外文翻譯--- 化工工業(yè)離心泵

1、<p>　　本科畢業(yè)設(shè)計(jì)（論文）外文翻譯譯文</p><p>　　學(xué)生姓名： </p><p>　　院 （系）： 機(jī)械工程學(xué)院 </p><p>　　專業(yè)班級(jí)： 過(guò)程裝備與控制工程 </p><p>　　指導(dǎo)教師： </p>&l

2、t;p>　　完成日期： 2012 年 3 月 19 日 </p><p>　　文獻(xiàn)名稱（中文） 化工工業(yè)離心泵</p><p>　　文獻(xiàn)名稱（外文）CENTRIFUGAL PUMPS IN THE CHEMICAL INDUSTRY</p><p>　　作者： J.M.Coucson , J.F.Richardson</p><p

3、><b>　　起止頁(yè)碼：～</b></p><p>　　出版日期（期刊號(hào)）：Chemical Engineering , 1995</p><p>　　出版單位：Butterworth-Heinemann Ltd</p><p><b>　　外文翻譯譯文：</b></p><p><b&

4、gt;　　化工工業(yè)離心泵</b></p><p>　　摘要 ：離心泵是通過(guò)葉輪的旋轉(zhuǎn)把液體的內(nèi)能轉(zhuǎn)換成動(dòng)能的一種旋轉(zhuǎn)裝置。液體由吸入口進(jìn)入蝸殼，通過(guò)高速旋轉(zhuǎn)的葉輪，液體呈放射狀加速?gòu)谋弥邢蛲廨敵?，這時(shí)葉輪附近留出一個(gè)真空，不斷吸引更多的流體進(jìn)入泵的葉輪附近，這樣由葉輪的旋轉(zhuǎn)來(lái)完成液體的進(jìn)出。這篇文章主要講述了關(guān)于離心泵的發(fā)展史，離心泵工作原理的分析，汽蝕的基本原理和預(yù)防汽蝕的措施等的一系列問(wèn)題。從而幫

5、助我們加深對(duì)離心泵的理解。</p><p>　　關(guān)鍵詞：離心泵 介紹 工作原理 汽蝕 汽蝕原理 預(yù)防措施</p><p><b>　　1.介紹</b></p><p>　　泵的提出，最先是用于轉(zhuǎn)移或壓縮液體和氣體的設(shè)備。在所有泵中，我們一步步采取措施來(lái)防止氣蝕，氣蝕將減少流量并且破壞泵的結(jié)構(gòu)。用來(lái)處理氣體和蒸汽的泵稱為氣體壓縮機(jī)，研

6、究流體的運(yùn)動(dòng)的科學(xué)稱為流體力學(xué)。</p><p>　　水泵是用管子連接的機(jī)械把水從一個(gè)地方傳到另一個(gè)地方。水泵的操作壓力從一磅到一萬(wàn)磅每平方英尺。日常生活中，泵是很多見(jiàn)的，有用于在魚(yú)池和噴泉使水循環(huán)和向水中充氣的電泵，還有用于從住宅處把水引走的污水泵。</p><p>　　離心泵的早期形式---螺桿泵，是通過(guò)一個(gè)管子連接一根螺桿組成的，它是利用螺桿的旋轉(zhuǎn)把水提升上去。螺旋泵經(jīng)常用在污水處理

7、廠中，因?yàn)樗鼈兛梢赃\(yùn)輸大量的水，而不會(huì)因?yàn)樗槠氯Ｔ谶h(yuǎn)古的中東，因?yàn)閷?duì)農(nóng)場(chǎng)進(jìn)行灌溉的需求，所以有一種強(qiáng)大的動(dòng)力去推進(jìn)水泵的進(jìn)程。在這些區(qū)域里，早期的泵是為了將水一桶一桶的從水源或河渠中提升到容器中。古希臘的發(fā)明家和數(shù)學(xué)家阿基米德被認(rèn)為是公元前3世紀(jì)首先提出螺旋泵的發(fā)明家。之后，古希臘發(fā)明家發(fā)明了第一個(gè)提水泵。在十七世紀(jì)末和十八世紀(jì)初，英國(guó)的工程師Thomas Savory，法國(guó)的物理學(xué)家Denis Pa]pin，和英國(guó)的鐵匠和發(fā)明家

