Driven by financial necessity, operators of steam boiler systems have sought ways to increase boiler cycles of concentration to reduce energy and water losses through boiler blowdown. Improvements in pretreatment technology, most notably in reverse osmosis, have provided the means, but new concerns have arisen as a result. When boiler cycles are low, feedwater contaminants are removed from the boiler through the blowdown before they deposit on heat transfer surfaces.
As boiler cycles increase, contaminants - mostly iron and copper returned with the condensate - remain in the boiler for longer periods of time, increasing the likelihood of deposition.
Chemical dispersants are often used to minimize deposition of iron and other contaminants, but do nothing to address root causes which often reside in the condensate system. Maximizing condensate return is a powerful cost reduction tool. Condensate can be extremely pure water. It often requires no pretreatment to make it acceptable for boiler feedwater, although use of condensate polishers is not uncommon. It is also free in the sense that the returned condensate takes the place of excess make-up water in the system.
Condensate can be contaminated by release of corrosion products through metal dissolution or by separation of accumulated oxide layers from the metal surface during system startups or shutdowns. Corrosion products tend to deposit on boiler heat transfer surfaces Figure 1? Condensate corrosion control is required to protect process equipment, lines and tanks as well as to maintain the condensate as a good quality source of feedwater.
In order to maximize the benefits of returned condensate, it is important to understand the condensate system corrosion mechanisms which can degrade condensate quality and, ultimately, boiler operation and reliability. This paper discusses those mechanisms, several case histories illustrating failures, a summary of corrosion protection technologies, and typical and state-of-the-art methods for corrosion monitoring in the condensate systems.
Sign In or Register. Advanced Search. Sign In. Skip Nav Destination Proceeding Navigation. It is important to realize that systems with frequent shutdown periods may experience accelerated rusting in all pipes if condensate is allowed to remain in the system during shutdown time. A good measure for preventing rust during operation is to employ the proper use of steam traps which will remove condensate as it forms and help to keep the steam dry.
Air should also be removed from the system via air vents so that opportunities for rust to form are minimized. When shutdowns occur, it is important to manually drain condensate from all collecting points which may not be drained automatically by steam traps. Copper is often utilized in tracing lines due to its low installation cost and ease of bending around equipment and flanges, but it is also vulnerable to corrosion under certain conditions.
High temperatures and low pH values in condensate can cause copper to degrade into copper ions which then dissolve into the condensate. When copper-laden condensate reaches a steam trap and is discharged, the lower pressure on the outlet side of the trap will cause part of the condensate to flash into steam; some of the dissolved copper ions may precipitate and accumulate as solid build-up around the valve seat, causing orifice blockage and lowered temperatures in the tracing line.
It is important to take extra precautions in treating water and pH monitoring to prevent this from occurring. Low dissolved oxygen content and a neutral pH of are ideal. Ammonia is sometimes used in water treatment to combat low pH levels, but in copper, it can actually catalyze the corrosion process and should therefore be avoided where copper is used. Stainless steel is often said to be a corrosion-resistant metal. In truth, however, it is not the metal itself that is actually corrosion-resistant.
Passivation refers to the formation of a thin layer of oxide on the surface of the metal upon contact with air. The oxide layer protects the metal, which retains its original color and luster. In the case of stainless steel, this layer forms naturally and is resistant to rust and other types of corrosion. Stainless steel can be used in industry for systems where resistance to corrosion, capability to handle high temperature, and high standards of sanitation are absolutely vital, such as in demanding heavy industry or medical applications.
TLV employs the use of stainless steel in many of its steam traps and offers stainless steel options for many other products to meet these needs. Though stainless steel products may command a higher purchase price, it is also important to consider the remarkable longevity and durability they can provide. Many companies are now requiring stainless steel for tracing systems to avoid blockages caused by corrosion.
Apart from clogging steam traps and causing pipe thinning, corrosion can also affect other parts of the steam system. As corroded metal separates from the pipe wall and is carried away by the fluid, it may erode piping further down the line. The resulting oxide layer is a much more dynamic system than that of iron. Soluble copper ions and particulate copper oxides are also formed by the normal oxidation processes.
The stability of the passivating iron or copper oxide layer is critically dependent on condensate pH. Any contaminants in the condensate system that cause the pH to decrease cause dissolution of the oxide layer and increased corrosion.
