NKG150-125化工离心泵设计(含12张CAD图纸)
NKG150-125化工离心泵设计(含12张CAD图纸),NKG150,125,化工,离心泵,设计,12,CAD,图纸
任务书 学院 班级 学生 设计题目 NKG150-125化工离心泵设计 课题来源 生产实践 起讫日期 年 月 日至 年 月 日 指导教师(签名) 教研室主任(签名) 课题依据:不锈钢长轴式端吸泵NKG为单级蜗壳式离心泵,本设计参数为:流量170m3 /h;扬程23m;转速1500r/min;汽蚀余量NPSHr3.8m。任务要求:1、资料检索,收集(写出不少于3000字的开题报告)。 2、外文资料翻译(不少于10000个外文单词)。 3、水力设计 (1)叶轮水力设计; (2)蜗壳水力设计; (3)设计计算。 4、结构设计 (1)总体结构方案的确定,轴承、联轴器等的选择与计 算,对1个主要部件的强度进行计算; (2)总装配图的绘制; (3)主要非标零件图的绘制。 5、设计说明书撰写。设计进度计划起讫日期工 作 内 容备 注2月20日3月17日 外文资料翻译。2月20日3月3日熟悉课题,收集资料、设计方案制定; 开题报告撰写。3月30日4月14日完成总装配图绘制。3月9日3月29日水力设计。4月15日5月12日零件图绘制,相关计算校核,全套图纸提交审查。5月13日5月19日图纸修改定稿,说明书编写。5月20日5月26日设计所有资料定稿提交,说明书查重,审核评分。5月27日6月2日毕业答辩准备,总结及毕业答辩。备注 NKG150-125化工离心泵设计学院名称: 专业班级: 学生姓名: 指导教师姓名: 指导教师职称: 年 月摘要: 根据由关醒凡教授所著现代泵理论与设计、现代泵设计手册中所阐述的有关离心泵的叶轮、蜗室的设计理论;键,轴,蜗壳和其他过流部件的机械设计和校核理论。综合其他主流设计理论,我做完了NKG150-125单级单吸化工离心泵的叶轮、蜗室的水力设计;绘制了叶轮、蜗壳的木模图;轴、泵后盖、轴承座等部件的结构设计;键、轴承的选取;最后对关键部件进行了强度校核。 关键字: 离心泵 叶轮 水力设计 强度校核Abstract: According to the design theories of the impeller and volute of the centrifugal pumps, strength-checking theories of key, shaft and volute which are talked in Modern Pumps Theory and Design、Modern Pumps Design handbook both written by Dr. Xingfan Guan,also combining classic and main desig theories. I successfully finished the hydraulic design of NKG150-125 single stage single suction centrifugal pump, drew the hydraulic drawing of impeller and volute as well as the structure drawing of shaft, cover and bearing pedestal as well as other components. At last, I completed the checking task of the strength of crucial components. Key Words: Centrifugal pump, Impeller, Hydraulic design, Strength check 目录设计简介5第一章 水力设计71.1 初始设计参数71.2 性能参数和泵型71.2.1 转速71.2.2 叶轮级数和泵型71.2.3 效率71.2.4 电机功率和轴功率81.2.5 泵进口直径81.2.6 泵出口直径81.2.7 泵进口和出口介质的流速81.3 叶轮几何参数91.3.1 泵的最小轴径91.3.2 叶轮进口直径101.3.3 叶轮出口直径101.3.4 叶片出口宽度101.3.5 叶片出口角101.3.6 叶片数10第二章 水力图绘制112.1 轴面投影图112.1.1 检查流道过水断面的面积变化112.1.2 绘制中间流线132.1.3 流线分点132.1.4 叶片进口角142.1.5 画方格网162.1.6 叶片加厚172.2 叶轮木模图172.2.1 画叶片工作面和背面的轴面截线172.2.2 绘制木模图182.3 压水室设计192.3.1压水室进口宽度202.3.2 基圆直径202.3.3 隔舌螺旋角202.3.4隔舌安放角212.3.5 断面面积212.3.6 断面形状212.3.7 蜗室平面图222.3.8 扩散管截线图23第三章 强度校核243.1键的强度校核243.1.1轴与叶轮配合处的键243.1.2 轴与联轴器配合处的键243.2 轴的强度校核243.2.1 轴向力253.2.2 拉应力253.2.3 弯曲应力253.2.4 切应力263.3 叶轮强度计算263.3.1 盖板强度校核263.3.2 盖板设计26第四章 联轴器选取27致谢28参考文献29附录:30设计简介泵是用于输送流体(大流量泵)或者使流体增压(高扬程泵)的机械,它把机械能(通常为电机转动的动能)转换为流体的能量(动能和压能)。泵按照比转速的不同可分为离心泵,混流泵和轴流泵。本次设计所完成的NKG150-125型泵为单级单吸式化工离心泵(以下简称为离心泵),其中“150”和“125”分别表示进口直径和出口直径(单位mm),“单级单吸”是指一个叶轮,一个进口吸入口。同理亦存在多个叶轮的多级泵,以及两个介质吸入口的双吸泵,而对单级单吸离心泵而言,该泵型则是泵类中应用最为广、品种繁多的产品。泵的主要性能参数包括流量,扬程,轴功率,转速和汽蚀余量。流量指单位时间内通过泵的介质数量,通常有体积流量和质量流量。扬程指单位重量介质经过泵之后获得的能量,通常用米表示。就国内目前的情况而言,单级单吸离心泵的相关性能参数范围相对还是较窄。流量从到不等,扬程从到不等。扬程时的最大流量为,流量时最大扬程为。目前国内单级单吸离心泵的主要缺陷:1、叶轮进口设计不合理,流道极其容易发生堵塞,从而白白耗费了能量,效率却得不到提高;2、大多数的压水室只是一个蜗壳,由于不具备双蜗壳的对称性,泵在运行时候很容易不稳定,产生振动噪声;3、整体结构以及零部件的结构设计存在缺陷和不合理的地方。4、性能曲线图只提供一条曲线,很大程度上局限住了泵的应用工况场合;国外单级单吸离心泵的主要优点:1、运行稳定噪声低,合理的结构设计以及对转动部件精准的动平衡和静平衡使得泵在运行的时候叶轮转动平稳不会窜动,介质不会在泵体内发生堵塞;2、泄漏量极低,得益于制造加工极为精密的机械密封;3、发生故障的概率较低,合理的结构设计以及合理的材料零件选择,加上电泳等高效的防腐蚀的措施导致泵的寿命非常的长久。本次设计主要分为四个章节。第一章水力设计,设计理论基于关醒凡教授所著现代泵理论与设计、现代泵设计手册阐述的一元设计理论进行,按照书中所述的步骤逐一完成了水力设计。第二章为水力图绘制,包括叶轮和蜗室木模图的绘制,同时参照格兰富水泵公司的结构样本完成了轴,叶轮,后盖,轴承座和其他部件的结构设计。第三章强度校核,应用材料力学的相关理论知识完成了对关键传动部件如轴,键的强度校核计算,同时反向计算出叶轮在进口和出口处的厚度。第四章联轴器选取。最后为致谢,参考文献和附录,附录包括了本次设计完成之后所有的图纸成果,总装配图,零件图和水力图均包含在内。 第一章 水力设计1.1 初始设计参数流量: 扬程: 转速: :1.2 性能参数和泵型1.2.1 转速确定转速时,需要把汽蚀比转数的对泵整体的影响纳入考虑中,由设计参数所给的转速和流量:,查阅设计手册,系数,故可以计算出在考虑汽蚀比转数条件下最大的转速为:又所给设计转速小于最大转速,即,故所给转速满足汽蚀余量的要求。1.2.2 叶轮级数和泵型由泵的比转速:查表,泵型和叶轮级数为:单级单吸离心泵。