Countering silica’s foul play1 July 2007
Silica is all too often a prime cause of reduced efficiency and failure in power generation plant. Only proper monitoring can alleviate the problem.
There's an old saying in industry that 'you can't control what you don't measure' and silica in power generation is no exception. Silica deposits can impair the performance of equipment to such an extent that it is imperative to keep it under tight control.
Silica is a major culprit behind the build up of hard and dense scales inside the boilers and turbines of power generation plants. At a time when power companies are anxious
to optimise their operations in line with business and environmental pressures, they can ill afford to operate plants suffering from the impaired heat transfer that results from this type of fouling. Although boiler feed water is treated to remove silica and other ionic contaminants, effective long-term control of silica can only be maintained by using the correct monitoring system.
Silica forms a dense porcelain-like scaling that cannot be removed with acid. Silica scaling also has a very low thermal conductivity. Because of its low thermal conductivity, a 0.5 mm build up of silica can reduce thermal transfer by 28%, reducing efficiency, leading to hot spots and ultimately ruptures.
Where to monitor
The first area of a power plant that can benefit from silica monitoring is the demineralisation plant responsible for removing ionic contaminants from the make-up water. This removal is typically achieved in three ion exchange beds. First a cation bed removes positive ions such as sodium, calcium and ammonium and replaces them with an H+ cation. An anion bed then strips out negative ions such as chloride, sulphate and nitrate, replacing them with hydroxyl ions (OH-). Finally a mixed bed removes residual contaminant ions to leave highly purified water.
Reactive silica is present in water as a weakly charged anion, which can be captured by the anion bed. However, the silica anion is held relatively loosely by the ion exchange resin and is therefore among the first species to break through the bed when it nears exhaustion. Monitoring the breakthrough of silica at the outlet from the anion bed is therefore a good indicator of when a bed needs regenerating. Regeneration is achieved by passing an alkali solution through the resin to reinstate the hydroxyl ions.
Monitoring silica at the outlet of the mixed bed again provides a useful check on the state of the anion exchange resin in the bed, as well as checking the quality of the water passing to the boiler as make-up water. The final level of silica in the boiler feedwater must be kept as low as possible to reduce the build up within the boiler drum and the subsequent carryover in the steam.
In drum boilers silica build-up is monitored inside the drum itself. Silica is distributed between the water and steam phases inside the drum, with the proportion in the steam rising as the temperature and pressure increase. In high pressure boilers in particular, appreciable levels of silica can be concentrated in the vapour and can be carried over and deposited on downstream equipment such as superheaters and turbine blades.
The level of silica in the drum is controlled using blowdown, but this ejects expensive treated water and energy each time it occurs. It is therefore important to monitor the build-up of silica to ensure that the blowdown cycle is optimised. The build-up of other contaminants is also taken into account with regard to blowdown cycles.
Measuring silica in the steam from the boiler, either at the superheater or at the entrance to the turbine, gives a good indicator of overall steam purity. Experience shows that there should be minimal scale deposition as long as the silica concentration remains below 20 ppb.
In addition to feed make-up water, the other main source of silica contamination is the water returning to the boiler from the condenser. The condenser cools the steam using locally sourced water that is not normally subjected to the same rigorous pre-treatment as process water. Unfortunately, many condensers are prone to leaks, which allows this cooling water to contaminate the resultant condensate.
Condensers operate at near perfect vacuum as the steam condenses back into water. This increases the likelihood of contamination problems occurring if there is even a small leak between the process side and the cooling water.
Many condensers are therefore fitted with polishing plants like the demineralisation plant already described. In the case of the condensate polishing plant however, the levels of contamination at the inlet are prone to wider fluctuations so it is even more difficult to predict when the ion exchange beds will be exhausted. Only continuous silica monitoring can eliminate the need for frequent, labour-intensive manual sampling and testing.
Unlike many other potential contaminants, dissolved silica is only very weakly ionised, so it cannot be detected using a simple conductivity measurement but instead requires a dedicated monitor.
On-line silica monitors measure the silica compounds dissolved in the water using standard Molybdenum Blue chemistry. This chemistry generates a blue coloured solution, the intensity of which is measured by colorimetry and is proportional to the silica concentration.
There are certain features to look for when choosing a monitor:
• The unit's liquid handling system should be designed carefully to minimise the need for any maintenance. For example, ABB's new Navigator 600 silica analyser (pictured) features precision-engineered multi-channel peristaltic pumps that only require annual maintenance.
• Because there are so many potential points in any power generation scheme that can benefit from silica monitoring, the ability of a monitor to handle multiple samples drawn from a number of different sources around the plant is another obvious advantage. Microprocessor-based electronics can provide programmable multi-stream switching between multiple sample points, along with a number of other useful features that can contribute to bringing down the cost of ownership.