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Monitoring & metering


Protecting power station performance with gap-free conductivity monitoring data


I


n an ideal world, continuous monitors would monitor continuously, but for some instruments downtime is necessary whilst


service operations are undertaken. To overcome this, Swan Analytical Instruments has developed a new technology for power and industrial water/steam monitoring systems to enable uninterrupted operation. Chemists at the South Humber Bank Power


Station (UK) have evaluated this innovative conductivity instrumentation, demonstrating its ability to avoid gaps in vital data streams, whilst also saving costs and lowering labour requirements. Swan’s AMI CACE (Conductivity After Cation Exchange), dispenses with the need to replenish the instrument’s cation exchange resin every month, which means that downtime is almost completely avoided. After almost one year of uninterrupted


operation with the AMI CACE, South Humber Bank Power Station chemist Paul Kelk says: “CACE is a vitally important measurement because it amplifies conductivity readings so that we can measure the potential threat of corrosion. We have found this new Swan instrument to be extremely reliable, with a fast response time, and it has saved significant costs because it does not require manual resin replenishment every month. However, for me, the main advantage is that the instrument does not have to go off-line; as is the case with conventional resin-based CACE instruments.”


18


South humber bank Power Station


The South Humber Bank Power Station is a 1,365 MW combined cycle gas turbine (CCGT) power station, owned by EP UK Investments, a daughter company of EP Power Europe. The plant was built in two phases in 1997 and 1999. Phase 1 consists of three 166 MW gas turbines with three heat recovery steam generators, and a 255 MW steam turbine. Phase 2 consists of two 169 MW gas turbines with two heat recovery steam generators, and a 171 MW steam turbine. The plant operates once-through cooling using a deep-water intake drawn from about one mile into the River Humber estuary, at a rate of around 23m3


per second. why monitor conductivity?


Conductivity, in this context, is the non-specific ability of water in the water/steam cycle to carry an electrical current. This is made possible by the presence of dissolved substances – anions and cations, which may be treatment chemicals, dissolved gases or contaminants. The presence of these contaminants represents a threat to power station assets because of their ability to accumulate at points in the water/steam cycle and because they may cause corrosion in locations such as the main boiler, the steam turbine header, the condenser tubes and the hot well. Corrosion in these vital assets would necessitate remedial action, causing partial or complete plant


shutdown with heavy financial consequences. Turbine blades would also be vulnerable to corrosion, and given that they travel at 3,000 rpm, any damage could be catastrophic. With the benefit of various conductivity


measurements, South Humber Bank Power Station has been able to operate largely problem- free for many years. However, with an eye to the future, Kelk says: “We are constantly on the lookout for new technologies that offer an opportunity to improve performance.”


corroSion Prevention


Corrosion occurs in the presence of water and oxygen, when metals revert to their oxidised, more stable state, so dissolved oxygen in cooling systems is avoided. Carbohydrazide is employed as a water treatment chemical at South Humber Bank Power Station. As a powerful oxygen scavenger Carbohydrazide promotes the formation of a protective magnetite layer without interfering with the action of other treatment chemicals. Magnetite is an iron oxide,


(Fe3O4).which forms as a thin layer on steel surfaces and passivates them; thereby protecting the surfaces from corrosion. Corrosion is an electrochemical process, and


any build-up of dissolved contaminants could harm the magnetite layer and expose steel surfaces to the risk of corrosion. This risk is amplified where high pressure and temperature accelerate the corrosion process. So, the monitoring of


April 2021 Instrumentation Monthly


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