Analysis of the causes of three-tube explosion in coal-fired boilers and countermeasures

Thread Content

Analysis of the causes of tube rupture in coal-fired boilers and corresponding countermeasures. At present, tube rupture accidents in large-scale power plant boilers have become a significant threat to the stable operation of power generation equipment. Moreover, as the operating time of these units increases, such accidents tend to rise year by year, becoming a major factor affecting safe production. Regarding the coal-fired boilers in our plant, since they began operation in 1999, boilers No. 1, 2, and 3 have experienced 13 tube leakage incidents in total, which severely disrupted the stable operation of the power plant as well as the entire company, resulting in substantial economic losses. In fact, when tube bursts occur, we often use so-called rapid repair methods such as replacing the tube segment with a new one or applying coatings for repairs, only for the tubes to burst again after some time. The repeated occurrence of tube failures on the same cross-section of the same tube, made of the same material, or in the same area of the boiler indicates that the fundamental issue behind tube failures has not yet been resolved; therefore, analyzing and understanding the root cause of these tube failures is the top priority at present. 1 Causes and preventive measures for overheater tube failures. The fundamental causes of overheater tube failures include overheating, wear, corrosion, and welding quality. Based on actual cases of overheater tube failures in boilers across various plants, it can be seen that about 30% of such failures are caused by metal overheating, 15% by wear, 10% by corrosion, 30% by poor welding quality, and 15% by other factors. Therefore, excessive heating of the heat-exchanging surfaces and poor welding quality are the main reasons for overheater tube failures. Next, we will analyze the causes of tube rupture mainly from these two aspects. 1.1 Overheater tube rupture caused by poor tube quality or poor welding quality. a. Poor tube quality. If the tubes have defects such as stratification or slag inclusion, and these defects expand due to the effects of temperature and pressure during operation, it can lead to tube rupture in the superheater. For example: In December 2000, a tube burst in the low-temperature superheater of Unit 1 at our company’s second thermal power plant during operation; the crack in the burst tube was peach-shaped, and the area at the crack had thinned significantly due to the explosion. Metallographic analysis of the cracked area revealed that the rupture of the superheater tube was caused by the poor quality of the tube material and its uneven microstructure, which was of incomplete normalized type. The base metal contained numerous microscopic pores, and these defects reduced the strength of the material, ultimately leading to the tube’s rupture. b、Poor welding quality. During manufacturing or maintenance, poor welding quality, which results in pores, slag inclusions, and weld protrusions within the welds, can lead to frequent tube failures. In 2001, the low-temperature superheater of Unit 1 in our plant experienced a leakage during operation. Macroscopic inspection of the area where the tube burst revealed that the welding quality of the welds was poor, with numerous weld beads present at the root of the welds. The leakage points were mostly located along the weld fusion line and in the heat-affected zone. Metallographic examination showed that the microstructure of the tube bundle was normal, as shown in Figure 1. However, the microstructure of the welds, fusion line, and heat-affected zone consisted of coarse Widmanstatten structure plus ferrite; this extremely coarse structure represented an abnormal condition, as shown in Figure 2. From the above analysis, it can be seen that tube bursting is caused by extremely poor welding quality of the tube welds, with defects such as coarse Widmanstatten structure and a large amount of non-metallic inclusions within the weld fusion zone. 1.2 Overheating of the heating surfaces leads to tube failure in the superheater. When metal is operated above its rated temperature, there are two types of overheating: short-term overheating and long-term overheating. As a result, tube failure due to overheating of the heating surfaces can occur as a consequence of either short-term or long-term overheating. When the heating surfaces overheat, the metal of the tubes exceeds the allowable operating temperature; this causes changes in the internal structure of the metal, reducing its allowable stress. Under the influence of internal stresses, the tubes undergo plastic deformation, which ultimately leads to tube failure due to overheating. 1.2.1 Short-term overheating of the heating surfaces: When the heat exchange conditions of the fluid inside the boiler’s heating surfaces deteriorate severely in a short period of time, the wall temperature rises sharply, which leads to a significant decrease in the strength of the steel. This can result in metal overheating and tube failure within a short time. The reasons for short-term overheating include severe uneven distribution of the steam and water flow within the pipes ； The local heat load inside the furnace is too high ； Severe scaling inside the pipe ； Foreign objects are severely blocking the tube ； Improper use of steel, etc. Figure 1: Microstructure of the pipe material. Figure 2: Microstructure of the weld area – pearlite + ferrite; coarse Widmanstatten structure + ferrite. 