鍋爐水牆管噴焊可行性評估

Abstract

國內某燃煤電廠之鍋爐水牆管於2007年4月歲修檢查時發現管壁厚度已不足，但是因為未達全檢週期，所以採用焊補的方式加以維護，不過此截焊補過的水牆管卻於2008年7月時發生破管的情形，從該破損爐管經斷面之觀察、硬度量測、及顯微鏡金相分析結果顯示；水牆管的破管原因是肇因於此管發生應力腐蝕破裂，應力可能源於焊補的殘留應力，所以最初在焊補之起焊端或是止焊端的內管壁產生了一些小裂縫，待這些小裂縫受到管內水壓、焊補的殘留應力以及腐蝕因子的綜合作用而繼續成長之後，裂縫最後產生了串聯而貫通管壁，於是管內高壓水噴出產生了高壓水柱，進而造成後續的其他破孔發生； 本文希望透過相關的測試與分析來釐清破管的發生原因，並利用高速火焰的熔射加工方式，將WC/Co 噴覆在鍋爐水牆管表面，並實際進行現場實況測試，三年後比較噴覆層厚度，作為日後評估鍋爐爐管全面噴焊可行性。

During the annual inspection in April 2007, one coal-fired power plant in Taiwan discovered that the thickness of the broiler’s tube wall was under the borderline. Because it was not the time yet for an overall inspection, this problem was handled by welding. Later on in July 2008, this welded waterwall tube ruptured. The section of the broken tube was examined by observing the cross-section of the broken tube, testing the rigidity, and performing a microscopic metallographic analysis. The results suggested that this matter may be stress and corrosion induced rupture, and the stress here may be the residual stress from wielding. It was suggested that some cracks were formed at the internal wall of the starting or stopping end of welding. Once these cracks received water pressure inside the tube, the residual stress from welding and corrosion factors acted together and grew bigger. At the end, cracks were formed throughout the tube wall, causing the high pressure water inside the tube to spout and create more rupture. The objective of this study is to clarify causes for tube rupture using relevant tests and analyses. High velocity oxy fuel will be used to coat the surface of the wall of broiler tubes with WC/Co. This treatment will be followed by onsite testing. Three years later, the thickness of the sprayed surface will be compared for assessing the feasibility of giving broiler tubes comprehensive welding in the future.