dieu khien qua trinh lan 2
DESCRIPTION
Dieu Khien Qua Trinh Lan 2Dieu Khien Qua Trinh Lan 2Dieu Khien Qua Trinh Lan 2TRANSCRIPT
Chng I
Chng 2
c tnh c bn ca thit b o v chp hnh trong iu khin qu trnh
Thit b o v chp hnh l thnh phn khng th thiu trong iu khin qu trnh. c tnh ca n nh hng rt ln ti n nh v cht lng iu khin qu trnh.
Trong chng ny chng ta s nghin cu cc c tnh chung c bn ca mt s thit b o v chp hnh vi mc ch phc v cho vic thit k v chnh nh h iu khin qu trnh. C th cc loi thit b o nhit , p sut , lu lng, cc loi van iu khin v cc my bm qut.2-1. Cu trc c bn ca thit b o cc i lng qu trnh trong cng nghipMt thit b o qu trnh c nhim v cung cp thng tin v din bin ca qu trnh v cho u ra l mt tn hiu chun. Cu trc c bn ca mt thit b o qu trnh c minh ha trn hnh 2.1.
Hnh 2.1 Cu trc c bn ca mt thit b o qu trnh ca mt thit b o.Thnh phn ct li ca mt thit b o l cm bin. Mt cm bin (sensor) c chc nng chuyn i mt i lng vt l v d nhit , p sut, mc, lu lng, nng sang mt tn hiu thng thng l in hoc kh nn. Mt cm bin c th bao gm mt hoc vi phn t cm bin (sensing element), trong mi phn t cm bin li l mt b chuyn i t mt i lng ny sang mi i lng khc d x l hn. Tn hiu ra t cm bin thng rt nh, cha truyn c xa, cha sai s do chu nh hng ca nhiu hoc do nhy km ca cm bin, phi tuyn vi i lng o. V th, sau phn t cm bin ngi ta cn cc khu khuych i, chuyn i, lc nhiu, iu chnh phm vi, b sai lch v tuyn tnh ha. Nhng chc nng c thc hin trong mt b chuyn i o chun (transmitter). Mt b chuyn i o chun ng vai tr l mt khu iu ha tn hiu, nhn tn hiu u vo t mt cm bin v cho u ra l mt tn hiu chun c th truyn xa v thch hp vi u vo ca b iu khin. Trong thc t, nhiu b chuyn i o chun c tch hp lun c phn t cm bin, v vy khi nim transmitter cng c s dng ch c thit b o. Tn hiu u ra t l vi lng o c chun ho theo tiu tn hiu in p 0 10V, 1 5V... hay theo tn hiu dng 0 20 mA. Hoc tn hiu truyn thng cng nghip.Cht lng v kh nng ng dng ca mt thit b o ph thuc vo nhiu yu t m ta khi qut l cc c tnh thit b o, bao gm c tnh vn hnh, c tnh tnh v c tnh ng hc. c tnh vn hnh bao gm cc chi tit v kh nng o, chi tit vn hnh v tc ng mi trng. c tnh tnh biu din quan h gia i lng u vo v gi tr tn hiu u ra ca thit b o trng thi xc lp, trong khi c tnh ng hc biu din quan h gia bin thin u vo v tn hiu ra theo thi gian. c tnh tnh lin quan ti chnh xc khi gi tr ca i lng o khng thay i hoc thay i rt chm. Ngc li, c tnh ng hc lin quan ti kh nng phn ng ca thit b o khi i lng o thay i nhanh.
2-2. c tnh vn hnh:
Phm vi o v di o
Phm vi o l phm vi gi tr danh nh ca i lng o m mt thit b o c s dng theo quy nh. Mt phm vi o c xc nh bi gii hn di (xmin) v gii hn trn (xmax). Gii hn di cn c gi l im khng. Di o c nh ngha l khong cch gia gii hn trn v gii hn di ca phm vi o.
c tuyn l tng ca mt thit b o nhit c th hin trn hnh 2.2 , trong quan h gia i lng o (nhit T) v tn hiu o (y) c coi l tuyn tnh. Phm vi o l t 100oC ti 400oC, di o tng ng l Sy = 300oC
Hnh2.2. Phm vi o v di o
Phm vi o v di o nh ngha nh trn da trn khong bin thin cho php ca gi tr o, v th cn c gi l phm vi u vo (input range) cng nh di u vo (input span). Ta cng c cc nh ngha tng t cho phm vi u ra (output range) v di tn hiu ra hay di u ra (output span). Phm vi u ra ca tn hiu o chnh l phm vi tn hiu ra ca b chuyn i o. a s cc b chuyn i o chun s dng trong iu khin qu trnh cho u ra theo chun dng 4-20mA, vi 4mA tng ng vi 0% v 20mA tng ng vi 100% phm vi o. Chun dng c mt s u im nh:
- Kh nng truyn xa, khng b nh hng bi tr khng cp truyn- Kh nng phn bit gia trng hp li t cp vi trng hp tn hiu ra t gii hn di.
- Kh nng mang ngun nui cho cc thit b mc ni tip phn gii:
Khi gi tr ca mt bin o bin thin lin tc trong phm vi o, mt s thit b o cho tn hiu ra thay i mt cch ri rc thay v lin tc. Trong trng hp ny, phn gii ca thit b o c nh ngha l mt bc thay i ca tn hiu ra. Khi kch c cc bc thay i khng c nh th bc thay i ln nht c gi l phn gii cc i. phn gii thng c biu din bng t l phn trm so vi di tn hiu ra ca thit b. phn gii trung bnh biu din theo t l phn trm ca di tn hiu ra c tnh theo cng thc:
phn gii trung bnh
Trong : N l s gi tr c ghi nhn
Di cht v c tnh trDi cht (dead band) ca mt thit b o l thay i nh nht ca i lng o theo chiu ngc li m thit b o c th p ng vi tn hiu u ra thay i. Di cht cn c gi l ngng nhy (threshold). c tnh tr (hysteresis) l s khc nhau trong p ng ra cng vi thay i u vo nhng theo hai chiu khc nhau. Hiu ng di cht v c tnh tr to ra hai ng c tnh khc nhau cho mi chiu thay i ca i lng o nh minh ha trn hnh 2.3Hnh2.3. c tnh tr tnh ca thit b o
nhy:
nhy hay h s khuych i tnh ca mt thit b o ti mt gi tr o l t l gia thay i tn hiu ra v s thay i i lng u vo tng ng trng thi xc lp:
nhy ph thuc vo im lm vic, tc ph thuc vo gi tr o hin thi. i vi mt thit b o c c tnh tuyn tnh, nhy l mt hng s v c xc nh bng t l gia di tn hiu ra v di o:
Cp chnh xc v sai s
Cp chnh xc (accuracy) ca thit b l gi tr o c gn nht vi gi tr qu trnh. Sai s (error) l s khc bit ln nht c th xy ra gia gi tr qu trnh v gi tr o c. Sai s c th c th hin bng nhiu cch. Cch chung nht l gi tr tuyt i (chng hn nh sai s ln nht cho php ca gi tr o nhit l (20C) nh phn trm gi tr thc t ca s bin i qu trnh hay phn trm thang o ca thit b o (thng c gi l FSD full- scale deflection- dng cho lch ca thang o).
V d nh thit b o v chuyn i p sut hnh v di vi phi tuyn c phng i ln. Thit b ny c sai s ln nht l 0,5 p.s.i tng ng vi 6,25 % gi tr thc t.
Hnh 2.4: Sai s khc bit ca b chuyn i gia p ng l tng v thc t Cc nh hng ca mi trng v thi gian.
Cc c tnh k thut ca mt b chuyn i thng th hin chi tit cc li c th xy ra i vi cm bin chnh nh trong cc iu kin vn hnh c nh (v d nh nhit mi trng xung quanh, in p ngun cp c nh). chnh xc ca b chuyn i s chu nh hng ca s thay i mi trng v suy gim theo thi gian. Nhng hiu ng ny c th hin nh mt s thay i im khng (hay li zero) hay mt s thay i nhy (li span). C hai u thay i theo thi gian v c bit n nh hiu ng tri.
Hnh 2.5: Cc nh hng ca mi trng thay i v thi gian
(a) Thay i im khng (b) Thay i spanCc nh hng ca mi trng thng c nh ngha nh l phn trm li do s thay i v mi trng. V d mt b chuyn i o p sut theo phng php chnh p c th b nh hng bi p sut tnh. Mt b chuyn i vi dng u ra 4-20 mA c th chu nh hng s qua bi ti. Hu ht nhy ca cc thit b u chu nh hng ca nhit mi trng xung quanh. Danh sch cc c tnh k thut v cc li c th xy ra i vi b chuyn i c th gip cho cc k s iu khin qu trnh xc nh cc li xy ra i trong nhng ng dng c th. Cc nh hng do nhit c th tc ng ch yu n cc thit b sn nh my ni nhit mi trng c th thay i t - 15 n + 500C nhng li gy ra rt t nh hng n cc thit b trong phng iu khin c nhit c iu ho.
Cc yu t mi trng tc ng n thit b c th c loi tr bng cch b thm cc gi tr thch hp. Cc yu t thi gian khng nh hng nhiu lm n cc thit b ngy nay do c cu trc chc chn, da trn c s cc vi mch in t so vi cc thit b kh nn hay n in t chn khng th h trc y. Cc nh hng do thi gian c th c loi tr bng vic tin hnh bo dng cng nh chnh nh li theo cc khong thi gian nh k. Cc c tnh k thut ca thit b s cho php cc k s t ra cc k hoch ph hp gi cc nh hng v thi gian trong nhng gii hn cho php.
CM BIN THNG MINH
a. Khi nim v cm bin thng minh
K thut o lng v iu khin t ng hin i ngy nay c nhng tin b vt bc nh vic s dng k thut vi in in t, vi x l (P) v vi iu khin C. thc hin c cc c tnh mi cho dng c o nh t ng chn thang o, t ng x l thng tin o, t ng b sai s ngi ta phi s dng cc b vi x l hay vi iu khin kt hp vi cc loi cm bin khc nhau gi l cm bin thng minh Smart sensor.
Trong h thng thng tin o lng v iu khin, cc loi dng c o, cm bin lm nhim v bin i t i lng o thnh i lng in. y l khu c bn nht quyt nh sai s ca h thng o hay dng c o bi v sai s ca cc loi cm bin ph thuc ch yu vo nguyn l cu to ca cm bin (da trn cc hiu ng vt l) ta gi l sai s phng php, ngoi ra cc yu t khch quan nh nhit , m ca mi trng, ch lm vic ca cm bin cng nh hng ng k n chnh xc ca dng c o v h thng o. V vy nng cao chnh xc ca php o, trc tin ta phi ci tin cc c tnh k thut ca cm bin. iu ny ch c th c thc hin c nu ta bit p dng nhng thnh tu ca cng ngh in t v tin hc.
Nh tnh nng cao ca mch vi x l, cc nh k thut o lng a ra c nhiu bin php, nhiu phng php o a nng ho cc cm bin, a ra nhiu bin php x l nng cao chnh xc, kh nng thng tin, tng tc ng nhanh, nng cao tnh n nh, tuyn tnh, hn ch n loi b cc yu t nh hng n chnh xc ca dng c o cng nh h thng o.
b. Cc u im ca cm bin thng minh
Cm bin thng minh c th thc hin c cc chc nng mi m cm bin thng khng thc hin c l:
- Chc nng thu thp s liu o t nhiu i lng o khc nhau vi cc khong cch o
khc nhau.
- Chc nng chng trnh ho qu trnh o, tc l o theo mt chng trnh nh sn (c
th thay i bng thit b lp trnh).
- C th gia cng s b kt qu o theo cc thut ton nh sn
- Tin hnh tnh ton a ra kt qu o khi thc hin cc php o gin tip.
- Hiu chnh sai s ca php o.
- B cc kt qu o b sai lch do nh hng ca s bin ng mi trng nh nhit ,
m...
- T ng chn thang o.
- M ho tn hiu o.
- Ghp ni vi cc thit b ngoi vi.
- Ghp ni vi bus trng Fieldbus.
- S dng P c th thc hin cc php tnh nh cng, tr, nhn, chia, tch phn, vi phn,
php tuyn tnh ho c tnh phi tuyn ca cm bin, iu khin qu trnh o, iu khin s
lm vic ca cc khu khc nh chuyn i tng t s A/D, cc b dn knh MUX
- S dng mP c kh nng pht hin nhng v tr hng hc trong thit b o v a ra cc
thng tin v chng nh ci t chng trnh kim tra v chun on k thut v s lm vic ca
thit b o.
c. Cu trc chung ca mt cm bin thng minh
Mt cm bin thng minh c s cu trc nh sau:
Hnh 2.6: Cu trc chung ca mt cm bin thng minh
T i tng o qua cm bin s cp, tn hiu to thnh sau cm bin o c s c a vo b chuyn i chun ho (transmitter). B chuyn i chun ho lm nhim v to ra tn hiu chun ri a vo b x l P. Tn hiu ra t P c th l t 4 n 20 mA hay l tn hiu s c truyn qua Fieldbus
2.3. Khi qut chung v thit b o nhit 2.3.1. Gii thiu chungVic o lng v iu khin nhit l mt trong nhng yu t quan trng nht trong iu khin qu trnh. Trong tt c cc i lng vt l, nhit c quan tm nhiu nht v nhit ng vai tr quyt nh n nhiu tnh cht ca vt cht. Dng c o nhit n gin nht l nhit k s dng hin tng gin n nhit.
o c nhit , ta cn phi nh ngha mt thang o nhit . n v o nhit tu thuc theo thang o nhit , c th l K theo thang Kelvin, oC theo thang Celsius, oF theo thang Fahrenheit. Thang Fahrenheit v Celsius (hay Centigrade) dng im ng lnh v im si ca nc lm hai im tham kho.
