spectrophotometers to modern portable analyzers, technological progress has made detection simpler. Key points to focus on when selecting an instrument include:
Detection Principle: Confirm it uses the standard molybdenum blue spectrophotometric method, as this is the foundation for data accuracy.
Degree of Automation: Manual reagent preparation and step-wise addition are not only time-consuming but also prone to human error.
Portability & Field Suitability: Is it necessary to bring samples back to the laboratory, delaying warning times?
Addressing these pain points, the ERUN-SP7-D2 Portable Silica Analyzer launched by Erun Environment provides an efficient solution. This instrument is designed for rapid field detection:
One-Button Operation: Built-in national standard method process, automatic reagent addition, mixing, and timing to avoid manual errors.
Fast Results: Completes the entire process from sampling to concentration display within 10 minutes.
Portable & Durable: Compact body, equipped with a rugged shell and built-in battery, suitable for field use in power plants and chemical plants.
Data Traceability: Stores a large amount of detection data and supports printing, facilitating quality tracking and analysis.
Background: A 650 MW supercritical coal-fired thermal power plant in East China experienced an unexplained slow decline in the efficiency of its high-pressure turbine cylinder.
Problem Investigation: Routine laboratory tests showed boiler water silicate at 0.08 mg/L, well below the national standard limit. However, intensive sampling and testing of the main steam using the ERUN-SP7-D2 Portable Silica Analyzer revealed that the instantaneous peak silicon content in the steam frequently touched the alarm line of 18 μg/kg.
Analysis & Solution: It was determined that "silica transport" occurred during load fluctuations, where trace amounts of silica in the boiler water were more easily carried by steam during pressure and temperature changes. Through rapid diagnosis with the portable instrument, the power plant promptly adjusted boiler operating parameters and blowdown procedures, stabilizing the steam silicon content below 10 μg/kg. This successfully prevented further scaling deterioration and avoided an unplanned shutdown.
Q1: Can phosphate cause interference during detection?
A: Yes. Phosphate can also react with ammonium molybdate to form phosphomolybdenum yellow, which is the main interference factor. The national standard method effectively masks phosphate interference by adding tartaric acid or oxalic acid, ensuring the specificity of the detection results.
Q2: Why is it emphasized to use polyethylene bottles for sampling?
A: Because glass bottle walls can leach trace amounts of silicon, contaminating the water sample and leading to falsely high results. Polyethylene material hardly absorbs or releases silicon, making it the standard container for silicon sample collection.
Q3: Is the data from portable analyzers reliable?
A: Yes, reliable. Taking the ERUN-SP7-D2 as an example, its core optical system is precisely calibrated, the detection process fully simulates the national standard method, and it uses a built-in curve for multi-point calibration. The data is highly consistent with laboratory analysis results, fully meeting the accuracy requirements for industrial monitoring.
Accurate monitoring of silicate in boiler water is a scientific means to foresee risks and ensure the long-term, safe, and economical operation of power equipment. Mastering the molybdenum blue method principle, understanding national standard requirements, and utilizing efficient portable analyzers like the ERUN-SP7-D2, can empower your team to shift from passive response to active prevention, firmly safeguarding the lifeline of steam quality.
