IJE TRANSACTIONS A: Basics Vol. 32, No. 1 (January 2019) 112-120    Article in Press

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G. H. Farrahi, M. Minaii, M. Chamani and A.H. Mahmoudi
( Received: November 20, 2018 – Accepted in Revised Form: January 03, 2019 )

Abstract    In a shell and tube heat exchanger, the failure of tube-to-tubesheet welds results in high-pressure water jet which erodes the refractory in front of the tubesheet. Finite element method was employed to simulate the welding process and post weld heat treatment (PWHT) to find the factors affecting the failure in tube-to-tubesheet weldments. Residual stresses in two different geometries of tube-to-tubesheet weldment were calculated through uncoupled thermal-structural analysis. The results showed that the values of residual stresses are higher in heat exchanger of site 1 than site 2 due to more weld passes and geometry of connection. Also, the maximum stress in site 1 occurs at the shellside face of tubesheet while it is on the weld toe in site 2. High tensile residual stresses, especially in Site 1, reduce the tubesheet life. Therefore, performing an efficient PWHT is vital. The PWHT simulation indicated that the process designed is effective for both sites by reducing the residual stress significantly. In addition, the effect of stress concentration was examined on both sites. Moreover, the stress concentration factor in site 1 is as twice as in site 2 and it is the main reason for more failures in site 1.


Keywords    Residual Stress, Post Weld Heat Treatment, Heat Exchanger, Stress Concentration Factor



در یک مبدل حرارتی پوسته و لوله خرابی در اتصال جوشی لوله به تیوب‏شیت منجر به ایجاد جت آب پرفشار می‏شود که سبب خرابی عایق مقابل تیوب شیت می‏گردد. جهت بررسی دلایل ایجاد این خرابی، اتصال جوشی لوله به تیوب شیت و عملیات حرارتی بعد از جوشکاری با استفاده از روش المان محدود مدلسازی شده است. با انجام تحلیل حرارتی-سازه‏ای غیرکوپل تنش‏های پسماند و اعوجاج لوله و تیوب شیت در دو مبدل حرارتی با هندسه اتصال لوله به تیوب شیت متفاوت محاسبه گردید. با بررسی و مقایسه تنش‎های پسماند در هر دو سایت مشخص شد که تنش پسماند در مبدل سایت 1 بیشتر از سایت 2 بوده که ناشی از تعداد پاس‏های بیشتر و هندسه اتصال است. همچنین بیشینه تنش در سایت 1 روی سطح تیوب شیت و در سمت لوله‎ها روی می‎دهد در حالی که در سایت 2 در محل گرده جوش بیشینه تنش مشاهده میشود. مقادیر بالای تنش پسماند کششی خصوصا در سایت 1 عمر تیوب شیت را کاهش می‎دهد، به همین جهت انجام یک عملیات حرارتی مناسب پس از جوش، امری اجتناب ناپذیر است. با شبیه سازی فرآیند عملیات حرارتی این نتیجه بدست آمد که عملیات حرارتی طراحی شده برای هر دو سایت بطور موثر عمل می‎کند و میزان تنش‎ها را بطور قابل ملاحظه‎ای کاهش می‎دهد. علاوه بر این با بررسی اثر ضریب تمرکز تنش در سایت 1 و 2 این نتیجه حاصل شد که ضریب تمرکز تنش در سایت 1 تقریباً دو برابر سایت 2 است که یکی از دلایل بروز خرابی بیشتر در مبدل سایت 1 می‫باشد.‬‬‬‬‬‬‬‬‬‬‬‬


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