sổ tay cdt In - Chuong 19 -Phep Do

8/6/2019 s tay cdt In - Chuong 19 -Phep Do

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1919.1 Php olu lng Richard ThornGii thiu

Php olu lng rt quen thuc vi cuc sng ca chng ta. Ta c th thy chng hngngy nh ccthit b o nc v kh.Tng t nh th, cc lu lng k cng c s dng tronghu ht cc ngnh cng nghip, t du kh ti sn xut v ch bin thc phm. Do khng ngc

nhin rng ngy nay, tng gi tr ca th trng lu lng k ton cu l hn 3000 triu USD v hyvng s pht trin n nh trong tng lai.Mc dphp olu lng c vai tr khng th ph nhn i vi nnkinh t,tuy nhin

chnh xc v cng ngh cacc lu lng kangs dng hin naytng ilc hu sovi ccthit b c s dng o cc i tng khc nh p sut hay nhit . V d nh lu lng kavi phun, l lu lng kc s dng nhiu nhttrong cng nghipch bin, ch c chnhxc ca gi tr o l2% v c s dngt nhngnm cuicath k 18.S bo th cacngnghipo lu lng th hin chcc k thut c in nh l avi phun, ng h Venturi, vlu lng k tit din thay i, hin vnc s dng rng ri, trong khi cc lu lng k sium, c pht trin vo nhng nm 1950, th vn c coi nh l mt thit b mi vi nhiu ngidng. Mc ny sgii thiunhng phng phpo lu lnghin ctrn th trng.

Thut ng Thut ng olu lngl mt thut ng chung, v trc khi chn la mt lu lng k ngidng phi xc nhr php olu lng no s c s dng. Vi mt dng cht lng chy qua

mt ng, vic olu lngc ththc hin theo 1 trong6 kiu osau.1. Php o vn tc im: vn tc dng chy ti mt im c nh ngang qua tit din

ngang cang (m/s)2. Php o vn tc dng chytrung bnh: vn tc trung bnh dng chy qua tit dinngang

cang(m/s)3. Php o lu lng dng chy:tc thay i ca th tch dng chy ngang qua ng

trong mt n v thi gian (m3/s)4. Php o th tch:tngth tch dng chy qua ng (m3)5. Php o mc khi lng:tc thay i khi lng ca dng chy qua ng trong mt

n v thi gian6. Php o tng khi lng - Tng khi lng chy qua ng (kg/s)

Mcd dng c bnnhtca php olu lng l odng cht lng chy qua mt ng dnkn,nhngcc php olu lng cho ccdng chytrongcc knh m cng l cn thit, c bitl cc ng dng trong x l nc v cht thi.

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S tay C in t

Cc c tnh dng chyLu chtc o thng l cht lng hoc kh, cn gi l dng mt pha. Tuy nhin, vic o

tc lu lngcacc cht lng hn hpcng rt cn thit(nh trong phn dng hai pha).C rt nhiu nguyn tc quan trng lin quan ti cc c tnh dng chy trong ng cn c

tm hiu trc khi chn la lu lng k m bo s ph hp v an ton, l ngha cahng s Reynolds vs quan trng ca lu vn tc dng chy.

Hng s Renolds Re l t s ca cc lc qun tnh trong dng chy( )vD vi cc lc nhttrong dng chy( ) , v n c th c dng xc nhmt dng cht lng l chy lp hocchy ri.

Hnh 19.52Cc dng vn tc dng chy ri v chy tng

Hng s Reynolds c th c tnh nh sau

.Re v D

= (19.65)

l t trng ca cht lng,v l vn tc trung bnh ca cht lng, D l ng knh ca ng, v l nhn ng hc ca cht lng. Nu Re nh hn 2000, cc lc nht trong dng chyl ln vdng chy tr thnh dng chy tng. Nu Re ln hn 4000, cc lc qun tnh ca dng chyl lnv dng chytr thnh dng chy ri. Nu Re gia 2000 v 4000th dng chy lchuyn tip, n c th l dng chy tng hoc dng chy ri. Ngi ta tnh hng s Re ch yu davo cc tnh cht ca cht lng v khng a vo cc yu t nh l g gh ca ng, cc gcln, v cc van,mc dchngcngnh hng ti tnh cht dng chy. Tuy nhin, hng s Re lmt ch dn tt xc nhdng dng chy, y l mt yu t quan trngtronghu ht cctrnghp.