FahrenheitCelsius
im ng320
im si212100
Trong mi lin quan gia hai thang o ny c th hin qua cng thc:
F = 9/5C + 32 v C = 5/9(F - 32)
n v SI ca nhit l thang Kelvin (lu rng khng nh 0F v 0C, k hiu nhit (0) khng c dng trong thang Kelvin. im nhit thp nht trong thang Kelvin l im khng tuyt i. y l im nhit thp nht trn l thuyt v c nh ngha l 0 K. so snh, 0 K = - 273,16 0C. im nhit th hai c nh ngha trong thang Kelvin l 273,16 K tng ng vi 0,01 0C.
K = 0C + 273,15
Trong nhng ng dng cng nghip, thang Celsius c s dng rng ri nht nhng vic chuyn i sang thang Kelvin i khi cng cn thit.
o c tr s chnh xc ca nhit l mt vn khng n gin. a s cc i lng vt l u c th c xc nh trc tip nh so snh chng vi mt i lng cng bn cht. Nhit l i lng ch c th o gin tip trn c s tnh cht ca vt ph thuc vo nhit .
Ni chung c bn phng php o nhit da trn cc c im vt l sau:
- S gin n ca cc vt liu theo nhit to ra s thay i v di, th tch hay p sut, c ng dng trong cc loi nhit k thu ngn
- S thay i in tr theo nhit , c s dng trong cc nhit in tr kim loi v bn dn.
- S thay i nhit im tip xc gia hai kim loi khc nhau, c dng trong cp nhit in.
- S thay i nng lng pht ra theo nhit , c ng dng trong cm bin quang2.3.2. Cc phng php o nhit v ng dng trong cng nghip
2.3.2.1. Phng php o nhit bng nhit in tr kim loiS chuyn ng ca cc ht mang in tch theo mt hng hnh thnh mt dng in trong kim loi. S chuyn ng ny c th do mt lc c hc hay in trng gy nn v in tch c th m hay dng dch chuyn vi chiu ngc nhau. S dn in ca kim loi da trn s di ng ca cc electron t do cn gi l my electron ch khng phi do cc ion. Mt s kim loi c s dn in rt tt v qua th nghim cho thy s dn in ca kim loi khng i i vi s dch chuyn vt cht. Ngi ta cn nhn thy rng cc electron ngoi bin cc nguyn t kim loi c kt ni rt lng lo d dng bin thnh cc electron t do. iu ny to nn s dn in rt tt ca kim loi cng nh cc hiu ng vt l khc: hiu ng quang in, hiu ng pht x electron do in trng hay nhit. Trong cc kim loi dn in tt nh bc, ng trung bnh mi nguyn t ng gp mt electron t do cho s dn in.
dn in ca kim loi c th c din t bi cng thc:
Phng trnh trn chng t dn in ca kim loi t l nghch vi nhit hay in tr ca kim loi c h s nhit dng. S lng n electron trong kim loi khng thay i theo nhit . Khi nhit tng, bin dao ng ca mng tinh th ln hn v lm cho s ln va chm gia cc electron v nguyn t cng gia tng. Nh vy on ng dch chuyn t do l ca cc electron b thu nh li. Trong khi vn tc nhit vth gia tng vi . 1. Nhit in tr platin
a. Nhit in tr platin theo DIN v IEC
Platin l vt liu cho nhit in tr c dng rng ri trong cng nghip. Cc tnh cht ca loi nhit in tr ny c quy nh theo tiu chun quc t DIN IEC 751. Theo tiu chun ny di o nhit ca nhit in tr platin trong khong t - 200 n 8500C.
Cho di o u tin t -200 n 00C ta c a thc cp ba:
R (t) = Ro( 1+ At+ Bt2+ C[t- 1000C]t3)
Cho di o t 0 n 8500C ta c a thc cp hai:
R (t) = Ro( 1+ At+ Bt2)
Cc h s c gi tr nh sau:
A = 3,90802.10-3 oC-1B = -5,802-7 oC-2C = -4,2735-12 oC-3R0 l tr s in tr nh mc 00C
R0 ca nhit in tr Pt-100 l 100(, ca Pt-500 l 500( v Pt-1000 l 1000(. Cc loi Pt500 v Pt1000 c h s nhit ln hn do c nhy ln hn: in tr thay i mnh hn theo nhit .
S thay i nhit khong:
0,4 (/ Kelvin vi Pt100
2 (/ Kelvin vi Pt500
4 (/ Kelvin vi Pt1000
Ngoi ra tiu chun DIN IEC 751 cn xc nh mt tr s c trng na, l h s nhit trung bnh gia 0 v 1000C. l t l gia s thay i in tr 0 v 1000C vi in tr nh mc R0.
Hnh 2.7. c tuyn ca nhit in tr Pt100
Vi platin nguyn cht, c tr s 3,925.10-3 0C-1.
Tr s ca nhit in tr platin theo DIN c s khc bit vi tr s ny. Theo tiu chun DIN vt liu platin dng lm nhit in tr c pha tp. Do khi b cc tp cht khc thm thu trong qu trnh s dng s thay i tr s in ca n t hn so vi platin nguyn cht. Nh n t n nh lu di theo thi gian v thch hp hn trong cng nghip. Vi cc thit b o nhit chnh xc vi n nh nh s khuych tn cc tp cht vo u o d vi mt lng kh nh cng ng mt vai tr kh ln. Do vi cc nhit k chnh xc, ngi ta dng dy platin c ng knh kh ln n 250m. Mc ch s khuych tn cc tp cht vo bn trong dy in tr c ngn cn bt hay c s tin trin cng chm cng tt. Cng v l do ny m trong cng nghip, cc u o thng phi c thay th mi c nh k chnh xc ca php o lun c m bo.
b. Sai s cho php
Khi tnh n sai s, tiu chun DIN IEC 751 phn bit hai cp dung sai A v B.
Cp A c gi tr cho nhit t -200 n 6500C v cho cc thit b o nhit t 3 n 4 dy o. Cp B c gi tr cho ton thang t -200 n 8500C.
Cp A: t = ( (0,15+0,002t)
Cp B: t = ( (0,30+0,005t)
Hnh 2.8 Dung sai ca Pt100
2. Cc cu trc ca cm bin nhit platin v nickel
- Nhit in tr vi v gm: si platin c gi cht bn trong ng gm s vi bt oxit nhm. Di o t -200 n 800oC.
- Nhit in tr vi v thu tinh: loi ny c bn c hc v nhy cao. Di o t-200 n 400oC c dng trong mi trng ho cht c n mn cao.
- Nhit in tr vi v nha: Gia hai lp nha polyamid dy platin c ng knh khong 30 m c dn kn. Vi cu trc mng, cm bin loi ny c dng o nhit b mt cc ng hay cun dy bin th. Di o t -80 n 230oC.
- Nhit in tr vi k thut mng mng: Trn mt nn oxit nhm mt lp platin dy khong 1m c ph ln bng phng php phun ion hay bc hi chn khng. Sau vi phng php quang khc hay vi tia laser, lp platin c hnh mt ng gp khc v c chun ho cng bng tia laser. Sau lp platin cng c ph bng mt lp thu tinh. Di o nhit t -50 n 6000C. Cc nhit in tr vi k thut mng mng u c thi gian phn hi rt nh v qun tnh nhit nh. Vi k thut mng mng, nhit in tr c n nh lu di, d thay th, di o rng v c th sn xut hng lot.
Bng dung sai nhit ca nhit in tr.
2.3.2.2. Phng php o nhit bng cp nhit in
1. Cp nhit in
a. Hiu ng nhit in v m hnh cp nhit in
Hiu ng nhit in c m t nh sau: hai kim loi khng ging nhau c ni vi nhau ti hai im nh hnh v. Nu ta t nng mt im n nhit t1 v u cn li c gi nhit thp hn t2 s xut hin mt dng trong mch. Dng in ny s ph thuc vo kim loi v nhit t1, t2. Hiu ng ny c nh khoa hc Seebeck tm ra v c gi l hiu ng nhit in hay hiu ng Seebeck v c th c dng nh mt phng php o chnh xc nhit . Cc thit b ng dng hiu ng ny gi l cc cp nhit in.
Hnh 2.9. Cp nhit in n gin
Hiu ng nhit in xut hin do s xut hin ca mt in th qua im tip xc ca hai kim loi khc nhau. in th ny ph thuc vo nhit im tip xc v xut hin do s khc bit v tnh cht in v nhit ca cc kim loi. Cc electron im c nhit cao t1 c nhiu nng lng nhit hn cc electron nhit thp t2 trong mi kim loi v s c mt s dch chuyn cc electron v pha c nhit t2. S khc bit v dch chuyn gia hai kim loi khc nhau s to ra mt in p. in p ny rt nh, thng ch vo khong vi chc milivon.
im tip xc A c in p Va l hm ca t1, tng t ti im B c in p Vb l hm ca t2. Dng in xut hin do Va khc so vi Vb. Hin nhin nu t1 = t2, s khng c dng chy qua. Do , c th gi cp nhit in l mt thit b o chnh nhit.
Hnh 2.10. Mch in vi thit b o
Mt cp nhit in hnh thnh khi hai thanh kim loi khc nhau c ni vi nhau. Nh vy trong hnh trn, khi cp nhit in c ni vi dy ng a in p nhit n mch o in, hai cp nhit in khc c hnh thnh. T im ni ca cp nhit in, dy ng (dy o) v mch o in ta khng c s thay i v nhit , in p nhit ca cc cp nhit in y coi nh khng c. Quan trng vn l nhit ti im ni, v d nhit ti im o l 200oC , ti im ni l 20oC v in p nhit o c l 9 mV. in p nhit ny tng ng vi s chnh lch nhit l 180oC. Trong khi nhit lun c so snh vi 0oC. Nh vy in p nhit thc t cn phi hiu chnh bng cch cng thm in p nhit 20oC.
V = V (t) + V (20oC)
in p nhit so snh vi 0oC o c ca mi ni. gii quyt vn ny, trong thc t mt mch o nhit vi cp nhit in c gii quyt nh sau: T cp nhit in, ng dy in ni tip n my o khng phi dy ng m l mt loi dy dn in c cng vt liu nh cp nhit in. Hoc v l do tn km ngi ra dng dy vi vt liu khc nhng c cng tnh cht nhit in, dy ny c gi l dy thay th hay dy b.
Hnh 2.11 Cp nhit in vi dy b
im ni vi cp nhit in khng c in p nhit in. in p nhit in ny c hnh thnh im ni vi dy ng ca thit b o. y chnh l ni gi l im so snh. y ngi ta t mt u o nhit , nhit v s thay i nhit im so snh c o v tr s ny c a vo mch in t so snh, tnh ton v b tr.
ng knh ca si dy trong cp nhit in khng nh hng n tr s in p nhit. V d vi dy NiAl v NiCr d ng knh ca chng l 3 mm hay 0,55mm, nhit 998oC ta lun c in p nhit in l 40 mV. Tuy nhin, ng knh si dy trong cp nhit in cng nh th kt qu o cng chnh xc v cng nhanh, c bit khi cn phi o nhit b mt. Khi o nhit kh cao gn vi ranh gii ca nhit si dy ca cp nhit in cn chu ng c, ng knh li cn kh ln khng b t. nhit cao v theo thi gian, cp nhit in s tr nn dn hn, d gy do s ti kt tinh, cu trc ca kim loi tr nn th hn. Cc mch o vi cp nhit in thng phi c mch bo ng khi cp nhit in b t.
b. Cc loi cp nhit in theo tiu chun quc t.
C rt nhiu hn hp kim loi c chn la ch to cp nhit in nhng ch c mt s t c la chn v c tiu chun ho. Vic tiu chun ho c nh hng theo cc c trng k thut, c bit l di in p v sai s cho php. Bng di y cho ta cc loi cp nhit in theo tiu chun quc t IEC 584-3.
Thnh phnKiuDng lin tcDng ngn hn
PtRh10%-Pt13%S14000C17720C
PtRh30%-PtRh6%B15000C18100C
NiCr-NiK12600C13500C
Cu-CuNiT3000C3710C
Fe-CuNiJ7600C12000C
NiCr-CuNiE8710C10000C
NiCrSi-NiSiN12000C13000C
c. S thay i tnh cht ca cp nhit in theo thi gian.
nhit kh cao, cc nguyn t bn ngoi v d nh N, S, O2 s khuych tn vo cp nhit in. Qua m hnh cp nhit in, ta c th hiu rng khi c s khuych tn cc nguyn t l, lc ht ca cc ion kim loi b thay i i v do in p nhit cng b thay i theo. Ngi ta gi l s lo ho hay s trt ca kt qu o c cp nhit in. Vi s khuych tn ny, sau mt nm nhit ch th thay v 900oC c th l 906oC hay 600oC mc d thit b o khng h b h hng.
Hnh 2.12. S lo ho ca cc cp nhit in
d. Mt s li thng gp phi.
- Ch th nhit phng: cp nhit in hay dy dn b t.
- Ch th ng tr s nhng du m: in cc b i.
- Ch th nhit qu cao hay b tri:
+ in cc ca dy thay th b ni nhm cc vi cp nhit in (nh vy s to thm 2 cp nhit in)
+ S dng dy thay th, dy b sai.
- Ch th nhit qu cao hay qu thp: nhit im so snh sai.
- Ch th ng nhng tri dn i: nhit im so snh khng n nh hay khng c o.
- Khi h mt nhnh ca cp nhit in vn ch th mt tr s no .
+ Nhiu in t u vo thit b o.
+ S cch in trong l ti.
+ Phn cch in ca dy thay th b m.
- Li thng thng nht nc ta, do vic dng dy dn in bnh thng bng ng ni gia cp nhit in v thit b o. Sai s s rt ln v thay i lin tc.
e. Dung sai
Theo DIN IEC 584, cc cp nhit in c ng knh t 0,25 n 3 mm c ba cp dung sai c xc nh khi cn mi.
