Hide Notes 

Slide 1: Steam Turbine Design
Slide 2: Impulse Turbine    Impulse steam turbine stage consists as usual from stator which known as the nozzle and rotor or moving blades Impulse turbine are characterized by the that most or all enthalpy and hence pressure drop occurs in the nozzle. The rotor blades can be recognized by their shape, which is symmetrical and have entrance and exit angles around 20o. They are short and have constant cross sections.
Slide 3: Single Stage Impulse Turbine      Pressure  Velocity It is usually called De-Laval turbine The steam is fed through one or several convergent-divergent nozzles The nozzles do not extend completely around the circumference of the rotor, so that only part of the blades are impinged upon by the steam. Pressure drop occurs in the nozzle and not in the blades. Maximum velocity and hence kinetic energy of the steam occurs at the nozzle exit Velocity change occurs in the rotor blades where the steam gives up its energy to the rotor blades. IMPLUSE STAGE Nozzles Blades
Slide 4: Compounded Steam Turbines      Compounded steam turbine means multistage turbine. Compounding is needed when large enthalpy drop is available. Since optimum blade speed is related to the exit nozzle speed. It will be higher as the enthalpy drop is higher. The blade speed is limited by the centrifugal force as well as needs of bulky reduction gear Compounding can be achieved either by velocity compounded turbine or pressure compounded turbine.
Slide 5: Velocity Compounded Impulse Turbine    The velocity compounded turbine was first proposed by C.G Curtis. It is composed of one stage of nozzles, as the single stage turbine, followed by two rows of moving blades instead of one. These two rows are separated by one row of fixed blades which has the function of redirecting the steam leaving the first row of the moving blades to the second row of moving blades.
Slide 6: Velocity Compounded Impulse ).Turbine )Contd
Slide 7: Velocity Compounded Impulse ).Turbine )Contd    In Curtis turbine steam leaving the nozzle is utilized in both rows of moving blades instead of single raw as in the de-Laval turbine. The velocity remain almost constant across the fixed blades. Using an analysis similar to that used for the single stage , The work of the Curtis turbine is as follows: mo wo  { Vs2  Vs2  Vr22  Vr2  Vs2  Vs2  Vr24  Vr2 } 1 2 1 3 4 3 2 First Row Second Row      
Slide 8: Velocity Compounded Impulse ).Turbine )Contd  Due to friction effect inlet and exist velocities for different rows are related as follows: Vr 2 Vr 2  Vr1  k v1 Vr1 Vs 3  Vs 2 Vr 4  Vr 3 Vs 3  kv 2 Vs 2 Vr 4  kv 3 Vr 3
Slide 9: Velocity Compounded Impulse ).Turbine )Contd     Although the Curtis stage is composed of two rows of moving blades, a velocity compounded turbine can be composed of any number of such rows. All these rows are sharing in the same kinetic energy of the incoming steam. These stages are usually built with successively increasing blade angles such that they become flatter and thinner blades toward the last row. Expression for the optimum speed is as follows: VB ,opt . V Cos1  s1 2n Three Stages Velocity Compounded Turbine
Slide 10: Velocity Compounded Impulse ).Turbine )Contd    The work ratio of the highest-to-lowest pressure stages in an ideal turbine is 3:1 for two stages turbine and 5:3:1 for the three stage turbine and 7:5:3:1 for four stages turbine. The lower pressure velocities stages produces little work compared with the added investment. This makes additional stages above two )Curtis) uneconomical. If blade speeds must be reduced below that afforded by Curtis turbine another type of compounding could follow the Curtis stage.
Slide 11: Pressure Compounding Impulse Turbine    Pressure compounding impulse turbine is a multistage impulse turbine where expansion in the fixed blades )nozzles) is achieved equally among the stages. This type of turbines is usually called as Rateau turbine Accordingly the inlet steam velocities to each stage is essentially equal, due to equal drop in enthalpy. htot Vs1  Vs 2  ...  2 n   Where n is the number of stages This equal enthalpy drop does not mean equal pressure drop
Slide 12: Pressure Compounding ).Impulse Turbine )Contd Two Stages Pressure Compounding Turbine Three Stages Pressure Compounding Turbine
Slide 13: Pressure Compounding ).Impulse Turbine )Contd    In reference to the previous velocity triangle the whirl of all stages is equal to zero )δ=90o). The kinetic energy from each stage should be neglected, because the nozzle of each stage must receive the steam discharged by the preceding stage. The pressure compounding has the advantages of:     It suffers from the following disadvantages:   reduced blade velocities reduced steam velocities )and hence friction). equal work among the stages as desired by the designer. Pressure drop across the fixed raw of nozzles which require leak tight diaphragms. Large number of stages  Accordingly pressure compounding is used for large turbine where efficiency is more important than the capital cost
Slide 14: Comparison Between Velocity and Pressure Compounding Impulse Turbines Velocity Compounding Not equal velocity drop for each stage No pressure drop per stage Non equal power per stage High friction losses due to high velocities Not recommended for more than two stages No problem with steam leak Pressure Compounding Equal velocity drop for each stage Not equal pressure drop per stage Equal power per stage Low friction losses due to reduced steam velocity Recommended for multistage Larger steam leak Suitable for small turbines as well as only Suitable for large turbines for the first stage in large turbine
Slide 15: Advantages of Impulse Turbines  No pressure drop in moving blades   low steam thrust low leakage losses at blade extremities and shaft ends spare parts unnecessary for stationary and mobile blades  Low consumption of spare parts    Compact design High operation flexibility
Slide 16: Reaction Principle  Reaction effect results from issuing a fluid at very high velocity from a nozzle. This results in a reaction which moves the nozzle in the opposite direction. F mV o   Pure reaction happens if the flow is accelerated from zero velocity to its exist velocity in the moving blades. Since this is not the case in turbines, thus there are no pure reaction turbine but it is usually a mix between impulse and reaction. Accordingly the term reaction turbine does not mean a full reaction turbine but a partially impulse and partially reaction.
