Sharing of coil leakage and thinning incidents in melamine gas-fired salt furnaces, along with exploration of the causes

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This post was last edited by vivian0330 on 2020-9-27 08:44. The 1400×104 kcal/h molten salt furnace produced by a certain company was put into use after passing the commissioning tests in September 2018. It was not until August 7, 2020, that salt leakage occurred while heating the molten salt. During that period, due to market conditions in the melamine sector, the molten salt furnace operated intermittently for a total of about 120 days; the remaining time was used solely for maintaining the temperature of the molten salt, with the furnace being turned on and off every seven days, in strict accordance with the operating procedures for such furnaces. Inspections revealed that the salt leakage was caused by cracks at two welds on the furnace top branch pipes. Thickness measurements of the coils inside the furnace showed that the wall thickness at all the elbows of the upper branch pipes was only about 2.5 mm; the thinnest part of the inner coil’s wall thickness was 3.14 mm, with an average wall thickness of around 3.5 mm. The average wall thickness for the coils in the upper and lower sections was also around 4.3 mm. The design parameters for this coil are 127*5, with the material being 12Cr1MoVG. The process flow of the gas-fired molten salt furnace system is as follows: The molten salt at 400–410°C, originating from the molten salt storage tank and composed of 53% KNO3, 7% NaNO3, and 40% NaNO2, is heated to 440–450°C in a molten salt heater before being fed into the reactor to provide the heat required for the melamine reaction. The burner at the bottom of the molten salt furnace burns natural gas to provide heat energy. Working pressure: 1.5 Mpa. One question? ? ? ? The figure above shows the wall thickness measurements of the upper, middle, and lower coils after the leak was detected; the thinnest part in the middle is 3.5 mm. This type of pipe is widely used in the triamine industry. Why then did its thickness decrease to such an extent in less than two years, while no similar thinning was observed in the pipes outside the furnace? Second question? ? ? Based on physical and chemical analyses, the composition of the material is generally satisfactory. However, preliminary analysis of the leak site in the elbow indicates that incomplete welding, edge misalignment, and erosion are the causes of the leakage. I would appreciate everyone’s insights on this matter. Three questions? ? ? Does the presence of chloride ions in the molten salt composition have an impact, and what is the mechanism? I’d like to hear the details – the four questions? ? ? Is it due to the corrosion of internal and external deposits, such as minor leaks of molten salt earlier on that then accumulated on the surface of the coils, and it wasn’t until the leak became more severe that inspections were carried out?