鍋爐汽包水位平衡容器的選型和壓力校正問題探討

1、 收稿日期:20021218作者簡介:高啟繁(1964 , 男, 甘肅靖遠(yuǎn)人,1987年畢業(yè)于福州大學(xué)應(yīng)用物理專業(yè), 現(xiàn)為珠江電廠熱工工程師, 從事火電廠熱工儀表與自動化設(shè)備的維修工作。鍋爐汽包水位平衡容器的選型和壓力校正問題探討高啟繁(廣州珠江電廠, 廣東番禺511458摘要介紹了差壓式水位計幾種平衡容器的結(jié)構(gòu)和性能, 分析了汽包壓力對水位測量的影響, 給出了幾種平衡容器的壓力校正方法和選型建議。關(guān)鍵詞汽包水位; 差壓式水位計; 測量; 平衡容器; 壓力校正中圖分類號TK223. 6文獻(xiàn)標(biāo)識碼A文章編號10023364(2003 090017041, 、蒸汽罩式(即帶中間抽頭的雙室平衡容器

2、4種。1. 1單室平衡容器單室平衡容器又叫簡單平衡容器, 如圖1所示, 其結(jié)構(gòu)簡單, 安裝方便, 但測量誤差較大。當(dāng)鍋爐在額定汽壓運(yùn)行, 水位為正常水位時, 其輸出的差壓p 比較穩(wěn)定, 測量較準(zhǔn)確; 當(dāng)汽壓下降時(即使此時的水位保持不變, 正壓側(cè)壓力(p + 變化不大 , 負(fù)壓側(cè)的壓力(p - 將顯著增大, 致使平衡容器輸出差壓減小, 水位表指示偏高。由圖1可以得到水位測量關(guān)系式:p =p +-p -=L (c -s g -H (w -s g (1 或H L (c -s g -p (w -s g(2圖1和式(1 、式中:c 為平衡容器內(nèi)水密度,kg/m 3; w 為汽包內(nèi)飽和水密度,kg/m

3、3; s 為汽包內(nèi)飽和汽密度,kg/m 3; g 為重力加速度,m/s 2; H 為汽包水位, m ; p 為平衡容器輸出差壓,kPa ; L 為水位計量程,m 。1正壓一次門2單室平衡容器3負(fù)壓一次門4汽包圖1配單室平衡容器的差壓式水位計測量示意由圖1可以看出, 正壓側(cè)壓力p +由恒定的水柱高度維持(汽包內(nèi)的蒸汽經(jīng)過正壓側(cè)一次門“1”注入平衡容器內(nèi)并凝結(jié)成水, 利用溢流原理將多余的水流回汽包。, 負(fù)壓側(cè)壓力p -則隨汽包水位變化而變化, 所以p 即隨汽包水位而變化。但是, 由于汽包內(nèi)的飽和水與平衡容器內(nèi)的冷凝水溫度不同(即密度不同 , 會導(dǎo)致測量誤差。為了減少此誤差, 通常是使平衡容器的安裝

4、標(biāo)高(正負(fù)壓取樣管的垂直距離L 與二次顯示儀表的刻度全量程一致, 并在二次表校驗(yàn)時, 按運(yùn)行額定參數(shù)和環(huán)境平均溫度,研究論文(| 考慮密度影響的修正值。單室平衡器一般用于測量低溫、低壓容器的水位, 在用于測量鍋爐汽包水位時, 要運(yùn)用水位測量的汽壓自動校正系統(tǒng)才能實(shí)現(xiàn)較準(zhǔn)確的測量。1. 2雙室平衡容器雙室平衡容器(簡單雙室平衡容器 的結(jié)構(gòu)如圖2所示 。1正壓一次門2雙室平衡容器3負(fù)壓一次門4汽包圖2雙室平衡容器的結(jié)構(gòu)和測量示意雙室平衡容器的正壓側(cè)與單室平衡容器一樣持恒定水柱高度, , 管下緣高10, , 。2根管內(nèi)水的溫度比較接近, 減少了采用單室平衡容器因正負(fù)壓取樣管內(nèi)水的密度不同所引起的測量

