Cathodic Protection of Seawater Pump Interior

摘要

A 500-MW combined cycle power plant was constructed immediately on a coastline. The main circulating water (CW) used was seawater. Two large vertical turbine pumps were utilized as CW pumps to circulate the seawater flowing in a once-through cooling water system. This article provides engineering details of pump internal cathodic protection (CP) and pump material selection. Seawater is very corrosive to most of the metallic materials that are commonly used. There may be other materials used for seawater handling pumps,1 but Type 316L stainless steel (SS) (UNS S31603), duplex SS (DSS) Alloy 2205 (UNS S32205), and super DSS Alloy 2507 (UNS S32507)2 are the most used materials for large seawater CW pumps in power generation applications. The pitting resistance equiva- lent number of the three SS are 23, 35, and 40, respectively. Among the three materials, only Alloy 2507 requires no additional corrosion protection measures, provided that proper heat treatment, surface finishing, and pickling/passivation have been properly performed. Both Type 316L SS and Alloy 2205 require additional corrosion protection measures, normally CP. From a cost standpoint, the three materials are not at the same level. Note that internal CP may not be applicable to smaller pumps due to pump configuration and pump internal space; hence, in cases where adding CP is not feasible, pump integrity would rely on pump materials, good fabrication practices, and proper operation patterns.