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204

Chng 6. Cng trnh tho l trong thn p

Bin son: PGS.TS Nguyn Phng Mu

6.1 Phn loi, iu kin xy dng I. Phn loi: C nhiu loi cng trnh tho l. Cn c vo cao trnh t cng trnh tho l trong thn

p, ta c th phn lm hai loi: cng trnh tho l di su v cng trnh tho l trn mt.

- Cng trnh tho l di su: c th t di y p v trn nn (cng ngm), i qua thn p (ng ng) khi iu kin a hnh, a cht cho php, c th tho c nc trong h cha bt k mc nc no, thm ch c th tho cn h cha. Loi ny khng nhng ch tho l m cn tu cao trnh, v tr v mc ch s dng cng trnh, c th dng dn dng thi cng trong lc xy dng, tho bn ct lng ng trong h cha, hoc ly nc ti, pht in. Do , tu theo iu kin c th m c th kt hp nhiu mc ch khc nhau trong mt cng trnh tho nc di su.

- Cng trnh tho l trn mt: thng t cao trnh tng i cao. Do cao trnh ca ngng trn cao, nn n ch c th dng tho dung tch phng l ca h cha. Cn c vo hnh thc cu to, cng trnh tho l trn mt li c th phn ra cc kiu sau y:

+ p trn trng lc ;

+ Xi phng tho l ;

i vi tng u mi cng trnh, chng ta cn phn tch k c im lm vic, iu kin, a hnh, a cht v thu vn, cc yu cu v thi cng, qun l khai thc, v,v... chn loi cng trnh tho l trong thn p thch hp nht.

II. iu kin xy dng Do iu kin lm vic, c im a hnh v tnh cht ca cng trnh ngn nc m c

th c nhiu cch b tr v nhiu hnh thc cng trnh tho l.

i vi cc loi p b tng v btng ct thp, ngi ta thng b tr cng trnh tho l ngay trn thn p. Nh cc h thng thu li Bi Thng, Lng, Thch Nham th p va dng nc, va trn nc.

Khi xy dng h cha nc, vn u t vo cng trnh tho l kh ln. Cc cng trnh tho l phi lm vic lu di, vng chc, n gin trong qun l v tha mn trong iu kin

kinh t. Mt trong nhng kiu cng trnh tha mn cc iu kin ny l xi phng. nhng ni nc l v nhanh khi c ma, nh min ni nc ta th vic ng dng xi phng tho l c tc dng rt ln v n lm vic t ng v m bo tho l mt cch nhanh chng.

Trn th gii xiphng c ng dng rng ri v c xy dng c trong p btng cao, c trong nhng p t khng ln lm. Nga, xi phng tho l c xy dng rng ri trong cc p t trn cc sng sui nh. Cc xiphng thng lm bng cc ng btng ct thp hoc ng thp c sn. Xi phng c cc u im sau:

-T ng tho nc: khi c l v, mc nc thng lu vt qu mc nc dng bnh thng mt tr s no , xiphng bt u lm vic c p hon ton.


205

-R tin: lu lng tho ca xi phng ln nn chiu rng ca xiphng nh hn rt nhiu so vi chiu rng cc cng trnh tho l kiu h khc. S chnh lch c bit ln khi cng trnh c lu lng l thit k cng ln.

-Khc vi cc kiu cng trnh tho l bng ng ng, xiphng khng cn ca van v cc thit b nng ca, do qun l n gin.

Do nhng u im nh vy, xiphng c ng dng rng ri khng nhng trong cc p m cn c xy dng trn cc knh, trong trm bm, nh my thu in, v.v...

Trong chng ny c trnh by cc cng trnh tho l trong thn p ch yu l p trn tho l, cng trnh tho l x su (cng ngm, ng ng qua thn p...), p trn kt hp x su.

6.2 p trn tho l p trn tho l chim mt v tr quan trng trong cc loi cng trnh tho l. Lc c

iu kin s dng th y l mt loi cng trnh tho l r nht.

Khong 50 60 nm trc y, ch mi c p trn tho l cao 50 70 m th ngy nay c p trn cao 150m. Xy dng c loi p trn cao l do iu kin a cht v kt cu cng trnh quyt nh.

I . B tr p trn Vic b tr p trn trong h thng u mi c quan h vi iu kin a cht, a hnh,

lu lng tho, lu tc cho php h lu,v.v...Khi lu lng tho ln, ct nc nh, nu lng sng khng n nh v nn khng phi , c cu to a cht phc tp th hnh thc v b tr cng trnh tho nc c ngha quyt nh. Khi ct nc ln, phi tiu hao nng lng ln, vic chn v tr ca p trn c ngha ln.

Khi thit k cng trnh tho l, cn c gng tho mn cc iu kin sau y:

1.Khi c nn , phi tm mi cch b tr p trn vo nn . Nu khng c nn hoc nn xu th c lc cng phi b tr trn nn khng phi l .

2.Cn to cho iu kin thin nhin ca lng sng khng b ph hoi, do trc tin cn phi nghin cu n phng n b tr p trn ti lng sng hoc gn bi sng. Trong trng hp cn rt ngn chiu rng p trn th tnh hnh thu lc ban u c th b ph hoi, do phi c nhiu bin php tiu nng phc tp. Tuy nhin trong nhiu trng hp, phng n rt ngn chiu rng p trn vn l kinh t nht. Nu lu lng tho nh hoc dng chy c iu tit tt th khng nht thit phi b tr p trn gia lng sng.

3.B tr p trn phi ph hp vi iu kin tho lu lng thi cng v phng php thi cng.

4.Nu p ngn nc khng ch phi l cng trnh btng, c bit lc phm vi nn khng rng, mun gim bt khi lng cng trnh th c th dng bin php tng lu lng n v rt ngn chiu rng p trn, ng thi c th kt hp hai hnh thc x mt v x su tho l, thm ch phi s dng k nng tho l ca mi cng trnh khc nh qua nh my thu in, u thuyn, v.v...


206

5.Khi c cng trnh vn ti thu, vic b tr p trn cn ch m bo cho dng chy v lu tc h lu khng nh hng n vic i li ca tu b.

6.B tr p trn cn m bo cho lng sng v hai b h lu khng sinh ra xi l m bo an ton cho cng trnh.

7.i vi cc sng nhiu bn ct, b tr p trn tho l cn trnh khng sinh ra bi lng nghim trng.

II . Chn v tr l trn v lu lng n v Tu tnh hnh c th, trn p trn c th c ca van hoc khng c ca van. Trng

hp lu lng thit k khng ln v chiu rng p trn ln th ngi ta khng b tr ca van.

Mt vn quan trng c lin quan n vn hnh ca h thng l chn v tr v kch thc ca l trn.

Khi xc nh kh nng tho ca h thng u mi thu li, cn phi xt ton din n cc lu lng trn mt, x su, qua tuabin v u thuyn,v.v...Lc thi cng, c th s dng cc cng trnh c iu kin tho lu lng thi cng. y l iu phi xt ti lc b tr l trn.

Ngy nay, lc thit k mt s p, ngi ta b tr l tho cc cao trnh khc nhau (v d trn mt kt hp x y), nh vy c mt phn kh ln lu lng qua x su. Qua kinh nghim vn hnh Nga v mt s nc khc, cho thy loi p ny lm vic kh tt. u im ca p tho l hai tng ny l c th gim chiu di p trn v gim c khi lng btng (c th gim t 10 15%) v c th ci thin iu kin tiu nng. Nhc im l cu to kh phc tp, phi b tr nhiu ct thp, ng thi trnh t thao tc ca van tng i phc tp. Ngoi ra cng cn ni thm, c th dng l y tho bt hoc tho cn h cng nh dng tho lu lng thi cng. Do mi l y cn c ca van linh hot v c th m vi mt m bt k.

Mt trong nhng vn quan trng khi thit k p trn l xc nh lu lng n v cho php. Nu phn ngn nc bao gm p t v p b tng, thng c gng tng lu lng n v gim di p trn. Trc lc chn lu lng n v, cn phi nghin cu k cu to a cht ca lng sng ,chiu su nc h lu, lu tc cho php cng nh hnh thc v cu to ca b phn bo v sau p v trnh t ng m ca van.

Ngy nay trong thit k p trn ngi ta thu c nhiu thnh tu v mt nghin cu tiu nng, do lu lng n v c nng ln.

Lc chn lu lng n v v lu tc cho php cui sn sau, ngi ta phi so snh vi lu lng v lu tc ln nht lc cha xy dng p, ng thi phi xt n su xi cc b c kh nng sinh ra m khng nh hng n an ton ca cng trnh.

Lu lng n v nht nh phi thch ng vi hnh thc ca b phn bo v sau p v kh nng m bo cho cng trnh khng b xi l. Do xc nh chiu rng p trn v cc thit b ni tip phi xut pht t lu lng n v qp b phn bo v sau sn tiu nng. Tr s qp c xc nh theo lu tc khng xi h lu hoc lu tc cho php phn bo v sau sn tiu nng bit, lu tc cho php khng xi [v] ng vi chiu su h h lu v lu lng thit k ca p trn Qp th lu lng n v thit k s xc nh theo biu thc :


207

p

pp B

Qh]v[q == ;

Nu lu tc dng chy h lu nh hn lu tc khng xi th khi lng cng trnh s kh ln, cho nn i vi nhng p khng ln lm, c lc ngi ta ly tr s lu tc trung bnh cho php [v]tb. Lc , phi d tnh n kh nng on sng sau sn th hai c th b xi, cn c bin php hn ch khng cho xi l tin n sn sau th hai. Theo quan im khai thc p trn, c th ly tr s [v]tb nh sau:

loi t ct [v]tb = 2,5 3,0 m/s ;

loi t st [v]tb = 3,0 3,5 m/s ;

loi na [v]tb = 3,5 4,5 m/s ; loi [v]tb = 5,0 m/s.

Nu khng xt n s khuch tn dng chy sau sn th hai, vi p trn c ca van phng hoc van cung, ta c th s b ly lu lng n v ca p trn nh sau:

q = (1,15 1,2)qp Sau cn nghim li vi iu kin dng nc nhy ngp ni tip h lu.

Nhiu khi phi da vo kinh nghim thit k xc nh qp, v d p c ct nc va (10 25m) vi nn t ct, c th ly qp = 25 40 m

3/s.m, vi nn st qp = 50 m3/s.m, nn

qp = 50 60m3/s.m, v.v...

Nu lu lng tho l ln nht l Qmax , lu lng chy qua cc cng trnh khc nh qua tuabin, u thuyn, v.v... l Qc , lu lng qua l x y l Ql th lu lng qua p trn s xc nh theo biu thc :

Q = Qmax - Qc - Ql. (6-1)

Tt nhin phi xt n trng hp khng phi tt c tuabin u lm vic, cc l tho c th b s c do ca van su, do bn ct, v.v...v th ch nn ly 80 90% lu lng qua l tho v 75 90% qua tuabin, u thuyn, v.v...

III. Cc loi mt ct ca p trn: 1. Phn loi: Tu theo tnh hnh c th v quy m ca tng h thng cng trnh u

mi, mt ct ca p trn c th c nhiu hnh thc khc nhau (hnh 6-1)

Hnh 6-1. Cc hnh thc mt ct ca p trn

a. p trn kiu mt bc; b. p trn kiu nhiu bc;

c,d. p trn kiu hnh cong; e. Ngng trn

a) b)

c) d) e)


208

Loi p trn kiu mt bc c ng dng lc nn mng chc chn, khng c loi ct si ht ln chy qua. Do c tia nc t do chy xung y sng v phn bo v nn ngi ta thng dng loi ny vi trng hp ct nc khng ln (3 4m) hoc c ct nc ln hn nhng c bin php tiu nng i vi nhng tia nc .

Loi p trn kiu nhiu bc t c ng dng hn, do cn c nn mng kh di v tn vt liu xy dng.

Loi p trn kiu hnh cong (p trn thc dng) c dng nhiu nht. Loi ny ni tip c thun v h s lu lng ln.

Loi ngng trn thng c dng khi ct nc thp v c ca van.

