35t／h 鍋爐熱管回收排煙余熱系統(tǒng)的設(shè)計(jì)【含11張CAD圖紙+文檔全套】

喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請放心下載，，有疑問咨詢QQ:414951605或者1304139763 ======================== 喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請放心下載，，有疑問咨詢QQ:414951605或者1304139763 ======================== 京 江 學(xué) 院 JINGJIANG COLLEGE OF J I A N G S U U N I V E R S I T Y 本 科 畢 業(yè) 設(shè) 計(jì) 調(diào) 研 報(bào) 告 學(xué)生學(xué)號： 3101105052 學(xué)生姓名： 孫俊 專業(yè)班級： J動力熱能1002 指導(dǎo)教師姓名： 王助良 指導(dǎo)教師職稱： 教授 2014年3月 余熱回收的項(xiàng)目的調(diào)研報(bào)告 摘要：這次調(diào)研的主要途徑是上圖書館查閱資料，上網(wǎng)了解現(xiàn)狀，咨詢老師?？偨Y(jié)了余熱回收的利用，余熱回收的現(xiàn)狀。本次設(shè)計(jì)主要是利用熱管來回收余熱的，本次調(diào)研還對熱管進(jìn)行了簡單的調(diào)查，分析了熱管的分類，原理和利用前景 關(guān)鍵詞：余熱回收 回收的工藝 熱管的利用 前言：眾所周知，能源是關(guān)系國計(jì)民生、與日常生活息息相關(guān)的大問題。在我國能源利用的方法和方式還是比較單一的，以熱能為主。風(fēng)能，太陽能，核能等只有在局部地區(qū)使用。改個開放三十多年，經(jīng)濟(jì)飛速增長，對能源的需求也在飛速增長。一邊是能源供需緊張一邊卻是工業(yè)余熱大量過剩及利用效率底下。各行各業(yè)中余熱資源占其燃燒消耗總量的16%到67%，可利用的余熱資源約為余熱總資源的60%。特別是在傳統(tǒng)的工業(yè)鍋爐中，排煙溫度每降低15℃鍋爐效率就提高1%。本次課程設(shè)計(jì)的主要內(nèi)容是通過煙氣余熱來加熱熱水，根據(jù)鍋爐企業(yè)給定的參數(shù)，結(jié)合書本知識設(shè)計(jì)出一款合格的余熱回收裝置。本次設(shè)計(jì)主要是利用熱管進(jìn)行回收余熱的。設(shè)計(jì)過程中重點(diǎn)分析了熱管的排數(shù)與結(jié)構(gòu)參數(shù)以及熱管的換熱系數(shù)，流動阻力等參數(shù)。經(jīng)過這次設(shè)計(jì)使我們的理論知識結(jié)合到實(shí)踐中去了，培養(yǎng)了我們獨(dú)立設(shè)計(jì)的能力。本次設(shè)計(jì)促使我們掌握了換熱設(shè)備基本結(jié)構(gòu)和換熱原理。促使我具備了分析數(shù)據(jù)的能力，并且使我們熟悉CAD等畫圖軟件。 1.余熱回收的利用 我國政府計(jì)劃到2020年將碳排放量減少40%-50% ，目前面臨著巨大的壓力。政府在推行各種節(jié)能減排的政策，其中余熱回收作為提高能源利用效率的主要途徑，目前國家出臺多項(xiàng)政策鼓勵企業(yè)回收余熱。政策一出，必然有許多企業(yè)響應(yīng)。 1.1 燒結(jié)余熱回收 通過對冷卻車罩子、落礦斗、冷卻風(fēng)機(jī)進(jìn)行部分改造，使廢氣溫度可提升到360℃到420℃之間，余熱鍋爐將產(chǎn)生1.25-2.45Mpa、330℃的過熱蒸汽，可以確保冷卻機(jī)余熱的充分利用。燒結(jié)余熱發(fā)電的優(yōu)點(diǎn)在于其不僅能夠回收余熱，同時也解決了余熱蒸汽的利用問題。另外鍋爐產(chǎn)生的過熱蒸汽為工廠帶了明顯的經(jīng)濟(jì)效益和社會效益，根據(jù)冷卻機(jī)的不同使用情況，余熱回收系統(tǒng)可分為但壓、雙壓、符合閃蒸等系統(tǒng)[4]。 在鋼鐵企業(yè)中，燒結(jié)工序耗能占總耗能的10%左右，僅次于煉鐵。所以燒結(jié)余熱回收具有很重要的意義。降低燒結(jié)工序能耗，節(jié)約資源，降低單位產(chǎn)值能耗，增加企業(yè)效益。減少火電廠排放的CO2、SO2、NOX、粉塵等大氣污染物，有助于改善當(dāng)?shù)氐哪茉唇Y(jié)構(gòu)。 1.2硫酸工業(yè)煙氣余熱回收 硫酸在生產(chǎn)過程中，位于焚硫爐出口的中壓余熱鍋所產(chǎn)生的9.6 t/h中壓蒸汽70%用于汽輪機(jī)拖動主鼓風(fēng)機(jī)，其余蒸汽經(jīng)減溫減壓用于硫磺儲罐，伴熱保溫以及送往低壓蒸汽蒸汽管網(wǎng)，位于轉(zhuǎn)化四段出口的抵押鍋爐產(chǎn)出的4t/h低壓蒸汽全部送到低壓蒸汽管網(wǎng)，以供化工廠生產(chǎn)和冬季采暖。全部利用余熱生產(chǎn)蒸汽就能滿足全場化工生產(chǎn)用氣并且無需啟動燃煤鍋爐。 1.3鍋爐煙氣余熱回收 鍋爐的主要余熱量是鍋爐的尾部煙氣，一般鍋爐都會在煙道的最后安裝省煤器和空氣預(yù)熱器，這兩種受熱面的煙氣溫度最低，所以稱他們?yōu)槲膊渴軣崦?。用于進(jìn)一步降低煙氣溫度，提高鍋爐效率 按水在省煤器中被加熱的程度，省煤器可分為非沸騰式和沸騰式兩類，省煤器出口水溫低于飽和溫度的叫做非沸騰式省煤器；在省煤器出口處水已經(jīng)被加熱到飽和溫度并產(chǎn)生部分蒸汽的叫做沸騰式省煤器。在中壓鍋爐中水的汽化吸熱量占的比例較大，為了提高鍋爐內(nèi)熱強(qiáng)度，需要提高給水溫度，所以采用沸騰式。隨著鍋爐壓力的提高，蒸發(fā)吸熱量比例減小，考慮到在汽包內(nèi)蒸汽的品質(zhì)，因此在高壓，超高壓和亞臨界壓力的鍋爐都采用非沸騰式省煤器，且具有一定的欠熱。應(yīng)用省煤器的目的有兩個，一 減少蒸發(fā)受熱面，以價(jià)格低廉的省煤器受熱面替代價(jià)格高昂的爐膛受熱面；二 給水進(jìn)過省煤器加熱后，溫度接近或達(dá)到汽包內(nèi)水的溫度，改善汽包的工作環(huán)境，延長汽包壽命。 電站鍋爐廣泛采用的空氣預(yù)熱器有管式和回轉(zhuǎn)式兩種，在中小型鍋爐中普遍采用管式空氣預(yù)熱器?；剞D(zhuǎn)式空氣預(yù)熱器與管式空氣預(yù)熱器相比，有以下優(yōu)點(diǎn)：1 結(jié)構(gòu)緊湊；2 節(jié)省鋼材；3 耐腐蝕性能好；4 受熱面受到磨損、腐蝕時，不曾加空氣預(yù)熱器的漏風(fēng)量，且為組裝受熱面，更換方便?？諝忸A(yù)熱器不僅能降低排煙溫度，提高鍋爐效率，且經(jīng)過預(yù)熱的空氣進(jìn)行燃燒能改善燃料的著火和燃燒條件，減少燃料的不完全燃燒熱損失，進(jìn)一步提高效率。這對于著火難的燃料尤為重要?？諝忸A(yù)熱器已經(jīng)成為現(xiàn)代鍋爐的重要受熱面。 2.換熱器 2.1 換熱器分類 近代尖端科技的發(fā)展（如高溫高壓、低溫、超低溫等），促使了高強(qiáng)度、高效率的緊湊型換熱器層出不窮。雖然如此，所有的換熱器仍按照他們的一些共同特征來加以區(qū)分[1]。 按照用途來分：預(yù)熱器、冷卻器、冷凝器、蒸發(fā)器等 按照制造換熱器的材料來分：金屬的、陶瓷的、塑料的、石墨的、玻璃的等。 按照冷熱流體的流動方向來分：順流式（并流式）：兩種流體平行的想著同一方向流動；逆流式：冷熱流體平行流動，但是他們的流動方向相反；混流式：冷熱流體既有順流部分，又有逆流部分。 按照傳送熱量的方式來分：間壁式、混合式、蓄熱式等三大類，這是換熱器的一種最主要的分類方式。 