Vn tc dng chy qua mt tit din ngangca ngkhng l hng s v ph thuc vo dngdng chy (Hnh 19.52). Trong dng chy tng, vn tc tm ng bng hai ln gi tr vn tctrung bnh chy qua tit din ngang ca ng v dng dng chy khng b nh hng big ghca thnh ng. Trong dng chy ri, cc nh hng ca thnh ng l nh v dng tc dng

chy ln nhhn, vi vn tc tm ngbng 1,2 ln vn tc trung bnh.Dngdng chy chnhxc trong dng chy ri ph thuc vo g gh ca thnh ng v hng s Reynolds. Trong ccng dng cng nghip him khi gp cc dng chy tng tr khicn o cc cht lu c nht ln.Phi lun lun tnh hng s ng Re v cmt vi lu lng k khng ph hp khi s dng trongciu kin chy tngv chy ri.

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Cm bin

Dngvn tc ca mt dng chy s ch ixngti im cui ca mt ng rt di. Cc gcln v s cn tr cacc van s lmdng vn tc b mohoc khng ixng. Do vic cnchnhcc lu lng k l nhy vidng cadng chy quathit b o, nnm bo s hotng tin cy calu lng kth dng cadng chyphin nh v c bit trc.

Phn loi cc lu lng kTrn th trng hin nay ct nht 80 loi lu lng k khc nhau, chng c phn loithnh

9 nhm chnh. Bng 19.3 a ra nhng v d v nhng lu lng k in hnh trong mi nhm.Cc k thut olu lng truyn thng hot ng da vos chnh lch p sut, sthay i din tch, s dch chuyn dng, v cc loi turbine. Cc k thut mi hn sdng in t, siu m, s dao ng v cc loi khi lng. Mcd cc gia tc khotng da trns chnhlchp sutvn c dng trong hu ht cc ng dng, c bittrong cclnh vccng nghipch bin, nhngnhn chung nhng phng php truynthng dn dn c thay th bng cc phng php mi hn. Nhng k thut trn ca thch hn vtrong hu htcc trng hp chng khng cn tr dng chy,ngoi rachng cnc chnh xc v tin cy cao.

Bng 19.3Cc mc chnh ca lu lng k h thng dn khp kn

Dng 1 cc lu lng k chnh p

a vi phun gc phng, a vi phun dy, a vi phun lch tm, Venturi,vi, ng Pitot, gp khc, hnh nm, nn ch V, ng Dall, ng dng Elliot- Nathan, Epiflo

Dng 2 cc lu lng k din tch thay iLu lng k dng phao, cht dng thon v c vi, xylanh v pittng, bia, m thay i

Dng 3 cc lu lng k dch chuyn dngVan trt, 3-roto, 2-roto, piston, bnh rng ovan, a lc, CVM, mngngn, kh m

Dng 4 cc lu lng k turbineTurbine trc, turbine trc hai roto, roto dng tr, bnh cng tc, bnh xePelton, Hoverflo, chn vt

Dng 5 cc lu lng k dao ngRi xoy, lu lng k dng xoy, lng

Dng 6 cc lu lng k in t trngT trngxoay chiu, t trngmt chiudng xung, dng ghp

Dng 7 cc lu lng k dng sng ngnDoppler, thi gian truyn qua n chiu, thi gian truyn qua a chiu,tng quan cho, lch

Dng 8 cc lu lng k khi lngCoriolis, nhit

Dng 9 cc lu lng k a nng

Thit b o gi laser, thit b o gi dy nng, cng hng t ht nhn Nhng phn tip theo sgii thiucc dng lu lng k ph bin nhtca8 nhmchnh

trong bng trn.Thng tin v cc dng lu lng k khc hoc nhm tng hp c thtm thytrongcc ti liukhc, v dnh [3-6].

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S tay C in t

Lu lng kchnh p Nguyn tc c bn ca cc lu lng k chnh pl nu t mt vt cn trong ngth s gim

p qua vt cn ny c quan h vi th tch lng cht lng chy qua ng.a gicl l dng n gin nht v r nht trong s cc loi lu lng k chnh p. N n

gin l mt a vi mt l c kch thc vv tr xc nh. a gicl c thckp gia hai mp bch trong ng (Hnh 19.53). Lu lng ca dng chy Q trong ng c cho bi phng trnh19.66:

2 1 2

4

2( )41

p pC Q d

=

(19.66)

p1 v p2 l p sut mi mt ca gicl a, l t trng ca cht lng pha u ngun trcgicl, d l ng knh l trong a gicl, v l h s t l ng knh d/D, D l ng knhtrong ng pha u ngun. Cc nhn t hiu chnh c nh theo kinh nghim l h s phngin C, v h s co gin . C b nh hng bi s thay i t s ng knh, hng s Re, ggh ng, sc nhnca cnhtrccaagicl, v imta gicl trong ng. Tuy nhin, nuc nh h s t l ng knhth Cch ph thuc vo h s Reynolds, do c th xc nh h sny trongcc ng dng c th. H s c dng tnh h s nn ca cht lng c o. C haih s v C c th cxc nht cc phng trnh v bng trong cc ti liu c cng

nhn rng ri nh l cc chun.