1.2.3 效率1)水力效率2)容积效率 3)机械效率 4)总效率1.2.4 电机功率和轴功率取介质水密度,取安全系数,由此查表,(采用直联方式),得轴功率为:计算电机的功率为:选用电机功率不必过大,就可以满足功率要求1.2.5 泵进口直径取泵进口介质的流动速度为,圆整取进口直径1.2.6 泵出口直径为了减小泵的整体体积以及减小排出口管路的直径大小,一般情况下出口直径要小于进口直径也就是,通常情况取1.2.7 泵进口和出口介质的流速 根据1.3所计算出的泵进口和出口直径可以算出进口介质流动速度:出口介质流动速度:整合以上所有数据如下表: 体积流量扬程轴的转速汽蚀余量泵的比转速泵型和叶轮级数单级单吸式离心泵水力效率容积效率机械效率总效率轴功率电机功率进口直径出口直径1.3 叶轮几何参数1.3.1 泵的最小轴径由轴功率和转速,利用材料力学知识,可计算轴上的扭矩为:由该扭矩查材料特性表,选用轴材料为,其许用扭转应力为,取安全系数为0.2,可得该泵设计的轴的最小直径为圆整取 ,即轴最细的地方为25mm1.3.2 叶轮进口直径叶轮进口的当量直径系数,为经验系数,那么可以计算出进口当量直径:对本次设计的单级单吸式离心泵:,那么进一步可以得1.3.3 叶轮出口直径由于之前已经算出来比转速为113.3,查手册,得经验系数,那么:圆整取为276mm1.3.4 叶片出口宽度由于比转速为113.3,查表8-13,得,那么:圆整取1.3.5 叶片出口角正常情况下取,本次设计选取1.3.6 叶片数查手册,比转速13.3介于120至300之间,取叶片数综合以上全部数据,叶轮几何参数如下表:参数名称初算数据(mm)圆整数据(mm)最小轴径20.425进口直径133.8134出口直径275.8276出口宽度23.724叶片出口安放角叶片数Z66第二章 水力图绘制2.1 轴面投影图第一章水力设计中已经计算出叶轮的一系列重要尺寸数据,在关醒凡教授所著现代泵理论与设计(以下简称设计)提供的水力模型中选取与所设计泵相近的为参照,选取原则为比转速相近,画叶轮的轴面投影图。2.1.1 检查流道过水断面的面积变化通过在流道内部绘制内切圆,用平滑曲线连接圆心得流道中线,而后作一系列辅助线。参照设计第二百六十四页至二百六十五页计算公式及方法。作图流程和流道过水断面的面积变化情况如下图所示:图中曲线变化平直且光滑,满足设计要求。2.1.2 绘制中间流线 中间流线是将流道划分成相等两块面积的线。 参照设计二百六十六页绘制理论,不断调整计算左右流道面积并对比误差。下图为本次设计中间流线的结果图,表格内容为最终流道左右面积及误差数据2.1.3 流线分点 本次毕设采用作图分点法,具体绘制过程见设计二百七十九页。具体情况如下图所示,表格为分点误差数据:2.1.4 叶片进口角叶片的进口角,通常选取大于液流角, 叶片的液流角按这个公式计算: 其中:介质的圆周速度; 介质的轴面分速度; 表示介质在该点的圆周分速度,此处因为吸入式为直锥型吸入室的缘故取排挤系数可以按如下公式计算: 其中:叶片数;计算点处的直径;计算点处的真实厚度;计算点轴面截线与轴面流线所成的夹角.具体计算步骤如下1.在图上面绘制过水断面,然后计算断面的大小:2.算出圆周速度:3.假设某,算出轴面速度,再按公式算出进口角,然后得到,把冲角考虑进去,自己选取一个4.校核一下最后的结果如下表所示2.1.5 画方格网方格网的画法按照设计第二百八十页绘制,分点的个数和包角可以自己确定,方格网的大小也可以自己定,竖线表示轴面流线的相应分点,横线表示轴面间的夹角。画好方格网后就可以在上面画流线了,通常先画中间流道再画其他两条流线,进出口角度的等于叶片安放角,然后画光滑曲线与进出口角度线相切。型线应该满足以下要求:光滑平顺;单向弯曲,不可出现型,以平直稍凸为好;各个型线要有一定的对称性。与下图为本次设计的方格网和流线。2.1.6 叶片加厚在轴面投影图上依照该公式进行叶片厚度的计算:代表流面厚度,代表实际厚度。下图为本次设计的叶片加厚的数据2.2 叶轮木模图2.2.1 画叶片工作面和背面的轴面截线根据设计第二百八十三页设计方法,在方格网上画出的三条流线就是叶片表面的三型线。用方格网中的竖线也就是轴面去截流线就相当于用轴面去截叶片。把竖线和流线的交点的位置按每隔一定角度的分布画到对应的轴面投影图上去,然后用光滑的曲线连接起来就是叶片的轴面截线了。轴面截线应光滑且有规律变化,并且应该尽量使轴面截线与轴面流线垂直。本次设计绘制的叶片工作面和背面的轴面截线如下图2.2.2 绘制木模图根据设计第二百八十三页设计原理,用一组等距的轴垂面去截叶片,每个截面和叶片有两条交线(工作面和背面)。具体方法如下:1、 画一组垂面,编号0,1,2,3,4,5,62、 在平面图中,画相应的轴面截线角度的轴面(一组射线)并按射线角度编号3、 作叶片平面投影轮廓线:工作面与前盖板的交线是把轴面投影图中各个截线与前盖板流线的交线,以此类推可以画出所有的投影轮廓线。画得本次设计的叶片的木模图如下所示,表格为叶片工作面和背面的径向数据。2.3 压水室设计压水室涵盖了叶轮的出口一直到出口法兰的部分。压水室的功能包括: 1、收集从叶轮中流出的介质,然后输送到泵出口或者下一级叶轮进口; 2、保证流出叶轮的流动是对称的,使叶轮稳定; 3、降低介质流速,动能转换成压力能;4、消除叶轮旋转造成的介质的旋转也就是环量,从而降低损失提高效率。压水室的设计原则:1、力求水力损失最小,并保证液体在压水室中的流动是轴对称的,以保证叶轮中的流动稳定;2、在能量转换过程中,轴对称的流动不能被破坏;3、消除叶轮出口介质的速度环量,也就是说在介质进入下一级叶轮之前,不能有速度环量4、在设计工况下,流入的液体对泵体无冲击损失。2.3.1压水室进口宽度通常压水室进口宽度要大于前盖板厚度加后盖板厚度加叶轮的出口宽度,不可大太多但至少要留一定的间隙,间隙可以保证叶轮在运转的时候不会左右震动撞到蜗壳。本次设计取。2.3.2 基圆直径基圆直径不可过大亦不可过小,太小会因为介质阻塞而引起噪声和振动,还有可能在隔舌处发生汽蚀。间隙增大的话可以减少叶轮外周流动的不均匀性,降低噪声和振动,并且减少汽蚀发生的可能性提高效率。但是如果太大一是会增大蜗壳的尺寸,而是介质在压水室里,就会有足够的空间活动,产生旋转的液流环,消耗一定的能量,使泵的效率大大下降,通常情况下取:,本次设计。2.3.3 隔舌螺旋角为了符合流动的规律,不造成介质液流对蜗壳的严重冲击,隔舌的螺旋角按绝对液流角算:那么。2.3.4隔舌安放角隔舌要放在蜗壳螺旋部分的起始点或者靠后一点的地方,隔舌很明显的把螺旋线的部分和紧接着的扩散管部分分离开来。通常情况下,过隔舌的头部的断面为第0断面,隔舌和第8断面之间所成的角度定义为隔舌的安放角,以表示。一般情况下的大小要和的大小成对应关系。根据设计第三百零七页的表,查到我的为113.3时候,取为。2.3.5 断面面积本次设计我采用速度系数法也就是按角度的比例算除第八断面之外的各个断面,查手册,选取,可以算得到在压水室里面介质的流动速度: 那么第8断面的面积就可以按照这个算: 那么除此之外的其他各断面的面积就可以按角度比例来算:断面IIIIIIIVVVIVIIVIII包角45 90135180225270315360面积/mm741.211482.412223.622964.833706.034447.245188.445929.65画出螺旋形的压出室的各断面以及扩散阶段的断面之后,即可以量取得到各截断面顶点到轴线的距离,而各个断面对应的包角也是知道的,所以就能够把各个顶点放置于平面图之上,然后用光滑的曲线去连接他们,或者用几个圆弧连接,就能够获得螺旋形的压出室的平面图了。2.3.6 断面形状 在算出上面各个断面对应的大小后,断面形状的选择其实关系并不大。所以在画断面的时候只需要力求实际面积与理论面积接近,这次设计采用梯形的断面形状本次设计画出来的各个断面的平面图如下图2.3.7 蜗室平面图 前面已经算出来基圆直径以及各个断面的轴面图,在平面图上一一对应画出来,就可以得到本次设计如下的蜗室平面图:2.3.8 扩散管截线图查设计,参考优秀模型的扩散管截线图,结合自己的实际情况用相似换算原理得到每个对应点的位置,然后用光滑的曲线连起来,力求平整光滑,此次毕设的扩散管截线图和径向数据如下:第三章 强度校核3.1键的强度校核本次设计采用的键都是普通平键(参照标准为GB/T10962003)。3.1.1轴与叶轮配合处的键 根据前面的选取,在和叶轮配合的轴段,直径是30mm,选用用A型圆头平键,标号为bhL =8mm7mm56mm ,计算挤压应力和剪切应力分别是: 选键的材料是45号钢,而45钢的许用正应力、许用剪切应力。 正应力和切应力均充分满足45号的强度要求。3.1.2 轴与联轴器配合处的键 根据前面的选取,在轴和联轴器配合的轴端,直径是34mm,选用A型圆头平键,标号为bhL =10mm8mm100mm ,计算挤压应力和剪切应力分别是: 选键的材料是45号钢,而45钢的许用正应力、需用剪切应力。 