Constant problems with cold spots and plugging led BP’s Texas City, Texas, plant to reengineer the thermal jacketing on its heat-traced sulfur pit vent line. The results include higher reliability and better environmental compliance with lower ownership and maintenance costs.The vent line is a critical part of the emission control system in the plant’s 1,100 tpd sulfur recovery operation. Powered by a steam eductor, the 200-ft.-long, 6-in.-diameter vent line draws sweeping air across the surface of the molten sulfur in the storage pit. That air entrains H2S and SOx vapors, airborne droplets of molten sulfur, and other particulates and carries them to an oxidizer.“With molten sulfur and its vapors, the trick is to keep the entire pipe wall temperature at 260°F or above,” says John Jondle, BP area operations supervisor. “Any decrease at any point along the pipeline can lead to condensation and dropout of H2S and SOx vapors. Such dropout impairs heat flow at that location, cooling it further and triggering still more dropout. In the vent pipe, this creates a snowball effect, leading to more localized buildup, choke points and eventual blockage that can ultimately halt refinery production.”
BP had experienced many such upsets with the previous enhanced tube tracing system, beginning very soon after startup. Enhanced tube tracing uses conductive mastic to enhance the heat path between the tube and the vent pipe. The mastic often pulls away from the main piping, causing chill spots. Crews can locate such spots with infrared thermometers and make the necessary repairs, but as the system aged, it had become an increasingly continuous Band-Aid process. The upgrade started with thermal analysis of the pipe-wall temperature profile along the vent pipe, rather than a bulk process temperature average. That way, chill spots and potential dropout areas could be addressed at the outset. The thermal analysis was done by Controls Southeast (CSI, www.csiheat.com), which also supplied the ControTrace bolt-on jacketing.The bolt-on recommendation came with a full thermal analysis that considered:* Insulation thickness and physical properties.* Piping size, wall thickness and physical properties.* Ambient site conditions including maximum wind velocities.* Process gas constituents, their properties and flow rates.By contrast, typical tube-trace sizing methods deal with only four criteria: ambient conditions, insulation thickness, pipe sizes and process temperatures. The number of tracers needed depends on the amount of bulk heat lost. How the heat transfers to all areas of the pipe is not considered. Therefore, simply adding more tracers to offset a calculated heat loss does not assure that all points on the pipe wall remain above the condensation temperature of the gas stream.The analysis led to a hybrid solution: 1. Heat the first 10 ft. with fully jacketed pipe to maintain 260°F at the inlet even when the pit is less full and pit air is at its coolest. 2. Heat the next 26 ft. with four-element bolt-on jacketing.3. Heat the remaining 165 ft. with two-element jacketing. 4. In the steam control system, provide independent and variable flow control to each section.5. Fit all valves and pumps in the line with bolt-on jacketing to eliminate possible chill zones.BP contractors applied heat mastic between the heating elements and the pipe and valves before bolting the elements in place. To facilitate inspection and maintenance, the layout of heating elements left all pipe welds exposed for easy viewing.The new bolt-on thermal maintenance system has performed as predicted for more than five years. There have been no chill spots, no plugging problems and no loss of mastic. The bolt-on elements have maintained thermal contact with the vent piping under all conditions, with negligible maintenance and inspection.
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