PEMP RMD M.S. Ramaiah School of Advanced Studies

Transcription

1 Steam Turbine Seion delivered by: Prof. Q.H. Nagpurwala

2 Seion Objective Thi eion i intended to dicu the following: Claification of team turbine Compounding of team turbine Force, work done and efficiency of team turbine Numerical example

3 Steam Steam i a vapour ued a a working ubtance in the operation of team turbine. I team a perfect ga? Steam poe propertie like thoe of gae: namely preure, volume, temperature, internal energy, enthalpy and entropy. But the preure volume and temperature of team a a vapour are not connected by any imple relationhip uch a i expreed by the characteritic equation for a perfect ga. Senible heat The heat aborbed by water in attaining it boiling point. Latent heat The heat aborbed to convert boiling water into team. et team Steam containing ome quantity of moiture. Dry team Steam that ha no moiture content. Superheated team Dry team, when heated at contant preure, attain uperheat The propertie of team are dependent on it preure. 3

4 Steam Propertie Enthalpy (H) kj/kg Entropy (S) kj/kg-k Internal energy (U) kj/kg Specific volume (v) m 3 /kg Denity (ρ) kg/m 3 Iobaric heat capacity (C p ) kj/kg-k M.S. Ramaiah School of Advanced Studie 3 4

5 Steam Power Plant Proce T hot Fuel Boiler Turbine Generator Pump Low Preure ater Cold Pump x x Exhaut Steam Low Preure and temp. Hot T cold 5

6 Steam Turbine Steam turbine convert a part of the energy of the team evidenced by high temperature and preure into mechanical power-in turn electrical power The team from the boiler i expanded in a nozzle, reulting in the emiion of a high velocity jet. Thi jet of team impinge on the moving vane or blade, mounted on a haft. Here it undergoe a change of direction of motion which give rie to a change in momentum and therefore a force. The motive power in a team turbine i obtained by the rate of change in momentum of a high velocity jet of team impinging on a curved blade which i free to rotate. The converion of energy in the blade take place by impule, reaction or impule reaction principle. Steam turbine are available in a few k (a prime mover) to 500 M Impule turbine are ued for capacity up to Reaction turbine are ued for capacity up to 6

7 Steam, Ga and Hydraulic Turbine The working ubtance differ for different type of turbine. Steam turbine are axial flow machine (radial team turbine are rarely ued) wherea ga turbine and hydraulic turbine of both axial and radial flow type are ued baed on application. The preure of working medium ued in team turbine i very high, wherea the temperature of working medium ued i ga turbine i high comparatively. The preure and temperature of working medium in hydraulic turbine i lower than team turbine. Steam turbine of 300 M ingle unit are available wherea larget ga turbine unit i 530 M and 85 M 7

8 Merit and Demerit of Steam Turbine Merit: Ability to utilize high preure and high temperature team. High component efficiency. i High rotational peed. High capacity/weight ratio. Smooth, nearly vibration-free operation. No internal lubrication. Oil free exhaut team. Can be built in mall or very large unit (up to 00 M). Demerit: For low peed application reduction gear are required. The team turbine cannot be made reverible. The efficiency of mall imple team turbine i poor. 8

9 Application Power generation Refinery, Petrochemical, Pharmaceutical, Food proceing, Petroleum/Ga proceing, Pulp & Paper mill, ate-to-energyto energy 9

10 Turbine Selection In all field of application the competitivene of a turbine i a combination of everal factor: Efficiency Life Power denity (power to weight ratio) Direct operation cot Manufacturing and maintenance cot 0

11 Rankine Cycle Saturated Rankine cycle Superheated Rankine cycle

12 Reheat on T- diagram T Note that T 5 < T 3. Many ytem reheat to the ame temperature (T 3 =T 5 ). q inhi 4 3 q inlo w outhi 5 w outlo w in Reheat i uually not in q offered for turbine le out than 50 M 6

13 Schematic of Rankine Reheat Cycle BOILER q inlo 4 Low 5 Preure TURBINE 3 High Preure TURBINE w outhi 6 CONDENSER w outlo q inhi w in PUMP q out 3

