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Fouling Enhancement Under Flow Boiling At Elevated Steam Qualities
Stan J. Klimas, John M. Pietralik, Keith Fruzzetti, and Robert L. Tapping

Under laboratory conditions of flow boiling in water at 272–285°C (5.7 to 7.0 MPa), it has been observed that fouling rates by colloidal iron oxides (“crud”) dramatically increase upon reaching a certain steam quality and mixture velocity. In loop tests, an increase in fouling rates by up to 3 orders of magnitude was repeatedly observed. This effect is called here “heavy fouling under elevated steam quality” (HFESQ). HFESQ is potentially very significant for once-through steam generators, and very large versions of recirculating nuclear steam generators, because it can lead to heavy fouling in the upper tube bundle. The mechanism of HFESQ is not certain, but its onset appears to be associated with significant droplet entrainment after the transition of flow to the annular pattern. The postulated connections between the flow pattern and the fouling mechanism will be discussed. This mechanism may also be the reason for an increased rate of flow-accelerated corrosion at high steam quality in piping and piping fittings. Experimental data will be shown suggesting that the onset of HFESQ is susceptible to the chemistry and size of the crud particles. This offers a route for possible mitigation of the fouling problem.

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