Causes of the nine common scaling components that lead to overheater scaling and tube failure, as well as solutions

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Nine common causes of scaling that lead to tube failure in superheaters, along with the reasons behind these scaling issues and corresponding solutions. The usual causes of scaling in superheaters are poor treatment of boiler water, with salt-laden steam depositing within the superheater; inadequate deoxidization, which results in the formation of oxide corrosion products; or problems with the boiler’s steam-water separation equipment or drum water level, all of which cause steam to carry boiler water and thus lead to scaling. I. The 3 major hazards of scaling in boiler superheaters. Hazard 1 of scaling in boiler superheaters: Whether it is sodium salt scale, calcium-magnesium scale, corrosion products in the form of iron oxide scale, or silicon oxide scale, all of these reduce the heat transfer rate. This hinders heat exchange between the high-temperature flue gas and the steam, resulting in higher flue gas temperatures, a decrease in the boiler’s thermal efficiency, energy waste, and increased operating costs for the boiler. Hazards of scaling in boiler superheaters: Scaling in boiler superheaters can impede the flow of steam within the superheating tubes. A reduced steam flow leads to higher steam temperatures and overheating of these tubes. Overheating of the superheating tubes can cause high-temperature corrosion. In severe cases of scaling, scale deposits may break off and block the superheating tubes, resulting in their premature overheating and rupture. Harm of scaling in boiler superheaters: The low heat transfer rate due to scale deposits means that less heat is carried away by the steam, resulting in overheating of the walls of the superheater tubes. This overheating can cause deformation and creepage of the material forming these tubes; prolonged overheating or severe scaling can lead to tube rupture in the superheater. ▲The image above shows a high-pressure boiler being used at low pressure by a coal chemical company in Shanxi; its design pressure is 42 kilograms, while the operating pressure is 10 kilograms ； The designed steam temperature is 420 degrees Celsius, while the operating temperature is 350 degrees Celsius. Trisodium phosphate with 99% purity is used as a scale inhibitor in the boiler. In October 2020, tube blockage led to tube rupture; after the tubes were cut open, a black phosphate scale mass was removed from them. There are nine common components that cause scaling and resulting tube ruptures in superheaters. Scale masses can usually be found in areas such as the headers and elbows of superheaters where scaling leads to ruptures. Analysis of the composition of these scale masses shows that common components include sulfides, sodium sulfate (Na2SO4), sodium silicate (Na2SiO3), trisodium phosphate (Na3PO4), sodium carbonate (Na2CO3), sodium chloride (NaCl), silicon dioxide (SiO2), iron oxide, calcium salts, and magnesium salts. Boiler operators typically add trisodium phosphate (Na3PO4) to prevent scaling and increase the alkalinity of the boiler water. To raise the pH level of the boiler feedwater or boiler water, they often use ammonia, sodium carbonate (Na2CO3), or sodium hydroxide (NaOH). The deposition of the nine common scaling components carried by saturated steam in the superheater is as follows: sodium triphosphate Na3PO4 scale, sodium sulfate Na2SO4 scale, and sodium silicate Na2SiO3 scale; the solubility of these three impurities decreases as the temperature rises. For example, when the temperature exceeds 120°C, the solubility of trisodium phosphate (Na3PO4) drops sharply. As the thermal load on the boiler increases, boiling becomes more intense inside the tubes, resulting in an increase in the salt concentration in the water near the tube walls. At this point, the solubility of trisodium phosphate further decreases, so its concentration in that area can easily exceed its solubility limit, leading to precipitation and deposition on the inner surface of the superheater tubes ； Sodium carbonate Na2CO3 scale in the superheater: Both sodium carbonate Na2CO3 and sodium hydroxide NaOH are impurities whose solubility increases as the temperature rises; as a result, they travel in the form of concentrated liquid droplets to the superheater and adhere to its tube walls. Sodium hydroxide NaOH reacts with CO2 to form sodium carbonate Na2CO3, which then deposits within the superheater. In superheaters, sodium