Steam Turbine The Impulse Principle Fixed flat plate

Steam Turbine

The Impulse Principle Fixed flat plate n The force on the plate, F is equal to the change in momentum of the jet in +x direction where m is mass-flow rate of the jet, lbm/s or kg/s Vs is velocity in horizontal direction, ft/s or m/s

The Impulse Principle Moving flat plate Force on the plate Power done by jet Efficiency is ratio of power to initial power of the jet where VB is the plate velocity, ft/s or m/s Power is 0 if VB = 0 or (Vs-VB)=0

The Impulse Principle n To find optimum VB, power is differentiated respect to VB Half of jet velocity Half of kinetic energy per unit time of jet

The Impulse Principle For 180 o curved blade For frictionless blade By impulse and momentum principle Double of value on flat plate case

The Impulse Principle n To find optimum VB, power is differentiated respect to VB Half of jet velocity Equal to kinetic energy per unit time of jet

The Impulse Principle n It is impossible to have 1800 curved blade in actual application n n jet exit will impinging on the back of next blade Blade entrance angle and blade exit angle cannot be zero, as shown in the figure below

The Velocity Diagram Relative velocity of fluid (as seen by an observer riding on the blade) Blade Absolute velocity of fluid leaving the entrance nozzle Nozzle angle Blade exit angle Blade velocity Fluid exit angle Absolute velocity of fluid leaving the blade Relative velocity of fluid leaving the blade

The Velocity Diagram Velocity of whirl, Vw

The Velocity Diagram

The Impulse Principle where H 1 and H 2 are the enthalpy entering and leaving the blade H 1 - H 2 is obtained by considering fluid flow relative to the blade (observer is on the blade), where only relative velocities and no work are observed. n From first-law of thermodynamics, n for adiabatic system and ΔPE = 0 Including friction, expansion or contraction

The Impulse Principle n In case of pure impulse (no friction, no expansion and no contraction), n H 1 = H 2 and Vr 1 = Vr 2 n Friction is described by, velocity coefficient, kv n Stage efficiency is the ratio of work of the blade divided by the total enthalpy drop for the whole blade

Impulse Turbine Blade is usually symmetrical. n Entrance angle (φ ) and exit angle (γ) are around 20 o. n Usually used in the entrance high-pressure stages of a steam turbine. n Enthalpy drop and pressure drop occur in the nozzle. n

The Single-Stage Impulse Turbine n De Laval turbine • Steam is fed through one or several convergent-divergent nozzles. • Pressure drop occurs in the nozzle (not in the blade) • Maximum velocity (kinetic energy) occurs at nozzle exit.

Compounded-Impulse Turbine n For single-stage impulse turbine n For modern boiler conditions, expansion in single nozzle stage gives 1645 m/s. Beyond the maximum allowable safety limits. (due to centrifugal stress) To overcome these difficulties, n n Velocity-compounded turbine Pressure-compounded turbine

Velocity-Compounded Impulse Turbine n Curtis stage turbine

Velocity-Compounded Impulse Turbine Nozzle angle Number of stages

Velocity-Compounded Impulse Turbine n Work ratio n n n for 2 stages turbine 3: 1 for 3 stages turbine 5: 3: 1 for 4 stages turbine 7: 5: 3: 1

Pressure-Compounded Impulse Turbine Δ htot = the total specific enthalpy drop of the turbine n = the number of stages Enthalpy drops per stage are the same n Rateau turbine Pressure drops are not

Pressure-Compounded Impulse Turbine Advantages of reduced blade velocity, reduced steam velocity (hence friction) Equal work among the stages. Disadvantages pressure drop across the fixed nozzles require leak-tight diaphragm to avoid steam leakage.

Reaction Principle n n Fixed nozzle, a rocket, a whirling lawn sprinkle and turbine are devices that cause a fluid to exit at high speeds. The fluid beginning with zero velocity inside, creates a force in the direction of motion F equal to

Reaction Turbine Nozzles with full steam admission Unsymmetrical blade Similar shape to fixed blade (opposite direction curve) pressure Pressure continually drops through all rows of blades (fixed and moving) Absolute velocity changes within each stage repeats from stage to stage Absolute velocity 50 % Degree of reaction -Half of enthalpy drop of the stage occurs at fixed blade -Half of enthalpy drop of the stage occurs at moving blade

Reaction Turbine

Reaction Turbine Fixed-blade (nozzle) efficiency Enthalpy Moving-blade efficiency Stage efficiency Entropy

Reaction Turbine Reaction stage has pressure drop across the moving blade. n Not suitable for high pressure stage because pressure drop is very high and results in steam leakage around the tips of the blades. n Impulse turbine is normally used for HP stages. n Reaction turbine is normally used for LP stages. n

Axial Thrust n Impulse turbine n Little pressure drop on the moving blade from friction Change in axial component of momentum of the steam from entrance to exit n n For pure symmetrical impulse blades, Vr 1 = Vr 2 and φ = γ, axial thrust is zero.

Axial Thrust Reaction turbine n Change in axial momentum is zero. n Large and continual pressure drop across the moving blade. n Axial thrust is quite large. n Thrust bearing to support axial thrust. n Dummy piston (rings) to balance axial thrust n

Steam Turbine

Twisted Blades n n n Reaction blades are high, especially in the latter stages. VB increases with radius from base to tip of blade. Vs 1 and θ do not vary in radial direction. Increase from root to tip decrease from root to tip

Twisted Blades

Combination Turbines Case 1 n Curtis stages (Velocity compounded impulse) n n First two-rows Rateau stages (Pressure compounded impulse) n Latter stages Case 2 n Curtis stages n n First one or two-rows Reaction stages

Combination Turbines n Impulse stage Suitable for high pressure n No pressure drop on moving blade n For same enthalpy drop, much larger pressure drop occurs at high pressure. n Higher pressure drop = more possibility for leakage between blade tip and casing n n Reaction stage n More efficient at low pressure

Turbine Configurations Tandem compound – single shaft n Cross compound – two parallel shaft n HP turbine – high pressure turbine n IP turbine – intermediate pressure turbine n LP turbine – low pressure turbine n LSB – last stage blade n

Turbine Configurations

Steam Flow Path Straight through Single reheat Extraction Induction (or mixed flow)

Turbine Rotors HP inlet HP Exhaust IP inlet IP Exhaust LP inlet LP Exhaust Almost all of turbines are placed face-to-face, especially in IP and LP turbine, which comprise of reaction stages. n What is the reason for this arrangement? n

What is the configuration type of this steam turbine?