What do you get when you put a stainless steel tank inside a carbon steel tank? It might sound like a construction-only riddle, but that’s exactly the situation that brought Jim Stotler and his five-person crew from Allen Blasting and Coating, Inc. (ABC) to an Iowa jobsite. It was a very unique project.
The project, which lasted 17 days, was solely done on the exterior carbon steel tank, which held the interior stainless steel one. “It was referred to as an envelope tank,” explained Stotler, western division manager for ABC. “The carbon steel tank was designed to be a berm if the stainless steel tank inside ever leaked.” The crew was to coat the exterior surface and line the interior surface of the exterior carbon steel tank. The interior stainless steel tank, which was built to hold fertilizer chemicals, was not part of the approximately 12,500-square-foot (1,161 m²) coating project. And in between the two tanks was a 6-foot-wide (2 m) corridor where the crew would eventually work.
Because the crew had to work on the exterior and interior of this tank, which was shop-primed at CF Industries’ plant and then field-welded, there were different challenges to overcome. The crew started on the exterior. And because of that, they first had to overcome some issues with the weather. “Weather’s always an issue,” said Stotler. “We ran into a few rain days up there. Environmentals are always an issue outside.”
When the weather was cooperating, the crew started by preparing the newly welded steel surfaces. First they washed the steel. Then, using 450 Ingersoll Rand compressors and Schmidt 600-pound (272 kg) blast pots, they blasted the welds to a NACE International No. 3/Society for Protective Coatings (SSPC) Surface Preparation (SP) 6: Commercial Blast Cleaning, and the rest of the steel received a NACE No. 4/SSPC-SP-7: Brush-Off Blast Cleaning. They used 20-40 coal slag from Black Beauty.
To catch the spent abrasive, the ABC crew laid tarps out 20 feet (6 m) from the edges of the tank. And later, after the entire job was complete, per the contract, “anything that didn’t get caught by the tarps — it was nonhazardous, so the customer just mixed it with the dirt,” explained Stotler.
The crew made sure to circle back to the prepped substrate and spot-prime any areas that were damaged. For that step they used a Graco 45:1 pump and aerial lifts to spray-apply Tnemec’s Hydro-Zinc 91-H20 at 2.5–3.5 mils (64–89 microns) dry film thickness (DFT). Then they applied a full intermediate coat of Epoxoline 11 N69 at 3–4 mils (76–102 microns) DFT, followed by a topcoat of Endura-Shield 11 series 1075 at 2–4 mils (51–102 microns) DFT.
“We spray-applied the zinc on the welds, but we rolled the intermediate and the top coat on the outside to prevent overspray,” Stotler said. The tank was “right dab smack in the middle of a plant, so there are constant cars running around, other tanks nearby, a brand new building within 15 feet [5 m] of it.” Not to mention the strong winds that the crew encountered at the jobsite. “It was very windy with no wind breaks around, and with fresh dirt, it was a challenge to keep the dust from going into fresh paint,” he continued. According to Stotler, it would seem that there were a lot of reasons for the crew to use the slower roller method instead of spraying. Because of that, it took the crew about two days to apply each coat. “It went pretty quick,” Stotler said. “The prep is 90 percent of the job, so once you get to the painting portion of the job it goes pretty fast.”
When the crew moved to the interior, things went a bit differently for the ABC crew. Here, there were no external elements, such as the wind, to combat; however, there were a few internal ones. The first was due to the fact that there were only two manways into the tank, which meant that proper ventilation wasn’t inherent. Because of that, the crew brought in their own ventilation equipment and completed air monitoring tests every morning. They used four-gas monitors from MSA to test for CO, O2, H2S, and SO2 prior to entering the confined space. The crew was also sure to test the air every 30 minutes or if there were any changes throughout the workday to recertify the site. They logged those results along with their entry and exit into the tank.
The crew also used hole watchers and wore harnesses, lifelines, full-face respirators, and Tyvek suits. As for gloves, Stotler explained that they use different types for different tasks along the way. “We switch on and off with gloves when we’re spraying or mixing. Grinding or blasting we wear blast gloves. If we’re over-masking or masking tarps, we use cut-resistant gloves,” he said. “There’s a variety of PPE [personal protective equipment] that we wear on all jobs.” As Stotler later said decidedly, “safety is taken very seriously by Allen Blasting.”
While in the corridor, the crew used rolling scaffoldings to access the full interior carbon steel surface. They blasted the weld seams the same as they did on the outside. But here, they primed the seams with a 2.5-to-3.5-mil (34–89 microns) layer DFT of Pota-Pox Plus N140. On top of that, the ABC crew applied two full layers of the same coating at 4–5 mils (102–127 microns) DFT each.
“That one went a little faster…’cause you could spray and not roll. You didn’t have to worry about the overspray,” Stotler said. They did have to protect the interior tank, though, so the crew was sure to hang a tarp on the exterior of the stainless steel tank and the corridor floor. It took them about one day to apply each coat before moving on.
“Since we started doing their work, we haven’t left that plant,” Stotler explained. ABC has worked at this facility on and off for about 3–7 months every year since, completing various coatings maintenance work depending on how hard they get hit by winter each year. The fact that this job was completed three years ago and the crew is still being called back in for maintenance work shows that the project was a successful one.
“That’s the first one I’ve ever done like that, and I’ve never seen one since then,” Stotler said. “I’ve never seen one quite like that.” It was a standard coatings job on a unique project — something that doesn’t come around very often…even for this seasoned coatings crew.