To restore the first digester lid at the Chambers plant, the old liner was removed and the applicator team abrasive blasted the headspace, removed the original surfacer filler and neutralized all surfaces.
Click here to enlarge imageA systematic check less than a year later found that the polyurea linings were beginning to lose their elasticity and adhesion to the substrate. The thick film was beginning to deform, blister and wrinkle, starting in the crest of each dome and moving outward to the edges. The owner’s warranty required the material to be replaced, so CU removed the delaminated lining from both lids and applied the polyurea over a different primer.
By the summer of 2003, the coating on both digester lids was failing once again above the waterline in the tanks’ head space. It was apparent to everyone that a new coatings solution was needed. The warranty work was no longer actionable, and the delays were costing a great deal of money. CU recommended a call to Tnemec’s Seattle-based coatings consultant.
New Coating
The utility emptied one digester and brought in a bank of intense lighting equipment. Looking up at the lid, it was immediately evident to the Tnemec consultant that the failure was likely due to elevated gas concentrations that had permeated the coating in the head space.
The environment in the head space of an anaerobic digester can be aggressive, often filled with methane, carbon dioxide and hydrogen sulfide gases. Those gases can permeate a liner, altering the physical properties of the film and ultimately affecting the adhesion to the substrate.
The recommended solution for the problem was application of a modified polyamine epoxy from Tnemec that is specifically designed for wastewater immersion and fume environments. Called Perma-Shield, the thick film, 100 percent solids, abrasion-resistant lining exhibits low permeation to gases including H2S, as well as protects against Microbiologically Induced Corrosion (MIC) and provides chemical resistance to severe wastewater environments.
Digester Lid Restoration
To remedy the situation, the consultant instructed the CU crew to remove the failed coating on the first lid. This meant pulling it off in sheets by hand and with scrapers. For the surface preparation, the CU team abrasive blasted per SSPC-SP13/NACE 6 Surface Preparation of Concrete, removing the lining system, the surfacer filler and neutralizing all surfaces so they were clean, dry and free of contaminants.