ABSTRACT

A computerized statistical method which calculates the current demand requirement based on potential measurements for cathodic protection systems is introduced. The method uses the regression and correlation analysis of statistical measurements of current and potentials of the piping network. This approach involves four steps: field potential measurements, statistical determination of the current required to achieve fill protection, installation of more cathodic protection capacity with distributed anodes around the plant and examination of the protection potentials. The procedure is described and recommendations for the improvement of the existing and new cathodic protection systems are given.

INTRODUCTION

The Petrochemical Complex of El Tablazo, which is an important area of development for PEQUIVEN, is located at the eastern shore of Maracaibo Lake, State of Zulia, Venezuela. All of its facilities, both service and production installations, are considered equally important since one of the principal characteristics of the petrochemical industry is the interdependence of the different plants and units. However the Olefins Plant has a special role in the Complex because its products provide the input to other plants which, in turn, produce the raw materials used in the manufacture of plastics and detergents for domestic market. Remaining production is exported.

The Olefins plant was designed by the M.W.Kellog Company, a US firm. It uses ethane and propane from MARAVEN natural gas plant as raw materials for its production.

The network of underground piping in the existing Olefins Plant which is 20 years old includes the cooling water system, process piping and the ductile iron drainage system. The impressed current system of Cathodic Protection (CP) was designed in 1985 and installed in 1988. This system with a design current of 320 amperes has 4 groundbeds installed at the periphery of the plant at the average depth of 3-5 meters and did not generate enough current to obtain the level of protection required [- 950 mV (Cu/CuSO4)], because the influence of Sulphate-Reducting-Bacteria determined by soil analysis.

Several leaks occurred in the cooling water system. These were particularly costly when water losses occurred underneath equipment and close to other structures. A preliminary potential survey of the piping network showed that pipelines located at the central part of the plant did not receive complete cathodic protection. Wherever the piping was accessible, the original coal tar coating was seeing in poor condition, what means it probably suffered degradation with time and soil corrosivity.

In 1991, an expansion project was undertaken in order to increase the capacity of ethylene and propylene production which included the construction of new underground cooling water lines, process pipelines, fire water system and drainage piping. The new project incorporated a new cathodic protection system considering only the new structures; nevertheless, it was decided to perform a comprehensive study of the existing piping network and cathodic protection system in order to obtain all the parameters required to upgrade the cathodic protection installations, because the old structures and the new ones would be interconnected.

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