MC LC

K THUT O V IU KHIN QU TRNH Bi m u. Gii thiu chung v k thut o v iu khin qu trnh

PHN I. H THNG O CNG NGHIPChng 1. Cc khi nim c bn v o lng 1.1. Khi nim o lng1.2. Cc phng php v bin php o lng c bn1.3. Phn loi dng c oChng 2. nh gi sai s trong o lng

2.1. Nguyn nhn v phn loi sai s trong o lng

2.1.1. Nguyn nhn gy sai s

2.1.2. Phn loi sai s

2.1.3. Cc biu thc din t sai s 2.2. Mt s khi nim c bn v o lng

2.3. Cc phng php kh sai s v xc nh kt qu oChng 3. Cm bin o v chuyn i o

3.1. Cc khi nim v c trng c bn

3.2. Cc cm bin v chuyn i o nhit 3.3. Cc cm bin v chuyn i o p sut

3.4. Cc cm bin v chuyn i o lu lng

3.5. o mc PHN II. H THNG IU KHIN T NGChng 1. Nhng khi nim c bn v iu khin qu trnh1.1. Khi qut chung v iu khin 1.2. C s v iu khin qu trnh1.3. Nhn bit cc bin qu trnhChng 2. Cc sch lc iu khin c s 2.1. iu khin truyn thng

2.2. iu khin phn hi

2.3. iu khin t l

2.4. iu khin tng

2.5. iu khin suy din

2.6. iu khin la chn

2.7. iu khin phn vng

Chng 3. c tnh cc thnh phn h thng

3.1. Thit b o

3.2. Thit b chp hnh v van iu khin

Ti liu tham kho[1]. Nguyn Vn Ha. C s t ng iu khin qu trnh. NXBGD, 2007.[2]. Hong Minh Cng. Gio trnh cm bin cng nghip, HBK Nng. NXBGD, 2007.

[3]. o Vn Tn. Gio trnh t ng ha v m hnh ha qu trnh lc du. Trng H M-a Cht. HN, 2007.[4]. Hong Minh Sn. C s h thng iu khin qu trnh. NXB Bch Khoa HN, 2006.

[5]. V Qu im, Phm Vn Tun, L Ph. C s k thut o lng in t. NXBKHKT, 2001.

[6]. Nguyn Minh Hu. Gio trnh t ng ha cc qu trnh cng ngh, 2004[7]. Nguyn nh Lm. Cc thit b ph tr cho qu trnh lc du.

[8]. Hong Dng Hng. Gio trnh t ng ha qu trnh nhit, 2004.

[9]. Page S. Buckley, William L. Luyben, Joseph P. Shunta. Design of distillation column control systems. Publishers Creative Services Inc., New York, 1985.Bi m uGii thiu chung v k thut o v iu khin qu trnh

Trong cng nghip ni chung v cng nghip du kh ni ring, ngi ta cn phi o v kim tra thng xuyn cc i lng nh: p sut, nhit , nng , lu lng, l nhng qu trnh o lin tc. Cc tn hiu o t cc cm bin c gi v cc trung tm x l s liu. Do m bo o chnh xc cc thng s ny c nh hng rt ln n qu trnh sn xut v ch bin du kh.iu khin qu trnh khng phi l mt lnh vc mi, nhng lun chim v tr quan trng hng u trong t ng ha cng nghip. Ngy nay cc nh my, x nghip v ang c trang b cc h thng iu khin t ng mc cao vi cc thit b tin tin. Bt c mt nh my xi mng, mt nh my in, hay mt nh my lc du no nht nh khng th vn hnh c nu thiu h thng iu khin t ng. H thng iu khin t ng m bo cho s hot ng ca quy trnh cng ngh t c kt qu mong mun. Cu trc cc h thng iu khin t ng cc qu trnh cng ngh rt a dng, nhng c th chia thnh hai mc: mc thp v mc cao. Mc thp l cc h thng iu chnh t ng m bo mi trng cn thit cho quy trnh cng ngh. Mc cao l h thng t ng ha iu khin quy trnh cng ngh. Mc ch ca h thng ny khng ch bo m mi trng cn thit cho quy trnh cng ngh m cn bo m sao cho quy trnh cng ngh t c cht lng tt nht (iu khin ti u).Mc t ng ha cc dy chuyn sn xut ngy cng phi c nng cao tng nng sut, cht lng sn phm v h gi thnh sn xut.Vic ng dng rng ri T ng ho iu khin qu trnh cc qu trnh cng ngh ang l mt trong cc yu t then cht thc y tin b k thut ca ngnh cng ngh. T ng ho l bc pht trin logic ca c kh ho sn xut. Nu c kh ho thay th lao ng c bp cho con ngi th t ng ho l bc tip tc pht trin thay th cc c quan cm gic v logic ca con ngi.