Hnh 19.53Php o lu lng s dng a c l

Cc chunny khng ch m t v C v , m cn v hnh dng hnh hc v cc iu kinthit lp cho a gicl cnh vung, v hai dng c bn khc ca lu lng k chnh p, ngVenturi v ming.Mc ch ca cc ti liu hng dn ci t l m bo rngtn ticc iukin dng chy ri trongkhu vco ca lu lng k. Chun thng xuyn c dng nht Chu u l ISO 5167-1 [7], trong khi M l API 2530 [8]. Do vy, mt trong nhng nguyn

nhn chnh cho vic tip tc s dng lu lng k a gicl lsai sca php o (thng l 2% ca kt qu o) c th c d bo trc m khng cn thit phi kim nh,min ln c snxut v thit lp ph hp vi mt trong nhng chun ca quc t.

Nhc im chnh calu lng k a gicl l s hnchv di o vrtnhyvis thayica dngvn tc dng chy.Do tc dng chy t l vi cn bc hai ca chnh p oc, nn t s ca s kt hp mt a/ mt b pht chnh p b gii hn gi tr 3:1. Tsng4


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Cm bin

knh(hay cn gi l t s Bta)ca a ph thuc votc ln nht ca dng chy cno v dica b thu p sut chnh sn c.

Phng trnh (19.66) gi sdng vn tc cadng chy hnh thnh y v n nh, do vic lp tthit b phi chnh xc, c bit lngng trcv sau lu lng k phi thng. S mi mn ca cnh dn a gicl c th lm thay i rt nhiu chnh xc ca php o,v do thit b ny thng ch c s dng vi cc cht lng sch.

Hai lu lng k chnh p khc theo tiu chun quc t l ng v vi Venturi. ng Venturi c

p sut c nh thp hn a gicl, v n t nhy cm hn i vi s n mn v s nhiu lon ungun. Hn ch ln nht ca n chnh l kch c v gi c. Vic sn xutgp nhiukh khn hnso vi gicl a, do vy gi thnh cao hn.

Cc vi phun c sgimp gingnha gicl nhng docthit krt phng nn chnggi c chnh xc trongmt thi gian di.Gi thnh sn xut cc thit b nylnhnagicl nhngnhhn ng Venturi.Hai dng thit k ph binnht ca vi phuntuntheo cc tiuchun quc t ISA-1932 c a thch chu u, v ASME M.

C nhiu dng khc nhau ca cc lu lng k chnh p, nh l nm phn on, ng hnhnn,ng gp khc, v ng Dall. Mi loiuc nhngu imvt tri so vi a gicl, ng Venturi,v vi phun trong nhngng dngc th. V d, c th s dng cc nm phn on vi cc dngchy c h s Re thp, v ng Dall c sgimp c nh thp hn so vi ng Venturi. Tuy nhin,nhng thit b ny cha c chun ho theo tiu chun quc t, do cn phi kimtra chnhxc ca chng.

Hnh 19.54Lu lng kcng hnh nn v phao ni thay i din tch

Lu lng k tit din thay iCc lu lng k tit din thay i da trn nguyn tc s dng mt vt cn dng to ra s

chnh lch p sut, ngoi tr trong trng hp nychnhp l hng s v tit din vt cn bin

thinkhi tc dng cn o thay i. C l loi ph bin nht ca lu lng k tit din thay il ng bp mng v kiu phaoni(Hnh 19.54). Loilu lng knyc mt phao hoc mt ntt trong mt ng thng ng. Cht lng cn o chy vo trong y ng v lm ni phao ln, do lm tng tit din khe h quanh phaochocht lng chy vo.Tc dng chy cng tng th phao cng dch chuyn ln trn, vv tr ca phao trong ng khi n nh l hm trc tip ca tc dng chy.

