Controlling microbially influenced corrosion through piping geometry

Power plants have experienced tuberculation and microbially influenced corrosion (MIC) in systems that use raw, or minimally treated, water. Piping geometry can have a significant effect on MIC. A geometry that allows the action of thermal convection to provide constant low levels of oxygen, under near static flow conditions, are known to promote tuberculation and MIC. The two geometries studied in this paper are: Piping geometry with horizontal secondary headers between a flowing ring header and a vertical riser (pipestand) which is susceptible to MIC; and, piping geometry with vertical secondary headers between a flowing ring header and a vertical riser (pipestand) which is not susceptible to MIC. A trend of historical failures due to pinhole leaks has been noted in pipes that are routinely stagnant or have low flow, but are connected to pipes that see regular flow. Accurate predictions of problem areas were made from our understanding of the failure mechanism. Initially, inspections were completed to determine the extent of degradation in the PNGS fire protection system. Radiographic inspections and visual inspections indicated that sections of the secondary headers that had an immediate vertical take-off from the flowing ring header (as opposed to the typical horizontal take-off) appeared to exhibit a much lower rate of subsequent tuberculation. This paper will present historical station data to support the concept that piping geometry changes can significantly lower the risk of MIC. An explanation will be presented demonstrating how convective mixing is reduced with the vertical configuration, thus reducing the tuberculation and MIC. Similar geometry changes are expected to significantly lower the incidence of tuberculation and MIC in other systems that use raw water with similar flow patterns. (author)