PEMP RMD & Cycle Performance. M.S.Ramaiah School of Advanced Studies

Transcription

1 Steam Se Turbine ub ecyces Cycles & Cycle Performance Session delivered by: Prof. Q.H. Nagpurwala 1

2 Session Objectives This session is intended to discuss the following: Basic construction and classification of steam turbines Steam turbine cycles Pressure and velocity compounding Subcritical and supercritical steam turbines Steam turbine cycle performance Combined cycle power plants 2

3 Steam Steam is a vapour used as a working substance in the operation of steam turbine. Is steam a perfect gas? Steam possess properties like those of gases, namely pressure, volume, temperature, internal energy, enthalpy and entropy. But the pressure volume and temperature of steam as a vapour are not connected by any simple relationship such as is expressed by the characteristic equation for a perfect gas. Sensible heat The heat absorbed by water in attaining its boiling point Latent heat The heat absorbed to convert boiling water into steam Wet steam Steam containing some quantity of moisture content. Dry steam Steam that has no moisture content. Superheated steam Dry steam when heated at constant pressure attains superheat. Superheated steam behaves like perfect gas. The properties of steam are dependent on its pressure. 3

4 Steam Properties Enthalpy (H) kj/kg Internal energy (u) kj/kg Entropy (s) kj/kg-k Specific volume (v) m 3 /kg Density ( ) kg/m 3 Isobaric heat capacity (c p ) kj/kg-k 4

5 Steam Turbine A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in The main components of a steam turbine are: Feed water pump Boiler Turbine stages, comprising nozzle/stator and rotor blade rows Condenser Steam from the boiler is expanded in the nozzle blade passages to produce high velocity jets, which impinge on the rotor blades mounted on a disc and shaft. The rate of change of momentum of steam flow across the rotor blades produces the required torque for the shaft to rotate. The conversion of energy across the blade rows takes place by impulse, reaction or impulse reaction principle. 5

6 Application of Steam Turbines Power generation Petrochemical refineries Pharmaceuticals Food processing Petroleum / gas processing Paper mills Sugar industry Waste-to-energy 6

7 Gas Turbine Power Plant Comb ustor Hot gas Compressor Exhaust gas 17-$Boiler_control 7

8 Steam Turbine Power Plant 17-$Boiler_control 8

9 Steam Turbine Power Plant Power Generation Efficiency i = energy out / energy in 17-$Boiler_control 9

10 Rankine Cycle Saturated Rankine cycle Superheated Rankine cycle 10

11 Reheat Cycle T Note that T 5 < T 3.Many systems reheat to the same temp p( (T 3 = T 5 ) Reheat is usually not offered for turbines less than 50 MW q inhi 3 w outhi 5 q inlo 2 w outlo w in 1 q 6 out 4 s 11

12 Schematic of Rankine Reheat Cycle q inlo 4 5 Low Pressure TURBINE BOILER 2 3 High Pressure TURBINE w outhi 6 CONDENSER w outlo q inhi w in 1 PUMP q out 12

13 Schematic of Rankine Cycle without Reheat 13

14 Steam Turbine Classification Steam turbines can be classified in several different ways: 1. By details of stage design Impulse or reaction 2. By steam supply and exhaust conditions Condensing or non-condensing Automatic or controlled extraction Mixed pressure Reheat 3. By casing or shaft arrangement Single casing, tandem compound or cross compound 4. By number of exhaust stages in parallel Two flow, four flow or six flow 5. By direction of steam flow Axial flow, radial flow or tangential flow 6. Single or multi-stage 7. By steam condition Superheated or saturated 14

15 Steam Turbine Classification Non-Condensing Turbine The entire flow of steam is exhausted to the industrial process or facility steam remains at conditions close to the process heat requirements ExtractionTurbine It has opening(s) in its casing for extraction of a portion of steam at some intermediate pressure for use in a process or in a CHP facility, or for feed water heating. The rest of the steam is condensed 15

16 Turbine Designation H - Single Flow HP Turbine K - HP/IP Opposite flow E - HP/LP Opposite flow N - Double flow LP Turbine M - Double flow IP Turbine 16

17 Steam Turbine Blade Rows 149 MW steam Single cylinder type turbine rotor turbine casing Siemens SST-900 steam turbine at Finspong plant in Sweden 17

18 Types of Steam Turbine Impulse Turbine Reaction Turbine Complete expansion of steam takes place in stationary nozzle blades and the kinetic energy is converted into mechanical work in rotor blades Expansion of steam takes place partly in nozzle / stator and partly in rotor. However, conversion of kinetic energy to mechanical work takes place only in rotor blades 18

19 Types of Steam Turbine Parsons Reaction Turbine De Laval Impulse Turbine. 19

20 Flow through Multistage Steam Turbine 20

21 Compounding of Steam Turbines Compounding is done to reduce the rotational speed of the impulse turbine to practical limits. Compounding is achieved by using more than one set of nozzle and rotor blade rows, in series, so that either the steam pressure or the velocity (after expansion) )is absorbed b dby the turbine in stages. Three main types of compounded impulse turbines are: a. Pressure compounded b. Velocity compounded c. Pressure and velocity compounded impulse turbines Pressure Compounding Involves splitting of the whole pressure drop into a series of smaller pressure drops across several stages of impulse turbine. The nozzles are fitted into a diaphragm locked in the casing that separates one wheel chamber from another. All rotors are mounted on the same shaft. 21