8、Tomas Newcomen,它們發(fā)明了用蒸汽驅(qū)動(dòng)活塞的水泵。蒸汽驅(qū)動(dòng)的水泵首先廣泛的被應(yīng)用是在從煤礦往外輸水過(guò)程中?，F(xiàn)在離心泵使用的例子，是來(lái)自于哥倫比亞河上使用的大古利水壩。這個(gè)泵有超過(guò)灌溉一百萬(wàn)英畝的土地能力。</p><p>　　離心泵被認(rèn)為是旋轉(zhuǎn)泵，它有一個(gè)旋轉(zhuǎn)地葉輪，葉輪上有葉片，葉片是侵入液體中的。液體也是由葉輪軸向進(jìn)入泵，并且旋轉(zhuǎn)的葉輪將液體甩向葉片根部。同時(shí)葉輪也給液體一個(gè)較高的速度，這個(gè)速度通

9、過(guò)泵的一個(gè)固定部件轉(zhuǎn)化成壓力。我們一般稱為擴(kuò)壓器。在高壓泵里，很多葉輪可以被系列選用，并且在一個(gè)葉輪后有一個(gè)擴(kuò)壓器，也可能含有導(dǎo)輪，可以逐漸的降低液體的速度。對(duì)于低壓泵來(lái)說(shuō)，擴(kuò)壓泵一般就是一個(gè)螺旋形的通道，成為蝸殼，作用原理是攔截面逐漸增加可以有效降低流體的速度。在泵工作前，葉輪必須被灌注，也就是在泵啟動(dòng)時(shí)，葉輪必須被液體包圍。也可以通過(guò)在吸入線上放另一個(gè)截止閥來(lái)實(shí)現(xiàn)，截止閥在泵停止工作時(shí)是液體保留在泵內(nèi)。如果截止閥泄露了，泵可以通過(guò)閥

10、的入口，從外面的水源比如說(shuō)蓄水池來(lái)取水灌注。一般離心泵在排水線的地方也有一個(gè)閥控制流體和壓力。對(duì)于小流量和高壓力來(lái)說(shuō)，葉輪作用很大部分是放射狀的。對(duì)于高速流體和低壓排水壓力，泵中流體的方向可以近似于與軸的軸向平行，這時(shí)泵有一個(gè)軸流。這時(shí)葉輪就近似于螺旋推進(jìn)器。從一種流動(dòng)的狀態(tài)轉(zhuǎn)換到另一種流動(dòng)的狀態(tài)是漸進(jìn)的，對(duì)于中間狀態(tài)，設(shè)備可稱為混流泵。</p><p><b>　　2.離心泵 </b>&l

11、t;/p><p>　　離心泵是化工和石油工業(yè)中應(yīng)用最廣泛的一種泵。它能輸送性能非常廣泛的液體和固體含量高的懸浮液，像泥泥漿，可以用多種抗腐蝕材料建造。泵的整個(gè)外殼可用像聚丙烯這樣的塑料來(lái)建造，或者用腐蝕襯里加工。由于它的高速運(yùn)轉(zhuǎn)，可將其直接耦合到電動(dòng)機(jī)上，由電動(dòng)機(jī)的規(guī)格大小決定流量高低。</p><p>　　在這樣的泵中，液體被吸入到旋轉(zhuǎn)葉輪的中心，通過(guò)離心作用向外流動(dòng)。由于高速旋轉(zhuǎn)，液體在吸

12、入口和因動(dòng)能轉(zhuǎn)化為壓能的出口側(cè)獲得較高的動(dòng)能和壓力差。</p><p>　　葉輪由一系列弧形葉片組成，因此能使液體的流動(dòng)盡可能平穩(wěn)。葉輪中葉片越多，則液體的流動(dòng)方向越好控制，那么液體循環(huán)流動(dòng)時(shí)因波動(dòng)引起的損失就越少。在開(kāi)式葉輪中，葉片被固定在中心輪轂上，而在閉式中葉片則是用兩塊鋼板支撐以減少漏液。由此可以看出，在很大程度上，葉片末端的角度決定了泵的工作特性。</p><p>　　流體通常在

13、軸向上通過(guò)葉片的上升進(jìn)入泵殼。在這種簡(jiǎn)單類型的離心泵中，液體由切向方向隨著橫截面逐步流到蝸殼中。圖（a）所示為旋渦型泵。圖（b）中，在渦輪泵中的液體隨移動(dòng)的葉輪在一系列固定葉片中形成擴(kuò)散環(huán)。</p><p>　　這種旋渦能逐漸改變流體的流動(dòng)方向，并有效地將動(dòng)能轉(zhuǎn)化成壓能。固定葉片前緣處的流體應(yīng)該沒(méi)有受到?jīng)_擊。沿著葉輪葉片，液體的流動(dòng)具有一定速度，同時(shí)，葉片末端相對(duì)于泵體有移動(dòng)。液體的運(yùn)動(dòng)方向相對(duì)于泵殼——和固定葉