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S tay C in t

Cc lu lng k dng phao rt n gin vng tin cy. Chng cgi tr u ra thay ituyn tnh vi tc dng chy (khng ging vi cc lu lng k chnh p)vi t l thngthngl 10:1 (so vi 3:1 lu lng k chnh p). chnh xc thng l 2% caton b thango, nhng n ph thuc vo di o v githnhca thit b. Thm vo ,vic cn chnh thang oca lulng kkhng b nh hng ca dngvn tc dng chy. Do ng c lm bng knhhoc nha trong sut,nn c th c cgi tr ca tc dng chy mt cch trc quanvkhng cn c ngun cungcp bn ngoi thc hinchc nng ny.V vycc lu lng k kiunyc dng rng ritrongccng dngsn xut vtrong phng th nghimni cnochtkhv cht lng. Nu o cc cht lng c nhit cao, p sut cao, v c kh nng n mn th ng calu lng k kiu phao phi c lm bng kim loi.Trong trng hp , cn phi c mt b phn xc nhv hin th v tr ca phao.

Hn ch chnh ca lu lng k kiu phao l n thngch c s dngtheo phng thngng, do vyvic lp t s rt kh khnnu ng cn o nm theo phng ngang. Mt s nh snxut ch tolu lng k dng phao til xo, c dngtrong mi v tr, tuy nhin, chnhxc lithp hn loi chun. Mt s hn ch khc l viccn chnh thang o calu lng k loiny ph thuc vo nht v t trng cht lng cn o, vvic a ra tn hiu intng ng vigi tr o lm tng thm phc tpca lu lng k. Tuy nhin, vic s dng cc cng tcgiihnquang hc hoc ttrong cclu lng kvi chc nngcnh bo dng cao hoc thpl rt ph bin.

Lu lng k dch chuyn dngCc lu lng k dch chuyn dng da theo nguyn tc o kh n gin. Dngcno c

di chuyn t u vo ca lu lng k n u rathng quahng lot cc ngn vi th tch bittrc.Tng th tch chy qua lu lng kc xc nh da trn s lng cc ngn, v nuthigiancng c o th c th tnh c lu lng dng chy.

Hnh 19.55Gia tc k dch chuyn dng vi bnh rng oval

Trn th trng hin nay cnhiuloilu lng kdch chuyn dng khc nhau. Vi cccht lng th thng dng nht l cc dng lu lng k piston, van trt, bnh rng oval, hai roto, ba roto, v a. Trongkhi cc dng thng dng i vi kh l r, ng thi (mng ngn), hoc dngCVM.Cc lu lng k dch chuyn dng k trnu da trn cng mt nguyn tchot ngv hu htl cc thit b c kh. u im ca cc lu lng k dng ny l kh nng o vi chnh xc cao (thng thng l

0,5%cagi tr oi vicht lng v 1%cagi tr oi vikh) vi mt di rng tc .Chng c th c s dng o cht lng vi nht v t trng cao. Thm vo , khng

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Cm bin

ging nh hu ht cc lu lng k khc, chngt ph thuc vos thay i cadng vn tcdng chy v do ngdntrc v sau lu lng kkhng nht thit phi thng vi di ln.

Hnh 19.55m tnguyn tc ca lu lng k bnh rng oval v minh ho nhng hn ch calu lng k dch chuyn dng. Chng l cc thit bc khtng i phc tp, phn di ng bmn theo thi gian. chnh xc ca chng ph thuc vo cht lng sn xut ban u vch bo tr nh k trong qu trnh s dng. Cht lng c okhngnn dng cc phn t c hn ch s mi mnca ccming m chng thmv gim yu cu bo tr qu mc cn thit.i vi cc ng dn c kch thc ln th cclu lng k dng nythng rt nngv gi thnhcng cao hn, v mt s dng thit k c th lm tccc ng dn nu cc lu lng k b kt cng.

Lu lng k turbineCng ging nh lu lng k dch chuyn dng, cc lu lng k turbine l cc thit b c

kh c kh nng t c chnh xc o cao. Nguyn tc hot ng ca loi ny l mt rotor nhiu l(multi-bladed rotor) c t trong dng chy v quay khi cht lng chy qua n.Tc quay carotor c o bngcm bin(thng dng loicm bint hoc c kh), v t l vi vntc dng chy qua thit b o.Cc lu lng k loi nyc th o vn tc trung bnhca cht lngchy trongng, vdo bit ng knh ng , nn c th tnhc lu lng dng chy.

Mcdcc loi lu lng kturbine lc so vi ccloi hinc trn th trng, nhng n vnl mt trong nhng lu lng k c chnh xccao v ngy nayvn c s dng. Sai s ca php o l nh hn0,1% khi o cht lng v 0,25% khi ochtkh. Do vy chng l mt trongnhng thit b c s dngnhiunht trong cc ng dng kim tra s truyndncht lng hockh. Chng c u ra tuyn tnh vdi ot nht l 10:1 v c th ti 100:1 i vi mt s ngdng.

