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An accident analysis of the perforation of a molten salt
In December 16, 2004 a Henan melamine manufacturers to carry out the maintenance plan, the molten salt system of intermittent warming to maintain the temperature of molten salt in the melting point (200 c), which is in the standby liquid.12 20 day at 8:10 in the morning, on-site inspection found that the molten salt furnace into the salt pipe and molten salt furnace bottom hole with molten salt leakage judge, molten salt coil leak. The leadership then decided to open the top of the furnace tube. Check the inspection process found in the molten salt furnace inner coil has three perforation, respectively located above the 10,12,15 ring coil. Tenth circle perforation shape is oval, 50mm along the furnace to the long axis, short axis axial direction along the furnace body 40mm, along the center line of the furnace tube gauge at 10mm; twelfth ring perforation round shape, diameter 15 mm; fifteenth ring perforation is elliptical, axis is 75mm, short axis 50mm, partial 10mm. The center line down uneven inner molten salt on the coil ,. There no of deformation uniform molten salt attached on the pipe wall, similar scar, scar greatly remove scar tube wall thickness for 3.4mm.3.5mm, lower temperature region of pipe wall thickness is good overall shape of furnace 3.9mm.
At the same time, the temperature of the molten salt furnace and the flue gas temperature interlock alarm were checked, and no abnormal was found.
Two, perforation cause analysis
From the molten salt furnace tube three perforated view is obviously serious local corrosion caused by, the corrosive medium should be for molten salt tube in molten salt medium and furnace tube containing corrosive high temperature flue gas generated by the combustion of fuel oil, possible corrosion process is:
1, the corrosion of steel in the air
Carbon steel in the air the corrosion rate increases with the temperature changes large, 400 DEG C, the corrosion rate for 45mg/dm2.d, rise in 1190 mg/dm2.d 700 DEG C, to 1000 DEG C L 13500mg/dm2.d. If the furnace tube local defects, in local will accelerate corrosion caused tube outside the hot air corrosion of furnace tube from the outer pipe perforation.
2, the corrosion of molten salt in the coil
Molten salt is KNO3, NaNO2, NaNO3 according to a certain proportion of eutectic, the three salts belong to the oxidant, at high temperatures can cause corrosion and oxidation on the surface of carbon steel, especially NaNO2 in high temperature can be decomposed into Na2O, Na2O on iron caused by corrosion even more, and the decomposition speed increased with the rise of temperature.
3, harmful ingredients in fuel oil!
S combustion of the fuel oil may lead to the formation of SO2, SO3, H2S and other corrosive gases for metal pipe corrosion caused tube perforation, measured, we use the sulfur content in fuel oil samples were 0.24%, 0.28%. In addition, fuel oil combustion ash in V, Na, s compounds. In the furnace tube, easy cause corrosion and scaling
At the same time, with domestic counterparts that, Sichuan Meifeng, NinGYang County in Shandong Province, such as melamine plant of molten salt furnace also occurred similar perforation, qualitative corrosion perforation.
Three, preventive measures
1, the regular detection of heavy oil in the S content, to ensure the quality of the fuel.
2, regular inspection of molten salt furnace, removal of furnace ash, wall thickness measurement, in order to find the problem in advance.
3, slow down the decomposition of NaNO2, and reduce the content of Na2O in molten salt by 5
The decomposition reaction equation of NaNO2 is as follows:
From the equations, we can see that the NaNO2 decomposition to generate N2, increasing the N2 partial pressure can effectively inhibit the NaNO2 decomposition, in order to increase of N2 partial pressure, of molten salt storage tank seal improvement, while increasing the N2 protected pipelines, the molten salt under N2 protection.
At the same time, the content of Na2O in the molten salt furnace was analyzed periodically. When the Na2O content reached a certain value, the molten salt was partially updated to reduce the Na2O content.
4, improve the operation
As far as possible to reduce the frequency of molten salt furnace stop feeding salt, so as to reduce the heat shock to the furnace tube, in addition, after the stop of the furnace is not immediately stop the molten salt pump and fan, in case of furnace cavity heat to accelerate the corrosion of molten salt furnace tube.