14、片所需的角度一樣——是兩個(gè)速度的合成方向。在圖c中，</p><p><b>　　c.</b></p><p>　　是液體相對(duì)于葉片的速度，是葉片上某點(diǎn)的切向速度；將這兩個(gè)速度合成即可得到液體的速度。因此，很明顯，在擴(kuò)散環(huán)中所需要的葉輪角由葉輪的產(chǎn)量、旋轉(zhuǎn)速度和葉片的角度決定。所以，泵在很嚴(yán)格的條件下才能有最大的運(yùn)行效能。</p><p>　　

15、2.1離心泵的有效壓頭</p><p>　　當(dāng)流體所剩余的動(dòng)能全部轉(zhuǎn)化為壓能時(shí)，壓力最大。如下文所述，有效壓頭和半徑的平方以及速度成正比，壓力更高時(shí)，必須使用多級(jí)泵。考慮到液體在離泵中心r到r+dr 的距離內(nèi)旋轉(zhuǎn)，如圖d</p><p><b>　　d.</b></p><p>　　所示。這一部分流體的質(zhì)量為dM =2πrdrdρ，其中ρ是流體

16、的密度，b是這部分流體的寬度。</p><p>　　如果流體在與切向方向成θ角上以速度u流動(dòng)，則這部分質(zhì)量流體的角動(dòng)量為</p><p>　　=dM(urcosθ)</p><p>　　流體通過(guò)泵所產(chǎn)生的扭轉(zhuǎn)力等于角動(dòng)量對(duì)時(shí)間的改變量</p><p>　　dτ=dM(u r cosθ)=2πr bρdr( u r cosθ)</p>

17、;<p><b>　　液體的體積流速為：</b></p><p><b>　　Q=2π r b</b></p><p>　　Dr=Qρd(u r cosθ)</p><p>　　因此，液體在泵中受到總的扭轉(zhuǎn)力由dτ在小標(biāo)1和2之間積分而得，下標(biāo)1引用的是泵入口處的條件，小標(biāo)2是出口時(shí)的條件。</p>

18、;<p>　　于是有：τ=Qρ（u2r2cosθ2 – u1r1cosθ1）</p><p>　　2.2離心泵的優(yōu)缺點(diǎn)</p><p><b>　　主要優(yōu)點(diǎn)有：</b></p><p>　?。?）制造簡(jiǎn)單，可用多種材料加工。</p><p><b>　?。?）無(wú)閥門。</b></

19、p><p>　?。?）高速運(yùn)轉(zhuǎn)（高達(dá)100赫茲），因此可直接耦合到電動(dòng)機(jī)上。一般地，速度越大，泵和電動(dòng)機(jī)的效率越小。</p><p><b>　　（4）能平穩(wěn)傳送。</b></p><p>　?。?）維修費(fèi)用比其他類型的泵少。</p><p>　?。?）輸送堵塞時(shí)，只要不是長(zhǎng)時(shí)間運(yùn)作，泵就不會(huì)被損壞。</p>

20、<p>　　（7）與其它泵相比，體積較小。因此，它可以利用電動(dòng)機(jī)做成密封裝置沉浸在吸收罐中。</p><p>　?。?）能容易輸送含有高比例懸浮固體的液體。</p><p><b>　　主要缺點(diǎn)有：</b></p><p>　?。?）單級(jí)泵不能提高壓力。而多級(jí)泵能提高壓頭，但價(jià)格昂貴而且由于它們的復(fù)雜性不能用抗腐蝕的材料加工建造。通常

21、用較高的速度來(lái)減少所需要的級(jí)數(shù)。</p><p>　?。?）只有在有限條件下才能以最高效能運(yùn)作：尤其是渦輪泵。</p><p>　?。?）它不能自動(dòng)注水。</p><p>　?。?）在輸送和吸收管道中，如果沒(méi)有止回閥，液體就會(huì)在泵停止瞬間倒流到吸入槽內(nèi)。</p><p>　?。?）不能有效處理粘性液體。</p><p>

22、;　　3. 離心泵中的汽蝕</p><p>　?。?）“汽蝕”一詞來(lái)源于拉丁語(yǔ)高弓足，這意味著一個(gè)中空的空間或空腔。韋氏詞典定義的字是在一個(gè)非常低的壓力區(qū)域流動(dòng)的液體腔內(nèi)迅速形成和崩潰的“腔”。</p><p>　　在離心泵中的任何地方像蒸氣泡沫，氣體泡沫，氣體破洞，氣泡等各種條件長(zhǎng)期作用都會(huì)造成汽蝕。這是一個(gè)各種結(jié)果同時(shí)作用的事情，不能簡(jiǎn)單地看待。汽蝕的形成討論如下。</p>