Bng di trnh by cc loi cp nhit in vi cc cp dung sai:LoiCp dung sai
J1
2
3-40 +7500C: ( 0,004.t
-40 +7500C: ( 0,0075.t(1,50C
(2,50C
T1
2
3-40 +3500C: ( 0,004.t
-40 +3500C: ( 0,0075.t
-200 +400C: ( 0,015.t(0,50C
(1,00C
(1,00C
K
N1
2
3-40 +10000C: ( 0,004.t
-40 +12000C: ( 0,0075.t
-200 +400C: ( 0,015.t(1,50C
(2,50C
(2,50C
E1
2
3-40 +8000C: ( 0,004.t
-40 +9000C: ( 0,0075.t
-200 +400C: ( 0,015.t(1,50C
(2,50C
(2,50C
B1
2
3 -600 +17000 C: ( 0,0025.t
-600 +17000C: ( 0,005.t(1,50C
(4,00C
2. ng dng trong cng nghip: Cp nhit in CM v CH ca Yokogawa.
Cp nhit in model CM v CH bao gm mt thnh phn cp nhit in (kiu K, E, J hay T) vi lp cch ly bng thu tinh, cha trong mt ng v bc bo v bng kim loi. Loi CM dng cho cc ng dng ni chung, loi CH dng trong vic o nhit ni c p sut cao.
Hnh 2.13 Cp nhit in CM v CHCp nhit in bao gm cc vt liu:
Kiu cp nhit inVt liu
Chn dng (+)Chn m (-)
KNiken-CromNiken-Nhm
ENiken-Cromng-Niken
JStng-Niken
Tngng-Niken
Bng dung sai nhit
KiuDung sai cp 1Dung sai cp 2Dung sai cp 3
Kiu T
Di nhit
Gi tr dung sai
Di nhit
Gi tr dung sai
-40 n 125oC-40 n 133oC-67 n 40oC
(0,5oC(1oC(1oC
125 n 350oC133 n 350oC-200 n -67oC
(0,004.| t |(0,0075.| t |(0,015.| t |
Kiu E
Di nhit
Gi tr dung sai
Di nhit
Gi tr dung sai
-40 n 375oC-40 n 333oC-167 n 40oC
(1,5oC(2,5oC(2,5oC
375 n 800oC333 n 900oC-200 n -167oC
(0,004.| t |(0,0075.| t |(0,015.| t |
Kiu J
Di nhit
Gi tr dung sai
Di nhit
Gi tr dung sai
-40 n 375oC-40 n 333oC-
(1,5oC(2,5oC-
375 n 750oC333 n 750oC-
(0,004.| t |(0,0075.| t |-
Kiu K
Di nhit
Gi tr dung sai
Di nhit
Gi tr dung sai
-40 n 375oC-40 n 333oC-167 n 40oC
(1,5oC(2,5oC(2,5oC
375 n 1000oC333 n 1200oC-200 n -167oC
(0,004.| t |(0,0075.| t |(0,015.| t |
Bng gii hn ca nhit vn hnh trong iu kin bnh thng v trong iu kin vt mc.
Kiu cp nhit inng knh dyNhit gii hn trong iu kin lm vic bnh thngNhit gii hn trong trng hp vt mc
K0,65650850
1,00750950
1,608501050
2,309001100
3,2010001200
E0,65450500
1,00500550
1,60550650
2,30600750
3,20700800
J0,65400500
1,00450550
1,60500650
2,30550750
3,20600750
T0,32200250
0,65200250
1,00250300
1,60300350
2.4. o p sut v Mc2.4.1. Gii thiu chung1. i cng v p sut
Cm bin c dng nhiu nht trong cng nghip l cc loi cm bin nhit , tip theo l cc cm bin p sut. Vic o c p sut ca kh, cht lng l cng vic cn thit bt buc trong thit k my mc c kh, cc dy chuyn sn xut Trc y, ngi ta dng cc loi cm bin p sut hon ton bng c hc. Hin nay v trong tng lai, ngi ta pht trin cc loi cm bin p sut vi vt liu bn dn r tin hn, bn b hn v chnh xc hn cho cc cng vic kim sot v iu khin t xa, cng vic t ng ho trong ton b h thng my mc, dy chuyn sn xut ca nh my. Mt s ln cm bin p sut hin ny c ch to t vt liu silic vi hiu ng in tr p in. Khc vi cc loi thit b o p sut c hc dng l xo v ch th trc tip qua mt h thng truyn ng c hc, cc loi cm bin p sut bn dn bin i lng vt l p sut thnh tn hiu in. Trong hu ht cc cm bin p sut u c mt phn t bin i tr s o t nng lng c hc thnh nng lng in gn trn mt mng n hi. Mng n hi ny c th c ch to vi vt liu kim loi, silic hay gm Hnh di cho ta cu trc nguyn tc ca mt cm bin o p sut: a) Mng n hi b un cong bi p sut; b) v c) Cu trc nguyn tc ca cm bin p sut tuyt i v hiu p.
p sut l i lng vt l biu din lc tc ng ln mt n v din tch c tnh theo s la chn vt liu cho mng n hi phi da theo mt s yu t sau:
Hnh 2.14 Cu trc ca mt cm bin o p sut
- S n hi cao nht
Trong khi ranh gii n hi ca mng kim loi cao nht c xc nh bi vng Hooke xa nht th mng tinh th silic c xc nh bi ranh gii gy vn.
nh lut Hooke: Gia bin dng v ng sut trong mt c th n hi ta c mt lin h tuyn tnh.
= /E
E c gi l t sut n hi hay modun n hi. nh lut ny c Robert Hooke tm ra t kinh nghim thc t v ng vi cc bin dng nh.
- T sut n hi: T sut n hi E quyt nh kch thc ca mng n hi, t sut cng nh, din tch mng cng ln.
- Nhit lm vic ti a
Ranh gii ca nhit lm vic thng b hn ch do cc vt liu tr nn do (s bin dng khng tr v hnh thi c) trc khi nhit t n chy lng.
- S mi ca vt liu
S mi ca vt liu lm n n hi nhiu hn v c bit sau khi b bin dng, n tr v trng thi c rt chm chp.
- S chu ng vi cc ho cht n mn
Ngy ny ngi ta s dng nhiu vt liu khc nhau lm mng kim loi n hi nh thp khng r, hp kim titan, nhm... Ngy nay loi mng n hi vi vt liu bn dn dn dn chim mt v tr rt quan trng trong vic sn xut cc cm bin p sut. V vi k thut bn dn mng n hi, phn t bin i p sut thnh tn hiu in v c mch khuych i c th c ch to trn cng mt chip. Vt liu u tin ch to mng n hi bn dn l cc phin silic dy t 0,3 n 0,5 mm. Cc phin silic ny c th c n mn vi phng php ho hc hay in ho c b dy t 1 n 2 m. dy ca mng n hi c xc nh chnh xc nh cc lp chm dt s n mn. Hnh di cho ta c tuyn vi vn tc n mn l hm s ca pha tp Bor. Gia pha tp Bor c tr s khong t 1019 n 1020 cm-3 ta c s thay i vn tc n mn c trm ln. Cc phin silic c chn lm vt liu khi im c pha tp p hay n vi nng N < 1018 cm-3.
Hnh 2.15 Cc cng on ch to mng mng bn dn
Trn b mt ca n ngi ta to mt lp rt mng c s pha tp Bor vi cng ngh khuych tn. K n ngi ta ph mt lp sn bo v ln trn b mt pha trc v trn hnh vnh khn mt sau. Cng on n mn bng ho cht c tin hnh vi vn tc kh nhanh vi lp bn dn c pha tp thp bn trong ng hnh tr. Khi n lp bn dn c pha tp cao, s n mn c ngn li mt cch t ng v lp ny vn tc s n mn b chm i hng trm ln. Sau lp sn bo v s c ty i. Tu theo khong o p sut, ngi ta s s dng cc loi cm bin p sut vo tng trng hp c th.2. nh ngha v n v p sut
nh ngha
p sut c nh ngha l lc tc ng trn mt n v din tch, theo cch vit vi phn
p = dF/dA
Trong : p: p sut
A: din tch
F: lc
o p sut ngi ta s dng mt nguyn tc ging nhau, p sut c cho tc dng ln mt b mt xc nh, nh th u tin p sut c bin thnh lc. Vic o p sut c a v vic o lc. Tt c lc tc dng ln mt mt phng xc nh l thc o p sut. Ta c:
p = F/A
o p sut c hiu l o p sut vi mt p sut tiu chun, p sut tiu chun c th l:
- p sut kh quyn: p sut c ghi nhn cho bit l p sut trn hay di p sut kh quyn.
- p sut chn khng: p sut c ghi nhn l p sut tuyt i.
- p sut khc: p sut c ghi nhn cho bit hiu s p sut gia hai p sut gi tt l hiu p.Hnh 2.16. Cc thang p sut
V d: 5 bar p sut cao hn p sut kh quyn tng ng vi 6 bar ca p sut tuyt i. 300 mbar p sut thp hn p sut kh quyn tng ng vi 700 mbar p sut tuyt i.
n v p sut
C mt di rt rng v kh phc tp cc n v o p sut c s dng hin nay. Theo l thuyt, p sut l mt lc tc dng ln mt n v din tch v c m t trong h SI l Newton trn mt mt vung (N/m2) v c gi l Pascal. n v ny khng c s dng rng ri m trong thc t, n v kiloPascal (kPa = N/cm2) c dng nhiu hn. lin hip Anh, n v p.s.i (pounds per square inch) c s dng rng ri hn. Trong nhiu ng dng thc t, n v atmosphere (14,7 p.s.i) hay bar (100 kPa) xp x mt atmosphere cng c s dng.
Ngoi ra cn c mt s n v c vn c dng cho vic o p sut trc y nhng nay khng c dng trong nhng ti liu chnh thc nh inches WG, Torr Bng di cho ta mi lin h gia cc n v ny.
Bng chuyn i cc n v p sut
1 p.s.i (lbf/in2)= 6,895 kPa
= 27,7 inch WG
1 lbf/ft2= 47,88 Pa
1 kgf/cm2= 98,07 kPa
1 ich WG= 249 Pa
= 5,2 lbf/ft2
= 0,036 p.s.i
1 foot WG= 2,989 kPa
= 62,43 lbf/ft2= 0,433 p.s.i
1 torr (mmHg)= 133,3 Pa
1 bar= 100 kPa
= 14,5 p.s.i
= 750 mmHg
= 401,8 inch WG
= 1,0197 kgf/cm2
1 atmosphere= 1,013 bar
= 14,7 p.s.i
1 kilopascal= 0,145 p.s.i
= 20,89 lbf/ft2= 4,141 ich WG
= 7,502 torr (mmHg)
= 0,01 bar
= 9,872x10-3 atm
3. o mc
Vi cc cht lng, lu tr cng nh vn chuyn, i hi vic hin th mc cht lng. C nhiu cng ngh c dng o mc trong iu khin qu trnh v vic chn la loi cm bin thch hp l nhu cu thit yu.
Mc thng lin quan n th tch. Vic o mc s khng phc tp khi thc hin vi thng cha c mt ct ngang ng dng nh thng cha trong hnh a) vi th tch c tnh bi cng thc:
V = h.A
Quan h gia th tch v mc s phc tp hn v khng tuyn tnh trong trng hp thng cha nh trong hnh b) v c).
Hnh 2.17 Quan h gia mc v th tch
(a) Thng cha hnh tr vi quan h tuyn tnh gia su v th tch
(b) Thng cha hnh cu; (c) Quan h gia su v th tch ca thng cha cuTrong rt nhiu ng dng, khi cn o th tch v khi lng, gii php n gin nht thng l o trng lng ca thng cha, vt cha bng cch treo ln cc loadcell hay gn ln cc cn o trng lng tuy nhin phi rt thn trng trnh cc va chm vi cc kt ni u vo v u ra ca ng ng. Vic cn o c th tch c th tch v khi lng m khng cn quan tm n hnh dng ca vt cha.
Cc iu kin bn trong vt cha khi o mc cng cn phi c quan tm n. Nhit , p sut v s n mn cng c xu hng lm nh hng n cc cm bin o mc hn l n cc nhit in tr v cp nhit in. Cc c tnh ca cht lng (hay cht rn) cng cn phi c quan tm. Mt lu cht lm m t hay bm vo cm bin s c th dn n vic hin th sai gi tr qu trnh. Vic o mc cng c th chu nh hng c hi bi s chuyn ng khng u hay s ni bt trn b mt. V vy, cn thit phi c v bc cho cm bin o mc.
Mt thng cha cht lng cng c th b cng hng. Mt thng cha vi ng knh L s dao ng vi chu k:
Ni chung, c bn loi b mt cn quan tm trong iu khin mc:
- Cht lng/ kh
- Cht rn/ kh
- Cht lng 1/ cht lng 2 (2 cht lng khng ho tan, v d nh nc/ du)
- Cht rn/ cht lng ( v d nh o su ca cn du trong thng cha du)
Trong hu ht cc ng dng, ta ch hay dng n loi mt v hai. Cc cm bin o mc nhn chung lm vic theo cch nhn dng v tr ca b mt.
2.4.2. Cc phng php o p sut, Mc v ng dng trong cng nghip
2.4.2.1. Cm bin o p sut
1. Cm bin p sut in tr p in
Ngy nay, loi cm bin p sut vi mch t hp in tr p in bn dn c sn xut nhiu cho vic o p sut v hiu p. Vi k thut bn dn ngi ta c th ch to mng o p sut hon ton bng vt liu silic. iu ny a ti vic ton b k thut ch to ra mt cm bin p sut c th c thc hin trn cng mt chip silic.
a. Hiu ng in tr p in trong bn dn (Piezoresistive Effect)
Di tc dng ca mt lc c hc ln tinh th, cc nguyn t b y lch i i vi nhau. iu ny lm thay i cc hng s mng v c cu trc cc vng nng lng: vng dn v vng ho tr.
