Impressed Current Cathodic Protection for Two Power Stations and a New Generating Unit, El Paso (2018-2023)

From 2018 to 2023 we designed, installed and commissioned impressed current cathodic protection systems at two power stations in El Paso, Texas, and at a new generating unit. Client details are confidential.

An equipment rack for the new unit, set in its footings before the rectifier and junction boxes were mounted. The racks go in first so the concrete can cure.

Where the current was going

A survey we performed in May 2018 at the first station found that the existing sacrificial anode system was not giving the buried piping adequate protection. The piping was not electrically isolated from plant metal, so most of the protective current went to the copper grounding grid and the structural steel instead of the pipe, and the anodes were being used up faster than expected. We recommended an impressed current system focused on the high-pressure gas lines, the most critical asset.

Design and permitting

Field current requirement testing with the piping shorted to ground was not possible with portable equipment, and there was no complete record of the buried plant metal. We specified two 40 V, 40 A rectifiers to overcome the mixed-metal couple while the owner worked on isolation. Deep anode wells place the anodes electrically remote from the plant, which spreads current more evenly. Each well was 8 inches in diameter and 250 feet deep, with ten mixed metal oxide anodes spaced 10 feet apart in coke breeze and sealed with bentonite. The Railroad Commission of Texas Groundwater Advisory Unit reviewed the wells against groundwater protection depths and approved both without restrictions. The design was approved in August 2018.

Installation

We mobilized in October 2018. Negative cables ran in trenches about 18 inches deep. Where no drain piping connection was close to the rectifiers, we added bonding jumpers between the air and drain piping at each unit so current could reach both. Before each pin-brazed connection, we measured the pipe wall by ultrasonic thickness testing, then ground the steel to a near-white finish, checked the braze with a strike test and coated it with epoxy. The drillers met gravelly sand in both wells, which made it hard to hold the hole open. The same approach was then applied at the second station.

Start-up and the isolation problem

After one day of operation, potentials across the first station had shifted an average of 38 mV in the negative direction, a sign that current was reaching the piping. Low readings near the gas supply station showed stray current from the supplier CP system, which saw the grounded plant piping as an easy path. By February 2019, isolating flanges were nearly complete at the four unit gas risers, yet the piping was still shorted to plant grounding. The cause turned out to be thermal insulating blankets with metallic mesh covers, which bridged flanges, sheathing and supports. Removing or replacing those blankets was the next recommendation.

A new unit, designed from experience

For a new generating unit in 2022, we designed the system from the start based on what we had learned at the original plant. The two deep wells were 10 inches in diameter and 150 feet deep, each with ten mixed metal oxide anodes spaced 5 feet apart. Every anode lead lands on its own shunt in the anode junction box, so the output of each anode can be read, and the box has room for resistors if one ever needs trimming. The rectifiers were energized on January 26, 2023, and the individual anode readings confirmed that every connection survived installation.

CP at generating stations

The design side of this work is described on our page for cathodic protection design. The start-up side is described on our page for start-up and commissioning.