Hn ch chnh ca chng l cc b phn c kh chnh nh cc roto s b mn khi s dngv lm gim lp li ca thit b v thay ithang oca n. Do vychngcn c bo tr vkim nh thng xuyn. Do nhng phn t cng trong cht lngc th lm tng tc mi mnca trc, nn cht lng cn o phi l cht lng sch. Thango ca lu lng k cngrt nhyvi s thay i ca nht cht lng v dng vn tc dng chy pha trc lu lng k.

Hnh 19.56 Nguyn tc ca lu lng k dng ri xoy

Cc dng khc ca lu lng k s dng nguyn tc turbine bao gm lu lng k bnh Peltonv loi chn vt, mc du chng c chnh xc thp hn cc loi quay quanh trc.

Lu lng k dng ri xoyLu lng k dng ri xoy, hay cn gi l lu lng k dng xoy, da trn hin tng ri

xoy, c Strouhal nghin cu vo nm 1878. Hnh 19.56 m t nguyn tc ca lu lng k7


8/13

S tay C in t

dng xoy.Mt vt cn c thnh thngngc t gia dng chy. Khi dng chy va phichng ngi vt n phi tch ra vt qua, vi cc lp cht lng gn b mt cavt cnchuyn ng chm hn cc lp xa. Do vy,sau khi vt qua vt cn, cc lp cht lng ti kthp, hin tng xoy c hnh thnh v to ra ln lt t cc mt cavt cn. Tn sto dngxoy lin quan ti vn tc dng chy u ngun v rng cavt cnv c nh ngha bi hs K ca lu lng k. Vi mtvt cn bit trc kch thc th h s K ca l tng i nnhtrongmt di rng hng s Re ca ng, v do trong nhng trng hp ny lu lng dngchy c quan h tuyn tnh vi tn s to dng xoy.

pht hin tn stodng xoy ngi ta tch hpcc cm binvo trong ccvt cn. Ccloicm binthng dng cho mc ch ny lcm binp sut, nhit,indung, vsiu m.

Lu lng dng xoyc kh nng o chnh xcchtkh v cht lng(sai sthng gpl 1%gi tr o) trnmt di o ti thiu 30:1.Chng cng c s dngtrnmt dirngnhit cachtlng, v do thng c dngo cc chtlngcng nghiptinhng ni cnhit cao,v d nh nit lnghoc hi nc. Vic cn chnh thang o ca thit b khng ph thuc vos thay i ca t trng cht lng, do thang chia ca mt lu lng k c th c s dngcho bt k cht lng no min l lu lng k c dng trong di hng s Re m nh sn xut ghi r. Loi lu lng k ny c cu to n gin v tin cy v nn c th c dng vi cc dngchy c cha cc phn t nh,mc dcc dng nhiu pha nh l h xi mngs lm tng tc mi mnca ccvt cnv gy rasai s ca php o.

Hn ch chnh ca loi ny l n ch c th c s dng trong cc iu kin dngchyhnlon. Do n khng thch hp vi cc ng c ng knh ln, hoc trong cc ng dng m vn tc dng chy thp hoc nht cao.Cc nh sn xutthng t hngs Renh nhtl10.000, y l gi tr mhu ht cc lu lng k c th lm vicc. Trong khi mt s lulng k cn hot ng hng s Re thp hn, dng xoy c to ra km n nh hn v chnh xc b gim. Khi hng s Re nh hn 3000 th s khng c dng chy ri, do lu lngk s ngng hot ng.Lu lng k dng xoy rtnhycmvi s thay ica dngvn tcdng chy pha trc lu lng k v cc nhiu dng xoy khc, v do khi ci t phi ch rng on ng pha trc v pha sau lu lng k phi thng vi di ln. Khng nn sdng lu lng k dng xoy ti nhng ni m ng ng b dao ng hoc nhiu ca cc ngunin cc b cao qu mc cho php, v cc hin tng s lm sai lch tn hiu xoy cn o, v cth a ra kt qu sai ngay c khi khng c dng chy.

