Precautions for steam condensate recovery
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Precautions for the recovery of steam condensateTechnical Service Center, Hangzhou Watt Energy Saving Engineering Co., Ltd.
Steam Technology Engineer: Li Shaopeng
Steam condensate is essentially pure, high-quality water; it does not cause corrosion to the tanks, pipes, pumps, valves, etc. used in its recovery, nor does it contaminate or affect the boiler feed water. In fact, in steam systems, we theoretically consider condensed water to be the most suitable boiler feedwater. However, in practical applications, steam contains various possible impurities for various reasons. In industrial steam, deoxygenation is absent or insufficient due to inadequate feedwater deoxygenation equipment ； Shutting down the steam pipeline allows air to enter the steam space through connections such as valves and flanges ； It dissolves into the condensate water during the vapor condensation process, and gases such as carbon dioxide are also introduced as the condensate water begins to be collected. Certain gases also dissolve during the transportation of condensate water. In high-temperature and humid environments, these gases corrode the heat exchange equipment and transmission pipelines, which serve as carriers for condensate water. Severe corrosion can result in a large amount of condensate water unable to be recovered, leading to significant waste of water resources and heat. Over the past 20 years, Hangzhou Watt Energy Saving has been investigating the causes of corrosion in its engineering projects, and by addressing these causes through a combination of prevention and treatment measures, it has been able to effectively prevent corrosion caused by condensed water, thereby avoiding waste of water resources and heat. II. Analysis of the causes of condensate corrosion 1. Oxygen corrosion of condensate 1.1 Sources of oxygen in condensate The prerequisite for oxygen corrosion is the presence of O2 in the condensate. There are two sources of O2 in condensate: one is the certain amount of O2 contained in steam, which dissolves into the condensate during the condensation process. The feed water for industrial boilers often contains a certain amount of O2, either due to suboptimal performance of the deoxygenation equipment or poor operational management. Once feed water containing O2 enters the boiler, the O2 moves into the steam as the water evaporates. Another source of O2 in the condensate water is the oxygen dissolved in the air during its transportation. To be more precise, most of the condensate water recovery systems currently in use are open-system types; that is, the steam condensate from the heat exchange equipment is collected and discharged into a condensate tank. When the liquid level in this tank reaches a certain height, the condensate is pumped back to the heat generation plant for reuse. The condensation tank has a vent hole that leads to the atmosphere. When the liquid level in the tank rises, the gas inside the tank is released into the atmosphere; when the liquid level drops, gas from outside the tank enters the tank. Due to the significant difference in oxygen concentration between the gas inside the tank and that in the air outside, oxygen from the atmosphere continuously enters the condensation tank as it breathes. According to Henry’s law, the oxygen concentration in water is proportional to the partial pressure of oxygen in the gas phase; therefore, oxygen from the gas phase continues to dissolve into the condensate water until the oxygen concentration in the water reaches equilibrium with that of the gaseous oxygen. If the boiler’s feed water is equipped with deoxidization equipment and it is functioning properly, then the O2 that enters the condensate from the condensate tank is the main source of O2 in the condensate. The form of oxygen corrosion in condensate water is oxygen depolarization corrosion, and its corrosion products vary depending on the material of the carrier. The pipelines used for transporting condensate are generally made of steel, and the corrosion products resulting from this are iron oxides. The reaction equations are as follows: Anodic reaction: Fe → Fe2+ + 2e; Cathodic reaction: O2 + 2H2O + 4e → 4OH-. The Fe2+ produced in these reactions further reacts with other substances in water, through the following processes: Fe2+ + 2OH- → Fe(OH)2; 4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3; Fe(OH)2 + 2Fe(OH)3 → Fe3O4 + 4H2O. Among these corrosion products, Fe(OH)2 is unstable in the presence of oxygen and can transform into α-FeOOH, γ-FeOOH, or