間壁式：冷熱流體之間有一固體壁面，冷熱流體分布在壁面的兩側(cè)，兩種流體不接觸，熱量通過壁面?zhèn)鬟f。 蓄熱式：這種換熱器也有固體壁面，當(dāng)熱流體流過時，把熱量儲存于壁內(nèi)，壁的溫度升高；當(dāng)冷流體流過時，壁面放出熱量，壁的溫度逐漸降低，如此往復(fù)進(jìn)行，已達(dá)到熱交換的目的。 混合式：這種熱交換內(nèi)依靠熱流體與冷流體的直接接觸進(jìn)行傳熱。 2.2 實(shí)際的換熱器 管殼式換熱器是目前應(yīng)用最廣的類型，管殼式換熱器又稱列管式換熱器。是以封閉在殼體中管束的壁面作為傳熱面的間壁式換熱器。這種換熱器結(jié)構(gòu)簡單，操作可靠，可以用各種結(jié)構(gòu)材料制造，能在高溫、高壓下使用，所以應(yīng)用的比較廣。 混合式熱交換器如圖1是依靠冷熱流體直接接觸進(jìn)行換熱的，這種方式避免了傳熱間壁以及其兩側(cè)污垢所形成的熱阻，只要流體間的接觸良好，就有較大的傳熱速率。 圖1 管殼式換熱器 混合式熱交換器的有點(diǎn)是結(jié)構(gòu)簡單、消耗材料少、接觸面大，并因直接接觸而是熱量利用的比較完全，因此它的應(yīng)用日漸廣泛。 目前用的比較多的混合式換熱器如圖2有：1 水冷塔 在這種設(shè)備中，利用自然通風(fēng)或機(jī)械通風(fēng)的方法，用空氣將熱水進(jìn)行冷卻降溫；2 氣體洗滌塔 工業(yè)上用這種設(shè)備來洗滌氣體有各種目的，例如氣體的除塵，增濕、干燥等。；3 噴射式換熱器 在這種設(shè)備中，使壓力較高的流體由噴管噴出，形成很高的速度，低壓流體被引入混合室與射流直接接觸進(jìn)行傳熱，并一同進(jìn)入擴(kuò)散管，在擴(kuò)散管的出口達(dá)到同一壓力和溫度后送給用戶。 圖2 混合式換熱器 在蓄熱式換熱器中，冷熱流體交替地流過同一固體傳熱面如圖3及其所形成的通道，依靠成傳熱面的物體的熱容作用，實(shí)現(xiàn)冷熱流體之間的熱交換。與間壁式相比，雖然都需要固體傳熱面，但是在間壁式中，熱量是在同一時刻通過固體壁面由一側(cè)的熱流體傳遞給另一側(cè)的冷流體。與混合式相比差別更大，混合式換熱器是通過冷熱流體直接接觸進(jìn)行換熱的和蓄熱式完全不同。 蓄熱式熱交換器常用于流量大的氣-氣熱交換場合，如動力、硅酸鹽、石油化工等工業(yè)中的余熱利用和廢熱回收等方面。 圖3 蓄熱式換熱器 3.熱管式換熱器 本次設(shè)計(jì)的換熱器主要利用的就是熱管式換熱器。熱管式換熱器作為一種新型的高效熱交換器，其應(yīng)用已從20世紀(jì)60年代末用于宇航的熱控制，擴(kuò)展到近期的余熱回收、電子工業(yè)、新能源、化學(xué)工程、石油化工及核電工程等領(lǐng)域。 3.1 熱管的結(jié)構(gòu) 普通的熱管從軸向看一般分為三段，蒸發(fā)段、冷凝段和絕熱段，如圖3-1所示。有時也把熱管的蒸發(fā)段叫做蒸發(fā)器，冷凝段叫做凝結(jié)器，絕熱段稱為傳送段。絕熱段作為流體的通道，把熱源和冷源分隔開。熱管的絕熱段長度可以很長也可以很短；其材料也可以自由選擇[1]。這樣使熱管能適應(yīng)布置任意需要的幾何形狀。但是，工業(yè)上節(jié)能熱管和余熱利用的熱管的絕熱段是比較短的，只是管殼的一小段。就是這很短的一段絕熱段仍然能把加熱段與冷凝段分隔開，而且在此處還得把通過加熱段的熱流體和冷凝段的冷流體分隔開來。所以，熱管的密封環(huán)都放在絕熱段里。 熱管內(nèi)部從徑向看也可以分為三個部分，管殼、吸液芯和工作液（氣）。對一般的熱管來說，熱流體從熱源到熱管總要先經(jīng)過管殼。如果把熱管傳送能量的過程假設(shè)為一條流水線的話，那么這條熱流線的途徑一定是先經(jīng)過管殼、吸液芯、蒸汽空間，然后再有蒸汽空間經(jīng)過吸液芯和管殼離開熱管。 圖4 熱管的工作原理圖 3.2 熱管的原理 熱管之所以能夠進(jìn)行高效的傳遞熱能，成為一個高效的換熱元件，原理在于熱管利用的是傳熱學(xué)中相變潛熱的吸收和釋放兩個最強(qiáng)烈的過程。熱管工作時利用相變吸收、放出潛熱和毛細(xì)泵或者重力作用下工作流體的循環(huán)，進(jìn)行輸送熱能，這就是熱管工作基本原理的核心如圖3。 熱管在運(yùn)行過程中，加熱段流體包含了大量的熱能，這些熱能在傳遞給工作液時是通過工作液的相變進(jìn)行的。工作液由液態(tài)逐漸吸熱，當(dāng)吸入一定量的熱能時就變成氣態(tài)。工作液在相變的過程中吸收大量的熱量。隨后，工作液把熱量帶到冷凝段工作液遇冷液化放出大量的潛熱。工作液由毛細(xì)泵吸收的蒸發(fā)段（重力熱管沒有吸液芯，是通過重力作用循環(huán)工作液的本次設(shè)計(jì)使用的是重力熱管）。 3.3 熱管的應(yīng)用和前景 在電子方面的應(yīng)用：晶體管散熱 今年來，大功率電子器件的冷卻都采用了熱管，而且受到比較好的效果。體積很小但是可以傳遞大量的熱量，現(xiàn)在的電子器件做的越來越小但是功率也是有增無減的。大功率小體積的電子器件安裝風(fēng)扇已經(jīng)不能滿足要求，所以熱管就在電子行業(yè)廣泛使用。 生產(chǎn)方面的應(yīng)用：熱管空氣預(yù)熱器 在生產(chǎn)中使用熱管空氣預(yù)熱器已經(jīng)收到了較好的預(yù)熱效果，熱管式空氣預(yù)熱器由管束，外殼和隔板組成。隔板用于將蒸發(fā)段和冷凝段隔開，是冷熱流體分別流過冷凝段和蒸發(fā)段。熱管的外壁加有翅片，以加強(qiáng)氣側(cè)的換熱效果。 科技方面的應(yīng)用：超音速熱管機(jī)翼 利用熱管可將超英速航天飛行器機(jī)翼的前沿進(jìn)行局部降溫。機(jī)翼的前沿相當(dāng)于熱管的蒸發(fā)段，前沿后側(cè)相當(dāng)于熱管的冷凝段。高速飛行時，由于空氣的快速摩擦導(dǎo)致機(jī)翼的前沿局部溫度很高，在該位置安放熱管的蒸發(fā)段將熱量轉(zhuǎn)移到冷凝段也就是機(jī)翼的后沿，這樣就起到了均溫的作用。 4.總結(jié) 進(jìn)過調(diào)查和參考文獻(xiàn)分析發(fā)現(xiàn)，熱管是當(dāng)今社會主流熱傳遞工具，高效、節(jié)能環(huán)保、使用壽命比較長由于工作液不與外界接觸不容易損壞。鍋爐煙氣余熱存在浪費(fèi)現(xiàn)象，響應(yīng)國家政策，余熱回收勢在必行。 參考文獻(xiàn)： [1] 史美中,王中錚.熱交換器原理與設(shè)計(jì).南京.東南大學(xué)出版社，2009. [2] 容鑾恩，袁鎮(zhèn)福，劉志敏，田子平.電站鍋爐.北京.中國電力出版社，1997. [3] 方彬，白文彬. 鍋爐與窯爐節(jié)能用熱管換熱器.哈爾濱.哈爾濱工業(yè)大學(xué)出版社，1985. [4] 周耘、王康、陳思明.工業(yè)余熱利用現(xiàn)狀及技術(shù)展望.無錫.科技情報(bào)開發(fā)與經(jīng)濟(jì)，2010. 江蘇大學(xué)京江學(xué)院 畢業(yè)設(shè)計(jì)（論文）任務(wù)書 能源與動力工程 學(xué)院 班級 電廠熱能1002 學(xué)生 孫俊 設(shè)計(jì)（論文）題目 35t/h鍋爐熱管回收排煙余熱系統(tǒng)的設(shè)計(jì) 課題來源 自選 起訖日期2014 年 2 月 24 日至 2014 年 6 月10日共 16周 指導(dǎo)老師（簽名） 王 助 良 教研室主任（簽名） 設(shè)計(jì)依據(jù)： 鍋爐煙氣排放溫度高于酸露點(diǎn)，還可以利用其余熱。設(shè)計(jì)余熱回收系統(tǒng)，保證管壁溫度在高于酸露點(diǎn)溫度下，最大幅度回收煙氣余熱。 設(shè)計(jì)參數(shù) 1. 