5、誤差, 但是, 由于平衡容器內(nèi)的溫度遠(yuǎn)遠(yuǎn)低于汽包內(nèi)的溫度, 故負(fù)壓管內(nèi)的水位比汽包實(shí)際水位偏低, 因而產(chǎn)生測量誤差。當(dāng)汽壓和平衡容器環(huán)境溫度變化時, 此誤差是個變數(shù)。雙室平衡容器的水位測量關(guān)系式與單室平衡容器相同。1. 3蒸汽罩式雙室平衡容器蒸汽罩式雙室平衡容器是采用汽包內(nèi)飽和蒸汽來加熱正、負(fù)壓側(cè)取樣管內(nèi)的水, 使之處于飽和溫度, 即c =w , 從而消除了簡單雙室平衡容器內(nèi)水溫度與汽包內(nèi)水溫度不同帶來的測量誤差, 其結(jié)構(gòu)如圖3所示。由于c =w , 蒸汽罩式雙室平衡容器的水柱差壓關(guān)系式為:p =(L -H (w -s g (3 或H =L -p/(w -s g(4蒸汽罩雙室平衡容器雖然消除了

6、環(huán)境溫度對水位測量的影響, 但當(dāng)汽壓p 降低時, 雖然實(shí)際水位不變 , 圖3蒸汽罩式雙室平衡容器水位測量示意但由于飽和水密度增加, 正壓管壓力p +增大, 平衡容器輸出差壓也將顯著增大, 導(dǎo)致水位指示偏低。1. 4蒸汽罩補(bǔ)償式平衡容器鑒于當(dāng)汽壓變化時, 單室平衡容器和蒸汽罩雙室平衡容器的輸出差壓變化方向恰好是相反的, 于是提出了蒸汽罩補(bǔ)償式平衡容器(即帶中間抽頭的雙室平衡容器 , 其結(jié)構(gòu)如圖4所示。1正壓恒位水槽2蒸汽罩3正壓管4疏水管5負(fù)壓管圖4蒸汽罩補(bǔ)償式平衡容器結(jié)構(gòu)和測量示意蒸汽罩補(bǔ)償式平衡容器正壓側(cè)取樣管的水柱改由2段組成, l 段保持飽和溫度, Ll 段保持室溫。適當(dāng)選擇2段的比例,

7、 即可獲得在某一特定水位(如正常水位 下平衡容器輸出的差壓值不受汽壓變動的影響。目前, 測量中小型鍋爐汽包水位時, 廣泛采用蒸汽罩補(bǔ)償式平衡容器, 用蒸汽罩對正壓恒位水槽加熱, 使槽內(nèi)的水在任何情況下都與汽包壓力下飽和水的密度相同, 不受環(huán)境溫度的影響。蒸汽罩的加熱蒸汽取自汽包的蒸汽室, 凝結(jié)水經(jīng)疏水管“4”流至鍋爐下降管。為了使平衡容器能迅速達(dá)到正常的工作狀態(tài), 在汽包與平衡容器的連接管之間加裝汽側(cè)一次門, 當(dāng)鍋爐開始升壓時, 要關(guān)閉該閥門, 使較高壓力的爐水由疏水管注入平衡容器, 并迅速充滿正壓恒位水槽。這樣,研究論文( 待儀表管路沖洗后, 打開該閥門, 水位表即可正常投入。在鍋爐參數(shù)變化

8、時, 為了保證汽包水位一定, 差壓與水位成單值函數(shù), 密度補(bǔ)償長度l 必須選擇合適, l 值的確定是在水位為正常水位H 0時求取的。因此, 只有當(dāng)汽包水位為H 0時, 才能進(jìn)行良好的密度補(bǔ)償。l 值的計算公式如下:l =(L -H (s -se /(w -we +H =(L -H +H(5式中:we 、se 為額定汽壓下飽和水和飽和蒸汽的密度, kg/m 3; w 、s 為與補(bǔ)償范圍下限壓力相應(yīng)的飽和水和飽和蒸汽密度, kg/m 3; 為比例系數(shù)。由于飽和水密度和飽和蒸汽密度與汽壓之間為非線性關(guān)系, 因此, 即使線性化后, 式(5 中的比例系數(shù)對于不同的汽壓范圍, 其數(shù)值也有所不同。如:汽壓為

9、4. 41MPa 時, =-0. 18; 汽壓為10. 78MPa 時, =-0. 26; 汽壓為14. 7MPa 時, =-0. 32。蒸汽罩補(bǔ)償式平衡容器的水位差壓轉(zhuǎn)換關(guān)系式如下:H (L -l (c -s g +l (w - g p(w -s g2鍋爐在正常運(yùn)行時, 汽包水位將保持在正常水位附近。從這個角度出發(fā), 對于中、小型鍋爐, 在汽包水位測量不采用汽壓自動校正(補(bǔ)償 系統(tǒng)條件下, 以采用蒸汽罩補(bǔ)償式平衡容器(帶中間抽頭的雙室平衡容器 為宜。對于采用汽壓校正(補(bǔ)償 的水位測量系統(tǒng)(高壓或超高壓鍋爐 , 當(dāng)平衡容器安裝現(xiàn)場環(huán)境溫度較穩(wěn)定時, 以采用單室平衡容器為宜, 因?yàn)槠浣Y(jié)構(gòu)簡單、安