Ta hiu T ng ho iu khin qu trnh l s ng dng cc dng c, cc thit b v cc my iu khin. Nhng phng tin k thut ny cho php thc hin cc qu trnh cng ngh theo mt chng trnh c to dng, ph hp vi nhng tiu chun cho trc (cc iu kin) m khng cn s tham gia trc tip ca con ngi.

Ni dung ca hc phn trang b cho sinh vin nhng kin thc c bn v h thng t ng o lng v iu khin t ng cc quy trnh cng ngh. Nhng kin thc ny rt cn thit cho cc k s cng ngh. N s gip cc k s cng ngh s dng tt cc h thng o lng kim tra v iu khin t ng phc v cho cng tc nghin cu cng nh tip cn vi h thng t ng ha cc quy trnh cng ngh hin nay. Cc ni dung c bn bao gm: Cm bin v chuyn i o H thng iu khin t ng

PHN I. H THNG O CNG NGHIP

Chng 1. Cc khi nim c bn v o lng1.1. Khi nim o lngo lng l khoa hc v cc php o, cc phng php v cc cng c m bo cho kt qu o t c chnh xc mong mun. Cc hng nghin cu chnh ca o lng bao gm:

- Cc l thuyt chung v php o.

- Cc n v vt l v h thng ca chng

- Cc phng php v cng c o, bin php nng cao chnh xc ca php o.- Phng php xc nh chnh xc ca php o.

- C s bo m cho vic thng nht gia php o v rt nhiu cng c thc hin n.- Cng c o chun v barem.- Cc phng php chuyn n v o t cng c chun hoc gc ra cng c lm vic.

Php o l cng vic thc hin chnh ca o lng, l vic tm ra gi tr vt l bng cch th nghim vi s tr gip c cc cng c k thut c bit. Gi tr tm c gi l kt qu ca php o. Hnh ng thc hin trong qu trnh o cho ta kt qu l mt i lng vt l gi l qu trnh ghi nhn kt qu. Tu thuc vo i tng nghin cu, vo tnh cht ca cng c o m ngi ta cn thc hin php o ghi nhn mt ln hay nhiu ln. Nu nh c mt lot ghi nhn th kt qu php o nhn c l kt qu khi x l cc kt qu t cc ghi nhn .

Php o c bn cht l qu trnh so snh i lng vt l cn o vi mt i lng vt l c dng lm n v. Kt qu ca php o c biu din bng mt s l t l ca i lng cn o vi n v . Nh vy thc hin php o, ta cn thit lp n v o, so snh gi tr ca i lng cn o vi n v v ghi nhn kt qu so snh c. n = Q/q

Trong : Q - gi tr i lng cn o

q - gi tr n v o

n - ch s o c

Nh vy ch s o c n khng ch ph thuc vo gi tr cn o Q m cn ph thuc vo gi tr n v o q.1.2. Cc phng php v bin php o lng c bnCc phng php c bn ca k thut o lng thng c chia thnh:

- Phng php o trc tip

- Phng php o gin tip

- Phng php o kt hp, tng quan

o trc tip l phng php dng cc my o hay cc mu o (cc chun) nh gi s lng ca i lng o c. Kt qu o c chnh l tr s ca i lng cn o, m khng phi tnh ton thng qua mt phng trnh vt l no lin quan gia cc i lng, gi tr i lng cn o c so snh trc tip vi gi tr n v o xc nh ch s o. Nh vy, s dng phng php o trc tip th phi tn ti gi tr n v o c th. Nu khng tnh n sai s, th tr s ng ca i lng cn o X s bng kt qu o c a: X = a

Cc v d v phng php o trc tip nh: o in p bng vn-mt; o tn s bng tn s-mt, o cng dng in bng ampemt,.... o trc tip th php o thc hin n gin v bin php k thut, tin hnh o c nhanh chng v loi tr c cc sai s do tnh ton.

o gin tip l phng php o m kt qu o c khng phi l tr s ca i lng cn o, m l cc s liu c s tnh ra tr s ca i lng ny. Ngha l y, X=F(a1, a2,..., an). Cc thng s cng ngh thng l nhng i lng khng in nh: nhit , p sut, lu lng dng chy, p lc, vn tc, m,Gi tr n v o cc thng s ny khng tn ti trong thc t m ch l gi tr tru tng. Do vy o cc i lng ny phi dng phng php o gin tip. Ngha l gi tr i lng cn o phi chuyn sang dng tn hiu khc v c so snh vi tn hiu n v xc nh gi tr i lng cn o.Cc v d v phng php o gin tip nh: o cng sut bng vn-mt v ampemt, o nhit theo sut in ng ca cp nhit, o lu lng ca dng chy bng phng php chnh p bin i, v.v....