Cc ng sut nn v ko lm thay i in tr sut ca bn dn silic p nh sau:
a) ng sut nn s h thp cc im cc i ca nng lng vng ho tr v th ta c s gia tng cc in tch theo hng nn trong tinh th.
b) Ngc li, ng sut ko s tc ng s nng ln cc im cc i ca nng lng theo hng ca n, do lm gim i cc in tch, in tr gia tng.
Trong mt cm bin p sut vi mch t hp in tr p in, s thay i in tr do s thay i kch thc hnh hc di tc dng ca p sut ch ng vai tr th cp (chim 2% s thay i in tr). Hiu ng c dng y l s thay i in tr sut theo cc ng sut c hc trong tinh th. Hiu ng ny c gi l hiu ng in tr p in c m t nh trn v c vit vi phng trnh nh sau:
(1)
Trong : : h s in tr p in
: in tr sut
: ng sut c hc
H s in tr p in ph thuc vo hng tinh th v iu kin o c.
Hnh 2.18 V tr mng sc co gin
Cc in tr c t cc im cc i ca ng sut nn v ko. Tng ng vi cc hng tng i ca cc thnh phn ng sut c hc, in trng E v mt dng in j ngi ta phn bit cc loi hiu ng:
- Hiu ng dc:
E // j //
- Hiu ng ngang:E // j
- Hiu ng ngang:(E j) //
Ch s cho cng ca hiu ng p in l h s K tu thuc vo vt liu. l h s t l ca s thay i in tr v di mt cch tng i.
(2)
Vi phng trnh (1), (2) v:
H s K c vit vi phng trnh
K =
: h s in tr p in
: modun n hi ca vt liu
K c tr s bng 2 cho kim loi v ln gp 50 n 100 ln cho bn dn tu pha tp. Cc cm bin vi k thut bn dn c cc u im nh nhy cao, tuyn tnh tt, tr ca p sut v nhit u nh, thi gian phn hi ngn, chu ng c s thay i ti cao do khng c mi ca mng silic n tinh th
b. Cu in tr v bin php hiu chnh cc thng s
T bo o p sut ca cm bin o p sut loi in tr p in l mt chip silic vung khong 6mm2 v c b dy sau khi c n mn vo khong vi m.
Hnh 2.19- Cm bin in tr p in
a) Chip silic vi in tr R1R4b) Mt ct ngang khi khng ti
c) Mt ct ngang khi c ti
vi vng gin (+) v vng co (-)
d) Mch in tr
e) Mt ct cm bin: 1. Mng thp; 2. Du;
3. Chip silic; 4. Ni dy
Mng silic c tc dng nh mt mt phng c cng ra, khi b un cong, trn b mt n xut hin nhng ni b cng v nhng ch b co vo. nhng ni ny, qua cng ngh khuych tn hay cy ion, cc in tr c cy vo. Cc in tr ny cng s b cng ra hay co vo mt cch tng ng. Ngoi ra cc in tr b tr nhit cng c hnh thnh trn cng mt chip. Cc in tr ny c ni vi nhau thnh mt cu in tr. C mt s cm bin, mch khuych i c t hp trn cng mt chip hay trn mt mch hn hp vi cu in tr. Nh in th ca cu in tr c khuych i ngay trn cm bin.
Hnh 2.20 - Cu in tr
Trong hnh trn ta c cu in tr ca cm bin vi 4 in tr ging nhau khi mng silic b un cong, R1 v R3 s gia tng tr s trong khi tr s ca in tr R2 v R4 s gim i. Do nhy ca cu c gia tng. in p ra U4 ca cu c tnh nh sau:
vi Ri(P) = Ri + Ri(P)
Vi mt k thut thch ng, ngi ta c th ch to sao cho cc in tr Ri c tr s ging nhau v s thay i Ri cng bng nhau. Phng trnh UA c th c rt gn nh sau:
vi l un c hc, K l h s t l
Hm s (p) cho un c hc ca mng silic theo php tnh gn ng bc 1 c s tuyn tnh, do gia in p ra v p sut cng c s lin h tuyn tnh.
UA = UCC.K.(p)
Khi un gia tng kh cao, ta s khng cn s tuyn tnh na. Vi hai in tr do s gia tng ca un, mt c tr s gia tng v mt c tr s gim i, vi s chn la tr s Ri hi khc i mt cht, hiu ng ny c th c trit tiu mt phn no.
c. Cc hiu ng nhit v ngun in thch ng
Cc cu in tr loi bn dn u b nh hng ca nhit kh mnh. Do vic b tr nhit cho cm bin p sut loi in tr p in l rt cn thit.
Hnh 2.21- S thay i ca in tr so vi nhit
Trong hnh trn, ta thy rt r:
a) S thay i tng i ca in tr i vi nhit . S thay i ny tng ng vi s gia tng in p qua cu in tr v tng ng vi s thay i tr s ca tng in tr.
b) Vi mt in th khng i, hiu th ca cu in tr ( nhy) gim mnh khi nhit gia tng. nhy c nh ngha l in th nh mc ng ra (mV) cho mi Volt ca ngun in cp cho cu in tr v tng n v p sut. nhy c h s nhit m, tr s ca n gim i khi nhit tng.
c) Khi cu in tr c cp in vi mt ngun in c cng dng in c nh, ta c s trit tiu ln nhau ca hai hiu ng a) v b), ta c ng biu din c).
Do cp in cho cc cm bin p sut in tr p in, ngun dng c nh l mt gii php kh tt b tr s nh hng ca nhit nh hiu ng b tr t ng nu trn.
2. Cm bin p sut in dung o hiu sut
Hnh di trnh by cu trc nguyn tc ca mt cm bin hiu sut in dung.
Hnh 2.22- Cm bin hiu p
1. Mng o; 2. Gm; 3. Du silic
4. in cc; 5. o nhit
Vi cc in cc (4) hnh thnh t in C1 v C2. C hai c in dung b thay i theo hiu sut p = p1 p2. in mi ca C1 v C2 l du silic (3). Khi p2 > p2 in dung C1 gia tng v C2 gim. S thay i th cp tip theo l s thay i hng s in mi v th tch ca du do s thay i nhit .
p = p1 p2 ~ 1/C1 1/C2p c th c tnh vi phng php tnh ton ca b vi x l trong h thng cm bin hiu sut. Trong hnh v, khi nhit c o vi (5), h thng cm bin c kh nng t kim sot.
2.4.2.2. o mc cht lng
1. o mc cht lng bng cch o p sut v hiu p
Cht lng trong mt bnh cha khng chu tc dng di mt p sut cho ta p sut di y bnh:
ph = gh
Trong : l t trng
g l gia tc trng trng
h l chiu cao ca mc nc cht lng
Nh vy ta c th xc nh c mc cht lng bng cch o p sut. Mt ct nc cao 10m vi t trng 1000kg/m3 c p sut y:
Ph =
Bnh cha chu tc dng mt p sut p0, xc nh mc cht lng, ta cn o hiu p: p sut ph + p0 v p sut p0. Ch th cho ta hiu p t l vi mc cht lng. Ta cn mt van kho cht lng trong trng hp cn kim sot s chnh xc ca ng h o p sut vi mt ng h chun. Trong trng hp o hiu p, ta cn 3 van kho cht lng. Hai van cho 2 ct cht lng v 1 van ni tt hai ct cht lng nhm kim sot im khng ca ng h o p lc.
2. o mc bng phng php o in dung
Mc cht lng khng dn in hay mc cht rn c th xc nh bng phng php o in dung. Hai bn cc ca mt t in c cho vo mi trng o nh hnh v. in dung c tnh vi phng trnh:
r l hng s in mi ca mi trng
3. o mc cht lng bng sng siu m
Phng php o bng sng siu m da trn sng m thanh tn s cao. Tn s 50 kHz l tn s in hnh trong nhng ng dng cng nghip (tn s cao hn c th dng trong nhng ng dng v y t nh my qut siu m). Tn s lm vic c chn tng ng vi tn s cng hng ca b pht v b thu.
Nguyn l hot ng ca thit b o mc bng sng siu m c m t hnh di. Mt b pht v thu sng siu m c t trn nh ca thng cha v mt chm tia sng siu m t b pht chiu xung b mt ca cht lng (hay cht rn) c o. Mc trong thng cha c tnh ra t cc tn hiu phn x li ln b thu.
Hnh 2.23- Nguyn l o mc bng sng siu m
Tn hiu phn x l mt dng tn hiu tr ca tn hiu pht i t b pht vi tr bng 2d/v trong d l khong cch t b mt cht c o v v l vn tc sng m thanh trong mi trng pha trn b mt .
V d vn tc ca sng m trong khng kh l khong 300 m/s-1, vi mt mc t 0 n 10 m, thi gian tr l 0 n 70 ms. C 2 cch o thi gian tr, vic chn la s c quyt nh tu theo di o so snh vi di sng m. Nu d ln (d thng ln trong cc ng dng o mc), mt xung m thanh hp c truyn i t b pht. B thu s nhn c 2 xung: mt t b pht v mt phn x li t b mt cn o. Thi gian tr c th c o nh hnh di v v tr ca b mt cht cn o hay mc s c tnh ton trc tip.
2.4.3. ng dng trong cng nghip: Thit b o p sut v mc EJA ca Yokogawa
Hnh 2.2 4- Thit b o v chuyn i tn hiu p sut v mc EJA ca Yokogawa
1. Chc nng
o p sut, mc cng nh lu lng ca cc cht lng v cht kh, Yokogawa cung cp thit b o v chuyn i p sut vi sai da trn cc cm bin cng hng o c s chnh p sut u vo.
2. Cu to ca cm bin cng hng.
Hnh 2.26- Cu to ca cm bin cng hng
1) Tm vt liu silicon
2) Cu cng hng hnh ch H t trn tm silicon.
3) Cu cng hng dao ng vi tn s 90 KHz khi khng c p sut a vo.
4) p sut a vo lm bin dng b mt tm silicon.
5) S bin dng ca tm silicon gy ra mt s thay i tn s u ra ca cu cng hng t l vi p sut a vo.
3. Nguyn l hot ng ca cm bin
Hnh 2.27a - Nguyn l hot ng ca cm bin cng hng
Nguyn l hot ng ca cm bin cng hng c m t nh sau: hai cu cng hng hnh ch H dao ng vi tn s 90 KHz c t trn mt tm silicon. Khi p sut c a vo, tm silicon s b bin dng. Do v tr v su chnh xc ca hai cu cng hng, 1 cu s trng thi b cng v cu cn li trng thi b nn. Lc ny tn s ca cu b cng s thay i t 90 KHz n 110 KHz v cu b nn thay i t 90 KHz n 70 KHz. nhn c hiu p u ra, b vi x l s tnh ton s thay i tn s ca 2 cu v cho ra tn hiu p sut u ra tng ng.
Hnh 2.27b- S thay i tn s cng hng2..5. o lu lng
2..5.1.Gii thiu chung Mt trong s cc tham s quan trng ca qu trnh cng ngh l lu lng cc cht chy qua ng dn. Lu lng vt cht l s lng cht y vn chuyn trong mt n v thi gian..
Trn thc t, ta thy vt cht tn ti ba dng ch yu sau: rn, lng ,kh . i vi mi loi vt cht ta c cc phng php o khc nhau, cc n v tnh hay dng l khc nhau , iu ny do bn cht tn ti ca tng dng vt cht l khc nhau.
- Lu lng cht lng kh thng c tnh bng th tch trn n v thi gian:
Qv = V/t
(m3/s)
- Lu lng cht rn thng c tnh bng trng khi trn mt n v thi gian:
Qm = m/t
(kg/s)
Trn y ch l cc n v o hay dng i vi tng loi ,nu ta bit chnh xc khi lng ring ca vt cht cn o ta c th chuyn i n v mt cch d dng gia 2 loi n v lu lng trn bng phng trnh:
Qm = Qv.
Qv =Qm/
Vic o lu lng c trnh by trong cc phn di y cng xin da trn t tng v s phn loai vt cht theo bn cht tn ti m phn ra lm hai phn c bn: o lu lng cht lng-kh, v o lu lng cht rn .
2.5.2. Cc phng php o lu lng cht lng - kh
1. Phng php o lu lng bng tn s dng xoy
a. Nguyn l o
Phng php o lu lng bng tn s dng xoy da trn hiu ng s pht sinh dng xoy khi mt vt cn nm trong lu cht. Cc dng xoy xut hin tun t v b dng chy cun tri i. Hin tng ny c Leonardo da Vinci ghi nhn v Strouhal trong nm 1878 c gng gii thch ln u tin. ng nhn thy rng mt si dy nm trong dng chy c s rung ng nh mt dy n. S dao ng ny t l vi vn tc dng chy v t l nghch vi ng knh si dy.
Theo dor von Karman tm ra nguyn nhn gy ra s dao ng ny: l s sinh ra v bin mt ca cc dng xoy bn cnh vt cn. Mt con ng dng xoy hnh thnh pha sau vt cn khi mt vt c t trong mt dng chy, pha sau n (pha khut gi) cc dng xoy khng lin tc c to ra.
Hnh 2.28a- Dng xoy xut hin sau vt cn
Cc dng xoy ny ri b vt cn tun t v tri i theo dng chy. Pha sau vt cn hnh thnh con ng ca dng xoy c t tn l con ng dng xoy Karman. Cc dng xoy 2 cnh bn ca vt cn c chiu xoy ngc nhau. Nh th vi s xut hin ca dng xoy, nng lng quay s b chuyn ch i mt.
Hnh 2.28b Phng php o lu lng bng dng xoy
Cui cng, lc tc ng vo vt cn c hnh thnh. Di nhng iu kin cn thit, mt dao ng pht sinh: cc dng xoy mnh m c sinh ra ri li ln lt bin mt theo dng chy. Tn s s bin mt ca dng xoy (v c s xut hin) l mt hiu ng dng o lu lng tnh bng th tch.
