Energy management: Silica in plants

The West Burton B Combined Cycle Gas Turbine (CCGT) power station in Nottinghamshire has three 435MW open-cycle gas turbines that generate enough power to supply roughly 1.5 million homes.

Like all combustion-based power stations, the West Burton B CCGT plant has a requirement to monitor silica levels within its water and steam processes.

As a CCGT power station we have an extra focus on accuracy because of the finer tolerances that feature in this plant

Sean Todd

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 used within power plants.

Silica deposition on plant equipment can cause pitting and other defects, however, it is also likely to cause an imbalance in the turbine blades, which in turn causes vibration and could result in failure.

Likewise, silica deposition in boiler tubes must also be avoided because it causes a loss of thermal efficiency, thereby reducing the efficiency of the whole plant.

During February last year, EDF Energy - who operates West Burton B - replaced the power station’s silica monitoring systems with HACH 5500sc Silica Analysers.

“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,” says Sean Todd, the plant’s Control & Instrumentation team leader.

“Over the last 10 months, the new monitors have proved to be extremely accurate and reliable, requiring very little maintenance,” he says.

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.

What’s more, the original monitors required frequent recalibration, causing a heavy maintenance requirement and resulting in excessive use of reagents.

The HACH 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.

The HACH 5500sc Silica Analysers take measurements from each sample stream every 15 minutes, providing Todd with almost live data on performance, which enables him to take appropriate action before alarm levels are reached.

To conduct the analysis, the 5500sc measures silica by reacting samples with molybdate ions under acidic conditions to form silicomolybdic acid complexes.

The addition of citric acid destroys the phosphate complexes, and amino acid reagent is then added to reduce the yellow silicomolybdic acid to an intense blue colour, which is proportional to the silica concentration, and measured optically at 815 nm.

Typically, just two litres of reagent are required for the analyser to perform unattended for up to 90 days.

“As a CCGT power station we have an extra focus on accuracy because of the finer tolerances that feature in this plant,” Todd says.

“Boiler chemistry is an important issue for us, so the development of accurate silica monitors has been an enormous benefit.”