Analysis, treatment and prevention measures of wat

Analysis, treatment and preventive measures of boiler water-cooled wall tube burst

1 overview

after a dzl4-l.25-a Ⅱ steam boiler was put into use in October 2002 in a spice chemical plant in our city, the water-cooled wall tube burst occurred within a year. During the on-site inspection on August 11, 2003, the author found that the blast hole was located at the bending part of the 11th water-cooled wall pipe next to the right furnace door (the furnace is a partial drum with compact structure, and the furnace is full of water-cooled walls, so a bend was used at the furnace door to prevent blocking the furnace door). The blast hole was longitudinal fracture, and its section was relatively sharp. The upper and lower ends of the thin scale breach with a thickness of about 0.5mm on the inner wall of the pipe are blocked with flake scale. There is a large amount of fallen scale accumulation in the right header connected to the pipe, and there is also a small amount of scale residue at the bottom of the drum

2 cause analysis

the author believes that the water circulation failure is the main reason for the short-term overheating and burst of the water-cooled wall tube

(1) the boiler is a natural circulation boiler. The working medium with different temperature has different weight, and the driving force of its difference is the power of water cycle. How to maintain the belt tension machine? Many belt manufacturers bought the belt tension machine but did not understand it, that is, the weight of the working medium in the downcomer is greater than that in the water-cooled wall. The dynamic force formed by its difference overcomes the rising and falling flow resistance of the working medium, so the water cycle is formed by using its natural characteristics. The working medium absorbs the heat of the heating surface wall in the cycle, which not only reduces the temperature of the heating surface wall, prevents the overheating of the heating surface wall, ensures the service life of the heating surface, but also becomes steam with certain quality, so that the function of the boiler can be realized

(2) during the operation of the furnace, the scale residue produced by the non-standard water treatment work is accumulated at the bottom of the drum. Under the action of water circulating power, the tensile test: the tensile strength of the three specimens shall not be lower than the specified tensile strength value of the grade of reinforcement; The tensile strength of the joint specimen of grade 3 reinforcement after waste heat treatment shall not be less than 570Mpa of the tensile strength of hot-rolled grade 3 reinforcement; At least two test pieces are broken outside the weld and sucked into the downcomer together with the boiler water and brought to the header. The loose scale with heavy mass and large particles is deposited at the bottom of the header under the action of gravity and inertia, and continuously accumulated into a hill shape, thus reducing the flow area of the working medium into the water-cooled wall pipe. According to the theory of hydromechanics, the flow rate of the working medium here increases and the static pressure decreases, As a result, the inlet pressure of the water wall tube decreases and the water circulation weakens

(3) the scale slag brought into the header with the downcomer boiler water is light in weight, and the thin loose scale will float in the flowing boiler water, and enter the water-cooled wall pipe from the header with the water circulation. When flowing to the bending part of the water-cooled wall pipe, it begins to stagnate and accumulate there under the action of the loose scale gravity and friction resistance, which further increases the flow resistance of the boiler water and slows down the flow rate of the working medium; On the other hand, with the continuous increase of the amount of loose dirt, the flow area in the water-cooled wall tube continues to decrease until it is completely blocked. In this way, the flow rate of the working medium at this place gradually tends to zero, that is, the water circulation stagnates. At this time, the water-cooled wall tube is in membrane boiling, and its elbow is in a dry burning state, resulting in a sharp rise in the temperature of the tube wall, forming a short-term overheating and tube explosion

3 repair and replacement treatment

because the water-cooled wall pipe burst, except for the elbow part with cracks and different degrees of expansion, no abnormalities were found in other parts, so it was decided to cut and replace the deformed pipe section. According to the requirements of relevant specifications and technical standards, the starting and ending parts of the cut and change pipe section are selected on the straight pipe section, which extends 50mm upward and downward from the bending starting point at both ends of the deformed elbow, and the actual length of the cut and change pipe section is about 720mm. Raw water cold wall pipe adopts 20 # pipe conforming to GB3087, with specifications ф 57mm × 3.5mm。

(1) bend fabrication. Take a new pipe with the same properties and specifications as the original furnace pipe, and use the pipe bender to cold bend it into a shape roughly the same as the cut pipe section, and ensure that the ovality, waviness and wall thickness reduction of the elbow meet the relevant requirements, and the surface is free of defects such as double skin and cracks

(2) welding repair. Grind the butt joint of the new and old pipe orifices and make a groove that meets the requirements, and the welder with corresponding qualifications shall weld according to the specifications in Table 1

Table 1 welding specification

see table

(3) post weld inspection. After welding, weld appearance, MT, Pt inspection and ball passing test were carried out for two pairs of joints according to relevant requirements, and the results were qualified

4 preventive measures

(1) manually and mechanically remove the scale on the boiler once, completely remove the loose scale and slag at the bottom of the drum and in the header, and flush the loose scale attached to the water wall pipe with a hose from top to bottom with a large amount of water and high water pressure

(2) strengthen the management of boiler water treatment, strictly enforce the discipline of water treatment process, make the boiler water quality meet the standard requirements of gb1576 "industrial boiler water quality", and prevent boiler scaling

(3) do a good job in the regular blowdown of the boiler during daily operation, and timely remove the loose dirt and slag in the drum and header. Every time the boiler is shut down for maintenance, thoroughly remove the scale and impurities of the boiler according to the above method

after taking the above measures, the boiler has been running for more than a year, and no abnormality has been found in the silent pay pipe of Jinmin entrepreneurship service department