正应力和切应力均充分满足45号的强度要求。3.2 轴的强度校核本次设计中水力设计部分以扭矩而计算得到的最小的轴径是25mm,而轴的材料选取为 3Cr13,承受的扭矩,这是轴已知的基本参数。 3.2.1 轴向力轴向力的产生原因包括前后盖板不对称;轴端的结构不合理;转子有重量;转子分布方式不合理以及动反力计算轴向力需要按照以下步骤进行:先计算叶轮在出口的势扬程为: 下一步算盖板上的轴向力为:3.2.2 拉应力 小于许用应力所以充分满足了强度要求。3.2.3 弯曲应力在叶轮与轴配合的地方根据轴键槽的的结构计算该出的抗弯截面系数是:估算叶轮的重力,大致计算这个重力在叶轮处的轴上产生的弯矩差不多是所以那么根据材料力学的相关知识,弯曲应力可以按这个公式计算: 3.2.4 切应力在叶轮与轴配合的地方根据轴键槽的的结构计算该出的抗扭截面系数是: 而扭矩之前计算出来为,那么就可以按照材料力学的原理计算切应力为: 此处我选用材料力学中的第四强度理论综合弯曲正应力和扭转切应力来折算应力:由于轴选用的材料是,查该材料的力学性质表可得,取安全系数。所以由,而,由此轴满足强度要求。3.3 叶轮强度计算前盖板和后盖板所承受的应力主要是因为旋转产生的离心力,半径越小,应力就越大。叶轮的材料选用ZG1Cr13,查该材料的物理和力学性质表可以得到密度,许用应力。3.3.1 盖板强度校核 盖板上的应力按如下的公式计算满足强度要求。3.3.2 盖板设计按照等强度的前提对叶轮的盖板进行设计,盖板任意直径处的厚度计算可以按照: 而经过之前水力设计部分的计算,出口处的厚度,所以代入上面的计算公式可以得到出口处的盖板厚度为: 经过之前水力设计部分的计算,进口处厚度,代入计算公式可以得到进口处的盖板厚度为:综上,经过设计计算的叶轮进口处的厚度为6.3mm,出口处的厚度为5mm。第四章 联轴器选取 采用弹性套柱销联轴器(参考标准为GB/T43231984) 因为轴传递的扭矩大小为,轴在联轴器段的直径为34mm,查阅选取标准,得联轴器型号为LT6 参数:公称转矩,选钢材料,许用转速为,轴孔径,Y型轴孔长度82mm,质量为10.36kg,转动惯量为。致谢本次设计是在陈教授和)工程师王老师的共同指导下完成的,持续时间大约半年。在此期间,两位导师广泛而渊博的专业知识,严谨的治学风范,高尚的品德以及孜孜不倦的教诲对我产生的深刻的影响。从一开始选题直到最后完成说明书的撰写和所有图纸的绘制都离不开两位导师的悉心指导。我要向他们表达最诚挚的感谢。与此同时,在毕设期间工厂对我提供了很大的便利和帮助,每当我有不理解的问题或者不清楚的结构时,总能够去现场查看我所设计的泵型,而现场的工程师为我提供了详细的解答和指导。正是这些给予我莫大帮助的人,才让我的本次设计成为不仅仅成为理论上的学习提升和总结,更是对生产实践上的理解。最后我要感谢我的同学,师长,遇到问题的时候同学总是第一个热情帮助我的,而大学期间给我传授专业知识,答疑解惑的师长更是有着非凡的意义。在此我也对他们表达深深的感谢。参考文献1关醒凡编著. 现代泵理论与设计. 北京:中国宇航出版社,2011.42关醒凡编著. 现代泵技术手册. 北京:宇航出版社,1995.93查森编著. 叶片泵原理及水利设计. 北京:机械工业出版社,1988.6 4张克危主编. 流体机械原理(上册). 北京:机械工业出版社,2000.55谈高明,袁寿其,刘厚林. 离心泵性能预测的研究现状及展望. 水泵技术,2005.(35)6关醒凡,商明华,张生昌.我国单级单吸离心泵的现状及发展方向. 通用机械,2010.(1) 7李必祥. 国内离心泵的发展. 化工设备设计,1998.(35)附录:以下为本次设计总装配图,各个部件的零件图和相关水力图纸。CAD图纸另外见附件。35 设计外文翻译 设计外文翻译 院(部): 专业班级: 学 号: 学生姓名: 指导教师: 原文原文The requirements for a successful pump installation are performance and life.Performanceis the rating of the pump head, capacity, and efficiency. Life is the total number of hours ofoperation before one or more pump components must be replaced to maintain an acceptableperformance. The initial performance is the responsibility of the pump manufacturer andis inherent in the pump design. Life is primarily a measure of the resistance of the mate-rials of construction to corrosion, erosion, wear, and other factors that can influence thematerials when the pump has been placed in service.The need to maximize reliability andextend the pump life makes the selection of appropriate materials of construction crucial.The selection of materials that are both cost-effective and technically suitable for theapplication requires a knowledge not only of the pump design and manufacturing pro-cesses,but also of the engineering properties of the material,particularly its corrosion andwear resistance properties when subjected to the conditions encountered in the pump.Suf-ficient information is available in the corrosion and metallurgical literature as well asfrom the experience of pump manufacturers to make appropriate material choices for vir-tually any pumping application.It is known that several factors lead to a long pump life.These include Neutral liquids at near-ambient temperatures Appropriate material selections for pumps in aggressive services The absence of abrasive particles Continuous operation at or near the maximum efficiency capacity of the pump An adequate margin of available NPSH over NPSH required as stated on the manu-facturers rating curve A low velocity (developed head/rotative speed)Pumping installations that satisfy all these criteria will have a long life. A typicalexample would be a waterworks pump. Some waterworks pumps with bronze impellersSECTION 5.1METALLIC MATERIALSOF PUMP CONSTRUCTION (AND THEIR DAMAGEMECHANISMS)COLIN O. McCAULRONALD S. MILLER5.35.4CHAPTER FIVEFIGURE 1A small fragment of Ductile Ni-Resist from the lower casing of a vertical pump. The microstructure isalso shown on the right side of this figure, illustrating the depth of the corrosions penetration. This is a classicexample of general corrosion (right photo at 100?).and cast-iron casings have a life of 50 years or more. At the other extreme might be achemical pump handling a hot corrosive liquid with abrasive particles carried in suspen-sion.The life of this pump might be measured in months rather than in years, despite thefact that construction was based on the most resistant materials available.Most pumping applications fall somewhere between these two extremes. The pumpdesigner needs to be familiar with the various types of degradation that can affect thecomponents of the pump and reduce its useful life.These can be grouped into the generalcategories of corrosion, wear, and fatigue, with corrosion and wear being the predominantlife-limiting mechanisms.TYPES OF CORROSION _General CorrosionGeneral corrosion is corrosion that proceeds without an appreciablelocalization of attack. This type of corrosion occurs on metals or alloys that do not developan effective passive film on the surface. Usually, the corrosion mechanism is oxidation withthe formation of metal oxide corrosion products.General corrosion is most often encounteredin pumps with carbon steels and copper base alloys.Cast irons also experience a specializedform of general corrosion,known as graphitic corrosion,which will be considered separately.Carbon steel does not develop a protective oxide film and will corrode at a rate depen-dent upon several characteristics of the water or other fluid, including temperature, oxy-gen content, pH, and fluid chemistry. Several empirical indices based on water chemistryexist and can be used to calculate the relative corrosivity of natural waters to carbon steeland similar ferrous alloys.The Langelier Index is best known.The rate of corrosion is alsovery dependent on velocity and increases with an increasing velocity.In most pump appli-cations,with the notable exception of hydrocarbons,the corrosion rate of carbon steel is toohigh for this material to provide a useful life.However,carbon steel is frequently used,par-ticularly in vertical pumps,with some form of protective coating to prevent corrosion.Coaltar epoxy is a preferred coating for many water services.Copper alloys,including both brasses and bronzes,are also subject to general corrosionin the water applications where they are most commonly used in the pump industry.Thecorrosion rate will be increased by the presence of small amounts of sulfides in the water.Copper alloys gradually develop a protective copper oxide corrosion film in most applica-tions. The corrosion rate gradually decreases over time as this film develops. The rate ofgeneral corrosion varies with the specific type or grade of copper alloy. Among the alloyscommonly used in pumps, nickel aluminum bronzes have the lowest corrosion rate andbest tolerance for higher velocities.The general corrosion of a Ductile Ni-Resist casing from a vertical pump is shown in Fig-ure 1.A metallographic cross section was removed to show the depth of the corrosion attack.