Design Criteria for LP BYPASS SYSTEM

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Design Criteria forLP BYPASS SYSTEM
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LP Bypass system provide a very effective means
of controlling reheat steam flow during

• Startup
• Low-load / part load operation
• Sudden load rejection

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It enhances fast starting capability of turbines
from all initial conditions e.g.

• Cold condition
• Warm condition
• Hot Condition

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LP steam-bypass system operate in parallel with
the turbine

• Protect Condenser from excessive temperature and
  pressure that could result from sudden injection
  of superheated steam

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• Ensure coordinated control between HP and IP
  turbine control valves and the steam bypass
  systems to operate at any remaining load
• Subsequently reloaded in a controlled manner

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Schematic Diagram of Bypass System
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• The controls for LP Bypass system are combination
  of electrical and well proven hydraulic system
• Electro-hydraulic converter provides the
  necessary link between hydraulic actuator and
  electrical system

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• The LPBypass Controller acts as a maximum
  pressure Controller
• It regulates the reheater pressure either to the
  fixed set value or suitably derived variable set
  value
• If the reheater pressure exceeds this set value,
  the Controller causes the electro-hydraulic
  converter to operate and initiate bypass operation

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Set Value formation

• The Two set values fixed and variable are
  formed for the LPBypass control system
• set value to be used is determined by a maximum
  value selector
• For the formation of variable set point, a
  pressure transducer is used to measure the steam
  pressure in HP turbine

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SEQUENCE OF EVENTS RESULTING FROM LOAD REJECTION

• Upon receiving load rejection signal from
  electrical system
• Turbine Speed Controller assumes control of the
  TG set
• It initiates fast closure of HP IP turbine
  control valves through position controller
• Fast closure of turbine control valves cause an
  immediate decrease in HP 1st. Stage pressure

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SEQUENCE OF EVENTS RESULTING FROM LOAD REJECTION

• Pressure increases in main and reheat steam
  systems
• These pressure changes are sensed by HP and LP
  Bypass controllers
• Respective bypass valves are opened

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• If LP steam- bypass system fails or the
  condenser cannot accept LP bypass steam , Safety
  Valves open and dump steam to atmosphere

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Hydraulic Mode of Operation

• The controller controls plunger coil of Electro
  Hydraulic Converter
• With increase in voltage, the jet pipe is
  deflected towards right
• Converter piston moves downwards leading to
  generation of LPB signal oil through Follow-up
  pistons provided in the hydraulic control system

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Hydraulic Mode of Operation

• Two Follow-up Pistons actuate and open first
  stage of water injection valve
• Remaining two Follow-up Pistons generate signal
  oil for opening LPBypass stop and control valve

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• ADJUSTMENT
• OF
• LPBYPASS
• CONVERTER

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Design of LPBypass Valves for 210 MW Steam Turbine

• Controlled by 8 bar mineral oil

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Design of LPBypass Valve for 500 MW Steam Turbine

• Controlled by 36 bar FRF

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Protection Devices

• Low Vacuum Condensor Protection
• Spray Water Pressure Switch
• Condensor Temperature Protection

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Protection Devices cause closure of bypass
station in case of following eventualities

• Condensor vacuum is low ( gt 0.6 bar absolute )
• Spray water pressure is low (lt 8.5 / 9.5 bar )
• Temperature of condenser body is high (gt 90 C )

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Water Injection Valves operate in two stages

• Second stage Water Injection Valves open when
  steam flow through LPBypass Valves increases more
  than 45

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CURVE FOR LPBYPASS CONTROLLER
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• Damping device

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