1.2.2 Prolonged overheating of the heated surfaces: Due to thermal imbalances, hydrodynamic effects, or issues such as scale accumulation, blockages, and incorrect use of steel materials, heat exchange within the tubes of a boiler’s heated surfaces is poor. As a result, the metal is subjected to prolonged overheating conditions. Under stress, the metal in the tubes undergoes creep (the tubes expand), until they eventually break. Long-term superheating mainly occurs on the fire-facing side of the outer ring of the high-temperature superheater, and it can also occur in the low-temperature superheater. For example, in 2001, during the major overhaul of Boiler No. 3 at our plant, the boiler inspection center conducted random inspections of the boiler’s heating surfaces. Metallographic analysis was performed on the tubes of the high-temperature superheater, and it was found that their structure consisted of ferrite plus carbides; the pearlite had been spheroidized to a degree of 3–4. The carbides were present in small spheres distributed along the grain boundaries of the ferrite, and the tensile strength had decreased significantly. It was recommended to replace those tubes of the high-temperature superheater. The main reason, as analyzed, is that the steam load across the entire plant has been low over the past few years; Boiler No. 3 has been operating at a low load for an extended period, resulting in a low steam flow velocity inside the superheater tubes. This leads to poor heat transfer, and the tube walls remain under excessive temperature conditions for long periods, thereby causing the internal structure of the superheater tubes to change due to prolonged overheating. 1.3 Causes of overheating in superheater tubes and measures to be taken. In terms of design, factors such as a low height of the boiler furnace, a flame center that is positioned too far back, poor hydrodynamic conditions, a low steam flow rate, and an unreasonable structure of the heat-exchanging surfaces can all lead to general overheating in the superheater or to localized overheating due to significant temperature differences ； Defects such as blockages caused by foreign objects inside the tubes, or the reversal of the partition plates in the screen superheater, can occur during manufacturing, installation, and maintenance; these defects lead to poor flow of the working fluid and result in overheating of the heating surfaces ； During operation, issues such as improper combustion control, flame rising, flame deviation, high flue gas temperature at the furnace outlet along with insufficient air flow, secondary combustion in the flue due to incomplete combustion, insufficient steam flow, improper operation of the desuperheater, and low utilization rate of the high-pressure heaters can also lead to overheating of the superheater tubes ； Furthermore, poor water quality leads to scaling and salt deposition inside the pipes, which affects heat transfer and can also cause the superheater tubes to overheat during operation. To prevent overheating of the superheater tubes, it is necessary to operate in strict accordance with the operating procedures during operation. When starting up or shutting down the boiler, it is essential to follow the specified start-up and shutdown curves, ensuring that the boiler parameters and the temperature of the superheater tube walls remain within acceptable limits ； Closely monitor key parameters such as boiler steam parameters, evaporation rate, and water level to prevent incidents of over-temperature, over-pressure, full water levels, and low water levels ； Properly adjust the boiler combustion to prevent flame deviation; pay attention to controlling the fineness of the coal powder, use air efficiently to avoid coking, reduce thermal deviations, and prevent reignition at the rear of the boiler ； Strengthen soot blowing and soot blower management to prevent severe ash accumulation on the heated surfaces ； Ensure normal quality of boiler feed water and qualified quality of steam and water during operation. 2 Causes of Boiler Tube Failure in Economizers and Preventive Measures There are several reasons for boiler tube failure in economizers: poor quality of feedwater, high oxygen content in the water, which leads to corrosion of the inner walls of the tubes ； Low-temperature corrosion of flue gas ； The quality of the pipes and the welding is poor ； Large variations in feedwater temperature and feedwater flow rate result in excessive thermal stress on the pipes ； Severe fly ash wear, etc. Among the above reasons, the main factors causing tube failure in the economizer are fly ash wear and welding quality. 2.1 Poor welding quality: Similar to the reasons for tube failures in superheaters, substandard welding quality during manufacturing or maintenance can also lead to frequent tube failures in economizers. In September 2000, the low-temperature economizer of Unit 3 at the thermal power plant of Wuhu Petrochemical Company experienced leaks at the same location on two separate occasions. Macroscopic inspection and metallographic analysis revealed that the first tube failure was caused by poor quality of the installation welds, with pores and inclusions present; the second tube failure occurred at the weld site where the tube section had been replaced previously, and the poor welding quality led to the second leak. 2.2 Fly ash wear Operational experience shows that, among the three main convective heating surfaces in a boiler—the superheater, the economizer, and the air preheater—the economizer suffers the most severe wear. Since the economizer is usually arranged in a staggered pattern, the flue gas exerts a strong scouring effect on the staggered tube banks, resulting in much greater wear compared to when the tubes