22 Compounding of Steam Turbines Velocity Compounding Entire pressure drop is achieved across the first nozzle. The high velocity is then reduced stage by stage across the following rotor blade rows. There is no expansion across the stator rows. Pressure-Velocity Compounding Pressure-velocity compounding is combination of pressure and velocity compounding. It gives the advantage of producing a shortened rotor compared to pure velocity compounding. In this design steam velocity at exit to the nozzles is kept reasonable and thus the blade speed is reduced. 22

23 Comparison of Impulse and Reaction Turbines Impulse Turbines An impulse turbine has fixed nozzles that expand steam flow to produce high velocity jets Rotor blade profile is symmetrical as no pressure drop takes place across these blades The design is suitable for efficiently absorbing high velocity and high pressure Steam pressure is constant across the blades and therefore fine tip clearances are not necessary Efficiency is not maintained in the low pressure stages (high steam velocity cannot be achieved in the low pressure stages) Reaction Turbines Reaction turbine makes use of the reaction force produced as the steam accelerates through the rotor blade passages Rotor blades have convergent passages allowing pressure drop to occur partly through them Efficient at the low pressure stages Fine blade tip clearances are necessary due to the pressure leakages Lower efficiencies in high pressure stages due to the leakage losses around the blade tips Fine tip clearances can cause damage to the blade tips 23

24 Coupling of Turbine Modules In power stations, different types of turbines -- high pressure, medium (intermediate) pressure and low pressure -- are used in series. This coupling leads to an excellent efficiency ffii (over 40%), which hihis even better than the efficiency of large diesel engines. 24

25 Steam Turbine Cycle (Subcritical) Rankine cycle with superheat Rankine cycle with reheat 25

26 Steam Turbine Cycle (Supercritical) There are inherent advantages of supercritical steam cycle: High pressures and temperatures enhance thermal efficiency; avoid use of steam dryers and steam separators; and reduce CO 2 emissions. 26

27 Development of Conventional Coal Fired Steam Power Plants 27

28 Potential Efficiency Improvements (Based on a 700MW bituminous coal fired plant, with a 40mbar condenser pressure) 28

29 Supercritical Steam Turbine Plant 29

30 Rankine Cycle Efficiency Work done by the turbine, W 1 = m (h 1 - h 2 ) Work input to pump, W 2 = m (h 4 - h 3 ) Net work output of the cycle = W 1 - W 2 Heat input in the cycle (boiler) Q 1 = m(h 1 -hh 4 ) Heat rejected in the condenser Q 2 =m(h 2 -hh 3 ) Thermal efficiency of Rankine cycle W 1 W Q 1 2 h h h h h 1 h2 h 1 h h 3 4 h

31 Energy Flow in Steam Turbine System Flue Gas 31

32 Steady Flow Energy Equations Boiler Turbine F+A+h + d = h 1 +G+hl + b, hence F+A = G+h 1 - h d +hl b h 1 = T + h 2 + hl t, hence 0 = T - h 1 + h 2 + hl t Condenser Unit W i + h 2 = W o + h w + hl c, hence W i = W o + h w -h 2 + hl c Feed dwater System h w + d e + d f = h d + hl f, hence d e + d f = - h w + h d + hl The four equations on the right can be arranged to give the energy equation for the whole turbine system enclosed dby the outer boundary 32

33 Steady Flow Energy Equations Energy of fuel (F) per unit mass of working agent (water) is equal to the sum of the mechanical energy available from the turbine less that used to drive the pumps (T - (d e + d f ) the energy leaving the exhaust [G - A] using the air temperature t as the datum. the energy gained by the water circulating through the condenser [W o -W i ] the energy gained by the atmosphere surrounding the plant Σ hl The overall thermal efficiency of a steam turbine plant can be represented by the ratio of the net mechanical energy available to the energy within the fuel supplied : 33

34 Steam Turbine Cycle on T-h Diagram The efficiency of the Rankine cycle AB'CDEA is The efficiency of the real cycle is 34

35 Steam Turbine Installation 35

36 Steam Turbine Power Plant 2x130MW Power Station (JSW Energy) Located at Jindal Vijaynagar Steel Plant complex Toranagallu, Bellary 36

37 Combined Cycle Power Plant Heat recovery steam generator Gas turbine plant Steam turbine plant Electrical energy Air Fuel Exhaust gas Life steam Gas turbine plant: Steam turbine plant: Condensate 12 1 Air intake 2 Compressor 3 Gas turbine 4 Heat recovery steam generator 5 Generator 6 Transformer 7 Steam turbine 8 Condenser 9 Feeding pump 10 Generator 11 Transformer 12 Circulating pump Electrical energy Fresh water Cooling tower Cooling air 37

38 Single Shaft Combined Cycle Plant 38

39 Multi Shaft Combined Cycle Plant 39

40 Combined Cycle Power Plant A 580 MW capacity combined cycle power plant at Ontario, Canada, comprising two natural-gas fired turbines, one steam turbine and two vertical heat recovery steam generators. 40

41 Typical Steam Turbine Materials Part name Casing Inner casing Shaft Blade high pressure Blade Low pressure Casing joint bolt Crossover pipe Valve spindle Valve body Valve seat Material Code/Composition IS:2063 GS 22Mo4 Shaft 30CrMoV121 X22CrMoV121 X20Cr3 21CrMoV57 ASTM 533 Gr.70 X22CrMoV121 GS17crmov511 21CrMo57 Source BHEL Hyderabad 41

42 Session Summary In this session the following aspects of steam turbines have been discussed: Constructional features of steam turbines Basic Rankine cycle and effect of reheat Velocity and pressure compounding of turbine stages Supercritical steam turbines Steam turbine performance parameters Combined cycle power plants 42

43 Thank you 43