23、;<p>　　在離心泵的蝸殼中，汽蝕意味著一個(gè)氣泡內(nèi)的液體，他們的形成，成長(zhǎng)和隨后通過(guò)泵的液體流動(dòng)崩潰所經(jīng)歷的動(dòng)態(tài)過(guò)程。</p><p>　　一般來(lái)說(shuō)，液體內(nèi)氣泡的形成有兩種類型：蒸汽氣泡或氣態(tài)空泡。</p><p>　　1.由于一個(gè)進(jìn)程正在進(jìn)行的液體汽化而引起的泡沫的形成。蒸汽氣泡的形成和崩潰引起的汽蝕條件通常被稱為霧狀氣蝕。</p><p>　　2

24、.泡沫形成的過(guò)程中，由于正在往泵中輸送的液體中溶解入氣體（一般空氣的存在，但可能是系統(tǒng)中的任何氣體），由這些氣體的形成和崩潰引起的汽蝕條件通常被稱為氣態(tài)空泡。</p><p><b>　?。?）重要的定義：</b></p><p>　　為了使汽蝕機(jī)制有一個(gè)清晰的認(rèn)識(shí)，我們對(duì)所要遵循的重要術(shù)語(yǔ)的定義進(jìn)行了探討。</p><p><b>

25、　　?1）靜態(tài)壓力，</b></p><p>　　?2） 動(dòng)態(tài)的壓力，</p><p><b>　　?3） 總的能頭，</b></p><p><b>　　?4） 靜壓頭，</b></p><p><b>　　?5） 速度頭，</b></p><

26、p><b>　　?6） 蒸氣壓。</b></p><p>　　靜態(tài)壓力：在流體中的靜態(tài)壓力是指單位面積上流體的的移動(dòng)邊界與流體的正常作用力之間的壓力差。它描述了系統(tǒng)內(nèi)部和外部的壓力之間的差異，而無(wú)視系統(tǒng)中的其他條件。例如，當(dāng)提到管道，靜壓是內(nèi)管和外管的壓力之間的差異，而不用管任何管道內(nèi)的氣流。在能源方面，靜態(tài)壓力是流體的勢(shì)能。</p><p>　　動(dòng)態(tài)的壓力：由

27、于流體的動(dòng)能（二分之一），一個(gè)移動(dòng)的流體流施加壓力如果高于靜態(tài)的壓力，這種額外的壓力就被定義為動(dòng)態(tài)壓力。動(dòng)態(tài)壓力流體流的動(dòng)能將轉(zhuǎn)化為勢(shì)能。換句話說(shuō)，它是在流體流已經(jīng)從它的速度'V'到'零'的速度減慢的過(guò)程中存在的壓力。</p><p>　　總能頭：定義為總靜壓和總動(dòng)壓的總和。它是衡量運(yùn)動(dòng)流體流的總能量。</p><p>　　速度頭：相應(yīng)的動(dòng)態(tài)壓力的能頭稱為速

28、度頭。</p><p>　　蒸汽壓：蒸汽壓力必須是保持在液體狀態(tài)時(shí)液體的壓力。這適用于液體表面的壓力不夠時(shí)保持分子之間非常接近，氣體或蒸汽的分子將是自由的的分離和漫游。蒸汽壓力取決于液體的溫度。溫度越高，蒸汽壓越高。</p><p>　?。?）汽蝕的危害：汽蝕可以摧毀泵和閥門，可立即引起泵的效率損失，并且設(shè)備會(huì)不斷增加的效率損失，加速對(duì)泵部件的侵蝕。因此，重要的是理解現(xiàn)象，充分預(yù)測(cè)和減少汽

29、蝕產(chǎn)生的危害，也給診斷汽蝕問(wèn)題找到切實(shí)可行的解決方案</p><p>　　1）汽蝕增強(qiáng)化學(xué)腐蝕</p><p>　　泵在汽蝕條件下變得更容易受到腐蝕和化學(xué)攻擊。金屬常用的氧化層或鈍化層，可以保護(hù)金屬的進(jìn)一步腐蝕。氣泡如果連續(xù)的產(chǎn)生，就可以去除氧化或鈍化層而暴露未受保護(hù)的金屬被進(jìn)一步氧化。然后這兩個(gè)進(jìn)程（空化及氧化）共同作用，加速腐蝕金屬泵殼和葉輪，這一進(jìn)程就是不銹鋼也無(wú)能為力。</p

30、><p><b>　　2）材料選擇</b></p><p>　　金屬，塑料，或人類現(xiàn)在已知的任何其他材料，幾乎沒(méi)有能夠承受較高能量釋放的熱量和壓力產(chǎn)生的汽蝕。然而，在實(shí)踐中材料可以選擇，提高材料汽蝕作用下的承受能力，提供更長(zhǎng)的使用壽命和經(jīng)濟(jì)價(jià)值的結(jié)果，所以，注意泵的結(jié)構(gòu)材料是重要的和富有成效的。</p><p>　　汽蝕的問(wèn)題，是一個(gè)可以預(yù)測(cè)的問(wèn)題

