省煤器换热管开裂失效分析

Failure Analysis on Cracking of Heat Exchange Pipe in Economizer

某化工企业一省煤器的底部、管程低温进水端的不锈钢换热管管束外壁翅片根部出现裂纹,部分裂纹穿透管壁导致泄漏。通过裂纹断口宏观形貌和SEM形貌分析、金相分析以及介质分析等手段研究泄漏发生原因。试验分析结果表明:裂纹起源于管束外壁翅片根部,为应力腐蚀开裂,腐蚀介质源于壳程烟气含有氯和硫的露点凝液;开裂位置多为烟气流通的死角处,烟气在这些位置几乎处于静止状态,且省煤器装置的进水温度并非恒温,开停车阶段会有低温水流入管程,这些都加剧了烟气露点凝液的形成。进一步分析计算了烟气中硫的最大体积分数为0.01%的工况下的酸露点,并指出应在管程上游位置添加一处恒温装置以避免低温水的流入;其次建议更改省煤器的结构,尽量避免出现烟气流通死角。

In a chemical enterprise, at the bottom of the economizer and the low temperature water inlet end of pipes side, cracks appeared at the root of the outer wall fins of the tube bundle of stainless steel heat exchange pipe, some of which penetrated the pipe wall and led to leakage. The causes of leakage were investigated by means of crack fracture macroscopic and SEM analysis, metallographic analysis, and media analysis. Experimental results showed that cracks were originated from the root of the outer wall fins of the tube bundle, which was stress corrosion cracking(SCC). The corrosion medium was the dew point condensate containing chlorine and sulfur of the flue gas in the shell side of the economizer. Most cracks were located at the dead corner of circulation of flue gas, where the flue gas was almost stationary. Moreover, the inlet temperature of the economizer device was not constant. During startup and shutdown stage, there would be low temperature water flowing into the pipe of the device. All the above aggravated the formation of dew point condensate in the flue gas. Furthermore, the acid dew point was analyzed and calculated under the condition that the maximum volume fraction of sulfur in the flue gas was 0.01%. Also, it was pointed out that a thermostatic device should be added at the upstream of the tube side to avoid the inflow of low temperature water;and the structure of the economizer was suggested to be changed to avoid the occurrence of dead corner of circulation of flue gas as far as possible.