Annual Cathodic Protection Surveys of Buried Natural Gas Piping at a Chemical Plant, La Porte (2019-2026)

Small plant systems need their annual checks as much as large ones. A specialty chemical plant in La Porte protects its buried natural gas piping with a single rectifier, and in October 2019 we ran its annual survey.

A compact system

Gas enters the plant through a 4-inch supply line that ends at a gas island near one corner of the site, and from there the piping branches to buildings and systems across the plant. The impressed current system had been installed by others; no design drawings or calculations were available, only the plant inspection procedure and a readings form from 2018. The rectifier is rated 12 V and 12 A. Its serial number suggested it was no more than seven years old, and we found it running normally at 3.0 V and 0.6 A. Five anode cables meet a header in a junction box beside the laboratory building, each through a shunt, which points to a conventional shallow groundbed. All five anodes were working, at 0.10 to 0.19 A each.

Twenty-one test points

The plant uses its pipe risers as test points, 21 of them. With a current interrupter cycling the rectifier 4 seconds on and 1 second off, we read each riser on both sides of its isolating fitting against a portable copper sulfate electrode, and checked each reading on the spot against the two criteria of NACE SP0169: -850 mV or more negative, or at least 100 mV of polarization. Every reading on the buried side met the criteria, with instant off potentials between -980 and -1,030 mV.

Small signs of age

The aboveground side of several risers read the same as the buried side, which means one or more insulating flange kits at the gas island, and possibly the one on a 3-inch control room riser, were no longer isolating. That was not hurting protection on the buried piping at the time. The dielectric unions on the 1-inch and 3/4-inch risers all worked. We noted atmospheric corrosion on two risers above grade, a reminder that cathodic protection only reaches the buried part of a pipe, and recommended periodic inspection and repair of the exposed piping.

The same system, 2023 to 2026

The annual survey has continued. In March 2023 the rectifier was still at 3.0 V and 0.6 A, all five anodes were working, and every buried test point met criteria, at -980 to -1,130 mV instant off. The failed flange kits at the gas island were still in place and still not hurting protection, and we recommended replacing them at the next outage. In September 2024 the plant told us the buried part of a 3/4-inch line had been replaced about a year earlier. The readings showed the new pipe shorted to the aboveground piping at both of its risers and no longer connected to the CP system, so it was getting no protection. We recommended new dielectric unions on those risers and a new wire to tie the pipe back in. The rest of the buried piping read -1,160 to -1,270 mV. In May 2026 the rectifier ran at 2.9 V and 0.30 A, the rest of the piping read -1,240 to -1,280 mV, and the 3/4-inch line was still waiting for its repair.

Related services

The method is explained under interrupted potential surveys. Plant and distribution gas piping is covered on our page for gas piping cathodic protection.