Fe3O4. α-FeOOH is yellow in color, γ-FeOOH is orange in color, while Fe3O4 is black in color ； Fe(OH)3 represents the hydroxide of trivalent iron; its chemical composition is not as simple as its chemical formula suggests. It is usually a mixture of various hydrated iron oxides, which can be expressed as Fe2O3·nH2O or simply as Fe2O3. Fe2O3 itself exists in two forms: α-Fe2O3 and γ-Fe2O3. α-Fe2O3 is colored from brick red to black, while γ-Fe2O3 is brown in color. Contaminated condensate water has a reddish-brown color, and the more severe the corrosion, the darker the color, due to the presence of these corrosive products in the condensate water. The oxygen corrosion of condensate water is a type of ulcerative corrosion; after it occurs, bulges form on the surface of the metal, with diameters ranging from 1 mm to 30 mm. The surface of these bulges is yellow-brown to brick-red in color, and it is composed of the various oxygen corrosion products mentioned above. Once these corrosion products are removed, the metal surface is covered with corrosion pits. Once oxygen corrosion of condensate water occurs, it is difficult to stop the continuation of this corrosion process. The reason is that at the site of corrosion, the diffusion of dissolved oxygen in water to that location slows down due to the presence of corrosion products; as a result, the oxygen concentration around the corrosion site is higher than that at the corrosion site itself. The area surrounding the corrosion site becomes the cathode, while the corrosion site (a specific point on the metal surface) becomes the anode. The metal at the anode (Fe) is corroded, and its products—Fe2+—diffuse slowly into the solution through the corrosion products, where they continue to react with other substances in the solution to form new corrosion products. This process of oxygen corrosion continues in this manner. Engineering practice shows that there are many factors affecting oxygen corrosion. The main factors influencing oxygen corrosion in condensate water include pH value, dissolved oxygen concentration, flow rate, temperature, etc., which will be briefly introduced below. The lower the pH value, the faster the corrosion rate; the higher the pH value, the slower the corrosion rate. When the pH value is near the neutral point, the corrosion rate remains constant, as the pH value has little impact in this case. The pH value is low, meaning there is a certain amount of H+ ions in the water. The corrosion of condensed water involves not only oxygen-induced corrosion but also acid-induced corrosion; as a result, the corrosion rate is high ； A high pH value (generally considered to be pH > 7) can form a protective film on the metal surface, preventing oxygen-induced corrosion and thus reducing the corrosion rate ； The corrosion that occurs near the neutral pH point is an oxygen depolarization reaction; the rate at which dissolved oxygen in water diffuses to the metal surface determines the rate of oxygen corrosion, so the corrosion rate has little relation to pH. The length of the horizontal section of the corrosion rate curve near the center point is related to temperature; the higher the temperature, the shorter this horizontal section. Based on this pattern, it can be inferred that when the temperature rises to a certain value, this horizontal section may disappear. The higher the O2 concentration in the condensate, the faster the rate of oxygen corrosion; however, when pH > 7, a dense protective layer forms on the metal surface, preventing further oxygen corrosion. The higher the temperature, the faster the rate of oxygen corrosion. The rate of oxygen corrosion is determined by the speed at which O2 diffuses to the metal surface; the faster the diffusion rate, the faster the oxygen corrosion. As the temperature rises, the diffusion rate of various substances (including O2) increases, and at the same time the resistance of aqueous solutions decreases, accelerating the electrode processes at the anode and cathode of the corrosion cell and thus speeding up oxygen-induced corrosion. The higher the water flow velocity, the greater the diffusion rate of various substances in the water. The double layer on the metal surface becomes thinner, and oxygen diffuses to the metal surface more rapidly, thereby accelerating oxygen corrosion. When a passivation film is formed on a metal surface, if the water flow velocity reaches a certain level, the mechanical action of the water flow can destroy the passivation film, resulting in erosion corrosion. The flow velocity of the condensate water is not too fast, so impact corrosion generally does