Overcoming the shrink-and-swell effect in water level control strategy on industrial boiler-drum

Transcription

1 Standard Scintific Rsarch and Essays Vol1 (1): , Fbruary Rsarch Articl Ovrcoming th shrink-and-swll ffct in watr lvl control stratgy on industrial boilr-drum makhai Lawrnc. Rcivd 07 January 2013; Accptd 21 Fbruary Abstract Powr gnration has bcom an incrasingly comptitiv arna. Th cost of oprating powr plants coms mostly from th ful bill, which runs in th billions of dollars annually. n ordr to minimis th ful bill and maximis pant fficincy, a plant s load-following capability must b optimizd, that is, to follow th dmand of powr closly. A grat dal of attntion must b givn to th controllr that rgulats watr lvl (and stam prssur) at th stam-gnrating unit th boilr-drum. Th control stratgy is complicatd by th non-linar and non-minimum phas charactristics of th boilr-drum. This papr illustrats our work in th optimization of load-following schm by applying a 3-lmnt controllr schm to rgulat th boilr-drum s watr lvl. Th controllr is thn cascadd with anothr fdback loop that rgulats stam prssur. Th rsults from th control schm hav shown considrabl improvmnt ovr a typical boilr-drum s watr lvl control stratgy. Ky words: Shrink-and-swll, control, boilr-drum s, optimization, boilr-drum. NTRODUCTON Th cost of running a MW coal-fird unit can vary btwn USD$75,000 pr day whil th annual ful bill is narly USD$5 billion (Waddington t al., 1987). Waddington t al. (1987) showd that mor than 99% of th cost in powr plant opration coms from ful-in this cas, coal. Th typical componnts of a powr plant ar furnac, boilr-drum, suprhatd and rhatr, and turbin units. Figur 4 illustrats th boilr-drum with th down comr and risr circulation tubs. Th cylindrical drums ar not hatd. Rathr, hat is supplid to th incoming watr in th risr tubs by dirct hat from th furnac gasss. Thr ar a larg numbr of risr tubs in th drum-down comr risr circulating loop in ordr to maximiz hat transfr. Th downcomrs ar largr in siz sinc no hat transfr taks plac. Flow around th circulating loop can b ithr natural du to prssur diffrnc or forcd with pumps. n th dsign of th circuit, it is vry important that sufficint circulation occurs at all tims. Th cost of plant opration is mainly on th ful consumption. n ordr to optimiz opration, powr gnration has to follow powr dmand vry closly, that is, load-following (Rs, 1997). Th complication in th load-following schm is mainly causd by th shrink and swll phnomnon that occurs whn drum prssur changs (DiDomnico t al., 1983; Rs, 1997). Th controllr action tnds to ract ngativly du to th mislading shrink and swll ffct. Th control schm that is adoptd has to countr th ffct of th shrink and swll phnomnon in ordr to optimiz th cost of oprating th powr plant.

