Controlling silica in boiler water is primarily achieved by removing it from the feedwater before it enters the boiler and managing its concentration within the boiler through controlled blowdown.
Silica is a common impurity found in water supplies and can cause significant problems in boilers, leading to deposits on heat transfer surfaces and turbine blades (silica carryover). Unlike other suspended solids that can be removed by simple filtration methods like coagulation and filtration, such processes have little effect in reducing the soluble silica content. This means specialized water treatment is necessary. Furthermore, silica is unique among common boiler water salts because it vaporizes at pressures below 2400 psig, potentially as low as 400 psig, leading to vaporous carryover into steam.
Here's a breakdown of how to control silica:
Controlling silica is crucial for preventing:
- Scaling: Silica can form hard, insulating deposits (scale) on boiler tubes, reducing heat transfer efficiency, increasing fuel costs, and potentially leading to tube failure due to overheating.
- Turbine Deposits: Vaporized silica carried over with steam can deposit on turbine blades, reducing turbine efficiency, causing imbalance, and potentially leading to forced outages.
- Corrosion: Although less direct than some other contaminants, silica deposits can hide corrosive conditions underneath.
Methods for Controlling Silica
Effective silica control involves treating the water before it enters the boiler and managing the concentration within the boiler.
1. Feedwater Treatment (Primary Control)
The most effective way to control silica in boiler water is to remove it from the incoming water supply (feedwater). Since coagulation and filtration primarily remove suspended matter and have little effect on soluble silica, advanced treatment methods are required.
- Demineralization (Ion Exchange): This is a widely used method. Water passes through ion exchange resins that capture dissolved minerals, including silica. Mixed-bed demineralizers are highly effective at reducing silica to very low levels required for high-pressure boilers.
- Reverse Osmosis (RO): RO uses a semipermeable membrane to separate dissolved solids, including silica, from water. RO can significantly reduce silica concentration, often serving as a pre-treatment step before demineralization for very high purity requirements.
- Electrodeionization (EDI): Often used after RO and/or demineralization, EDI further polishes the water, using electricity, ion exchange resins, and membranes to remove remaining ions, including silica.
Example: For high-pressure boilers (e.g., > 900 psig) where silica carryover is a major concern due to its vaporization characteristics, extensive feedwater treatment using RO followed by mixed-bed demineralization is common practice to reduce silica in the feedwater to part-per-billion (ppb) levels.
2. Boiler Water Management (Secondary Control)
Even with effective feedwater treatment, some silica may enter the boiler. Managing its concentration within the boiler water is the secondary control method.
- Blowdown: This involves periodically or continuously draining a portion of the concentrated boiler water and replacing it with lower-silica feedwater. Blowdown is the primary way to control the concentration of all dissolved solids, including silica, in the boiler water. The required blowdown rate depends on the silica concentration in the feedwater and the maximum allowable silica concentration in the boiler water (which varies with boiler pressure).
Example: If the feedwater has a low level of silica, but it still enters the boiler, controlled blowdown is performed to keep the silica concentration in the boiler water below the point where excessive scaling or carryover would occur. Higher pressure boilers require lower silica concentrations, thus often requiring more precise blowdown control or higher quality feedwater treatment.
Target Silica Levels
The acceptable level of silica in boiler water depends heavily on the boiler operating pressure. Higher pressures require lower silica concentrations to prevent vaporization and carryover.
| Boiler Pressure (psig) | Recommended Maximum Boiler Water Silica (ppm SiO₂) |
|---|---|
| 0 - 300 | 150 |
| 301 - 450 | 90 |
| 451 - 600 | 50 |
| 601 - 750 | 35 |
| 751 - 900 | 25 |
| 901 - 1000 | 15 |
| 1001 - 1500 | 2.5 |
| 1501 - 2000 | 1.0 |
| 2001 - 2400 | 0.5 |
| > 2400 | 0.1 (ppb levels required) |
Note: These values are general guidelines and may vary based on specific boiler design and steam purity requirements.
By implementing robust feedwater treatment methods to remove soluble silica and maintaining controlled blowdown based on boiler pressure and monitoring, operators can effectively control silica levels in boiler water and ensure reliable, efficient boiler operation.
