Tucked in a hard-to-reach area, an unfilled joint was the culprit of a persistent leak that stymied the restoration team. After successfully filling the joint, the digester tank held pressure.
The fully restored and sealed cover enabled the digester tank to hold pressure and satisfy the project specifications.
to the team of SEH (Savage, Minn.), industrial sandblasting and painting contractor Champion Coatings (Savage, Minn.), and general contractor Wapasha Construction (Winona, Minn.) to restore the digesters. After the city drained and cleaned the 50-foot-diameter by 24-foot-tall tanks, engineers from the city, SEH, and Champion Coatings entered the structures to assess their condition. The inspection revealed that the concrete used in the 1930s and 1950s was still in great condition, but concrete used in the 1980s to rebuild the digesters’ mixer bases was severely deteriorated. In addition, inspectors observed cracking at the wall/cover interface of both vessels. After reviewing options, SEH specified two Sherwin-Williams products for the repairs — Dura-Plate 235 Multi-Purpose Epoxy and SherFlex Elastomeric Polyurethane. Making repairs under difficult conditions The rehabilitation team initiated restoration of the first digester in mid- December 2014. Throughout the project, the contractors faced difficult operating and weather conditions, as well as a challenging tank design. Initiating the restoration process, personnel from Champion Coatings removed loose debris and sandblasted the 4,000-square-foot interior of the tank to prepare the surface of the deteriorated concrete to the SSPC-SP 13/NACE 6 standard for the coating and lining applications. Next, they applied a coat of Dura-Plate 235 as a primer for the en- tire structure, including both concrete and steel surfaces, as well as to control any potential outgassing. The coating provides exceptional performance in immersive and corrosive environments like a digester. It can be applied at temperatures as low as 0° F, making it highly suit- able for the wintertime application. The cold-season application was especially difficult due to frigid con- ditions. Inside the digester, crews ran heaters to make the environment more palatable for the coating applications. While the heat helped with application consistency, it created a minor problem of its own — the formation of some pinholes, which developed due to the heated envi- ronment encouraging outgassing.
Cold conditions also contributed to difficulties in applying the SherFlex coating and lining system. The product requires the use of a heated plu- ral-component sprayer. With temperatures below zero, contractors had to wait up to two hours for the material to heat up before beginning applica- tions. They then sprayed the product on the undersides of the concrete cover and the upper portion of the tank sidewalls to form a monolithic surface that would seal cracks in the concrete. The corrosion-resistant elastomeric polyurethane lining material bridges hairline cracks with a thick, flexible film that expands and contracts with the concrete substrate to minimize the chance of cracks forming in the lining. The construction of the concrete tank cover caused additional chal- lenges for the coating and lining applications, as the cover had numerous concrete “fins” that were difficult to coat and inspect. The fins were a remnant from the narrow wood forms used to construct the tank eight decades ago. “We needed to build up a fully coated surface without pinholes to seal leaks and ensure waste material couldn’t get through to the concrete and promote corrosion,” said Gary Johnson, president, Champion Coatings. “Sealing off every sharp edge was tough, especially with the difficulties of getting the sprayer gun into tight spaces.” Testing — and retesting — the work The project team understood that a successful digester repair meant the tank would no longer leak and therefore would hold pressure. SEH’s specification required the tank to be filled with water and pressurized to make this determination. Unfortunately, the first pressure test failed. After draining the tank and performing troubleshooting, the team added additional coatings to the roof/wall juncture, thinking that the leak must be at that interface. The team also recoated some isolated pinholes. The crew then filled the tank with water a second time. Again, it would not hold pressure. Engineers from SEH next tried a smoke test from the interior to see if they could identify leaks by standing on top of the cover and watch-
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csengineermag.com
june 2018
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