Analysis of the cause of the perforation of the ec

Cause analysis of perforated economizer tube of boiler

a kctk35/402100 boiler in a unit was put into operation at the end of 1994, and the economizer tube burst and leaked many times in more than two years of operation. In 1996, water leakage occurred in the economizer tube on the third floor of the boiler, and water leakage occurred again in the economizer tube on the second floor after two months of operation. The working pressure of the boiler is 3.9Mpa; Evaporation capacity 3.5t/h; The economizer tube is made of 20g steel with a specification of 28mm × 3mm。 In order to prevent the recurrence of similar accidents, the causes of the perforation of the second layer economizer tube were analyzed and studied

1 physical and chemical inspection

1.1 macro inspection

the perforated pipe section is located at the lower part of the economizer, and the oxide skin on the outer wall of the pipe is flaky, which is more serious near the perforation, as shown in Figure 1. There are many circular corrosion spots and corrosion pits with different sizes and depths on the inner wall of the pipe, and the size of the corrosion pit is generally 5mm × 1.2mm～ 1.5mm × 0.3mm, with black corrosion products in the pit; In addition, there are round corrosion bulges of different sizes on the inner wall of the pipe, with a diameter of 1 ~ 5mm. The surface of the bulge is brick red, and there are black powder inside. Peeling off the bulge, you can see an ulcer pit, in which there are black corrosion products, and the more corrosion products, the deeper the corrosion pit

the wall thickness of the pipe at and near the perforation is thinned, the thickest is 2mm, and the thickness is thinned by 1mm; The wall thickness at the perforation is only 0.2mm, which is reduced by 2.8mm; At the place where the inner wall corrosion is not serious, the wall thickness also has the phenomenon of thinning, which indicates that the thinning of the wall thickness is caused by the outer wall corrosion. The scale on the inner wall of the whole section of the pipe is very thin, and the scale thickness is about 0.5mm

1.2 chemical composition analysis

samples were taken from perforated tubes for chemical composition analysis (mass fraction), and the results are listed in Table 1. It can be seen that the chemical composition of the perforated pipe meets the technical requirements of 20g steel in gb/t standard

1.3 metallographic examination

the longitudinal and transverse samples are cut at the perforated part. After grinding, polishing and observation under the optical microscope, the non-metallic inclusions in the pipe material are evaluated as A1 and D1 (the total amount is level 2) according to gb/t "methods for the objective evaluation of micro crushed test blocks of non-metallic inclusions in steel"; After the cross-sectional sample was eroded by 4% nitric acid alcohol solution, it was observed that there were corrosion pits on the inner wall of the pipe, and the pits were filled with gray black oxide

1.4 scanning electron microscope and energy spectrum analysis

xl230 scanning electron microscope (SEM) and dx24x-ray energy spectrum dispersion spectrum (EDS) of Philips company were used to observe the different areas of the inner and outer walls of the perforation in the experimental method mode. For example, the position and direction of sampling, the shape and size of the sample, the force application characteristics during the experiment, including the loading speed, the composition and temperature of the environmental medium, will affect the results of the experiment. The corrosion bulge and corrosion pit on the inner wall contain more oxygen by energy spectrum analysis. The corrosion product observed by the electric microscope is oxide, indicating that oxygen corrosion has occurred on the inner wall of the pipe. Figure 6 shows the morphology of corrosion bulge, and Figure 7 shows the morphology of corrosion products at the corrosion pit

oxygen and sulfur, which reduce labor intensity more, are detected on the outer wall of the perforated pipe. The electron microscope morphology of the corrosion products is shown in Figure 8, indicating that the outer wall of the pipe is corroded by oxygen and sulfur