Lu lng k in t Lu lng k in t hot ng da trn nh lut cm ng Faraday(v d: khi mt vt dn

chuyn ng vung gc vi mt t trng, th in p cm ng trn vt dn t l vi tc chuyn ng ca n). Trong trng hp ca lu lng k in t, dy dn s l cht lng cn o,trong khi inp cm ng c o bng cc in cc trn thnhng. Do trong hu ht cc ngdng, thnhng ca cc lu lng k thngclm bng cc cht dn in nh thp khng g,nncn phic mtlp cch in bn trongcch lycc in cc v hn ch s hao tn ca tnhiuin pvo thnhng. Cc cun dy bn ngoi ng c dng to ra mt t trngngang qua cht lng, vi cc k thut kch thch mt chiu dng xung thay th cho cc phng php kch thch cun dy xoay chiu b nh hng bi hin tng lch khng.

Lu lng k in t c nhiu u im hn so vi cc k thut o lu lng truyn thng, vc mt s c tnh ca lu lng k l tng. N khng c phn chuyn ng, khng lm tc ng,

di kch thc ng rt rng, v c th dng o dng lng hng. chnh xc c th t cl 0,5%trnmt diti thiul 10:1. chnh xc cng khng b nh hng bi s thay ica nht v t trng cht lng, v c th dng o cc cht trnphc tpnh ximng hay giynho.

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Cm bin

Hn ch chnh calu lng k in tl ch cth cs dng vi cc cht lngc sutdn ln hn5 /S cm , mc d mt sthit k c bit c th s dngvicc cht lng c sutdn thp mc0,1 /S cm . Do vychngkhngthch hp trong cc ngdngo cht kh,hinc, hoc cc cht lng khng dn in nh du. Viccn chnh thang onhy cm vi s thayi cadngdng chy, mcd di caon ngthng trc thit bo l ngn hn so vikhi dng lu lng k gicl hoc turbine. Mc d cc lu lng k in t khng yu cu bodng c bit, nhng vn phi ch trong qu trnh s dng m bo rng lp cch in bn

trong khng b h hng, v cc cht cn bn khng tch t trn cc in cc, nu khng chng slm thay i thang o hoc trong mt vi trng hp lm cho lu lng k b ngng hot ng. Ngay c khi cc hiu ng o c hn ch ti mc nh nht, th chng vn phi c cn chnhli thang o theo nh k bng cc k thut truyn thng hoc bng cc thit b cn chnh in tc cc nh sn xut cung cp km theo thit b o.

Lu lng k siu mMong c sn xut ra c mt lu lng kkhng tip xca nng tc ng ti s hnh

thnh ca nhiu loi lu lng k siu m khc nhau. Cc lu lng k siu m hin c trn thtrng c chia lm hai loi: Doppler vthi gian truyn qua.

Loi Doppler da trn nguyn tc dch Doppler. Sng siu m tn s 1 MHz c truyndi mt gc vo dng cht lng cn o. Mt phn nng lng ny s b phn hi bi cc im

gin on m thanh nh ccht nh, bong bng, hoc ccdngxoy hn lon. S khc bit tn sgia tn hiu truyni v tn hiu nhnc( dchtn s Doppler) t l trc tip vi vn tcdng chy.

Cc b chuyn i siu m, dng truyn v nhn sng siu m, thng c t trong mthp bo vc nh bnngoingng.Do , c th thc hin cvic ch tomt lu lngkdng kp,lp td dngv hon ton khngtip xc.

Hnh 19.57 Nguyn tc ca lu lng k siu m thi gian truyn qua

Sng siu m phn hikhng phi l sng mt tn s mgm mt di tn sgy ra dos phnx t cc im gin on c kch thc khc nhau. Cc im gin on di chuyn vi cc vntc khc nhau trong vngo. Vi cc cht lng, tn s ca sng siu m c truyn c gi tr t500 kHz ti mt vi MHz. Ti tn s 500 kHz, cc im gin on phi c ng knh xp x50 m phn x sng siu m v thit b thu. Vic tng tn s hot ng s cho php pht hinra cc phn t nh hn, nhng s lm gim kh nng xm nhp ca tn hiu truyn vo trong cht

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S tay C in t

lng. Lu lng k nhy vi nhng s thay i ca dng vn tc dng chy v phn b khng gianca cc im gin on trong dng chy. Do , chnh xc ca cc lu lng k Doppler lthp, sai s thng mc 5% ca gi tr cn o. Tuy nhin, iu ny c th c ci thin quavic cn chnh thang o trc tip trong qu trnh hot ng. Do s lch m gia thp v khng khl rt ln, nn khng nn s dng cc lu lng k Doppler dng kp o cc dng kh, hoc cccht lng hon ton sch do chng khng c cc phn t phn x hoc cc bong bng sinh ratn hiu Doppler tin cy.

