Nh chng ta bit, trong cng ngh lu lng dng vt cht l mt i lng rt cn thit cng ngh, c bit l cng ngh ho cht v thc phm. N l mt i lng cn c bit, c o v c iu khin ph hp. Chnh v th trong mt thit b, mt dy chuyn sn xut u c nhng thit b o lu lng. Lu lng c chia ra lm hai loi: 1. Lu lng th tch: l lng vt cht tnh theo th tch di chuyn qua trong mt n v thi gian, n v thng dng l m3/s, m3/pht, m3/h, cubic foot/hour (CFH hay S*CFH), cubic foot/minute (CFM hay SCFM), cubic foot/second (CFS hay SCFS), gallon per hour (GPH hay SGPH), gallon per minute (GPM hay SGPM), gallon per second (GPS hay SGPS),... (*) S-standard Ngi ta c th dng: IGPM vi I- inlet; AGPM vi A-Actual,... 2. Lu lng khi lng: l lng vt cht tnh bng khi lng chuyn qua trong mt n v thi gian, n v tnh bng kg/s, kg/pht, kg/h, ppm,.... Chnh v s cn thit ca lu lng nn cc nh nghin cu a ra rt nhiu loi thit b o lu lng vi c s tnh ton v thit k khc nhau. Trong bi ny ti a ra vi mc ch gii thiu cc nguyn l v cc thit b o khc nhau kt hp vi gii thiu cho cc bn cc hng sn xut ln trn th gii hin nay. Lu lng cn o ca dng dng knh kn (ng dn) hay knh h (mng, mng,...). o dng knh h c cc nguyn l o c bn sau: + o dng nh o chnh lch p + o dng theo nguyn tc in t + o dng theo s thay i tit din dng o dng trong knh h da trn nguyn tc... PHN I GII THIU Nh chng ta bit, lu lng dng vt cht l mt i lng rt cn thit, n tn ti trong hu ht cc thit b hay cc h thng thit b. Lu lng l mt i lng cn c bit, hiu, nh lng v c iu khin. Chnh v s cn thit ca n nn c rt nhiu hng sn xut i nghin cu v sn xut rt nhiu loi thit b o lu lng da trn nhiu nguyn l khc nhau. Trong ti liu ny ti mun gii thiu ti cc bn cc nguyn l o lu lng, cc loi lu lng k ca mt s hng sn xut trn th gii hin nay. Lu lng ca dng vt cht gm c 2 loi: 1. Lu lng th tch: L lng vt cht tnh theo th tch chuyn ng qua tit din ang xt trong mt n v thi gian.

Qv = V/T (1)

Trong : Qv: l lu lng th tch, m3/h V: l th tch ca vt cht, m3. T: l thi gian m V m3 vt cht chuyn ng qua, h (gi). n v ca lu lng th tch thng c dng l: m3/h (hay CMH - cubic meter per hour); m3/pht; m3/s; GPH (gallon per hour- galng trn pht), GPM, GPS; CFH (cubic foot per hour); CFM, CFS, ... Ngoi ra ngi ta cn dng SCMH (S-standard); SCFH; ICMH (I - Inlet), ACMH (A - Actural),... V d: Mt ng dn kh khai thc (kh ng hnh) t m du ngoi bin khi c vn chuyn vi nng sut l 3000 m3/h. iu c ngha l ta xt mt tit din ng trong khong thi gian 1 gi th c 3000 m3 kh ng hnh c chuyn qua. 2. Lu lng khi lng: L lng vt cht tnh theo khi lng chuyn ng qua tit din dng ang xt trong mt n v thi gian.

Qm = m/T (2)

Trong : Qm : l lu lng khi lvt cht, kg/h. m: l khi lng vt cht chuyn ng qua, kg . T: l thi gian m V m3 vt cht chuyn ng qua, h (gi). n v ca lu lng khi lng thng dng l: kg/h; kg/pht; kg/s; pound per minute,... Vit Nam, thng dng cc n v theo h thng n v SI. V trn th gii dng rt nhiu h n v khc nhau nn cc nh sn xut thng ci t vo trong thit b o nhiu n v o p ng vi cc i tng s dng. Cc hng sn xut lun lun i tm phng n thit k v sn xut cc loi thit b vi di o ln, chnh xc cao v h gi thnh sn phm. V th ra i rt nhiu loi lu lng k, tuy nhin, cc thit b thng c da trn cc nguyn l c bn sau: i vi knh kn (ng dn):

+) o dng theo phng php chnh lch p (da vo phng trnh Bernoulli) +) o dng theo phng php tit din dng thay i +) o dng theo nguyn tc in t i vi dng knh h (mng, mng, rnh, ng dn nhng dng vt cht khng chon y): +) o dng nh p chy trn +) o dng chiu su cui cng +) o dng da vo phng trnh Bernoulli. Sau y ta nghin cu chi tit tng phng php o cng vi vic gii thiu cc loi thit b tng ng.

