Silica Analysis in Power Plant:
1 - Silica monitoring in boiler feedwater and steam
Development of modern power plants using new types of boilers has led to steadily higher operational pressures. Critical pressure of water is approached and, in some cases, exceeded.
Because of this, many problems, insignificant in the past, have now assumed critical importance. Disregarding purely technical and design problems, a whole spectrum of physical/chemical problems have come to the fore, concerning mainly the steam solubility of inorganic compounds. Among the many contaminants in the steam/water circuit, silica plays a special role because of its high solubility in the steam. In fact, silica is a very weak acidand is not completely dissociated (ionized). i.e. at pH 10, 50% of the silica present in boiler water is undissociated. The undissociated silica is the part which is soluble into the steam. In the case of a two phases water/steam, the solubility is dependent on the pressure. At a given pressure an equilibrium is established which results in given concentration distribution of silica in the respective phases: steam and water.
The steam, when passing through the turbine, comes into contact with the turbine blades, is cooled down and the silica dissolved in the steam deposits upon the blades. This coating is very difficult to remove and can unbalance the whole system. The possible consequences can be a loss of efficiency or, worse, a forced stop of the plant for repair or blades exchange. Experience has enabled the industry to specify the allowable concentration of SiO2 in steam to avoid turbine damage. I.e. for a 180 bar operating pressure in order to get 5 ppb max. of SiO2 in the steam, boiler water should not contain more than 100 ppb SiO2 ... if ideal boiler conditions are met (drum boiler type, mechanical , vaporous carry-over is not considered, neither hide out effects in the boiler nor water treatment plant and condensate polishing). Once-though boilers require SiO2 concentration to be lower than drum boilers, since all water (and impurities it contains) is converted into vapour and there is no possibility for blow-down. As explained above, an excessive silica concentration in the boiler water can have dramatic impact on the power plant efficiency. It is therefore logical that this parameter be closely monitored.
Silica concentration can be measured at the following process steps which may vary from one plant to the other depending on their architecture and on the way the plant is managed.
- Boiler blowdown (drum boilers only),
- Economizer outlet,
- Steam,
- Make-up water,
- Condensate polishing,
- Demineralisation plant (cf.§2)
2 - Silica monitoring at the demineralization stage
The performance of anion exchangers and mixed-bed is generally monitored with the silica as the indicative parameter. Both the resins efficiency and exhaustion (break-through) may be monitored with high sensitivity and reliability. The benefits of such a practice are considerable:
- Follow-up of the demineralisation process performance,
- Better utilization of the resin capacity,
- Optimization of regeneration cycles.
3 - Range of measurement
Normal operating condition will, in most cases, result in SiO2 concentration of around:
- 5 to 20 ppb at the anion exchangers output,
- 2 to 5 ppb at the mixed-bed and economizer output,
- Less than 5 ppb in the steam,
- Blowdown water can contain up to several thousands of ppb of silica.