What to do if the alkylation pipeline is severely corroded? Thank you, I really need help
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Thinning was severe in the alkylation low-flow acid addition line; leaks occurred twice in the pump outlet pipeline in November 2016. Thickness testing was conducted on the pump outlet elbow on-site, with the thinnest area measuring 3 mm and the thickest area 4.5 mm. This section of pipeline was replaced as part of the 15-year major maintenance, with an annual corrosion rate of not less than 1 mm/year. Among them, the pump outlet buffer section is severely thinned. Cause analysis: (1) The low-flow acid addition pump is a diaphragm plunger pump; the flow of sulfuric acid in the acid addition line is unstable, and during the back-and-forth compression of the medium, some areas experience higher flow rates leading to more severe corrosion. The pump outlet buffer pipe section has become severely thinned due to repeated erosion by the medium. The sulfuric acid flow rate in the outlet pipeline of P6912AB is calculated to be approximately 0.43 m/s. According to authoritative NACE guidelines, when carbon steel is used with 98% concentrated sulfuric acid, the flow rate should be kept below 0.6 m/s at room temperature. Since the pump is a diaphragm plunger pump, it is possible that the actual local flow velocity is too high, leading to increased corrosion. (Ideal actual flow velocity can reach 0.8 m/s.) (2) The heat tracing for the new acid line is provided by hot water, and the high temperature, along with localized hot spots, can exacerbate corrosion. The appropriate operating temperature for 98% sulfuric acid is generally below 50°C; however, as the temperature rises, the corrosion rate of sulfuric acid increases significantly, and lowering the temperature can markedly reduce this corrosion rate.
An analysis has been conducted on the main causes of corrosion in the acid injection pipeline used in alkylation processes. This has been a persistent problem over the years as I’ve worked in this field; however, based on comparisons among different manufacturers, it is recommended that the following changes be made: first, replace the acid injection pump with a magnetic pump, as it is resistant to corrosion, does not leak, and ensures stable and controllable flow rates； II. Electric heating is used for heating, rather than hot water. Generally, to save costs, companies use hot water for heat tracing, but the temperature of hot water is difficult to control; when it is too high, it can accelerate the corrosion rate of the dense oxide film inside. When using electric heating for temperature control, be sure to install a temperature controller and a leakage protection device – remember this carefully.
Electric heating is used instead of hot water for heating; is this a practice adopted by manufacturing enterprises in the north, or is it necessary to use heating in certain stages of a production process? ?