燃煤鍋爐蒸發(fā)量 35t/h 2. 排煙溫度 160℃ 3. 酸露點(diǎn) 100℃ 4. 余熱生產(chǎn)熱水溫度 85℃ 任務(wù)要求： 1. 熟讀鍋爐原理與計(jì)算、鍋爐結(jié)構(gòu)與設(shè)計(jì)、鍋爐手冊、換熱器原理等方面的書籍,了解我國鍋爐的發(fā)展，掌握相關(guān)設(shè)計(jì)手冊的使用和設(shè)備的選型； 2. 調(diào)研報(bào)告和開題報(bào)告各一份（不少于5000字）； 3.相關(guān)外文資料翻譯一份（不少于15000英文單詞）； 4.畢業(yè)設(shè)計(jì)論文一份； 5.設(shè)計(jì)圖紙符合畢業(yè)設(shè)計(jì)任務(wù)要求（0號圖紙3張）。 畢業(yè)設(shè)計(jì)（論文）進(jìn)度計(jì)劃： 起訖日期 工作內(nèi)容 備注 1～2周 3～4周 5～9周 10～15周 16周 查閱資料，調(diào)研，翻譯英文資料； 完成開題報(bào)告、讀書調(diào)研報(bào)告和英文文獻(xiàn)翻譯； 課題方案的論證，初步設(shè)計(jì)計(jì)算； 設(shè)備選型，完成設(shè)計(jì)說明書和圖紙； 畢業(yè)設(shè)計(jì)答辯。 Thermal constraint considerations in design of a heatrecovery boilerR. Caligiuri, J. Foulds*, R. Sire, S. AndrewExponent Failure Analysis Associates, 149 Commonwealth Drive, Menlo Park, CA 94025, USAReceived 23 September 2005; accepted 8 October 2005Available online 9 February 2006AbstractHeat recovery boilers are typically designed to conform to the rules of the ASME Boiler and Pressure Vessel Code, Sec-tion I on Power Boilers. While the unfired pressure vessel design rules of Section VIII of the ASME Code have evolved toaccommodate a wide range of vessels and loading conditions (including system thermal loads), Section I of the Coderemains characteristically simple, specifying explicit rules only for primary loading (pressure mainly). By and large, thecombination of boilermaker experience and Section I rules makes for robust boilers. However, departure from conven-tional design, although in conformance to the explicit rules of Section I, can result in premature failures. This paperdescribes one such design of a heat recovery boiler in a hardwood Kraft pulp mill, wherein system thermal stresses inducedon superheater tube-to-tube tie welds were high enough to result in weld toe cracking and early tube leaks. A finite elementstress analysis of the design showed that operational thermal stresses due to constraint imposed by the tie welds were inexcess of the tube material yield strength and above a level that would be considered acceptable for design. The analysishighlights the need for considering effects of thermal constraint in designs that otherwise meet the basic Code requirementsfor boilers.? 2005 Elsevier Ltd. All rights reserved.Keywords: Heat recovery boiler; Tie weld; Superheater; Thermal constraint; Design1. Introduction and backgroundHeat recovery boilers are commonly used in a variety of processes involving combustion and production ofexhaust heat. While gas turbine-based combined cycle power plants represent a relatively recent, popularapplication of heat recovery boilers (commonly referred to as heat recovery steam generators or HRSGs),other chemical and industrial processes having heat recovery boilers to produce steam and/or electric powerhave been in existence for well over 50 years. Kraft pulp mills comprise one such industrial process utilizingheat recovery boilers. A Kraft pulp mill heat recovery boiler is the subject of this paper, although the lessonslearned apply to boilers in any application.1350-6307/$ - see front matter ? 