31、，常見(jiàn)的材料，如鑄鐵和青銅就很適合泵的結(jié)構(gòu)。鑄鐵和青銅泵在工作20年或以上的時(shí)間，是沒(méi)有任何問(wèn)題的，即使這些泵遇到一些腐蝕。</p><p>　?。?）汽蝕的機(jī)理：氣蝕現(xiàn)象是一個(gè)循序漸進(jìn)的過(guò)程，如圖所示（下）</p><p>　　步驟之一，流動(dòng)液體內(nèi)氣泡的形成。</p><p>　　內(nèi)部形成氣泡的液體，當(dāng)它從液體到蒸氣揮發(fā)即發(fā)生相變。但液體汽化過(guò)程中是怎么發(fā)生的呢？

32、</p><p>　　任何一個(gè)封閉的容器內(nèi)的液體汽化發(fā)生在任何液體表面上產(chǎn)生的壓力減小，這樣就變成等于或小于液體的蒸汽壓力，在工作溫度下，液體溫度上升，蒸汽壓力提高，這樣就變成等于或大于在液體表面的壓力。例如，如果在室溫下的水（約77℉）保存在密閉容器中，系統(tǒng)壓力降低到它的蒸氣壓（約0.52 PSIA），水迅速變?yōu)檎羝?。此外，如果工作壓力是恒定保持在約0.52 PSIA和允許的溫度上升77華氏度以上，那么水迅速變

33、為蒸汽。</p><p>　　試想在一個(gè)密閉容器中，液體汽化，可發(fā)生在離心泵低于液體的蒸氣壓在泵的輸送溫度時(shí)，它的靜態(tài)壓力降低。</p><p>　　第二步，氣泡生長(zhǎng)，在沒(méi)有其他條件變化的情況下，新的泡沫不斷形成和舊泡沫規(guī)模的擴(kuò)大。形成的液體氣泡從葉輪眼沿葉片后緣尖流向葉輪出口。由于葉輪旋轉(zhuǎn)作用，氣泡達(dá)到非常高的速度，并最終達(dá)到葉輪內(nèi)的高壓地區(qū)，在那里他們開(kāi)始崩潰。據(jù)估計(jì)，在0.003秒的

34、時(shí)間是泡沫的生命周期。</p><p>　　第三步，泡沫的破滅，由于蒸汽氣泡沿葉輪葉片移動(dòng)，氣泡周圍的壓力開(kāi)始增加，直到達(dá)到一點(diǎn)泡沫外面的壓力大于氣泡內(nèi)部的壓力。泡沫崩潰。這個(gè)過(guò)程不是爆炸的過(guò)程，而是一個(gè)內(nèi)爆（向內(nèi)爆裂）。大約每個(gè)葉輪葉片上都有數(shù)百相同點(diǎn)會(huì)同時(shí)產(chǎn)生泡沫的破滅。泡沫的破滅是非對(duì)稱的，周圍的液體無(wú)秩序地填補(bǔ)這一空白，形成了液體微噴。微噴后泡沫破裂產(chǎn)生的力量，使氣泡爆破。曾經(jīng)有泡沫破滅的壓力大于1 GP

35、A（145x106磅）的報(bào)道。高度化的爆破效果破壞了泵的葉輪。汽蝕效果示意圖在這個(gè)數(shù)字說(shuō)明。</p><p>　　泡沫崩潰后，沖擊波散發(fā)向崩潰的邊緣。這是我們稱之為“汽蝕”其實(shí)就像我們聽(tīng)到的什么沖擊波。氣泡內(nèi)爆破，簡(jiǎn)而言之，汽蝕機(jī)制是所有有關(guān)泵輸送液體內(nèi)部氣泡的形成，生長(zhǎng)和崩潰的過(guò)程。但如何才能使汽蝕機(jī)制的知識(shí)能真正幫助解決汽蝕問(wèn)題。了解它的機(jī)制和概念，可以幫助確定氣泡的類型和他們的形成和崩潰的原因。</p

36、><p>　　(5) 解決方案和補(bǔ)救辦法：對(duì)于汽化（蝕）問(wèn)題</p><p>　　1. 防止汽化問(wèn)題，必須增加吸頭，降低流體的溫度，或減少NPSH（汽蝕余量）必需的。我們應(yīng)看看每個(gè)可能性：</p><p>　　1）增加吸頭：提高水箱中的液位</p><p><b>　　?提高通進(jìn)口的位置</b></p><

