Twelve causes of industrial boiler accidents and their preventive measures – very comprehensive and detailed

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This post was last edited by yinkuilin6868 on November 16, 2015, at 22:37. It covers twelve causes of industrial boiler accidents and their preventive measures; the information is very comprehensive and detailed. Boilers operate under adverse conditions of high temperature and pressure; improper operation or equipment defects can lead to overpressure or overheating, resulting in explosions or detonations. Boilers have many components and are large in size, featuring complex systems for steam, water, air, and smoke. If not properly maintained, issues can arise with combustion, auxiliary equipment, as well as pipes and valves, which may lead to forced shutdowns. 　　Boiler explosion accidents are often catastrophic incidents that cause damage to equipment and buildings, as well as casualties. If the boiler unit stops operating, resulting in a sudden interruption of steam power, it will lead to severe consequences such as production and operational shutdowns. The occurrence of these accidents causes tremendous losses to the national economy and the safety of people’s lives. Therefore, preventing boiler accidents is of great significance. I. Classification of accidents: Boiler accidents can be classified according to their severity into: boiler explosion accidents, major accidents, and general accidents. 　　A boiler explosion accident is an incident that occurs during boiler operation, in which components such as the boiler drum and header are damaged, and a large pressure release opening appears, causing the operating pressure to drop to atmospheric pressure in an instant. Such accidents have a great explosive power and cause significant damage. 　　Major accidents refer to various incidents that occur during operation, such as explosions, tube ruptures, severe deformation, furnace collapse, furnace wall failure, and overheating of the steel framework, which force the shutdown of the furnace for major repairs. 　　General accidents refer to incidents in which a malfunction occurs during operation, forcing the boiler to be shut down; however, it can be quickly restored to normal operation. 　　If boiler accidents are classified by the location where they occur, there are explosion accidents resulting from the sudden cracking of pressure-bearing components with large water capacities such as the boiler drum, tube burst accidents, economizer accidents, superheater accidents, and accidents involving pipes, flues, and furnace walls ； Accidents occurring in safety accessories, water supply equipment, combustion equipment, etc. 　　In terms of the causes of boiler accidents, there are water-level monitoring errors leading to low-water or overfill accidents; accidents caused by poor water quality management; accidents resulting from defective design, manufacturing, installation, or inspection; accidents due to improper maintenance, which in turn cause corrosion and the accumulation of scale and soot; and accidents stemming from poor combustion control. II. Accident Prevention 1. It is necessary to establish and improve various relevant rules and regulations such as boiler operation procedures, safety operation protocols, post responsibility systems, inspection quality standards, and shift handover procedures, and to enforce them strictly. 　　2. The treatment of water used in boilers should be improved; the quality of feedwater must meet specified requirements, softened water should conform to quality standards, and the alkalinity of boiler water should not be too high. There must be a system in place for waste discharge; the interior of the heat-exchanging surfaces should remain free from scale or only have a thin layer of it. Scale should be removed regularly by mechanical or chemical means to prevent the steel plates or tubes from overheating. 　　3. Materials that meet the requirements specified in the drawings should be used during installation and inspection. 　　4. Adopt a fair boiler structure. During manufacturing, installation, inspection, and technical modifications of boilers, attention should be paid to improving their unreasonable structures so as to achieve rational or basically rational conditions. 　　5. Systematically organize training for boiler operators and maintenance staff to improve their skills in safe operation and management. Based on a thorough understanding of the equipment’s performance, the boiler operators ensure safe and efficient operation, thereby preventing accidents. The boiler operators must remain at their posts, and in the event of an accident, they should take calm and swift action to address it. III. Common boiler accidents In recent years, boiler explosion accidents have occurred from time to time; water shortage accidents are the most common and pose significant hazards. Another issue is the overheating and damage of heating surfaces such as furnace tubes caused by inadequate water quality treatment. When describing common boiler accidents, apart from boiler explosions and accidents caused by lack of water, excess water, or steam-water mixing, the other accidents are described separately based on the location where they occur. (1) Boiler explosion accident: A boiler explosion occurs when the boiler drum (either a steam-water drum or a water drum) ruptures, resulting in the instantaneous release of enormous amounts of energy as several tons, or even dozens of tons, of pressurized saturated water and steam stored within the drum are released. 　　The disasters caused by a boiler explosion mainly stem from two aspects: one is the energy released due to the expansion of water and steam inside the boiler drum ； The second is the disaster caused by the high-pressure steam and some of the saturated water in the pot rapidly evaporating, thereby generating a large amount of steam that spreads throughout the area. 1. Energy released during boiler explosion: When a boiler explodes, the drum suddenly ruptures, causing the pressure inside the boiler to drop rapidly from its operating pressure (the pressure prior to the explosion). The process of steam expansion occurs instantaneously; therefore, it can be regarded as an adiabatic process. In this way, the energy released by the steam can be calculated as the work of perfect adiabatic expansion. 　　