1 May 2006
What's in your Tank?
Users seek answers to expensive corrosion puzzles in power, paper industries
By Ellen Fussell Policastro
Where there's steam there's water, and where's there's water there's probably corrosion. In the power industry, it is a persistent phenomenon that is difficult to control. In the paper industry, harsh chemicals are the corrosion culprit. 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 deposition, scaling, and deposits that fall out of water and collect on condensers and heat exchangers. Corrosion can occur more rapidly as these impurities concentrate in under-deposit corrosion (deposits that stick to the metal surfaces inside these systems). "We always talk about corrosion and deposition together, and in water chemistry, it's difficult to control," said Brian LaBelle, power industry manager at Emerson Process Management in Irvine, Calif. In a power plant, it can change fast because large quantities make up steam losses.
These power plants boil tons of water to turn turbines to contain steam. As the steam goes away, they need to replenish with makeup water. Ground and city water have elements that, left untreated, will cause corrosion. Chemicals such as ammonia added to water to raise pH levels can create a slight alkaline environment to add protective coating on metallic surfaces that come in contact with water.
The power industry is responding to this by monitoring dissolved oxygen, LaBelle said. Companies use sensors sensitive to parts per billion (ppb) levels of oxygen to help control the pH, but adding ammonia to raise pH can create another corrosion problem. Using deaerators to get rid of oxygen raises water temperatures, which introduces oxygen scavengers such as hydrazine.
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 before you put them into service, 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, more than what you use in a power plant. The faster it can consume oxygen in air during maintenance, the better the sensor," 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 Peter 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 these advanced sensors, we eliminated the use of the treatment chemicals that removed the oxygen, so we're not introducing chemicals to scavenge oxygen," he said. The sensors allow users to know they're in an area where oxygen is within these low levels. "If that's the case, we know we don't need to use this chemical," he said. "If we didn't have reliable instrumentation to tell 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.
While oxygen is the offender in power plants, paper plants see the biggest problem as storing and conveying different liquors and process chemicals. Since the paper industry is made up of power plants, pulp mills, and chemical plants, it deals with corrosion across the board. Issues with steam in power plant recovery boilers are some of the hottest issues at International Paper – Georgetown Mill, in Georgetown, S.C. "In our power area, we have two types of boilers: power boilers that burn coal like any major utility; and recovery boilers, a boiler in which we burn black liquor (a by-product from the cooking process), which we burn for fuel," said Mike Richardson, the plant's senior department engineer. "We have unique corrosion issues, but the top three are liquor storage, bleaching chemicals, and steam."
In recovering chemicals from the process, corrosion is a continuous threat. In the past, the tanks were made of carbon steel. Now, as new construction occurs, the team considers temperatures, pressure, and chemicals stored to better select materials for storage. "We've gone to a variety of stainless steels that have great corrosion resistance to alkaline materials," Richardson said.
Duplex stainless steel has some benefits over regular carbon, he said, "but sometimes it can be overkill. A common stainless steel is 304 (austenitic) for storage in black liquor, which offers good corrosion benefits in an alkaline environment," he said. A good example of carbon steel's suitability is soap storage, a by-product of black liquor. It rises to the top, and you have to skim that off. "I make all our soap storage tanks out of carbon steel because soap is not very aggressive," he said.
A good example of duplex stainless steel's suitability is for strong black liquor. "Now the duplex is a combination of materials in one; it takes the benefits of stainless steel plus the benefits of carbon steel and puts them into one material, which gives it good corrosion resistance but also greater strength," he said. "I've been using it for various applications, but there's an added cost. The larger the diameter, the more cost effective the duplex can be versus regular carbon. Is there something particular that vessel is holding? Where it fits in process might be better justification."
While the cost of duplex stainless steel is more expensive, Richardson said the materials market fluctuates from day to day. "Today, I'll find it's more expensive than regular stainless steel, but two weeks from now, it could be less expensive. But there are different grades of duplex. The lower grade may be less expensive than regular stainless steel," he said.
Part of the decision-making process lies in what elements are added (chromium, nickel, and molybdenum). Supply and demand is also a factor, he said, in raw ingredients as well as the consumer's needs. Pricing is also a function of tonnage. "How much are you ordering? Carbon steel is still a pretty good material. I always try to weigh out lifecycle costing," he said. "I might be able to build a carbon steel tank for a certain value and compare it to the cost of stainless steel or duplex. That cost will be higher, but the life expectancy of the tank might be four times the cost of the carbon steel. That's how I justify the higher alloy materials—by lifecycle cost."
Amperometric: A detection method in which the current is proportional to the concentration of the species generating the current.
Austenitic: A nonmagnetic, solid solution of ferric carbide or carbon in iron, used in making corrosion resistant steel.
Black liquor: A recycled byproduct formed during the sulfate process of chemical wood pulping in the paper industry.
Molybdenum: A hard silvery-white metallic element used to toughen alloy steels and soften tungsten alloy.
