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7/27/2019 LP ByPass System
http://slidepdf.com/reader/full/lp-bypass-system 1/43
24 December 2013 PMI Revision 00 1
LP BY PASSSYSTEM
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Presentation outline• Introduction of LP Bypass system
• Overview of LP Bypass
• System details and lay out
• Control scheme of LP bypass system•
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L P BYPASS SYSTEM
• LP bypass bypasses steam to IPT & LPT & maintain flow
through RH.
Function of LP bypass:-
1. To monitor the pressure in reheat system and to control it
under certain operating condition.
2. The volume of steam not utilized by the IPT & LPT during
start up & shut down must pass through condenser.
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LP Bypass system provide a very effective means ofcontrolling reheat – steam flow during
Startup Low-load / part load operation
Sudden load rejection
LP BYPASS SYSTEM
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It enhances fast starting capability of turbines from allinitial conditions e.g.
Cold condition
Warm condition
Hot Condition
LP BYPASS SYSTEM
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LP steam-bypass system operates in parallel with theturbine
Protects Condenser from excessive temperature andpressure that could result from sudden injection of
superheated steam
LP BYPASS SYSTEM
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• The controls for LP Bypass system are combination of electricaland well proven hydraulic system
• Electro-hydraulic converter provides the necessary linkbetween hydraulic actuator and electrical system
LP BYPASS SYSTEM
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SEQUENCE OF EVENTS RESULTING FROM LOAD
REJECTIONUpon receiving load rejection signal from electricalsystem:
Turbine Speed Controller assumes control ofthe TG set
It initiates fast closure of HP & IP turbinecontrol valves through position controller
Fast closure of turbine control valves cause animmediate 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 LPBypass 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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System details and layout
• The controls for LP Bypass system are combinationof electrical and well proven hydraulic system
•
Electro-hydraulic converter provides the necessarylink between hydraulic actuator and electricalsystem
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Control scheme
• Set point formation
• Pressure control for LP bypass system
•
LP bypass HYDRAULIC System• Protection system
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• The LPBypass Controller acts as a maximumpressure Controller
• It regulates the reheater pressure either to thefixed set value or suitably derived variable setvalue
• If the reheater pressure exceeds this set value,the Controller causes the electro-hydraulic
converter to operate and initiate bypassoperation
Control scheme
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Set Value formation
The Two set values – fixed and variable are formed for theLPBypass control system
set value to be used is determined by a maximum valueselector
For the formation of variable set point, a pressuretransducer is used to measure the steam pressure in HPturbine
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Bypass Controller Block Diagram
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LP Bypass System
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Features of LPBP Controller
• LPBP controller maintains pressure after reheater
by controlling LP bypass valves
• It acts on the coil of the LPBP E-H converter to
position the control valves.
• Two setpoints, fixed and variable are used in
maximum selection circuit
• Variable setpoint is derived from HP first stage
pressure
• Water spray is in two stages depending on steam
flow through LPBP stations.
• LPBP valves cannot open until adequate spray
water pressure and required vacuum is there and
condenser wall temp is not high.
• Valves can also be operated manually
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LP BYPASS SETPOINTS
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CONTROLLER
OUTPUT
CHARACTERI
STICS
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Hydraulic Mode of Operation
• The controller controls plunger coil of ElectroHydraulic 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-uppistons provided in the hydraulic control system
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Hydraulic Mode of Operation
• Two Follow-up Pistons actuate and open first stage ofwater injection valve
• Remaining two Follow-up Pistons generate signal oil foropening LPBypass stop and control valve
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DJUSTMENT
OF
LPBYP SS
CONVERTER
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Design of LPBypass Valves for 210 MW
Steam Turbine
• Controlled by8 bar mineraloil
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Design of LPBypass Valve for 500
MW Steam Turbine
• Controlled by 36bar 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
a) Condensor vacuum is low ( > 0.6 bar absolute )
b) Spray water pressure is low (< 8.5 / 9.5 bar )c) Temperature of condenser body is high (> 90° C )
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Water Injection Valves operate in two stages
• Second stage Water Injection Valves openwhen steam flow through LPBypass Valvesincreases more than 45 %
CURVE FOR LPBYPASS
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CURVE FOR LPBYPASS
CONTROLLER
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Damping device
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Actuator Bypass
Stop Valve
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ActuatorLPBPCV
500 MW
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Pilot valve
for Water
Injection Valve
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Water
InjectionValve
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Combined Bypass
Stop & Control
Valve 210 MW
Feedback for Pilot valve of
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Feedback for Pilot valve ofBypass Control Valve 210 MW
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