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfailanal.2005.10.003*Corresponding author. Tel.: +1 650 326 9400/688 7226; fax: +1 650 326 8072/328 2990.E-mail address: (J. Foulds) Failure Analysis 13 (2006) 13881396Fig. 1 is a schematic of the heat recovery boiler used to recover heat generated in the burning of blackliquor, a part of the hardwood Kraft pulping process. Of particular interest to this study is the pendant super-heater, consisting of three banks as identified the low, intermediate, and high temperature sections. The boi-ler comprises many such repeat units or elements, all connected to common headers. For size perspective, theintermediate section is about 43 feet (13.1 m) tall.Heat transfer is counter-flow for the low and intermediate sections and parallel-flow for the high temper-ature section. Each section consists of four tubes arranged in serpentine loop fashion from an inlet to an outletheader. Fig. 2 is a schematic of the tube arrangement for the intermediate section. Also seen in the schematicare horizontal lines to indicate locations of tube-to-tube tie welds.Within 18 months of the start of operation, leaks developed in the superheater tubes. Inspections followingboiler shutdown revealed that tube leaks had occurred from cracks that had initiated at the toes of tube-to-tube tie welds and propagated into the tube base material, breaching the tube wall. The intermediate and hightemperature sections exhibited cracking. Fig. 3 shows macrophotographs of typical tie welds after cleaningand a metallographic cross-section showing tube cracking at a tie weld.2. Design and performance specificationsThe pendant superheater was designed per ASME Boiler and Pressure Vessel Code, Section I: PowerBoilers 1 (ASME I). Nominal performance specifications on the superheater were as follows: inlet steam(to the low temperature section) at 712 psig (4.9 MPa), 505 ?F (263 ?C)1and outlet steam (from the hightemperature section) at 630 psig (4.3 MPa), 752 ?F (400 ?C). Steam temperature gradients were nominallyFig. 1. Schematic of heat recovery boiler showing the pendant superheater that is the focus of this study.1US customary units were specified in the design and were used in all analyses; SI units are reported in parentheses.R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 13881396138983 ?F (46 ?C), 115 ?F (64 ?C), and 117 ?F (65 ?C) across the low, intermediate, and high temperature sec-tions, respectively, with a steam attemperation stage in the crossover between the intermediate and hightemperature sections.Most of the pendant superheater tubes had a nominal outside diameter of 2 in. (50.8 mm). Tube wall thick-ness varied, being 0.165 in. (4.2 mm), 0.180 in. (4.6 mm), or 0.220 in. (5.6 mm). Tube materials were, perASME I, SA-192 (CMn steel), SA-209 (MnMo steel), and SA213 (T11) (CrMo steel). The 3 in.(76 mm) long tube-to-tube tie welds were shop-produced gas metal arc welds (GMAW). Reported design tem-perature in all cases was 800 ?F (427 ?C). For SA-192, the lowest-strength material at design temperature, andthe one representing the majority of tubes, the minimum yield strength ranged from about 26 ksi (179 MPa) atroom temperature to about 14.5 ksi (100 MPa) at 800 ?F (427 ?C). The applicable ASME I-allowable stress at800 ?F (427 ?C) was 9 ksi (62 MPa).3. Finite element stress analysesPrior to performing a detailed three-dimensional (3D) finite element (FE) stress analysis, a relatively simpletwo-dimensional (2D) elastic analysis was conducted to quantify the effect of pressure and thermal gradientacross the tube wall. The 2D analysis naturally could not provide any indication of the effect of thermal gra-dients along the length of individual tubes. The stress analyses results and conclusions made here are for theintermediate and high temperature sections.Fig. 2. Schematic of intermediate superheater section showing tube arrangement; horizontal lines indicate tie-weld locations.1390R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 138813963.1. Local 2D analysisFig. 