2 Stand. Sci. Rs. Essays Lawrnc 020 METHODOLOGY Mod of oprations n ordr to work around th non-linar bhaviour of th plant along its oprating rgion (that is, 0-100% load), it will b subdividd into thr diffrnt rgions-dtrmind by thr diffrnt load lvls. Th valus shown in Tabl 1 ar dtrmind by th powr plant nginrs in such a way that thy rprsnt th most common oprating mods of th boilr-drum. Th PD-basd control mods of th boilr-drum. Th PD-basd control schm will only work if w trat th non-linar systm as bing linar (locally) at ach rgion. Tabl 1. Mod of Oprations Paramtr Low load load Mdium load High load Prssur, p (mpa) Powr Lvl (MW) Opn loop rsponss To illustrat th dynamic bhaviour of th modl, w will simulat th rsponss-to-stp changs in th inputs. Th modl was simulatd using th following paramtrs, basd on Orsund Powr Station in Swdn: m t =,000 kg; m r = 20,000 kg; A d = 20 m 2 ; V d = 40 m 3 ; v r = 37 m 3 ; v dc = 11 m 3 ; V sd0 = 8 m 3 ; C p = 650; C fw = 4.18; k = 25;b =0.3. Th stam tabls usd in th simulation ar approximatd using quadratic approximation. Th quadratic approximation implmntation is givn in Fawnizu (1). As dscribd arlir, th drum variabls ar non-linar. nitial incras of drum watr lvl du to shrink and swll phnomnon. Th comparison of rsponss to 10 kg/stam flow rat chang at diffrnt oprating conditions. Th swlling ffct on th watr lvl is largst at low load, as illustratd by th gratst ovrshoot. This is mainly causd by th gratr variation of stam contribution at low load than at high load. Th non-minimum phas charactristics of th drum watr lvl prov to b non-trivial. Any control schm that w us to control th watr lvl will b affctd by th non-minimum phas charactristics. n addition, th varying snsitivity of th paramtrs at diffrnt oprating condition will impos xtra difficulty to th control dsign. Singl lmnt control Singl lmnt fdwatr control uss only th drum lvl procss variabl as fdback. Th masurd drum lvl is compard to th drum lvl st point. Whn th diffrnc dviats from zro (that is, non-zro rror), th fd watr control valv will b adjustd by th proportional-plus-intgral (P) controllr to compnsat for th rror. Th intgral componnt of th controllr rgulats th drum lvl rror to zro. This control mthod nsurs that th watr lvl stays as clos as possibl to st point valu. This schm prforms satisfactorily undr constant or small changs in load (stam flow) and saturation prssur in th drum. f th stam flow incrass fastr than th hat input, th drum prssur drops quickly and causs th saturation condition to chang rapidly from on stat to anothr (Astrom t al., 0). This phnomnon is known as swll ffct bcaus of th swlling of stam bubbls undrnath watr surfac. This would caus th watr lvl to ris initially. On th contrary, a suddn dcras in stam flow would caus th stam bubbls undrnath watr surfac. This would caus th watr lvl to ris initially. On th contrary, a suddn dcras in stam flow would caus th stam bubbls to shrink and th watr lvl to drop. f th transint is not too svr, th lvl will vntually rturn to th stpoint. n ths circumstancs, mor complicatd control is ncssary. Thr-lmnt control Thr-lmnt control schm (Figur 1) uss fd watr flow masurmnts and stam flow masurmnts as inputs to th controllr in addition to watr lvl fdback signals. This improvd control schm adds prdictability by anticipating chang in load by using stam flow as fd forward and fd watr flow rat as fdback rgulation. Figur 2 shows th comparison btwn singl lmnt control and thr lmnt control rsults on watr lvl and stam prssur. Th improvmnt shown by th improvd control mthod is vry significant. Thr is hardly any oscillation shown by th solid lin, xcpt during th shrink and swll (non-minimum phas) ffct.

3 Lawrnc 021 hat input Lvl + - P P Fdwatr Valu Boilr Drum Modl Stam flow Stam prssur stpoint Watr lvl Figur 1. Thr-lmnt controllr structur. Hat control loop is not shown Drum Watr Lvl, Stam Prssur, P (MPa) Tim (s) Figur 2. Singl-lmnt controllr (dottd) and 3-lmnt controllr(solid). Stp rsponss to 10 kg/s chang of stam flow rat on boilr-drum modl at mdium load nput Fdwatr, wfw (kg/s nput powr, Q (MW) Tim (s) Figur 3. Singl-lmnt controllr (dottd) and 3-lmnt controllr (solid). Stp rsponss to 10 kg/s chang of stam flow rat on boilr-drum modl with dlayd input at mdium Load Figur 3 shows th fdwatr and powr input corrsponding to th watr lvl and stam prssur rsponss in Figur 2. Th thr-lmnt controllr input fdwatr liminats th initial rduction shown by th dottd lin. This ffct is

