Water Technologies & Solutions technical bulletin Steamate* technology superior protection against condensate system corrosion problems in steam distribution systems The steam distribution system can be thought of as the "business end" of the boiler system. The boiler plant operator makes a substantial investment in the "front end" of the system in terms of the capital cost of the boiler, associated equipment, and fuel. The steam produced is the renewable energy source that drives the production process, whether it involves electrical generation, heating or drying, or a myriad of other uses. Maintaining the efficiency and reliability of steam distribution system components is critical in controlling overall energy and maintenance costs. Problems arising from excessive corrosion of steam condensate system surfaces include the following: Loss of Capital/Increased Maintenance Costs: Corrosion can not only result in the loss of expensive equipment, but also in greatly increased maintenance and repair costs. Reduced Efficiency: The buildup of corrosion product deposits on the surface of heat exchange equipment can dramatically reduce heat transfer efficiency. This can lead to higher energy costs, and in severe cases, limit production capacity. In addition, untimely outages caused by equipment failure can be extremely expensive in terms of lost production time. Increased Boiler Scale Formation: Condensed steam represents a valuable resource as a high temperature, high purity supply of boiler feedwater. However, condensate returned to the boiler with high levels of iron and copper corrosion products can result in the formation of metal oxide scale on the boiler heat transfer surfaces. This can dramatically reduce efficiency and, in severe cases, lead to overheating and tube failure. condensate system corrosion While many complex and interrelated factors are involved with the corrosion of metal surfaces in steam condensate systems, the two primary corrosive agents are carbon dioxide and dissolved oxygen. Carbon dioxide (CO 2 ) is the most common cause of condensate system corrosion. CO 2 is produced in the boiler as a result of the chemical conversion of natural alkalinity, principally bicarbonate ions, which enter with the feedwater. The reactions are as follows: 2 NaHCO 3 HEAT Na 2 CO 3 CO 2 H 2 O Na 2 CO 3 H 2 O HEAT 2 NaOH CO 2 The first reaction proceeds to completion in the boiler, while the second reaction is approximately 80% complete under typical conditions. Thus, each ppm (mg/l) of bicarbonate alkalinity (expressed as the CaCO 3 equivalent) will result in the formation of approximately 0.4 ppm (mg/l) of carbon dioxide. CO 2 is extremely volatile and leaves the boiler with the steam. At points of condensation in the steam distribution system, some fraction of the carbon dioxide present in the steam enters the condensate, forming carbonic acid: CO 2 H 2 O H 2 CO 3 H HCO 3 It takes very little carbon dioxide to create a corrosive, acidic ph, as shown in Table 1. This is because of the high purity and low buffering capacity of the condensate. As shown in the equation above, the carbonic acid hydrolyzes to produce hydrogen ions which cause acidic corrosion of iron and copper alloy surfaces. The initial corrosion reaction for iron is shown below: Find a contact near you by visiting www.suezwatertechnologies.com and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries. 2017 SUEZ. All rights reserved. Jan-06
2H 2 CO 3 Fe Fe (HCO 3 ) 2 H 2 The ferrous bicarbonate formed is soluble and thus has no ability to protect the metal surface against further corrosion. Table 1: Effect of CO 2 on the ph of Pure Water ppm (mg/l) CO 2 ph 0 7.00 1 5.49 2 5.34 5 5.14 10 4.99 20 4.84 Carbonic acid corrosion most frequently manifests itself as generalized metal loss rather than highly localized corrosion such as pitting. Typical corrosion patterns include thinning or grooving of the lower diameter of return line piping, thinning of threaded pipe fittings, and general corrosion on the downstream side of steam traps and control valves where abrupt pressure changes occur. Dissolved oxygen is another major cause of condensate system corrosion. There are several means by which oxygen contamination can occur, including systems under vacuum; leaking heat exchangers; inefficient or improper feedwater deaeration; frequent start-up and shutdown cycles; and air leakage at pump seals, receivers, and flanges. Oxygen can have two distinct effects on the corrosion rates of iron and copper alloys under condensate conditions. First, traces of dissolved oxygen can significantly accelerate the rate of carbon dioxide corrosion. This results both from the oxidation of the protective ferrous hydroxide (or magnetite, Fe 3 O 4 ) film to nonprotective ferric hydroxide and the tact that oxygen accelerates the rate of the acidic corrosion reaction. 