Hnh 19.57 m t nguyn tc c bn ca lu lng kthi gian truyn quasng siu m. Hai b chuyn i siu m c gn trn hai mt ca ng, do sng siu m c th c truynngang qua dng chy trong ng. Sai lch thi gian mt xung siu m i t b chuyn i 1 n b chuyn i 2( xui dng) v t b chuyn i 2 n b chuyn i 1( ngc dng) c xcnh nh sau

2 2 2

2 coscos

lvT c v

=

(19.68)

Do vn tc ca m thanh trong cht lng c ln hn nhiu so vi vn tc cht lng v, nn

2

2 coslvT c

= (19.69)

Do , nu vn tc ca m thanh trong cht lng l hng s, th c mt mi quan h tuyn tnhgia T v v.Mc d v nguyn tc phng php ny l n gin v d hiu, nhng trong thc t c rt

nhiu kh khn do T qu b, v s thay i caT xy ra khi vn tc dng chy thay i thmch cn nh hn (thng l phn l ca micro giy trn mt). Hn na, phng trnh 19.68 chra rng, nu nhit ca cht lng thay i th c s thay i. V vy, cn phi o s thay i canhit cht lng hiu chnh. Cc lu lng k dngthi gian truyn quayu cu php otrng thi tn hiu v x l phc tp.

Vn tc dng chy v, xc nh bi phng trnh 19.68, l vn tc trung bnh dc theo ngtruyn gia hai b chuyn i, v do vy vic cn chnh thang o ca lu lng k ph thuc vodng vn tc dng chy. S ph thuc vo ch dng chy c th c gim ng k bng cchs dng cu hnh gm nhiu chm sng siu m song song v ly gi tr trung bnh ca cc vn tc

o c dc theo mi chm sng tnh vn tc dng chy. iu ny tng t vi vic tch hpcc s, v rt nhiu cu hnh a chm sng c a ra, mi cu hnh u c nhng u imring.

Cc lu lng k loi ny c chnh xc khong2% ca gi tr o trn mt di t nht l100:1. Mc d chng c th c s dng vi kh v cht lng, nhng thit k dng treo th ch cth s dng cho cht lng. Loi lu lng k a chm c chnh xc c ci thin ng k,nhng gi thnh li rt cao. Tuy nhin, c th s dng chng trong mt s ng dng c gi tr caonh theo di qu trnh truyn kh t nhin. Khng ging nh cc loi lu lng k khc, gi calu lng k dng thi gian truyn qua tng khng ng k khi ng knh ca ng tng. Chngc d nh dng cho cc cht lng sch, tuy nhin chng vn c th hot ng nu tn ti mtlng nh cc cht bn trong dng chy.

Lu lng k CoriolisLu lng k Coriolis c th c s dng trc tip o khi lng mt dng chy. Nguyn

tc hot ng da trn hiu ng Coriolis (c Gustave Coriolis tm ra vo nm 1835). Cht lngcn o chy qua mt ng dao ng, v khi lc Coriolis s tc ng ln n, cht lng s ctng tc khi n hng ti im dao ng ln nht v gim tc khi n i xa im . iu ny xyra ti im un ca ng, s lng im un t l trc tip vi khi lng dng chy ca cht lng.10


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S tay C in t

c nhng vn khc nhau, v kh c th gii thiu ht cc phng php o c th ca cc loidng trong quyn sch ny. c thm thng tin chi tit v cc k thut o dng hai pha hyxem trong [11].

Lp t lu lng kD lu lng k c tt nh th no i na th vic lp t n chun xc l rt cn thit nu

dng o vi mt sai s c nh sn xut ghi r. Vic cn chnh thang o ca cc lu lng kl nhy cm vi s thay i ca dng vn tc dng chy v trong nhng trng hp ny th chngcn phi c lp t m bo rng dng vn tc dng chy hnh thnh y v n nh tiim o. Cc ti liu k thut ca nh sn xut a ra hng dn v di cn thit ca on ngthng trc v sau lu lng k m bo s lp t hp l. C th dng cc b iu tit dngchy (b nn dng) hiu chnh cc dng vn tc b thay i v loi b cc nhiu xoy trongnhng ng dng m on ng thng khng c di cn thit. Tuy nhin, hn ch ca tt ccc b iu tit dng l chng lm cn tr dng chy v sinh ra s gim p khng th b li trongng ng.

