Abstract

The use of natural gas as a fuel has been an attractive alternative in both economic and environmental perspectives. Natural gas pipelines show usually low internal corrosion rates. Hydrogen sulfide, carbon dioxide, water and other contaminants are removed to specified limits during processing stage to assure quality and safe use of natural gas to customers. Nevertheless, the presence of contaminants dissolved in condensed water can result in internal corrosion processes (ICP), production losses and equipment damage. In order to assess ICP, the use of corrosion monitoring techniques has been extensively studied and improved. Electrical resistance probes and weight loss coupons have been successfully used, fitted in specific points of the pipelines where corrosion is mostly expected, to monitor corrosion progress.

Corrosion rates are expected to be higher in bottom-of-the-line (BOL-six o'clock position) where condensed water flows by gravity. But depending on gas velocity different flow patterns can be encountered and top-of-the-line (TOL-twelve o'clock position) corrosion is also found. When gas velocity is high the flow pattern is usually annular. At very high velocities and low liquid contents, a mist flow can be achieved. The stratified is more common when gas velocities are lower. This work presents the first approach and results of internal corrosion monitoring in a specific Brazilian natural gas pipeline using electrical resistance probes placed at three selected TOL sites of a 357 km length, API 5LX 65 steel, 16" nominal diameter natural gas pipeline.

Internal Corrosion Natural gas is a safe, economic, non-toxic product and its physical and chemical property allows it Monitoring to be a high quality fuel, when free of impurities such as CO2, H2S, water and oxygen. The presence of these compounds in natural gas not only reduce power recovery and production losses but may induce internal corrosion processes, which can result in equipment damage and even accidents. These processes take place when residual water even in low limits of specification condense inside the pipelines. Contaminant gases tend to dissolve in BOL relatively stagnant regions forming a localized acid zone. Once in contact with the metal ICP can occur. In order to try to understand the effects caused by the eventual presence of contaminants in natural gas, electrical resistance probes are usually inserted in selected points of natural gas pipeline which water is expected to condense. The electrical measurements are converted to corrosion rate, and so it is possible to evaluate internal corrosion processes.

Water is expected to condense in lower parts of pipel

This content is only available via PDF.
You can access this article if you purchase or spend a download.