2Fe(OH) 2 H 2 O 1/2O 2 2Fe(OH) 3 condensate treatment technologies The three primary methods of condensate system treatment are neutralizing, filming, and passivating programs. Neutralizing amines are the predominant technology and are volatile organic bases that readily enter the steam phase and distribute throughout the system. Filmer treatments adsorb onto metal surfaces and provide a physical barrier between the corrosive environment and the metal surfaces. Passivators work by promoting the formation of a tightly-adherent protective magnetite layer on steel surfaces even when some oxygen is present in the condensate. A neutralizing amine serves a dual function as a corrosion inhibitor. First, it neutralizes the acidity imparted to the condensate by carbon dioxide. - R-NH 2 H 2 CO 3 R-NH 3 HCO 3 After it has neutralized the carbonic acid, the amine elevates the ph of the condensate into the alkaline range. R-NH 2 H 2 O R-NH 3 OH - These reactions can help promote the stabilization of the protective magnetite (Fe 3 O 4 ) layer on steel surfaces and minimizes the corrosion of copper alloy surfaces. In systems where oxygen may be present in the condensate, passivating and neutralizing treatments may be combined to provide an extra level of protection against condensate system corrosion. The ph control range normally recommended for softened water systems is 8.0 to 8.5, while for demineralized systems with both iron and copper alloys, a ph range of 8.8 to 9.2 is typically recommended for maximum protection of all surfaces. The differing ranges are a matter of economics. As a result of the formation of a bicarbonate buffer system in condensate arising from softened makeup, a significant increase in amine feedrate and cost is often associated with raising the ph from 8.0-8.5 to 9.0. Fe 1/2 O 2 2 H Fe 2 H 2 O In this case, the corrosion patterns observed are characteristic of normal carbonic acid attack, except that the severity is significantly increased. Dissolved oxygen can also cause pitting, which begins at weak points in the protective magnetite film. The stationary nature of pitting can result in rapid failure of the affected component. Page 2
There are several important physical properties which determine the effectiveness of a neutralizing amine molecule. These include the (1) neutralizing capacity, (2) basicity, (3) distribution ratio, and (4) thermal stability. Table 2 summarizes the key properties of the amine molecules commonly used for steam condensate system treatment. The neutralizing capacity measures the quantity of amine required to neutralize a given quantity of acid, in this case carbonic acid (H 2 CO 3 ). The smaller the number, the greater the capacity of the amine to neutralize carbonic acid. The neutralizing capacity is a function of both the molecular weight and the number of amine groups on the molecule. For simple reasons of economy, it is obviously desirable for an amine to have a high neutralizing capacity. As shown in Table 2, Diamine, by virtue of its dual amine functionalities, has nearly double the neutralizing capacity of the other commonly used molecules. Table 2: Physical Properties of Neutralizing Amines Neutralizing Base Distribution Ratio at Pressure Capacity Strength 15 psig 50 psig 100 psig 200 psig Amine (Note 1) (Note 2) (1.1 kg/cm 2 ) (3.5 kg/cm 2 ) (7 kg/cm 2 ) (14 kg/cm 2 ) Aminomethylpropanol 2.0 66 -- -- 0.5 1.0 Morpholine 2.0 3.4 0.6 0.8 1.0 1.2 Diamine 1.2 200 0.7 1.3 1.7 2.0 Methoxypropylamine 2.0 102 --- 1.6 2.5 2.4 Diethylaminoethanol 2.7 68 6.8 5.9 5.3 4.5 Cyclohexylamine 2.3 489 23.7 19.2 15.9 12.3 Notes: 1. Neutralizing capacity: ppm (mg/l) amine required to neutralize 1 ppm (mg/l) of carbonic acid (expressed as CO 2). 2. Base Strength at Room Temperature: Expressed as the Basicity Constant (Pkb) X 1,000,000. The base strength, or basicity, of an amine is a measure of its ability to elevate the ph of the condensate after all carbonic acid has been neutralized. It corresponds to the degree of dissociation of the amine in water, and is expressed numerically as the basicity constant, K b. Amines with large basicity constants, such as cyclohexylamine and the Diamine, are more effective in elevating the condensate ph per ppm (mg/l) of material fed. As shown in Table 2, the basicity of the Diamine at room temperature is over fifty times greater than that of morpholine, while cyclohexylamine is over a hundred times greater in base strength. It is important to realize that the base strength of an amine is a function of the water temperature. The values given in Table 1 were measured at room temperature. The temperature/basicity profiles for several amines are shown in Table 3. Due to its dual amine functionalities, the temperature/basicity profile of the Diamine is more complex. However, it retains excellent ph elevation capabilities at boiler temperatures. Table 3: Amine Base Strength at Temperature 72 F 298 F 338 F (22 C) (148 C) (170 C) Morpholine 3.4 4.9 3.8 Cyclohexylamine 489 61 32 Diethylaminoethanol 68 11.3 9.2 The distribution of the neutralizing amine between the steam and liquid phases is as important as the neutralizing capacity and basicity in determining the effectiveness of a particular molecule. The distribution ratio is simply a measure of the ratio of the concentration of the amine in the steam to its concentration in the liquid when the two phases are in contact with one another. Distribution Ratio = ppm (mg/l) amine in steam ppm (mg/l) amine in water Page 3
