Four-tube Boiler Explosion Causes and preventive measures

[Abstract] boiler "four" crack total thermal power generating units causes all kinds of unplanned outage in the first, serious impact on power plant safety, economic operation. Summary power under the anti-"four" leakage management experience, the boiler "four tube "Crack analysis of the causes and propose preventive measures.

[Keywords:] boiler "four" crack, cause analysis, prevention measures.


The so-called boiler "four" means the boiler water wall, superheater, reheater and economizer, the boiler to prevent the traditional sense of the four tube leak, is part of the furnace above the metal pipe to prevent leakage. Boiler four covers All the boiler heating surface, they bear the internal working fluid pressure and the role of some chemical components, external to withstand the high temperature, erosion and wear of the environment, water and fire, to reconcile, is where the concentration of energy transfer, so it is easy failure and leakage occurs. According to incomplete statistics, the boiler over the years, "four" crack total thermal power generating units of various reasons the first unplanned outage. boiler in the event of "four" crack, increasing loss of unplanned outages, increasing overhaul work, and sometimes may lead to accidents, serious impact on power plant safety, economic operation. caused by the boiler, "four" causes more leakage, including wear, corrosion, overheating, crack is the main cause leakage of four reasons. summary power under the anti-"four pipe" leak management experience and wear the next bomb squad last 10 years, electrical, prop up power, plate power, Zhang thermal, hot stone experience in power plant, boiler, "four" Explosion analysis of the causes and propose preventive measures.

First, the boiler "four tube" Crack Analysis
1. Wear
Pulverized fly ash wear and mechanical wear, is a long-term safe operation of the main boiler. Ash wear mechanism was carrying ash particles and the incomplete combustion of fuel gas through the high-speed granule heating surface, the particles on the heating surface from each impact will comb out of a trace amount of metal, thereby gradually heating surface wall thinning, gray smoke the higher-speed particles on the wall the greater the impact force, of gray smoke particles carrying more (fly ash concentration increased, the more the number of impacts, the result will accelerate the heating surface of the wear and tear. a long period of wear and thinning of the wall, the intensity decreases causing the tube leakage. heating surface wear of fly ash leakage, explosion possession of significant macro-features, wall thinning, surface smooth. running a serious leak, can be found in both sides of the flue gas temperature difference, not timely shutdown processing, tend to increase the leakage areas, and bring disaster to other heated surface # 3 safety .2009 electric furnace temperature Nianxia wear and tear Economizer leak, first discovered the side of the flue gas temperature decreased, water and steam flow deviation large, that shut down after the bursting of the economizer tube wear. causes serious wear and tear of fly ash are structural factors, design, installation and maintenance deficiencies may lead to increased wear and tear. in the economizer tube and the furnace wall side row between the elbow and the furnace wall between the economizer, reheater, and both sides of the wall between There is a smoke corridor. The gas flow resistance region due to the small, local smoke-speed can be increased to twice the average smoking rate, or even greater, resulting in severe wear and tear where the tube. economizer in the flue gas corridor, and then Thermal control of the elbow, under the elbow and superheater tubes cards near the row of pipes and wear Bian wall tube wear part of it is the more serious parts of fly ash, especially in the economizer zone, flue gas temperature is low, gray granulite hard, wear more prominent. jet fuel, soot blower, and tertiary air nozzles, etc. near the water wall is more serious wear and tear parts of pulverized coal. in the installation, operation and maintenance process, if not heat the tube or pipe fixed firmly card thermal deformation, vibration will occur tube row card collision with the tube wear, but also cause mechanical wear and leakage. prevention method is to reduce the wear and tear gas corridor, uniform airflow, installation of heating surface protection windward iron pipe or coated with wear-resistant coatings.

