Extensive online chemical monitoring of modern power stations’ water/steam cycle and water treatment plant has become a widely established practice. This monitoring is key to enabling tight water chemistry control, achieving peak efficiency, and minimizing the downtime linked to excessive boiler scaling or corrosion.
Well-balanced water chemistry is vital for optimizing the efficiency of the steam-raising and distribution plant. Image Credit: ABB
Well-balanced water chemistry helps to optimize the availability and efficiency of boiler plant machinery in power station applications. If the plant chemistry varies from specified limits, expensive plant outages can occur, potentially incurring costs of over $1,000,000 per day.
Therefore, accurate and reliable monitoring of water quality covering a range of parameters is essential to ensure the continuous and efficient operation of power-generating equipment.
The Application
The accurate measurement of dissolved oxygen is dependent on the efficient, cost-effective operation of boiler plants with minimal downtime. Oxygen is highly corrosive to most metals in its dissolved form, particularly the mild steel employed in boiler tubes.
Even small quantities of dissolved oxygen in boiler water can considerably hinder a boiler’s operation, corroding its vital components and drastically reducing its working life.
One notable issue is pitting. This form of localized corrosion can rapidly cause extensive damage to a specific point or area in the boiler tubes and surface. Pitting is worsened in cases where there are also high levels of highly corrosive chloride, and this is magnified in boilers with high chloride levels.
Chlorides are also a major cause of corrosion in power applications. As an oxidizer, chlorides, acting with excessive oxygen levels, can attack and damage the boiler's weakened parts.
To minimize damage caused by corrosion, dissolved oxygen must be reduced to the lowest possible level, typically five parts per billion or less.
The Challenge
An efficient deaeration process can substantially reduce oxygen levels in boiler feedwater. This involves relaying a mix of returned condensate from the steam turbine and make-up water to the deaerator via an extraction pump, which heats the mixture under vacuum.
This process can generally lower dissolved oxygen concentrations from several parts per million to a few parts per billion, but the risk of oxygen ingress via leaks in the deaerator or extraction pump glands means that dissolved oxygen levels can still remain above desirable levels following deaeration.
Therefore, some form of secondary treatment is administered to further reduce levels, generally by adding oxygen-scavenging chemicals such as sodium sulfite or hydrazine.
The Solution
Monitoring should be implemented in any setting with a risk of oxygen ingress into boiler feedwater. An appropriate monitoring system should be able to measure dissolved oxygen at a range of key points, such as the de-aerator inlet and outlet, the extraction pump discharge, and the economizer or boiler inlet.
It is possible to measure deaerator efficiency by monitoring at both the deaerator inlet and outlet. Discrepancies in oxygen levels between the inlet and outlet may indicate leaks in the deaerator casing, glands, or fittings.
Monitoring at the extraction pump discharge helps to identify whether oxygen has entered water through the extraction pump glands or via leaks in the condenser.
Operators can assess the efficiency of dosing regimes in settings where hydrazine or alternative oxygen scavenging chemicals are used by measuring for dissolved oxygen at the boiler inlet or economizer, addressing any fluctuations by increasing or reducing the dose quantities.
An effective monitoring system involves measuring dissolved oxygen at key points prior to the boiler. Image Credit: ABB
ABB’s Monitoring Solutions
An analyzer able to deliver a rapid response across both high and low dissolved oxygen concentrations is necessary to accommodate the combination of significant variations in oxygen levels during a plant's load cycle and the different levels required for different boiler chemistry regimes.
The Navigator 500 Low Level Dissolved Oxygen online analyzer from ABB accurately measures dissolved oxygen levels in process feed water by employing a galvanic-type sensor. This accurate, reliable instrument requires no maintenance and can monitor dissolved oxygen concentrations up to 20 parts per million.
Part of ABB’s Navigator 500 analyzer family boasts a reliable, compact, and accurate range of instruments specifically designed for high-purity water treatment and boiler chemistry monitoring applications.
The Navigator 500 Hydrazine, Dissolved Oxygen, and Sodium analyzers provide continuous online monitoring of boiler feedwater quality, helping to ensure optimal efficiency of boiler plant and ancillary equipment throughout the steam distribution loop.
ABB’s Navigator 500 low-level dissolved oxygen analyzer. Image Credit: ABB
The entire range features a separate sensor and transmitter section design, meaning that all analysis and signal conditioning take place within the sensor section and are transported digitally to the transmitter. Each transmitter can collect data from up to four sensing systems, facilitating comprehensive monitoring at multiple points without the expense of purchasing and fitting separate transmitters.
The system’s four transmitter inputs can be used to acquire signals on one parameter, or these can be mixed and matched, allowing multiple parameters to be fed to a single unit. The transmitter can also accommodate digital communications protocols, including Ethernet, allowing data to be efficiently relayed to a central control room.
ABB’s Navigator 500 and 600 online analyzers allow complete monitoring of all key boiler chemistry parameters.
Acknowledgments
Produced from materials originally authored by ABB Measurement & Analytics.
This information has been sourced, reviewed and adapted from materials provided by ABB.
For more information on this source, please visit ABB.