Trong k thut o lng, thng thng ngi ta mun trnh phng php o gin tip, v trc ht n yu cu tin hnh nhiu php o (t nht l hai php o) v thng l khng nhn bit ngay c kt qu o. Song trong mt s trng hp th khng th trnh c phng php ny.Trong php o kt hp kt qu nhn c bng cch gii h phng trnh, c xc lp trn c s cc php o trc tip (v d: xc nh h s nhit ca l xo p k). Php o gin tip c ng dng rng ri trong cng nghip hn c, cn php o kt hp ch ng dng trong iu kin th nghim v trong nghin cu.

Hin nay, k thut o lng pht trin nhiu v phng php o tng quan. N l mt phng php ring, khng nm trong phng php o trc tip hay phng php o gin tip. Phng php tng quan dng trong nhng trng hp cn o cc qu trnh phc tp, m y khng th thit lp mt quan h hm s no gia cc i lng l cc thng s ca mt qu trnh nghin cu. V d: tn hiu u vo v tn hiu u ra ca mt h thng no . Khi o mt thng s ca tn hiu no bng phng php o tng quan, th cn t nht l hai php o m cc thng s t kt qu o ca chng khng ph thuc ln nhau. Php o ny c thc hin bi cch xc nh khong thi gian v kt qu ca mt s thut ton c kh nng nh c tr s ca i lng thch hp. chnh xc ca php o tng quan c xc nh bng di khong thi gian ca qu trnh xt. Khi o trc tip, tht ra l ngi o phi gi thit h s tng quan gia i lng o v kt qu rt gn 1, mc d c sai s do quy lut ngu nhin ca qu trnh bin i gy nn. Ngoi cc phng php o c bn ni trn, cn mt s cc phng php o khc thng c thc hin trong qu trnh tin hnh o lng nh sau:

-Phng php o thay th: Php o c tin hnh hai ln, mt ln vi i lng cn o v mt ln vi i lng o mu. iu chnh hai trng hp o c kt qu ch th nh nhau.

-Phng php hiu s: Php o c tin hnh bng cch nh gi hiu s tr s ca i lng cn o v i lng mu.-Phng php vi sai, phng php ch th khng, phng php b, cng l nhng trng hp ring ca phng php hiu s. Chng thng c dng trong cc mch cu o hay trong cc mch b.-Phng php o thng: kt qu o c nh lng trc tip trn thang ca thit b ch th. Tt nhin s khc ca cc thang ny l c ly chun trc vi i lng mu cng loi vi i lng o.

-Phng php ri rc ho (ch th s): i lng cn c o c bin i thnh tin tc l cc xung ri rc. Tr s ca i lng cn o c tnh bng s xung tng ng ny.1.3. Phn loi dng c oDng c o c th phn loi theo nhiu du hiu khc nhau. Hin nay thng thng ta phn loi dng c o theo cch thng bo kt qu o, theo i lng o v theo cp chnh xc (theo cc cp khc nhau).

Theo cch thng bo kt qu o cc dng c o c phn thnh: dng c t ch, dng c t ghi v ng h tng ghi.

cc dng c o t ch gi tr ca i lng o c th c thng bo bng kim hoc hin s.

Dng c t ghi c trang b phng tin cho php t ng ghi kt qu o trn giy hoc trn cc phn t nh. Dng c t ghi c loi mt im o v loi nhiu im o.

ng h tnh tng (b m) cho kt qu tng ca gi tr o trong mt khong thi gian nht nh. B m thc hin php tnh tch phn gi tr tc thi ca i lng o theo hm thi gian.

Theo i lng o (thng s o) cc dng c o phn thnh dng c o nhit , p sut v chn khng, lu lng v s lng, mc, thnh phn cht lng v trng thi ca cht (khi lng ring, nht, m v.v) v cc thng s khc.

Chng 2. nh gi sai s trong o lngM u

o lng l mt phng php vt l thc nghim nhm mc ch thu c nhng thng tin v c tnh s lng ca mt i tng hay mt qu trnh cn nghin cu. N c thc hin bng cch so snh i lng cn o vi i lng chn dng lm tiu chun, lm n v. Kt qu o c biu th bng s hay biu ; kt qu o c ny ch l gi tr gn ng, ngha l php o c sai s. Chng ny s nghin cu v cch x l cc tr s gn ng tc l cn nh gi c chnh xc ca php o.

2.1. Nguyn nhn v phn loi sai s trong o lng

2.1.1. Nguyn nhn gy sai s Khng c php o no l khng c sai s. Vn l khi o cn phi chn dng phng php thch hp, cng nh cn chu o, thnh tho khi thao tc..., hn ch sai s cc kt qu o sao cho n mc t nht.

Cc nguyn nhn gy sai s th c nhiu, ngi ta phn loi nguyn nhn gy sai s l do cc yu t khch quan v ch quan gy nn.