2. Mt ct chn khng v khng chn khng:

Vi loi p trn thc dng khng chn khng, dng chy trn p m, p sut dc mt p lun lun dng. Vi p trn thc dng c mt ct chn khng, nh p c p lc chn khng. Lc chn khng ln, c th sinh ra hin tng kh thc. H s lu lng ca p trn chn khng ln hn p trn khng chn khng khong 7 15%. m bo an ton cho cng trnh, trnh hin tng kh thc, ngi ta khng cho php tr s chn khng qu ln, thng khng ln hn 6 6,5 m ct nc.

Loi mt ct ca p khng chn khng Kriger Ofitxrov, c ng dng rng ri, ni chung mt ct ngang c dng nh hnh 6-2.

Xc nh cc tr s aB, aH v a cn cn c vo mt ct c bn ca p, iu kin thi cng v xt n nh hng ca cc i lng ny i vi h s lu lng. Bn knh R ni tip vi sn sau h lu khng c quan h g vi h s lu lng, m c lin quan n vic ni tip gia dng chy ra vi mt ct h lu. Nu ni tip tt th ly R theo bng 6-1 trong Htr l ct nc trn nh p. Trong thc t c khi ly R nh sau:

-i vi p thp trn nn mm, c ct nc trn nh ln:

R (0,50 1,00).(Htk + Zmax) ; (6-2) -i vi p cao trn nn , ct nc trn nh nh hn 5m:

R (0,25 0,50) .(Htk + Zmax) ; (6-3) trong : Zmax - chnh lch ct nc ln nht thng h lu ;

Htk - ct nc trn thit k trn nh p.

Hnh dng mt cong CDE (hnh 6-2a) phi cn c vo Htk m xc nh. Tr s Htk thng l ct nc thit k hoc ct nc ln nht trn nh p. Trong qu trnh lm vic, ct nc lun lun thay i, nn khi H < Htk th trn mt trn CDE khng th xy ra hin tng chn khng, nu H > Htk th s th xy ra hin tng chn khng trn mt p. Ngoi ra cn ch l gn im B trn on BC c th c chn khng khi H Htk.

C my loi hnh dng mt ct nh sau : khng c tng thng ng AB tc l a =0 (hnh 6-2b) ; khng c on thng DE (hnh 6-2c) ; mt thng lu thng ng, aB = 90

0 (hnh 6-2d); mt thng lu nh ra (hnh 6-2e).


209

a) c)

b) d) e)

Hnh 6-2. Cc dng mt ct ca p trn khng chn khng.

Bng 6-1. Tr s ca bn knh ni tip R(m)

Htk Chiu cao p (m)

1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0

10 3,0 4,2 5,4 6,5 7,5 8,5 9,6 10,6 11,6 20 4,0 6,0 7,8 8,9 10,0 11,0 12,2 13,3 14,3 30 4,5 7,5 9,7 11,0 12,4 13,5 14,7 15,8 16,8 40 4,7 8,4 11,0 13,0 14,5 15,8 17,0 18,0 19,0 50 4,8 8,8 12,2 14,5 16,5 18,0 19,2 20,3 21,3 60 4,9 8,9 13,0 15,5 18,0 20,0 21,2 22,2 23,2

Mun v mt ct p trn, trc ht xc nh Htk , sau cn c vo to cc im trong bng 6-2 vi trc to nh hnh 6-2 m v ng cong CC0DD

'; tip v ng thng BC v DE tip tuyn vi ng cong cng vi ng thng nm ngang to thnh cc gc aB v aH. Cc tr s ca gc aB v aH cn cn phi cn c vo mt ct c bn ca p xc nh cho ph hp. Cui cng v ng thng AB v ng cong EF bn knh R.

Bng 6-2. To cc im trn ng bin ca mt trn khng chn khng

Kriger - Ofitxrov

Tn im x y Tn im x y

1 0,0 0,126 21 2,0 1,235

2 0,1 0,036 22 2,1 1,369

3 0,2 0,007 23 2,2 1,508

4 0,3 0,000 24 2,3 1,653

5 0,4 0,006 25 2,4 1,894

6 0,5 0,027 26 2,5 1,960

D'

E

D

Co

R

H F

xO

y

CB

A

B B

a

C

a

aCo

BC

A, B

a=0 ; DE =0 C

D,ER

FHa

Ba

A, B

BC

D

Co

C

R

F

E

Ba Ha

a=0

BC

D

yD'

ER

C

Co

F

x=90Ba

Ha=90Ba

Ay

BC

C

Co

F

D

ER

x

BB'


210

7 0,6 0,060 27 2,6 2,122

8 0,7 0,100 28 2,7 2,289

9 0,8 0,146 29 2,8 2,462

10 0,9 0,198 30 2,9 2,640

11 1,0 0,256 31 3,0 2,824

12 1,1 0,321 32 3,1 3,013

13 1,2 0,394 33 3,2 3,207

14 1,3 0,475 34 3,3 3,405

15 1,4 0,564 35 3,4 3,609

16 1,5 0,661 36 3,5 3,818

17 1,6 0,764 37 3,6 4,031

18 1,7 0,873 38 3,7 4,249

19 1,8 0,987 39 3,8 4,471

20 1,9 1,108 40 3,9 4,689

Ch dn bng 6-2. Cc tr s to trong bng ng vi H = 1, khi s dng phi nhn vi ct nc trn Htk

Loi mt ct ca p chn khng c thng lu l mt phng thng ng, h lu l mt nghing (h s mi thng l 3 : 2), nh p hnh elip (c khi l hnh trn), trc di hnh elip l 2e song song vi mt h lu p (hnh 6-3), trc ngn l 2f .

a) b) c)

d) e) f)

Hnh 6-3. Cc dng mt ct ca p trn chn khng.

a,b,c. Mt ct ca p trn chn khng ; d,e,f. Mt ct kinh t ca p trn.

Bng 6-3 ghi to cc im ng cong mt trn ca 3 loi p chn khng c t s e/f khc nhau. Mun v, trc ht v vng trn c bn knh rj ni tip vi 3 cnh AB, BC,

BC R

B C

D'

A

rjC

A

B

D

R

B C

e f

1

x

y

T (11)

y

D

EA

B

C

Ox

Ey

C

B

D

xO

B'

O'

RE'

F

h

H

A'A

1

tk

E

y

A'A

C

B'B

RE"

D

O'O

y'

C'

h

H

1

tk


211

CD. Bng 2-3 ng vi trng hp rj = 1; khi rj >1 hoc rj < 1 th cc to x,y trong bng phi nhn vi rj.

im gc to l im cao nht ca nh p (hnh 6-3c). im ny nm trn ng BC (hnh 6-3b), trong bng 6-3 l im 7 (khi e/f = 1 v e/f = 3) hoc im 11 khi (e/f =2). Ni tip gia mt h lu v sn sau cng ging nh mt ct khng chn khng.

Loi mt ct kinh t ca p trn xc nh nh sau: sau khi da vo iu kin n nh, cng v kinh t, xc nh c mt ct kinh t p khng trn ABOE (hnh 6-3d) v da vo mt ct c bn xc nh c mt trn CD (v theo to trong bng 6-2 hoc bng 6-3). Mt trn CD phi tip tuyn vi mt p khng trn DE ti im D. To cc im ca mt trn rt c th vt ra tam gic c bn AOE (hnh 6-3e), bi v vi p trn trn nn , theo yu cu v n nh v cng , chiu rng y p rt hp. Trng hp ta cn dch tam gic c bn v pha h lu mt on (hnh 6-3e), sao cho mt p DE' ca tam gic c bn A'O'E' tip tuyn vi mt trn ti D. Nh vy mt trn CDE'F tho mn iu kin thu lc. i vi iu kin n nh v cng th tam gic A'O'E l m bo, do ta c th gim bt khi ABB'A' (hnh 6-3e) nhng cn phi h1 0,4Htk khi nh hng n kh nng dng chy. Trng hp p trn c ca van sa cha, trn nh p cn c on nm ngang CC' (hnh 6-3f) d b tr ca van. Lc to cc im ca mt trn phi di i mt on n cui on nm ngang. Ch rng trn nh trn c on nm ngang nh vy th h s lu lng s gim.

Bng 6-3. To cc im ca ng cong mt trn kiu chn khng,

nh p hnh elip (khi rj = 1)

To cc im e/f = 3,0 e/f = 2,0 e/f = 1,0 Tn im

x y x y x y 1 -0,472 0,629 -0,700 0,806 -1,000 1,000 2 -0,462 0,462 -0,694 0,672 -0,960 0,720 3 -0,432 0,327 -0,670 0,519 -0,880 0,525 4 -0,370 0,193 -0,624 0,371 -0,740 0,327 5 -0,253 0,072 -0,553 0,241 -0,530 0,152 6 -0,131 0,018 -0,488 0,162 -0,300 0,046 7 0,000 0,000 -0,402 0,091 0,000 0,000 8 0,194 0,030 -0,312 0,046 0,200 0,020 9 0,381 0,095 -0,215 0,012 0,400 0,083 10 0,541 0,173 -0,117 0,003 0,600 0,200 11 0,707 0,271 0,000 0,000 0,720 0,306 12 0,866 0,381 0,173 0,025 0,832 0,445 13 1,022 0,503 0,334 0,076 1,377 1,282 14 1,168 0,623 0,490 0,147 2,434 2,868 15 1,318 0,760 0,631 0,223 3,670 4,722 16 1,456 0,890 0,799 0,338 5,462 7,410 17 1,584 1,021 0,957 0,461 - - 18 1,714 1,163 1,107 0,595 - -


212

19 1,855 1,320 1,243 0,731 - - 20 1,979 1,467 1,405 0,913 - - 21 2,104 1,628 1,551 1,098 - - 22 2,240 1,792 1,688 1,282 - - 23 2,346 1,943 2,327 2,246 - - 24 2,462 2,106 2,956 3,189 - - 25 2,575 2,272 4,450 5,430 - - 26 3,193 3,214 5,299 6,704 - - 27 4,685 5,452 - - - - 28 5,561 6,766 - - - -

IV. Kh nng tho nc ca p trn : Lu lng chy qua p trn c mt ct thc dng tnh theo biu thc:

2/30n Hg2mBQ es= , (6-4)

trong : B = Sb , B - tng chiu rng nc trn ;

b - chiu rng mi khoang ca ;

sn - h s ngp (trng hp khng ngp th sn = 1) ;

e - h s co hp bn ; m - h s lu lng ;

H0 - ct nc trn nh p trn c k n lu tc tin gn.

Nu trn nh p c ca van, khi khng m ht v nc chy di ca van (hnh 6-4), lu lng tho qua p c tnh theo biu thc:

)aH(g2aBQ o a-em= , (6-5)

trong :

a - m ca van ;

a- h s co hp ng do nh hng ca m (bng 6-4) ;

q-+-=m cos)357,025,0(186,065,0H

a

H

a ; (6-6)

cc k hiu xem hnh 6-4.

Hnh 6-4. Mt ct ca p trn c ca van

Khi ca van m ht hon ton, biu thc tnh lu lng tr v dng (6-4).

aH = 90


213

Bng 6-4. H s co hp ng a khi nc chy di ca van

a/Ho 0,1 0,2 0,4 0,5 0,6 0,7 a 0,61 0,62 0,633 0,645 0,66 0,69

Mun tnh Q theo biu thc (6-4), cn phi xc nh c cc h s sn , e v m. Sau y s trnh by cch xc nh cc h s i vi cc trng hp c th.

1. H s ngp sn.

a) H s ngp sn ca p trn c mt ct thc dng c chn khng:

Khi h lu c nc nhy xa th p trn lun lun khng ngp, sn = 1,0. Nu h lu c nc nhy ngp th nc chy qua p trn c th khng ngp hoc ngp, lc sn ph

thuc vo t s 0

n

Hh

(hn - chiu su nc ngp, tc l khong cch t mc nc h lu n

nh p trn, nu mc nc h lu thp hn nh p th hn c tr s m).

