Cause analysis and preventive measures of overheat

Cause analysis and preventive measures for overheating deformation of boiler drum

1 overview of boiler defects

a DZL in a unit in Nanping City The evaporation capacity of 25-w11 boiler is 4t/h, and the working pressure is 1.25mpa. It has been used for 6 years. On January 24, 2010, the author found that a large amount of flake scale accumulated on the water side of the bottom of the drum of the equipment during the internal inspection of the equipment. After removing the scale, two bulge deformation defects were found at the bottom of the drum. One place is 1030mm away from the front tube plate, with an area of 400mm and 360mm, which is oval, and the bulging deformation height is 60 ~ 70mm; the other place is 1830mm away from the front tube plate, with an area of 390mm and 335mm, which is oval, and the bulging height is 60 ~ 70mm. Through further inspection, it was found that the boiler drum, smoke pipe, water-cooled wall pipe, front and rear tube plates and other main pressure components had scale with a thickness of 2 ~ 3mm

2 on site inspection and analysis of defect causes

according to the analysis and judgment of the on-site inspection results, the direct cause of the defect is: first, due to the external fire heating of the drum and the accumulation of internal scale, the local material in the high-temperature area at the bottom of the drum is overheated and overtemperature (the formation of scale greatly slows down the heat transfer speed of the metal, and slowly unloads the load to produce overburning), so that the strength decreases and softens (lower than the yield limit of the material), Bulge deformation occurs under the action of steam pressure inside the drum; Second, a large number of flake scale falling off the smoke pipe accumulated at the bottom of the drum and failed to be removed in time

the indirect reasons for this defect are: first, the boiler managers of the enterprise do not pay attention to the thought, are careless, so that the management is not in place, the boiler operators and water treatment operation managers do not operate according to the requirements, resulting in unqualified boiler water quality treatment and unscientific and inadequate boiler sewage discharge, resulting in a large amount of scale on the main pressure components of the boiler, such as the smoke pipe, drum and water wall pipe. Referring to several representative boiler water sample test reports of the enterprise within half a year, the hardness of feed water is 0.8 ~ 1.8mmol/l, and its standard value is 4mmol/l, The pH (25 oc] value of feed water is 6.6 ~ 6.8, which is lower than the standard value of 7.0 ~ 10.0; the phenolphthalein alkalinity of boiler water is 2.8 ~ 5.6mmol/l, which is lower than the standard value of 6.0 ~ 18.0mmol/l; the total alkalinity of boiler water is 3.7 ~ 6.0mmol/l, which is lower than the standard value of 8.0 ~ 26.0mmol/l; the dissolved solids are 3.03 103 ~ 3, 67 103 mg/L, which is 5.0 103 mg/L; the pH (25 ℃) value of boiler water is 9.6 ~ 10.2, which is lower than the standard value of 10.0 ~ 12.0, which is tested from the water quality.According to the data, In addition to the hardness of the feed water and the dissolved solids of the boiler water, the phosphate radical, phenolphthalein alkalinity, total alkalinity of the boiler water and the pH (25 ℃) value of the feed water did not meet the standards. Second, the failure to do a good job of scientific and regular sewage discharge has accelerated the formation of scale on the main pressure components such as boiler drum, smoke pipe and water wall pipe. According to the analysis of the on-site inspection, the main reasons for this defect are that the boiler operation management is not in place, the boiler water quality treatment is not up to standard, the boiler produces a large amount of scale and is not removed in time, and the regular sewage discharge is unscientific and not in place

3 rectification plan and implementation

according to the relevant provisions of the boiler inspection rules and the requirements of technical standards, the two defects shall be repaired and repaired. The location and size range of the repair shall be determined according to the magnetic particle testing and hardness testing results (to confirm whether the material strength around the bulge deformation decreases). The welded plate shall be the same as the drum material on the original boiler quality certificate (including material thickness), The welding rod shall also meet the standard requirements. The professional maintenance and construction team with boiler installation and maintenance qualification shall carry out maintenance in strict accordance with the repair plan, and the boiler can be put into operation only after passing the inspection

4 inspection experience and inspection technology experience

in order to avoid missed inspection or wrong inspection, inspectors should pay attention to the following aspects in the inspection process: first, carry a flashlight (or safety work light provided by the enterprise), inspection hammer, five times magnifying glass, digital camera, welding joint ruler, steel tape, steel ruler and other inspection tools and original inspection records. Second, measure the scope, specific position and height of bulge deformation with steel tape and weld ruler. Third, magnetic particle testing should be carried out within a certain range around the drum to check whether there are cracks and whether the scope of patching should be expanded; At the same time, the hardness tester should be used to test the hardness of the communication part at the deformation edge of the drum, so as to confirm whether the material strength at the deformation edge of the drum has decreased and whether the scope of patching and repair should be expanded. Fourth, for boilers with serious scaling (the average scale thickness is more than 1.5mm), It is also important to check whether the front and rear tubesheets and water-cooled wall tubes have cracks in the front and rear tubesheets caused by local overheating due to scaling or whether the water-cooled wall 6 is operated in good faith, and the tubes are thickened or bulged (the front and rear tube sheets are heated unevenly on the high-temperature flue gas side due to incrustation on the ice side, causing local uneven thermal expansion and cold contraction of the front and rear tube sheets and cracks. In addition, a large amount of flake incrustation that falls off is bridged and accumulated in the tubes, blocking the water-cooled wall tubes, causing poor water circulation, overheating or dry burning, causing the water-cooled wall tubes to increase rent or bulge and burst tubes). Therefore, it is recommended to dredge the water-cooled wall tubes one by one before boiler operation, Qualified enterprises had better use the special pipe washing machine for boiler to dredge, so as to ensure that the water-cooled wall pipe is unobstructed. After cleaning the surface of the flue gas side of the front and rear tube plates, use a five times magnifying glass for visual inspection, and use magnetic particle testing for suspicious parts to check whether there are cracks

5 conclusion

scaling caused by unqualified water treatment, resulting in bulge deformation or expansion and explosion of boiler drum, water wall pipe or smoke pipe, is common in industrial boilers with evaporation capacity of 4t/h. In order to prevent such defects or accidents, the user unit should take the following measures: first, strengthen the safety management of boiler, improve and strictly abide by the rules and regulations of boiler room and boiler water treatment, and strictly do a good job in boiler feed water The treatment of boiler water shall meet the standard requirements of industrial boiler water quality (GBI), discharge sewage scientifically, and make boiler operation records as required, so that the main pressure components of the boiler do not form scale or thin scale (the scale thickness is 0.5mm); Second, after the boiler is repaired and repaired, it is necessary to carry out descaling treatment (chemical descaling or manual mechanical descaling) before operation. In particular, it is necessary to manually and timely remove a large amount of scale falling off the smoke pipe and the scale of the water-cooled wall pipe, so as to prevent the scale from accumulating at the bottom of the boiler drum or the scale from blocking the water-cooled wall pipe in the technical embodiment of external thermal insulation of many external walls, To prevent accidents such as bulge deformation at the bottom of the drum of the boiler serial acquisition Shulman and citadel plastics Inc. or water-cooled wall tube explosion from happening again; Third, strengthen the safety awareness, heart and safety technology knowledge education of boiler managers and boiler operators

I believe that as long as enterprises learn from experience and lessons, strictly abide by boiler operating procedures, water treatment management rules and other standards and norms, and enhance the safety awareness and safety technical ability of management and operators, the boiler will operate safely and economically