Performance of Low Pressure Turbine Stages
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Performance of Low Pressure Turbine Stages. P.M.V. Subbarao Associate Professor. Department of Mechanical Engineering Indian Institute of Technology, Delhi. More Problems and Higher Outputs…. L P Turbine Stages. LP Turbine Exhaust System .

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Performance of Low Pressure Turbine Stages

P.M.V. Subbarao

Associate Professor

Department of Mechanical Engineering

Indian Institute of Technology, Delhi

More Problems and Higher Outputs…..



Lp turbine exhaust system
LP Turbine Exhaust System

  • In a condensing steam turbine, the low-pressure exhaust hood,consisting of a diffuser and a collector or volute!, connects the

  • last stage turbine and the condenser.

  • The function of the hood is to transfer the turbine leaving kinetic energy to potential energy while guiding the flow from the turbine exit plane to the condenser.

  • Most of exhaust hoods discharge towards the downward condenser.

  • Flow inside the hood therefore must turn about 90 deg from the axial direction to the radial direction before exhausting into the condenser.

  • The 90-deg turning results in vortical flow in the upper half part of the collector and also high losses.

  • The exhaust hood is one of the few steam turbine components that has the considerable aerodynamic losses.

  • It is a challenge for engineers to operate a hood with high pressure recovery and low total pressure loss in a compact axial length.




Steam turbine exhaust size selection
Steam Turbine Exhaust Size Selection

  • The steam leaving the last stage of a condensing steam turbine can carry considerably useful power to the condenser as kinetic energy.

  • The turbine performance analysis needs to identify an exhaust area for a particular load that provides a balance between exhaust loss and capital investment in turbine equipment.


Typical exhaust loss curve :

Annulus restriction loss

Annulus velocity (m/sec)

SP.Volume

Total Exhaust Loss

50

Condenser flow rate

Turn-up loss

40

Gross hood loss

Annulus area

Exhaust Loss of dry flow

30

20

Actual leaving loss

Percentage of Moisture at the Expansion line end point

10

0

120

240

180

240

300

360

Annulus Velocity (m/s)



Problems in low pressure turbine
Problems in Low pressure turbine

  • In the case of condensing turbines the last few stages operate under wet steam conditions.

  • This results in the formulation of minute droplets of water.

  • These droplets under the influence of centrifugal force are thrown out towards the periphery.

  • At the same time these droplets of water receive an accelerating force from the steam particles in the direction of flow .

  • Thus some of the kinetic energy of the flowing steam is lost in accelerating these water droplets.

  • The absolute velocity of the steam is considerably greater than that of the water droplets into the moving blade passages.

  • The water droplets are deflected onto the back of the moving blades as a result of which the moving blades experience an impact force caused by impingement of the moving blades.

  • As a result of this moving blades experience an impact force caused by the impingement of water droplets on their backs.


  • The practical investigations that the blade tips are subjected to wear from one side water droplets present in the last few stages can also result in erosion damage of turbine blades and nozzles .

  • One of the loss mechanisms in the steam turbine is the kinetic energy of the steam as it leaves the last stage blade.

  • The lower the kinetic energy, the higher the steam turbine efficiency will be.

  • The magnitude of loss is proportional to the square of the ratio of the volume flow rate of the steam through the last stage of the steam turbine and the annulus area of the turbine exit.

  • To decrease the loss, a larger turbine exit annulus area is needed.

  • An increase in the last stage blade annulus area can be accomplished by either using shorter blades mounted on a larger diameter rotor (larger “hub”) or

  • by using longer blades mounted on a smaller diameter rotor.


  • The low-pressure turbine exhaust end is one of the important factors affecting the turbine performance.

  • The size of the exhaust end is determined by the number of exhaust flows and the length of the last stage blades.

  • In general, the larger the exhaust ends, the lower the full load net heat rate.

  • Under the part-load conditions, turbines with a large exhaust end will deteriorate more rapidly in performance.


Cumulative loss for turbine stages at designed condition
Cumulative Loss for Turbine Stages at Designed Condition factors affecting the turbine performance.


Simplified performance analysis
Simplified Performance Analysis factors affecting the turbine performance.

Governing Stage Efficiency Calculations

where

= Correction coefficient for governing stage efficiency.


Calculations of HP and IP Turbine Efficiencies factors affecting the turbine performance.

where

kg/sec.

= average steam flow rate =

= Steam flow rate at entry of group in kg/sec,

= Steam flow rate at exit of group in kg/see

And similarly

=

m3/kg

is the available enthalpy drop of the group


Calculations of LP Turbine Efficiency factors affecting the turbine performance.

where correction for wetness fraction

= 0.8 for peripheral moisture separation design.

Exit velocity loss is given by


= axial surface area at the exit from last stage moving blades

average diameter to blade height ratio is

,

i = No. of flows in LP turbine


is exit velocity loss coefficient = blades

= Nozzle exit angle

Z = No. of stages in group,


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