Causes and prevention of four tube explosion and l

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Causes and prevention of boiler four tube explosion and leakage in thermal power plants

the "four tube" explosion and leakage of boiler in thermal power plants is a major problem that has troubled the safety production of thermal power plants for a long time. Due to its high accident rate, it has a great impact on power generation and technical and economic indicators. According to the statistics of a certain electric power group company in China, in the first quarter of 2003, there were 57 class I obstacles in the whole company, and the boilers in thermal power plants accounted for 30 times. Among them, 21 times were boiler "four pipe" leakage, accounting for 70% of the total number of boiler obstacles, accounting for 36.8% of the class I obstacles in the group company, which seriously affected the safe and economic operation of thermal power plants. Two 125 MW units of Nanchang power plant in Jiangxi Province were put into operation in October, 1988 and September, 1989 respectively. They have been running for a long time, and they burn low-quality coal all the year round. The boiler equipment is seriously worn. In the mid-1990s, the "Four Tubes" burst and leak frequently. According to the actual situation of Nanchang power plant, corresponding countermeasures and preventive measures were taken to reduce the occurrence of "four pipe" explosion and leakage

1 the main cause of "four pipe" explosion and leakage

1.1 burning inferior coal and increasing wear

in recent years, due to the increasingly tight supply and demand in the coal market, the coal quality has been declining year by year. In recent three years, the annual average calorific value and ash content of Nanchang Power Plant were 16.467 mj/k in 2000 respectively, but on the whole, G and 40.13%; In 2001, it was 16.15 mj/kg, 43.01%; In 2002, it was 15.52 mj/kg, 45.91%; In the first quarter of 2003, it was 15.04mj/kg, 46.57%. As the two units of Nanchang power plant have been operating for more than 10 years, and the ash content of coal has increased, the wear of boiler heating surface has intensified

1.2 the load of the unit rises and falls frequently, resulting in thermal fatigue of furnace tubes. The unit undertakes the task of peak load regulation. The average load of the two 125 MW 'units in the three years was 95.18 MW, 90.94mw and 101.30mw respectively. The unit starts, stops or load changes frequently. When the load is high, it reaches 125 MW, and when it is low, it is only 65mw. There is a great difference between the high and low loads, which leads to the thermal fatigue of furnace tube metal and tube explosion. For example, the economizer pipe of the front partition wall was broken during operation. For another example, when the boiler four corner water-cooled wall tube and the burner bellows are in the hot state, the uneven expansion causes the bellows rib plate weld to crack the water-cooled wall tube for many times

1.3 the coal quality changes greatly and is prone to overtemperature

the design low-level heating of Nanchang power plant coal is 14.66 mj/kg. Because the coal quality changes frequently and deviates too much from the design value, the adjustment of the operating air volume can not meet the needs of the change, resulting in short-term over temperature accumulation and overheating in the long-term period, leading to pipe explosion. In 2000, the low calorific value of man-made boiler coal reached 19.05mj/kg at high temperature and only 13.64mj/kg at low temperature; In 2001, the maximum was 20.03mj/kg, and when it was low, it was only 13.72mj/kg; In 2002, it was also 18.55mj/kg at high level and only 12.35mj/kg at low level

1.4 the design of flue gas circulation parts is improper.

the design layout or structure of the equipment is unreasonable, which increases the smoke velocity in some flue gas corridors and intensifies the erosion and wear. For example, the lower header of the economizer on the front partition wall is too close to the elbow of the low-temperature reheater, so that the flue gas collides with the lower header to form a vortex to scour the elbow of the low-temperature reheater, and the pipe bursts for many times

1.5 furnace tube corrosion and crater quality problems

tube corrosion and poor welding quality during installation and maintenance are also the causes of "Four Tubes" explosion and leakage. After combustion, Na, K, s and other elements contained in coal-fired power plants produce oxides, which condense on furnace tubes and react with SO3 in flue gas to form sulfate. The sulfate is viscous and forms slagging. SO3 in flue gas passes through the ash and Fe2O3 on the surface of furnace tubes and reacts with sulfate to form low melting point composite sulfate, When the sulfate covering the outer surface of the tube wall and the oxide formed after the oxidation of the tube form a low melting point liquid eutectic, a 4-phase 3-interface system of "base metal oxide film molten salt layer sulfur-containing flue gas" will be formed, resulting in thermal corrosion of the furnace tube by electrochemical process; There is also severe erosion wear in some parts

the quality of the welded junction can not be ignored. The welded junction of the short pipe of the lower header of the main economizer of the boiler has leaked twice. Through inspection and analysis, it is determined that the quality of the original welded junction is defective, and small hole leakage has occurred after years of corrosion