chloride (NaCl) scale forms; when the pressure is above 9.8 MPa, the solubility of sodium chloride is very high, and it often dissolves in the superheated steam and is carried to the turbine or other equipment that uses steam. Silica SiO2 scale forms in the superheater as a result of these two impurities present in the boiler water: metasilicic acid H2SiO3 or orthosilicic acid H4SiO4. When heated and deprived of water, these substances transform into silica SiO2 scale. Silica SiO2 has a high solubility in superheated steam, and it is generally carried along to the turbine or other equipment that uses steam. Iron oxide scale forms in the superheater due to low deoxidization efficiency of the boiler or large fluctuations in the temperature of the deoxidizer. Overheating of the superheater tube walls, as well as oxygen corrosion, stress corrosion, or under-scale corrosion, can all lead to the formation of iron oxide scale. Calcium and magnesium salt scales form in the superheater; impurities such as calcium and magnesium hardness present in the boiler water are carried into the superheater by the steam, where they adhere to the walls of the superheater tubes and form calcium and magnesium salt scales. Based on the above, the deposits in the superheaters of medium and low-pressure boilers are mainly sodium compounds, such as sodium sulfate Na2SO4, sodium silicate Na2SiO3, trisodium phosphate Na3PO4, sodium carbonate Na2CO3, and sodium chloride NaCl ； The deposits in the superheater of high-pressure boilers are mainly Na2SO4 and Na3PO4, with very low levels of other sodium salts ； The amount of salt deposits in the superheater of ultra-high pressure boilers is very small. Colleagues and fellow students, aside from these nine common components that cause overheater tube failure and scaling, are there any other components that can lead to such problems? How was that solved again? Regarding issues such as superheater scaling, superheater tube failures, scaling and tube failures in the superheater, condenser corrosion and scaling, red-colored boiler drainage water, adjustment of the pH value of boiler feedwater, red-colored boiler water, hardness in steam condensate water, boiler corrosion, scaling, and tube failures, corrosion in the steam system, excessive iron content in steam condensate water, yellow-colored steam condensate water, as well as online cleaning and descaling technologies that allow for removal of scale without shutting down the equipment, and online descaling techniques for boiler scaling without interrupting operations, Yan Hui from Beijing University of Chemical Technology at I86OO475З86 is always available to discuss these issues, exchange experiences and insights. Everyone is welcome to share their experience in managing and using boiler equipment, so as to learn from one another in addressing various practical problems related to boilers. Nine common causes of scaling that lead to tube failure in superheaters, as well as measures to address these issues. III. Seven common solutions for superheater scaling. 1. Ensure the quality of boiler feed water, prevent excessive amounts of trisodium phosphate (Na3PO4), and avoid too high an alkalinity in the boiler water, which can cause azeotropy between steam and water, leading to steam carrying water droplets along with it ； Next is to ensure that the components of the steam-water separation equipment inside the drum are in good condition, in order to prevent inadequate steam-water separation ； During operation, the water level in the drum should not be too high to prevent steam from carrying boiler water. These measures prevent steam from carrying impurities that could lead to the formation of scale on the walls of the superheater tubes, consisting of eight common substances: sodium sulfate Na2SO4, sodium silicate Na2SiO3, trisodium phosphate Na3PO4, sodium carbonate Na2CO3, sodium chloride NaCl, silicon dioxide SiO2, calcium salts, and magnesium salts. 7 common solutions for superheater scaling: 2. Ensure the efficiency of boiler deoxidization or reduce fluctuations in the deoxidizer temperature, in order to prevent overheating of the superheater tube walls and resulting oxidative corrosion; such measures can help avoid the formation of iron oxide scale inside the superheater tube walls. 