Cathodic Protection System Considerations

Lightning Protection Information

Summary

Lightning protection ideas for active corrosion inhibiting systems. DC isolated pipe flange protection. Pipe line protection.

Cathodic Protection System Considerations (Pipe Line Companies)

A cathodic protection system reduces the corrosive effects of soil on buried steel pipe. Soil conditions such as porosity, conductivity, dissolved salts, moisture, and pH have a corrosive effect on steel. Protection is achieved by the use of either an anode impressed electrical current driving the buried steel gas pipe to a negative potential with respect to the soil, or through the use of a more active metal in the galvanic series as a sacrificial anode. Although both methods are used, anode impressed electrical current is most widely utilized.

A remote dc power supply, with up to 100 volts at 100 amps capacity, is connected to a number of below grade anodes (+) arranged in a pattern related to the buried steel pipes to be protected. The anodes set up an electrical field in the soil with the steel pipes acting as a cathode (-). The pipes are driven to a negative potential with respect to the surrounding soil. The consistent negative potential over the surface area of the pipe overrides the local galvanic interaction with the soil.

Reliable measurement of actual impressed pipe voltages to local ground are difficult to obtain without the use of a standard copper/copper sulfate cell to substitute for the missing hydrogen side of the theoretical electrolysis cell formed by the anode - cathode current passing through moisture laden soil.

Typical voltage to ground measurements on the pipe as it surfaces are from 300 millivolts to 3 volts. Occasionally, close to the anode field, the voltage can go up to 5 volts. The typical current required for cathodic protection is approximately 2 milliamps per square foot of exposed buried steel pipe. Most newer pipe runs are wrapped with a corrosion resistant, insulated barrier. Insulation effectiveness is estimated to be 98% (40 microamps per square foot required for cathodic protection).

The non-wrapped buried steel pipe can be an excellent earth ground. However, it is directly connected via the (-) conductor back to the cathodic protection power supply. Depending on the point of the (-) attachment to the pipe, and the length/inductance of the insulated (-) lead back to the power supply, the risk of damage to the power supply should be minimal. A gas tube device (e.g.), bridging the cathodic protection flange insulator, would conduct surge energy into the non wrapped pipe and earth ground only after its turn on voltage was exceeded. There would be a dynamic path to earth ground without violating the cathodic protection circuit.

A gas pipe insulated flange uses insulating gaskets and bolt sleeves to isolate cathode protection currents from other ground returns such as power/signal conductors to and from remote measurement/control systems. 

Without insulating flanges, additional loads would be placed on cathode protection power supplies and measurement signals could be degraded. Insulated tubing connectors (dielectric fittings) and insulated remote sensors also block cathode protection currents. Any lightning protection device bridging the insulator must turn on before the flange or sensor breakdown voltage is approached (assuming 2kv per nanosecond surge pulse risetime).