Galvanic Cathodic Protection Design and Start-Up for 12,000 Feet of Insulated Hot Water Piping at a Wastewater Treatment Plant, Dallas (2018-2019)

A wastewater treatment plant in Dallas replaced its buried hot water supply and return piping during a major rehabilitation. The new system is about 12,000 feet of 2- to 6-inch steel pipe, wrapped in closed-cell polyurethane foam and an HDPE jacket. The general contractor brought us in to design its cathodic protection, sealed by a professional engineer as the specifications required, and to see it through start-up.

Hot pipe in moderate soil

Wenner four-pin tests at seven locations put the layers at pipe depth, 2.5 to 5 feet, at 1,500 to 3,500 ohm-cm, which matches the clays and loamy sand the USDA soil survey maps there. On its own that is moderately corrosive. The water runs at 180 °F, though, and heat speeds the corrosion reaction; we expected rates three to four times those of the same soil at normal temperature, and treated any bare steel as severely at risk.

Sizing the anodes

The lead designer had specified sacrificial anodes, and the soil and the well-coated pipe suited them. Typical design current for bare steel in moderately corrosive soil is 1 to 2 mA per square foot; for this pipe we used 8. We assumed 99.5% coating efficiency and added about 68 anchors, steel rings welded to the pipe and cast in concrete, at 1 mA per square foot, since they run cooler and the concrete protects them. That came to 1.21 A. For a 30-year life with 17-pound magnesium anodes the numbers called for 45; we specified 48, in twelve groundbeds of four vertical anodes about 10 feet apart, each with its own test station. Isolating flanges went aboveground at the building tie-ins, and dielectric unions with test stations where new steel meets existing copper below ground.

The design report also named the limit of this approach. If water gets into a void between the pipe and its insulation, the foam and jacket shield that steel from the CP current, and hot wet steel corrodes quickly.

Construction and start-up

In June 2018 we watched the crew make thermite welds for test leads and an anchor bond, and talked through surface preparation until their welds came out clean. In January 2019 we checked anode spacing in two open trenches, asked for 5 to 10 gallons of water around each anode during backfill, found all lead resistances within 0.3 ohm, and confirmed by the moving ground method that the dielectric unions isolated the copper from the steel.

The start-up visit on March 4, 2019, a week after the last anode station was connected, found the system working. Each station put out 120 to 260 mA in saturated ground, and every one showed more than 100 mV of polarization from its baseline. Two problems needed fixing: flange kits at the building tie-ins were missing their washers on one side, which shorted the pipe to building steel, and the bond jumpers in both vaults were on the wrong side of the non-metallic couplings, breaking continuity. We suggested bonding to steel brackets bolted to the flange faces, since there was no room to weld to the pipe.

Related services

The design work is described under cathodic protection design. The visits during and after construction are part of start-up and commissioning.