Philadelphia - "An Old System Revisited"

Abstract The recent return to direct current powered mass transit systems throughout the country has resulted in a renewed interest in stray earth currents and resulting underground corrosion of metallic facilities. While in some areas of the country stray current corrosion of underground utilities represents a new problem for corrosion engineers, the major utility companies in the greater Philadelphia area have had to contend with this problem since the advent of original mass transit construction during the 1900 to 1940 period. Little consideration was given to stray current corrosion control during this period by the transit industry and as such, established the basis for the corrosion control programs of area Philadelphia utilities. This aspect, in conjunction with planned abandonment of many urban streetcar systems during the 1950's, caused utility operators to adopt a somewhat tolerant or a "live with" approach in dealing with stray current corrosion. This paper will examine the present corrosion control programs of the Philadelphia Gas Works and City of Philadelphia Water Department relative to stray current corrosion of their underground gas and water transmission and distribution systems. Current engineering approaches for each utility will be discussed with examples of stray current effects and the impact of mitigative measures. To more fully appreciate the problem and corresponding solutions, the transit system configuration is also reviewed.