Inside the boiler, in addition to steam, there is also a large amount of saturated water, whose temperature corresponds to that of saturated water at the operating pressure of the boiler. This temperature is much higher than the boiling point of water at atmospheric pressure. When the boiler drum ruptures and the pressure inside the boiler drops suddenly to atmospheric pressure, the saturated water releases heat rapidly, and part of it evaporates into steam, which then continues to expand and do work – this is what is known as a \"steam explosion\". 　　The energy released when a boiler explodes is used, aside from a small portion that goes into displacing the broken pieces or the entire boiler from its original location (often only about 1/10 of the energy required for the explosion is needed to throw the boiler more than a hundred meters away), for generating shock waves that travel through the air and damage the buildings in the surrounding area. When a boiler explodes, the tearing of components such as the boiler drum also consumes some energy, but this amount is very small and can be ignored. 2 The volume of steam generated upon a boiler explosion. When a boiler explodes, the pressure inside the boiler drum drops, causing the high-pressure steam already present to expand into steam at atmospheric pressure, resulting in a rapid increase in volume. At the same time, due to the drop in pressure, the saturation temperature of the water in the register drops from that at operating pressure to the saturation temperature at one atmosphere, releasing a large amount of heat and causing part of the saturated water to evaporate into steam. In this way, when the boiler explodes, a large amount of steam is generated instantly, and employees in the area affected by it will be scalded. 3 Main causes of boiler explosions: (1) When the boiler has been deprived of water for an extended period, causing the steel plates to become red-hot and their mechanical strength to drop drastically, boiler operators violate operating procedures by introducing water into the furnace, thereby triggering an explosion ； 　　(2) In riveted boilers, long-term leakage from the boiler shell or drum, combined with a high alkalinity of the boiler water, can cause caustic embrittlement of the steel plates at the rivet seams or flanges, leading to explosion accidents ； 　　(3) Severe overpressure causes explosion ； 　　(4) Boiler explosions occur due to reasons such as the failure of safety accessories, inadequate structural design, or deterioration of the material. 4 Preventive Measures (1) Special attention should be paid to the materials and strength of the main pressure-bearing components of boilers with large water capacities, such as the boiler shell, head, tube sheet, and furnace chamber. It is also necessary to ensure that the connection methods, welding, and cold working processes used in their design and manufacture comply with relevant regulations and standards. Due to factors such as the relatively large diameter of the drum and the numerous pressurized components inside it, which make the connection types rather complex, extra attention must be paid to this issue in fire-tube boilers. 　　(2) When inspecting and repairing boilers, great caution must be exercised regarding caustic embrittlement, severe corrosion and deformation, as well as grooved cracks in the boiler drum. Inspections need to be thorough and meticulous, and repairs must ensure high quality to prevent sudden tearing due to insufficient strength or crack propagation. 　　(3) The boiler operators must keep in mind that in the event of a severe water shortage, no more water should be added, to prevent the boiler steel plates from suddenly contracting and cracking due to cooling when they are overheated and red-hot. 　　(4) The safety accessories of the boiler, especially the safety valves, must always be sensitive, accurate, and reliable. Most small boiler explosion accidents share one important common cause: the absence of safety valves or the failure of these valves, which leads to overpressure. If the safety valve functions properly and the system operates at a relatively low pressure, explosion accidents can be completely avoided. 　　(5) Attention should be paid to easily overlooked weak points. There are many explosion accidents involving boilers used for cooking, bathing, heating, and warming food; such incidents also frequently occur with hot water boilers and tea boilers. These boilers are small in size and operate at low pressures; they are often located in residential areas, which means they tend to go unnoticed and underappreciated. As a result, they can easily become weak points and vulnerabilities in terms of boiler safety management, so special attention should be paid to them. (II) Water shortage accidents Boiler water shortage accidents are the most common type of accident that occurs in boilers. The harm caused by severe water shortage incidents is often considerable. In mild cases, it causes overheating and deformation of large areas of the heated surfaces, leakage at the joints, collapse and damage to the top walls and partition walls of the furnace, and excessive temperature of the superheated steam, which in turn damages the steam engine ； In severe cases, this can cause the tube to burst, the tube to come loose, and a large amount of steam and flames to be ejected, thereby injuring people ； The most serious consequence is that improper handling may lead to an explosion. Severe water shortage incidents often cause extensive damage to boilers; those that are severely overheated and deformed are very difficult to repair ； In cases where the overheating-induced deformation is mild, it often takes a long time to restore the equipment to normal operation, which frequently results in shutdowns of the facilities using it. 1. Phenomena of water shortage accidents (1) A white appearance on the glass plate (tube) of the water level gauge ； Alternatively, place a pencil-shaped object or a slanted line board behind the water level gauge and observe through it; if no broken lines are visible but only continuous rod-shaped shapes or slanted lines, it indicates that there is no water left inside the water level gauge ； 　　(2) The water level gauge remained stationary with no slight fluctuations; the boiler operator failed to detect this false water level condition in a timely manner ； 　　(3) The high and low level alarms and other low water level warning devices issue low water level alarms or signals. (4) The steam flow rate is greater than the water supply rate ； 　　(5) Sudden rise in superheater steam temperature ； 　　(6) A burnt smell was detected in the boiler room ； 　　(7) Collapse of the furnace top wall ； 　　(8) Overheating and deformation of heating surfaces such as boiler drums, furnaces, and furnace tubes ； 　　(9) When filling water, abnormal impacts are heard from the economizer, or water suddenly starts leaking from the flues surrounding the economizer ； 　　(10) White water vapor smoke coming from the chimney ； 　　(11) Burst pipes and expansion joint pipe detachment are detected. 　　