Lord Rayleigh tm thy s lin h gia kch thc hnh hc vt cn b, vn tc lu cht v v tn s bin mt ca dng xoy f- tn s dng xoy. S lin h ny c din t trong tr s Strouhal:
Trong : b : ng knh ca vt cn
f : tn s dng xoy
v : vn tc dng xoy
Ngoi ra, Karman tm thy rng,h s S li bin thin rt t khi h s Reynolds(Re) thay i trong mt khong kh rng.
Hnh dng ca vt cn phi c cu to sao cho trong mt khong tr s Reynolds kh rng m tr s Strouhal vn l hng s. Hnh di cho ta s lin h gia tr s Strouhal v tr s Reynold vi hai vt cn khc nhau. Vi vt cn c hnh dng lng knh, ta c tr s S kh n nh trong sut mt di tr s Re kh rng.
Hnh 2.28c- Tr s Strouhal l hm ca tr s Reynol
Trn thc t ta thng tm thy trn th trng cm bin loi ny vi hnh dng vt cn c dng hnh lc lng,loi vt cn ny c c im l s thay i ca h s S theo Re l rt t .
Tn s dao ng ca vn tc c th c o vi nhiu phng php khc nhau: Nhit in tr un nng c dng n, n c th c gn pha trc, gia hay pha sau vt cn, s thot nhit ca nhit in tr thay i theo hng ca lu cht. Ngi ta cng c th o s dao ng p sut vi mng sc co gin hoc o cc dng xoy vi sng siu m.
Khi y lu lng c th tnh qua cng thc sau:
Phng php o lu lng vi tn s dng xoy rt kinh t v c tin cy cao:
Tn s dng xoy khng b nh hng bi s d bn,s h hng nh ca vt cn.
ng biu din ca n tuyn tnh v khng thay i theo thi gian s dng. Sai s php o rt b. Khong o lu lng tnh bng th tch t 3 n 100%.
Mt tnh cht rt c bit na ca php o bng dng xoy l n c lp vi cc tnh cht vt l ca mi trng dng chy. Sau mt ln chnh nh, sau ta khng cn chnh nh li vi tng loi lu cht. 2. Cc lu lng k kiu xoy (Vortex Flow Metter) hay gp trong cng nghip.
a. Nguyn l lm vic.
Khi mt thanh chn to xoy (vortex shedder) c t trong lng mt ng dn cht lng, cc xoy kiu Karman s xut hin xui theo thanh chn nh hnh v.
Hnh 2.29 - Nguyn l lm vic
Gi s tn s ca cc xoy c to ra bi thanh chn l f, vn tc dng chy l v v rng ca thanh to xoy l d, ta thu c cng thc sau:
f = St.
Cng thc ny cng c p dng cho lu lng k c lp t trn mt ng ng.
Q : lu lng t l
D : ng knh bn trong lu lng k
St : tr s Strouhal
T (1) v (2) , ta tnh c lu lng t l:
Tr s Strouhal (St) l mt s khng th nguyn ph thuc vo hnh dng v kch c ca thanh to xoy. Do vy, bng cch chn la mt hnh dng thch hp, tr s Strouhal c th c gi khng i trn mt di o rng ca tr s Reynolds.
Hnh di minh ho mi quan h gia tr s Reynolds v tr s Strouhal.
Hnh 2.30 - Quan h gia tr s Reynolds v tr s Strouhal
Do vy, khi bit tr s Strouhal, c th tnh c lu lng t l bng cch o tn s xoy.
Hnh 2.1.2a3- Nguyn l to xoy
Hnh 2.31- Nguyn l to xoy
b. Cu trc
1. B chuyn i
2. Ming m
3. Thnh phn cm bin
4. Thanh to xoy
5. Mn hnh hin th tn hiu u ra
6. Dy tn hiu cm bin.
c. Nhiu v chng nhiu
Tn hiu ng sut v nhiu ng sut.
Mt cm bin p in rt nhy cm vi nhiu, c bit l do s rung trn cc ng ng, c thit k hn ch n mc ti a tc ng ca nhiu k c vi phn cng v phn mm.
Hnh 2.33- Cc nhiu tc ng
d. Cu hnh phn cng ca cc b bin i
in tch thay i c to ra cc thnh phn p in do cc dng xoy c bin i thnh tn hiu in p xoay chiu bi hai b bin i in tch. Nhiu c loi tr bi b cng v b lc tch cc ca trng thi tip theo, sau tn hiu c a vo trig Schmidt v s cho ra mt dy xung tng ng vi mi tn s dng xoy.
Hnh di trnh by cu hnh phn cng ca cc b bin i:
Hnh 2.34- Cu hnh phn cng
B vi x l s ly tn hiu l dy xung ny, thc hin vic tnh ton. Trong khi , bin ca tn hiu c bin i thnh in p mt chiu ti mch chnh lu qua mt b cng v b lc u tin. Tn hiu in p mt chiu ny c bin i thnh tn hiu tn s bi b bin i V/F v c a qua b vi x l tch nhiu. Dy xung sau khi i qua b vi x l, c bin i thnh in p tng t lm u ra tng t bi b bin i F/V qua mt mch cch ly v sau c chuyn thnh tn hiu 4 n 20mA qua mt b bin i V/I.
3. Phng php o bng siu m
a. Nguyn l o
Tn s ca siu m cao hn tn s m thnh gic ca con ngi c th cm nhn c. Trong k thut, tn s hu ch ca siu m tri di t 20 kHz n 10 MHz. Tn s, di sng v vn tc truyn sng c lin kt vi nhau.
Co = f.
Vn tc truyn sng l thuc vo c tnh ca mi trng v c bit vo nhit ca mi trng. nhit bnh thng, sng m thanh lan truyn vi vn tc 344 m/s trong khng kh v 1483 m/s trong nc. Nh th vi sng siu m 100 kHz c di sng l 15mm trong nc. Hnh di trnh by cu trc ca mt cm bin siu m p in.
Hnh 2.35- Cu trc ngun pht siu m
Sng siu m c to nn bi cc vt liu p in. Trng hp ng vai tr pht sng nng lng in c bin thnh nng lng c hc. Di mt in p xoay chiu phin p in rung vi tn s ring v sng siu m c pht ra thng gc vi b mt. Vi nhng cu trc c bit, ngi ta c th to nn nhng mt sng nm nghing.
thu nhn sng siu m, ngi ta dng linh kin c cu trc nh ngun pht. Trng hp ny nng lng c hc c bin thnh nng lng in. Cc sng siu m lm rung phin p in. Vi hiu ng p in ngc li ny, ta c tn hiu in t cc sng m thanh. Trong thc t, thng vi cng mt linh kin va ng vai tr pht v thu.
Hnh di trnh by mt cu trc dng o lu lng. Cc cm bin siu m nm cch nhau mt khong L trong ng dn c lu cht dch chuyn mt vn tc v. Cm bin 1 pht sng v cm bin 2 thu sng, vn tc truyn sng c gia tng thm thnh phn vcos. Vi phng php o sng siu m ta c vn tc v ca dng chy v sau khi nhn v vi din tch mt ct ngang ca ng, ta thu c lu lng tnh bng th tch.
Hnh 2.36- Cu trc ng o lu lng bng siu m
o c thi gian truyn sng mt cch chnh xc, cc cm bin siu m phi hot ng nhanh. Cc cm bin ny cn phi pht c cc sng c sn dc thng ng. C 2 cm bin i din nhau pht cng lc mt sng siu m. C 2 hot ng u tin nh ngun pht v sau hot ng nh hai cm bin thu sng siu m ca nhau. Vn tc dng chy c xc nh rt nhanh chng vi phng php ny.
Hnh 2.37- ng o lu lng vi siu m trong cng nghip
b. Cc lu lng k siu m hay gp.
Trong phn ny ta s tp trung vo lu lng k siu m ca Siemens
Hnh 2.38- Lu lng k FUS ca Siemens
a. Nguyn l lm vic
Hnh 2.39- Nguyn l lm vic ca lu lng k sng siu m
Hnh v trn m t nguyn l lm vic ca lu lng k sng siu m. Trong :
A, B: 2 b chuyn i sng siu m.
R: vt phn x
L: khong cch gia 2 b chuyn i.
VM: vn tc ca dng chy
tAB/VAB: t s gia thi gian v vn tc truyn sng m t A n B
tBA/VBA: t s gia thi gian v vn tc truyn sng m t B n A.Tc truyn m thanh v ca sng siu m trong mt mi trng ph thuc vo vn tc m thanh CM trong mi trng v vn tc dng chy VM (hiu ng mang).
VAB = CM + VM
VBA = CM - VM Hiu ng ny c Siemens ng dng trong lu lng k siu m. 2 b chuyn i sng siu m s ln lt gi cho nhau nhng tn hiu sng siu m v o c 2 i lng thi gian thc tAB v tBA qua li gia cc tn hiu ny.
tAB = L/(CM + VM)
tBA = L/(CM - VM)
Nu c dng chy vo, thi gian qua li gia cc tn hiu s nhanh hn vi chiu xui v chm hn vi chiu ngc dng chy. S chnh lch v thi gian ny chnh l gi tr vn tc ca dng chy VM.
Kt qu ny khng ph thuc vo tc m thanh trong mi trng v do khng ph thuc vo mi trng o.
b. c im k thut- Lu lng k sng siu m c dng o cc lu cht l cht lng vi chnh xc:
Lu cht chnh xc
Cht lng( 0,5 2 %
- Cc yu t cn quan tm khi s dng lu lng k sng siu m:
+ Cch lp t (Ta c th tham kho ti liu ca nh sn xut)
+ Nhit lu cht
4. Phng php o lu lng bng cm ng in t
a. Nguyn l o
Theo nh lut cm ng in t ca Faraday, mt dy dn in pht sinh mt in th khi n di chuyn trong mt t trng. in th ny t l vi vn tc di chuyn ca dy dn in v cng ca t trng.
Hiu ng ny c dng trong phng php o lu lng cc cht lng dn in c cha cc ion mang in tch.
Nh th theo nh lut Faraday, khi cc ion mang in tch q dch chuyn cng vi lu cht vi vn tc v c hng thng gc vi t trng B, cc ion ny b ko lch v mt bn vi lc Fm = qvB. Do s phn b cc ion ta c mt in trng E pht sinh v to ra mt lc Fe = qE tc dng ln ion. Vi in trng E ta c mt in th U o c hai in cc nm trn ng lu cht c ng knh D
(1)
Hnh 2.40- a) Nguyn tc o lu lng bng hiu ng cm ng in t; b) Mch in thay thVi vn tc v, ta c lu lng tnh theo th tch khi din tch mt ct ngang ng l (Qv =
Qv =
Nh th in th o c l thc o ca lu lng cht lng c tnh bng th tch trn n v thi gian.
Trong phng trnh (1) ta thy khng c thnh phn dn in . N khng ng vai tr no cho tr s hiu in th U, tuy nhin n li xc nh tr s in tr trong Rq ca ngun in. l in tr ca ct cht lng gia hai in cc. Vi D l ng knh ca ng dn lu cht, A l din tch ca in cc ta c:
Rq =
Thit b o lu lng vi cm ng in vi cm ng in t l mt ngun in c in tr trong rt ln (hnh 2.3.1b). o c in th ca n ta cn mt mch khuych i c in tr vo cng kh ln.
a. T trng mt chiu
Vi phng php o lu lng bng cm ng in t, ta c tn hiu hu ch o c khong vi mV. Tn hiu ny b mt in th mt chiu bt i xng che lp. Nguyn nhn ca in th nhiu ny l do s khc nhau ca cc th in ho t:
- Vt liu ca in cc khc nhau do nhim bn hay s ko lch cc mng tinh th.
- B mt c s khc nhau do b oxi ho hay b bm bi mt lp bn.
- S khc nhau ca s phn b s bt kh v cc vt th khng gian.
b.T trng xoay chiu hnh sin
Hn na vi t trng mt chiu, cc in th phn cc pht sinh cng s che lp cc in th cm ng v lm tr s o b sai lch. Do ngi ta thng dng n t trng xoay chiu. in th o c c tr s nh vi mV. Tuy nhin vi thnh phn t trng phn tn ca nam chm, in th cm ng pht sinh trong cc dy o in trng c tr s ln hn in th U cn o. Nh th ta cn phi tch tn hiu nhiu do t trng phn tn v tn hiu cn o. Tn hiu cn o t l vi cm ng B = , in th nhiu t l vi o hm theo thi gian dB/dt =
in th hu dng v in th nhiu c s lch pha 900. Nh th ta c th dng mt mch chnh lu ng b vi in th o c in th u ra sau khi ly tr s trung bnh khng cn c thnh phn in th nhiu b lch pha (/2. Tuy nhin vi t trng xoay chiu cc kh khn gp phi khi o lu lng vi cm ng in t vn cha c gii quyt ht. Ta cn phi ch l tn hiu o ch c vi mV trong khi tn hiu cm ng nhiu ln gp 105 n 106 ln. Tn hiu nhiu trong thc t c pha v bin thng khng n nh. Lu cht c th l nc thi khng c dn in c nh, nhiu lu cht trng thi ni t khc nhau.
c. T trng xung mt chiu
Vi t trng xung c i cc ngi ta c th loi b nh in th nhiu mt chiu bt i xng. Vi phng php ny mt nam chm in cho ta t trng mt chiu. T trng ny c c ng m o cc theo nhng khong thi gian nht nh. Trong hnh di vi s o cc ca t trng, trong nhng qung thi gian c gch sc tng v hiu ca tn hiu nhiu v tn hiu o c v ghi li trong b nh. Ngi ta li dng s kin l khi o cc, tn hiu o cng o cc trong khi tn hiu nhiu khng o cc. Tn hiu o c tch ra khi ta ly tng s hai tn hiu tr i hiu s hai tn hiu.Cc tnh cht k thut :My o lu lng cm ng in t cho ta mt in th t l vi lu lng trong mt t trng ng nht khi m hnh dng dng chy c i xng xoay. Mt u im ca phng php o lu lng bng cm ng in t l mt ct ca dng chy khng b thu hp bi cc ng blende hay ng phun.
