Water, steam sampling strategies for cycling plants

If you can’t measure a process, you can’t control it. And although good sampling systems are critical for any plant, they become more critical during flexible operation. This was the summary message from John Powalisz of Sentry Equipment Corp. His presentation focused on sampling techniques to protect assets, maintain output, predict failure, and prepare systems for startup and potential layup.

Steam and water sampling were covered first. As EPRI tells it in Report CS-5164, “Fossil Plant Cycle Chemistry Instrumentation and Control—State-of-Knowledge Assessment,” “The primary objective of any sampling system is to transport and condition a sample without altering the characteristics of interest. The system parameters which need to be controlled are velocity, pressure, and temperature.”

If the integrated steam and water sampling system fails to control flow, pressure, or temperature (secondary cooling); if it fails to give consistent online flows during startup or low load (night shift); or if it doesn’t help mitigate high iron transport during startup or load changes, impacts will be negative. In general, if flow is too high (startup), system components could be stressed and some readings might be inconsistent. If too low, data errors and air ingress are common.

Powalisz emphasized the most controllable areas: procedures, training, and technology. These become even more critical when cycling. The question: What is the best approach for each particular plant?

Attention turned to a series of manual-panel best practices requiring no capital investment. For example:

      • Monitor blowdown samples containing high levels of particulates for safety issues.

      • At startup, establish sample flow at minimum level to feed analyzers.

      • Close valved rotameters as soon as possible after a shutdown/cycle off to hold liquid in the flow cells and to keep probes wet.

      • Set VREL® valves (high-pressure sample flow-control valves) to the fully closed position during shutdown.

Simple low-cost upgrades also can improve sampling consistency:

      • Increase the size of primary sample coolers to allow more “forgiveness” (for improper settings).

      • If missing, add total flow indicators so that total sample flows can be set properly.

      • Use preset combination back-pressure regulator/relief valves to improve flow consistency to analyzers and eliminate lost flow through pressure-relief valves/lines.

      • Add ability to valve-in flush water to keep probes wet during shutdowns.

      • Add a degassed cation-conductivity panel to identify contamination during low-load operation.

      • Add a large-surface-area high-pressure magnetic trap or similar device to capture magnetite as far upstream as possible in lines with high particulate counts.

Alternative and supplemental-action discussions followed. The presentation then concluded with these recommendations:

      • Automate critical sample points to mitigate staffing issues.

      • Consider outsourcing maintenance to improve instrument reliability and alleviate small-staff challenges.

      • Address high iron transport/plugging by installing a high-pressure magnetite trap or by implementing automated blowdown practices—or do both.

Good sample conditioning systems should increase safety for both personnel and instruments, provide representative samples to the analyzers, be easy to set up and use, be intuitive, and have low maintenance costs.

Available standards and guidance documents were then listed for ASTM, ASME, EPRI, IAPWS and VGB.

The bottom line was this: You might not be able to control operational modes or numbers of employees, but you can control procedures, training, and choice of technology.

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