are arranged in a straight line. Moreover, the flue gas temperature at the economizer is relatively low, and the ash particles in the flue gas are relatively hard. These two factors contribute to significantly greater wear of the economizer compared to the superheater and air preheater. Tube failures due to wear in the economizer mainly occur in the following areas: a) the second and third rows of tubes in the economizer. Since the economizer is usually arranged in a staggered pattern, the tubes in the first row of the economizer are subjected to a lower smoke velocity – namely the velocity of the smoke in the ducts before it enters the economizer. Once the smoke reaches the tubes in the second row, the cross-sectional area through which the smoke flows decreases, causing the velocity to increase suddenly; as a result, the impact force of the ash particles in the smoke is greater. Therefore, the tubes in the second and third rows suffer more wear compared to the tubes in the lower rows. b. At the elbow of the economizer tubes. The gap between the elbows of the economizer tubes and the side walls of the shaft flue forms a flue gas corridor; due to the low resistance, the flue gas flow velocity is high, resulting in severe wear. c. Serpentine tube array on the rear wall. In L-type boilers, the flue gas turns from the horizontal flue into the vertical flue; due to centrifugal force, most of the ash particles in the flue gas accumulate against the back wall of the vertical flue. As a result, the several rows of serpentine tubes located against this back wall suffer severe wear. In 2002, a leak occurred in the serpentine tubes located at the rear wall of the low-temperature economizer of Boiler No. 3 in our plant. Macroscopic inspection revealed that the crack originated from the weld area, and there were obvious signs of thinning on the outer wall of the tube. Metallographic analysis of the cracked area showed that the metallographic structure was normal. From the above analysis, it can be seen that the tube of this economizer burst due to a significant reduction in its strength, resulting from the erosion of its outer wall by flue gas. 2.3 Causes of economizer wear and preventive measures The causes of wear include: high ash content in coal, with hard particles present in the fly ash ； The flue gas flow rate is too high, or the local flue gas velocity is too high ； The ash concentration in the flue gas is unevenly distributed, with excessively high ash concentrations in certain areas. Fly ash wear is typically prevented by reducing the number of times fly ash impacts the tubes, lowering the flue gas flow rate, or increasing the wear resistance of the tubes. For example: ① Control the flue gas flow velocity, especially that in the flue gas ducts; therefore, during installation and maintenance, the distance between the economizer tubes and the wall should be minimized, while ensuring that the distances between the various coiled tubes are as equal as possible ； ②In areas where the local smoke flow velocity is too high, anti-wear devices should be installed at the parts of the tubes that are prone to wear, such as elbows, the second and third rows of tubes, and the rows of tubes near the back wall ； ③Cold spraying of a high-temperature wear-resistant coating on the economizer tubes is used to enhance the wear resistance of these tubes. 3 Causes of Water Wall Tube Failure and Preventive Measures 3.1 Water wall tube failure is mainly caused by the following factors: a. Local blockages in the tubes due to debris, which lead to poor water circulation, resulting in overheating of the tube walls and the formation of bulges or cracks. This type of tube failure is a short-term overheating-induced tube failure. b. Unqualified water quality, resulting in scaling inside the pipes. c. Improper operation of the furnace during ignition and voltage increase. d. When the boiler is severely underwater, the sudden injection of a large amount of water creates excessive stress that damages the pipes. e. The pipe material is substandard or the welding quality is poor. 3.2 Measures to prevent tube rupture in water cooling walls: Ensure that the quality of feedwater and boiler water is satisfactory, and reduce scaling and corrosion inside the water cooling wall tubes ； Adjust the combustion so that the flame is even and not skewed, thereby preventing uneven heating of the water wall tubes and avoiding disruption in the water circulation ； Prevent external wear of water wall tubes ； When increasing temperature and pressure, follow the procedures strictly and control the rate of pressure increase. To effectively prevent three-tube explosion, in addition to taking the above measures during installation, maintenance, and operational adjustments, it is also necessary to carry out metal surveillance on the boiler’s heating surfaces, which mainly includes the following aspects: ① Regular inspection of the heating surfaces for conditions such as creep, deformation, and wear ； ②For heating surfaces that have long suffered from overheating problems, thermal temperature sensors are installed for monitoring and control ； ③Regular tube inspection should be carried out, including metallographic testing of the tubes in the high-temperature and low-temperature superheaters, as well as inspection of scaling and corrosion on the inner surfaces of the water wall tubes in the areas with the highest heat load in the furnace. By identifying the root cause of tube explosions and implementing effective preventive measures, it is possible to resolve the issue of boiler tube failures once and for all, and to effectively prevent such accidents from occurring