When a water shortage occurs, not all of the above phenomena appear; generally, only the first three occur, while the latter ones do not. This may indicate a slight water shortage, but the possibility of a severe water shortage cannot be ruled out ； If the first three phenomena occur simultaneously along with the latter ones, it is generally considered to be a severe water shortage incident. 2. Determination and handling of water shortage incidents There are two types of water shortage incidents: one is a mild water shortage, where although the water level cannot be seen on the gauge, the water level inside the boiler drum has not dropped below the water connection pipe; in this case, what appears on the gauge is an artificial water level. This can be achieved by using a steam stopcock on the closed level gauge, which causes the steam inside the gauge to condense and create a vacuum pressure that draws the water, which has not yet dropped below the water connection pipe, into the level gauge. This method is commonly known as “calling for water”. If the water level is still not visible even after operating the water control, it indicates that the water level is at least below the water connection pipe, and it is likely to be even worse; in such a case, a severe water shortage has occurred. 　　If it is confirmed to be a minor water shortage incident, since the heating surfaces have not yet been subjected to “dry firing”, water can definitely be added to restore the normal water level. If the cause is unknown, if the water level does not rise despite water supply, or if there is a malfunction in the water supply equipment, the boiler must be shut down immediately. If severe water shortage is detected, the boiler should be shut down immediately on an emergency basis, the load should be reduced, and the feedwater valves should be closed. 　　The most important issue in dealing with water shortage incidents is to strictly prohibit adding water to the boiler before it can be determined that there is only a slight shortage, and after it has been confirmed that there is a severe shortage. 　　After the furnace is shut down due to severe water shortage, wait for the furnace to cool down gradually before conducting a thorough inspection of the furnace chamber and other heat-exchanging surfaces, as well as the furnace walls and steel framework. If, thanks to prompt action, the water shortage was not too severe and there are no major problems (such as only slight deformation of the tubes), it is necessary to identify and eliminate the causes of the incident, and then put the furnace back into use after a successful hydraulic test ； If the overheating is severe, resulting in leakage at expanded joints, serious deformation of the tubes, or severe overheating and damage to the steel material (and metallographic examination may be conducted if necessary), the tubes must pass inspection before they can be used. 3 Causes and Prevention of Water Shortage Incidents (1) Lack of monitoring of water level or failure by operating staff to pay attention to it. 　　(2) The water level gauge was not flushed in a timely manner as required, the steam and water connection pipes became blocked, and the operating staff failed to detect the false water level in time or failed to identify it as such. 　　(3) Both the water supply automatic regulator and the water level alarm device have failed ； Or a disruption in the water supply, or damage to the water supply equipment. 　　(4) Severe leakage from the drain valve and serious water leakage in other areas. 　　(5) Incorrect operation during discharge: Excessively long discharge time ； The operating staff failed to monitor the water level properly ； After draining, the drain valve was forgotten to be closed. 4. For the above reasons, the key points in preventing water shortage incidents are: (1) Strengthen the training of operating personnel, enhance their sense of responsibility, and improve their technical capabilities in handling accidents ； 　　(2) The flushing of the water level gauge and blowdown operations must be carried out strictly in accordance with the post responsibility system and operating procedures ； 　　(3) A dedicated person must be assigned to inspect, calibrate, adjust, and maintain the automatic water supply system as well as the water level alarm and signaling devices on a shift basis ； To prevent excessive and total reliance on automated devices, maintenance and management must be strengthened during operation ； 　　(4) Operating staff should get enough rest; generally, they should not work long shifts of more than eight hours ； 　　(5) The water level gauge must be installed in the correct position, and the steam and water pipes must not be tilted, so as to accurately reflect the water level inside the furnace. During operation, the water level gauge should be maintained and cared for properly to prevent blockages and false readings. (III) Full-water accident A full-water accident is also a common type of accident that occurs during boiler operation. Severe full-water accidents can cause water hammer in the steam pipes, leading to damage or even cracking of valves, flanges, and steam pipes. This can severely damage the turbine blades and bearings, and may even result in the breakage of those blades ； After a full-water accident in the boiler, steam contains a high amount of water, resulting in poor steam quality. The superheater is prone to the accumulation of scale, which can cause overheating and damage; this may have serious effects on the equipment using the steam as well as on the quality of the products. 1. Phenomena of a full-water accident (1) The glass plate (tube) of the water level gauge turns dark, and the water level line disappears ； 　　(2) The high/low alarm or other high water level alarm device emits a high water level signal ； 　　(3) The water supply flow rate is significantly greater than the steam flow rate ； 　　(4) Superheater temperature decreases ； 　　(5) There are abnormal impacts and vibrations in the steam pipes and turbines, and steam and water droplets are leaking from flanges, shaft seals, valves, etc. 2 Handling of full-water accidents (1) First, it is necessary to confirm whether a full-water accident has occurred by checking the water level and comparing it with the readings from various water-level indicating devices; if there is no water hammer in the steam pipes, it is considered a normal full-water accident ； Conversely, it can be determined to be a severe flooding accident. 　　