4 illustrates the FE model used for both heat transfer and stress analysis. The nominal tube dimensionsof 2 in. (50.8 mm) outside diameter and 0.165 in. (4.2 mm) wall thickness were used in the analysis. A 2D planestrain analysis was performed using the ANSYS program 2. The assumed conditions were: internal steamFig. 3. (a) Photograph of tie welds (after cleaning) and (b) metallographic cross-section showing tube cracking at tie weld (arrows).Fig. 4. 2D FE model of tube-tie weld.R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 138813961391pressure of 630 psig (4.3 MPa), steam temperature of 700 ?F (371 ?C), gas temperature of 1425 ?F (774 ?C),convective heat transfer coefficients of 1.93E-05 BTU/s/in2/?F (0.057 kJ/s/m2/?C), and 4.07E-04 BTU/s/in2/?F (1.2 kJ/s/m2/?C) for the gas and steam side, respectively. The coefficients were computed using the methodsin Ref. 3, with inputs for fluid velocities from plant performance specifications.The analysis showed metal temperature varying between 721 and 739 ?F (383 and 393 ?C) with a max-imum through-wall gradient of 5 ?F (3 ?C). The stress analysis results showed stress due to pressure atthe tubetie-weld interface was only about 4 ksi (27.6 MPa), well below the ASME I allowable of 9 ksi(62 MPa) at 800 ?F (427 ?C). The through-wall gradient was such that resulting stresses at the tube-weldinterface were compressive (due to higher tube outside surface temperature). In the worst case, where cooldown could produce tensile stresses at the outer tube surface, the contribution from the thermal gradientwas estimated at no more than 0.5 ksi (3.4 MPa). In effect, the pressure and tube wall thermal gradient stres-ses were determined to be low and of no design concern. This finding is not surprising since boiler tubedesigns conforming to the ASME I requirements are expected to have acceptable pressure stresses (bythe very intent of ASME I), and thermal gradients across tube walls even in high heat flux regions of boilershave not generally been a source of integrity problems. What is of particular interest here is the effect ofthermal gradients along individual tubes that are tied together and configured in such a manner that thelongitudinal differential expansion between adjacent segments causes high stresses at the tie welds. The3D analysis sought to quantify this effect.3.2. Global 3D analysisA global 3D analysis was used to obtain relative stress levels throughout the superheater to identify tie-weldlocations exhibiting the highest global stresses. Again, the ANSYS program 2 was used. Refer to Fig. 2 for aschematic of the global model. The model is composed primarily of pipe and beam elements. The tubes aremodeled using ANSYS pipe element Type 16, a specialized beam element adapted to model tubular geome-tries. The U-bends were modeled using ANSYS curved pipe element, Type 18, and the tie welds using beamelement, Type 4. Typical weld dimensions were used in computing sectional properties (area and moment ofinertia) of the beams. At the top, truss bar elements, Type 