ABSTRACT

Significant AC interference was discovered on a new tape coated steel water transmission main which was tied into a concrete coated steel water transmission system. The AC interference was evidenced by shock incidents during construction, and failure of dielectric isolation flanges within one year after construction. The safety concerns and cathodic protection system failures led to an investigation to determine the causes of the interference. Interim safety procedures were developed to minimize the safety hazard. The goal of the investigation is to define and implement mitigation measures which will maintain electrical isolation and cathodic protection of the pipeline, and reduce the AC interference to acceptable levels.

INTRODUCTION

In January, 1994, an operational safety problem was discovered in Pleasant Hill, California, on the Taylor Boulevard transmission main relating to AC interference voltages. This report describes the Contra Costa Water District transmission pipeline, and corrosion control systems. The introduction of dielectric tape coated steel pipeline into the system is documented. The problems associated with these types of pipelines regarding AC interference are identified and work specific to the Taylor Boulevard pipeline is described. Conclusions and recommendations are given to mitigate future affects of AC interference on the pipeline. Personnel safety procedures are presented. This document describes the efforts of the Contra Costa Water District corrosion control staff to resolve the problem over a two year period.

Transmission Pipeline System and Corrosion Control System Design

Pipeline System. Until 1985, the District had installed approximately seventy five miles of transmission main larger than 12 in (31cm) diameter. The preferred pipeline material until 1985 was cement mortar coated and lined steel cylinder pipe of various configurations. Many of these systems were installed with dielectric coated steel and cast iron appurtenances, which were not dielectrically isolated horn the mains. In 1985, the District began installing dielectric tape coated, mortar lined, steel pipe for new transmission mains. The reasons for the change included economic advantages and trends in the local water industry. The District has installed ten transmission main expansion projects since 1985 using dielectric tape coated steel pipe. The transmission system currently consists of about 85 mi. (135km) of larger diameter pipeline. Of the total, 75 mi. (120km) are concrete mortar coated pipe, and 10 mi, (16 km) are dielectric tape coated pipe.

Corrosion Control. The District has taken a proactive stance regarding pipeline corrosion control since 1972. Corrosion related failures of the dielectric mated appurtenances of the transmission mains were a primary concern. Preliminary corrosion control surveys were contracted to a corrosion engineering consultant who performed the work with assistance of District personnel. Widespread soil corrosivity studies were performed leading to design and installation of cathodic protection on many of the transmission mains in the system. Approximately 85 percent of the transmission system is catholically protected primarily with impressed current systems. Of these cathodically protected steel pipelines, 55 mi. (88 km) are concrete mortar coated and 10 mi. (16 km) are dielectric tape coated.

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