Like the base strength and neutralizing capacity, the distribution ratio is an intrinsic property of the amine molecule and is a function both of the system temperature/pressure and the ph of the liquid phase. An amine with a low distribution ratio, like morpholine, will tend to concentrate at the initial condensation sites in a steam distribution system. Materials with higher volatility, such as cyclohexylamine and diethylaminoethanol (DEAE), tend to concentrate in the steam and will effectively chase carbon dioxide, which has a very high distribution ratio, to the far ends of long distribution lines or units receiving flash or cascaded steam from higher pressure sources. In practice, the best protection is provided by a blended product which contains the proper ratios of component amines covering a broad range of distribution ratios. This is particularly true in complex and/or extensive distribution systems where the steam is used for a variety of process and heating applications. It is very important that the proper blend is selected to provide effective ph elevation at all points in the condensate return system. Another important property of a neutralizing amine is its thermal stability. Not only is the temperature/pressure profile for thermal decomposition of amines a key consideration, what the molecule breaks down to is equally important. The amines selected for Steamate* provide excellent thermal stability over a wide range of pressure and temperatures. As an example, molecules which decompose to produce significant levels of ammonia would be unacceptable in a system with extensive copper alloy surfaces, where formation of the soluble ammonia copper complex can increase corrosion rates. In designing a Steamate treatment program, the loss of amine to blowdown must be considered. Although amine blowdown losses are often overstated, it is a significant concern in lower pressure, lower cycle systems. The quantity of amine lost to the blowdown is a function of two variables: the distribution ratio of the amine (which depends on pressure/temperature), and the boiler cycles of concentration. Table 4 compares the blowdown losses for several amines at a pressure of 100 psig (7 kg/cm 2 ). Note that the blowdown loss is expressed as a percentage of the amount of amine fed to the boiler. Some loss of amine (usually minor) also occurs in the deaerator. Fortunately, because of its extremely high volatility, a much larger fraction of carbon dioxide is removed with the vent dearator steam than neutralizing amine. Page 4 Table 4: % Blowdown Loss of Amine Versus Feedwater Cycles at 100 psig (7 kg/cm 2 ) 10 25 50 Amine Cycles Cycles Cycles Morpholine 10.0 4.0 2.0 Diamine 6.1 2.4 1.2 DEAE 2.1 0.8 0.4 Cyclohexylamine 0.7 0.3 0.1 Steamate series SUEZ Water Technologies & Solutions offers a complete line of condensate treatment programs through our Steamate series products: Steamate NA Series- Blends of volatile neutralizing amines primarily designed to combat carbonic acid and low ph corrosion. Steamate PAS Series- Product combinations which include both neutralizing amines and passivating chemicals to provide protection against both carbonic acid and oxygen corrosion. Steamate FM Series- Filmer treatments which can protect against both carbonic acid and oxygen corrosion. Steamate NF Series- Combination filmer and neutralizer treatment blends which protect against carbonic acid and oxygen corrosion. SUEZ Water Technologies & Solutions Research and Development has worked intensively for years to develop an advanced, real world, computer-based Condensate Modeling System* (CMS) that realistically reproduces the complex behavior and interactions of neutralizing amines and carbon dioxide in even the most sophisticated boiler systems. This capability, combined with SUEZ s vast practical experience developed over decades of treating thousands of steam distribution systems, has culminated in the development of the Steamate series of products. Some of the key benefits to our customers include: The Steamate products represent the most technically-advanced, cost-effective condensate treatment programs available in the industry. Our customers benefit from superior protection of critical heat transfer equipment, reduced maintenance costs, and increased system reliability and availability. Our BoilerCalc computer program allows us to choose the most effective and economical Steamate blend for each specific application.
Steamate products offer superior ph elevation due to the high basicity of the component amines and excellent thermal stability for maximum effectiveness, even in high pressure systems. Steamate condensate treatment programs assist in maintaining cleaner, more efficient boiler internal surfaces by minimizing the return of corrosion product with the condensate. The value of the condensate as a renewable energy source is maximized. Steamate products are concentrated for maximum cost effectiveness. By delivering less water, we save our customers money and reduce chemical inventory and container disposal concerns. Automated, drum-free delivery, and "hands off" chemical feed systems are available in a wide range of tank volumes and configurations, custom-designed to meet the needs of your system. Only SUEZ offers this range of technology and delivery system flexibility. key to chemical symbols NaHCO 3 Sodium bicarbonate Na 2 CO 3 Sodium carbonate CO 2 Carbon dioxide NaOH Sodium hydroxide H 2 CO 3 Carbonic acid - HCO 3 Bicarbonate ion Fe Iron metal Fe(HCO 3 ) 2 Ferrous bicarbonate H 2 Fe(OH) 2 O 2 Fe(OH) 3 H Fe 2 R-NH 2 R-NH 3 OH Molecular hydrogen (dissolved gas) Ferrous hydroxide Molecular oxygen (dissolved gas) Ferric hydroxide Hydrogen ion Ferrous ion Primary amine (general) Primary ammonium ion Hydroxide ion Page 5