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.5FIGURE 2The interface between the advancing graphitized front and the sound base metal. Graphitic corrosionpropagates along the path of the graphite flakes (50?).DealloyingDealloying is the preferential removal of one phase from a multi-phasealloy, or one element from a material. Several types of dealloying occur in the pumpindustry. One of the most common is the graphitic corrosion of gray cast iron.This mate-rial is low cost, easy to machine, and well suited for a variety of applications, especiallyin the waterworks industry. It is probably the most widely used material in the pumpindustry.Gray cast iron corrodes by a fundamentally different mechanism than carbon steel orductile cast iron.The structure of gray cast iron consists of interconnected graphite flakesin a matrix that is predominantly iron. In the presence of an electrolyte, which is usuallywater, a galvanic cell is established between the iron and graphite.The iron corrodes, andthe corrosion products are largely flushed away with the fluid passing through the pump.The original casting is gradually reduced to a porous graphite structure that may containsome iron oxide corrosion product.This is frequently referred to as graphitization.The sur-face of a gray iron casting that has suffered graphitic corrosion will retain its originalshape and dimensions, but the surface will be largely graphite, which can be cut with aknife.The casting will lose some fraction of its mechanical properties and become increas-ingly susceptible to brittle failure, resulting from modest shock or impact loads. This isalso the corrosion mechanism for Ni-Resist in seawater. Figure 2 shows the interfacebetween the sound base metal and the graphitzed front.It is important to recognize that the rate of graphitic corrosion varies with the waterchemistry,and that this type of corrosion can occur in both fresh and salt waters.The highconductivity of salt water corresponds to a higher corrosion rate. Graphitic corrosion willproceed at a slower pace in waters that have a high mineral content.Minerals tend to plugthe graphitic layer on the surface, sealing off the base metal from exposure to the fluid,thereby reducing the corrosion rate.As the surface of a cast-iron component, such as a pump casing, gradually graphitizes,the galvanic relationships with other components within the pump will be altered. It hasbeen observed that the bronze impeller originally supplied in a cast-iron pump handlingseawater will provide a significantly longer life than bronze impellers that are installedafter the pump has been in service for several years. The reduced life of the replacementimpellers is caused by an altered galvanic relationship with the pump casing.Initially,thecasing was cast iron,which is anodic to a bronze impeller.With time,as the casing graphi-tizes, it gradually becomes cathodic, due to the influence of the graphite. The bronzeimpeller is now the anode and corrodes at a much higher rate.This example highlights theinfluence that graphitic corrosion can have on other components within the pump and theimportance of carefully selecting materials for use in conductive fluids,such as salt water.Several other types of dealloying can also occur in pumps.Brass and bronze alloys con-taining more than about 14 percent zinc are subject to a form of dealloying known as dez-incification.The zinc is preferentially corroded from the matrix of the material, leaving aspongy, copper-rich residue. Dezincification can occur either uniformly in a shallow layer5.6CHAPTER FIVEFIGURE 3The dealloying of a vertical turbine pump impeller. Note the change in color across the cross section.The unaffected bronze (light color) material is surrounded by a dezincified layer (1.3?).over the surface of the casting or as a distinct plug confined to a small area.Plug-type dez-incification is a more serious problem