1．蒸發罐液位測量的原理
    目前制鹽(yan)蒸發(fa)(fa)罐(guan)液(ye)(ye)(ye)位的(de)測量(liang)常規方(fang)案還(huan)是采用(yong)單插入法蘭液(ye)(ye)(ye)位變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)或單平遠傳(chuan)法蘭液(ye)(ye)(ye)位變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)來檢(jian)測液(ye)(ye)(ye)位(安(an)裝(zhuang)示意圖見圖1)。為保(bao)證液(ye)(ye)(ye)位變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)正／負(fu)(fu)(fu)壓(ya)(ya)室的(de)氣(qi)相(xiang)平衡．需在(zai)(zai)變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)負(fu)(fu)(fu)壓(ya)(ya)側(ce)接(jie)一負(fu)(fu)(fu)壓(ya)(ya)管與蒸發(fa)(fa)罐(guan)上部的(de)氣(qi)相(xiang)相(xiang)接(jie)。因在(zai)(zai)運行(xing)時，負(fu)(fu)(fu)壓(ya)(ya)管中(zhong)會有二次(ci)蒸汽冷凝，形成一定高度(du)的(de)液(ye)(ye)(ye)柱(zhu)．對變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)的(de)負(fu)(fu)(fu)壓(ya)(ya)室造成附加的(de)壓(ya)(ya)力，故(gu)在(zai)(zai)初次(ci)安(an)裝(zhuang)時，確定變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)量(liang)程(cheng)后，在(zai)(zai)空(kong)罐(guan)狀態需將低壓(ya)(ya)側(ce)管路充滿水．并將由此產生的(de)差壓(ya)(ya)遷(qian)移掉，即負(fu)(fu)(fu)遷(qian)移。經過量(liang)程(cheng)遷(qian)移后，在(zai)(zai)運行(xing)時，變(bian)(bian)(bian)送(song)(song)(song)器(qi)(qi)檢(jian)測到的(de)差壓(ya)(ya)值轉換成4 20 mA輸出信(xin)號．就與液(ye)(ye)(ye)位零位至滿量(liang)程(cheng)成一一對應關系了。 

圖1 液壓變送器安裝示意圖
 
2．液位測量系統伴熱的特殊性
    在制鹽工程中．由于蒸發罐大多是負壓．有一定的真空，因此水的沸點較正常大氣壓下要低得多。例如在鹽硝聯產工程中．正常工況下，EV一303罐的絕壓約為17 KPa左右，EV一304罐的絕壓約為8 KPa左右。在EV一303、304罐中，當液位變送器負壓管伴熱溫度分別達到58℃、42℃以上時，變送器負壓管中的冷凝水便會閃發．從而造成液位檢測故障。我們原先使用蒸汽伴熱時．由于用來伴熱的蒸汽溫度較高，加上被伴熱的變送器負壓管本身管徑較細，熱容小，又沒有設置伴熱的溫度檢測儀表．伴熱時間一長，變送器負壓管中的冷凝水便沸騰閃發，形成汽／液兩相，導致液位顯示值劇列波動，冷凝水被蒸發掉一部分后，又會造成液位測量值出現偏高的現象。
使用蒸汽伴熱．有可能導致伴熱溫度太高。而使用伴熱電纜，在(zai)EV一(yi)303、304罐(guan)液位伴(ban)熱(re)上(shang)也(ye)有(you)可能(neng)(neng)出現(xian)伴(ban)熱(re)溫(wen)(wen)(wen)度(du)(du)(du)過高(gao)(gao)現(xian)象。雖然我(wo)(wo)們(men)使(shi)用(yong)低溫(wen)(wen)(wen)型自限溫(wen)(wen)(wen)伴(ban)熱(re)電纜(最(zui)高(gao)(gao)維持溫(wen)(wen)(wen)度(du)(du)(du)65℃)，也(ye)就是(shi)說如果在(zai)保溫(wen)(wen)(wen)沒有(you)熱(re)損(sun)失(shi)的(de)(de)情況(kuang)下．變送(song)器負壓(ya)管(guan)的(de)(de)溫(wen)(wen)(wen)度(du)(du)(du)最(zui)高(gao)(gao)將能(neng)(neng)達到6O℃多一(yi)些就不(bu)再上(shang)升．但(dan)實踐表明即使(shi)在(zai)負壓(ya)管(guan)上(shang)平行單根鋪(pu)設(she)伴(ban)熱(re)電纜．其(qi)伴(ban)熱(re)溫(wen)(wen)(wen)度(du)(du)(du)也(ye)有(you)可能(neng)(neng)超(chao)過5O℃ (視環(huan)境溫(wen)(wen)(wen)度(du)(du)(du)及保溫(wen)(wen)(wen)狀況(kuang)導致熱(re)損(sun)失(shi)的(de)(de)大小(xiao)而有(you)所不(bu)同)。由于液位測量系統伴(ban)熱(re)的(de)(de)特殊性(xing)．故我(wo)(wo)們(men)在(zai)應用(yong)伴(ban)熱(re)電纜時．還是(shi)很有(you)必要對其(qi)伴(ban)熱(re)溫(wen)(wen)(wen)度(du)(du)(du)加以(yi)檢測并進行控(kong)制。