14 Steam Turbine Claification Steam turbine can be claified in everal different way:. By detail of tage deign Impule or reaction.. By team upply and exhaut condition Condening, or Non-condening (back preure), Automatic or controlled extraction, Mixed preure Reheat 3. By caing or haft arrangement Single caing, Tandem compound or Cro compound 4. By number of exhaut tage in parallel: Two flow, Four flow or Six flow. 5. By direction of team flow: Axial flow, Radial flow or Tangential flow 6. Single or multi-tage 7. By team upply Superheat or Saturated 4

15 Steam Turbine Stage A turbine tage conit of tationary tator row (guide vane or nozzle ring) and rotating rotor row. In the guide vane high preure, high temperature team i expanded reulting in high velocity. The guide vane direct the flow to the rotor blade at an appropriate angle. In the rotor, the flow direction i changed and kinetic energy of the working fluid i aborbed by the rotor haft producing mechanical energy 5

16 Type of Steam Turbine Impule Turbine Reaction Turbine Proce of complete expanion of team take place in tationary nozzle and the velocity energy i converted into mechanical work on the turbine blade. Preure drop with expanion and generation of kinetic energy take place in the moving blade. 6

17 Type of Steam Turbine Paron Reaction Turbine De Laval Impule Turbine. 7

18 Impule Reaction Turbine Modern turbine are neither purely impule or reaction but a combination of both. Preure drop i effected partly in nozzle and partly in moving blade which are o deigned that expanion of team take place in them. High velocity jet from nozzle produce an impule on the moving blade and jet coming out from till higher velocity from moving blade produce a reaction. Impule turbine began employing reaction of upto 0% at the root of the moving blade in order to counteract the poor efficiency incurred from zero or even negative reaction. Reaction at the root of reaction turbine ha come down to a little a 30% to 40% reulting in the reduction of the number of tage required and the utaining of 50% reaction at mid point. It may be more accurate to decribe the two deign a Dic and diaphragm turbine uing low reaction blading Drum rotor turbine uing high reaction blading 8

19 Flow Through Steam Turbine Stage 9

20 Compounding of Steam Turbine Thi i done to reduce the rotational peed of the impule turbine to practical limit. Compounding i achieved by uing more than one et of nozzle, blade, rotor, in a erie, keyed to a common haft; o that either the team preure or the jet velocity i aborbed by the turbine in tage. Three main type of compounded impule turbine are: a. Preure compounded b. Velocity compounded c. Preure and velocity compounded impule turbine. Involve plitting up of the whole preure drop into a erie of maller preure drop acro everal tage of impule turbine. The nozzle are fitted into a diaphragm locked in the caing that eparate one wheel chamber from another. All rotor are mounted on the ame haft. 0

21 Compounding of Steam Turbine Velocity drop i acheived through many moving row of blade intead of a ingle row of moving blade. It conit of a nozzle or a et of nozzle and row of moving blade attached to the rotor or the wheel and row of fixed blade attached to the caing. Preure velocity compounding give the advantage of producing a hortened rotor compared to pure velocity compounding. In thi deign team velocity at exit to the nozzle i kept reaonable and thu the blade peed (hence rotor rpm) reduced.

22 Comparion between Impule & Reaction Turbine Impule Turbine An impule turbine ha fixed nozzle that orient the team flow into high peed jet. Blade profile i ymmetrical a no preure drop take place in the rotor blade. Suitable for efficiently aborbing the high velocity and high preure. Steam preure i contant acro the blade and therefore fine tip clearance are not neceary. Efficiency i not maintained in the lower preure tage (high velocity cannot be achieved in team for the lower preure tage). Reaction Turbine Reaction turbine make ue of the reaction force produced a the team accelerate through the nozzle formed by the rotor Blade have aerofoil profile (convergent paage) ince preure drop occur partly in the rotor blade. Efficient at the lower preure tage Fine blade tip clearance are neceary due to the preure leakage. Inefficient at the high preure tage due to the preure leakage around the blade tip. Fine tip clearance can caue damage to the tip of the blade.