because the plug is weak and will cause leakage ifit penetrates a pressure boundary, but it should be emphasized that copper alloys con-taining less than 14 percent zinc are not susceptible to this form of corrosion. Conse-quently,the requirement often imposed upon pump manufacturers for zinc-free bronzes toavoid dezincification is without technical justification.Figure 3 shows the dealloying of animpeller.The final type of dealloying that occasionally occurs in pumps is dealuminification inaluminum bronzes. These are metallurgically complex materials. Some compositions canform an aluminum-rich phase that can be preferentially corroded in aggressive fluids,especially seawater. The detrimental phase can be mitigated by a special heat treatmentknown as temper annealing. This heat treatment must be specified by the designer forsusceptible compositions, because it is not a mandatory requirement of national materialspecifications. The chemistry of some aluminum bronze alloys from Europe has beenadjusted to preclude the formation of the detrimental aluminum-rich phase without theneed for the temper annealing heat treatment. The temper anneal can serve as a stressrelief operation for fabricated aluminum bronze structures, which is a secondary benefitfor products in this category.Galvanic CorrosionGalvanic corrosion refers to the corrosion that occurs when onealloy is electrically coupled to another and exposed in a conductive liquid.Usually,the cor-rosion rate of the more noble alloy will be less than if it were exposed uncoupled.The cor-rosion rate of the less noble material will be greater than if it were exposed uncoupled.Several factors influence the rate of galvanic corrosion of both metals.This corrosion isgreatly influenced by the conductivity of the fluid.In a fluid such as fresh water,which hasa low conductivity, galvanic corrosion will be less severe and generally confined to theimmediate location where the metals contact one another.However,in a highly conductivefluid, such as seawater, galvanic corrosion will be more severe and will occur over a widerarea. The pump designer needs to consider the possibility of such corrosion when usingdissimilar metals in a conductive fluid.Galvanic corrosion problems in seawater and other conductive fluids can be avoided bythe careful use of materials. Galvanic corrosion is related to the area ratios of the coupledmetals.It is always desirable to have the area of the anode,or less noble metal,equal to orgreater than that of the more noble metal. In this way, the additional corrosion experi-enced by the less noble metal will be spread over a relatively large area and will not beexcessive because of being coupled. An example of the effective use of this galvanic rela-tionship involves centrifugal pumps having a Ni-Resist casing and austenitic stainlesssteel internals.This combination is often specified for seawater services.The Ni-Resist is5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.7anodic to the stainless steel and will protect it from localized corrosion when the pump isshut down and contains stagnant water.The area of Ni-Resist is considerably larger thanthat of stainless steel. The increased galvanic corrosion of the Ni-Resist is spread over alarge area and is negligible.The amount of corrosion that will occur in a galvanic couple also depends on the freelycorroding potentials of the coupled metals. Less corrosion-resistant metals, such as zinc,cast iron, and steel will usually have more negative potentials when measured against astandard reference electrode.More corrosion-resistant metals,such as