Thng tin sn phm: Mng Parshall

M t - Tnh nng k thut:

Mng Parshall c pht trin trong nhng nm 1920, ch yu o lu lng nc ti. Tuy nhin hin nay, mng Parshall c s dng rng ri trong cc h thng mng rnh cng nghip, mng thot nc th, v mng dn nc trong cc nh my x l nc thi.Trong nm 1992, Tin s Ralph L. Parshall ca C quan Dch v Bo tn M thc hin mt s thay i cn bn trong vic thit k mng Venturi hin c (vi dng chy di ti hn). S thay i c tnh cht quan trng ny l to ra dng siu ti hn thng qua hng mng. Thit b ny c y ban Thy li ca Hip hi k s dn s Hoa K t tn l mng o Parshall.

im chnh ca mng Parshall l hng mng c co hp li, to ra mi quan h tng quan gia cao v lu lng dng x. Phn u mng c thit k dng y dc hng mng sao cho ngp nc cao m khng nh hng n tc dng chy. Dng thng ngun ca phn ny gip tng vn tc dng chy vo mng, c kh nng lng cn rn v trm tch. Kch thc mng Parshall c thit k da vo chiu rng hng mng, k hiu l W. Kch thc c sn cho mng vi rng hng khc nhau, t 0,0254 n 15,2 m. Da vo kch thc, mng Parshall c phn loi thnh ba nhm chnh: + Nhm mng c kch thc rt nh: 0,0254; 0,0508 v 0,0762 m + Nhm mng c kch thc nh: 0,152 2,44 m + Nhm mng c kch thc ln: 3,05 n 15,2m. Cc mng c mc ng dng theo lu lng t 0,948 m3/h n 306000 m3/h Dng chy trong mng Parshall thng c to thnh 2 trng thi. Trng thi u tin l trng thi dng chy t do, xy ra nhng ni m chiu su nc t ng khng lm gim lu lng. Lc ny, ch c chiu cao ti v tr o dng thng lu l thng s cn thit xc nh lu lng x thng qua bng tra. Trng thi dng chy t do l hin tng nhy thy lc hay cn gi l sng ng ca dng h lu. Trng thi dng chy th hai l dng chy ngp, xy ra khi su b mt dng h lu cao lm gim lu lng. Khi lu lng tng ln n gi tr cao hn gi tr ti hn, sc chng ca dng nc ti h lu mnh lm gim vn tc dng chy, lm tng chiu su dng chy v gy ra hin tng t ng nc trong mng. xc nh lu lng, cn phi xc nh chiu su ti thng lu (Ha) v chiu su ti hng mng (Hb). T l gia chiu su dng h lu v chiu su dng thng lu (Hb/Ha, tnh theo t l %) c gi l t l ngp nc. Th nghim hiu chun cho thy rng lu lng trong mng Parshall khng gim (ngha l mng hot ng theo trng thi t do) cho n khi t l ngp vt qu gi tr nh sau: + 50 % ng vi mng c knh thc chiu rng l 0,0254m; 0,0508m v 0,0762 m + 60 % ng vi mng c knh thc chiu rng l 0,152m; 0,0508m v 0.229 m + 70 % ng vi mng c chiu rng t 0.305m n 2,44 m + 80 % ng vi mng c chiu rng t 2,44m n 15,2 m. Khi t l ngp nc vt qu gi tr c lit k trn, th mng hot ng trong

iu kin ngp nc, v s dng bng lu lng ngp nc tnh ton lu lng. Lu lng ti thiu v ti a cho dng chy t do trong mng Parshall c xc nh bng thc nghim. Phng trnh lu lng ca dng chy t do trong mng Parshall c dng: Q = KHn Trong : Q = lu lng H = chiu cao c xc nh ti im Ha K = hng s, da vo chiu rng hng mng v n v Mng c th c lm bng g, b tng, tm kim loi m km, si thy tinh hoc nhng vt liu khc. Mng ln thng c xy dng, mng nh th c th c gia cng sn ri lp t. Mt thun li chnh ca mng Parshall l kh nng t lm sch dng nc, mc gim p thp, v kh nng hot ng trn phm vi v lu lng rng m ch cn 1 php o cao xc nh lu lng. Nhng c im ca cc mng Parshall lm cho n c bit thch hp trong vic o lu lng knh rch thy li, knh t nhin, v h thng mng rnh.