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Corrosion Failure of a Spool Piece

Background: A pipe spool piece that was operated at 75 PSI and temperatures between 80 and 90 oC experienced numerous leaks. Plant operations noted that the leaks tended to occur during or immediately following acid cleaning of the spool piece interior. After each leak, the pipe was repaired with a fiberglass wrap and placed back in service. KML was contracted to determine the cause of the repeated leaks in the pipe.

Figure 1: Photograph of the Inside of the Spool Piece with Surface Features and Non-Service Water Level

Summary: The failure of the spool piece was the result of erosion corrosion at the water line. When not in operation, the spool piece was allowed to partially drain, resulting in a water line within the spool (Figure 1). Corrosion rates along this line were very high, resulting in significant material removal. Where the spool piece narrowed in diameter, the constriction of flow resulted in increased interaction between the fluid and the spool surface, causing the flow to remove the corrosion products from the surface and continuously expose clean metal surfaces which then continued to corrode. The combination of the high corrosion rates when not in service and the erosion when in service led to rapid through thickness corrosion of the spool piece. Additionally, when not in service, the poor water quality led to the precipitation of thick calcium carbonate and iron oxide layers (the columnar deposits at the bottom of the pipe in Figure 1). The removal of these deposits by acid washing exposed the holes that had formed in the metal surface, leading to leaks during or immediately following the acid washes.

Take Away: When operating a pipe intermittently it is important to try to prevent partial drainage of the pipe if possible. If that is not possible then corrosion resistant materials and close control of water chemistry are necessary in order to prevent significant corrosion damage.