stainless steels,willhave less negative potentials.The corrosion potentials for many commonly used engineering alloys in slowly movingseawater are shown in Table 1.The alloys are listed in the order of the potential that theyexhibit in flowing seawater. Certain alloys (indicated by solid colored boxes preceding thename of the alloy) in low-velocity or poorly aerated water and at shielded areas maybecome active and exhibit a potential near ?0.5 volts.The extent of galvanic corrosion thatwill occur when two metals are electrically coupled will depend on the potential differencebetween the metals.The corrosion rate of zinc coupled to stainless steel will increase dra-matically because of the large potential difference between these two metals.A nickel alu-minum bronze coupled to austenitic stainless steel will experience little galvanic corrosionbecause the potentials of these two metals are close to one another. The pump designerneeds to be aware of the corrosion potentials of dissimilar metals used in conductive flu-ids in order to avoid unanticipated galvanic corrosion problems.The use of coatings can decisively alter the galvanic relationships in a pump. If themore anodic component,such as a steel casing,is coated,one can expect a high rate of cor-rosion at those locations where the coating eventually begins to fail. This will be causedby a very unfavorable area ratio, with a small area of exposed carbon steel coupled to alarge area of some more noble metal, such as stainless steel or bronze. For this reason,coatings should be employed with caution in pumps handling conductive fluids that areconstructed of dissimilar metals.It is generally advisable in these applications not to coatthe anodic component. Figure 4 documents the galvanic corrosion on the interior diame-ter of a carbon steel flange connected to a stainless steel shroud. The accelerated corro-sion is due to the unfavorable ratio of stainless steel to carbon steel in this component.Stress Corrosion CrackingStress corrosion cracking (SCC) is a particularly danger-ous form of corrosion because it is not easily detected before it has progressed to such anextent that it can cause sudden catastrophic damage. Although relatively uncommon inpumps, it can occur in several classes of materials. The pump designer should be awareof the potential combinations of material and environment that can cause SCC.Stress corrosion requires that several factors be present. These include tensile stress,which can be either residual or applied,a susceptible material,an environment capable ofcausing stress corrosion, and time.The materials used in the pump industry that may experience SCC include austeniticand martensitic stainless steels, some copper base alloys, and, occasionally, Ni-Resist.Theaustenitic stainless steels are susceptible to stress corrosion in aqueous chlorides at tem-peratures above about 140F (60C). Cast alloys, which contain some fraction of ferrite inthe microstructure,are significantly more resistant to stress corrosion than their wroughtcounterparts.The possibility of cracking is increased in situations where chlorides are con-centrated, as by evaporation. High residual stress, often present in as-welded structures,also enhances the possibility of cracking. Increasing nickel content in austenitic stainlessalloys enhances the resistance to SCC.The high nickel grade,commonly known as Alloy 20,is often used in chemical applications where the optimum resistance to stress corrosion isnecessary.The SCC of austenitic stainless steels in pumps is relatively uncommon.Martensitic stainless steels are