10、裝方便, 同時還可避免在采用雙室平衡容器時汽壓驟然下降, 造成飽和水汽化而喪失正壓頭的危險。但如果平衡容器安裝現(xiàn)場環(huán)境溫度變化較大時, 以采用蒸汽罩式雙室平衡容器為宜。3汽包水位測量的壓力校正(補(bǔ)償3. 1單室平衡容器、簡單雙室平衡容器由式(1 可知, 如果將差壓信號p 與反映密度變化的信號(c -s g L 代數(shù)相減, 再除以密度變化信號(w -s g , 則測量系統(tǒng)的輸出為水位H 。平衡容器的汽包壓力自動校正系統(tǒng)如圖5所示 。圖5單室平衡容器、簡單雙室平衡容器汽包壓力自動校正系統(tǒng)圖中f (x 1 和f (x 2 為函數(shù)轉(zhuǎn)換器, 其輸出分別為(c -s g L 和(w -s g , 二者能自

11、動跟隨汽包壓力變化而變化, 達(dá)到自動校正汽包壓力的目的。由于采用單室平衡容器時c 仍隨環(huán)境溫度變化, 因此, 測量上仍有一定的測量誤差, 該誤差的大小取決于c 隨環(huán)。. , 蒸汽罩雙室平衡容器內(nèi)凝結(jié), 不受環(huán)境溫度的影響。由式(4 可得到其壓力校正系統(tǒng)(圖6 。圖6蒸汽罩雙室平衡容器的壓力校正系統(tǒng)函數(shù)轉(zhuǎn)換器f (x 2 接受汽包壓力信號, 輸出為(w-s g , 經(jīng)除法器與差壓信號相除, 再送入減法器與代表L 的定值電壓相減, 便得到汽包水位H 。3. 3蒸汽罩補(bǔ)償式平衡容器的壓力校正由式(6 可得到蒸汽罩補(bǔ)償式平衡容器水位測量壓力校正系統(tǒng)(圖7 。綜上所述, 在汽包壓力校正系統(tǒng)中, f (x

12、 1 用來描述(c -s 與汽壓的關(guān)系, f (x 2 用來描述(w -s 與研究論文( 圖7蒸汽罩補(bǔ)償式平衡容器的壓力校正系統(tǒng)汽壓的關(guān)系。這些關(guān)系在實(shí)際應(yīng)用中可采用多折線函數(shù)發(fā)生器來實(shí)現(xiàn); 在采用微處理器或計算機(jī)控制系統(tǒng)的情況下, 可采用較復(fù)雜的計算式, 亦可采用按汽壓分段的多段計算式以提高計算精度。此外, 還可利用計算機(jī)控制系統(tǒng)強(qiáng)大的存儲功能, 采用查表的方法實(shí)現(xiàn)汽包水位的壓力自動校正(補(bǔ)償 。但由于汽包水位測量涉及的因素較多, 壓力校正的精度要求適可而止。而且, 以上討論都是在假定凝結(jié)水溫度恒定的情況下給出的。在火電廠運(yùn)行現(xiàn)場, 凝結(jié)水溫度是隨環(huán)境溫度和運(yùn)行狀況而變化的, 因此, 單純提

13、高壓力補(bǔ)償精度而忽略環(huán)境溫度的影響也是不合適的。4結(jié)語(1 鍋爐汽包水位平衡容器的選型應(yīng)因地制宜。(2 平衡容器有多種壓力校正方式, 其校正精度可酌情而定, 但要適度。(上接第12頁含氧化合物、潔凈煤氣化聯(lián)合循環(huán)發(fā)電(IG 得到較為成功的應(yīng)用。T T 貴, , 、原料煤的灰。3水煤漿的發(fā)展須慎重水煤漿替代燃油, 符合中國國情, 但要真正實(shí)現(xiàn)水煤漿的產(chǎn)業(yè)化, 決不可盲目樂觀, 對油改水煤漿項(xiàng)目一定要慎重, 其原因是:(1 水煤漿技術(shù)是一項(xiàng)涉及多門學(xué)科的技術(shù), 需要整個技術(shù)配套。就目前的技術(shù)條件, 并不是所有的燃油設(shè)備都能夠成功地燃用水煤漿, 其應(yīng)用領(lǐng)域是有限的。(2 水煤漿制備對煤的質(zhì)量要求較高

