液位變送器測量原理和雙液位計新的選型方法。
1 雙液位計簡介
石油化工自控(kong)設(she)計(ji)人員對(dui)一些較重要的塔、槽、釜、罐的液(ye)位(wei)測量和控(kong)制采(cai)用(yong)雙液(ye)位(wei)計(ji)的設(she)置,如(ru)圖1和圖2所示。這樣設(she)計(ji)一是為(wei)了將被測量液(ye)位(wei)轉(zhuan)換成顯示和控(kong)制所需(xu)的信(xin)號,二是為(wei)了方便(bian)操作人員在生(sheng)產(chan)現場進行(xing)巡(xun)回檢查(cha)時能方便(bian)地看(kan)到被測量液(ye)位(wei)指示值(zhi),三是為(wei)了方便(bian)維護。其中之一液(ye)位(wei)計(ji)發生(sheng)故障(zhang)可以無顧慮(lv)地進行(xing)檢液(ye)位(wei)計(ji)修,不(bu)會(hui)影(ying)響生(sheng)產(chan)正常進行(xing)。

2 存在的問題
以上的雙液位計的設置應該說考慮得十分周全,可是多數測量液位變送器都和密度Q有關,這就造成在實際使用過程中往往會出現這兩臺液位計指示值不能吻合,有時誤差較大,尤其在一個生產裝置開停工過程中或操作系統不穩定的情況下,兩者之間誤差甚至會達到30%左右。這時生產操作人員就會為液位測量儀表的準確性向儀表維修人員提出疑問,例如圖1,某苯塔塔底液位測量,它采用差壓測量液位變送器和玻璃板液位計相結合的雙液位計,它的設計使用工況條件是塔底壓力80kPa,溫度150e,介質密度Q:0.69g/cm3,差壓變送器測量液位的工作原理是$P=QH,H=Q/$P,當生產穩定時,Q=0.69g/cm3,也就是說相當于一個常數,液位高度H只和$P有關,差壓變送器所測的液位應該是正確的,也能和玻璃板液位計顯示的液位高度一致。但在裝置送苯過程中,塔底注入的純苯,其密度為0.878g/cm3,塔底壓力和溫度均為常溫常壓,遠遠偏離設計值,這時就會出現玻璃板液位計所顯示的液位高度和差壓變送器傳送到中控室的液位指示值相差甚大。在注苯過程中差壓變送器所測的差壓$P注=Q注H注=0.878H注與設計值$P設=Q設H設=0.69H設相差甚遠,因為差壓變送器無法識別所測的介質密度。這時它的差壓輸出為$P注=$P設,這樣就出現0.69H設= 0.878H注、H設=0.878/0.69H注=1.27H注,這就是在注苯過程中差壓變送器顯示的液位高度遠遠大于玻璃板液位計液位顯示高度的原因。
如圖2,某一鍋爐液位測量,它采用外浮筒液位變送單元和玻璃板液位計相結合的雙液位計,它的設計使用工況條件是壓力3MPa、溫度230e、介質密度0.823g/cm3。外浮筒液位變送器測量液位的原理是基于阿基米德定律,浮筒浸在液體里所受到的浮力大小等于浮筒所排開液體的重量:
F=Q.AH
F-浮筒所受的浮力;
Q-液體密度;
A-浮筒截面積;
H-浮筒排開液體的液柱高度(被測液位的高度)。
對于一臺外浮筒液位變送器,它的浮筒截面積A是恒定不變的,可視為一個常數,那么浮力只和被測液體的密度Q和高度H有關,在正常生產的情況下,被測液體的密度Q基本上是一個常量,即被測液位的高度H和浮力F成為一個線性關系: F設=Q設.AH設。外浮筒液位變送器所測的液位高度(即中控室顯示的液位高度)就能和現場玻璃液位計顯示的液位高度一致。可是鍋爐開工初期進水、升壓升溫過程中,所有的工況值都大大偏離設計值。這時水的密度Q開=0.98g/cm3,就會出現F開=Q開.AH開,按設計要求F設=F開,也就是Q設.AH設=Q開.AH開,H設=Q開/Q設H開,代入開工和設計的不同密度值,就會得到H設=0.98/0.823H開=1.19H開,這就是鍋爐開工初期出現的外浮筒液位變送器所測的液位高度(即中控室顯示的液位高度)大大高于現場玻璃液位計顯示的液位高度的原因。這是兩個較為典型的雙液位計之間誤差偏大的問題,誤差也只是暫時的,等到開工正常后,各項參數尤其是液體密度Q達到設計值時,兩臺液位計的指示就會一致,一般情況下不必進行處理。但是出現儀表維修人員不能清楚地向生產操作人員解釋這一現象、或生產操作人員一定要求兩臺液位計指示一致的情況下,只能將液位變送器或差壓變送器的零位一直下調到兩表一致,待開工正常之后,再將液位變送器或差壓變送器的零位調回去,使兩臺液位計在正常生產情況下指示值一致。有時候在生產過程中也會出現由于生產控制過程發生變化,尤其是溫度發生較大變化的情況,這時就會造成被測液體的密度Q發生較大的變化,從而使液位變送器指示值出現偏差,造成兩臺液位計之間誤差較大。在雙液位計之間發生指示偏差較大。在處理這類故障時,一定要首先了解生產工況是否有較大變化,一定要慎重處理,以免帶來很多不必要的工作。
3 磁翻板雙液位計的原理及(ji)優點(dian)

20世紀末,磁翻板現場指示液位計和磁翻板液位變送器的出現給自控設計者在液位計的選型使用上增添了余地。磁翻板液位計的工作原理如圖3所示。
磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)隨著被測液(ye)(ye)(ye)體(ti)上(shang)下移(yi)動,翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)標尺中(zhong)(zhong)的(de)(de)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban),受到磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)的(de)(de)吸(xi)引而翻(fan)(fan)(fan)轉,翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)上(shang)正反兩(liang)面涂有(you)紅(hong)(hong)、綠兩(liang)色(se)(se)。紅(hong)(hong)色(se)(se)顯(xian)(xian)示(shi)(shi)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)指(zhi)示(shi)(shi)值,綠色(se)(se)為(wei)(wei)(wei)空值,這樣可(ke)(ke)以(yi)清(qing)晰(xi)地顯(xian)(xian)示(shi)(shi)出液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)的(de)(de)高度(du)的(de)(de)現(xian)(xian)象(xiang)。磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)清(qing)除了(le)(le)被測液(ye)(ye)(ye)體(ti)對玻(bo)(bo)璃(li)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)的(de)(de)污染造成看不清(qing)被測液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)高度(du),及(ji)玻(bo)(bo)璃(li)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)取樣角(jiao)閥易堵塞和(he)選(xuan)位(wei)(wei)(wei)(wei)(wei)不當所(suo)造成的(de)(de)假(jia)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)現(xian)(xian)象(xiang),而被廣(guang)泛地替代(dai)玻(bo)(bo)璃(li)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)。磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)變(bian)(bian)(bian)送器(qi)(qi)(qi)采(cai)用(yong)磁(ci)(ci)(ci)(ci)耦合工(gong)作(zuo)原理,傳感器(qi)(qi)(qi)內裝有(you)一(yi)組干簧管和(he)精(jing)密(mi)電(dian)阻(zu),當磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)隨液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)變(bian)(bian)(bian)化上(shang)下移(yi)動時,位(wei)(wei)(wei)(wei)(wei)于液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)處的(de)(de)干簧管依次接通(tong),使(shi)傳感器(qi)(qi)(qi)的(de)(de)輸出電(dian)阻(zu)發生變(bian)(bian)(bian)化(如(ru)(ru)圖4所(suo)示(shi)(shi)),再(zai)通(tong)過轉換電(dian)路模塊將電(dian)阻(zu)值轉換4~20mA電(dian)流輸出。磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)玻(bo)(bo)璃(li)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)和(he)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)變(bian)(bian)(bian)送器(qi)(qi)(qi)都(dou)(dou)以(yi)磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)為(wei)(wei)(wei)基礎,因為(wei)(wei)(wei)磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)與(yu)被測液(ye)(ye)(ye)體(ti)的(de)(de)密(mi)度(du)Q有(you)關,這就給雙(shuang)(shuang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)的(de)(de)選(xuan)型增加了(le)(le)新(xin)方(fang)法。我(wo)們(men)分別對塔、槽、廢熱(re)鍋爐如(ru)(ru)圖1、圖2的(de)(de)雙(shuang)(shuang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)均采(cai)用(yong)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)現(xian)(xian)場(chang)指(zhi)示(shi)(shi)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)和(he)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)痊變(bian)(bian)(bian)送器(qi)(qi)(qi)的(de)(de)方(fang)式進(jin)行改造,取得了(le)(le)良好(hao)的(de)(de)效果,兩(liang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)之間測量誤差(cha)可(ke)(ke)控制(zhi)(zhi)在(zai)20-30mm。