(2) In the event of a normal water-overflow accident, supply of water must be stopped immediately, combustion must be reduced, and the drain valve must be opened to release water ； Simultaneously open the drain valves on the superheater and steam pipes, as well as the drain valves on the steam-using sections, to enhance drainage. Operation can be resumed only after the water level returns to normal, and the cause of the full water level has been identified and eliminated. 　　(3) In the event of a severe water-overflow accident, the boiler should be shut down immediately, feed water should be stopped, water should be drained quickly, the load should be reduced, and drainage systems should be activated more intensively. Only after the water level returns to normal and relevant components such as pipeline valves have been inspected and found to be in good condition, and after the cause of the water accumulation has been identified and the associated risks eliminated, can operations be resumed. 3 Causes and Prevention of Full Water Accidents The main cause of full water accidents is inadequate monitoring of water levels by the operating staff ； Next is false water level caused by a clogged water gauge ； Furthermore, there are issues with the high water level alarm signal device and the failure of the automatic water supply control equipment. 　　The preventive measures are the same as those for water shortage incidents. (IV) Foaming accident 1. Phenomena of foaming accident: Foaming refers to a condition in which the surface of the boiler water is covered with significant amounts of foam. When the load increases, combustion intensifies, and steam-water separation becomes more severe, the foam layer on the surface of the boiler water begins to churn and fluctuate up and down; many bubbles and foam appear in the water level gauge, resulting in an unclear reading of the water level. When foaming occurs, similar to a water-overflow accident, the amount of water carried by the steam increases sharply; water hammer may occur in the steam pipes, and the temperature of the superheated steam drops. Boiler water with a high salt concentration in the steam will severely affect the safe operation of the superheater and turbine. 2 Handling of simultaneous steam and water flooding accidents (1) Reduce combustion, lower the load, and close the main steam valve ； 　　(2) Strengthen the drainage of steam pipes and superheaters ； 　　(3) Fully open the continuous sewage discharge valve ； Open the drain valve to discharge water, and at the same time supply water in order to reduce the salt content in the boiler water and improve its quality. Pay attention to water level changes when draining or filling water ； 　　(4) Once the water quality improves and the water level becomes clear, normal operation can be gradually resumed. 3 Causes and Prevention of Steam-Water Boiling Over Accidents (1) Under normal conditions, due to steam-water separation, the salt concentration in the water layer 100~200 mm below the boiler water evaporation surface is relatively high. When the feedwater has high alkalinity, many impurities, and insufficient drainage, the salt content in the surface layer of the boiler water is often very high. More foam forms on the evaporation surface, the viscosity of the boiler water increases, and the resistance to the rise of steam bubbles rises as well. When the load increases and vaporization intensifies, a large number of bubbles accumulate on the surface of the boiler water due to slow separation of vapor from the water, striking the evaporation surface and disturbing the foam layer, which causes severe fluctuations and turbulence in the water level. 　　(2) When the water level is too high, the main steam valve opens too quickly, or the load increases suddenly, a sudden drop in the pressure in the steam space leads to more intense vaporization. The temporary negative pressure in the steam space often results in the \"water hanging\" phenomenon, which promotes and intensifies foaming. 　　Preventive measures include strengthening water quality monitoring, strictly controlling the salt content in boiler water, and properly carrying out waste discharge ； When the boiler water has a high salt content and many impurities, and a foam layer begins to form without any improvement, it is necessary to reduce the load, weaken the combustion, and slowly open the main steam valve. (5) Furnace tube explosion accidents Furnace tube explosion accidents mainly refer to the explosions of water wall tubes and boiling tube banks, with explosions of water wall tubes subjected to high heating intensities being particularly common. Tube explosion accidents are serious incidents that occur during boiler operation. The massive release of steam and water creates positive pressure in the furnace, causing steam along with fire to burst out suddenly through the furnace doors and other openings; this often results in injuries. If not dealt with promptly, such accidents can also lead to water shortage issues. After a tube explosion, the boiler has to be shut down for inspection, which disrupts normal production processes and has serious consequences. Phenomena and handling of furnace tube explosion accidents: When the explosion of the furnace tube is minor, and the steam pressure is high as well as the combustion is intense, there is an abnormal sound of steam spraying inside the furnace; the flame color in the furnace becomes dimmer, the area of the rupture becomes more apparent, combustion deteriorates, the furnace temperature drops, and white smoke resembling water vapor emerges from the chimney. In more severe cases, the water level begins to show obvious abnormalities. If the steam pressure is low and combustion is weak, the fire bed in the furnace at the location of the tube rupture turns black, and it is possible to clearly see steam and water spraying from the furnace tubes. In such a situation, if the normal water level can be maintained, the combustion should be reduced immediately, and the units that use steam should be informed to prepare for shutdown inspections. 　　When the furnace tubes rupture severely, large amounts of steam and water, along with flames and sparks, are ejected out through the openings in the furnace wall, causing severe abnormalities in the water level and combustion. In the event of a severe tube rupture, the furnace must be shut down urgently. 