2. Corrosion
Boiler "four" main heating surface of pipe corrosion and water quality of corrosion failure of chemical corrosion caused by the tube. When corrosion is severe, can lead to corrosion of tube rupture occurred. Smoke on the wall of high temperature corrosion, mainly Ash alkali metal at a high temperature distillation, and the SO3 in the flue gas generated composite sulfate, was in the range of 550-710 �� liquid condensation in the wall, the destruction of the wall surface of the oxide film, that is, high temperature corrosion occurs. lead heating surface temperature corrosion is mainly due to poor combustion and flue gas dynamic field unreasonable, the local gas temperature control to ensure the wall is not over-temperature, corrosive compounds attached to prevent the low melting point metal surface, the flue gas flow and reasonable to minimize the thermal deviation is an important measure to reduce high temperature corrosion. waterwall slagging on if they have, in the surrounding at a certain temperature and reducing gas conditions, will produce more serious water wall tube corrosion. waterwall corrosion and high temperature reducing gas is closely related to the existence, CO concentration of corrosion on the big big place. wall temperature is also a great influence on the corrosion in the range of 300 ~ 500 ��, the outer surface of the wall temperature is increased by 50 ��, corrosion level is doubled. WALL DURING site more in the high heat load, the higher wall temperature areas such as near the burner. superheater, reheater reducing gas areas lower than the furnace, the corrosion rate than the general Wall smaller. but large-capacity boiler superheater and reheater wall temperature is high, especially around both sides of the flue gas temperature difference is large, can cause serious corrosion. In the temperature range of corrosion, in addition to selection of corrosion-resistant alloy steel and austenitic steel, the furnace exit gas temperature should be controlled and smoke rise of temperature deviation and other factors, in order to avoid leaving the local high wall temperature increases the corrosion rate. low-temperature corrosion is sulfuric acid vapor condensation in the tail heating corrosion of the surface occurs, which is also known as sulfuric acid corrosion. It usually appears in the low-level air preheater cold end. When SO3 flue gas flows through the tail with a heating surface, when the rear lower heating surface of the wall temperature When the acid dew point in the wall of water vapor condenses into water, flue gas SO3 gas dissolved in water to form H2SO4, which is the low-temperature corrosion of corrosion of metal wall. low temperature corrosion prevention method most commonly used method is to improve the entrance air temperature, to ensure that the end of Heating Surface temperature above the acid dew point, usually in boilers burning high-sulfur fuel used in the installation of air heater or air recirculation, but the higher the inlet air temperature, exhaust gas temperature will be higher ranked smoke the greater the heat loss, so in order to ensure economic operation of the boiler, exhaust gas temperature increase will be limited. normal operation, the boiler and not cause pipe corrosion and fouling. quality is good to water with a small amount of impurities, water into water through the furnace slag or jelly, dissolved in water through the sewage discharge. When the water quality is poor, the furnace water Fe, Cu, Ca, Mg, SiO2 and other impurities in the evaporation heat is concentrated plane, and freed from the pan in the water attached to the inner surface, the formation of scale, the scale of the heat transfer coefficient of steel is only 1 / 200, affect the heat transfer, and to the wall temperature rise, leading to overheating of wall bulging or rupture. boiler heating surface in the disabled with unqualified access to water or humid air, by air, O2, CO2 and SO2 will have control of the chemical corrosion. excessive oxygen in the water, they will have spots economizer wall oxygen corrosion. prop electric furnace # 5 occurred in 2006, failed due to boiler water chemical corrosion of water wall tube wall caused large bulge events, after being forced to replace all the water wall heating surface.

3. Overheated
Superheater and reheater heating surfaces of boiler pressure in the working fluid temperature, the highest temperature and metal parts, while the steam side heat transfer and relatively poor, so much overheating in the heating surface in two. Overheated heating surface , the tube metal temperature exceeds the limit permitted temperature, an internal organizational change, reducing the allowable stress, pressure pipes, including plastic deformation, significantly reduced service life, leading to over-temperature burst. Therefore, over-temperature result in overheating, reduced safety factor of the equipment, should be strictly controlled steam temperature limit. overheated overheating and short-term overheating points, long-term overheating is the wall temperature over a long period of lower than design temperature under the critical temperature of the material, but time is not over temperature range long, carbide ball of the boiler tubes occurs, oxidation wall thinning, decreased rupture strength, creep speed, so that uniform diameter expansion of crude and finally the weakest part of the pipe burst pipe caused the phenomenon of brittle fracture. long-term super- temperature heat pipe explosion occurred in the outer ring and the high temperature superheater reheater Xianghuo surface. superheater temperature, low temperature side reheater Xianghuo are likely long-term over-temperature pipe explosion. long term overheating of the break-shaped pipe explosion appearance, with the general characteristics of creep rupture, brittle fracture characteristics of tube rupture, cracking-rough, uneven edges blunt edge, blasting the wall thinning small mouth. short-term overheating is when the wall temperature exceeds material under the critical temperature, the material strength decreased significantly, including the pressure, the occurrence of the phenomenon of inflation and tube thick. short-term overheating often occur in superheater Xianghuo face contact and direct and direct flame of the heating surface by radiation heat pipe on.