Linear polarization: Confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation.
Stainless steel soothes corrosion headaches
By John Grocki
With higher alloys rising in price due to cost of raw materials, industry's interest in stainless steel is starting to rise because lower alloy content is so much lower in cost. The duplex family of stainless steel isn't new to the marketplace, but it has been under-used because of the way their chemistries are formulated—they have less high-priced alloy elements. Using a duplex alloy as opposed to austenitic would give you the same or better performance but at a lower cost. It's not just the dollars per pound for the alloy; you've got mechanical and physical properties advantages to help reduce the cost of your item, particularly in pressure vessels and large tanks.
The duplex family of alloys, especially the 2304 grade, has outstanding engineering properties in comparison with similar grades of the austenitic family. Duplex stainless steel structures are approximately 50/50 austenite and ferrite. Physical properties are a combination of the ferritic and austenitic grades.
General corrosion resistance can vary greatly with changes in concentration, pH, temperature, and impurities. The greatest advantage for duplex stainless steels is their improved resistance to chloride stress corrosion cracking (CSCC) when compared to the austenitic grades. Only the 25% nickel grades have similar CSCC resistance.
Duplex stainless steels have roughly twice the yield strength of their counterpart austenitic grades. This allows equipment designers to use thinner gauge material for vessel construction. In heat transfer, they provide a 5% advantage compared to austenitic grades. Higher strength means you can apply higher cyclic stress without fatigue failures. By using duplex grades, you enhance fatigue and fatigue corrosion resistance. The duplex grades have higher mechanical properties, such as chromium content and duplex microstructure. Because of its high ferrite content, the duplex stainless steel has a ductile, brittle transition temperature of -50°F.
Stainless steel's strength allows you to decrease wall thickness, so you reduce the amount of material you need for a project. You also reduce building and structure costs, as well as the costs associated with labor and transportation.
About the Author
John Grocki is a consultant at Advantage Resources Consulting in Enfield, Conn.
New techniques allow real-time corrosion monitoring
Tom Henke, senior corrosion specialist at Albemarle Corp. in Baton Rouge, La., a maker of polymers and fine chemicals, hasn't used probes for quite a while because of the problems with linear polarization and corrosion resistance probes. Since his plant doesn't run a continuous process, "a lot of our processes are variable, but someone with fixed environments could make [those probes] work," he said. "With all the changes in electronics and control systems, maybe there's something new out there."
Corrosion can wreak havoc not only in the chemical industry, but in pulp and paper and the power industry. Wherever these industries are using "sacrificial pieces of metal, there are new techniques to provide online, real-time corrosion monitoring," said Tyler Redslob, vice president, sales at Pepperl+Fuchs, Inc. in Twinsburg, Ohio. Three technologies enable corrosion monitoring of general or localized corrosion online and in real time: linear polarization resistance (LPR); harmonic distortion analysis (HDA); and electrochemical noise (ECN). "The issue in the past has not been that you couldn't measure corrosion rates electronically," Redslob said, "but the information appeared in manufacturer specific products and software outside the normal process control."
LPR involves measuring the polarization resistance of a corroding electrode to determine the corrosion current. Since the voltage-current response of a corroding element tends to be linear over a small range, determination of the polarization resistance allows determination of the corrosion current. The slope of the response (the polarization resistance) is inversely proportional to the corrosion current, thus you can calculate a corrosion rate.
HDA is similar to LPR and measures the resistance of the corrosive solution by applying a low frequency sine wave to the measurement current. Using harmonic analysis, you can determine the solution resistance and combine it with the polarization resistance of the LPR method to calculate a more accurate general corrosion rate.
Developed in the early 1980s, ECN evaluates the fluctuation in current and voltage noise generated at the corroding metal-solution interface. This technique detects non-uniform or localized corrosion.
All three methods have seen use in laboratory equipment for years, "yet the instruments used required extra software and hardware to interpret data to provide corrosion rates," Redslob said. "Incorporating these technologies into a field transmitter enables users to control processes, as they are no longer tied to laboratory equipment."
If the new methods (harmonic distortion and electrochemical noise) can live up to their promises, Henke said his plant might be trying out one of the probes. One process in which Henke said he believes the new probes would work well, "where even the old ones would work, is dry corrosive gas," he said. "You can do that in carbon steel. If it gets wet, it's really aggressive. The LPR probes work in conducting electrolytes, something that can conduct a fluid. The electrochemical resistance probes, a whole different concept, will work on non-conducting fluids."
P049-"Performance Improvement for Cooling Water Systems; Corrosion – The New Process Variable," course by D C Eden, Honeywell InterCorr LLC
P037-"Dissolved Oxygen Measure-ment using Luminescent Tech-nology," course by Scott Crosier, Hach Company
NACE Corrosion Engineer's Reference Book, Third Edition, by R. Baboian
"Watch the Water" by Dave Anderson
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