2 Causes of tube rupture and prevention (1) Excessive scaling on the tubes leads to overheating and burnout, resulting in rupture. This is most common in the sections of the water wall tubes with high heat intensity. 　　(2) During operation, the boiler contains some larger flaky or lumpy deposits. As the water circulates and enters the boiler tubes, these deposits tend to get trapped in the sections where the tube diameter is smaller and where there is greater resistance to flow, thereby reducing the pipe diameter. Moreover, sediment and other impurities in the circulating water are likely to be blocked by these deposits, which accumulate over time until the pipes become completely blocked, leading to overheating, damage, and even explosion. The larger flaky and lumpy precipitates are mainly the existing old scale in the furnace, or the scale that has fallen off during operation ； Or, after tannin treatment, alkali cooking, and acid cleaning to remove scale, although the hard scale has become loose, it is not completely removed, and large pieces of it often fall off during operation ； Then there are the remaining tools, cotton yarn, and so on. 　　(3) Severe corrosion and wear thinning of the pipes reduce their pressure-bearing capacity, leading to pipe rupture. 　　(4) Tube overheating and bursting caused by severe water shortage. 　　(5) If the normal water circulation is disrupted due to reasons such as water shortage, improper waste discharge, furnace coking, or incorrect operation of the burners, the pipes affected by this water circulation failure may overheat and burn out, leading to pipe rupture. 　　(6) Improper design, installation, and operation lead to the pipes operating under conditions of uneven thermal expansion, severe temperature fluctuations, or an inability to expand freely over time, which results in cracks at the welds and circumferential cracks in the expanded sections of the pipes, ultimately causing failure. 　　For the above reasons, to prevent furnace tube explosion accidents, emphasis should be placed on improving water quality control to avoid scaling. The scale must be removed completely, with special attention paid to prevent blockages in the pipes caused by the falling of flaky scale. During operation, care must be taken to prevent thermal imbalances in the furnace and to ensure a reliable water circulation. During the technical inspection of boilers, attention should be paid to checking for corrosion of the furnace tubes, wear and thinning, as well as potential cracks. (VI) Economizer damage accidents Damage to the economizer is mainly caused by pipe ruptures and cracks, as well as leaks resulting from damaged flange joints. 1. Phenomena of economizer damage accident (1) The water level drops abnormally, the water supply volume increases significantly and exceeds the steam volume, and the water pressure at the economizer inlet decreases ； 　　(2) The flue gas temperature decreases, and the water temperature at the economizer outlet increases ； 　　(3) Abnormal noises around the economizer ； 　　(4) Steam emission, moisture, or even water leakage at the ash hopper below the economizer and at the furnace wall. 2. Handling of economizer damage incidents (1) For boiling economizers 1) Increase the boiler feed water to maintain a normal water level ； 　　2) Reduce combustion, quickly lower the load, communicate with the steam-using sections, and prepare for shutdown inspections ； 　　3) Close all drain valves of the boiler; it is prohibited to open the recirculation valve between the boiler drum and the economizer ； 　　4) Pay attention to the increase in the flue gas temperature at the inlet of the induced draft fan and the steam temperature at the outlet of the superheater, as well as their control ； 　　5) If the water level in the boiler drum cannot be maintained, the boiler should be shut down urgently. 　(2) For non-boiling economizers: 1) Open the economizer bypass flue, and close the inlet and outlet dampers of the economizer flue ； 　　2) Open the feedwater bypass valve that allows water to enter the boiler drum directly without passing through the economizer, and close the inlet and outlet valves of the economizer ； 　　3) Once the smoke and water have been completely separated, water should be released immediately, and the air valve should be opened or the safety valve lifted ； 　　4) If the flue dampers are airtight, inspections and resumption of operation should be carried out under strict conditions to ensure personal safety; otherwise, the furnace should be shut down for inspection as soon as possible. Causes and Prevention of Economizer Damage in 3 Provinces The main causes of economizer damage are: (1) Failure to deoxidize the feedwater, which leads to severe oxygen corrosion on the inner walls of the economizer tubes. This is a very common problem with steel tube economizers ； 　　(2) Fly ash wear and low-temperature acid erosion on the outer wall of the economizer tubes ； 　　(3) The intense vibrations caused by water hammer accidents and flue explosion accidents often severely damage the economizer, or even cause it to crack ； 　　(4) Overpressure and overheating caused by incomplete or malfunctioning safety accessories of the economizer ； 　　(5) Cracks and leaks caused by quality issues in aspects such as the welding of economizer tubes, castings, and connection installation. 　　In light of the above, the key to preventing damage to the economizer is to ensure the quality of its manufacturing and installation ； The safety accessories at the inlet and outlet of the economizer must be complete, sensitive, and reliable ； Oxygen removal from feedwater is mandatory for steel tube economizers ； During operation, it is necessary to prevent water hammer accidents in the economizer as well as explosions in the flue ； When inspecting the boiler, it is necessary to check for corrosion and wear on the outer walls of the economizer tubes. (7) Overheating and tube rupture accidents The main damage to the superheater is tube rupture. Phenomena and handling of superheater tube rupture accidents (1) Abnormal noises around the superheater ； 　　(2) The negative pressure in the furnace suddenly decreases; in some cases, positive pressure causes steam to be ejected and smoke to appear ； 　　(3) The steam flow rate decreases significantly; it is abnormally lower than the feedwater flow rate ； 　　(4) The flue gas temperature drops significantly. 　　After a tube explosion occurs in the superheater, the boiler should be shut down promptly for repair, to prevent the high-temperature steam escaping from damaging adjacent tubes and causing the accident to escalate. If analyzed from the perspective of the accident’s symptoms, when the situation is not very serious and is unlikely to worsen or spread rapidly, it is possible to delay shutting down the furnace according to load requirements, but the duration should not be too long. 2 Causes and prevention of superheater tube rupture accidents