susceptible to cracking in the presence of hydrogen sul-fide and is often referred to as sulfide stress corrosion cracking (SSC). These steels, par-ticularly CA-15 and CA-6NM, are commonly used in pumping applications in oilproduction and refining where hydrogen sulfide can be present. SCC can be avoided bygiving these materials a special heat treatment intended to reduce hardness below a cer-tain threshold level, below which cracking will not occur.This has also been correlated tothe yield strength of a material. It is often seen in literature that ferrous materials used5.8CHAPTER FIVEVolts: Saturated Calomel Half-Cell Reference Electrode+0.3+0.2+0.10-0.1-0.2-0.3-0.4-0.5-0.6-0.7-0.8-0.9-1.0-1.1-1.2-1.3-1.4-1.5-1.6-1.7MagnesiumZincBerylliumAluminum AlloysCadmiumMild Steel, Cast IronLow Alloy SteelAustenitic Nickel Cast IronAluminum BronzeNavel Brass, Yellow Brass, Red BrassTinCopperPb-Sn Solder (50/50)Admiralty Brass, Aluminum BrassManganese BronzeSilicon BronzeTin Bronzes (G & M)Stainless Steel - Types 410, 416Nickel Silver90-10 Copper-Nickel90-10 Copper-NickelStainless Steel - Type 430Lead70-30 Copper-NickelNickel-Aluminum BronzeNickel-Chromium alloy 600Silver Braze AlloysNickel 200SilverStainless Steel - Types 302, 304, 321, 347Nickel-Copper alloys 400, K-500Stainless Steel - Types 316, 317Alloy “20” Stainless Steels, cast and wroughtNickel-Iron-Chromium alloy 825Ni-Cr-Mo-Cu-Si alloy BTitaniumNi-Cr-Mo alloy CPlatinumGraphiteTABLE 1Corrosion potentials in flowing seawater (813 ft/s, 5080F/2.44.0 m/s,1026C)in these services should have a hardness no greater than 22 Rcor a yield strength nohigher than 90,000 lb/in2(620MPa). Technical standards, including API 610 and NACEMR-01-75, can be used to specify appropriate requirements for martensitic steels, whichwill be used in environments containing hydrogen sulfide.5.1 METALLIC MATERIALS OF PUMP CONSTRUCTION5.9FIGURE 4Galvanic corrosion is evident on this pump section. Note the high corrosion rate on the interiordiameter of the carbon steel flange that is attached to the stainless steel shroud.Copper alloys are susceptible to SCC in the presence of ammonia, although consid-erable variations take place in the susceptibility of the various types of bronzes, withaluminum bronzes being the most resistant. Polluted natural waters can containammonia, and for this reason, bronze pumps are usually not a good choice for theseapplications.High-strength manganese bronzes are susceptible to cracking in natural waters. Castimpellers in these alloys have been known to suffer severe cracking.Residual stress in thecasting may also be sufficient to induce cracking.These alloys should not be used in pumpsbecause of their susceptibility to such problems.Ni-Resist is an austenitic cast iron that contains 15 to 20% nickel.This material is com-monly used in large, seawater vertical pumps. Experience has shown that it is subject toSCC, especially in the diffuser section of these pumps, unless the castings are furnacestress-relieved. This must be specified by the purchaser, as it is not a requirement ofnational material specifications.Hydrogen EmbrittlementHydrogen damage is a form of environmentally assisted fail-ure that results from the combined action of hydrogen and residual or applied tensilestress. Hydrogen damage to specific alloys or groups of alloys manifests itself in manyways, such as cracking, blistering, hydriding, or as a loss of tensile ductility. Collectively,these various forms of damage are often referred to as hydrogen embrittlement.Damage caused by hydrogen is
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