Hnh 6-5 cho cc ng cong xc nh sn theo th nghim ca Rozanov :

- ng cong I: i vi p trn c mt ct chn khng ; khi 15,0Hh

0

n - th sn = 1,0 ;

- ng cong II: i vi p trn khng chn khng Ofitxrov; khi 0Hh

0

n th sn = 1,0

- ng cong III : p trn khng chn khng c nh m rng hoc trn nh rng.

a) b) c)

Hnh 6-5. Cc ng cong xc nh sn ca p trn mt ct thc dng

Hnh 6-6. Cc dng mp vo ca tr bin

b) H s ngp sn ca p trn c mt ct thc dng khng c chn khng: Ch tiu ngp ca p trn c mt ct thc dng ging nh ca p thnh mng:

k

n

PZ

PZ

oh

-0,2 0,0 0,2 0,4 0,6 0,8 1,00,0

0,2

0,4

0,6

0,8

hH

n1,0

n

o

I

II

III

y = 1 y = 0,7 = 0,7y

45


214

Tr s kP

Z

c xc nh theo (bng 6-5):

trong : P chiu cao p so vi y ca lng dn h lu.

H ct nc trn, tc chiu cao mc nc thng lu so vi nh p.

mo h s lu lng bao hm yu t ct nc lu tc ti gn.

Bng 6-5. Tr s kP

Z

xc nh trng thi phn gii chy ngp ca p trn thnh mng

v p trn c mt ct thc dng.

H/P

mo 0,10 0,20 0,30 0,40 0,50 0,75 1,0 1,5 2,0 3,0

0,42 0,89 0,84 0,80 0,78 0,76 0,73 0,73 0,76 0,82 1,0

0,46 0,88 0,82 0,78 0,76 0,74 0,71 0,70 0,73 0,79 1,01

0,48 0,86 0,80 0,76 0,74 0,71 0,68 0,67 0,70 0,78 1,02

H s chy ngp sn trong cng thc (6-4) c xc nh theo (bng 6-6): trong : hn chiu cao mc nc h lu

Ho ct nc ton phn

g2v

HH2o

oa

+= ;

vo lu tc ti gn, tc lu tc dng chy thng lu khi n gn p; H v vo c ly v tr cch p mt khong bng 3H;

Bng 6-6. H s ngp sn ca p trn c mt ct thc dng khng c chn khng

o

n

Hh

sn o

n

Hh

sn

0,0 1,00 0,50 0,972 0,05 0,999 0,55 0,965 0,10 0,998 0,60 0,937 0,15 0,997 0,65 0,947 0,20 0,996 0,70 0,933

0,25 0,994 0,75 0,9110,800 0,30 0,991 0,80 0,760 0,35 0,988 0,85 0,700 0,40 0,983 0,90 0,590 0,45 0,978 0,95 0,410

1,00 0,000


215

2. H s co hp bn e.

* Trng hp 1b

Ho th h s e c xc nh theo biu thc sau y:

- i vi p trn khng c tr pin gia (ch c mt khoang):

e = 1 - 0,2 b

Hoyx , (6-7)

trong : yx - h s gim, xt n hnh dng mp vo ca tr bin.

hnh 6-6 Krigr cho cc tr s yx ca ba loi ca mp vo khc nhau.

- i vi p trn c nhiu tr pin chia thnh nhiu khoang ging nhau:

e = 1 - 0,2 n

)1n( py x-+xb

Ho, (6-8)

trong n - s ca (khoang);

px - h s gim, xt n hnh dng ca tr pin.

hnh 6-7 ofixrov cho cc tr s px i vi

cc dng khc nhau ca tr pin.

* Trng hp b

Ho>1 th dng biu thc (6-7)

hoc (6-8) nhng phi ly: b

Ho=1

Hnh 6-7. Cc dng tr pin

3. H s lu lng m. Theo N.N Pavlovxki, h s lu lng m ca p trn tnh theo biu thc.

m = mr.sh.sd , (6-9) trong : mr - h s lu lng dn sut, xc nh bng th nghim ;

sH - h s hiu chnh ct nc, v khi thit k mt ct p dng Htk (xem mc III-6.2), khi lm vic th ct nc H trn nh p thay i ;

sd - h s hnh dng. Ta xc nh m theo tng trng hp sau y. 1. i vi p trn khng chn khng Kriger- Ofitxrov, biu thc (6-9) c dng:

m=0,504.sh.sd , (6-10) trong : sd - tra bng 6-7 (cc i lng trong bng xem hnh 6-2) ;

sh - tra bng 6-8. Nu trn nh p c on nm ngang rng khong 0,5H th m gim i 3% so vi kt

qu tm c theo biu thc (6-10). Trng hp p c mt thng lu nh ra (hnh 6-2e) th m ly nh sau: - Nu chiu cao on CB' > 3H th ly nh p c mt ct hnh 6-2d tc l phn lm

ca p khng c nh hng g n lu lng; nu CB' < 3H th m ly nh hn 2% so vi p c mt ct nh hnh 6-2d.

d d

r R=1,708d 1,2

08d90

d

p= 0,8 = 0,45 = 0,25

a) b) c)


216

Bng 6-7. H s hnh dng sd ca p khng chn khng Kriger - Ofxrov.

a / CB aB () aH()

0,0 0,3 0,6 0,9 1,0

15 15 0,880 0,878 0,855 0,850 0,933 30 0,910 0,908 0,885 0,880 0,974

45 0,924 0,922 0,899 0,892 0,993 60 0,927 0,925 0,902 0,895 1,000

25 15 0,895 0,893 0,880 0,888 0,933 30 0,926 0,924 0,912 0,920 0,974 45 0,942 0,940 0,928 0,934 0,993

60 0,946 0,914 0,932 0,940 1,000 35 15 0,905 0,904 0,897 0,907 0,933

30 0,940 0,939 0,932 0,940 0,974

45 0,957 0,956 0,949 0,956 0,993 60 0,961 0,960 0,954 0,962 1,000 45 15 0,915 0,915 0,911 0,919 0,933

30 0,953 0,953 0,950 0,956 0,974 45 0,970 0,970 0,966 0,973 0,993

60 0,974 0,974 0,970 0,978 1,000 55 15 0,923 0,923 0,922 0,927 0,933 30 0,962 0,962 0,960 0,964 0,974

45 0,981 0,981 0,980 0,983 0,993 60 0,985 0,985 0,984 0,989 1,000

65 15 0,927 0,927 0,926 0,929 0,933

30 0,969 0,969 0,968 0,970 0,974 45 0,987 0,987 0,986 0,988 0,993

60 0,993 0,993 0,993 0,995 1,000 75 15 0,930 0,930 0,930 0,930 0,933 30 0,972 0,972 0,972 0,972 0,974

45 0,992 0,992 0,992 0,992 0,993 60 0,998 0,998 0,998 0,999 1,000

85 15 0,933 0,933 0,933 0,933 0,933 30 0,974 0,974 0,974 0,974 0,974 45 0,993 0,993 0,993 0,993 0,993

60 1,000 1,000 1,000 1,000 1,000 90 15 0,933 - - - 0,933

30 0,974 - - - 0,974

45 0,993 - - - 0,993 60 1,000 - - - 1,000

Ch dn bng 6-7: Khi aH > 600 , tr s sd c ly vi aH = 60

0


217

Bng 6-8. H s hiu chnh ct nc sH ca p trn khng chn khng

(theo ti liu ca N.P. Rozanov v A.X. oftrov)

aB ()

tkHH

20 30 40 50 60 70 80 90

0,2 0,893 0,886 0,897 0,872 0,864 0,857 0,850 0,842 0,3 0,915 0,909 0,903 0,897 0,892 0,886 0,880 0,874 0,4 0,932 0,928 0,923 0,919 0,914 0,909 0,905 0,900 0,5 0,947 0,943 0,940 0,936 0,933 0,929 0,925 0,922 0,6 0,960 0,957 0,954 0,952 0,949 0,946 0,943 0,940 0,7 0,971 0,969 0,967 0,965 0,963 0,961 0,959 0,957 0,8 0,982 0,980 0,979 0,978 0,977 0,975 0,974 0,973 0,9 0,991 0,991 0,990 0,989 0,989 0,988 0,987 0,987 1,0 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,1 1,008 1,009 1,009 1,010 1,011 1,011 1,012 1,012 1,2 1,016 1,017 1,018 1,019 1,020 1,022 1,023 1,024 1,3 1,023 1,025 1,025 1,028 1,030 1,031 1,033 1,035 1,4 1,030 1,032 1,035 1,037 1,039 1,041 1,043 1,045 1,5 1,037 1,040 1,042 1,044 1,047 1,049 1,052 1,054 1,6 1,043 1,046 1,050 1,052 1,055 1,058 1,061 1,064 1,7 1,050 1,053 1,057 1,060 1,065 1,067 1,070 1,074 1,8 1,056 1,059 1,063 1,067 1,071 1,074 1,078 1,082 1,9 1,061 1,065 1,070 1,074 1,078 1,082 1,086 1,091 2,0 1,067 1,071 1,076 1,080 1,085 1,089 1,094 1,099

2.i vi p trn c mt ct chn khng, nh elip (hnh 6-3b) th m ly theo bng 6-9

Bng 6-9. H s lu lng m ca p chn khng, nh elip

(theo ti liu ca Rozanov)

e/f

jrH0

3,0 2,0 1,0

1,0 0,495 0,487 0,486 1,2 0,509 0,500 0,497 1,4 0,520 0,512 0,506 1,6 0,530 0,521 0,513 1,8 0,537 0,531 0,521 2,0 0,544 0,540 0,526 2,2 0,551 0,548 0,533 2,4 0,557 0,554 0,538 2,6 0,562 0,560 0,543 2,8 0,566 0,565 0,549 3,0 0,570 0,569 0,553 3,2 0,575 0,573 0,557 3,4 0,577 0,577 0,560


218

V. Bin php tiu nng v tnh ton tiu nng sau p trn Dng chy sau khi qua p trn xung h lu c nng lng rt ln. Nng lng

c tiu hao bng nhiu dng khc nhau: mt phn nng lng ny ph hoi lng sng v hai bn b gy nn xi l cc b sau p, mt phn tiu hao do ma st ni b dng chy, phn khc do ma st gia nc v khng kh. Sc cn ni b dng chy cng ln th tiu hao nng lng do xi l cng nh v ngc li. V vy ngi ta thng dng bin php tiu hao nng lng bng ma st ni b dng chy gim kh nng xi l dng sng v dng hnh thc phng xa lm cho nc hn hp v ma st vi khng kh c tc dng tiu hao nng lng v gim xi l. t c cc mc ch trn, thng dng cc hnh thc tiu nng sau y: tiu nng dng y (hnh 6-8a) ; tiu nng dng mt (hnh 6-8c) ; tiu nng dng mt ngp ( hnh 6-8d) ; tiu nng phng xa (hnh 6-8e).

a) b)

c) d) e)

Hnh 6-8. Cc hnh thc ni tip dng chy h lu.

Nguyn l c bn ca cc hnh thc tiu nng trn l lm cho dng chy tiu hao bng ma st ni b dng chy, ph hoi kt cu dng chy bng xo trn vi khng kh, khuch tn theo phng ng v gim lu lng n v. Cc hnh thc tiu nng c lin quan ln nhau. Khi mc nc h lu thay i cc hnh thc c th chuyn ho ln nhau.

1. Tiu nng dng y: c im tiu nng dng y l li dng sc cn ni b ca nc nhy, l hnh thc thng dng nht. iu kin c bn ca hnh thc tiu nng ny l chiu su nc h lu phi ln hn chiu su lin hp th hai ca nc nhy hh > hc

'' m bo sinh nc nhy ngp v tiu nng tp trung.

Trong tiu nng y, lu tc y rt ln, mch ng mnh lit, c kh nng xi l, v th trong khu vc nc nhy (sn sau) phi bo v bng b tng. Khi nn xu, on ni tip theo sau sn sau (sn sau th hai) cn c bo v thch ng. Mun tng hiu qu tiu nng th thng trn sn sau thng xy thm cc thit b tiu nng ph nh m, ngng, v.v.... cho s xung kch ni b dng chy cng mnh lit v ma st gia dng chy vi cc thit b cng c th tiu hao mt phn nng lng. Bin php ny c hiu qu tt v c ng dng rng ri. Tiu nng dng y thng c dng vi ct nc thp, a cht nn tng i km.Khi ct nc cao, hc

" rt ln, yu cu chiu su nc h lu ln, nh vy phi o su sn sau v cn c bo v kin c hn. Lc , hnh thc tiu nng y thng khng kinh t.

hh

h

hh

c

a aa


219

Sau y s trnh by phng php tnh chiu su co hp hc ca dng chy, chiu su

lin hip hc" v chiu di nc nhy ln ca n. hnh 6-9, mt phng so snh O-O c tnh

t y h lu. Tn tht ct nc hf t mt ct 1-1 n mt ct 2-2 ( tc l mt ct co hp C - C) biu th bng biu thc :

g2

vh

2c

f x= , (6-11)

trong : vc - lu tc trung bnh mt ct co hp ;

x - h s tn tht tng ng ;

a) b)

Hnh 6-9. S tnh tiu nng dng y.