Population explosion large plastic deformation, significant expansion diameter thick wall thinning was blade-like, in general, larger explosion was horn mouth, thin lips was a typical burst-shaped mouth of the bursting of the mouth of the micro-explosion for the dimple ( fracture constituted by a number of pits), burst pipes around the mouth was significantly higher hardness, burst around the mouth, the wall thickness of oxide depends on short-term over-temperature tube burst long before the extent of over-temperature, long over-temperature The more serious, the thicker oxide. If there is a high degree of design of the furnace is low, the flame center side, the larger heating surface, heating surface is not enough margin or wrong selection of materials, power condition is poor, low and heat the steam mass flow rate surface structure is irrational and other factors, will cause heating surface temperature or the existence of a large deviation and local over-temperature heat, in the manufacture, installation and maintenance, if there is tube blockage caused by poor working fluid flow, open circuit, short circuit situation will lead to super-heated surface temperature, combustion control operation if there is improper, the flame after the shift, the high furnace exit gas temperature or the furnace heat load large deviations, caused by the incomplete combustion combustion flue, heated by cast stop inappropriate tube fouling, etc., can also cause overheating of heating surface. Strengthening operation adjustment and monitoring, control, over the wall temperature is the main measures to prevent overheating. under the 2005 electric furnace Boiler # 2 Superheater Tube replacement unit started after the explosion occurred the non-stop, blocking the tube after the examination revealed foreign body due to pipe bursting overheating. reposted elsewhere in the Research Papers Download http://www.hi138.com 4. weld quality and crack
Boiler heating surface by the welding assembly together, each heating surface of each tube has multiple weld root, a large boiler heating surface of the weld the entire quantity of up to tens of thousands. The heating surface is exposed high temperature and pressure equipment, welding defects are cracks, incomplete penetration, incomplete fusion, undercut, slag, porosity, etc. These defects exist in the heating surface of the metal matrix, the matrix is fragmented, resulting in stress concentration. in the medium micro-crack under pressure tip, incomplete penetration, incomplete fusion, undercut, slag, porosity and other defects at high stress cracking under the base and gradually develop into macro cracks, eventually leading through the wall heating surface Explosion accident. the generation of welding defects for many reasons, it is the stress and structure, groove, base metal, welding materials, welding parameters, heat treatment and welding technology and so on. welder must be strengthened to ensure the welding quality management and quality inspection and assessment of welding technology. For welder boiler and pressure vessel welding, should be certified by the examination of welding, the actual position of welding, pipe types, sizes and certification should be permitted under the welding line project, with special attention to alloy steel, dissimilar steels, focus on welding preparation, welding, heat treatment, all aspects of post-weld inspection, to strengthen the metal supervision to prevent misuse of steel and welding materials, especially on the weld metal to fully carry out inspection.

The so-called "crack" refers to the boiler after several starts and stops after the tube - the tube, tube - seals, tubes - rigid beam connections between parts of shrinkage due to thermal expansion is not synchronized, displacements are not synchronized, nor sufficient compensation capacity of the resulting crack pipe leakage. These sites are outside the furnace insulation, furnace tube row is often intensive, staff found it difficult to pre-screen, it is difficult to install monitoring equipment. to avoid the tube and - as superheater tubes tube row holding tube, positioning tube, welded tube within the panel between the screen and so should be considered in the design installation of "transition board" to avoid direct contact between pipe and tube, pipe screen, part of the furnace, between the tube without welding, so that the tube has some ability to compensate, "transition board" and the connection between the tube weld, ranging from strong to be that height should be slightly lower than the welded pipe wall thickness. pipe seals at a crack occurred in superheater and reheater The Management Office through the wall, water wall, including wall connecting the end of such tube and fin. The main problem should be resolved during the design stage and installation, hanging devices must support the boiler, the boiler expansion dead, expansion direction, expanding the amount of consideration clearly, there must be self-compensation capability. compensation section is appropriate, reserved for expansion space and the direction is correct, after the furnace wall pipe should curvature, so that it has sufficient ability to self-compensation. If you find Top Superheater collapse or serious distortion, should be treated to restore its branch hanging device, sealing device. pipe between a rigid beam crack, in the modern large-scale boiler furnace and tail are equipped with rigid beam, rigid beam pipe through the transition part and connected, rigid beam is an important component of the boiler, it ensures that the overall shape and rigidity of the boiler, the boiler when the explosion or outside the blast to protect the boiler from damage. Therefore, we must find out all parts of the boiler and boiler expansion of the rigid beam system design concept the tubes and the rigid beam expansion between the "dead spots" and expansion direction. pipe and rigid connections between beams should be intact, there should be no cracking, severe deformation and jamming phenomenon, rigid frame between the inside of the pipe insulation material to fill the gap, to prevent heat generated inside the rigid beam bending, resulting in additional stress. "crack" due to the larger proportion of leakage, should carefully check the reduction of "crack" leakage, mainly to prevent crack eliminate stress concentration. Photo # 2 furnace heat maintenance shutdown in 2009 and found reheater tube wall leaks, check that the wall again with the seal plate should be free expansion of the electrical construction installation, but when welding as one of the lead crack. under the power # 3 boiler in 2007 occur burner corner crack fin boiler starts leaking water wall led to violent incidents.

