Yet Another Four Losses in Turbines - 1

P M V SubbaraoProfessor

Mechanical Engineering Department

A Set of Losses not Strictly due to Geometry of Blading….

Losses by stage and section for a 700 MW turbine.Source: Toshiba

Other Losses in Steam Turbines

• Disc Friction & Windage Losses : Miscellaneous Losses• Partial Admission Losses• Wetness Losses• Leaving Losses

Diaphragms & Interaction with Steam

Generation of Windage Vortices

An Unwanted Fluid Mechanics : Churning?

• The rotors act like a giant food mixer stirring the flow.

• The blades are moving so fast that they create a lot of heat in the steam.

• This causes the steam temperature to rise and the blades become hotter and they expand.

• The LP casings become hotter which affects the differential movements between the rotor and casing.

• Water sprays are fitted in the exhaust of large LP turbines, to keep the blades and casings cool

Losses due to Disc Friction and Windage

• Frictional forces appear between the rotating turbine disc and the steam enveloping it.

• The rotating disc drags the particles near its surface and imparts to them an accelerating force in the direction of rotation.

• A definite amount of mechanical work is spent in overcoming the effect of friction and imparting this acceleration.

• The enegry dissipated (heat generated ) per blade row is proportional to ρ, N3 h D4

• Considerable heat can be generated in the LP stages by windage !!!

Estimation of Windage Losses

• The magnitude of these losses is calculated using Stodola’s Empirical Formula:

turbinewind PU

dlDP 6

25.1

12

10161.007.1

Pwind: Power los in overcoming friction and windage.λ : fluid Coefficient: 1 for air or highly superheated steam, 1.1 – 1.2 for ordinary superheated & 1.3 for saturated steam.D : Mean diameter of the disc.ε: Degree of partial admission.l1: Heght of blades, in cm.U : Velocity of blade at mean diameter, m/s.ρ: density of steam, kg/m3.

Partial Admission in Turbines

• Partial admission applied as control stage yields high part load efficiency and high specific work output due to a maintained high inlet pressure for the turbine in the fully admitted sectors.

Role of Valves on Turbine Enthalpy Diagrams

Full load at Design Conditions

Macro Thermodynamic Model for hp Drum

Three simultaneous Equations

availablehmP

1

1in

exitinavailable p

pTh

in

exitin

T

ppm

222

Part Load Conditions

The thermodynamics of partial admission can be explained by a comparison to simple throttling valve.

Deliberate Partial Admission at Full Loads• Partial admission is sometimes deliberately used at full load in small

scale turbine stages.

• This helps in avoiding short blades in order to reduce the tip leakage loss and losses induced by endwall flows.

• Radial dimensions of turbine blades and flow channels are primarily a function of the volumetric flow rate throughout the machine, and consequently become reduced for small turbines.

• The physical size of the turbines has a great deal of importance for the isentropic turbine efficiency.

• In an “ideal” machine where all the geometrical parameters could be held at a constant ratio to blade cord length, the small size would have very little impact on turbine efficiency.

• This in according to similarity rules, only a decrease in Reynolds number may affect the losses.

• In reality these ratios are not possible to practically uphold and the losses become large for small machines.