Vic lp t phi lun lu cht hon ton in y ng ti im o v khng c s tham giaca pha khng mong mun th hai. Trong cc dng cht lng, kh ln vo c th c sinh ra tnhiu ngun khc nhau, v d nh cc l hng v cc mi r r. Trong khi vi mt dng kh, phalng khng mong mun c th c sinh ra t cc nguyn nhn nh s ngng t. Trong tt c cc

trng hp, lu lng k c th a ra kt qu sai nu nh tn ti pha th hai trong dng chy.Mc d c th loi b pha khng mong mun ra khi dng chy t trc im o, nhng tt hnht hy thn trng trong qu trnh ch to, thit k ng ng, v lp t lu lng k nhm m bo rng tnh hung ny khng xy ra.

La chn lu lng kKhng c mt lu lng k no thch hp vi mi ng dng, cng vi rt nhiu lu lng k

v cc ti liu k thut hin c trn th trng lm cho vic la chn lu lng k ph hp i khigp nhiu kh khn. C th c nhiu lu lng k ph hp vi mt ng dng c bit, nhng chn ci thch hp nht th khng d cht no. Ni chung, ci ph hp nht chnh l ci p ngc cc yu cu k thut c th vi gi thp nht (bao gm gi thit b v chi ph bo dng).

C nm nhn t nn tham kho khi chn la lu lng. l loi cht lng cn o, iu kin

hot ng, iu kin lp t, cc tnh nng hot ng cn c, v yu t kinh t. Thng tin cn thitkhi xem xt loi cht lng c o bao gm: n l dng mt pha hay cha ng pha th hai nhkh hoc rn, nht v t trng cht lng, cht lng c phi l cht n mn khng, v n l khkh hay kh m. Cc yu t cn xem xt khi nh gi iu kin hot ng bao gm: nhit cang v p sut, v cc iu kin xung quanh ng ng. Cc iu kin lp t bao gm cc thngtin nh l ng knh ng, hng s Re ca ng, hng ca ng lm vic ti im cn o, dica on ng thng, dng chy c gin n hay khng, cc yu cu cho bt k iu kin dng chyno, ngun cung cp bn ngoi c sn khng, v mi trng thc hin php o c nguy himkhng. Cc tnh nng hot ng yu cu ca lu lng k bao gm chnh xc, kh nng lp li,di o, v p ng ng hc yu cu. Cui cng, cc yu t kinh t cn quan tm bao gm gi banu ca thit b, gi lp t, gi bo dng, v hnh thc o to.

Cc ti liu gii thiu v php o lu lng u c cc bng la chn lu lng k (v d nh[3,4,6]), v cc chun quc t cho vic chn la v ng dng cc lu lng k [12].

Tham kho[1] Flow Measurement and Instrumentation, Oxford: Elsevier Science.

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Cm bin

[2] Grant, D. M., Open channel ow measurement, in D. W. Spitzer (Ed.), Flow Measurement: Practical Guides for Measurement and Control , 2nd ed., Research Triangle Park, NC: ISA,2001.

[3] Miller, R. W., Flow Measurement Engineering Handbook , 3rd ed., New York: McGrawHill, 1996.

[4] Baker, R. C., Flow Measurement Handbook, Cambridge: Cambridge University Press,2000.

[5] Webster, J. G. (Ed.),Mechanical Variables Measurement: Solid, Fluid, and Thermal , BocaRaton: CRC Press, 2000.[6] Spitzer, D. W. (Ed.), Flow Measurement: Practical Guides for Measurement and Control ,

2nd ed., Research Triangle Park, NC: ISA, 2001.[7] International Organisation for Standardization, ISO5167-1, Measurement of Fluid Flow by

Means of Pressure Differential DevicesPart 1: Orice plates, nozzles and Venturi tubesinserted in circular cross-section conduits running full, Geneva, Switzerland, 1991.

[8] American Petroleum Institute, API 2530, Manual of Petroleum Measurement StandardsChapter 14 Natural Gas Fluids Measurement, Section 3Orice Metering of NaturalGas and Other Related Hydrocarbon Fluids, Washington, 1985.

[9] Lynnworth, L. C.,Ultrasonic Measurements for Process Control: Theory, Techniques, Applications, Boston: Academic Press, 1989.

[10] National Engineering Laboratory, UK, Effects of Two-Phase Flow on Single-Phase Flowmeters, Flow Measurement Guidance Note No. 3, 1997.

[11] Rajan, V. S. V , Ridley, R. K., and Rafa, K. G., Multiphase ow measurement techniques a review, Journal of Energy Resource Technology, 115, 151-161, 1993.

[12] British Standards Institution, BS7405, Guide to the Selection and Application of Flowmeters for Measurement of Fluid Flow in Closed Conduits, London, 1991.

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