37、;p><b>　　?放置在一個(gè)坑泵</b></p><p>　　?減少管道損失。這些損失發(fā)生的各種原因，包括：</p><p>　　1.此系統(tǒng)的目的是不正確。有太多的配件或管道直徑太小一個(gè)管道內(nèi)襯已經(jīng)崩潰。</p><p>　　2.固體已經(jīng)堵在了管內(nèi)。</p><p>　　3.吸入管倒塌，尤其是它在重型車輛運(yùn)行的時(shí)

38、候。</p><p>　　4.一個(gè)過(guò)濾網(wǎng)被堵塞。</p><p>　　5.務(wù)必通水口是開(kāi)放的，不妨礙。通料口可以在寒冷的天氣凍結(jié)。</p><p>　　6.東西卡住管道，它要么增長(zhǎng)，或留在最后一次打開(kāi)系統(tǒng)。也許止回閥被打破，東西卡在管內(nèi)。</p><p>　　7.管道的內(nèi)部，已經(jīng)出現(xiàn)的腐蝕。</p><p>　　8.一

39、個(gè)更大的泵已經(jīng)被安裝，現(xiàn)行的制度有太多的損失，增加容量。</p><p>　　9.一個(gè)截止閥，用來(lái)取代閘閥。</p><p>　　10.加熱凍結(jié)和崩潰的管道。</p><p>　　11. 增加泵的轉(zhuǎn)速。</p><p>　　12.安裝一個(gè)增壓泵</p><p><b>　　13.加壓罐</b>&l

40、t;/p><p><b>　　2）降低流體的溫度</b></p><p>　　?注射少量液體冷卻器往往是不切實(shí)際的。</p><p>　　?絕緣管道想著太陽(yáng)的一面。</p><p>　　?小心放電循環(huán)線，他們可以加熱吸收液。</p><p>　　3）降低必需N.P.S.H. 量</p>

41、<p>　　?使用雙吸泵。這可以減少N.P.S.H.R.，或在某些情況下，它將使你由41％提高泵的轉(zhuǎn)速降低達(dá)27％</p><p>　　?使用較低速度的離心泵</p><p>　　?使用具有較大的葉輪泵。</p><p>　　?如果可能的話，安裝誘導(dǎo)裝置。這些誘導(dǎo)裝置可以減少N.P.S.H.R.近50％。</p><p>　　?使用

42、幾個(gè)較小的泵。三個(gè)半容量泵可以比一個(gè)大的泵便宜，加上備用。這也將保存較輕負(fù)載的能量。</p><p><b>　　2.吸蝕：</b></p><p>　　1．減少吸入管道的碎片。 </p><p>　　2．接近泵源槽/油底殼。</p><p><b>　　3．增加吸線直徑。</b></p>

43、;<p><b>　　4．降低吸程要求。</b></p><p>　　5．安裝較大的泵運(yùn)行速度較慢這將降低泵（NPSHR值）所需的汽蝕余量。</p><p><b>　　6．增加的壓力。 </b></p><p>　　7．全開(kāi)吸入管路閥門。</p><p><b>　　3.放

44、電蝕：</b></p><p>　　1．改變放電線位置 2.減少排放管線長(zhǎng)度為3.增加放電線直徑。 4.減少排放靜壓頭的要求。 5.安裝更大的泵，它會(huì)保持無(wú)放電空泡所需流動(dòng)。 6.完全開(kāi)放的排放管路閥。</p><p><b>　　4.對(duì)于再循環(huán)蝕：</b></p><p>　　1.降低特定的速度泵再循環(huán)點(diǎn)以上的流動(dòng)能力和經(jīng)營(yíng)范圍。

45、 2.提高吸頭。</p><p>　　CENTRIFUGAL PUMPS IN THE CHEMICAL INDUSTRY</p><p>　　Abstract : A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the liquid by a revo

46、lving device - an impeller. The most common type is the volute pump. Fluid enters the pump through the eye of the impeller which rotates at high speed. The fluid is accelerated radially outward from the pump chasing. A v

47、acuum is created at the impellers eye that continuously draws more fluid into the pump . This article stresses on a series of centrifugal pumps，F(xiàn)rom a brief</p><p>　　Keywords : centrifugal pump ,Introductio

48、n ,Working principle , Cavitation ， Mechanism of Cavitation ，Solution and Remedies</p><p>　　1. Introduction</p><p>　　Pump ,device used to raise ,transfer ,or compress liquids and gases .Four

49、general classes of pumps for liquids are described below .In all of them ,steps are taken to prevent cavitation (the formation of a vacuum) ,which would reduce the flow and damage the structure of the pump .Pumps used fo

50、r gases and vapors are usually known as compressors .The study of fluids in motion is called fluid dynamics.</p><p>　　Water pump ,device for moving water from one location to another ,using tubes or other ma

