餘熱回收單元熱交換管件管壁減薄原因分析

Investigation of Wall Thinning for Heat Exchange Tubes in a Waste Heat Recovery Unit

本文針對石化工廠一座餘熱回收單元，在燃氣與媒油兩種流體對熱交換管材料皆無明顯腐蝕行為下，探討管壁仍發生減薄的原因。分析檢測項目包括：背景資料蒐集、目視觀察、顯微鏡微觀觀察、X-ray繞射分析及硬度量測。分析結果顯示：餘熱回收單元的熱交換管於進行熱交換時，管壁發生媒油結焦現象，而結焦物質逐漸增多時，將降低熱媒油的流動性，嚴重阻礙熱能的移除，當管內溫度超過熱媒油之自燃點，將引發燃燒產生更高溫的環境，進而使管壁表面發生局部熔解而減薄。此外，結焦的物質因管內熱媒油汽化或是流經管件形狀不規則處時發生嚴重擾流，將造成熱媒油伴隨結焦物質與石墨不斷沖蝕管壁，進而導致管壁發生減薄現象。

A waste heat recovery unit of petrochemical plant was utilized to heat up the heat transfer oil by hot flue gas. A significant wall thinning of the heat exchanger tubes was observed even though there is no obvious corrosion caused by heat transfer oil and flue gas. To comprehend the thinning causes of the tubes, the investigation works including collection of background information, visual inspection, microscopic analysis, X-ray diffraction and hardness measurement was performed. The analysis results revealed the heat transfer oil was thermally decomposed and changed into coke deposits on the internal surface of the heat exchange tubes. As a large amount of the coking particles formed and deposited on interior surface of tubes, the flowability of heat transfer oil was greatly diminished and the heat transfer efficiency was consequently deteriorated. The heat transfer oil would be ignited if tube temperature exceeded its ignition temperature. As a result, an overheating and even partial melting and wall thinning of tube material might occur. In addition, the countless tiny particles of coking substance and graphite would flow with vaporized heat transfer oil at high velocity and led to erosion and wall thinning at turbulent flow locations.