Hnh 2.41- T trng xung mt chiu
Phng php o ny c th thc hin o lu lng cc cht lng d bn, cht kem sn st hay ngay c vi mi trng c tnh cht n mn cao nh acid, kim T 10 n 100% thang o ta c sai s php o nh hn 1%. Hnh di l hnh cm bin o lu lng bng phng php cm ng in t trong cng nghip.
Trong th c t
Tm li vi phng php cm ng in t, ta c s lin h rt tuyn tnh gia th tch tnh trn n v thi gian v in th cn o. Khong o rt rng v tr s o khng b nh hng bi cc thng s ca lu cht nh nht, t trng, p sut, nhit . Trong ng dn lu cht khng c vt cn. Cm bin c ch to c c rng chun t vi milimet n vi mt. Trong cng nghip, thit b o lu lng vi phng php cm ng in t c dng nhng ni p sut cao v nhit rt cao cng nh nhng ni d gy chy n, ngp lt Phng php o lu lng bng cm ng in t d c t tin v phc tp hn cc phng php khc nhng n c nghin cu v th nghim rt lu trong cng nghip. N t ra bn b, t b h hng d phi lm vic vi nhng lu cht c hi, n mn cao nh nc thi, ho cht.
Tuy nhin ,nhc im chnh m ta cha khc phc c loi lu lng k ny , l vic o c ca n ph thuc vo mt ion cn c trong cht cn o .C l l y l nhc im khng khc phc oc v vic s dng ion trong cht cn o chnh l nguyn l lm vic ca lu lng k loi ny.5. Cc lu lng k in t hay gp trong cng nghip
Trong phn ny ta s tp trung phn tch cc c im ca lu lng k in t ADMAG AE ca Yokogawa.
Hnh 2.43- Lu lng k in t ADMAG AE
a. Nguyn l lm vic
Lu lng k t in h ADMAG ca YOKOGAWA hot ng da trn nh lut cm ng in t ca Faraday. Khi c dy dn chuyn ng trong t trng ct cc ng sc t, trong dy dn cm ng mt sc in ng t l vi tc chuyn ng ca dy dn. Nu ta dng mt cht lng dn in chy qua gia 2 cc ca mt nam chm v o sc in ng sinh ra trong cht lng th c th xc nh c tc dng chy hay lu lng th tch.
Hnh 2.44- Nguyn l lm vic
Gia 2 cc N v S ca mt nam chm, ngi ta t mt on ng kim loi khng t tnh vung gc vi ng sc ca t trng. Mt trong ca ng ph mt lp vt liu cch in (gm, sn emay, thu tinh hu c). Trong mt phng vung gc vi ng sc, ngi ta t 2 in cc. Sc in ng cm ng trong t trng khng i l:
E =B.V.D
Trong :
B: cng t cm
V: tc trung bnh ca dng chy
D: ng knh trong ca ng dn.
Khi B bng hng s th sc in ng ph thuc tuyn tnh vo lu lng th tch ca cht lng. T ta tnh c lu lng ca lu cht chy qua ng dn. Lu lng k in t ch o c lu cht l cht lng dn in.
b. Nhiu v chng nhiu
Khi o cc cht lng bng lu lng k in t, chnh xc ca lu lng k c th thay i ph thuc vo mt ion c trong cht lng cn o hay ni cch khc n b nh hng bi dn in ca cht lng cn o. Mt khc ta cng bit rng , dn in ca cht lng ph thuc vo rt nhiu yu t khc nhau nh nhit ,trong thi gian gn y khi ta khc phc c im mt s im yu ny th c th loi lu lng k ny s rt ph dng, c nhiu tin ch v a nng.
Mt nguyn nhn khc cng dn n s sai lch v chnh xc ca php o l nh hng ca cc cht n mn do trong qu trnh hot ng lu ngy s bm ln in cc gy ra nhiu ng k:
Mi quan h gia kch thch v tn hiu lu lng:
Hnh trn ch ra mi quan h gia dng sng kch thch v dng sng tn hiu. ng c tnh ca lu lng k t khc nhau tu thuc vo phng php kch thch. Do sc in ng t l vi cng t trng B, c ngha l dng sng cu tn hiu cng ging nh dng sng kch thch. V vy nu kch thch c dng sng hnh sin, tn hiu cng c dng hnh sin. iu ny c p dng chung cho tt c cc phng php kch thch.
Sc in ng E cng t l vi vn tc dng chy V, c ngha l bin ca tn hiu lu lng s t l vi vn tc dng chy, v vy dng sng ca tn hiu s nh nu vn tc dng chy nh v ngc li. Nh trn hnh v, dng sng ca tn hiu cng dng vi dng sng kch thch. Tn hiu lun i km theo cc thnh phn nhiu, chnh v vy nu dng sng hay tn s ca tn hiu c dng khc bit vi nhiu th s rt d pht hin v loi tr.
* c im k thut
- Lu lng k in t c dng o cc lu cht l cht lng dn in vi chnh xc cao:
Lu cht chnh xc
Cht lng( 0,2 1 %
- Cc yu t cn quan tm khi s dng lu lng k in t:
+ Cch lp t (Tham kho ti liu ca hng)
+ Nhit lu cht
+ dn in ca lu cht
6. Phng php o lu lng bng nghn tiu chun (hay nguyn l thay i gim p sut)
a. Nguyn l o
L thuyt c bn
Mt trong nhng nguyn tc ph bin o lu lng cht lng, kh v hi l nguyn tc thay i gim p sut qua ng thu hp. Ta c th thy trong hnh v di y, nu mt thit b thu hp c t trong lng ng ng c lu cht chy qua, khi c lu cht chy qua s c s chnh p sut trc v sau l thu hp. chnh p sut ny ph thuc vo lu lng chy qua ng. Nguyn l n gin ny c ng dng rt rng ri trong cng nghip o lu lng da trn cc thit b nh tm nghn (blende) hay cc loi ng phun venturi. c dng nh mt thit b o lu lng, ta cn tnh ton lu lng da trn chnh p sut. iu ny c thc hin bng cch quan tm n nng lng ca lu cht. Nng lng ca mt n v khi lng ca lu cht gm ba thnh phn:
- ng nng do chuyn ng, c tnh bi v2/2 vi v l vn tc.
- Th nng gh vi g l gia tc trng trng, h l chiu cao theo mt s mc o.
- Nng lng p sut ca lu cht, tng t nh th nng v c tnh bng P/ vi P l p sut v l t trng nhit v p sut ca lu cht.
Nng lng ca mt n v khi lng lu cht ti bt k im no cng bng tng ca cc thnh phn sau:
(1)
y l phng trnh c bn ca lu lng k chnh p.
Hnh 2.46 - Lu lng k o chnh p: do Q1 = Q2, vn tc dng chy phi tng ti A2 v gy ra s chnh p gia P1 v P2b. Dng chy ri v h s Reynolds
Nu phng trnh (1) c dng nh phng trnh c bn o lu lng bng phng php o chnh p, ta c th thy c rng vn tc ca lu cht ti bt k im no qua ng ng l nh nhau. Tuy nhin, vi nhng dng chy nh, iu ny c th khng chnh xc do hiu ng ma st ca thnh ng dn n m t v vn tc dng chy nh hnh v di v c gi l dng chy thnh lp. Nu vn tc dng chy tng ln, s xut hin s hn lon v mc ln, vn tc tr nn bng nhau ti mi im trn ng ng. Nhn chung, cc lu lng k o chnh p thng ch c s dng vi dng chy ri.Vi mt lu cht cho trc, trng thi ca dng chy c ch th bi h s Reynolds- Re- v c cho bi:
Trong : v: l vn tc lu cht
D: ng knh ng
: l t trng ca lu cht
: nht ca lu cht
nht ng hc / thng c s dng nhiu hn l cc gi tr ring l v . H s Renolds l h s t l do khng c th nguyn. Gi tr ca h s Re cng ln th mc hn lon ca dng chy cng cao. Nhn chung, nu Re < 2000, dng chy l dng chy thnh lp. Nu Re > 105, dng chy l hon ton ri.
c. o theo chnh lch p sut :
Xut pht t phng trnh bo ton lu lng trong ng chy ni tip ta c :
Q1=Q2
Ta c th thay vo phng trnh trn biu thc tnh Q =V.S nn ta c phng trnh mi nh sau:
V1.S1 =V2.S2By gi, n gin cho cc biu thc tnh ton, ta c th t nh sau :
Khi kt hp hai biu thc trn ,ta c th vit nh sau :
Ta tip tc t mt h s c trng mi theo biu thc di y :
Theo phng trnh thy ng lc hc Becnuli,v p dng cho ng nm ngang (khng thay i v th trng trng do cng cao), ta c phng trnh cn bng sau :
EMBED Equation.DSMT4 Trong :
P1 ,P2 tng ng l p sut vo ,v ra thit b o ca dng cht lng .
V1 ,V2 tng ng l vn tc vo ,v ra thit b o ca dng cht lng .
l mt ca dng cht lng .
,m l cc h s
Khai trin sau khi chuyn v ,ta c phng trnh sau:
Ta c th lp biu thc tnh V2 nh sau :
EMBED Equation.DSMT4
Khi y lu lng ti cc tit din khc nhau trong ng c th tnh nh sau :
Trong h s k c tnh nh sau :
Theo phng php ny ,ta c th phn ra mt s thit b o kiu ny nh sau:
ng Blender(tm nghn tiu chun)
ng phun
ng Venturi
Cc ng ny khng ch khc nhau v cu to m cn khc hn nhau v c tnh k thut c bit l v sai s ca php o ,trong cc loi ng trn ch c ng Venturi l ng cho sai s php o t nht v : ng c cu to nh mt ci qung u ra ,v vy n c dng xoy (chy cun) l t nht ,v th gy st p gia hai u trc v sau thit b o l t nht ,do vy kt qu o l chnh xc nht .
Sau y s l biu phn b p sut dc theo chiu di ca ng ,v st p sut trn thit b o (P=P1-P1 )
V mt l thuyt vi thit b o l tng th ta s khng c s st p ,tuy vy trong thc t gp phi mt s cc vn v sai s trong phng php ny v P0 .
Lu : Nu chnh p c tnh bng chiu cao h th
Sai s ny cn thay i theo m (h s t l tit din ng),khi y ta cng thy ng Venturi cng t ph thuc vo h s ny nht v vy vic b sai lch l kh quan nht. C th s ph thuc ca cht lng tng loi ng vo m v c tnh ca n c trnh by nh hnh di y :
Nu ta tng m th s gim sai lch ,th nhy php o s gim , iu ny rt d hiu v khi ta cho m tng th h s s tng , y chnh l vn ta cn cn i mt cch hp l .
d. K thut o chnh pVic chuyn i tn hiu chnh p thnh tn hiu in i hi phi c mt b bin i p sut vi sai (p). B bin i p c kt ni qua mt khi ng phn phi nh hnh v. Khi ny cho php b bin i c cch ly tho lp hay bo dng thng qua cc van B v C. Van A l van cn bng v c s dng trong qu trnh iu chnh im khng khi van B v C ng cn van A m. Trong qu trnh vn hnh bnh thng, van A lun ng. Khi vn hnh cc van ny, phi m bo rng p sut khng th i qua c trong ton b ng ng c th dn n h hng mng ca b bin i. Lun phi m bo th t: van A m, van B, C ng hay m van B, C, ng van A.
Vic lp t ng ng dn p sut cng phi thn trng trnh cc vn xy ra do s ngng t ca cc cht lng (i vi o kh) v cc ti kh (i vi o cht lng). L tng nht l mt h thng o lu lng kh nn t b bin i p trn tm nghn vi ng ng thng ly p sut i xung di. Tng t nh vy, vi h thng o lu lng cht lng, b chuyn i p nn t di tm nghn vi ng ng thng ly tn hiu p sut i ln.
Nu nh v iu kin v tr mt bng khng cho php thc hin c cch lp t nh trn, ta phi b tr ng ng vi cc vi thot (ng dn kh hay ng thot cht lng).
2.5.3. Cc phng php o lu lng cht rn
1. Nguyn l o
Ngoi vic o lu lng ca cht lng, cht kh, trong cc dy chuyn sn xut ca mt h thng t ng ho, vic o lu lng ca cht rn cng ng mt vai tr quan trng trong cc nh my cng nghip. V d nh vic xc nh lu lng nhin liu than cp vo l ca nh my nhit in hay xc nh lu lng clinker, thch cao hay ph gia trong cc day chuyn sn xut xi mng.v.v. o c lu lng ca cc vt liu rn ny, trong cc nh my cng nghip, ngi ta s dng cc cn bng ti.
Cn bng ti l thit b cung cp nhin liu kiu trng lng, vt liu c chuyn ch trn bng ti m tc ca n c iu chnh nhn c lu lng tng ng vi gi tr t trc ca ngi vn hnh (gi tr setpoint).
Tc ca bng ti chnh l tc ca vt liu c truyn ti v (m/s). Ti ca bng truyn chnh l trng lng ca vt liu c truyn ti trn mt n v chiu di (kg/m). Cc cn bng ti c trang b b phn o trng lng (load cell) o (ti bng truyn) v b iu khin iu chnh tc bng ti sao cho im nhin liu, lu lng dng chy vt liu bng gi tr t do ngi vn hnh t trc.
Lu lng dng chy vt liu c tnh bng tch gia trng lng vt liu trn mt mt chiu di bng ti vi vn tc chuyn ng di ca bng ti (cng chnh l vn tc chuyn ng ca vt liu).
