Where there's steam there's water, and where there's water there's probably corrosion. In the power industry it is a persistent phenomenon that can be difficult to control. Preventing corrosion doesn't call for a one-size-fits-all solution. Suppliers, integrators, and users agree, corrosion prevention means billions of dollars a year, and they offer strategies and techniques for their varying corrosion conundrums.
Along with corrosion come scaling and deposition… contamination that falls out of water and collects on condensers and heat exchangers. Under-deposit corrosion can occur rapidly as impurities concentrate on heat transfer and other metal surfaces inside these systems. "We always talk about corrosion and deposition together and they can be very difficult to control in power plant steam cycle chemistry," said Brian LaBelle, power industry manager at Emerson in Irvine, California, “especially considering the large quantities of water introduced continually to make-up for blow down and steam losses.”
Power plants boil tons of water to drive steam turbines. Raw water sources contain elements that, if left untreated, will cause corrosion. Beyond initial demineralization a typical treatment strategy includes ammonia or similar chemical addition to facilitate the formation of tough oxide films to protect the base metals of wetted equipment from corrosion. Some of these chemical additives themselves become corrosive in the presence of oxygen. Therefore, power plant chemists often go to great lengths to eliminate oxygen from their systems, LaBelle said. Companies use special sensors sensitive to parts per billion (ppb) levels of oxygen, two or three orders of magnitude lower than atmospheric oxygen.
Some avant-garde ways to measure dissolved oxygen are sensitive and less expensive sensors with self-depleting designs, also called membrane-based amperometric sensors. The self-depleting design means there is oxygen inside the sensor that it has to consume. "When you put these things together for the first time and after periodic recharging there's atmospheric oxygen to consume," LaBelle said. Some of the more recent innovations are the sensors that deplete themselves rapidly of internal oxygen. "Every time you take the sensor out to clean or calibrate, you expose it to atmospheric oxygen. The faster the sensor can come down to accurately measure trace oxygen the better," he said.
But totally eliminating oxygen isn't always the best way to go. "Since we run pure water systems in high-pressure power plants, we found it's no longer necessary to totally eliminate oxygen from boiler feed water," said Pete Lovallo, chemical engineer with Detroit Edison, Belle River Plant, China, Mich. "A small amount of oxygen in the 10 ppb range is helpful in minimizing corrosion," which helps avoid using oxygen scavengers. "We physically remove the oxygen by using vacuum pumps, and the end result is we end up with oxygen in the range of 0-10 ppb," low levels of oxygen that can optimize corrosion rates. "As long as you know you're below 10ppb, you don't need to take any further action," he said.
"None of this is huge dollars in comparison to the amount of money a power plant makes," Lovallo said, "but for a particular unit, we'll spend $100K a year in oxygen control. With Emerson’s advanced sensors we gained the confidence to eliminate the use of treatment chemicals to remove oxygen," he said. The sensors tell us when oxygen is within these low levels. "And if that's the case, we know we don't need to use oxygen scavenging chemicals," he said. "If we didn't have reliable instrumentation to confirm whether we were at the low levels, we'd take the conservative approach and treat to eliminate the oxygen, which means we'd spend the $100K a year" to avoid destroying the boiler pressure system.