4 Lawrnc 022 du to th fd forward (prdiction) stam flow signal, which incrass controllr flow vn whn watr lvl is incrasing. Th controllr knows that watr lvl will vntually dcras. Thrfor, it did not ract to th mislading changs in watr lvl. Th oscillation in input fd watr right aftr th stp input is du to th multi-cascadd structur of th controllr fdback and fd forward loops. DSCUSSON Shrink and swll phnomnon Figur 4 shows th simplifid diagram of th boilr-drum and sown comr-risr circulation loop. Whn powr dmand incrass, stam flow rat is rapidly incrasd, causing th stam prssur to drop momntarily. This drop of prssur causs th air bubbls to incras in siz and th watr lvl to incras. Th phnomnon is trmd swll ffct. Th principl of mass balanc, howvr, dictats that th incras of stam flow rat laving th drum will caus rduction of th total mass insid th drum. Thus, by kping th fdwatr input constant, th mass of watr insid th drum will vntually b dcrasd, causing th watr lvl to drop. This is shown in th opn-loop rspons in Figur 5. On th othr hand, if stam dmand is rducd, stam bubbls shrink initially and th watr lvl will vntually incras du to th incrasing mass of watr and stam in th drum. Th combind phnomnon is trmd shrink and swll. FEEDWATER BOLER DRUM STEAM FLOW RSER HEAT DOWNCOMER Figur 4. Drum Downcomr-Risr circulation loop Boilrs-drum dynamics Drum boilr modl (Astrom t al., 0) is formd using th basic thrmodynamics mass and nrgy balanc quations. Blow ar th stat quations of th boilr-drum dynamics. Th four stats ar: drum prssur p, total watr volum v wt, stam quality at th risr outlt sd and volum of stam undr th liquid lvl in th drum v sd. Th quations hav bn drivd using mass and nrgy balancs togthr. Th rsulting physical quations hav thn bn manipulatd into th following form: 11 dv wt + 12 dp = q f q s dt dt 21 dv wt + 22 dp = Q + q f h f q s h s dt dt 32 dp + 33 d r = Q - r h r q dc dt dt 42 dp + 43 d r + 44 dv sd = p s (V 0 sd V sd ) + h f h w q s (1) dt dt dt T d h c Whr th cofficints ij ar givn by: 11 = p w - p s

5 Lawrnc = V wt dp w + V st dp s dp dp 21 = p w h w - p s h s 22 V wt h w dp w + p w dp w + V st h s dp s + p s dp s - V t + m t C p dt s dp dp dp dp dp 32 p w dp w + r h c dp w ( 1- v ) V r + (1- r ) h c dp s + p s dh s v V r dp dp dp dp + (p s + (p w -p s ) r ) h c v r d v - v r + m r C p dt s dp dp 33 = ((1- r ) p s + r p w ) h c v r d v dp 42 V sd dp s + 1 p s V sd dh s + p w V wd dh w v sd + m d C p dt s dp h c dp dp dp + r (1+β) V r v dp s + (1- v ) dp w + (p s p w ) d v dp dp dp dp 43 = r (1+β) (p s -p w ) V r d v d r 44 = p s (2) Th paramtrs notations ar, xampl, pm is spcific dnsity of mtal. Paramtrs V: volum; p: spcific dnsity; u: Spcific intrnal nrgy; h: Spcific Enthalpy; t: Tmpratur; q: mass flow rat; q: hat input; r : stam quality; v : stam volum ratio; C p : spcific hat of mtal; M t : total mass for mtal tub and drum. Subscripts s: Stam; w: watr; f: fdwatr; m: mtal; d: drum; dc: Downcomr; t: total. Modlling with simulink This modl is implmntd in Matlab in th form of an S function, in ordr to b usd in Simulink to build th non-linar systm in block diagrams. Th working S function is givn in Fawnizu t al. (1). Th S-function is thn maskd to bcom a 4 input, 2 output MMO systms shown in Figur 5. Hat input Fdwatr flow Q qf Drum watr lvl Fdwatr tmpratur Stam flow tf qs Stam prssur Boilr-Drum Figur 5. Maskd MMO systm of a drum-boilr Using S function

6 Lawrnc 024 CONCLUSON Th simpl, pur fdback singl-lmnt PD controllr prforms satisfactorily to rgulat watr lvl rror to zro whn oprating lvl stays constant. n today s powr industry, comptition has drivn powr plant oprators to maximiz profit by optimizing powr gnratd-powr dmandd ratio. n ordr to follow th changing dmand, powr plant boilr has to chang its stam production rapidly; thrfor th load-following problm. Thr-lmnt control schm is a good altrnativ to th simplr singl lmnt control structur. Th prformanc shown by Figur 2 indicats how thr-lmnt controllr is a much bttr altrnativ. Rfrncs Astrom K J, Bll R D (0). Drum Boilr Dynamics, Automatica vol. 36 Pp Di Domnico, Ptr N (1983). Practical Application of Fdwatr Controls for a Utility Typ Drum Boilr. Amrican Control Confrnc, San Francisco, Pp Fawnizu A H, Rs N W (1). Drum Watr Lvl Control: A study by simulation. MEngSc thsis submittd to UNSW, Australia. Rs N W (1997).Advancd Powr Plant Control For Larg Load Changs and Disturbancs, FAC/CGRE Symposium on Control of Powr Systms and Powr Plant, Bijing, China. Pp Waddington J, Mapls G C (1987). Th Control of Larg Coal-And Oil-Fird Gnrating Units. Powr Enginring Journal. Pp