Slide 17: Reaction Turbine     Reaction turbine has been invented by C.A. Parson Turbine with 50% reaction is the turbine where 50% of the enthalpy drop happens in the stator and the other 50% occurs in the rotor. It is important to mention that this does not mean equal pressure drops. Pressure drop is usually higher for the fixed blades and greater for the high pressure conditions, where the pressure drop per unit of enthalpy drop is higher at the high pressure The rotor blades of a reaction turbine are not symmetrical as in the impulse turbine, they are similar to those of the stator but curved in the opposite direction.
Slide 18: ).Reaction Turbines )Contd   Reaction Ratio “RR” or )Degree of Reaction): is the ratio of enthalpy drop in the rotor to the total enthalpy drop in the stage. Accordingly impulse turbine could be considered as reaction turbine with Zero degree of reaction hstage htotal  n hrotor RR  hstage hstat .  hstage * (1  RR) hrotor  h * RR
Slide 19: Two Stages Reaction Turbine
Slide 20: Analysis of Reaction Stage Vs1  2hstage (1  RR)    Vr22  Vr2 1 hrotor1  2 Vs2  Vs2 2 hstator  3 2 Vr24  Vr2 3 hrotor 2  2 F  m o Vr1 cos   Vr 2 cos  
Slide 21: ).Analysis of Reaction Stage )contd F  m Vs1Cos1  VB  Vr 2Cos  o o P  m VB Vs1Cos1  VB  Vr 2Cos  m P Vs2  Vs2  Vr22  Vr2 1 2 1 2 o m P  hstat  hrotor  2 o   
Slide 22: Optimum Blade Velocity for Reaction Turbine  For the case of similar fixed and moving blades θ=γ Vr 2Cos  Vs1Cos P  m oVB  2Vs1Cos  VB  dP  m o  2Vs1Cos  2VB   0 dVB VB  Vs1Cos Pmax  m Vs1Cos   m oVB2 o 2
Slide 23: Efficiency of the Reaction Turbine  The efficiency of the reaction turbine depends of the efficiency of the fixed and the moving blades. 2 Vs2  Vso ho  h1 stat  1  2hstat ,s ho  h1,s rotor  P Vs2 m 1  hrotor ,s 2 o  P Vs2  m o  1   h1  h2 s   2  P stage  o m  ho  h2 ss 
Slide 24: Efficiency of the Reaction Turbine ).)Contd  It is clear that the reaction turbine is an efficient machine  ho  h1s    h1  h2 s    ho  h2 ss   This can be explain in the light of the steam velocity where for the same VB, where: VB  Vs1Cos Vs1Cos VB  2 1 Vs1R  Vs1I 2 for for Re action Im pulse
Slide 25: Disadvantages of Reaction Turbine   The main disadvantage of the reaction turbine that it is not suitable for large pressure drop, where ΔP/Δh is high at high pressure, and consequently high potential of steam leak. The usual design for large turbine at high boiler conditions is to make the first stage of impulse time )velocity compounding) to reduce the pressure and then continue with reaction stages.
Slide 26: Axial Thrust  The turbine rotor is subjected to axial thrust due to the pressure drop as well as the change in the axial momentum. Faxial  m o Vr1 sin   Vr 2 sin      For impulse turbine and since there is no pressure drop in the rotor blades, the axial thrust is minimum. )Vr1≈Vr2 & Φ=γ). In the reaction turbines the effect of pressure drop is added to the thrust force. A technique to reduce the thrust force is the use of double flow steam turbine. This technique has the following advantages: Steam In   Canceling the thrust force Reduce the thrust due to the reduction in the blades height
Slide 27: Twisted Blades h=1/3Dm Dm Providing that Vs and θ do not change while Φ increases and γ decreases with height due to the increase in VB. This means that the blade will have a twisted shape. This makes the degree of reaction changes along the blade height )impulse at the base and maximum reaction at the top The blade is designed for optimum conditions at the midpoint. VB  ND