2 countermeasures for "four pipe" explosion-proof

2.1 strengthen maintenance management, formulate and implement anti-wear plan

formulate anti-wear plan according to the wear of equipment. First, take advantage of the overhaul opportunity to renovate the boiler heating surface in a planned way; Make full use of the opportunity of unit overhaul to focus on the treatment of the boiler heating surface, and timely replace the tubes that are seriously worn, expanded and thermally corroded. Some key anti-wear or anti-corrosion areas are brushed with anti-wear coating, and anti-wear protective tile devices or metal spraying are added to enhance their anti-wear performance. In addition, the main economizer of No. 10 boiler was completely replaced in 2000, the low-temperature section of the low-temperature reheater of No. 11 boiler was completely replaced in 2002, and the two sides of the high-temperature section were partially replaced, thus ensuring the long-term continuous operation of the boiler. The second is to track and inspect the easily worn parts of the unit by means of minor repair or standby shutdown of the unit, so as to ensure that the inspection is carried out every time the unit is stopped. Replace the deformed and severely worn pipes in time, take corresponding anti-wear measures for the easily worn parts to improve their wear resistance, especially focus on the inspection of high and low temperature reheaters, convection superheaters, front and rear partition economizers, side and rear package superheaters, and eliminate the pipe explosion in the bud

2.2 strengthen operation management

operate carefully during operation, implement red line operation assessment, prohibit boiler overload operation, carefully increase and reduce load according to regulations, strengthen monitoring of pipe wall temperature, and reduce pipe wall overheating. Strengthening the rapid analysis and prediction of man-made boiler coal provides a basis for operators' careful operation and frequent adjustment. The fly ash carbon measurement system is more conducive to the timely adjustment of primary and secondary air by furnace operators. The application of DCS and other systems not only reduces the power consumption, but also makes the pulverized coal particles more uniform and safer to burn, avoiding the wear and leakage caused by incomplete combustion of pulverized coal particles scouring the water wall tubes. The operating personnel shall carefully adjust the combustion to minimize the thermal corrosion of furnace tubes caused by the original atmosphere which reflects the development direction of the industry

2.3 strengthen the work of coal blending

the average low calorific value of coal fired in Nanchang power plant boilers is 14.66mj/kg. There are a wide variety of coal factories, nearly 40 coal supply mines, and the coal quality of each mine varies greatly. For example, Henan Yima, Huainan Lizui, Ping Industrial aluminum profiles, automobile exhaust catalysts, lithium battery materials are expected to benefit from xiangtianyu and other coal mines, with an average low calorific value of more than 20mj/kg. If directly put into the furnace, it will cause boiler coking, For example, Pingxiang Changlong, Gaokeng sublimation and other mineral point coals have an average low calorific value of only 11 mj/kg. If they are directly used in the boiler, they may cause major accidents such as boiler fire extinguishing and blasting

according to the requirements of boiler design calorific value and the measurement of man-made coal 3. The principle of beam displacement measurement is roughly the same as that of deformation measurement, different coal types are stored in piles, and each coal type is matched with man-made boiler. The coal blending work is well done to meet the requirements of boiler combustion, which is conducive to safe and economic operation

2.4 carry out technical transformation for equipment

carry out technical transformation for remaining design problems in a timely manner. For example, the lower header of the economizer on the original rear partition wall of two 125 MW unit boilers is improperly arranged, which is easy to cause eddy current of flue gas to seriously scour the elbow of the nearby low-temperature reheater, resulting in wear and pipe explosion. During the overhaul of No. 10 boiler in 1996 and No. 11 boiler in 1997, the lower header of the rear partition economizer was moved down to eliminate the phenomenon of flue gas vortex. Since then, no leakage has occurred at the low-pressure reheater elbow near the lower header of the partition economizer, and the effect is good

in view of the problem that the water-cooled wall pipe near the air box of the four corner burner was cracked due to uneven expansion during the hot state of the boiler, the water-cooled wall pipe close to the air box was replaced by two circles, and the air box supports and hangers were readjusted or replaced to synchronize the expansion of the water-cooled wall pipe and the air box. Through the above treatment, no similar water-cooled wall pipe was cracked for more than three years

2.5 pay close attention to the welding of pressure pipes

argon arc welding with good quality shall be used as much as possible. After welding, nondestructive testing shall be carried out for the welded junction. If it does not meet the requirements, it shall be reworked again, which effectively improves the welding quality and prevents leakage accidents caused by welding quality problems. Spot check and test the original welded junction, and timely handle the wear, corrosion or unqualified process

2.6 strict assessment

in order to mobilize the enthusiasm of employees to do a good job in the "four pipe" explosion prevention and improve the reliability and economy of boiler equipment operation, Nanchang power plant has formulated the "four pipe" anti-wear and anti explosion assessment regulations for boilers, which reward and punish relevant units and individuals according to the length of continuous operation of the unit, reward the pipes that are found to be worn or bulged beyond the standard after boiler shutdown inspection, and repair the fitter process, the welding quality of welders The metal room inspection has implemented corresponding assessment methods, and the timeliness and accuracy of boiler leakage report, inspection report and over temperature report are also assessed. In order to ensure the quality of overhaul, the overhaul assessment method has been formulated to implement the 100 point system assessment, with safety and quality in the first place, including the length of continuous operation time of the unit. For departments with problems, the assessment regulations shall be strictly implemented without mercy

due to the above effective measures taken by Nanchang power plant, in recent years, the number of Unit Outages affected by "four pipe" explosion and leakage has been greatly reduced than before, and good results have been achieved

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