T phng trnh Bernouilli vit cho cc mt ct 1 -1 v 2-2 rt ra :

)hE(g211v c0c -

x+=

hoc )hE(g2v c0cc -j= (6-12)

Mt khc ta c th vit:

c

cQvw

= , (6-13)

trong : wc - din tch mt ct co hp: wc = f(hc). Gii h phng trnh (6-12) v (6-13) ta s c hai n hc v vc . H phng trnh c

th s dng xc nh hc trong c hai trng hp p c ca van v khng c ca van (hnh 6-9a,b), cng nh i vi lng sng c mt ct bt k.

Sau y ta xt mt s trng hp.

* i vi lng sng hnh thang, ta c.

)mhb(h ccc +=w ; (6-14)

)hE(g2)mhb(hQ ccccc -+j= ; (6-15)

C

C

OO

C

v

ec

C'

C'

2gv2o

e

1

c

1

HH

EE

v

hc

2

2

hh

C

O

E E

1

1

H H

h

C2

h h O

2 CC

h

ccc

o

oo

th

o o

2gv2o

vo


220

trong :

m - h s mi dc ca sng ;

b - chiu rng y sng.

Dng phng trnh (6-15) tnh hc bng phng php th dn hay bng phng php tra bng ca A. N. Rakhmanov.

* i vi lng sng ch nht:

Q= qb ; cc bh=w ;

c

c hqv = . (6-16)

Thay biu thc (6-16) vo biu thc (6-12) ta c

)hE(g2hq

ccc

-j= ; (6-17)

hoc:

2c

2c

2

co gh2qhE

j+= . (6-18)

Gii phng trnh (6-18) bng phng php th dn. Nhng phng trnh (6-18) l phng trnh bc ba nn c ba p s, ta phi chn tr s thc ca hc tho mn iu kin :

0 < hc < hpg , (6-19)

trong : hpg - chiu su phn gii.

Khi E0 rt ln, c th b qua s hng u v phi ca (6-18) v biu thc tnh hc c dng n gin hn :

0c

cgE2

qhj

= (6-20)

Biu thc (6-20) ch s dng khi :

5hE

pg

o (6-21)

Tnh hc vi lng sng ch nht c th dng phng php tra bng ca N.N.Pavlopski m trong cc sch thu lc gii thiu.

Tr jc trong cc phng trnh (6-12), (6-15), (6-18) v (6-20) xc nh nh sau:

- i vi trng hp hnh 6-9a : jc = 0,85 0,95 ;

- i vi trng hp hnh 6-9b : jc = 0,90 0,98 ;

- i vi p c mt trn khng c trn : jc = 0,80 0,90 ;

Trng hp p cao, ly jc nh hn cc tr s trn. Xc nh chiu su lin hp hc

" ca hc. Vi iu kin bi ton phng, mt ct lng sng ch nht, hc

" c tnh theo biu thc:


221

-+= 1

ghq81

2h

h 3c

2c''

c (6-22)

p trn thng c nhiu ca, c lc ch cn m mt hoc mt s ca hoc lc chiu rng B ca p trn nh hn chiu rng L ca lng sng h lu, lc cn tnh theo bi ton khng gian. Khi L > 5B, chiu su lin hp th hai ca nc nhy (hc

") trong iu kin khng gian c tnh gn ng theo M.Z.Abramov :

''ckg''c h3

2)h( (6-23)

trong : hc'' - chiu su lin hp th hai tnh theo bi ton phng.

.I. Kumin ngh tnh (hc")kg tng i chnh xc hn :

2

A2

A 222

221

1

2 x+

x=

x+

x, (6-24)

trong : pg

c1 h

h=x ;

pg

kg''c

2 h)h(

=x ;

LBA = nu

n5,111

LB

+> ;

n

A5,11

1+

= nu n5,11

1LB

+< ;

B

hn pg= ;

hpg - chiu su phn gii.

Chiu di nc nhy ln c th tnh theo biu thc thc nghim sau y:

ln = 5(hc'' - hc) , (6-25)

hoc: ln = 4,5(hh - hc), (6-26)

trong : hh - chiu su nc h lu.

Ni chung sau p trn phi m bo c nc nhy ngp hc'' < hh ; nu khng ngi ta

dng bin php nh o b, xy tng hoc b tng kt hp v cc thit b tiu nng khc, v.v... to thnh nc nhy ngp. Sau y gii thiu mt s bin php v phng php tnh ton.

a. B tiu nng. S tnh ton b tiu nng c biu th (hnh 6-10) v c xem xt trong bi ton phng.

1. Gi thit chiu su l thuyt d0 ca b tiu nng.

2. Tm E0' :

E0' = E0 + d0 , (6-27)

3. Tnh c chiu su co hp tng ng hc' .

4. Tnh chiu su lin hip (hc) ca hc

' .


222

Hnh 6-10. S tnh ton b tiu nng.

5. Sau khi c b, dng chy cui b ging nh p trn nh rng chy ngp c h thp ct nc Z' ca dng chy.

Sau khi xc nh c (hc") v cho rng hc

"=hh' , ta c th tm c Z' :

Z' = (hc") - d0 - hh , (6-28)

6. Xc nh lu tc tin gn v0' cui b tiu nng:

'"c

t'0 )h(

qv = , (6-29)

trong : BQq t = ; (6-30)

qt - lu lng n v qua p trn ;

Q - lu lng qua p trn ;

B - chiu rng p trn.

7. Tnh h thp ct nc sau ngng trn (cui b) Z0' c k n lu tc ti gn :

g2v

ZZ2'0''

0a

+= . (6-31)

8.Theo biu thc tnh lu lng qua p trn nh rng chy ngp, xc nh lu lng n v cui b tiu nng theo biu thc :

'0h gZ2hq j= . (6-32)

trong : j - h s lu tc ( j 0,95). 9. Tip tc gi thit cc tr s d0 khc v tnh ton theo cc bc nh trn, tm c cc

tr s ca q tng ng theo biu thc :

q = f(d0). (6-33)

ng quan h q = f (d0) v c nh hnh 6-11a. R rng, trn th ng vi lu lng n v qt qua p trn, ta xc nh c su (d0)t cn tm ca b tiu nng.

n gin tnh ton, c th b qua h thp ct nc Z' v cho rng, mc nc trong b bng mc nc h lu, tc l :

'hh = hh + d0 . (6-34)

O'

EE'

1

1

C

C

h

O h"

Z'Z'

hd

hov

q

qt

C

o

vo

oo

t

c

co

O'

O


223

Trc ht theo biu thc (6-34), gi thit cc tr s d0 v tm c cc tr s hh' tng

ng. V ng quan h hh' = f1(do) (hnh 6-11b). Nh vy, mi tr s ca d0 v theo phng

php tnh nu trn, ta hon ton xc nh c quan h (hc") = f2(d0). Hai ng cong

(hnh 6-11) ct nhau ti mt im. im cho ta su (d0)t cn tm ca b.

Cn ch rng, chiu su ln nht ca b tiu nng khng phi nhn c tng ng vi lu lng ln nht chy qua p trn, m tng ng vi lu lng tnh Qtt no . Do , khi thit k ta phi tnh vi nhiu tr s lu lng khc nhau v tm c quan h d0 = f(Q). Trn th (hnh 6-11c), d dng tm thy chiu su cn tm (d0)t ca b (chiu su ln nht) v Qtt .

a) b) c)

Hnh 6-11. Cc ng quan h xc nh su ca b tiu nng.

Chiu su ca b c tnh ton nh trn gi l chiu su theo l thuyt ca b tiu nng. Chiu su ng vi mc ngp A = 1 (hnh 6-10). Trong thit k, chng ta phi tm chiu su cn thit d ca b c nc nhy ngp vi mc ngp A = 1,05 1,10 :

d= (1,05 1,10)d0 + (0,05 0,10)hh ; (6-35) b. Tng tiu nng. Do iu kin kt cu v thi cng, khi lm b tiu nng khng thch hp th nn dng tng tiu nng. S tnh ton nh (hnh 6-12), trong C0 l chiu cao l thuyt ca tng tiu nng. Tng tiu nng lm vic nh mt p trn v trng thi chy qua tng l :

- chy ngp nu 'nh > 0 ;

- chy khng ngp nu 'nh < 0 ;

iu kin ngp c xc nh ging nh p trn thnh mng. Ni chung, sau tng tiu nng khng cho php c nc nhy xa. Lu lng n v qua tng tiu nng c xc nh theo biu thc :

2/3' 0''

n Hg2mq s= , (6-36)

trong : g2v

HH2'0''

0a

+= ; (6-37)

=s '

o

'n'

n Hhf ; (6-38)

q

d

q=f(d )

q

(d )

t

t

o

o

o Q tt

d =f(Q)

Q

(d )to

do

o

(h" )'chh '

(h" )'=f (d

)c

do

(d )to

2o

(h' ) =

f (d )

h

1o


224

Hnh 6-12. S tnh ton tng tiu nng.

Tnh ton tng tiu nng n gin hn tnh ton b tiu nng v khi tnh ton b vi s thay i d0 th cc tr s hc v hc

" khng thay i, cn tng tiu nng khi C0 thay i th cc tr s hc v hc

" thay i (E0 = const). Khi xc nh chiu cao l thuyt C0 ca tng tiu nng, cn phn bit hai trng hp dng chy qua tng l chy ngp v khng ngp. Khi dng chy qua tng tiu nng khng ngp th c th tnh C0 mt cch trc tip nh sau:

1. Bit E0 v lu lng n v qua p BQq t = , xc nh hc ;

2. Dng phng trnh nc nhy, xc nh hc" ;

3. Tnh lu tc ti gn "c

"c h

qv = , do tm c ct nc lu tc g2v 2'0a ;

4. Gi thit q trong biu thc (6-36) bng qt , cho sn' = 1 v cn c vo biu thc

tm H0' :

3/2

't'

o g2mq

H

= ; (6-39)

trong : m' - h s lu lng ; m' 0,40 0,44 ( thng ly m ' 0,42 ). 5. Bit H0

' ta tnh c H' trn nh tng :

g2v

HH2'o

0' a-= ; (6-40)

6. Cui cng xc nh c C0 :

C0 = "ch - H

' , (6-41)

Khi dng chy qua tng tiu nng l chy ngp th tnh C0 phi th dn v bng th.

Trc ht, tng t trn, ta xc nh trc tip cc tr s q t , hc , hc" , v0

' , g2v 2'0a ; sau

tip tc cc bc sau y :

1. Gi thit C0 ;

EE

O

tq

Z'

h"h'n

C

h CC

O

H'

hh

o

c

vo

v'o

2g(v' )2o q

co


225

2. Tnh H' = hc" - C0 v H0

' = H' + g2v 2'0a ;

3. Tnh 'nh = hn - C0 ;

4. Xc nh tr s ngp s'n theo ng cong H hnh 6-5, ph thuc vo t s h'n /H

'0

( m bo an ton nn thng ly s'n ln hn tr s xc nh c theo ng cong II ) ; 5. Xc nh lu lng n v q theo biu thc (6-36).

Tip tc thit cc tr s C0 khc v thc hin cc bc nh trn, cui cng ta tm c quan h q = f(C0), biu th nh hnh 6-13.

Lu lng n v q qua tng phi bng lu lng n v qt qua p trn :

q = qt (2-42)

Do chng ta xc nh c chiu cao cn tm (C0)t ng vi qt (hnh 6-13)

.

