The Optimal Steam Pressure of Thermal Power Plant in a Given Load

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1 Eergy ad Power Egieerig, 013, 5, 78-8 doi:10.436/ee b054 Published Olie July 013 (htt:// The Otimal Steam Pressure of Thermal Power Plat i a ive Load Yog Hu, Ji-zhe Liu, De-liag Zeg, Wei Wag, Ya-zhe Li North Chia Electric Power Uiversity, State Key Laboratory of Alterate Electrical Power System with Reewable Eergy Sources, Beijig, Chia huyog198610@ceu.edu.c Received Jauary, 013 ABSTRACT As the large chage of the grid load, may large caacity uits of our coutry had to chage the load i order to meet the gird eed. Whe a thermal ower lat receives a give load istructio from the grid, it is ecessary to set a otimal steam ressure to maitai the high efficiecy of the lat. I the ast otimizatio methods, durig the rocess of calculatio, the outut of the turbie ofte chaged, it was hard to maitai the outut costat. Therefore, i combiatio with the theory of variable coditio of turbie, calculatio of goverig stage ad the matrix equatio of thermal ower system, a otimizatio method were ut forward ad a otimal solutio was got i a give load. Keywords: ive Load; Pressure Otimizatio; Variable Coditio; Thermal Power Plat 1. Itroductio As the develomet of ecoomy i chia, cosumtio level of the eole have ehaced, which leads to a large roortio of electricity ower is cosumed i our daily life, resultig i the differece betwee eak ad valley of grid load icreased year by year. Ad i our coutry, large-caacity thermal ower lats have a large ercetage i the total istalled caacity of ower lats, which makes the large-caacity uits with a basic load have to articiate i the load regulatio. The uits have to deviate from the origial desig coditio ad eve ru i the low load area for a log time, which makes thermal efficiecy of the uits decrease greatly. Amog the factors that affect the thermal efficiecy of ower lat, oly the ruig modes ad oeratig arameters ca be adjusted by oeratig ersoel. Therefore the research of thermal ower lats i the off-desig coditio is of great sigificace i the selectig of ruig modes ad oeratig arameters. I a give load, whe the uit rus i a high steam ressure, the ideal ethaly dro of turbie will icrease ad the outlet ressure of feed-water um will rise simultaeously. I order to maitai the give uit load, it is ecessary to reduce the steam flow rate through decreasig the oeig degree of regulatig valves, this will icrease the throttlig loss of the goverig stage. Whe the uit rus i a low steam ressure, the theoretical thermal efficiecy of the uit will reduce, but the lower steam ressure will make the goverig stage to maitai higher iteral efficiecy, ad the outlet ressure of feed-water um will decrease. I order to maitai the uit load, it has to elarge the oeig degree of regulatig valves to icrease the steam flow rate. Therefore, i trackig of the grid give load, the uit usually deviates from the desiged coditio, how to select the otimal steam ressure ad the ruig mode has a great ifluece o the iterest of the ower lat. I the traditioal method of the ressure otimizatio, it usually assumed the steam ressure was aroximately roortioal to the steam flow, whe the steam ressure chaged, it calculated the steam flow firstly ad the calculated the back ressure of goverig stage accordig to the Flugel formula [1], carried o variable coditio calculatio of the goverig stage ad the whole turbie. Fially it determied the efficiecy of the uit uder the chaged steam ressure []. But i the ractical oeratio of the thermal ower lat, it must guaratee the load of the uit equal to the istructio from the grid, whe the steam ressure gets higher, it eeds to decrease the oeig degree of the regulatig valves, lower the steam flow to esure the stability of the load, ad vice versa. I the traditioal method, due to the aroximate roortioal relatioshi betwee steam ressure ad the flow, it leads to the load chaged i roortio, ot ivariable. I some other literatures, i order to esure the load uchaged, it iteratively calculated the steam flow usig the turbie ower equatio [3], which igored the characteristic of the goverig stage ad caused the deviatio of the results.