23 Loe in Steam Turbine Profile lo: Due to formation of boundary layer on blade urface. Profile lo i a boundary layer phenomenon and therefore ubject to factor that influence boundary layer development. Thee factor are Reynold number, urface roughne, exit Mach number and trailing edge thickne. Secondary lo: Due to friction on the caing wall and on the blade root and tip. It i a boundary layer effect and dependent upon the ame conideration a thoe of profile lo. Tip leakage lo: Due to team paing through the mall clearance required between themoving tip and caing or between the moving blade tip and rotating haft. The extend of leakage depend on the whether the turbine i impule or reaction. Due to preure drop in moving blade of reaction turbine they are more prone to leakage. Dic windage lo: Due to urface friction created on the dic of an impule turbine a the dic rotate in team atmophere. The reult i the forfeiture of haft power for an increae in kinetic energy and heat energy of team. 3

24 Loe in Steam Turbine Lacing wire lo: Due to paage blockage created by the preence of lacing wire in long blade of LP Stage. etne lo: Due to moiture entrained in the low preure team at the exit of LP turbine. The lo i a combination of two effect; firtly, reduction in efficiency due to aborption of energy by the water droplet and econdly, eroion of final moving blade leading edge. Annulu lo: Due to ignificant amount of diffuion between adjacent tage or where wall cavitie occur between the fixed andmovingblade.theextentoflo i greatly reduced at high annulu area ratio (inlet/outlet) if the expanion of the team i controlled by a flared caing wall. Leaving lo: Due to kinetic energy available at the team leaving from the lat tage of LP turbine. In practice team doe low down after leaving the lat blade, but through the converion of it kinetic energy to flow friction loe. Partial admiion lo: Duetopartial filling of team, flow between the blade i coniderably accelerated cauing a lo in power. 4

25 Nomenclature V U V a =V f = V m V V w α β h Abolute velocity of team Blade velocity Relative velocity of team Axial component or flow velocity hirl or tangential component Nozzle angle Blade angle enthalpy Suffix Inlet Outlet 5

26 Velocity Triangle The three velocity vector namely, blade peed, abolute velocity and relative velocity in relation to the rotor are ued to form a triangle called velocity triangle. Velocity triangle are ued to illutrate the flow in the blading of turbomachinery. Change in the flow direction and velocity are eay to undertand with the help of the velocity triangle. Note that the velocity triangle are drawn for the inlet and outlet of the rotor at certain radii. 6

27 Steam Turbine Blade Terminology 7

28 Inlet Velocity Triangle Vw U V Va 8

29 Outlet Velocity Triangle Vw U Va V 9

30 Combined Velocity Triangle Vw Vw U Va V V Va ΔVw For 50% reaction deign 30

31 ork Done Impule Steam Turbine If the blade i ymmetrical then β = β and neglecting frictional effect of fthe blade on the team, =. In actual cae, the relative velocity i reduced by friction and expreed by a blade velocity coefficient k. Thu k = / From Euler equation, work done by the team i given by; t = U(V w ± V w ) () Since V w i in the negative r direction, the work done per unit ma flow i given by, t = U(V w +V w ) () If V a V a, there will an axial thrut t in the flow direction. Aume that t Va i contant then, t = UVa (tanα + tanα ) (3) t = UV a (tanββ + tanββ ) (4) Equation (4) i often referred to a the diagram work per unit ma flow and hence the diagram efficiency i defined a 3

32 ork Done Impule Steam Turbine Diagram work done per unit ma flow ηd = (5) ork available per unit ma flow Referring to the combined diagram ΔV w i the change in the velocity of whirl. Therefore: The driving force on the wheel = mv w (6) The product of the driving force and the blade velocity give the rate at which work i done on the wheel. From equation (6) Power output = muδv w (7) If Va-Va V = ΔVa, the axial thrut t i given by; Axial thrut = mδv a (8) The maximum velocity of the team triking the blade V = {(h0-h)} (9) here h0 i the enthalpy at the entry to the nozzle and h i the enthalpy at the exit, neglecting the velocity at the inlet to the nozzle. The energy upplied to the blade i the kinetic energy of the jet and the blading or diagram efficiency; V 3

33 ork Done Impule Steam Turbine ηd = η Rate of work performed per unit ma flow Energy upplied per unit ma of team UΔV w ηd = ( U Δ Vw ) x = V V Uing the blade velocity coefficient (k= / ) and ymmetrical blade (β = β ), then; Δ V w = V co α U Hence ΔV w = ( V coα U ) U And the rate of work performed per unit ma = ( V co α U ) U Therefore; η d = ( V coα U ) U V 4( V U coα U ) U 4U U ηd = V = coα V V U where i called the blade peed ratio V (0) () () 33