Second, the boiler "four-tube" Crack prevention
Boiler "four pipe" leak more factors must be for different reasons from the operation and maintenance, and management, to take preventive measures.

1, strengthen maintenance management plan developed and implemented wear
Top-down awareness, to strengthen the "four pipe" inspection and supervision. Wear proof staff to make full use of large and small repair on the four macro-examination, adhere to the "must stop every principle of" master boiler "four" metal long-running performance in law, discovery and elimination of metal accident risk. on the high-temperature corrosion, wear, bulging thick, drum kits, stress concentration, etc. to strengthen inspection and found the problem timely and thorough manner. strengthen the metal monitoring and chemical analysis, heat load take a more focused part of the cut pipe inspection and chemical analysis, coke and easy, easy to wear, easy to blow soot blower parts of the thin side of thick and inspection, superheater, reheater and so easy to over-temperature parts of the metallographic analysis. to strengthen four attachment checking, such as wear-watt, pipe clamps and retaining body iron row, finishing the formation of tube row prevent smoke gas corridor. of doing NDT weld inspection and repair, welding quality control of the good relations. At the same time to raise the level of maintenance , the heating surface for the replacement of aging equipment and other technological transformation.

2 strengthen the operational management and supervision of boiler water
Operating personnel to serious action, according to a point of order lifting the load. Everyday closely monitoring four alarms to prevent heating surface, "four" over-temperature, wall temperature to strengthen the monitoring, reduction of wall-temperature. The fuel into the furnace of coal to enhance professional rapid analysis of forecasting for the operating personnel careful operation, regular adjustments to provide a basis to facilitate timely adjustment of operating personnel, wind, reasonably adjust the air volume of powder and excess air so that combustion of coal in the furnace. to ensure that mill group running at peak performance, strict control of coal fineness reduce equipment wear and tear. seriously combustion adjustment, the control center did not tilt the flame, and strictly control the parameters of the components, non-overrun and strengthen heating surface blowing, to prevent local over-temperature coke. strengthen the boiler water monitoring, quality assurance compliance and so on.

Third, the conclusion
To prevent the "four" crack is a comprehensive project, we should start operation, maintenance, technical improvements, etc. to strengthen supervision, management and inspection, considering. Xiahuayuan Power has always been attention to the boiler "four pipe" leak prevention established production director, head of the boiler, "four" special treatment wear proof working group on a regular basis on the unit operation, maintenance issues in the thematic analysis revealed the activities, advance planning and preparedness, revised <<to prevent the "four "Crack regulations>> to develop realistic goals, technical measures, and levels of personal responsibility. operating personnel to adjust according to changes in coal combustion, control and reasonable speed, control, over-temperature wall, strengthen the supervision and operation of equipment and chemicals supervision. maintenance personnel to improve maintenance quality and metal supervision. wear proof to the last 10 years the working Group entrusted power, plate power, Zhang thermal, hot stone out for special inspection of power plants, has accumulated rich experience in large unit checks and applied to proof under the power of the wear inspection work, adhere to the company "will review every stop" approach, as much as possible to expand the scope of the inspection, to strengthen the hidden parts of the inspection found a number of problems and timely treatment, greatly reduce the boiler non-stop to ensure safety and stability of power plant operation.

References:
1, Cheng Shaobing, etc. <"Big capacity boiler heating surface temperature and the temperature of Failure POLICY"> Guangdong Electric Power, January 2004
2, Fan from the vibration <<Boiler Principle>> Water Power Press in May 1986 the first edition
3, Huang Jian Yi <<Analysis of four tube leaks and boiler blow Cause "> Links in Guangdong Electric Power Research Institute papers free download http://www.hi138.com