2 Y. HU ET AL. 79 Therefore, o the base of variable coditio calculatio method of goverig stage ad variable coditio theory of turbie, usig thermal ecoomic matrix equatio [4], i order to solve the roblems metioed above, a ew calculatio method of otimal steam ressure i a give load was ut forward, the otimal steam ressure ad ruig modes was got uder differet loads.. Model of Steam Pressure Otimizatio.1. Calculatio of the overig Stage I the variable coditio calculatio of the goverig stage, the steam flow through the fully oeed regulatig valves ad the artly oeed valve ca be exressed as: A (1) 10 v A v0 The mai steam flow rate ca be exressed as: I which is the steam flow through the fully oeed valves; is the steam flow through the artly oeed valve; A is the flow area of fully oeed valves; A is the flow area of artly oeed valve; 0 is the ressure of mai steam; v0 is the secific volume of mai steam;, is the fuctio of / 0, / 0; 0 is the steam ressure behid fully oeed valve; 0 is the steam ressure behid artly oeed valve; is the back ressure of goverig stage; is the efficiecy of goverig stage; x a is the seed ratio of goverig stage. I geeral, whe the steam flows through the fully oeed valves, the throttle loss is smaller, so it ca be assumed ; whe the steam flows through the artly oeed valve, the oeig degree of artly oeed valve is x( x [0,1] ), sice the aular chamber after the ozzle is i commuicatio with each other, the steam ressure 1 behid the ozzle of each ozzle grou are the same, the steam ressure behid the uoeed regulatig valve (i.e., the ressure before the ozzle of uoeed valve) is also equal to 1. Whe the oeig degree of valve x gradually chages from 0 to 1, the ressure 0 behid artly oeed valve will chage from 1 to I order to facilitate the calcula- tio, 0 ( ) x 1 is assumed (this assumtio is oly coveiet to calculate 0, it has o effect o the otimizatio results). So whe the oeig degree of all the regulatig valves is kow, the mai steam flow ca be exressed as: 0 () (3) f(, x, ) (4) Therefore, the mai steam flow ca be determied by, x ad 0, the o basis of the variable coditio calculatio of goverig stage, the steam ethaly of goverig stage ca be got. h tj.. Calculatio of the Itermediate Stage ad Last Stage I the variable coditio calculatio of turbie, because the flow area of itermediate stage is costat, whe the load of the uit is chaged, if the variatio of temerature before all stages is igored, the ressure before itermediate stage is roortioal to the steam flow of this stage, so ressure ratio is ivariat, the efficiecy of itermediate stage is uchaged, the ideal ethaly dro of each stage is also uchaged [5]. Therefore, whe the arameters of goverig stage are kow, the steam ethaly of each extractio oit ca be exressed as: h( i1)1 hi 1( hi0 h( i1)0) (5) I which hi is the steam ethaly of ith stage; subscrit 0 reresets the desiged coditio; subscrit 1 reresets the variable coditio. For the last stage of steam turbie, we calculated from the last stage to the rior stage, foud the suerheated steam extractio oit ad set it as ith stage. The steam after the ith stage does adiabatic exasio i the turbie, so the etroy is costat. Combied with the steam ressure of extractio oit, the ideal steam ethaly of this stage could be got, accordig to (6), we could get the steam ethaly of this stage ad calculated oe stage by oe stage util to the last stage. h( i1)1 hi 1 i, i1( hi 1 h ( i 1)1) (6) I which h ( i1)1 is the ideal steam ethaly of ( i 1) stage, ii, 1 is the efficiecy of ( i 1) stage..3. Calculatio of the Boiler Feed-Water Pum Turbie Whe the mai steam ressure ad flow rate chage, the outut of Boiler Feed-Water Pum Turbie (BFPT) will chage too. Therefore, the ifluece of BFPT o the thermal efficiecy caot be igored. From the outlet of feed-water um to the mai steam valve, the hase of workig fluid chaged. I this rocess, there exists the loss of resistace alog the way ad the loss of local resistace [5], both loss ca be exressed as: C (7) I which is the ressure dro; is the average desity of fluid; C is the flow rate of fluid; is the loss coefficiet which deeds o the characteristic of ie. We use subscrit d rereset the arameters of