34 ork Done Impule Steam Turbine Differentiating equation () and equating it to zero provide the maximum diagram efficiency; d( ηd) = 4co α 8 U = 0 V d U V U V coαα = or (3) i.e., maximum diagram efficiency = coα coα η = 4co α 4 α or d co Subtituting thi value in equation (7), the power output per unit ma flow rate at the maximum diagram efficiency P = U 34 (4) (5)

35 Degree of Reaction Degree of reaction i a parameter that decribe the relation between the energy tranfer due to the tatic preure change and the energy tranfer due to dynamic preure change. Degree of reaction i defined a the ratio of tatic preuredropintherotor to the tatic preure drop in the tage. It i alo defined a the ratio of tatic enthalpy drop in the rotor to the tatic enthalpy drop in the tage Λ= Degree of reaction = Static enthalpy change in rotor Total enthalpy change in tage = h -h h 0 -h The tatic enthalpy at the inlet to the fixed blade in term of tagnation enthalpy and velocity at the inlet to the fixed blade i given by h 0 h 00 V 0 C p = imilarly Subtituting Λ = h h = h 0 V0 C ( h h ) h V 00 0 p C p 35 C p V (6)

36 Degree of Reaction But for a normal tage, V 0 = V and ince h 00 = h 0 in the nozzle, then; Λ = ( h h ) ( h h ) 0 h 0 V w V h = e know that (h 0 h 0 ) = ( ) ( ) w + 0 Subtituting for (h - h ) in equation (7), Λ = ( V V ) = w w [ ( h h ) ] [ U ( V w Vw ) ] 0 0 h ( V V ) w Auming the axial velocity i contant through out the tage, then Λ = Λ = ( V V ) w [ U ( U + V + V U )] w w w ( Vw Vw )( Vw + Vw ) [ U ( V + V )] w w w 36 (7) (8) (9)

37 Degree of Reaction (0) Λ = ( β + tan ) V a tan U β From the velocity triangle it i een that V = U + V V V U w w w = w Therefore equation (0) can be arranged into a econd form: Λ = V a + U ( tan β + α ) tan Putting Λ = 0 in equation (0), we get ( β = β ) And V = V and for Λ = 0.5, ( β = α ) Zero reaction tage Let u firt dicu the pecial cae of zero reaction. According to the definition of reaction, hen Λ =0, equation (6) reveal that h =h and equation (0) that β = β. 37 ()

38 Degree of Reaction Mollier diagram Velocity Diagram Now h 0r0 =h 0r0 and h =h for Λ =0.Then =. In the ideal cae, there i no preure drop in the rotor and point and on the mollier chart hould coincide. But due to irreveribility, there i a preure drop through the rotor. The zero reaction in the impule tage by definition, mean there i no preure drop through the rotor. The Mollier diagram for an impule tage i hown in Fig..a, where it can be oberved that the enthalpy increae through the rotor. 38

39 Degree of Reaction From equation (6) it i clear that the reaction i negative for the impule turbine tage when irreveribility i taken into account. Fig..a Fifty percent reaction tage From equation (6) for Λ =0.5α = β and the velocity diagram i ymmetrical. Becaue of ymmetry, it i alo clear that α = β. For Λ=/, the enthalpy drop in the nozzle row equal the enthalpy drop in the rotor. That i h 0 -h =h -h 39

40 Degree of Reaction β = tanα U + Subtituting into equation () V a V a ( tan α tan ) Λ = + tan α U Choice of Reaction and Effect on Efficiency Equation (7) can be rewritten a Λ = Thu when α = α, the reaction i unity (alo V =V ). The velocity diagram for Λ =i hown in Fig. with the ame value of V a, U and ued for Λ =0 and Λ = ½. It i obviou that if Λ exceed unity, then V <V 0 (i.e., nozzle flow diffuion). V + Cw can be eliminated by uing thi equation V = V yielding U w w V U w w V Λ = + U U 40 w ()