14、, 須低灰、低硫精煤; 水煤漿替代煤粉燃燒毫無經(jīng)濟(jì)價值, 只有替代油料才能體現(xiàn)出其效益。(3 水煤漿的灰分較高, 火焰和煙氣的潔凈度不夠, 使其難以滿足大多數(shù)工業(yè)加熱窯爐的要求。(4 現(xiàn)有燃油工業(yè)鍋爐、窯爐改燒水煤漿, 需要一筆可觀的改造費(fèi)用。(5 水煤漿是潔凈煤技術(shù)的一種, 具有許多優(yōu)越性, 但缺點(diǎn)也很突出, 如水煤漿的制備和運(yùn)輸要消耗較多的電力h 。水煤漿穩(wěn)定60天。水煤漿使用管道, 一旦水煤漿的銷路出現(xiàn)問題, 管道運(yùn)輸能力就會閑置, 等等。(6 美國、瑞典、俄羅斯、日本等二十幾個國家, 對水煤漿均有一定的研究并取得相應(yīng)的應(yīng)用業(yè)績, 但迄今并未大量推廣采用, 主要作為未來能源供應(yīng)的一項(xiàng)備選

15、措施。參考文獻(xiàn)1王璋保. 我國能源(石油 供應(yīng)的安全問題J.工業(yè)加熱,2002, (2 :59.2楊松君, 陳懷珍. 動力煤利用技術(shù)M.北京:中國標(biāo)準(zhǔn)出版社,1999:138163.3孫成功, 李保慶. 水煤漿技術(shù)研究開發(fā)現(xiàn)狀及面臨的問題J.煤炭轉(zhuǎn)化,1995,18(3 :15.4朱成章. 水煤漿代油發(fā)電要慎重J.煤炭經(jīng)濟(jì)研究,2002,(2 :1416.5仝德良, 呂應(yīng)運(yùn), 劉波. 北京地區(qū)發(fā)展水煤漿的前景分析J.節(jié)能與環(huán)保,2001, (3 :2024.6周萬杰. 走出誤區(qū)正確認(rèn)識水煤漿J.佛山陶瓷,2001,(7 :3637.7許波. 水煤漿氣化技術(shù)在我國的應(yīng)用和發(fā)展J.煤化工,2001

16、, (2 :36.研究論文µ(THERMAL POWER GENERATIONV ol. 32N o. 9Sep. 2003Contents &AbstractsEVA L UATION OF ENERG Y AN D POWER ENGINEERING PR OJECTS FR OM THE VIEW POINT OF NATURA L RESOURCES YE H an -dong et al (2I n present -d ays o f giving increasingly prominence to environmental protection proble

17、ms , it is neccess ary to adopt a comprehensive evalu ation method , w hich can take tech nical , e 2conomical and environmental factors into accou nt , for evalu ating energy and pow er engineering projects. The air , w ater , fuel and so on consumed in production (including th at h ave been pollut

18、ed should been convrted into natural resources , the definition o f final evalu ation index , w hich is called the efficiency o f natural resources , being a ratio o f the energy sup 2ply production to the sum o f values concerning consumed natural resources. The principle o f this comprehensive eva

19、lu ation method h as been presented. T aking the combined cycle heat and electricity co -generation system o f a prep aratory natu al -gas firing gas turbine u nit as example , a calculation in comp arison with a coal firing heat and electricity sep arate generation system h as been roughly corried

20、out.OPTIMA L LOAD DISTRIBUTION IN POWER P LANT BASE D ON GENETIC A L G ORITH M DAI Yi -ping et al (6The b asic principle o f genetic algorithm h as briefly been described. U nder application o f the s aid algorithm in pow er plants , the problem o f optim al load distribution among multiple u nits i

21、n operation h as been solved. I t can be fou nd from results o f simulated calculation as example , th at the optimizing algorithm adopted features fast speed o f computing operation and being easy to use. The online dynamic optimization o f u nits load can be realized , improving economic aspects o

22、 f u nits operation.PRESENT SITUATION AN D LIMITATIONS OF TECHN OLOG Y USING COA L -WATER SL URR Y AS SUBSTITUTE FOR OI L -FU 2E L JIA Ming -sheng et al (10A new tech nology o f using coal -w ater slurry (CWS h as been widely spread in recent year. The features o f CWS , the problems to be faced , a

23、nd the limitation o f using CWS h ave been analysed , putting forw ard the feasibitity o f using CWS as a substitute for oil -fuel u nder certain conditions , and the applicabity o f CWS is limited. The wide application and m arket future o f CWS can t be blindly optimistic , the s aid tech nology s