3 80 Y. HU ET AL. desig-coditio, the the outlet ressure of feed-water um ca be exressed as: ( ) ( ) (8) d 0 d 0d d I which is the outlet ressure of feed-water um; is the mai steam ressure; 0 is the mai steam flow. Whe the feed-water flows through the um, the ressure of feed-water will rise because of the workig of um, this will make the feed-water ethaly rise. This rocess ca be regarded as isetroic flow [6], so the ethaly- rise of feed-water ca be exressed as: v( 1) h (9) I which 1 is the ilet ressure of feed-water um; v is the average secific volume of feed-water; is the efficiecy of feed-water um. Accordig to the law of coservatio of eergy, the extractio flow for BFPT ca be got. ( 1) v DBFPT (10) ( h h ) 4 c j I which h 4 is the ilet steam ethaly of BFPT; h c is the exhaust ethaly of BFPT; j is the efficiecy of BFPT. 3. Otimizatio Method of Steam Pressure i a ive Load 3.1. Otimizatio Method I order to maitai the outut of the uit ad overcome the shortcomigs of traditioal otimizatio methods, i the rocess of otimizatio, we adoted the sequetial calculatio method, combiig with assumtio, verificatio ad iterative adjustmet. If the load ad a iitial steam ressure were give, we could get the steam flow, oeig degrees of regulatig valves, back ressure of goverig stage ad thermal efficiecy of the uit, the a uique maig relatioshi was formed amog them. Ste1. Accordig to the load istructio, use Figure 1 to determie the feasible rage of steam ressure [7] ad the umber of fully oeed valves. Ste. Assume a certai back ressure of goverig stage ad the degree of artly oeed valve to determie the mai steam flow, the ethaly ad temerature of goverig stage. Ste3. Accordig to the ethaly ad temerature of the goverig stage, carry o the variable coditio calculatio of itermediate stage ad last stage, get the outut of turbie. Ste4. Judge the arameters of goverig stage usig (11). Equatio (11) is the Flugel formula [1]. If the equatio does ot hold, adjust the back ressure of goverig stage, ad the go to ste. T T c d d cd d (11) Ste5. Judge the outut of turbie. If the outut of turbie is ot equal to the load istructio, adjust the degree of artly oeed valve ad go to ste. The flow chart of the ressure calculatio is show i Figure. Figure 1. The feasible rage of steam ressure. [ ] 0 mi max i 0 i 0 mi 0.1 T x h tj T T c d cd d d Pe N Figure. The flow chart of the otimizatio method. x

4 Y. HU ET AL Alicatio Examles We took the Orietal steam turbie N /600/600 as a examle, the imact of the overla of regulatig valves was ot cosidered ad we igored the ifluece of evirometal factors o the thermal ecoomy of the uit. I the ideal coditio of 100% load, there were three regulatig valves fully oeed ad oe valve closed. First, we took the 100% THA coditio as a examle, aalyze ad validate the otimizatio method, the results were show i Table 1. I the 100% THA coditio, i order to maitai the uit load, as the declie of mai steam ressure, the regulatig valves had to be oeed larger to icrease the mai steam flow, ad the ower cosumed by feed-water um was decrease too. The thermal efficiecy of the uit was declie as the steam ressure became lower. But