Throughout the oil refining process, water used to cool production units can reach temperatures in excess of 140 degrees F, creating a need for numerous cooling tower structures that adjoin the processing units. Heated water sent to these cooling towers to be cooled and stored for reuse contain minerals and salt that can become concentrated, causing corrosion on steel surfaces in basins, walls and structural steel supports. Highly corrosive surfaces are typically located in or near the intake and updraft sections at the base of the towers, where large volumes of contaminated air are drawn past steel and concrete supports to meet water falling from the top of the tower.
Typically, maintenance is performed on cooling tower components whenever its processing unit is scheduled for a “turnaround,” which is a planned, periodic shutdown to inspect, repair or replace equipment that is worn out or broken. According to the Energy Information Administration (EIA), routine turnarounds on key fuel production units are planned for every 3-to-5 years and can result in a unit being offline for several weeks to several months. “During a major unit turnaround, as many as 1,500 to 2,000 skilled contractor workers may be brought on site to perform a myriad of interrelated jobs that require significant coordination and safety measures,” the EIA explained.
During the turnaround of an alkylation (alky) unit at a major Gulf Coast oil refinery in Louisiana, workers discovered four badly corroded headers in the unit’s cooling tower. “The protective coating system that was previously used on the headers had not only failed, but it had failed prematurely, resulting in serious corrosion and pitting of the steel,” recalled Tnemec coating consultant Eddie Borne. “There was blistering, rust and corrosion, so the steel was abrasive blast-cleaned down to bare metal and primed.”
The headers were prepared in accordance with SSPC-SP 10/NACE No. 2 Near White Blast Cleaning to remove all the rust and corrosion, then primed with Series 90G-1K97 Tneme-Zinc, an advanced technology, single component zinc-rich polyurethane. The coating system was scheduled to receive a 15.0 to 20.0 mils DFT topcoat of Series 46H-413 Hi-Build Tneme-Tar, a corrosion-resistant polyamide coal-tar epoxy, but the alky unit was put back into service before it could be applied.
Six years later, during another scheduled turnaround of the alkylation unit, the headers were inspected and found to be “nearly perfect with no corrosion on them,” Borne reported. “The performance of the Tneme-Zinc primer was so good that it stopped the corrosion on those headers, even after six years of service in a fairly aggressive environment containing corrosive chemicals and salts. It’s even more remarkable when you consider that some of the pitting in the headers was 1/8 of an inch, or 125 mils, which is 40 times the DFT of the primer. So even though all of these deep pits were sprayed with only 3.0 mils of primer, the fact there was no rust on the headers indicated the primer sprayed on uniformly and adhered evenly around the edges of the pitted steel.”
The corrosive effect of water from the cooling tower isn’t limited to the structure’s interior, but can also result from the mist that’s released into the atmosphere. “When the cooling tower is in service, it’s putting out what looks like a big fog that can cover the entire refinery, depending on the direction of the wind,” Borne explained. “When this mist containing salts and contaminants settles on steel surfaces, it can lead to corrosion, which is what happened on the floating roofs of steel storage tanks at a major oil refinery in Texas. The coatings on those roofs failed, causing enough corrosion that the refinery ended up replacing the roofs at a cost of $1 million for each replacement.”
In addition to cooling tower fallout, roof and sidewall coatings are subject to ultraviolet (UV) light, rain water accumulation and ponding, and elevated temperatures for crude and distillate storage of 120 to 140 degrees F. “These tanks experience the widest range of corrosive conditions and offer the greatest potential for advanced technology coatings to be used,” Borne added.
For additional details on the Gulf Coast refinery in Louisiana, contact Eddie Borne.