Hnh 6-13. ng quan h q = f(C0) xc nh chiu cao tng tiu nng.

Cn ch rng, trc khi tnh ton chng ta cha bit c dng chy qua tng tiu nng l ngp hay khng ngp. Do , trc tin gi thit l dng chy khng ngp. Sau cn c vo cc yu t tm c m kim ta li thc t c phi l chy khng ngp khng. Nu l dng chy ngp th chng ta cn lp li t u tnh ton tng theo dng chy ngp.

Chiu cao tng Co cng ging nh chiu su b do cn c tnh vi nhiu cp lu lng khc nhau cui cng tm c C0 ln nht. Sau khi xc nh kch thc ca tng, cn phi th li sau tng c nc nhy xa na khng, nu c th cn phi thit k thm tng tiu nng th hai, v.v...

Chiu cao C0 c tnh ton trn y l chiu cao l thuyt ca tng. Chiu cao thc t C ca tng tiu nng c xc nh theo biu thc :

C = C0 + (0,05 0,10 )"ch (6-43)

c. Xc nh chiu di sn sau cho trng hp c b tiu nng v tng tiu nng.

Hnh 6-14 l s dng chy sn sau trong cc trng hp khc nhau.

Trng hp 1: c b hoc tng sau p trn mt ct thc dng (hnh 6-14 a,b), chiu di ca sn sau Ls c tnh t mt ct co hp C - C ngay st chn p trn.

Trng hp 2 : c b hoc tng sau p trn thnh mng (hnh 6-14 c, d)

Chiu di sn sau Ls khng phi bt u t mt ct C - C m n bao gm mt on l0 trc mt ct C - C (l0 chiu di nc ri). Trng hp ny, khi tnh ton cn phi k n chiu di nc ri.

t(C )

q

C

qt

o

q=f(C )o

o


226

a) b)

c) d)

Hnh 6-14. S dng chy khi c b hoc tng tiu nng.

y chng ta ch xem xt cho trng hp 1. Chiu di sn sau trong trng hp ny c th vit nh sau:

'ns llL += , (6-44)

trong : ln - Chiu di nc chy t do khng ngp;

l ' - Chiu di nc xon h lu.

Th nghim cho thy rng biu thc (6-44) hon ton nhn c t l thuyt trong thc t trn sn sau khi c b hoc tng tiu nng s hnh thnh nc nhy khng t do, nn chiu di ca n nh hn chiu di nc nhy t do. Nhiu tc gi ngh tnh L s v cc kt qu tnh c nh hn tnh theo biu thc (6-44). c bit M. . Tstouxov ngh xc nh Ls theo cng thc kinh nghim.

Ls = b.ln (6-45) trong :

ln - chiu di nc nhy t do khng ngp, c tnh theo biu thc(6-25) hoc biu thc (6-26) ;

b- h s thc nghim, ly bng 0,7 0,8. Cng cn phi ch thm rng, chiu di sn sau phi di, nu ngn qu s khng

hnh thnh nc chy v a n xi l h lu.

Hnh dng b tiu nng trong mt phng thng ng l hnh ch nht (hnh 6-15a) th hiu qu tiu nng tt. Nhng dng chy c th bo mn cc cnh gc, nht l khi trong nc c nhiu bn ct, nn thng thit k b c dng hnh thang (hnh 6-15b). Khuyt im ca b tiu nng l khi lng o ln , cao trnh y p thp nn khi lng p tng; v th ngi ta dng b v tng kt hp (hnh 6-15c) gim khi lng o v khi lng p.

h

h

C

C

d

H'

L

l l'Z'

H

h

n

s

c

l

L

h

C

H

C

C

H'

l'

hh

s

c

n

L

h

C

H

C

d

H' h

Z'

l

h

c

s

o

Ll

H'

Ch

C

H

C

hh

s

o

c


227

a) b)

c)

Hnh 6-15. Cc dng b v tng tiu nng

Hnh dng tng tiu nng lm thnh mt ct trn v thun trnh ph hoi do bo mn, hay kh thc (hnh 6-16).

Hnh 6-16. Tng tiu nng

d. Cc thit b tiu nng trn sn sau. Trn sn sau thng b tr cc thit b tiu hao nng lng dng chy nh m, ngng v.v... (hnh 6-17) lm cho dng gy nn lc phn kch v gim c hc

" , rt ngn c chiu di sn sau. Th nghim chng minh rng nu b tr thch hp cc thit b th c th gim c hc

" t 20-25%, c khi n 30%.

a) b)

c) d)

Hnh 6-17. Hnh thc cc thit b tiu nng (kch thc trong hnh ghi theo m ).

dhh

ll no

dhh

hd

hCo

P L

C

105

5

37

3

P

CC/2

12

22

2

P

7,5

2 ,5

15

35

B

C

1: 0,75

111,0103,0

125,0

134,0

C

P

X

L


228

Phn tch tnh hnh dng chy khi c thit b tiu nng trn sn sau (hnh 6-18) v vit phng trnh ng lng cho hai mt ct 1-1 v 2-2, ta c:

,2h

Rqvg2

hqv

g

22

20

21

c0 g++

ga=

g+

ga (6-46)

trong :

R - phn lc ca thit b tiu nng ;

C - h s, ph thuc vo tnh hnh dng chy v hnh dng m tiu nng xc nh bng th nghim ;

w - din tch hnh chiu ng ca m tiu nng ; q - lu lng n v ;

ao - h s phn b c th ly bng 1. a) b)

Hnh 6-18. S tnh hnh dng chy khi c thit b tiu nng trn sn sau.

Sau khi c thit b tiu nng, do tng xung lng mt ct 2-2, gim c chiu su lin hip nc nhy, tc l gim c su o b tiu nng hoc gim chiu cao tng tiu nng. Trong iu kin nh nhau, m bo ngp ging nhau, nhng nh c thit b tiu nng trn sn sau nn hnh 6-18b gim c chiu su o b mt tr s l d0 = h2

" - h2 so vi hnh 6-18a.

Thit b tiu nng thng b tr nhng ni c lu tc ln nn xung quanh m d sinh p lc m. Lu tc cng ln, nu m khng thun th p lc m cng ln , gy ln kh thc, ph hoi btng lm cho iu kin lm vic ca thit b tiu nng khng tt. Sau y s nu mt s hnh thc, kch thc v b tr thit b tiu nng.

a) Ngng tiu nng (hnh 6-17a). Ngng tiu nng ngp trong nc nhy c tc dng phn kch mnh i vi dng chy c lu tc cao, gim chiu su nc nhy hc

" . Th nghim cho thy rng, gc nghing mi thng lu ca ngng nh hn 900 v phi ln hn 600 th khng c nh hng ti hiu qu tiu nng, nhng ci thin c trng thi dng chy rt ln. Mun tng lc phn kch th cn tng chiu cao ngng vi nguyn tc l khng sinh ra nc nhy sau ngng. Chiu cao ngng c th ly bng 1,9 (hc

- hh). V tr ca ngng nn t chnh gia chiu di sn sau. t gn pha trc th lc phn kch R ln hn, nhng dng chy bin ng ln. t gn pha sau th mc ngp ca nc nhy km, c khi khng ngp.

b) M tiu nng (hnh 6-17b, c, d). M tiu nng thng b tr gn ni bt u ca sn sau, ti khu vc dng chy c lu tc cao, cch chn p mt on di hn chiu su phn gii ca dng chy. Kch thc v v tr m tiu nng c nh hng ln i vi dng chy, nhng cho n nay cha c phng php no tnh ton chnh xc, thng phi thng qua th nghim quyt nh. Theo th nghim, kch thc m c th ly nh sau: chiu cao m dn

h'2t

d

th h

2

2

1

1

o 12


229

( 0,75 1,0) hc, chiu rng m bn =(0,5 1,0) dn, khong cch Bn gia mp ca hai m gn nhau Bn bn ; kch thc c th cn ph thuc vo hnh thc v cu to .

Nu b tr hai hng m th hiu qu tiu nng tt hn so vi b tr mt hng khong cch gia hai hng m Ln ( 2 3) dn b tr cc m theo hnh hoa mai chn s hnh m cn ph thuc vo hnh thc m c lc b tr hai hng lu tc phn b khng c tt. C nhiu hnh thc m tiu nng (hnh 619): mt thng lu ca m thng ng (hnh 619a) gy nn lc phn kch ln hn so vi m c mt thng nghing, nhng dng chy bin ng ln. Hiu qu tiu nng ca loi nh hnh 6 -19d khng tt bng cc loi khc (hnh 619a,b,c). Mt hai bn ca m l thng ng th d sinh p lc m rt ln, gy nn kh thc. Mun gim hoc tiu tr p lc m, thng cc gc cnh ca m lm thnh gc trn(hnh 6 19a,b) hoc hnh thc khuch tn (hnh 6 -19c,d) lm nh vy tuy t nhiu c gim hiu qu tiu nng, nhng rt cn thit. Mi thng lu ca m nghing c th gim c p lc m nh m.

a) b) c) d)

Hnh 6-19. Cc hnh thc m tiu nng.

g) M phn dng (hnh 6 - 20): dng m phn dng c th lm cho dng chy c lu tc cao chn p to thnh trng thi dng chy c li. gia cc m phn dng c dng chy y v trn mt c xoy, trn cc m phn dng c dng chy mt v st nh m c xoy, hai loi dng chy y tc dng tng h nhau c th tiu hao nng lng nhiu hn.

Ni chung sau m phn dng nn c m tiu nng (hnh 6 -17d). Do gia cc m phn dng chy tp trung, sau gp tc dng phn kch ca m tiu nng cng lm cho hiu qu tiu nng tng thm.

Hnh 6-20. M phn dng.

e. M rng dn sn sau (hnh 6 -21) nu sn sau c m rng dn th dng chy c khuch tn sang hai bn gim c lu lng n v, do gim c hc

, nh vy nng cao c cao trnh sn sau.

Gc khuch tn b khng nn ln qu, nu ln qu th dng chy b tch khi tng bn v to nn dng xoy hoc dng chy xin gy xi l:

dh

H'

b

b

b

b

b

1c

1

1

1


230

101

81tg


231

Trong thc t, quan h gia lu lng v ct nc h lu Q ~ hc thng xut hin

sn sau nh hnh 6- 23.

- ng cong I: Vi lu lng bt k u c

nc nhy xa. Trng hp ny cn dng b hoc

tng tiu nng hoc b tng kt hp cho

ng cong I lun lun nm di ng Q~ hh.

- ng cong II: Nc h lu rt su, c

nc nhy ngp ln, c th lm sn sau dc

thun hoc lm ngng cui sn sau hoc c

th tng lu lng n v qua p .

- ng cong III: Lc lu lng nh khng c nc chy ngp, cn c thit b tng chn h lu dng mc nc sn sau. Nh vy khi lu lng nc ln c th tng ngp, nhng do tc dng ca tng nn khng sinh dng ngp ln.

- ng cong IV: ngc li vi trng hp III, dng cc thit b m tiu nng hoc ngng tiu nng ci thin tnh hnh dng chy.

2. Hnh thc tiu nng mt:

Dng chy hnh thc tiu nng ny trng thi chy mt (hnh 6-8 c,d). Kinh nghim cho bit, hiu qu tiu nng trong hnh thc tiu nng mt so vi hnh thc tiu nng y

khng km nhiu (c th t 65%), nhng chiu di sn sau ngn hn 51

21

ln, ng thi

lu tc y nh nn chiu dy sn sau c th b, thm ch trn nn cng khng cn lm sn sau. Ngoi ra cn c u im l c th tho cc vt ni qua p m khng hng sn sau.

Hnh 6-24. Trng thi dng chy h lu p trn c bc tht.

Khi mc nc h lu thay i, trng thi dng chy h lu p trn c bc tht (hnh 6-24) khc nhau. Sau y ta s phn bit trng thi dng chy .