not occur. The acidic substances in condensate water are mainly CO2 dissolved in it, which forms a weak electrolyte—H2CO3; H2CO3 then decomposes into H+ and HCO3-. CO2 + H2O = H2CO3; H2CO3 = H+ + HCO3-. The CO2 present in condensed water comes mainly from steam, and the source of CO2 gas in the steam is the free CO2 and carbonates present in the make-up water for steam boilers, which decompose when heated in the boiler. The reaction equation is: 2NaHCO3 → Na2CO3 + CO2 + H2O. Once CO2 enters the condensed water, it forms carbonic acid (H2CO3). H2CO3 is a weak acid; it releases only a small amount of H+ ions when ionizing in water. However, condensed water is relatively pure with low salt content and poor buffering capacity, so even a weak acid like H2CO3 can cause a significant drop in the pH value. When the CO2 concentration in pure water is 1 mg/L, the pH value of this water drops from 7.0 to 5.5. At the same time, as H+ is continuously consumed during corrosion and the ionization equilibrium is disrupted, the reaction proceeds to the right, with more H+ being ionized to supply the corrosion process, until H2CO3 is completely exhausted. The anodic and cathodic reaction equations for CO2 corrosion are as follows: Anodic reaction: Fe → Fe2+ + 2e; Cathodic reaction: 2H+ + 2e → H2. The corrosion products resulting from CO2 corrosion are soluble and do not deposit on the metal surface; therefore, CO2 corrosion is a uniform type of corrosion that does not result in the formation of a protective layer. CO2 not only corrodes iron but also copper; when there is only CO2 present, it causes zinc loss corrosion in copper pipes ； When CO2 and O2 are present simultaneously, it also causes corrosion of the copper metal in copper pipes, with the corrosion product being soluble Cu2+. Due to copper’s low thermal resistance, copper tubes are generally used for the heat exchange tubes in heat exchangers. The corrosion of copper tubes by CO2 occurs mainly on the surface of the tubes above the level of the steam condensate (drainage water), that is, at the inlet end of the secondary water. At this point, the temperature of the incoming secondary water is relatively low, resulting in the formation of a supercooled water film on the vapor-side wall of the heat exchange tubes – the copper tubes. The temperature of water in this film is lower than that of the drainage water, and a large amount of CO2 dissolves into this film; together with the oxygen that also dissolves therein, this leads to corrosion of the copper tubes. Experiments show that when the CO2 concentration in the steam entering the heat exchanger is 8 mg/L, the CO2 content in the water film inside the copper tubes above the hydrophobe is 500–600 mg/L. In light of the causes and characteristics of condensate corrosion mentioned above, Watt Energy Saving takes targeted measures to prevent and address it. The measure to prevent condensate corrosion is to reduce or eliminate CO2 and O2 gases in the condensate. Removal of free CO2 from boiler feedwater: The free CO2 in boiler feedwater mainly comes from the boiler make-up water. Softening systems are generally used for treating make-up water in industrial boiler rooms, while hydrogen-sodium systems are employed for treating make-up water in large industrial steam boiler rooms. Desalination systems are virtually not used in industrial boiler rooms. If a hydrogen-sodium system is used for water treatment, the carbon remover in such a system will remove most of the CO2, leaving its concentration below 5 mg/L. On the other hand, when a single-sodium softening system is used for water treatment, the CO2 content in the feed water is relatively high, generally exceeding 20 mg/L ； The free CO2 in the make-up water enters the deaerator along with the boiler feedwater; if a thermal deaerator is used (either an atmospheric thermal deaerator or a vacuum thermal deaerator), CO2 gas is removed along with O2. Therefore, the free CO2 in the make-up water will not be introduced into the boiler, provided that the deoxygenation method is appropriate. The main way to reduce the CO2 generated by the thermal decomposition of carbonates in the furnace is to decrease the amount of carbonates present there, primarily NaHCO3; in other words, it is necessary to control the alkalinity of the boiler water. The standard \"Quality Requirements for Water Used in Industrial Boilers\" (GB1576—2001) specifies clear requirements regarding the alkalinity of boiler water: when the rated pressure of the boiler is ≤1.0 MPa, the total alkalinity of the boiler water should be between 6 and 26 mmol/L; when the pressure is 1.0 MPa