Recent advances in water laboratory instrumentation—from improved sample conditioning to advanced online instruments—have reached the market. Here’s a look at the equipment you’ll find in the best-equipped power plant laboratory this year.
Steam cycle chemistry analysis begins with the sampling methods and ends with the analytical instruments used. These tools don’t last forever.
The average lifespan of a sample and analytical panel is 10 to 15 years. After that, online analyzers degrade, become less reliable, and replacement parts become increasingly difficult to source. If your power plant was built in the 1990s and you haven’t already overhauled your sample and analytical panel on the unit, you are probably long overdue for an upgrade. The sampling system should also be overhauled if you are receiving a hot sample or if the sample stream is arriving as a trickle. A state-of-the-art analyzer that’s provided with poorly conditioned samples is of little help. Quality sampling and analysis are critical for monitoring power plant chemistry.
There have been many innovations in making sample streams more consistent and representative. Instruments that monitor a power plant’s water and steam chemistry have made significant advances over the past few years, especially by lowering the detection limits for critical impurities. The net result is more precise water and steam chemistry and fewer chemistry-related failures caused by corrosion and contamination when the monitoring is heeded and contamination is prevented.
Recent advances are the result of the new normal: Most utilities no longer have staff trained in chemistry at the plant 24/7 to monitor critical steam and water cycle parameters. Additionally, more of the analyses must be automated, instead of using grab samples, in order to achieve real-time results with the required accuracy.
An accurate understanding of your plant’s steam cycle chemistry depends upon following the current guidelines for collecting and processing water and steam samples and being aware of the latest advances in online analyzers.
Follow the Leaders
The recommendations of the Electric Power Research Institute, American Society of Mechanical Engineers, and, most recently, the International Association of the Properties of Water and Steam (IAPWS) require online continuous analysis of all parameters that are critical to minimizing corrosion and deposit formation in the boiler or turbine. Each of the water chemistry parameter limits set by these industry organizations links directly back to specific failure mechanisms in the steam and water cycles. These published recommendations give us guidance as to the parameters monitored and their concentration levels. This information also tells us the type of equipment and the accuracy requirements of equipment purchased for the plant water laboratory. For example, the IAPWS recently published recommendations for the concentrations of key parameters used to monitor the steam cycle (Table 1).
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| Table 1. Summary of minimum key instrumentation requirements for power plant water analysis. Source: IAPWS Technical Guidance Document: Instrumentation for Monitoring and Control of Cycle Chemistry for the Steam-Water Circuits of Fossil-Fired and Combined-Cycle Power Plants, September 2009. |
In the past, the concentration of some of the critical water treatment contaminants that can damage the turbine could not be measured directly at the part per billion (ppb) levels, so indirect measurements were substituted. For example, the familiar cation conductivity analyzer has been a required parameter by turbine manufacturers because it provides a simple and robust means of indirectly detecting ppb levels of chloride and sulfate. Another indirect measure is sodium. Sodium ions per se don’t damage turbine components, but sodium hydroxide does. We can’t measure the ppb levels of hydroxide, so we measure sodium instead, and we assume that all of the sodium in the steam is there in the form of caustic (sodium hydroxide) as a worst case.
But these indirect measurements can be fooled by other contaminants that may not be harmful. For example, dissolved carbon dioxide in the form of bicarbonate will raise cation conductivity, though it has been shown that the dissolved carbon dioxide in the steam is not harmful to the turbine. During commissioning, this has caused many a battle between the owner and turbine manufacturer about the warranty and how reasonable or unreasonable the cation conductivity limits are.
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