8, were used to simulate the boiler ceiling penetra-tion, and were assigned temperatures to permit free expansion of the model. While the elements used here arecapable of describing out-of-plane loading, this form of loading was considered minor and excluded in theanalysis, for practical reasons.Global model loading included pressure, dead weight, and a linear thermal gradient along each tube withmetal temperature at inlet and outlet equal to the steam temperature of the performance specification. Theserpentine tube arrangement (Fig. 2) results in varying temperature differentials between adjacent tubes,depending on location. The outer loop locations expectedly experienced the most severe tube-to-tube gradi-ents, and the analysis captured this effect.The outputs of primary interest were the forces and moments on the tie welds. These forces and moments atthe tie welds were then used as boundary conditions on a local 3D model to obtain detailed stresses around thetie welds.3.3. Local 3D analysisA local 3D analysis (also using the ANSYS program 2) was used to obtain the through-wall stress distri-bution in the tubes at the tie-weld locations. Hoop stress was used as a severity metric since the predominantcracking orientation was normal to the hoop direction.A generalized method was used to make the local analysis results useful in computing stresses at any of thetie-weld locations without having to perform repeated FE analyses for each location. The forces and momentsfrom the global 3D analysis were applied to the three-tube model using 24 generalized degrees of freedom asshown schematically in Fig. 5.The generalized method required analysis for each of the degrees of freedom independently, performed byapplication of a unit displacement at the degree of freedom while constraining displacements in all other gen-eralized degrees of freedom.1392R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 13881396This suitably accounts for reaction forces, so that the result of 24 analyses provides the interdependencebetween forces and displacements per:F K ? U1where F is the 24 1 matrix of generalized force; K is the 24 24 stiffness matrix constructed from the reactionforces of each degree of freedom analysis; and U is the 24 1 matrix of generalized displacements.Eq. (1) may be rewritten as:U K?1? F2defining the individual contributions of each degree of freedom to the applied loading.Fig. 6 shows the local FE model used to analyze the 24 generalized degrees of freedom results. The FEmodel is seen to physically represent only one-eighth of the three-tube geometry shown in Fig. 5. To reducemesh size and enhance computational efficiency, initially a quarter-symmetry segment of the three-tube modelwas considered. A further reduction in model size was made by slicing the quarter-symmetry geometry in half,front to back.Note the relative mesh refinement in the tie-weld region, more so near the weld crown. Symmetric or anti-symmetric displacement boundary conditions were applied to the planes of geometric symmetry to appropri-ately simulate response to the various unit load cases. The internal tube pressure load case was conductedseparately.3.4. Analysis resultsThe local 3D analysis gave detailed through-wall hoop stress distribution estimates at the various tie-weldlocations in the intermediate and high temperature superheater sections. The maximum hoop