A1258149933ncmEG

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Estimation of Losses in Large Turbines
P.M.V. Subbarao Associate Professor
Department of Mechanical Engineering Indian
Institute of Technology, Delhi
Accounting of Losses is Saving of Losses
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Sequence of Energy Losses
Steam Thermal Power
Blade kinetic Power
Steam kinetic Power
Nozzle Losses
Stage Losses
Moving Blade Losses
Isentropic efficiency of Nozzle
Blade Friction Factor
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Losses in Turbine Stage

• Losses in Regulating valves The magnitude of
  loss of pressure due to throttling with the
  regulating valves fully open is
• Dpv 3 to 5 of pmax.
• Loss in nozzle blades.
• pressure loss in moving blades.
• Loss due to exit velocity.
• Loss due to friction of the disc and blade
  banding
• Loss associated with partial admission.
• Loss due to steam leakages through clearances.
• Loss due to flow of wet steam.
• Loss due to exhaust piping.
• Loss due to steam leakage in seals.

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Losses in Nozzles

• Losses of kinetic energy of steam while flowing
  through nozzles or guide blade passages are
  caused because of
• Energy losses of steam before entering the
  nozzles,
• Frictional resistance of the nozzles walls,
• Viscous friction between steam molecules,
• Deflection of the flow,
• Growth of boundary layer,
• Turbulence in the Wake and
• Losses at the roof and floor of the nozzles.
• These losses are accounted by the velocity
  coefficient, f.

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Turbine pressure profile at designed condition
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Temperature profiles at designed condition
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HP Turbine per stage Enthalpy drop profiles at
designed condition.
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Nozzle Moving Blade Losses for HP Stages at
Designed condition
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IP Turbine per stage Enthalpy drop profiles at
designed condition.
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Nozzle Moving Blade Losses for IP Stages at
Designed condition
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LP Turbine per stage Enthalpy drop profiles at
designed condition.
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Nozzle Moving Blade Losses for LP Stages at
Designed condition
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GLAND Leakage Flows
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• Gland leakage losses
• The steam leaked out from the system does
  not work on the blades, it represents energy loss
• 1.Diaphragm leakage
• It takes place in stages through the radial
  clearance between the stationary nozzle diaphragm
  and the shaft or drum.
• 2.Tip leakage
• It occurs in stages through the clearance
  between the outer periphery of the moving blades
  and the casing due to the pressure difference
  existing across the blade.
• 3.Shaft leakage
• Shaft leakage occurs through radial clearance
  between the shaft and casing at both high and low
  pressure ends of turbines.
• At the high pressure end , steam leaks out to
  the atmosphere, whereas at the LP end, the
  pressure being less than the atmospheric , air
  leaks into the shell

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Loss by leakage through diaphragm gland,
Loss by leakage through banding gland,
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Leakage Losses for Turbine Stages
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• Turning loss
• These occurs as the steam turns in the blade
  passage.
• Disc friction loss
• When the turbine disc rotates in the viscous
  steam, there is surface friction loss due to
  relative motion between the disc and steam
  particles. Due to centrifugal force , steam
  thrown radially outward.
• The moving disc exerts a drag on the steam, sets
  it in motion from root to tip, and produces a
  definite circulation.
• Some part of Kinetic energy of steam is lost due
  to this friction.

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Profile Losses for Turbine Stages at Designed
Condition
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• Losses associated with partial admission of steam
• Partial admission of steam to turbine stages is
  employed in cases when the volume flow rate of
  steam is not high (ie. Turbine of low capacity)
• In turbine with partial admission ,steam is fed
  onto the moving blades only an arc of length ,
  rather than along the entire circumference.
• Along the arc , there is no active flow of steam
  , and the blade passage opposing this arc are
  filled with stagnant steam from the disc chamber.
• Owing to the rotation of the disc , the steam
  filling this passage is entrained by centrifugal
  force and moves from the roots to tips of moving
  blade

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• Steam can even flow from one side of the blade to
  the opposite side

Diagram of windage currents in a partial
admission turbine stages

• The work associated with this motion of the steam
  in blade passages of the inactive portion of the
  arc of moving blades, is lost-
• Usual energy of the turbine stage is decreased
  by the energy loss associated wit this motion (
  windage) of steam in blade passages

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• Residual velocity loss
• Steam leaving the last stage of the turbine has
  certain velocity, which represent the amount of
  kinetic energy that cannot be imparted to the
  turbine shaft and thus it is wasted
• Exhaust end loss
• Exhaust end loss occur between the last stage of
  low pressure turbine and condenser inlet.
• 2. Exhaust loss depends on the absolute
  steam velocity.
• Turbine Exhaust end loss Expansion-line -end
  point - Used energy end
• point.

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Typical exhaust loss curve
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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.

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• 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.

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• 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.

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Cumulative Loss for Turbine Stages at Designed
Condition