-Trng thi th nht : khi mc nc h lu thp hn nh bc tht, tc l hh < a, dng chy h lu l dng chy phng xa (hnh 6-8e).

h"

0Q

hh

Q ~hh

II

I

III

IV

c

Hnh 6-23. Quan h Q ~ "ch v Q ~ hh

a

R

E E

vo

hhC

i = 0

q

h

o


232

-Trng thi th hai : khi ct nc h lu hh nh hn su gii hn th nht hghI : dng chy trng thi chy y (hnh 6-8b), lc c th l nc nhy ngp hoc nhy xa tu

theo "ch v hh.

hh < hghI , (6-50)

-Trng thi th ba : gi l dng chy mt khng ngp, khi ct nc h lu trng thi gia su gii hn th nht hghI v su gii hn th hai hgh2 (hnh 6-8c) :

hghI < hh < hghII , (6-51)

-Trng thi th t : dng chy mt ngp, khi ct nc h lu ln hn su gii hn th hai (hnh 6-8d) :

hh > hghII (6-52)

Trng thi th nht s c trnh by mc V-3 6-2 v tiu nng phng xa, cn trng thi th hai trnh by mc V-1 6-2 ni v tiu nng y, y ni v trng thi dng chy th ba v th t. Khi hh = hghI dng chy t trng thi y chuyn sang dng chy mt khng ngp. Khi hh = hghII dng chy t trng thi chy mt khng ngp chuyn sang dng chy mt ngp.

a)Hnh thc dng mt khng ngp. Nh trn trnh by, dng chy trng thi ny xy ra khi hghI < hh < hghII , lc su nc h lu hh cn phi ln hn hc

ca nc nhy y, ng thi hh > a (a-chiu cao bc tht, xem hnh 6-24). Chiu cao a =(0,25 0,35) chiu cao p ( vi p cao a= 0,25 ; vi p thp a=0,35).

Gc nghing q chn p ln hay nh c nh hng rt ln n trng thi dng chy. Nu q ln qu c th sinh chy phng xa, q nh qu c th xut hin dng chy y. Thng dng q < 10 150 l thch hp.

Thit k hnh thc tiu nng mt khng ngp thch hp vi p trn c tho cc vt ni trnh cc vt ni va chm vo sn sau hoc chn p.

Nhc im ca hnh thc tiu nng ny l lm vic khng n nh khi mc nc h lu thay i ln ; h lu c sng lm nh hng khng tt lm nh hng n s lm vic ca trm thu in, vn ti thu v xi l b sng; yu cu mc nc h lu phi su.

b) Hnh thc dng mt ngp. Khi hh > hghII th c dng mt ngp. Hnh thc ny c nhc im gy lc xung kch ln mi chn p v c nh hng ti n nh p. Sau y ta s ni n su gii hn th nht hghI v th hai hghII .

Kt qu xc nh hghI v hghII bng l thuyt a n nhng biu thc tnh rt phc tp v khi lng tnh ton ln. T.N Axtafitsva ngh xc nh cc tr s bng cc biu thc thc nghim sau y :

pgh

ghI hCa244,2a82,0h

-+= , (6-53)

pgh

ghII hCa55,250,2a22,1h

-+= , (6-54)

trong : hpg - chiu su phn gii ; cc k hiu khc nh hnh 6-24.


233

Cc biu thc (6-53) v (6-54) c tnh vi trng hp khi ca van trn nh p m

hon ton v ct nc trn nh p H hC32

; nhng cng c th tnh gn ng cho

trng hp m ca van vi mt m no . Biu thc (6-54) ch ng vi iu kin

2,0Ca

h

, l trng hp trong thc t thng gp nht. Nu 2,0Ca

h

< , T.N Axtafitsva

ngh dng biu thc :

pgh

ghI hCa744,3a82,0h

-+= (6-55)

Khi thit k tiu nng hnh thc dng chy mt cn ch mt s im sau y:

1. Theo th nghim, khi chiu cao bc tht a nh th c th ch xy ra trng thi chy y m khng sinh ra dng mt.

2. Khi a < 0,2Ch , trng thi ni tip dng chy h lu khng n nh v th chiu cao a khng c nh hn 0,2Ch.

3. Cn ch rng, khi su nc h lu tho mn iu kin (6-52) nhng h lu c th xy ra v dng mt, va dng y, tc l trng thi dng mt st p, trng thi dng y sau p (hnh 6-25).

4. S thay i gc q trong phm vi ( 00 150 ) hu nh khng nh hng n cc tr s hghI v hghII.

3. Hnh thc tiu nng phng xa (hnh 6-8e)

a) c im ca tiu nng phng xa.

Tiu nng phng xa c li dng mi phun chn p h lu dng chy c lu tc ln phng xa khi chn p. Dng chy c khuch tn trong khng kh, sau xung lng sng. Do dng chy c tiu hao nng lng rt ln trong khng kh nn gim nng lc xi lng sng, ng thi dng chy c phng khi chn p tng i xa nn d c gy xi l cc b y sng h lu cng t nh hng nguy hi n an ton ca p.

Tiu nng hnh thc ny bao gm hai qu trnh : mt b phn tiu nng trong khng kh v mt phn tiu nng lng sng. Dng chy cng khuch tn ln trong khng kh v trn ln nhiu khng kh th nng lng tiu hao cng ln, do gim c xi l lng sng h lu.

Dng chy phng xung h lu gy nn xi l, sau khi h xi t n mt su nht nh th nng lng tha ca dng chy c hon ton tiu hao bng ma st ni b, cho nn nu chiu su nc h lu cng ln th cng gim c xi l dng sng.

Chiu di phng xa cng ln, cng c li. i vi p trn cao, chiu di c th n hng trm mt. Tri li, vi p thp, nc s phng gn chn p, nn vic dng hnh thc tiu nng ny b hn ch.

a

H

hh

Hnh 6-25. Trng thi dng mt st p, dng y sau p.


234

Cu to hnh thc tiu nng phng xa n gin, khng cn thit b tiu nng sn sau, gim khi lng o , rt ngn thi gian thi cng. Cho nn i vi p trn c ct nc c ct nc cao v iu kin a cht tt, dng hnh thc tiu nng ny l hp l v kinh t.

b)Hnh thc kt cu tiu nng phng xa.

i vi nhng p trn cao khng nn dng hnh thc tiu nng ngay chn p (tiu nng y, dng mt, v.v...), bi v lu tc rt ln, gy nn hin tng kh thc v mch ng ln. Ti trng ng tc dng ln sn sau tng ln rt nhiu lm nguy him n kt cu sn sau. Ni tip h lu ca cc p trn cao trn nn thch hp nht l hnh thc tiu nng phng xa.

C nhiu hnh thc kt cu mi phun. Sau y gii thiu mt s dng thng gp.

* Mi phun lin tc.

Hnh thc mi phun ny (hnh 6-26a) v c u im l cu to n gin v khong cch phng xa ln, nhng dng chy khuch tn trong khng kh km v xi l dng sng su. C th lm cc tng phn dng ni lin vi cc tr pin ko di n gn mi phun (hnh 6-26b) cho dng chy tp trung trn mt trn v gim tn tht thu lc. Nh vy chiu di dng phun s tng v mc khuch tn dng chy trn mt bng cng c m rng. Thit k mi phun lin tc bao gm cc vn sau y:

1.Gc nghing a ca mi phun c xc nh cn c vo iu kin chiu dng phun xa, ng thi th tch btng chn p tng tng i t. Thng dng a 300 350 l hp l.

2.Cao trnh mi phun phi cao hn mc nc cao nht h lu t nht l 1m m bo cho dng chy phun vo khng kh v trnh nc h lu ngp mi phun.

3.Bn knh cong R ni tip gia mt p v mi phun cn m bo sao cho dng chy khng tch khi mt p v mi phun, trnh hin tng p lc thay i t ngt, ng thi c chiu di dng phun xa. Bn knh R khng c nh hn 6h v khng ln hn 10h (h- chiu su ca nc ti mt ct co hp trn mi phun ).

a) b)

Hnh 6-26. Mi phun lin tc.

* Mi phun khng lin tc (hnh 6-27). Theo ti liu th nghim ca Khoa Thu li Trng i hc Thin Tn Trung Quc, mi phun kiu khng lin tc c ci tin hn loi lin tc. Dng chy trn mi phun khng lin tc c phn thnh hai dng : trn nh rng v gia khe rng. Theo phng thng ng dng chy c khuch tn nhiu hn so vi hnh thc mi phun lin tc, ng thi c s va chm gia cc tia dng nn c th tiu hao mt phn nng lng, gim kh nng xi l h lu, chiu su h xi c th gim c

R=

6h


235

35% so vi hnh thc mi phun lin tc, nhng c nhc im l chiu di phng xa nh hn.

a) b)

Hnh 6-27. Mi phun khng lin tc.

Theo th nghim, kch thc hp l i vi mi phun khng lin tc c rng hnh ch nht (hnh 6-27a) nh sau: a1 a2 5

0 100, t s gia chiu rng khe v chiu rng rng

32

21

ba

. Nu khng nh hng n iu kin khuch tn, nn tn lng gim tr s hd

(h- chiu su nc trn mi phun ) tng chiu su tng i, ng thi gim c p lc

m hai bn rng. Khi lu tc ln hn 20m/s th thng dng 0,1hd5,0 =

(R- bn knh thu lc ca mt ct tnh ton).

2

1

d

ba

35

30

A

A A - A


236

Sau y gii thiu phng php xc nh ng mt nc v lu tc ti mt mt ct bt k trn mt trn theo quy phm Nga Tnh ton thu lc ca p trn trng lc cao BCH-01-65 .

Hnh 6-28. S tnh ton thu lc cho p trn

1.Trng hp gc nghing q mi h lu p c cotgq = 0,7 0,8 v dng chy trn mt p khng c hm kh ( Fr < 45), ta c phng trnh :

g2

vcoshyT 2

i

2i

iioi j+q+= , (6-58)

trong :Toi - khong cch t mc nc thng lu n mt phng so snh ;

yi - to ca mt ct so vi mt phng so snh ;

hi,vi - chiu su v lu tc dng chy ti mt ct tnh ton ;

ji - h s lu tc.

H s lu tc ji ti mt ct bt k trn mt p c xc nh theo biu hnh 6-29, ph thuc vo lu lng n v q v khong cch l theo mt p k t nh p n mt ct tnh ton.

Hnh 6-29. Quan h gia h s lu lng j trn mt trn vi lu lng n v q v

khong cch l theo mt p k t nh p n mt ct tnh ton.

Mun tm chiu su v lu tc ti mt ct bt k trn mt p ta da vo phng trnh (6-59). Trc ht da vo s mt ct p v v tr tnh ton c th tm c T0i v yi, sau

L t

lo ayVX o

R

CB

A

H

PT

h

ZZ

g11

1

q

o

050 100 150 200 250 l (m)

10

20

30

40

50

60

70

80

90

100

=0.65

=0.72=0.75=0.78

q (m /s.m)

=0.68=0.70

=0.80

=0.82

=0.85

=0.88

=0.90

=0.

92

=0.

95

=0.

98

3

=0.

99


237

da vo biu hnh 6-29, tra c ij . Tnh hi v vi bng phng php th dn (gi

thit hi v tnh viih

q= ), thay hi v vi vo phng trnh (6-59), nu tho mn l ng, nu

khng th cn gi thit li hi tnh li. Hnh 6-29 cho ta xc nh c h s lu tc ij ti

mt mt ct bt k trn mt p, ring mt xc nh h s lu tc 0j ti mi khoan phun

cng trong trng hp cotg q = 0,7 0,8 v dng chy khng c hm kh th da vo biu (hnh 6-30).

2.Trng hp cotgq 0,7 0,8 v dng chy trn mt p c hm kh (Fr 45), xc nh ng mt nc theo phng trnh Bernuoilli (hoc bng phng php Tsanomxki) :

g2

v.h

lg2

vcoshyg2

vcoshy2tb

tb

ii

21i

1i1i

2i

iiD

l++q+=+q+ +++ ; (6-59)

q = hivi ; (6-60)

trong :

hi , vi , yi , hi+1 , vi+1 v yi+1 - chiu su, lu tc dng chy v to so vi mt phng so snh ca hai mt ct cch nhau mt on l Dli ;

li - h s cn trong on Dli ;

htb ,vtb - chiu su trung bnh v lu tc trung bnh trong on Dli ;

2vvv 1iitb +

+= ,

tb

tb vqh = ,

Tr s li c xc nh theo biu thc :

25,4Rlg41

i

+D

=l

; (6-61)

trong : R- bn knh thu lc ;

D - nhm tng i . Trn mt trn c kh nng xut hin chn khng, nn mt p thng lm bng btng

tng i nhn, tr s D 1,5mm. Khi xc nh lu tc on cong ca mi phun c bn knh cong R1 th trong phng

trnh (6-60) cn k n nh hng ca lc ly tm, tc l bt u t mt ct u tin ca on cong (mt ct i qua im B hnh 6-28) n mt ct thp nht ca on cong (mt ct i qua im C hnh 6-28) :

g2v.

