Cathodic Protection of Pipeline Using Distributed Control System

Distributed control system (DCS) is available in most of the compressor stations of cross-country pipeline systems. Programmable logic controller (PLC) is used in all the intermediate pigging (IP) stations/sectional valve (SV) stations to collect the field data and to control the remote actuated valves. This paper presents how DCS or PLC can be used for cathodic protection of gas pipelines. Virtual instrumentation (VI) software is used here for simulation and real-time implementation purpose. Analog input channels available in DCS/PLC can be used to measure pipe to soil potential (PSP) with the help of half-cell and voltage transducer. Logic blocks available in DCS can be used as low selector switch to select the lowest PSP. Proportional-integral (PI) controller available in DCS/PLC can be used for taking the controlling action. PI controller output varies the firing angle of AC phase controller. Phase controller output is rectified, filtered, and fed to the pipeline as cathodic protection current. Proposed scheme utilizes existing infrastructure to control pipeline corrosion. 1. Introduction Corrosion is a phenomenon by which metal is etched away naturally contributing to material loss. Corrosion reduces the life of metal structures. Methane rich natural gas is being transported through underground metal pipeline at high pressure. Corrosion in pipeline leads to material loss, gas leakage, and interruption in gas supply. Underground metallic pipelines are primarily protected by coatings. Impressed current cathodic protection is used to protect pipelines from coating defects. When pipeline is laid underground, soil acts as electrolyte in a corrosion cell and corrosion occurs in metal pipeline primarily due to differential corrosion cell. By impressing current to the pipeline, the entire structure is made to become a cathode of the corrosion cell. Impressed current corrosion controller should be dynamic enough to protect pipelines from ill effects due to variation in coating defects, soil resistance, soil pH, temperature, and so forth. The main objectives and requirements of cathodic protection (CP) systems are to prevent external corrosion throughout the design life of the pipeline by providing sufficient current to the pipeline to be protected. Distributed control system (DCS) is available in most of the compressor stations of cross-country natural gas pipeline systems. Programmable logic controller (PLC) is used in all the intermediate pigging (IP) stations to collect the field data and to control the remote actuated valves. Impressed current cathodic