24、hould be cautiously developed.COMPARISON OF DESIGN FEATURES AN D PERFOR MANCE TEST METH ODS FOR COMBINE D CYC LE HEAT RECOVER Y BOI LERS Wei -guo et al (13Directing against the structural features o f gas turbine s heat recovery boiler , the design ch aracteristics o f s aid boiler h ave been analys

25、ed , summ methods for measuring and calculating the utilization coefficient o f heat recovery boiler and the flow rate o f inlet gas. Combined with the practice , a b for heat recovery boiler h as been put forw ard , coming to a conclusion o f using di fferent method according to di fferent situ ati

26、on the DISCUSSION ON PR OB LEMS OF CH OOSING WATER -LEVE L AN D G AO Q i -fan (17Structure and perform ance o f several equilibrium containers o f al -, the influence o f drum pressure upon the mea 2surement o f w ater level being analysed. The and o f some equilibrium containers are also given.TEMP

27、ERATURE AN L ANA LYSIS OF A 50MW STEAM TURBINE S CYLIN DER IN A POWER P LI Yi -min et al (21A three dimentional o field and the therm al stress field regarding a 50MW steam turbine s high pressure cylinder in a pow r plant u nder di f 2ferent operation conditions carried The results show, the positi

28、on o f calculated m ax. stress tallies completely with th at o f the cracked cylinder , providing important tech nical b asis for evalu ating the steam cylinder s s afety and calculating the s aid cylinder s residu al service -li fe.STRENGTH ANA LYSIS OF CRACKE D PART ON CYLIN DER WA LL IN STEAM TUR

29、BINE G AO Yu an -hui et al (25Applying shell theory , the internal pressure stress and the therm al stress o f the cracked p art on cylinder w all in a 51-50-1type steam turbine h ave been analysed and calculated , and the s afety o f cracks being evalu ated. At the s ame time , some recommend ation

30、s for the cylinder defects and the steam turbine operation h ave been suggested. I t is considered from analysis th at the s aid cracks are s afe , th at is to s ay a single -stage brittle fracture can not occur.TEST AN D STU DY ON TEMPERATURE FIE LD OVER CR OSS -SECTION OF LARGE -SCA LE BOI LER YAO

31、 Bin et al (29The flame im age in the furnace o f a largescale boiler h as been detected by using four CC D flame detectors. Through processing o f the flame im age , the detection o f temperature dis 2tribution over the furnace cross -section o f a boiler h as been realized , giving the temperature

32、 distribution over s aid cross -section u nder various operation conditions , and the results being analysed and comp ared. The results o f furnace temperature field created by the flame im age show th at the temperature field over the cross -section above the zone o f burners appears as a typical s

33、ingle -peak distribution ch aracter , and its skew ness is b asically consistent with the operational experience in the s aid pow er plant.CAUSE ANA LYSIS AN D REME DIA L MEASURES FOR SOMEWH AT HIGHER CARBON CONTENT IN F LY ASH OF BOI LERSHE Jing et al (32The causes leading to somew h at higher carb

34、on content in fly ash o f boilers h ave been analysed from several aspects , namely fuel , prim ary air , terti ary air , and oxigen rate etc. . Through retro fit o f the fine pulverized coal sep arator and adjustment o f coal sorts , prim ary air velocity and air temperature , as w ell as burning a

35、dmixture o f other fuel , the carbon content in fly ash during the time o f burning single coal h as been decreased from an original value o f 8%to about 2. 4%,and during the time o f burning simutanuously admixture o f blast -furnace gas , the carbon content in fly ash can be controlled to below 8%

36、,enh ancing the boiler efficiency from 89. 4%in 1998to 92. 4%in 2002.RETR OFIT OF TWO 130t/h PU L VERIZE D COA L BOI LERS IN YI LI THER MA L POWER P LANT N O. 2DUE TO HIGH EXH AUST G AS TEMPERATURE THEREOF ZHI H uo -ting et al (34I n view o f troublesome problems o f high exh aust gas temperature an

37、d high hot air temperature h aving existed after putting tw o CG -130/3. 82-M 12type boilers into operation at Yili Therm al Pow er Plant N O. 2, corresponding retro fitting schemes h ave been studied and w orked out , namely renewing 107m 2area o f superheater in the boiler , increasing heating sur

38、face area o f a p arty o f the economizer , adding 8steam soot -blow ers in the heat recovery gas duct. The operation after retro fitting show s , the exh aust gas temperature h aving been low ered by 30, the hot air temperature by 100, obtaining good results o f operation.CAUSE ANA LYSIS OF OI L LEAK AGE FR OM BEARING BOXES OF BOI LER AUXI LIARIES AN D REME DIA L METH OD THEREOF ZH OU Ai -min et al (36C ause analysi