Carbon dioxide CO 2 is the primary cause of decreased condensate pH. Carbon dioxide enters the system with air leaking into the condenser or from decomposition of feedwater alkalinity. Although part of the feedwater alkalinity is removed by a properly operated deaerating heater, most is converted to CO 2 in the boiler and released into the steam.
The net results are release of 0. As the steam is condensed, carbon dioxide dis-solves in water and depresses the pH by increasing the hydrogen ion concentration as shown in the following reaction sequence:. The overall reaction is:. Low pH causes a generalized loss of metal rather than the localized pitting caused by oxygen corrosion.
Pipe walls are thinned, particularly in the bottom of the pipe. This thinning often leads to failures, especially at threaded sections Figure In order to reduce low pH-induced condensate system corrosion, it is necessary to lower the concentration of acidic contaminants in the condensate.
Feedwater alkalinity can be reduced by means of various external treatment methods. Less feedwater alkalinity means less carbon dioxide in the steam and condensate. Venting at certain points of condensation can also be effective in removing carbon dioxide.
Other contaminants in the condensate system can affect corrosion rates of iron and copper even when the pH is correctly maintained. By complexing and dissolving iron and copper oxides, contaminants such as chloride, sulfide, acetate, and ammonia for copper can dissolve part or all of the oxide layer.
Ammonia is the most common contaminant and is usually present in low concentrations. Ammonia contamination is usually caused by the breakdown of nitrogenous organic contaminants, hydrazine, or amine treatment chemicals. Sometimes, ammonia is fed to control condensate pH. In these systems, ammonia feed rates must be carefully controlled to minimize the attack of any copper-bearing alloys Figure Condensate systems can be chemically treated to reduce metal corrosion.
Treatment chemicals include neutralizing amines, filming amines, and oxygen scavenger-metal passivators. These amines hydrolyze when added to water and generate the hydroxide ions required for neutralization:. By regulating the neutralizing amine feed rate, the condensate pH can be elevated to within a desired range e. Many amines are used for condensate acid neutralization and pH elevation.
The ability of any amine to protect a system effectively depends on the neutralizing capacity, recycle ratio and recovery ratio, basicity, distribution ratio, and thermal stability of the particular amine. Neutralizing Capacity.
Neutralizing capacity is the concentration of acidic contaminants that is neutralized by a given concentration of amine. The neutralizing capacity of an amine is inversely proportional to molecular weight i.
Neutralizing capacity is important in treating systems with high-alkalinity feedwater. Table provides a measure of the neutralizing capacity of commonly employed amines. Neutralizing capacity is not the only measure of a required product feed rate. Recycle Ratio and Recovery Ratio. In determining product feed rates, recycle and recovery ratio are important factors. In Figure , the recycle factor is the concentration of amine at point x divided by the amine feed rate at point z.
Because some amine is returned with the condensate, the total amine in the system is greater than the amount being fed. Recovery ratio is a measure of the amount of amine being returned with the condensate. It is calculated as the amine concentration at site y divided by the amine con-centration at site z. An amine's ability to boost pH after neutralizing all of the acid species is termed "basicity. The dissociation constant Kb is a common measure of basicity. As the value of Kb increases, more OH is formed after all of the acid has been neutralized and pH increases.
Distribution of Amines between Steam and Liquid. In condensate systems, the distribution of amines between steam and liquid phases is as significant as basicity or neutralizing capacity.
As the steam condenses, acidic contaminants can either remain in the steam or dissolve in the liquid phase. Some contaminants, such as carbon dioxide, stay mainly in the steam phase while others, such as hydrochloric acid, go largely into the liquid phase. Neutralizing amines must be chosen according to their distribution characteristics to "chase" acidic contaminants.
This choice must be tailored to the condensate system and the process contaminants. For example, morpholine is an amine that primarily distributes into the liquid phase. If this amine is fed into a CO 2 laden steam system with three consecutive condensation sites, it will go into the liquid phase at the first condensation site while most of the carbon dioxide remains in the steam.
With a high concentration of morpholine, the liquid phase has a high pH. At the next condensation site, the concentration of morpholine is lower, so the condensate pH is lower.
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