LP ByPass System

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24 December 2013 PMI Revision 00 1

LP BY PASSSYSTEM




Presentation outline• Introduction of LP Bypass system

• Overview of LP Bypass

• System details and lay out

• Control scheme of LP bypass system•




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.




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




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

Cold condition

Warm condition

Hot Condition

LP BYPASS SYSTEM



24 December 2013 6

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




• 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




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




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




• If LP steam- bypass system fails or the

condenser cannot accept LP bypass

steam , Safety Valves open and dump

steam to atmosphere




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




Control scheme

• Set point formation

• Pressure control for LP bypass system

•

LP bypass HYDRAULIC System• Protection system




• 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




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




Bypass Controller Block Diagram




LP Bypass System




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




LP BYPASS SETPOINTS




CONTROLLER

OUTPUT

CHARACTERI

STICS




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




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




DJUSTMENT

OF

LPBYP SS

CONVERTER




Design of LPBypass Valves for 210 MW

Steam Turbine

• Controlled by8 bar mineraloil




Design of LPBypass Valve for 500

MW Steam Turbine

• Controlled by 36bar FRF




Protection Devices

Low Vacuum Condensor Protection

Spray Water Pressure Switch

Condensor Temperature Protection




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 )




Water Injection Valves operate in two stages

• Second stage Water Injection Valves openwhen steam flow through LPBypass Valvesincreases more than 45 %

CURVE FOR LPBYPASS




CURVE FOR LPBYPASS

CONTROLLER




Damping device




Actuator Bypass

Stop Valve




ActuatorLPBPCV

500 MW




Pilot valve

for Water

Injection Valve




Water

InjectionValve




Combined Bypass

Stop & Control

Valve 210 MW

Feedback for Pilot valve of




Feedback for Pilot valve ofBypass Control Valve 210 MW



THANK YOU

LP ByPass System

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