SILICA MONITORING SUPPORTS POWER STATION EFFICIENCY

Silica deposition on power station turbine blades can cause defects and an imbalance in the blades, which in turn causes vibration and could even result in failure. Silica deposition in boiler tubes is also to be avoided because it causes a loss of thermal efficiency, thereby reducing the efficiency of the whole plant.

West Burton B CCGT is a Combined Cycle Gas Turbine power station near Nottingham in England. It has three open-cycle gas turbines capable of generating enough electricity for approximately 1.5 million UK homes. In common with all combustion based power stations, the CCGT has a requirement to monitor silica levels in the Plant's water and steam processes.

Sean Todd, the Plant's Control & Instrumentation (C&I) Team Leader, says: "Boiler chemistry is one of the major challenges facing power plant managers, and the monitoring systems that were originally installed at this plant met the required specification but failed to deliver the accuracy and reliability that we need.

"At the Start of February 2014, we replaced the monitors with HACH 5500sc Silica Analysers and have been absolutely delighted with the results; over the last ten months, the new monitors have proved to be extremely accurate and reliable, requiring very little maintenance."

Prior to the installation of the HACH 5500sc Silica Analysers, the unreliability of the original monitors meant that manual samples had to be taken twice daily from multiple points. Some of these samples were tested in a site laboratory, but others had to be sent to a sister laboratory approximately 50 miles away. This incurred excessive costs and delays. Furthermore, the original monitors required frequent recalibration, causing a heavy maintenance requirement and resulting in excessive use of reagents.

The importance of Silica
Making up a large percentage of the earth's solid crust, silicon (Si) is second only to oxygen in natural abundance. Silicon is present in small concentrations in all natural water supplies, usually as dissolved silica or as small suspended silicate particles (colloidal silica). Silicon dioxide, also known as silica, is a chemical compound that is an oxide of silicon with the chemical formula SiO2. Silica is highly soluble in steam, so if sufficient quantities are present, it may deposit as a glass-like substance on the surfaces of turbine blades and boiler tubes.

Silica deposition on turbine blades can cause pitting and other defects, however, it is also likely to cause an imbalance in the blades, which in turn causes vibration and could even result in failure due to the fine tolerances between turbine blades and outer casings. Turbines cost tens of millions so effective inspection and maintenance is a high priority. Silica deposition in boiler tubes is also to be avoided because it causes a loss of thermal efficiency, thereby reducing the efficiency of the whole plant.

Monitoring systems
Silica concentration data is required to demonstrate compliance with the required levels for boiler feedwater and saturated steam. However, the data is also necessary for operational purposes; to control pH in the boilers with ammonia dosing for example. In addition, the data is required for insurance purposes.

In order to avoid silica deposition, high levels of treatment are necessary to produce almost totally demineralised water. CCGT draws its water supply from the River Trent and a variety of treatment processes are necessary before the water is pure enough for use. These processes include sedimentation, filtration, flocculation and ion exchange. The silica analysers draw treated water samples from the plant to ensure that levels are within acceptable limits. Online samples are also drawn from strategically located points in the plant to ensure that silica levels remain at acceptable concentrations throughout the process.

Online monitoring is conducted in two locations; in the Water Treatment Plant for monitoring the efficiency of the ion-exchange plant and in the 'boiler analyser house' which contains two Hach 5500sc silica analysers, a Hach 9240 sodium analyser, two HACH ORBISPHERE K1100 LDO optical dissolved oxygen probes, five pH probes and twenty one conductivity probes. Continuous water samples of steam and blowdown steam are delivered to the analyser house by a network of sample tubes from around the plant.

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HACH 5500sc Silica Analysers