1hR2

2g2

v.h

lcoshyg2

vcoshy2tb

fb

1

2tb

tb

ii1i1i

2i

ii

-+

Dl+q+=+q+ ++ ; (6-62)


238

Hnh 6-30. Quan h gia h s lu tc j0 ti mi phun vi lu lng n v q v chiu di S theo mt p k t nh p n mi phun.

6.3 p trn tho l kt hp x su

I. Kh nng tho nc. Kh nng tho nc ca p trn kt hp x y bao gm lu lng trn qua nh p

v lu lng tho qua ng ng. Khi c hoc khng c ng x y, lu lng chy nh p trn u khng thay i v tnh theo biu thc (6-56). Cn khi c v khng c p trn ng thi cng lm vic th lu lng tho qua ng x y c khc nhau. Sau y ch yu xem xt kh nng tho nc ca ng ng x y di p trn khi c v khng c nc trn qua nh p trn (hnh 6-31).

Lu lng chy qua ng ng c p xc nh theo biu thc:

gZQ 2mw= ,

Ta xc nh lu lng n v ca ng ng theo hai trng hp sau y :

-Khi khng c nc chy qua p trn (trng hp 1) :

gZaq 21 m= ; (6-63)

-Khi c nc chy qua p trn (trng hp 2) :

'2 2gZaq m= ; (6-64)

trong : m - h s lu lng ph thuc vo tnh cht hnh hc ca ng ng.

x+

ww

a

=m2

1

rr

;

100 150 200 250 300S(m)

10

20

30

40

50

60

70

80

90

100

90

40

20

30

10

50

60

70

80

100

=0.

94

=0.

92=

0.90

=0.

88=

0.85

=0.8

2 =0.

80

=0.7

8=

0.75

=0.72

=0.70

=0.68

=0.65

=0.62

=0.60

q (m /s.m)3 q (m /s.m)3

jojo

jojoj o

j o

j o

j oj o

j oj oj o j o

j oj o


239

a- chiu cao ca mt ct ;

Z v Z' - ct nc tc dng, tc l chnh ct nc o p hai mt ct 1-1 v 2-2 (hnh 6-31) trong hai trng hp trn.

Do nh hng ca nc chy qua p trn, ct nc o p ti mt ct 2-2 s ln hn so vi khi khng c nc chy qua p trn, ngha l Z' < Z v do q2 < q1 .

Hnh 6-31. S tnh ton kh nng tho nc ca p trn kt hp x y.

Nh vy, mun xc nh kh nng tho nc ca ng ng x y thao cc biu thc (6-64) v (6-65), chng ta cn xc nh Z v Z'. Vit phng trinh Bernouilli cho hai mt ct 1-1 v 2-2 (hnh 6-31):

wo h

g

vh

g

vE +

a+=+

22

20

2

, (6-65)

trong : h - ct nc o p ti mt ct 2-2 ;

hw - tng tn tht ct nc ca ng ng ;

whg

v+

a2

20 - ct nc tc dng ;

Theo B.M Tsikvasvili, i vi trng hp 1, ct nc tc dng xc nh theo biu thc :

Z = E0 - h1 , (6-66)

y nu ca ra l mt on thng th : h1 = acosb , Nu ca ra l on cong c bn knh R1 (hnh 6-31) th cn k n thnh phn ph do

nh hng ca lc ly tm :

+

22

1

1

aR

RdR

gR

v ,

Gi thit v = const, biu thc (2-67) c th vit nh sau:

+

--= 22

01

1 2cos

aR

RdR

g

vaEZ ,

hay l:

D

DD


240

.2lncos1

2

0 R

aR

g

vaEZ

+--= (6-67)

Theo kt qu th nghim ca B.M Tsikvasvili, biu thc (6-67) hon ton ph hp vi thc t, sai s khng vt qu 2 4% .

Trong trng hp 2, ct nc tc dng Z' ca ng ng c p xc nh theo biu thc:

HR

aR

g

vaEHZZ 2lncos'

1

12

0 -+

--=-= , (6-68)

Mun xc nh Z' cn phi bit quy lut thay i ca DH ph thuc vo hnh dng, kch thc v cc yu t thu lc khc.

B.M Tsikvasvili th nghim nhiu m hnh ng ng c hnh dng khc nhau qua thn p trn v kt lun DH l

hm s ph thuc vo a

P ,

1q

q t trong iu

kin bi ton phng v cho biu thc

nghim xc nh Z

H (hnh 6-32).

=

1

,q

q

a

Pf

Z

H t ; (6-69)

trong : qt - lu lng n v mt trn, xc nh theo biu thc (6-56) :

qt = gm 22/3

0H ;

q1 - xc nh theo biu thc (6-63);

P, a - xem hnh 6-31.

Khi bit cc tr s a

P v

1q

q t , t biu hnh 6- 32 tra c Z

H, trong Z tnh

theo biu thc (6-67), do xc nh c HD .

II. Tnh ton ni tip gia dng mt v dng y trn p trn. Chng ta xt s ni tip dng chy trn v x y nh hnh 6-33. Gi thit rng

chuyn ng ca dng chy qua cc mt ct cho tho mn iu kin chuyn ng thay i u.

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

qt

1q0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

ZH

aP =1.1

0.7

0.4a=0P

Hnh 6-32. Biu

=

1

,q

q

a

Pf

Z

H t trong iu

kin bi ton phng.


241

Hnh 6-33. S ni tip gia dng mt v

dng y trn mt p trn.

ng dng l thuyt thay i ng lng i vi cc mt ct 1-1, 2-2 v 3-3 v chiu ln phng ca trc x, cho 030201 a=a=a , b qua lc ma st v thnh phn rt nh ca phn lc, ta c :

-+++-+ sincoscos)2

(cos2 3

22

1

22

2

21 hl

ga

q

gh

qaah

hc

t

0)(cos

2 3

22

23 =

+--

gh

qqh t (6-70)

trong : h1, h3 - chiu su dng chy ti mt ct 1-1 v mt ct 3-3 ;

a - chiu cao ng ng ti mt ct ra ;

h2 - ct nc o p ti mt ct ra :

h2 = E0 - (Z - H ) ; Z v H - xc nh theo biu thc (6-68) v biu hnh 6-32 ; qt , q2 - lu lng n v trn mt v x su ;

lc - chiu di on ni tip, theo th nghim ly bng 5(h1 + a) ;

- gc gia cc hng ca dng chy.

Gii phng trnh (6-70) ta c quan h gia chiu su nc trc v sau khi ni tip gia hai dng chy. Sau khi khai trin phng trnh (6-70), c dng:

0323

33 =+-- EBhAhh (6-71)

trong :

A=5,0

C; C=lcsin ; B= 5,0

D;

E=g

qq t5,0

)( 22+ ;

D = coscos)2

(cos2

22

1

22

2

21

ga

q

gh

qaah

h t ++-+ .

h"h

d

ah

3

3

1

1

h2

h

23

1

cc


242

Sau khi xc nh c chiu su h3, tip chiu su ca phn p trn cn li c th xc nh theo phng trnh thay i u ca cht lng vi lu lng n v l (qt+q2), v tnh c chiu su v lu tc dng chy cui mt trn v h lu p; kt qu cho ta tin hnh tnh ton ni tip h lu p trn kt qu tnh ton trn ph hp vi thc t khi

25,02

q

q t ; cn khi 25,02

Kpg (6-80)

Tr s ca K v Kpg c quy nh cho tng loi vt b dng chy bao quanh nh sau.

1. Ca vo cc cng x su: c trnh by chng 8 ng hm thu cng.

2. Cc mu g gh trn b mt cng trnh tho nc.

a) Cc dng mu g gh c trng.

Do nhiu nguyn nhn khc nhau, trn b mt lng dn c th tn ti cc mu g gh cc b. y l mt trong nhng ngun pht sinh kh ho c th dn n kh thc ph hng thnh lng dn. Mt s dng mu g gh c trng v h s kh ho phn gii (Kpg) ca chng c nu bng 6-12.

b) Kim tra kh nng kh ho dng chy ti cc mu g gh.

kim tra, cn s dng cc cng thc (6-78), (6-79), trong ct nc v lu tc c trng (HT v VT) c xc nh nh sau:

* Ct nc c trng: l su dng chy h hay ct nc o p dng chy c p ti mt ct c mu g gh (tnh n y lng dn) khi khng xt n nhiu ng do mu g gh sinh ra.

* Lu tc c trng: VT =Vy , trong y l khong cch t mt c bn ca thnh lng dn n v tr tnh lu tc cc b Vy .

- Vi bc tht theo chiu dng chy (loi mu s 2, bng 6-12).

y = d + D ; (6-81) - Vi cc dng mu khc (bng 6-12)

y = Zm+ D ; (6-82)

trong : d - chiu dy lp bin ca dng chy ;

D - chiu cao mu nhm trung bnh ca thnh lng dn ; Zm- chiu cao mu g gh cc b.

Tr s d ph thuc vo v tr mt ct, tc gi tr L tnh t u dng chy n v tr mt

ct tnh ton. Quan h

D=

Dd Lf theo kt qu ca bi ton lp bin c th hin trn hnh

6-47.

Tr s Vy c xc nh km theo kt qu ca bi ton lp bin nh sau:

Vy = (v

TBVj

) 21xx ; (6-83)

trong : VTB - lu tc bnh qun mt ct;


253

21,xx - cc hm ph thuc vo Ds

v Dy

(hnh 6-46)

vj - h s biu th quan h gia lu tc bnh qun mt ct v tr s lu tc cc b ln nht trong dng chy khi chiu dy lp bin v dng mt ct ngang ca dng chy cho.

- Vi lng dn c mt ct ngang hnh ch nht c kch thc B x h:

vj = Bh1

{(h- d )( d- 2B )+3ln

2

+Ddd

[ 5ln2)2(lnh2B -Dd

-+Dd

d+

]}; (6-84)

- Vi dng chy c p mt ct trn bn knh r :

vj = 2r1{(r- d )2+

3ln21 2

+Ddd

[ 5ln2)2(lnr4 -Dd

-+Dd

s]}; (6-85)

Bng 6-12. Cc dng mu g gh c trng v tr s Kpg tng ng

No Loi mu S Kpg

1 Bc li theo chiu dng chy (u lp gia c b mt, ch ni ct pha,...)

0,125a0.65

(a tnh bng )

2 Bc tht theo chiu dng chy

1 khi Zm d 4/3

mZ

dkhi Zm < d

3 Nhm u t nhin trn mt vi chiu cao mu bnh qun D

1

4 thoi trn mt phng u (mi hn...)

2,24(Lm/Zm)-0,5

5 Mu li n c c b mt sc cnh (du vt do ni ct pha,...)

2

Z

TV

H

max =V

20<


254

6 Cc mu li cc b ring r (cc hn ct liu ln, u ct thp nh ra,...)

2. khi mp lng trn

3,5 khi mp sc

Hnh 6- 46. Biu quan h )y(f1 D=x ; ( )Dd= f 2 ; )L(f D=D

H

Z

VT T

H T

Z

T V

m

m

101

234568

102

2

34568

103

2

34568

104

2

34568

105

2

34568

106

2

34568

101 2 3 4 5 6 8 10 2 3 4 5 6 2810 2 3 4 5 6 8 10 3 2 3 4 5 6 8 10 4 21-1

10

-210

-510

1 2 3 4 5 6 8 10 2 3 4 5 6 2810 2 3 4 5 6 8 10 3 2 3 4 5 6 8 10 4 21 Y /

L/ 12

=f( )2

=f(

)

L /

=f( )

1Y/


255

Nu kt qu tnh ton cho thy ti mu g gh c kh nng pht sinh kh ho th cn p dng cc bin php chng kh thc (xem mc III).