stress varied as aFig. 5. Three-tube model showing the generalized degrees of freedom in applied forces and moments derived from the global model.R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 138813961393function of location, with the peaks in the range of 5560 ksi (379414 MPa) generally at the uppermost tiewelds (Level A in Fig. 2) and at the outer-loop tubes, where the tube-to-tube temperature differences werethe highest. Additionally, the peak stresses occurred at the weld toe in the crown region. Fig. 7 shows typicalhoop stress contours at the surface in the vicinity of the tie weld. Both the global location of predicted max-imum hoop stress (Level A, outer loop) and the local weldment location of stress (weld crown), coincide withareas of observed cracking.Fig. 8 is a graphical representation of the elastically computed through-wall stress distribution at one of themost highly loaded tie-weld locations in the high temperature superheater section.The hoop stress distribution was linearized in order to eliminate the local effect of the weld toe geometry,and evaluate the overall hoop stress distribution against the material yield strength and the ASME Codeallowable stress for the material. The linearized hoop stress distribution produces the same net axial forceand moment on the tube wall as does the actual predicted stress distribution. The membrane stress is11.3 ksi (81.4 MPa), and the linearized maximum bending stress is 30.8 ksi (212 MPa) at the OD surface, mak-ing the near-surface stress, exclusive of the weld toe geometry effect, 42.1 ksi (290 MPa).Fig. 6. Outside and inside views of one-eighth symmetry FE model of three-tube configuration.1394R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 138813964. Discussion and conclusionsA number of interesting facts are apparent from the hoop stress distribution of Fig. 8.? The predicted peak stress at the outside tube surface, even without the geometric effect of the weld toe, isabout 42 ksi (290 MPa), nearly three times the yield strength of SA-192 tube material.? The peak linearized stress exceeds the ASME I allowable for SA-192 at 800 ?F (427 ?C) by more than afactor of 4.5 and even the room temperature allowable by nearly a factor of 4.? The linearized hoop stress exceeds the material ASME I 800 ?F (427 ?C) allowable over more than half thetube wall, and exceeds the corresponding material yield strength over more than one-third of the tube wall.? While not explicitly shown in Fig. 8, one can easily deduce that the major contribution to the hoop stress isfrom thermal constraint, given the primary stress due to pressure is roughly 4 ksi (27.6 MPa), only about10% of the peak stress. Further, in the absence of the thermal constraint, stresses are well within designallowables.-20-1001020304050600.0000.0150.0300.0450.0600.0750.0900.1050.1200.1350.1500.165Distance Through Wall from OD (in)Stress (ksi)HoopAxialRadialHoop (linearized)Fig. 8. Through-wall elastically computed stress distribution for one of the most highly loaded high temperature superheater section tubesat a tie-weld location; the hoop stress linearization was performed to analytically remove the geometric effect of the weld toe.Fig. 7. Typical local analysis results showing hoop stress contours in vicinity of the tie weld. Units in ksi with maximum of 56 ksi(386 MPa) and minimum of ?20 ksi (?138 