Particle Analysis
Traditional monitoring of metal transport and generation in the steam cycle relies primarily on periodic wet tests such as "patch" tests or photometric analysis. This approach, though valuable, leaves significant holes in the data history. Every thermal, chemical, or hydraulic event liberates or generates metal oxides in the steam cycle called a "crud burst," and these events occur frequently yet randomly.
Time-based testing (iron sampling at a specific frequency, for example) is important, but it cannot detect the majority of these events. Thus, plants are flying blind during normal operation. The only reliable method for determining metal transport is periodic inspections and tube deposit weight density measurements. These methods are important and must continue, but they can only detect a metal transport problem after it has occurred. What’s missing is an intermediate step where plant operations can be tuned to minimize metal transport before damage is done.
Particle analysis provides a near-real-time measurement of metal liberation and transport. Two different technologies can be used: particle size analysis and particle counts.
The particle monitor, such as that sold by Chemtrac Systems Inc., detects particulate contamination in a flowing, liquid sample by dynamic light obscuration (DLO). As particles pass through the illuminated sensing area, they obstruct a portion of the transmitted light. A sensitive photodetector converts these light fluctuations into a root mean square signal representative of particulate size and concentration. The online flow-through sensor eliminates sampling/testing variances encountered in the more-familiar manual Millipore analyses used to count particles. Multiplexed sensors provide complete, real-time monitoring of individual condensate streams or any other component of the boiler system.
Use the AT Tools
An ideal monitoring program would correlate the interplay of the oxidizing environment with all of the possible corrosion stressors across the ORP space and relate that data to corrosion product transport. Combining particle counts with AT ORP technology closes the loop on steam cycle metal transport. This combination offers two windows into the process: an extremely accurate measurement of the oxidizing environment (AT ORP technology) with control capabilities and a measurement of the impact of that operating environment (particle counts and metal transport). AT ORP technology and particle analysis also can provide visibility into previously undetectable events. The two technologies can be used to correlate particle counts with metal (iron and copper) transport test results and operating environment (AT ORP technology).
It is possible to generate a set of plant-specific corrosion product transport curves using particle analysis and traditional monitoring techniques. Once that relationship has been established, then the particle counter output can be scaled to match the text results. Thus, particle analysis could generate an output proportional to the actual concentration of particulate metal oxides in ppb levels.
The next step is to use the particle counter and AT ORP technology to evaluate the impact of metal liberation and transport. Event analysis can be used to identify those plant evolutions that result in the greatest metal transport and the greatest change in the oxidizing environment. With this list, plants can then tune operation to minimize metal transport during these events. The monitoring program can then be used for real-time metal transport and oxidizing environment monitoring once the initial metal transport mitigation system has been implemented. Plants can use these monitoring tools to detect any significant increase in metal transport, identify the cause, and take corrective action.
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