51、chinery .Water pumps operate under pressures ranging from a fraction of a pound to more than 10,000 pounds per square inch .Everyday examples of water pumps range from small electric pumps that circulate and aerate water

52、 in aquariums and fountains to sump pumps that remove water from beneath the foundations of homes . </p><p>　　One type of modern pumps used to move water is the centrifugal pump .Early version of the centrif

53、ugal pump ,the screw pump ,consists of a corkscrew-shaped mechanism in a pipe that ,when rotated ,pulls water upward .Screw pumps are often used in waste-water treatment plants because they can move large amounts of wate

54、r without becoming clogged with debris .In the ancient Middle East the need for irrigation of farmland was a strong inducement to develop a water pump .Early pumps in this region were</p><p>　　Also known as

55、rotary pumps ,centrifugal pumps have a rotating impeller ,also known as a blade ,that is immersed in the liquid .Liquid enters the pump near the axis of the impeller ,and the rotating impeller sweeps the liquid out towar

56、d the ends of the impeller blades at high pressure .The impeller also gives the liquid a relatively high velocity that can be converted into pressure in a stationary part of the pump ,known as the diffuser .In high-press

57、ure pumps ,a number of impeller may be used in</p><p>　　2.The Centrifugal Pump </p><p>　　The centrifugal pump is by far the most widely used type in the chemical and petroleum industries .It wil

58、l pump liquids with very wide ranging properties and suspensions with a high solids content including ,for example ,cement slurries ,and may be constructed from a very wide rang of corrosion resistant materials .The whol

59、e pump casing may be constructed from plastic such as polypropylene or it may be fitted with a corrosion-resistant lining .Because it operates at high speed ,it may be directl</p><p>　　In this type of pump ,

60、the fluid is fed to the centre of a rotating impeller and is thrown outward by centrifugal action .As a result of the high speed of rotation the liquid acquires a high kinetic energy and the pressure difference between t

61、he suction and delivery sides arises from the conversion of kinetic energy into pressure energy .</p><p>　　The impeller consists of a series of curved vanes so shaped that the flow within the pump is as smoo

62、th as possible .The greater the number of vanes on the impeller ,the greater is the control over the direction of the liquid and hence the smaller are the losses due to turbulence and circulation between the vanes .In th

63、e open impeller ,the vanes are fixed to a central hub ,whereas in the closed type the vanes are held between two supporting plates and leakage across the impeller is reduced .As wi</p><p>　　The liquid enters

64、 the casing of the pump，normally in an axial direction，and is picked up by the vanes of the impeller．In the simple type of centrifugal pump，the liquid discharges into a volute，a chamber of gradually increasing cross—sect

65、ion with a tangential outlet．A volute type of pump is shown in Fig.(a)．In the turbine pump[-Fig．(b)]the liquid flows from the moving vanes of the impeller through a series of fixed vanes forming a diffusion ring．</p&g

66、t;<p>　　This gives a more gradual change in direction to the fluid and more efficient conversion of kinetic energy into pressure energy than is obtained with the volute type．The angle of the leading edge of the fi

67、xed vanes should be such that the fluid is received without shock．The liquids flows along the surface of the impeller vane with a certain velocity whilst the tip of the vane is moving relative to the casing of the pump．T

68、he direction of motion of the liquid relative to the pump casing--and the r</p><p><b>　　c.</b></p><p>　　is the velocity of the liquid relative to the vane and is the tangential velo

69、city of the tip of the vane；compounding these two velocities gives the resultant velocity of the liquid．It is apparent，therefore，that the required vane angle in the diffuser is dependent on the throughput，the speed of ro

70、tation，and the angle of the impeller blades．The pump will therefore operate at maximum efficiency only over a narrow range of conditions．</p><p>　　Virtual head of a centrifugal pump</p><p>　　The

71、 maximum pressure is developed when the whole of the excess kinetic energy of the fluid is converted into pressure energy. As indicated below．the head is proportional to the square of the radius and to the speed，and is o

72、f the order of 60m for a single—stage centrifugal pump；for higher pressures，multistage pumps must be used．Consider the liquid which is rotating at a distance of between r and r+dr from the centre of the pump(Fig．d)．</

73、p><p>　　d </p><p>　　The mass of this element of fluid dm is given by 2πrdrdρ，where ρ is the density of the fluid and 6 is the width of the element of fluid。</p><p>　　If the

74、fluid is traveling with a velocity u and at an angle θ to the tangential direction．The angular momentum of this mass of fluid</p><p>　　= dM (urcosθ)</p><p>　　The torque acting on the fluid dτ is

75、 equal to the rate of change of angular momentum with time，as it goes through the pump</p><p>　　Dτ = dM α/αt(urcosθ)=2πrbρdrα/αt(urcosθ)</p><p>　　The volumetric rate of flow of liquid through th