MPa).R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 138813961395It is evident from the preceding discussion, that tube stresses arising from thermal constraint imposed bythe tube-to-tube tie welds are very high, well in excess of pressure stresses and the material ASME Code-allow-able levels, and remain so over a significant portion of the tube wall. These conditions strongly suggest zero ornegative margins against failure, and are indicative of a significant design deficiency. This underscores the factthat when thermal constraint is ignored in application of ASME I, a deficient design may appear to be Code-compliant. This is because ASME I, while it does have rules (PW-43) for limiting loads on rigid attachments2(used generally to accommodate primary loads), it has no explicit rules for designing to accommodate signif-icant thermal or system loads. This is also a prime example of where the following, from the foreword of theASME Code, applies: The Code is not a handbook and cannot replace education, experience, and the use of engi-neering judgment.Nevertheless, established thermal design methods for accommodating system loads are available, and onesuch method is part of the ASME Boiler and Pressure Vessel Code, Section VIII for pressure vessels 4(ASME VIII).Section VIII, Division 2 of the Code, Alternative Rules Rules for Construction of Pressure Vessels, pro-vides means by which a manufacturer can demonstrate that a design detail is as safe as otherwise providedby the rules in the Code. Although fired process tubular boilers are outside the scope of Section VIII, the anal-ysis methodology and allowable stresses in Section VIII may be utilized to provide a quantitative indication ofthe design suitability or lack thereof.In this case, the predicted maximum stress intensity (defined as the difference between the maximum andminimum principal stresses) of 40 ksi (276 MPa), is compared against the Code-specified allowable stressintensity given by three times the average of the allowable stress at the extremes of the operating cycle tem-perature. The latter translates to roughly 3X (average of 11.8 ksi at room temperature and 9 ksi at 800 ?F)or 31.2 ksi (215 MPa). In effect, the peak stresses at the superheater tie welds exceed the ASME VIII-specifiedlimit by about 30%. This exercise confirms the deficiency in superheater design related to the constraintimposed by the tube-to-tube tie welds. The actual cracking experience was entirely consistent with the resultsof this analysis, and not at all surprising in light of this work.AcknowledgmentsDr. T. Kim Parnell, formerly with Exponent Failure Analysis Associates, is acknowledged for contribu-tions to the FE analysis effort. Domtar Inc. is thanked for supporting this research.References1 ASME boiler and pressure vessel code, Section I: Power boilers. New York (NY), American Society of Mechanical Engineers, 1983.2 ANSYS engineering analysis system users manual, Revision 4.4. Houston (PA), Swanson Analysis Systems Inc., 1990.3 Chapman AJ. Heat transfer. 3rd ed. New York: MacMillan; 1974.4 ASME boiler and pressure vessel code, Section VIII: Pressure vessels. New York (NY), American Society of Mechanical Engineers,1983.2The effective (thermal) load here is more than an order of magnitude greater than the PW-43 allowed attachment load.1396R. Caligiuri et al. / Engineering Failure Analysis 13 (2006) 13881396