The main causes of superheater tube rupture accidents are:
(1) Poor quality of boiler water, excessive moisture carried by steam, or water overflow incidents can lead to salt deposition on the superheater, resulting in thermal damage ； 　　(2) The superheated steam temperature is too high, causing damage to the superheater ； 　　(3) During shutdown, the superheater is prone to water accumulation due to the carelessness of the operators, which can lead to thinning of the superheater tube walls ； 　　(4) The steam nozzle of the soot blower is aimed directly at the superheater tubes, causing rapid damage to those tubes ； 　　(5) The superheater tubes are not made of heat-resistant steel, and the welding quality during assembly is poor (mostly involving heat-resistant alloy steels and all-position welding, which have high welding requirements). 　　(6) As preventive measures, boilers equipped with superheaters should have adequate steam and water separation devices; the quality of steam must be controlled, and high water levels should be avoided as much as possible during operation to prevent steam-water slugging and flooding accidents ； Pay attention to controlling and adjusting excessively high superheated steam temperatures caused by various factors ； It is necessary to ensure the quality of manufacturing and installation of the superheater. (8) Gauge failure – The rupture or damage of the glass tube (plate) of the water level gauge is also a common accident during boiler operation. Although an incident involving damage to a water level gauge cannot be considered a serious accident, it often affects the normal operation of the boiler. Moreover, such incidents frequently result in injuries during their occurrence and handling. Commonly seen are incidents of glass tube rupture on small boilers. 1 Causes and prevention of water level gauge damage accidents (1) Poor quality of glass tubes (plates) and mica ； 　　(2) Improper installation of the glass tube, such as deviation in the vertical alignment, over-tightening of the tightening nuts, or cracks at both ends of the glass tube during cutting ； 　　(3) When the water level gauge was used for the first time, it wasn’t properly preheated ； Or when flushing the water level gauge, the steam-water valve is operated too abruptly, causing a sudden change in temperature ； 　　(4) During operation, cold air or cold water points come into direct contact with the level gauge, causing a sudden change in temperature. 　　To prevent damage to the water level gauge, it is essential to ensure the quality of its glass tube (plate) and mica. It is advisable to conduct temperature change tests prior to installation ； Secondly, pay attention to the installation quality to avoid any risks of pipe bursts ； Additionally, during operation, sudden temperature changes should be avoided as much as possible, and the flushing procedure must be carried out with caution. 2 Handling of gauge damage incidents When a gauge damage incident occurs and a large amount of steam and water is ejected, the operating staff should not panic. Instead, they must quickly close the steam and water valves while wearing protective gear to avoid burns. When replacing a water level gauge, it should be thoroughly preheated before opening the steam and water cocks. 　　In cases where both water level gauges are damaged simultaneously, provided that the water level alarm signaling device and feedwater equipment are functioning normally, it is not necessary to shut down the boiler; however, prompt reinstallation should be carried out. (9) Water hammer accidents – A boiler water hammer accident is a phenomenon in which there is a violent impact of water flow occurring in the boiler drum, steam and water pipes, and economizers. During water hammer, loud noises and vibrations often occur. Severe water hammer can damage components; it may cause valves and flanges to leak or crack, and even lead to pipe rupture. 　　One type of \"water hammer\" is caused by the formation of a local vacuum due to the condensation of steam, which leads to water streams colliding with each other under sudden pressure differences. This type of water hammer occurs most often in steam pipes, economizers, and boiler drums equipped with steam pressurization devices. Another type of \"water hammer\" is caused by the large inertial force of the water flow hitting the pipe components after the high-speed flow of water is suddenly stopped; it generally occurs in water supply piping systems. The following are described separately. 1 Causes and handling of water hammer accidents in steam pipelines: This is the most common type of water hammer accident. The main reason is that, during overheating of the steam pipes, insufficient water drainage, or in cases of steam-water slugging and water filling accidents, a large amount of water accumulates inside the steam pipes. 　　After a water hammer incident occurs in the steam pipeline, the main steam valve should be closed to reduce steam supply, and drainage should be increased immediately to mitigate or eliminate the water hammer effect ； At the same time, it is necessary to specifically check whether the supports for the pipeline components have been damaged by the shock. 2 Causes and Handling of Water Hammer Incidents in Economizers. There are two reasons for water hammer incidents in economizers: one is caused by the sudden cold contraction of steam when superheated steam from a non-boiling economizer comes into contact with feedwater at very low temperatures. Another possibility is that the check valve in the feed water pipeline at the economizer inlet is not functioning properly, opening and closing intermittently, which causes inertial shock from the high-speed flowing feed water. After the occurrence of the former type of accident, the bypass flue should be opened immediately; once the outlet temperature of the economizer returns to normal, and there are no leaks or other abnormalities, normal operation can be resumed ； In the latter type of accident, it is necessary to check the operation of the check valve in the water supply pipeline; if it has failed, it should be replaced. 