Q = .v
Trong : Q: lu lng dng chy vt liu (kg/s)
: trng lng vt liu trn mt n v chiu di (kg/m)
v: tc bng ti (m/s)
Cu trc chung ca mt h cn bng ti gm cc thit b c minh ho hnh di y:
Hnh 2.49- Cu to cn bng ti
Cc b phn c bn:
(1) Bng truyn; (2) Phu cp liu; (3) Cng iu chnh cao ca liu trn bng ti
Cc b phn truyn ng cn bng:
(4) ng c c iu chnh tc ; (5) Cm bin tc ng c; (6) Hp gim tc
(7) Tang b ng; (8) Bulng c kh; (9) Tang truyn ng
(10) Load cell
2. ng dng trong cng nghip: Cn bng ti vi b iu khin lu lng
Hnh 2.50 - S cu trc ca h thng iu chnh lu lng cht rnHnh trn l s cu trc mt h thng iu chnh lu lng cht rn s dng cn bng ti. Ngoi cc thit b c bn ca mt h cn bng ti, h thng iu chnh ny cn bao gm thit b nh v ca bng ti t trn bng ti mt du hiu bng kim loi gi l belt index gip cho vic phn on bng ti v lm tng chnh xc ca cn, ng thi xc nh s trt ca bng ti.
B iu khin iu chnh tc bng ti nhn lu lng nhin liu im tng ng vi gi tr t. Lu lng liu l tch s ca ti trn mt mt di bng truyn vi tc bng truyn.
Giao din SWS (smart weighing system) ni gia cn bng ti v b iu khin. Tt c cc tn hiu t u ra ca cc cm bin s c ni vi h SWS t trn bng ti. H SWS thu thp cc tn hiu ri gi v b iu khin theo th tc truyn tin ni tip.
o ti ca bng truyn, h dng mt loadcell bin i trng lng thnh tn hiu in. Tc bng ti o c nh mt my pht tc kiu xung t gia ng c v hp gim tc bnh rng. S xung pht ra t l vi tc ca ng c.
Nh vy, c hai yu t chnh cn quan tm trong h iu chnh lu lng cn bng ti l o trng lng ca nhin liu trn bng ti v iu chnh tc ng c.
a. o trng lng ca nhin liu trn bng ti
Trng lng o c t tn hiu ca loadcell bao gm c trng lng ca bng ti v trng lng ca nhin liu trn bng. V vy, c c trng lng ca nhin liu, ta phi tin hnh tr b. B iu khin xc nh c trng lng ca nhin liu nh t ng tr b cc phn on bng ti.
Nguyn l ca trnh t tr b nh sau: bng ti phi c chia thnh cc phn on c di nht nh. Trong thao tc tr b bng ti rng (khng c nhin liu trn bng), trng lng mi phn on phi c ghi vo b nh. Khi vn hnh bnh thng, trng lng ca nhin liu trn mi phn on c xc nh bng cch ly trng lng o c trn mi phn on tr i trng lng ca phn on bng ti tng ng ghi trong b nh. iu ny gip cho vic cn nhin liu c chnh xc ngay c khi dng bng ti c dy khng ng u trn ton b chiu di. Vic hiu chnh cn phi c ng b ho vi v tr ca bng ti, sau khi khi ng cn, B iu khin i cho ti khi gp belt index mi bt u ng b ho tr b. Khi ngng cn, v tr ca bng ti c gi li trong b nh, do ln khi ng tip theo vic tr b c thc hin tc thi.
b. iu chnh tc ng c
iu chnh tc ng c, cc h thng cn bng ti thng s dng bin tn. ng c s dng l loi ng c xoay chiu 3 pha rotor lng sc. Tc ng c o c nh my pht xung. S xung pht ra t my pht xung t l vi tc ng c v c a ti SWS, sau khi c x l c truyn ti b iu khin. B iu khin c nhim v tnh ton lu lng bng cch nhn ti trn bng truyn thi im liu vi tc bng truyn. B iu chnh PID trong b iu khin s chnh nh tc bng truyn ph thuc vo lu lng cu cn. B iu khin m bo lu lng khng i ca dng nhin liu im liu ca bng cn.
2.6. Van iu khinThit b chp hnh ph bin nht trong lnh vc iu khin qu trnh l van iu khin. Van iu khin iu chnh lu lng cht lng nh gas, hi nc, nc, hoc hp cht ha hc.
Van iu khin in hnh bao gm thn van, phn chnh bn trong, mt c cu chp hnh cung cp lc dn ng vn hnh van, v cc ph kin van khc nhau bao gm b nh v van, thit b chuyn i, b iu chnh p lc, chng rung hoc cng tc gii hn.
Van iu khin l mt phn thit yu ca mch vng iu khin. Khng chnh xc khi ni rng van iu khin l phn quan trng nht trong mch vng, mch vng iu khin nh mt chui cc thit b, ging nh bt k mt chui no khc, ton b chui cng tt nh mt lin kt yu nht. iu quan trng l m bo rng van iu khin khng phi l lin kt yu nht.
1. Cu trc c bn ca van iu khin
Hnh 2..51 Cu trc tiu biu ca mt van cu kh nn
Mt van iu khin bao gm thn van c ghp ni vi mt c ch chp hnh cng vi cc ph kin lin quan. Trn hnh Error! Reference source not found. l hnh nh mt ct ca mt van cu kh nn vi c ch truyn ng mng rung l xo.
Phn thn van cng cc ph kin c gn vi ng ng, ng vai tr l phn t iu khin. m van v lu lng qua van c xc nh bi hnh dng v v tr cht van.
2. Cc cch Phn loi van iu khin :
Phn loi van da theo chc nng iu khin ta c:
Van iu khin ng ct (van ON/OFF)
Van iu khin tuyn tnh
Van iu khin tuyn tnh c mch vng v tr (servo van)
Phn loi van da theo thit k v kiu chuyn ng ca cht van:
Van cu (global valve): Cht trt c u hnh cu hoc hnh nn, chuyn ng ln xung.
Van nut (plug valve): cht xoay hnh tr (c c cc l theo chiu ngang) hoc mt phn hnh tr.
Van bi (ball valve): cht xoay hnh cu (c c cc l theo chiu ngang) hoc mt phn hnh cu.
Van bm (butterfly valve): cht xoay hnh a.
C cu chp hnh van c nhim v cung cp nng lng v to ra chuyn ng cho cht van thng qua cn van (i vi chuyn ng trt) hoc trc van (chuyn ng xoay). Phn ln van iu khin cng nghip c cp ngun kh nn, song mt s ngun nng lng khc nh in, in t hoc thy lc cng c th c s dng.
Phn loi van da theo c ch truyn ng:
Van kh nn: Loi ph bin nht, truyn ng kh nn s dng mng chn / l xo hoc piston. Tn hiu u vo c th l kh nn, dng in hoc tn hiu s (bus trng). Nu tn hiu iu khin l dng in, ta cn b chuyn i dng in kh nn (I/P) tch hp bn trong hoc tch ring bn ngoi.
Van in: C ch chp hnh s dng ng c servo hoc ng c bc, c iu khin trc tip t tn hiu ra b iu khin, thng thng l dng in tng t 4-20mA hoc tn hiu s. Van in c s dng trong nhng ng dng cng sut nh i hi chnh xc cao.
Van thy lc: c ch chp hnh s dng h thng bm du kt hp mng chn hoc piston, bm du c iu khin bi tn hiu ra t b iu khin. Van thy lc c s dng cho cc ng dng cng sut ln.
Van t: C ch chp hnh cun ht kt hp l xo, lc nn yu v chnh xc km, ch ph hp vi cc bi ton n gin.
3. Phn loi cc van iu chnh lin tc Van iu chnh c chia lm 2 nhm chnh da vo s chuyn ng ca phn t ng van: chuyn ng thng v chuyn ng xoay. Nu cn c vo hnh dng v kt cu ca phn t ng van, van iu chnh c th chia thnh rt nhiu loi nhng ph bin nht l van cu, van bm, van ca v van bi.
Hnh 2.52. Mt s dng ph bin ca van iu chnh
Van cu l loi van chuyn ng thng v l loi thng dng nht trong s cc loi van nu trn khi iu khin cc qu trnh cng ngh. a van c dng hnh cu, iu khin dng chy theo hnh ch S. c im ca van cu l tr lc i vi dng chy ln (lm cho dng chy khng phng), cn nhiu nng lng hn ng/m van (khng ph hp vi kch thc ln).
Van bm l loi van chuyn ng xoay thng dng nht. Phn t ng ca van c hnh dng ging con bm. Van ng kht v c th s dng lm van iu chnh. c im ca van l tr lc i vi dng chy thp (dng chy phng), ti u vi trng hp mmen vn hnh thp, gc vn hnh l 900. Van c kt cu gn, nh v c ch to vi cc kch thc t vi chc milimt n vi mt. i vi nhng ng c ng knh rt ln, iu khin lu lng mi cht th ch c gii php s dng van bm l cho hiu qu kinh t nht.
Hnh 2..53. Kt cu loi van bm xoay ng
Van ca c phn t ng l mt tm phng (hnh ch nht hoc hnh trn) hoc tm hnh nm c th chuyn ng vo hoc ra khi ng i ca dng chy. Loi van ny thng c s dng vi mc ch ng/m hon ton mi cht (khng ph hp lm van iu chnh), nhng c mt s thit k c s dng tit lu dng chy. c im ca van l tr lc i vi dng chy thp khi m hon ton (dng chy phng), hnh trnh di i hi thi gian ng/m, khng ph hp cho tc ng nhanh.
Hnh 2..54. Kt cu loi van ca hnh trn Van bi l loi van chuyn ng xoay, phn t ng ca van c dng vin bi vi ng thng bn trong. Van thng c s dng rng ri trong ch ng/m i vi cc qu trnh m (khng ph hp l van iu chnh). c im ca van l tr lc i vi dng chy thp khi m ht (dng chy phng), gc vn hnh l 900, i hi cng ngh nng cao ch to bi v vt liu chn kn.
Ngoi cc loi van trn, trong iu khin qu trnh i khi cn s dng mt loi van na gi l van mng. Van mng l loi van chuyn ng thng vi mng mm do l phn t ng ca van. Van mng ch yu s dng i vi nhng cht lng c tnh n mn hoc dng bn, va. Thn van c th c lt mt lp thy tinh, cht do, cao su hoc teflon. 4. c tnh ca van iu khin:
Kiu tc ng ca van:
ng an ton (fail closed- FC): mng chn ca van dch chuyn n v tr ng khi ngun nng lng ca c cu chp hnh mt.
M an ton (fail open - FO): mng chn ca van dch chuyn n v tr m khi ngun nng lng ca c cu chp hnh mt.
D phng an ton (fail-safe): c tnh ca van v c cu chp hnh ca n: nng lng cung cp cho c cu chp hnh b mt s dn n mng chn ca van c ng hon ton, m hon ton hoc gi li v tr trc c xc nh cn bo v qu trnh. D phng an ton c th lin quan n vic s dng kt ni iu khin ph n c cu chp hnh.
S la chn kiu tc ng ca van iu khin nh hng ti vic la chn h s khuych i ca b iu khin phn hi. Van ng an ton c m van ln hn khi tn hiu iu khin tng, trong khi van m an ton c m an ton nh hn khi tn hiu iu khin tng. Chiu tc ng thun c nh ngha l m van gim khi tn hiu iu khin tng (c ngha l cn van chuyn ng theo chiu thun t trn xung di i vi van trt). Chiu tc ng nghch c nh ngha l m van tng ln khi tn hiu iu khin tng (cn van chuyn ng theo chiu nghch t di kn trn i vi van trt).
c tnh iu khin vo ra:
c tnh iu khin vo ra ca van l quan h gia lu lng qua van v m van trng thi xc lp khi xt m van thay i t 0 n 100% gi tr danh nh. i vi mt van nht nh, lu lng qua van c th coi l t l vi cn bc hai ca chnh lch p sut qua van.
Ta phn bit gia c tnh iu khin vo ra c bn v c tnh iu khin vo ra lm vic. c tnh iu khin vo ra c bn ca van l quan h tnh gia lu lng qua van v m van trong iu kin p sut st qua van khng i. c tnh c bn ca van ch ph thuc vo kch c van v thit k cht van. Ngc li c tnh iu khin vo ra vn hnh l c tnh thc t khi van c lp t, c ngha l cn ph thuc vo ch vn hnh.
c tnh iu khin vo ra vn hnh:
Lu lng mt lu cht qua ng ng, c iu tit bng cch thay i m van. Nhng khi lu lng thay i th p sut tn tht qua ng ng v qua cc thit b qu trnh thay i, p sut ca bm cp cng c th thay i theo, dn ti chnh lch p sut qua van cng thay i. Do , mt van c c tnh tuyn tnh c bn sau khi lp t s khng cn tuyn tnh na. Tri li mt van c c tnh phn trm bng nhau nu c tnh ton v la chn hp l li c th cho c tnh vn hnh gn tuyn tnh trong phm vi lm vic yu cu.
Phm vi iu chnh:
Phm vi iu chnh l vng van c th iu chnh c. Phm vi iu chnh ca mt van c nh ngha l t l gia cc gi tr lu lng ti v tr m van 95% v 5%. Phm vi iu chnh ni ln kh nng iu chnh, can thip ca h thng iu khin vo qu trnh. Phm vi iu chnh cng ln, vic iu khin cng thun li.
c tnh ng hc:
S cu trc ca mt van iu khin c minh ha trn hnh 2.55. u vo ca van l tn hiu iu khin v u ra l lu lng. Van iu khin c hai thnh phn c bn l c cu chp hnh van v thn van. C cu chp hnh quyt nh quan h gia tn hiu iu khin v m van, trong khi thn van quyt nh quan h gia m van v lu lng qua van. Do ng hc ca dng chy nhanh hn rt nhiu so vi ng hc ca c cu chp hnh, c tnh ng hc ca van ph thuc ch yu vo c cu chp hnh van. Trong khi c tnh tnh ca van ph thuc ch yu vo kch c van v thit k cht van. thun tin trong vic phn tch v thit k iu khin, tn hiu iu khin m thng c tnh theo phn trm hoc theo phn n v ca di tn hiu chun. m van cng c tnh theo phn trm hoc phn n v ca hnh trnh ln nht. Khi quan h gia tn hiu iu khin v m van trng thi xc lp c th hin qua cng thc cho van ng an ton v cho van m an ton.
