Reliable steam quality measurement is important for reliable operation of an OTSG. Even though the quality measurements from the inline devices are frequently calibrated against measured samples, the online measurements show a considerable amount of drift from recent calibrations. False low steam quality measurements may lead to undetected dryout and potential damage. False high steam quality measurements may lead to spurious OTSG trips, and the subsequent hazards of lighting a major piece of fired equipment. Reliable steam quality measurement and control becomes even more important as we push for higher thermal efficiency, such as with rifled tubes, and operate closer to the edge of dry-out. The theory behind a standard Venturi-based steam quality measurement will be reviewed first. The accuracy of common steam quality correlations will be compared under steady state operation. A Computational Fluid Dynamics simulation for the water side dynamics will then be used to demonstrate how “shrink” and “swell” manifests along the tube length of an OTSG tube pass. These processes will be shown to affect the actual and measured steam quality for the pass, reminiscent of how shrink and swell affects drum level dynamics in drum boilers. These water-side dynamics are used to justify process control strategies that target more uniform steam quality.

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