7 common solutions for superheater scaling: 3. It is necessary to strictly control the temperature of the superheated steam in the boiler, maintain a stable heat load, and prevent insufficient steam flow in some of the superheating tubes when the boiler operates at low load levels. This reduces the amount of heat carried away by the steam, leading to overheating of those tubes and even tube rupture; special attention should be paid to controlling the temperature of the superheated steam in the high-temperature sections. In addition, it is necessary to strengthen the supervision of boiler water quality to prevent substandard quality of desuperheating water. 7 common solutions for superheater scaling: 4. Strengthen the maintenance of boilers that are not in use to prevent corrosion of the superheater tubes when they are not operational. 5 common solutions for superheater scaling: Add an additional underwater orifice plate below the centerline of the steam drum, in order to prevent steam from concentrating in certain areas and escaping to the surface. This prevents severe fluctuations at the steam-water interface, which would otherwise result in numerous small water droplets floating in the steam space. 7 common solutions for superheater scaling: 6. Install an additional cyclone separator inside the boiler drum to improve the primary steam-water separation capability, thereby preventing the steam-water mixture from entering the superheater inlet header via a short circuit. 7 Common Solutions to Superheater Scaling 7. To prevent scaling in the superheater from causing tube failures and thereby affecting the safe and efficient operation of the boiler, it is necessary to take preventive measures against scaling in the boiler’s superheater as early as possible. Scaling in the superheater system cannot be removed by acid cleaning; instead, regular backwashing is employed. Generally, the superheater is washed every six months to one year, and the indication that the washing was successful is that the salt content in the wastewater from the washing process is within acceptable levels. To fundamentally solve the problem of salt in steam and the issue of tube failure due to scaling in boiler superheaters, technical upgrades are a prerequisite, while equipment management ensures effectiveness; adopting both approaches yields significant results. IV. Cases of tube rupture caused by superheater scaling: A thermal power company in Shandong operates a 75-ton steam boiler, with a superheating temperature of 220°C and a pressure of 1.2 Mpa. The boiler feed water is obtained by treating groundwater from underground wells through reverse osmosis to produce demineralized water, which is used as the feed water. Trisodium phosphate is added to the boiler to prevent scale formation, and ammonia is added to raise the pH value of the feed water in order to prevent corrosion. The steam is used as a heat source for users in the surrounding area of the development zone, and the steam condensate is not reused. ▲The image above shows the scale deposits taken from the superheater header. Tests revealed that the composition of these deposits is as follows: iron Fe 55.87%, calcium Ca 5.097%, phosphorus P not detected, magnesium Mg 6.357%, silicon dioxide SiO2 4.32%, sulfides S 8.95%, sodium Na 2.25%, and the remainder being sludge. The superheater tubes burst 3 times at low boiler load; the superheater was replaced in 2023. The scale deposits taken out from the superheater header are shown in the image above, while those taken out from the bent sections of the superheater where the tubes burst are shown in the image below. ▲The image above shows the scale deposits taken from the elbow section of the superheater where tube rupture occurred. Tests revealed that the composition of these deposits is as follows: iron Fe 48.92%, calcium Ca 17.30%, phosphorus P 0.062%, magnesium Mg 3.22%, sodium Na 1.63%, and silicon dioxide SiO2 6.25%. ▲The image above shows the characteristics of the ruptures in the superheater tubes of a 75-ton boiler. At present, this 75-ton boiler is operating at a low load level, and the sodium ion concentration in the superheater is above the acceptable level. The gas production capacity of this 75-ton boiler is sufficient only to power units of around 20 tons; sometimes, exhaust venting to the atmosphere is necessary. Colleagues and classmates, what do you think are the reasons for the tube failure in this boiler’s superheater? What is the cause of scaling in the boiler superheater? What do you think should be done to address the issue of frequent tube failures in this boiler? Yan Hui invites colleagues and peers to share their opinions, views, and suggestions in the comments section below. Nine common causes of scaling that lead to overheater tube failure, along with solutions (Yan Hui)