兩(liang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)指(zhi)示(shi)(shi)基本一(yi)致(zhi),方(fang)便了(le)(le)生產操(cao)作(zuo),同(tong)時省去開(kai)停工(gong)及(ji)正常(chang)生產時對儀表零位(wei)(wei)(wei)(wei)(wei)的(de)(de)反復調(diao)整。在(zai)改造試(shi)驗過程中(zhong)(zhong)體(ti)會到,采(cai)用(yong)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)現(xian)(xian)場(chang)指(zhi)示(shi)(shi)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)和(he)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)變(bian)(bian)(bian)送器(qi)(qi)(qi)作(zuo)為(wei)(wei)(wei)雙(shuang)(shuang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)設(she)(she)計(ji)(ji)(ji)(ji)方(fang)案時,兩(liang)臺(tai)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)最(zui)好(hao)由同(tong)一(yi)制(zhi)(zhi)造廠同(tong)時生產,這就可(ke)(ke)以(yi)保(bao)證(zheng)磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)的(de)(de)重量一(yi)致(zhi)性(xing)(xing)(xing)(xing),也就是(shi)介質密(mi)度(du)Q的(de)(de)一(yi)致(zhi)性(xing)(xing)(xing)(xing),減少磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)重量差(cha)異帶來的(de)(de)測量誤差(cha)及(ji)零位(wei)(wei)(wei)(wei)(wei)的(de)(de)偏差(cha)。磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)外(wai)表要(yao)(yao)光滑無毛刺(ci)(ci),要(yao)(yao)有(you)足夠的(de)(de)磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing),因為(wei)(wei)(wei)磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)變(bian)(bian)(bian)送器(qi)(qi)(qi)的(de)(de)兩(liang)種變(bian)(bian)(bian)換原理,干簧管驅動有(you)阻(zu)式和(he)磁(ci)(ci)(ci)(ci)致(zhi)伸縮式,都(dou)(dou)與(yu)磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)浮(fu)子(zi)(zi)(zi)(zi)的(de)(de)磁(ci)(ci)(ci)(ci)場(chang)強(qiang)弱有(you)關,磁(ci)(ci)(ci)(ci)場(chang)強(qiang)度(du)足夠、磁(ci)(ci)(ci)(ci)性(xing)(xing)(xing)(xing)分布(bu)均勻、外(wai)表沒有(you)毛刺(ci)(ci)都(dou)(dou)可(ke)(ke)清(qing)除測量液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)瞬間回(hui)零現(xian)(xian)象(xiang),保(bao)證(zheng)系統(tong)的(de)(de)穩定性(xing)(xing)(xing)(xing)。由于磁(ci)(ci)(ci)(ci)翻(fan)(fan)(fan)板(ban)(ban)(ban)(ban)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)相對價格便宜、維修簡便、性(xing)(xing)(xing)(xing)能可(ke)(ke)靠,成為(wei)(wei)(wei)雙(shuang)(shuang)液(ye)(ye)(ye)位(wei)(wei)(wei)(wei)(wei)計(ji)(ji)(ji)(ji)設(she)(she)計(ji)(ji)(ji)(ji)方(fang)案的(de)(de)首(shou)選(xuan)。