3 Causes and Solutions for Water Hammer Incidents in the Drum There are two types of water hammer incidents in the drum: one occurs in boilers without economizers, where a large amount of low-temperature feedwater enters when the water level in the drum is below that of the feedwater inlet pipe, resulting in the condensation of steam in the steam space ； Another scenario is caused by too fast steam inlet and heating rates when the drum is heated by steam. Both of these water hammer phenomena can be exacerbated by loose supports or weak connections for the water inlet pipes and air inlet pipes inside the boiler. 　　The corrective action is to immediately reduce the water inflow and steam supply; once the water hammer has subsided, the flow rates can be increased appropriately. During the shutdown inspection, ensure that the brackets for the water inlet and steam supply pipes are properly tightened. 　　The causes and handling of water hammer accidents in the feedwater pipes are the same as those in the second scenario of water hammer accidents in the economizer. Furthermore, when the feedwater temperature changes drastically, water hammer may also be caused by the sudden thermal expansion and contraction of the feedwater. After a water hammer occurs in the water supply pipeline, in addition to a loud noise caused by the strong flow of water, the pointer of the pressure gauge at the outlet often swings sharply and significantly. Depending on the circumstances of the accident, measures such as suspending water supply, replacing check valves, and adjusting the water supply temperature should be taken. (10) Furnace explosion accident: A furnace explosion is an accident in which fire or gas suddenly bursts out from within the furnace. Flaming occurs when the combustible mixture inside the furnace ignites spontaneously or catches fire suddenly when exposed to heat, outside the lower ignition limit; as a result, large amounts of smoke cannot be expelled in time, creating positive pressure in the furnace that causes flames to be ejected outward. A gas explosion occurs when the concentration of the mixture of combustible materials and air in the furnace is within the explosive limit, and this mixture is ignited by an open flame. Flames erupting from the furnace can easily injure people, and gas explosions within the furnace can cause cracks and collapses in the furnace walls and flues; in severe cases, the boiler itself may be damaged, posing a serious threat to human safety. Furnace explosion accidents mainly occur in kerosene furnaces and pulverized coal furnaces. Causes of furnace explosion accidents: (1) Combustible materials remaining in the furnace before ignition. There are three scenarios in this case: First, turn on the burner before ignition to introduce combustible materials into the furnace ； Secondly, when ignition failed last time, there was a large amount of combustible material inside the furnace ； Third is the residual combustible gas in the furnace. 　　When there is oil mist or other flammable gases in the furnace, igniting it without first ventilating to remove them can very easily lead to a flame explosion. 　　(2) During boiler operation, if the furnace fire goes out suddenly for some reason and the fuel supply is not cut off in time, the combustible materials present in the furnace, under the high temperatures there, may catch fire on their own, resulting in a burst of flames or an explosion. Handling of furnace explosion accidents After a furnace explosion, the oil supply must be cut off immediately, air supply and exhaust must be stopped, and the dampers in the flue and air ducts must be closed. If flames are still seen in the flue, they should be extinguished immediately. Once it is confirmed that there are no flames in the flue, especially in the economizer and air preheater areas, the exhaust fan can be started, and it is necessary to check thoroughly whether all components of the boiler, the furnace walls, the flue, as well as various holes and doors, are in good condition. Once everything is normal, the flue must be ventilated for 5 to 10 minutes before re-ignition. Prevention of furnace explosion accidents (1) Before ignition, fuel-fired and pulverized coal boilers must be checked for any oil leaks in the burners inside the furnace; such issues should be addressed immediately if they are detected. At the same time, allow air to flow in for 5 to 10 minutes to remove any combustible gases present in the furnace. If ignition does not occur, stop attempting to ignite it immediately without delay. After ensuring adequate ventilation, light the fire following the correct steps. 　　(2) When starting the ignition, air should be turned on first, followed by igniting the flame, and then fuel should be supplied. It is strictly prohibited to perform the incorrect procedure of supplying fuel to the burner first, then opening the air valve, and finally igniting the flame. 　　(3) During normal shutdown, the fuel supply should be stopped first, and it is necessary to ensure there are no leaks before turning off the blast air and exhaust air. 　　(4) During operation, in the event of a sudden fire suppression or an unexpected shutdown due to an accident, the fuel supply must be stopped first. It is best to be equipped with automatic fire suppression systems. (11) Re-ignition accident in the flue tail: Re-ignition in the boiler’s flue tail occurs when some of the combustible materials that have not been completely burned in the furnace adhere to the heated surfaces at the tail area, and under certain conditions, they reignite and burn again within the flue tail. Also known as secondary combustion. During tail re-ignition, the air preheater, induced draft fan, and even the economizer can often be damaged. It is a relatively serious accident that is common in fuel-fired boilers and pulverized coal boilers. 1. Phenomena of post-combustion accidents at the flue tail (1) The exhaust gas temperature rises significantly, and the temperature of the hot air also increases ； 　　(2) There are significant changes in the negative pressure in the flue; sometimes the explosion-proof door at the rear operates ； 　　(3) Sparks or smoke escaping from the flue door openings or the shaft seals of the exhaust fans, etc ； 　　(4) The chimney emits black smoke or sparks. 2 Handling of post-flue tail re-ignition accidents (1) Immediately stop fuel supply and perform an emergency shutdown of the furnace. Seal the smoke and air dampers as well as all door openings tightly to prevent air leakage and thus cut off the source of oxygen. It is strictly prohibited to start the induced draft fan; otherwise, it will not only fail to extinguish the fire but will also intensify the combustion and exacerbate the accident. 　　