Cc nguyn nhn khch quan v d nh: dng c o lng khng hon ho, i lng o c b can nhiu nn khng hon ton c n nh...

Cc nguyn nhn ch quan, v d nh: do thiu thnh tho trong thao tc, phng php tin hnh o khng hp l...

V c cc nguyn nhn v ta khng th tuyt i loi tr hon ton c nh vy, nn kt qu ca php o no cng ch cho gi tr gn ng. Ngoi vic c gng hn ch sai s o n mc t nht, ta cn cn nh gi c xem kt qu o c sai s n mc no.

2.1.2. Phn loi sai s

Cc sai s mc phi trong php o c nhiu cch phn loi. C th phn loi theo ngun gc sinh ra sai s, theo quy lut xut hin sai s hay phn loi theo biu thc din t sai s.

Phn loi theo quy lut xut hin sai s c chia lm hai loi: sai s h thng v sai s ngu nhin.

Sai s h thng

Sai s ny do nhng yu t thng xuyn hay cc yu t c quy lut tc ng. N khin cho kt qu o c sai s ca ln o no cng nh nhau, ngha l kt qu ca cc ln o u hoc l ln hn hay b hn gi tr thc ca i lng cn o. Tu theo nguyn nhn tc dng, m sai s h thng c th phn thnh cc nhm sau y:

Do dng c, my mc o ch to khng hon hoV d: kim ch th ca thit b ch th khng ch ng v tr ban u, my mc khng c chun li thang vi cc my chun ... Do phng php o, hoc l do cch chn dng phng php o khng hp l; hoc khi x l kt qu o, khi tnh ton cho n gin hn t b qua mt s yu t no y. Do kh hu, v d nhit , m khi tin hnh o khc vi iu kin kh hu tiu chun quy nh trong quy trnh s dng my o...

Sai s ngu nhin

Sai s ngu nhin l sai s do cc yu t bin i bt thng, khng c quy lut tc ng. Tuy ta c gng thc hin o lng trong cng mt iu kin v chu o nh nhau, nhng v do nhiu yu t khng bit, khng khng ch c, nn sinh ra mt lot kt qu o khc nhau. V d: do in p cung cp ca mch o khng n nh, do bin thin kh hu ca mi trng chung quanh xy ra trong qu trnh o lng...

Ngoi hai loi sai s trn, khi o ta cn nhn c nhng kt qu cc ln o c cc gi tr sai khc qu ng (sai s th). N thng do nhng yu t ch quan ca ngi o gy ra, nh thiu chu o; hay do cc tc ng t ngt ca bn ngoi. Cc kt qu o ny qua suy xt ch quan, ta c th bit c n l cc gi tr v ngha v ta c th loi b ngay c. Thng ngi ta gi cc kt qu o ny l cc tr s o sai.

2.1.3. Cc biu thc din t sai sThng thng cc sai s hay c phn loi theo biu thc din t. Theo cch phn loi ny th c hai loi sau: sai s tuyt i v sai s tng i.

Sai s tuyt i

Ngi ta nh ngha sai s tuyt i l tr tuyt i ca hiu s gia hai gi tr o

c v gi tr thc ca i lng cn o.

Nu gi a l gi tr o c, X l tr thc ca i lng cn o th:

(x*=|a-X| [1]l sai s tuyt i.V d, nu dng c o ch gi tr nhit l a = 440C v gi tr thc ca nhit o bng dng c o c chnh xc cao hn l:

X= 450C, th sai s ch th ca dng c o l 0C.Trn thc t, v cha bit c X, nn khng nh lng c th c (x*. a ra: Sai s tuyt i gii hn ((x) : (x (x*.VD: Xc nh sai s tuyt i gii hn ca s a=3,14 thay cho s

V 3,14< < 3,15 nn a- < 0,01 suy ra (x=0,01

V 3,14< < 3,142 nn a- < 0,002 suy ra (x=0,002 Nn (x khng n tr.

Nhng cn c vo dng c o v kh nng t c chnh xc ca php o, cng nh thc hin cch o nhiu ln, ta c th tm c gii hn cc i ca (x*: (x*(x v ly (x l sai s tuyt i.

Sai s tng iSai s tuyt i v sai s tuyt i gii hn khng th hin mt cch y mc chnh xc ca php o hoc tnh ton.

Sai s tng i l t s ca sai s tuyt i v tr s thc ca i lng cn o:

[2]Sai s tng i c biu th di dng phn trm (%). Sai s tng i nh biu

thc (2) l sai s tng i chn thc, n ng theo nh ngha. Tuy vy, n khng c gi tr trong thc tin tnh ton, v cha bit c X.Sai s tng i gii hn () : xTrong trng hp (x