whe the mai steam ressure reduced to.76 Ma, four regulatig valves were all fully oeed, the throttlig losses was least at this momet, so the efficiecy of the uit rebouded a little. From the dates of Table 1, the variatio tedecy of ressure ad efficiecy cosistet with the theoretical aalysis, so this method ca be used to otimize other coditios of the uit. Figure 3 shows the thermal efficiecy chage as the umber of fully oeed valves chage from to 4 i differet load. As the icreasig of the oeig degree, the Table 1. The aalysis of efficiecy i a desig coditio. Steam Mai Pressure Steam (Ma) Flow(t/h) Degree of Regulatig Valves The Eergy Thermal Cosumtio of Efficiecy Feed-water Pum (MW) % % % % % % % % % % % % 3.31 Thermal efficiecy(%) % 70% 60% 50% The oeig degree of the adjustig valves(%) Figure 3. The relatio betwee thermal efficiecy ad valve oeig. Thermal efficiecy(%) Steam ressure of mode 1 16 Steam Thermal ressure efficiecy of mode of mode 46.5 Thermal efficiecy of mode 1 14 Thermal efficiecy of mode The uit load(%) Figure 4. The comariso curves of the two modes. mai steam ressure droed, the thermal efficiecy declied, but i the fully oeed oits, there existed a local otimal oit. Figure 4 shows the efficiecy of the uit i differet slidig ressure oeratio mode. I mode 1, the uit took a fixed ressure oeratio mode with 5Ma steam ressure first, whe the outut of the load decreased to the 80%, the uit took the slidig ressure oeratio mode with regulatig valves fully oeed. I mode, the uit took the slidig ressure oeratio mode with 3 regulatig valves fully oeed begiig from the 100% THA coditio. It ca be see from the icture, the efficiecy of mode 1 was higher tha mode, esecially i the low load regio. 4. Coclusios Basig o the variable coditio calculatio method of goverig stage ad variable coditios theory of turbie, a ew otimizatio method was ut forward for the otimal oeratio of thermal ower lat, ad we took the Orietal steam turbie as a examle, got the otimal steam ressure of differet load ad the otimal slidig ressure curve. Therefore, oeratig ersoel ca adot this method, combied with the characteristic of the uit ad the factor of eviromet, drawig the otimal ressure curves, which ca be used as a referece i the ractical oeratio. 5. Ackowledgemets This work was suorted by the Natioal Basic Research Program of Chia ( 973 Project) (rat No. 01CB- 1503) ad the Natioal Natural Sciece Major Fud Project (rat No ) REFERENCES [1] C. F. Zhag ad Y. H. Cui, The Distiguishig Theory of Critical State of Turbie ad Imroved Flugel Formula, Sciece i chia Series E, Vol. 33, No. 3, 003,. Steam ressure(ma)

5 8 Y. HU ET AL [] L. X. Zhou ad M. Hua, Method for Calculatig Mai Steam Pressure ad Heat Rate Correctio Curves Uder Off-desig Oeratig Coditios, Joural of Egieerig for Thermal Eergy ad Power, Vol. 6, No. 3, 011, [3] Z. P. Yag ad Y. P. Yag, Sesitivity Aalysis o Eergy Cosumtio of Exhaust Steam Pressure of 1000 MW Steam Turbie Uit, East Chia Electric Power, Vol. 39, No., 011, [4] S. L. Ya ad C. F. Zhag, The Steam-Water Distributio eeral Matrix Equatio of Thermal System for the Coal-Fired Power Uit, Proceedigs of the CSEE, Vol. 0, No. 8, 000, [5] N. Zhao, The Research of the Relatio betwee Pressure Rate ad Flow ad the Target Value of the Thermal Parameters uder Variable Workig Coditios of Steam Turbie, North Chia Electric Power Uiversity, 008. [6] P. Li ad M. Hua, Research o Target Value of BFPT Parameters for Slidig Pressure Oeratio Uit, Thermal Turbie, Vol. 39, No. 4, 010, [7] C. F. Zhag ad H. J. Wag, Quatitative Research of Otimal Iitial Oeratio Pressure for the Coal-fired Power Uit Plat, Proceedigs of the CSEE, Vol. 6, No. 4, 006,