41 Degree of Reaction In Fig. the total to tatic efficiencie are hown plotted againt the degree of reaction. hen /U =, η t i maximum at Λ = 0. ith higher loading, the optimum η t i obtained with higher reaction ratio. A hown in Fig. for a high total to total efficiency, the blade loading factor hould be a mall a poible, which implie the highet poible value of blade peed i conitent with blade tre limitation. It mean that the total to tatic efficiency i heavily dependent upon the reaction ratio and η t can be optimized by chooing a uitable value of reaction. 4

42 Blade Height in Axial Flow Machine The continuity equation m = ρav may be ued to find the blade height h. The annular area of flow = πdh. Thu the ma flow rate through an axial flow turbine i m = ρπdhv a h = m ρπdv a Blade height will increae in the direction of flow in a turbine and decreae in the direction of flow in a compreor. 4

43 Effect of Reheat Factor & Stage Efficiency The thermodynamic effect on the turbine efficiency can be bet undertood by conidering a number of tage, ay 4, between tate and 5 a hown in Fig. Total expanion i divided into four tage of the ame tage efficiency and preure ratio 43

44 Effect of Reheat Factor & Stage Efficiency P P P3 i. e., = = = P P P 3 4 P P 4 5 Let η 0 i the overall efficiency of expanion and i defined a the ratio of actual work done per kg of team to the ientropic work done per kg of team between and 5. h a h 5. e., η = i. e., η0 = ' h h5 i 0 The actual work done per kg of team a = η 0 () Ientropic or ideal value in each tage are Δ, Δ, Δ 3, Δ 4. Therefore the total value of the actual work done in thee tage i, a = Σ(-)+(-3)+(3-4)+(4-5) Alo tage efficiency for each tage i given by η = actual work done/kg of team Ientropic work done in tage = 44 a

45 F t Effect of Reheat Factor & Stage Efficiency For tage '.,. a a a or h h h h e i Δ = Δ Δ = = = η η [ ] a a Δ + Δ + Δ + Δ = Σ = ΣΔ Δ η η η η For ame tage efficiency in each tage 4 3 η η η η = = = [ ] a ΣΔ = + Δ + Δ + Δ Σ Δ = η η 4 3 (3) From equation () and (3), ΣΔ = ΣΔ = η η η η (4) η η 0 The lope of contant preure line on h- plane i given by h M.S. Ramaiah School of Advanced Studie 3 45 T h p =

46 Effect of Reheat Factor & Stage Efficiency Thi how that the contant preure line mut diverge toward the right. Therefore ΣΔ >. For expanion proce. It i obviou bi that the enthalpy hl increae when we move toward right along the contant preure line. Hence the ummation of Δ Δ etc., i more than the total ientropic enthalpy drop The ratio of ummation of ientropic i enthalpy drop for individual id tage to the totalt ientropic enthalpy drop a a whole i called Reheat factor. Thu RF = [ ] [( ) ( ) (3 ) (4 )] ' ' ' ' + Δ + Δ + Δ Σ + a + b + c Σ Δ RF = ΣΔ = 3 4 Therefore the overall efficiency of the expanion proce, A ( 5) η 0 =η tage RF (6) RF = ( ΣΔ / ) > the overall efficiency i of the turbine η0 i greater than than tage efficiencie i i η i. e., η > η for turbine (7) 0 46 (5)

47 Merit and Demerit of Reheating Advantage of Reheating. There i an increae in output of turbine.. Eroion and corroion problem in team turbine are reduced. 3. There i an improvement in overall thermal efficiency of the turbine. 4. Condition of team in lat tage are improved. Demerit. Capital cot required for Reheating. The increae in thermal efficiency i not appreciable compared to expenditure incurred in reheating for maller capacity turbine. 47

48 Material of Steam Turbine Part name Caing Inner caing Shaft Blade high preure Blade Low preure Caing joint bolt Croover pipe Valve pindle Valve body Valve eat Material Code/Compoition IS:063 GS Mo4 Shaft 30CrMoV XCrMoV X0Cr3 CrMoV57 ASTM 533 Gr.70 XCrMoV GS7crmov5 CrMo57 Source BHEL Hyderabad 48

49 Seion Summary In thi eion the tudent would have learnt about orking principle of team turbine Claification of turbine Type of compounding ork done and efficiency of Impule team turbine tage ork done and efficiency of reaction team turbine tage Solution of ome numerical example related to team turbine 49

50 Thank you 50