76、e pump：</p><p>　　Q=2πrbα/αt</p><p>　　Dr =Q ρ d(urcosθ)</p><p>　　The total torque acting on the liquid in the pump is therefore obtained integrating dτ between the limits denoted by

77、suffix 1 and suffix 2，where suffix 1 refers to the conditions at the inlet to the pump and suffix 2 refers to the condition at the discharge．</p><p>　　Thus，τ=Q ρ(cos- cos)</p><p>　　The advantage

78、s and disadvantages of the centrifugal pump</p><p>　　The main advantages are：</p><p>　　(1) It is simple in construction and can，therefore， be made in a wide range of materials</p>&

79、lt;p>　　(2)There is a complete absence of valves．</p><p>　　(3)It operates at high speed(up to 100 Hz)and，therefore，can be coupled directly to</p><p>　　an electric motor. In general，the higher

80、 the speed the smaller the pump and motor for a give n duty．</p><p>　　(4)It gives a steady delivery．</p><p>　　(5)Maintenance costs are lower than for any other type of pump．</p><p>

81、　　(6)No damage is done to the pump if the delivery line becomes blocked，provided it is not run in this condition for a prolonged period．</p><p>　　(7)It is much smaller than other pumps of equal capacity．It c

82、an，therefore，be made into a sealed unit with the driving motor and immersed in the suction tank．</p><p>　　(8)Liquids containing high proportions of suspended solids are readily handled．</p><p>　

83、　The main disadvantages are：</p><p>　　(1)The single—stage pump will not develop a high pressure．Multistage pumps will develop greater heads bat they are very much more expensive and cannot readily be made in

84、 corrosion—resistant material because of their greater complexity．It is generally better to use very high speeds in order to reduce the number of stages required． </p><p>　　(2)It operates at a high efficien

85、cy over only a limited range of conditions; this applies especially to turbine pumps． </p><p>　　(3)It is not usually self-priming.</p><p>　　(4)If a non-return valve is not incorporated in the

86、 delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops．</p><p>　　(5)Very viscous liquids cannot he handled efficiently．</p><p>　　3. Cavitation in ce

87、ntrifugal pump</p><p>　?。?）The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collap

88、se of cavities in a flowing liquid in regions of very low pressure. </p><p>　　In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of t

89、he term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.</p><p>　　In the context of centrifugal pumps, the term cavitation impli

90、es a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump. </p><p>　　Generally, the bubbles that form inside the liquid are of

91、two types: Vapor bubbles or Gas bubbles. </p><p>　　1.Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of v

92、apor bubbles is commonly referred to as Vaporous Cavitation. </p><p>　　2.Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in

93、the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation. </p><p>　?。?）Important Definitions: To enable a clear understanding o

94、f mechanism of cavitation, definitions of following important terms are explored.</p><p>　　·         Static pressure,</p><p>　　·    &#

95、160;    Dynamic pressure,</p><p>　　·         Total pressure,</p><p>　　·         Static pressure hea

96、d,</p><p>　　·         Velocity head, </p><p>　　·         Vapour pressure.</p><p>　　Static pressure ：T

97、he static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in t

98、he system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static p

99、ressure is a measure of the potential ener</p><p>　　Dynamic pressure：A moving fluid stream exerts a pressure higher than the static pressure due to the kinetic energy (½ mv2) of the fluid. This addition

100、al pressure is defined as the dynamic pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fl

101、uid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.</p><p>　　Total pressure：The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of tot

102、al energy of the moving fluid stream. i.e. both potential and kinetic energy.</p><p>　　Velocity head：Vapor pressure is the pressure required to keep a liquid in a liquid state. If the pressure applied to the

103、 surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liqu

104、id. Higher the temperature, higher will be the vapor pressure.</p><p>　?。?） Cavitation Damage：Cavitation can destroy pumps and valves, and cavitation causes a loss of efficiency in pumps immediately, and al

105、so a continuously increasing loss of efficiency as the equipment degrades due to erosion of the pump components by cavitation. Therefore It is important to understand the phenomena sufficiently to predict and therefore

106、reduce cavitation and damage from cavitation, and also to diagnose and find practical solutions to cavitation problems。</p><p>　　1）Cavitation Enhanced Chemical Erosion</p><p>　　Pumps operating u

107、nder cavitation conditions become more vulnerable to corrosion and chemical attack. Metals commonly develop an oxide layer or passivated layer which protects the metal from further corrosion. Cavitation can remove this

108、 oxide or passive layer on a continuous basis and expose unprotected metal to further oxidation. The two processes (cavitation & oxidation) then work together to rapidly remove metal from the pump casing and impell