Hnh 2.55. Quan h gia cc thnh phn ca van iu khin
Nu van c nh c tt th quan h gia lu lng ra v m van c th c coi l tuyn tnh, m hnh ng hc van iu khin c th c a v mt khu qun tnh bc nht:
Hng s thi gian (v ca van ph thuc ch yu vo c cu chp hnh, h s khuych i kv c th c tnh ton nh sau:
C cu chp hnh c th coi l tuyn tnh trong ton b di lm vic, nn
d/dm = 1 cho van ng an ton (FC)
d/dm = -1 cho van m an ton (FO)
Ta c:
Nu van c nh c tt th ta c th coi kv l hng s trong ton di lm vic
Servo van:
Van iu khin thng thng c chnh xc khng cao v nhiu l do nh di cht, tr, ma st thay i do bi bn, thiu bi trn v han g, p sut lu cht thay i cung nh do c tnh phi tuyn ca c cu chp hnh. Sai s m van trong nhiu trng hp c th ti 5%. Mc d trong mt h thng iu khin phn hi th y khng hn l mt vn ln, nhng nng cao cht lng iu khin ta c th s dng mt trong hai bin php:
S dng mt b nh v van (valve positioner)
B sung mt vng iu khin lu lng th cp
B nh v van thc cht l mt b iu khin th cp trong cu trc iu khin tng. B nh v s dng tn hiu o v tr cn van (tc l m van thc) v so snh vi gi tr t t tn hiu iu khin, trn c s a ra tc ng ti c cu chp hnh iu chnh li m van. Thng thng b nh v ch s dng lut t l vi h s khuych i tng i ln. S dng b nh v khng nhng cho php ci thin chnh xc m cn tng tc p ng ca van. V d sai s v tr c th gim xung ti 0.5%, di cht 0.1-0.5% v hng s thi gian 0.5 n 2 giy.
a. c tnh kt cu ca van iu chnh
c tnh kt cu ca van iu chnh l mi quan h gia tit din cho qua tng i v m tng i ca van, tc l:
Trong : F din tch tit din cho qua ng vi dch chuyn L ca van iu chnh,
Fmax din tch tit din cho qua ng vi dch chuyn cc i Lmax.
Bng cch to bin dng tng ng cho c cu tit lu ca van iu chnh, ngi ta c th thu c cc c tnh kt cu bt k, ph hp vi iu kin lm vic c th ca h thng t ng (Hnh 2.56)
Hnh 2.56. Cc c tnh kt cu ca van iu chnh
b. c tnh lu lng ca van iu chnh
c tnh lu lng ca van iu chnh l mi quan h gia lu lng tng i ca mi cht i qua van iu chnh v m tng i ca ca van , tc l:
Trong : lu lng tng i ca mi cht i qua van, l t s gia lu lng mi cht i qua van 1 m no so vi lu lng cc i mi cht c th i qua van;
m tng i ca ca van, l t s gia hnh trnh ca trc van 1 m no so vi ton b hnh trnh ca van.
i vi s lm vic ca b iu chnh th c ngha chnh l c tnh lu lng ch khng phi c tnh kt cu ca van iu chnh. Chnh v vy c tnh lu lng cn gi l c tnh lm vic. c tnh lu lng ca van iu chnh ph thuc vo hnh dng cu to ca a van v ci van iu chnh. a van c dng phng, dng tr v dng loe. Mi loi li c phn thnh cc loi khc nhau theo dng c tnh ca chng nh: loi c dng c tnh tuyn tnh, dng c tnh % u, dng c tnh Parabol v c tnh m nhanh. Hnh 2.57. ch r cc dng a van v cc loi c tnh lu lng l tng ca chng.
Hnh 2.57. Cc dng a van v c tnh lu lng l tng ca chng
Hnh 2.58. ch r v d mt s loi a van tr c hnh dng khc nhau to nn cc van c c tnh lu lng khc nhau.
Hnh 2.58. V d mt s loi a van tr c hnh dng khc nhau
Hnh 2.59.ch r cc ci van c hnh dng khc nhau to nn cc van c c tnh lu lng khc nhau (khung dn hng cho a van v cc l thng trn khung c hnh dng khc nhau ng vai tr l ci van).
Hnh 2.59. Cc ci van c hnh dng khc nhau
Van m nhanh c s dng nhiu hn trong iu khin ng/m. Mt s chuyn ng tng i nh ca tr van c th gy nn s thay i cc i ca lu lng i qua van. V d, van m nhanh c th cho php 85 % lu lng cc i i qua ch vi 25 % hnh trnh ca tr van.
Van tuyn tnh c lu lng t l tuyn tnh vi v tr ca tr van. Quan h ny c th biu din nh sau:
Loi van ny c s dng rng ri iu khin cht lng chy vo bnh (b) cha trong h thng t ng iu khin mc ca n.
Van % u c ch to sao cho khi v tr ca tr van thay i bao nhiu % th lu lng qua van s thay i by nhiu %. Quan h ny c th biu din nh sau:
hoc
Trong : k = const h s t l.
c tnh lu lng l tng
c tnh lu lng l tng (cn gi l c tnh lu lng t nhin) l c tnh lu lng c c vi iu kin chnh p sut trc v sau van iu chnh khng i, khi lu lng thay i. iu ny ch c th xy ra trong iu kin th nghim, khi duy tr c chnh p sut trc v sau van iu chnh khng i.
T hnh 2.57b. thy rng, i vi cc c tnh lu lng, khi m ca van bng 0, lu
lng tng i qua van l nh nht nhng khc 0 (do r r), t s gia lu lng nh nht ny so vi lu lng ln nht l 3,3%, hoc l t s gia lu lng ln nht so vi lu lng nh nht l 30 %.
dc ca ng c tnh lu lng kiu tuyn tnh bng hng s v khng ph thuc vo lu lng tng i, tc l:
d/ d = K
Trong K h s khuch i ca van iu chnh, chnh l dc ca ng c tnh tnh.
H s khuch i ca ng c tnh lu lng kiu % u l h s t l gia dc ca c tnh so vi lu lng tng i:
d/ d= K
Khi lu lng tng i tng, th h s khuch i ca c tnh cng tng. c tnh lu lng ca ng Parabol nm trung gian gia c tnh lu lng % u v c tnh lu
lng kiu tuyn tnh.
c tnh lu lng l tng ca van 3 ng cho Hnh 2.18. Khi c tnh lu lng ca van 3 ng c dng ng thng, bt k lu lng no, tng lu lng i qua 2 ca van l khng i, tc l lu lng tng khng i. c tnh lu lng % u l thay i, khi m 50%, th tng lu lng l nh nht. c tnh Parabol trung gian gia 2 c tnh trn.
Hnh 2.60. c tnh lu lng l tng ca van 3 ng
( m 2 a van ngc nhau) c tnh lu lng thc t Trong thc t, van iu chnh lp trn h thng ng ng c tr lc (Hnh 2.61), chnh p sut trc v sau van khng th gi khng i, n thay i theo lu lng. V vy, tuy cng 1 m, lu lng i qua van khng th bng lu lng tng ng trong trng hp l tng. Quan h gia lu lng tng i qua van iu chnh v m tng i ca van c tnh n s thay i ca chnh p sut trc v sau van gi l c tnh
lu lng thc t hay cn gi l c tnh lu lng lm vic.
Hnh 2.61. M hnh lp van iu chnh trn ng ng Gi (P1 l chnh p sut ngay trc v sau van iu chnh, (P2 l chnh p sut (tr lc) ca ng ng v thit b mc ni tip. t:
S = =
Trong : (P1m l chnh p sut ca van iu chnh khi m ht;
(P2 l chnh p sut ca ng ng v thit b mc ni tip;
S l h s tr lc ca van.
S biu th t l ging p trn van so vi ging p trn ton b h thng ng ng. Khi m van khng i, tr lc ng ng khng i, gi tr S s khng i. Khi tr lc ng ng tng, S gim dn, nu k hiu Q100 l lu lng qua van iu chnh khi m hon ton trn ng ng c tr lc ( Q100 < Qmax ), th Q/ Q100 s gi l lu lng tng i ca van iu chnh so vi Q100 (so vi lu lng khi m ht), xem Hnh 2.20.
Hnh 2.62. c tnh lu lng lm vic ca van iu chnh khi lp trn ng ng
T Hnh 2.62 ta thy rng, khi S =1 , tc l ton b tr lc ng ng u ging trn van iu chnh, v khng i (chnh p u vo h thng khng i), khi ny c tnh lu lng l l tng. Khi gi tr S gim dn (tc l tr lc thit b v ng ng tng), lu lng qua van iu chnh khi m ht cng gim dn (tc l khi lu lng qua ng ng v tht b ln nht), nhng lu lng tng i Q/ Q100 , mt m tng i no , s tng, khi S gim . Bi vy, c tnh lu lng c im un , to thnh 1 h ng cong phn li hng ln trn. c tnh tuyn tnh l tng c xu hng tin ti c tnh m nhanh, cn c tnh % u c xu hng tin ti c tnh tuyn tnh.
Vo/ra phn tn
t1
(cao)
Nhit o c
Di nhit lm vic
Dung sai
Nhit (oC)
Lp A
LpB
Nhit (oC)
in tr (( )
4-20 mA
V
V
EMBED Visio.Drawing.11
M
Fieldbus
Nhit oC
M
M p sut
V p sut
15
10
5
Dung sai
Lp A
Lp B
Nhit oC
(b)
15
Hnh 2.42- Cm bin o lu lng bng cm ng in t
Encoder
AC
Q
Q
Thit b cm tay
My tnh
BIN
TN
B iu
khin
SWS
Dy bng
Lu lng
AC motor
FT
Hp gim tc
Tang truyn ng
Load cell
Cng iu chnh
cao liu
40 KHz
Tn s ca cu cng hng
Tn s (KHz)
p sut
EMBED MSGraph.Chart.8 \s
Cu cng hng
su
p sut a vo
4
5
3
2
1
Hnh 2.25 a- o p sut v lu lng
L
H
EMBED PBrush
Hnh 2.25b - o p sut v mc
10
5
Sai s ln nht 0,5 psi 8 psi
Cc phn x t y v bn trong
Phn x li t b mt o
Trc tip qua
khng kh
B thu
B pht
Thi gian tr
d
H
B thu
B pht
p sut
1
2
3
4
Mng hnh khn
Mng hnh trn
A
V
h
h
h
p ng
thc t
Hiu p
p sut di
(p sut kh quyn)
p sut trn
(p sut kh quyn)
Chn khng
p sut kh quyn
p sut tuyt i
Mng mng
Sn cn quang
Lp nn
Lp pha tp p
c)
b)
a)
(a)
F
p sut cn o P1
p sut cn o P1
p sut cn o P2
Chn khng hay kh bo v
Cm bin
Mng mng
Mng mng
Nhit (oC)
EMBED Visio.Drawing.11
p ng l thuyt M = V
t2
t1
t2
t1
B
A
I
Vb
Va
t2
(thp)
Tang b ng
NP LIU
2
3
10
6
5
4
1
7
8
9
Nhin liu cp vo
Vi thot cho kh
Vi thot cho hi ngng
Hnh 2.48- ng ng vi vi thot cho cht kh v hi ngng
Hnh 2.47- B tr cc van
B mt A2
B mt A1
Dng chy
Q2
Q2
p sut P2
p sut P1
Kch thch l ngun cp xoay chiu
Kch thch l ngun cp mt chiu
Kch thch tn s kp
Dng sng tn hiu
Thp
Cao
Dng sng kch thch
Nhiu
Tn s
S dao ng tn hiu u ra do nhiu
Hnh nh nhiu ca tn s
Chiu dng chy
Hnh 2.45- Nhiu do in cc b n mn xy ra gy ra cc dao ng u ra lin tc
Vt rn p vo in cc
Vt rn lm v lp mng oxit
Lp mng oxit c ti to li
Ngun cp
xoay chiu
hay mt chiu
Vn tc V
ng D
T trng B
Sc in
ng E
Cc in cc
Dng kch thch
ng
Lu cht
i in cc
T trng
Nam chm
Chu k o
Chu k o
b)
a)
G hnh
vnh khn
Phin p in
Vt liu tng thch m hc
Ngun cp
Tn hiu iu khin t b vi x l
B chuyn
i V-F
Ch th
Xung
Ti
B chuyn
i F-1
Analog
B vi x l
Trig Schmidt
B lc
B cng
Mch tch nhiu
Mch suy gim nhiu tn s cao
Cn bng nhiu
B bin i in tch 1
B bin i in tch 2
Van
My bm
ng c
My nn kh
Nhiu do rung ng
c kh
Nhiu do rung ng
Hnh 2.32- Cu trc ca lu lng k xoy
Chiu dng chy
Tr s Reynolds
Tr s Strouhal
Di vn hnh bnh thng
Di o c
Tr s Strouhal
Tr s Reynold
Hnh lng knh
Hnh tr
EMBED Visio.Drawing.6
42
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CuNi
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_1316482825.unknown
_1316465042.vsdi tng o
Cm bin s cp
Chuyn i chun ho
B vi x l
Chuyn i D/A
Giao din Fieldbus
Hin th v iu khin
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fr
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