3. Cc m tiu nng, tng hay m phn dng.

Cc m ny c b tr cui ng tho hay trong b tiu nng ci thin iu kin tiu nng v phn tn dng chy trn mt bng .

Mt s loi m c trng v tr s Kpg ca chng c trnh by trn hnh 6-47.

I

II

III

IV

V

VI

VII

I - Kpg =2,1

II - K1pg =2,1;

K2pg =1,4 1,6.

III - Kpg =1,4 1,6

IV - Kpg =0,9 1,15

V - K1pg =0,70,8 ;

K2pg =0,66 0,75

VI - Kpg =0,7 0,85

VII -Kpg =1,5 1,7

VIII

IX

X

XI

XII

VIII - Kpg =1,45

IX - Kpg =0,95

X - Kpg =1,2

XI - Kpg =1,05

XII - Kpg =0,68 (khi r=0)

Hnh 6-47. S mt s loi m tiu nng, phn dng v tr s Kpg ca chng

C

2,5C

1:11:0,5

1,0C 2,3C

2,5m 1:1

2CC0,5C

C=

2,5m

1,6C2,4C

1:11:0,5

C C C0,

8CC

K1 K2

0,75C

1,75C

C/2

C

C

C=

3,2m

1,1C C 1,1C

2,5C

h

h

1,2C

2C

90

0,43C 0,1C

K

K

m

1

2

C

0 ,25

C

0 ,67C

C C/3

2,5C C

C

3h"

1,23

C1,

23C

0,37

C

C

c

C

0,3C

C

C

C

3h"

5

5

c

C

0,3C

CC

C

3h"

r=0,13C

5

r

c

C

3h"c

90

90

1 20

2 ,35

C

1,75

C

0,3C 4C

C=

3mC

CC

0,75

C1,

25C

0,75

C

VHKhi r=0


256

Khi xc nh h s kh ho hin ti (K), cc gi tr HT v VT ly nh sau:

- Ct nc c trng: HT= Ha+h, trong h l chiu dy lp nc trn nh m. i vi m tiu nng t cui b, ly h=hb c ; vi hb - su nc trong b, c- chiu cao m. i vi m t cc v tr khc, h=f(x/ln), trong x- khong cch t m n mt ct co hp; ln- chiu di nc nhy. Khi x/ln = 0,25, c th ly: h=(0,75 - 0,85)(hb - c).

4. Cc b phn ca bung van.

Bung van l mt b phn ca cng trnh tho nc, ti y c t cc ca van to ra s bin dng t ngt ca dng chy, cng l ni d pht sinh kh ho v kh thc.

a) Kim tra kh ho khi van m hon ton.

Kh ho c th pht sinh mt trc v mt sau ca khe van phng hay bn khe trn tr van cung. Mt s dng khe van in hnh nh trn hnh 6-48.

* H s kh ho phn gii:

- Vi mt trc ca ca khe:

Kpgt = Kpgy[1+0,65( 1tW

- )] Kd ; (6-86)

- Vi mt sau ca khe:

Kpgy = AKWKB[1+0,65( 1tW

- )]; (6-87)

trong : Kpgy - h s kh ho phn gii khi c bc tht (loi mu s 2, bng 6-12).

W - b rng khe;

t - chiu su khe;

Kd - h s sa cha do thay i b rng lng dn pha sau khe van; tr s Kd ph

thuc vo wd

(bng 6-13) ;

A, KW, KB - cc h s, xem hnh 6- 48.

Bng 6-13. Tr s Kd trong cng thc (6-87)

wd

0 0,04 0,08 0,12

Kd 0,55 0,75 0,90 1,00


257

I II

a) Kd ~ d/W

A = 1,2 (1-10 )Wd

0 08,0Wd

b) Kd = 1

c) Kd = 1

A=0,30 R/W 0,3 0,4 0,8 1,7

A 0,5 0,35 0,2 0,08

d) Kd = 1

a=0,2W

b/a 1 2 3 4

A 0,65 0,53 0,27 0,52

a=0,4W

b/a 1 2 3

A 0,35 0,17 0,08

e)

f)

Hnh 6-48. Mt s dng khe van v cc thng s tng ng.

* H s kh ho hin ti: xc nh theo (6-78), trong : VT ly bng lu tc bnh qun ti mt ct c khe van; HT - p lc tuyt i bnh qun ti mt ct c khe van; d - chiu dy lp bin, xc nh theo cc ch dn nu trn.

t d

W

t

W

0,1Wi=

2%

R=0,2W

0,1W

i=12%t

W

R

t0,1W

Wb

a

3 5 7 90

1

2

3

B

tt

KB

B/t0 10 20 30 40

0,2

0,4

0,6

0,8

1,0WK

W/

W

0 1,0 2,0

W/


258

b) Kim tra kh ho khi van m mt phn.

Trong trng hp ny, kh ho c th pht sinh t 2 ngun: s tch dng khi cc b phn c nh ca bung van v s tch dng khi b phn ng ca van. Ngun th nht l nguy him vi mi ch chy di ca van (c p hay khng p, ngp hay khng ngp). Ngun th hai ch nguy him khi chy c p hoc chy ngp khng p.

a) b) c)

Hnh 6-49. Tr s Kpg khi chy tch dng khi ca van

a) Van phng khi chon > 75% : Kpg = 1,6 2,0 ;

b) Van cung t xui : Kpg = 1,2 1,5 ;

c) Van cung t ngc: Mp h lu sc : Kpg = 1,9 2,0 ;

Mp h lu t : Kpg = 1,3 1,6 ;

*Trng hp tch dng khi ca van, tr s Kpg cho trn hnh 6-49. Cn khi tnh tr s K, ly tr s HT trn ng dn gn mt ct h lu van (gn v tr mt ct thu hp); VT ly l lu tc bnh qun ti mt ct co hp sau ca van:

VT = VC =0,95 pgH2 ; (6-88)

trong : Hp l ct nc tnh ton ca van ( h thp ct nc qua van).

*Trng hp tch dng khi khe van, bn khe, bc tht:

- Khi van c b phn kn nc pha sau : Kpg = 1,6 2,2;

- Khi van c b phn kn nc pha trc : Kpg = 1,2 1,6;

- i vi bn khe, bc tht : Kpg 1; c) Kim tra kh ho u cc tr pin trong bung van.

Vi s u tr pin nh trn hnh 6-50, h s kh ho phn gii cho trn bng 6-14.

Hnh 6-50. S u tr pin (mt bng)

H

ha Vo

o

V

h

H

a c

o H

a

h

Vc

o

R

R

R

d/2 b

lo1

2

3


259

Bng 6-14. Tr s Kpg ca cc u tr pin.

u trc dng 3 cung lin hp Thng s hnh dng

u tr dng trn Loi 1 2 3 4

d/l0=l 2,50 1,25 1,00 1,15 2,00

b/d=b 0,125 0,25 0,50 0,40 0,50

d/R1 0,50 5,15 1,48 2,10 9,20

d/R 2 - 1,48 0,70 0,75 1,60

d/R 3 - - 0,15 0,15 0,15

pgK 1,15 0,75 0,22 0,21 0,25

Tr s ca h s kh ho hin ti c tnh vi HT v VT ly ti mt ct lng dn cui on cong ca u tr.

III. Cc gii php phng chng kh thc. 1. Lm ng bin cng trnh thoi.

trnh pht sinh kh ho cn lm ng bin cng trnh thoi tho mn iu kin (6-79). V d lm ca vo cng x su vi cc h s Ks , Kr ln; khng ch g gh cc b (Zm) trn b mt lng dn; ln trn mp cc khe van, bc tht, m tiu nng, tng phn dng ...

Cc bin php loi ny thng dn n kch thc cng trnh ln, yu cu khng ch cht lng thi cng b mt lng dn phi tt.

Trong mt s trng hp, nu vic tho mn iu kin (6-79) dn n kch thc cng trnh qu ln th c th xem xt phng n cho php hnh thnh kh ho nhng khng ch giai on u, kh nng xm thc yu hoc khng c. Khi thay cho iu kin (6-79), ngi ta ch khng ch :

K > 0,85 Kpg (6-89)

Cc b phn cng trnh c ng bin c thit k theo iu kin ny cn c nh k kim tra tnh hnh xm thc v c th sa cha khi thy cn thit.

2. La chn vt liu theo bn kh thc.

Cc gii php loi ny chp nhn s pht sinh kh ho ti nhng v tr m lng dn c ng bao khng thun, nh cc khe van, m tiu nng, phn dng, hay cc khp ni trn mt lng dn m do ln khng u c th to ra cc g gh cc b ln hn mc khng ch ban u.

Vt liu thnh lng dn ti cc vi tr d kin c kh ho cn c chn theo tiu chun bn v kh thc. i vi vt liu btng, khi la chn bn ca n c th s dng cc

quan h nh hnh 6-51: Rb = f(Vng,S). y Rb cng btng (MPa); Vng- lu tc


260

ngng xm thc, ly theo lu tc c trng (VT) ca tng loi vt chy bao (xem mc II); S - hm kh trong nc. Khi khng c ti liu tin cy v tr s ca S, an ton trong tnh ton ly S = 0.

3. Dn khng kh vo vng h p.

Vic dn khng kh trc tip vo cc vng b gim p (pht sinh chn khng) trong lng dn c tc dng lm hn ch mc chn khng, tng hm kh trong nc v do tng c lu tc ngng xm thc (xem hnh 6-51). Cc s tip kh vo dng chy nh sau.

a) i vi bung van (t cc cng x su) :

- Tip kh vo khong khng pha trn dng chy thng qua ng dn kh chnh (ging khng kh).

- Tip kh thng vo cc v tr c tch dng nh khe van, bn khe, ngng y, bc tht. Cc ng tip kh loi ny c b tr ngm trong thnh btng ca lng dn, ni t ng dn kh chnh n cc v tr d kin c tch dng.

b) i vi dc nc, p trn.

Cc dc nc, mt p trn c dng chy lu tc cao thng c ch ng tip khng kh vo dng chy phng kh thc. Cc s tip kh nh trn hnh 6-52.

a) b)

a) Thng kh kiu h b) Thng kh kiu kn

Hnh 6-52. Cc s thit b khng kh trn mt trn, dc nc

a. dng mt nghing y v thnh bn ;

b. dng mt nghing v bc tht y ;

Hnh 6-50. Tr s lu tc ngng xm thc ca cc loi b tng (ti liu th

nghim ca MISI Nga)

10 15 20 25 30 35V

(m/s)10

20

30

40

R (MPa)

S=02%

4%6%

8%

b

ng

L

Z

LT

i= Z/L

A

A

L

LT

Z

A - A

Z

Z


261

dng thng kh kiu h, thng kh s t ng c ht vo nh cc khong h do tch dng thnh bn v y (hnh 6-52a). dng thng kh kiu kn (hnh 6-52b), cn b tr cc ng dn lun t thnh bn xung y lng dn. Ca vo ca cc ng dn ny c b tr tng bn v tr pin nu c, v tr cao hn mc nc ln nht ti mt ct c b tr ng.

c) i vi cc m tiu nng, m phn dng.

Vic tip kh trong trng hp ny c t ng thc hin nh ng dn ni vi kh tri v c cc l thng ra mi ca m tiu nng v tr c chn khng (hnh 6-53).

Hnh 6-53. S cc m phn dng ca ng trn Travc Nga.C ng thng kh ti min chn khng (d1

= 30cm; d2 = 5cm;d3 = 18cm)

Hnh 6-54. S m tiu nng c cc ng thng nc t mt trc ra mt

bn ca m. 8,07,0K;6,0dd

pgv

r ==

4. Dn nc vo vng h p.

V d v gii php loi ny nh trn hnh 6-54. Nc t vng p lc dng mt trc c dn qua h thng ca thng vi vng chn khng trn cc mt bn. T l ti u ca ng knh ca ra v ca vo ca cc ng thng nc l: d r /dv= 0,6.

d 10 d

d

d

200

2 10

860

d

22

3

1

1

drdv

chuong 6-cong trinh thao lu trong than dap(mo)-nguyen phuong mau.pdf

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