(2) Install a steam fire extinguishing system, or use other steam nozzle devices such as steam soot blowers to extinguish the fire. 　　(3) Strengthen the boiler’s water inlet and outlet systems to prevent the economizer from being damaged. 　　(4) The door opening can be opened for inspection only when the exhaust temperature is close to the temperature of the injected steam and has remained stable for more than one hour. 3 Causes of post-combustion accidents in the flue tail (1) The fundamental cause is the deposition of combustible substances such as carbon black, oil, and coal powder on the heated surfaces at the tail end. Reasons for deposition: First, the furnace temperature is low during ignition or shutdown, which makes combustion unsafe; a large amount of unburned combustible material is carried into the tail flue ； Secondly, the negative pressure in the furnace is too high, causing unburned combustible materials to be carried into the tail flue ； Third, poor fuel atomization or overly coarse coal powder makes complete combustion difficult, resulting in emissions into the tail flue ； Fourth, fuel that is added when ignition fails, or fuel that leaks in when the furnace is shut down, is carried into the flue. 　　(2) The main reason for the increase in the temperature of the exhaust flue at the rear is that, with combustibles present in the heated area at the rear, the heat transfer efficiency decreases, resulting in an increase in the exhaust gas temperature ； Secondly, these combustibles continuously oxidize and generate heat at high temperatures ； Thirdly, at low loads, especially when the furnace is shut down, the flue gas flow is very low or even stops, resulting in poor heat dissipation. This allows the heat generated by the oxidation of combustible materials to accumulate, gradually raising the auto-ignition temperature of those materials. 　　(3) Air leaks in at various points along the flue, especially at the flue door openings at the rear, as well as through the air and smoke dampers, which helps to fuel the combustion. 　　Combustion begins when combustible materials, temperature, and oxygen are all present simultaneously. 9 Prevention of post-flue combustion accidents (1) Improve combustion control to ensure complete combustion. The number of times the furnace is shut down for ignition should be minimized ； Long-term operation at low load should be avoided ； Ensure good fuel atomization and fine coal powder fineness ； The negative pressure in the furnace should not be too high, and in general, efforts should be made to minimize the accumulation of combustible materials in the tail flue. 　　(2) When it is observed that soot accumulation on the heated surface at the tail increases, soot removal must be carried out promptly, such as **soot cleaning or shutting down the furnace for flushing. 　　(3) When shutting down the furnace, the supply and exhaust air flow must be stopped promptly; within ten hours, the smoke and air dampers as well as all various openings must be tightly sealed to prevent air from leaking in. Coal-fired and oil-fired boilers should be equipped with reliable fire extinguishing systems in the tail flue. After the furnace is shut down, it should be monitored by a dedicated person; if an increase in exhaust gas temperature or black smoke coming from the chimney is detected, prompt action should be taken to address the issue. (12) Damage to furnace walls and arches Damage to furnace walls and arches mainly refers to cracks, collapse, and deformation of the furnace walls, as well as sintering, coking, and falling off of the refractory bricks in the furnace chamber, along with deformation of the arches and falling off of their refractory bricks. Such accidents often cause damage to normal combustion, overheating and burnout of the heated surfaces, redening and deformation of the steel framework, severe air leakage in the furnace chamber, as well as damage to the furnace walls, forcing the shutdown of the furnace. 1 Causes and Prevention of Damage to Furnace Walls and Arches (1) The material quality of bricks and mortar used in constructing furnace walls and arches does not meet the required standards, or the bricks are poorly shaped; as a result, the strength and fire resistance of the brick wall are insufficient. Additionally, insufficient gaps are left between the bricks during construction, which hinders the wall’s ability to expand and contract freely with temperature changes. 　　(2) After construction, the furnace was not dried as required; the time allocated for lighting the fire, raising its temperature, and cooling it down was too short, operations were carried out too hastily, the load changed suddenly frequently or was often overloaded, and ignition and shutdowns occurred frequently. As a result, the strength of the brick wall could not withstand the rapid thermal expansion and contraction, leading to cracking and deformation. 　　(3) Excessively high furnace temperature, an offset flame center, and fuel quality that tends to cause melting and slag formation lead to severe coking of the brick walls; in such cases, the slag corrodes the bricks and damages the furnace walls ； Second, the furnace wall is damaged due to improper operation during coking removal. This is particularly common in the furnace wall and the high-temperature section arch of furnaces without water-cooled walls. 　　(4) In the event of severe water shortages, water circulation failures, furnace explosions, and other accidents, the furnace walls and arches are often severely damaged. 　　For the above reasons, the key points to prevent damage to the furnace walls and arch are: ensuring the quality of the materials and the masonry work, drying the furnace strictly in accordance with requirements; being careful to avoid too rapid or large temperature changes during startup, shutdown, and operation; preventing severe coking by carrying out regular coking and slag removal; and preventing serious water shortages and furnace explosions. 7. Handling of furnace wall and arch damage: When the furnace wall develops minor cracks or gaps, these can be sealed with asbestos rope or covered with refractory mortar; slight deformations can be corrected through reinforcement measures. However, if the damage is severe, the surface temperature of the furnace wall rises significantly, the steel framework overheats and deforms, or there are serious protrusions or collapses in the furnace wall flues and arches that endanger safe operation, the furnace must be stopped immediately for repair.