low pressure governing system

PRESENTATION ON

GOVERNING SYSTEM

PRESENTED BY:BALGOPAL RAJU

ASSISTANT MANAGER (E&I)PERSONAL NO. :08303/08789

OPERATION DEPARTMENTCAPTIVE POWER PLANT (CPP)

NATIONAL ALUMINIUM COMPANY LIMITED (NALCO)

WHAT IS GOVERNING SYSTEM ROLE OF GOVERNING SYSTEM METHODS OF GOVERNING TYPE OF GOVERNING SYSTEMS CHARACTERISTICS OF GOVERNING SYSTEM HIGH PRESSURE GOVERNING SYSTEM LOW PRESSURE BYPASS SYSTEM

WHAT IS GOVERNING SYSTEM ?

WHAT IS GOVERNING SYSTEM

IT IS A GROUP OF DEVICES PROVIDED IN STEAM TURBINE

SYSTEM WHICH WORK IN COHERENCE TO MAINTAIN THE

SPEED OF T-G SET CONSTANT.

THIS CAN ALSO BE DEFINED AS :

IT IS A GROUP OF DEVICES WHICH INITIATE THE CORRECTIVE

ACTION TO RECTIFY THE DEVIATION OCURRED DUE TO

VARIATION IN SET RELATION BETWEEN INPUT AND OUTPUT.

BLOCK DIAGRAM OF GOVERNING SYSTEM

ROLE OF GOVERNING SYSTEM

ROLE OF GOVERNING SYSTEM

MAINTAIN THE TURBINE SPEED REASONABLY CONSTANT AT ALL

LOADS IN ORDER TO MAINTAIN THE GRID FREQUENCY WHEN

TURBINE IS CONNECTED TO GRID .

PROVIDE VERY FAST CONTROLLING ACTION.

SUCCESSFULLY LIFT AND CLOSE THE CONTROL VALVES ON

WHICH VERY LARGE STATIC AND DYNAMIC FORCES ACT.

ROLE OF GOVERNING SYSTEM (contd.)

PROVIDE A LINEAR RELATIONSHIP BETWEEN VALVE LIFT V/S

LOAD.

TO BE VERY SENSITIVE TO SPEED CHANGES OF THE ORDER OF

0.06% OF RATED SPEED (ie 1.8 rpm).

TO BE FREE FROM HUNTING AND NOT TO HAVE INDUCED

OSCILATIONS. (FOR ENSURING GOOD STABILITY A FEEDBACK

SIGNAL IS PROVIDED AT EVERY STAGE OF SIGNAL AMPLIFICATION).

TYPES OF GOVERNING-I

Nozzle Governing

Throttle governing

Bypass Governing

Governing is effected by varying the amount of steam which is further

done by changing the positions of control valves.

Any Throttling of Steam will lead to losses . In

governing system along with the change of quantity of steam the

quality of the steam also changes. Therefore Throttling should be

minimum.

NOZZLE GOVERNING

IN NOZZLE CONTROLLED TURBINES, STEAM FLOW IS

CONTROLLED BY SEQUENTIAL OPENING OR CLOSING OF

CONTROL VALVES ALLOWING STEAM TO FLOW TO ASSOCIATED

NOZZLE GROUPS.

THROTTLE GOVERNING

IN THROTTLE CONTROLLED TURBINES, STEAM FLOW IS

CONTROLLED BY SIMULTANEOUS OPENING OR CLOSING

OF ALL CONTROL VALVES ALLOWING THE STEAM TO FLOW

TO THE GROUP OF NOZZLES LOCATED ON THE ENTIRE

PERIPHERY.

THROTTLE GOVERNING

NOZZLE GOVERNIMG

All the valves in a set open or close

simultaneously / together.

The Valves in a set opens or closes

consequentially or in a sequence.

It is a Full arc Admission Turbine. It is a Partial Arc admission turbine

except at full Load.

It is most suitable for full load or

based Load plants.

It is good for Low load or variable

load turbines.

No operational Problems. May have operational Problem at

partial Loads.

BYPASS GOVERNING

It is employed in small capacity turbines running on high pressure

conditions and with small blading dimensions.

Here the loading up to approx. 80% ( Economic loading ) is met by

normal control valves feeding the First stage. For higher loading , to

supply more steam which is not possible due to small blading

dimensions ( can lead to operational problems) the extra quantity of

the steam is fed to the intermediate section of turbine bypassing the

initial high pressure stages.

TYPES OF GOVERNING-II

Constant Pressure Mode: Here pressure upstream of control valves is

kept constant and change is made by changing the position of control

valves.

Variable Pressure mode : Here control valves are in full open position and

pressure upstream of control valves varies proportionately with the load

requirement.

Response of Constant pressure Mode is much faster than Variable

pressure mode, but Constant pressure leads to more losses.

TYPE OF GOVERNING SYSTEMS

TYPE OF GOVERNING SYSTEM

MECHANICAL

HYDRO-MECHANICAL

HYDRAULIC

ELECTRO-HYDRAULIC

HIGH PRESSURE GOVERNING

MECHANICAL

HERE THE SPEED TRANSMITTER IS MECHANICAL

CENTRIFUGAL TYPE SPEED GOVERNOR WHICH ACTUATES

CONTROL VALVES THROUGH MEHANICAL LINKAGES.

PURE MECHANICAL GOVERNING SYSTEM ARE NOT USED FOR

UTILITY TURBINES NOW A DAYS.

HYDRO-MECHANICAL

IN HYDRO MECHANICAL GOVERNING SYSTEM SPEED

TRANSMITTER IS MECHANICAL CENTRIFUGAL TYPE SPEED

GOVERNOR. IT IS CONNECTED TO A HYDRAULIC SYSTEM

EITHER HYDRAULICALLY OR MECHANICALLY.

IN THE HYDRAULIC SYSTEM, THE SIGNAL IS AMPLIFIED SO THAT

CONTROL VALVES SERVOMOTORS CAN BE ACTUATED.

HYDRAULIC

IN HYDRAULIC GOVERNING SYSTEM SPEED TRANSMITTER IS

CENTRIFUGAL PUMP WHOSE DISCH. PR. IS PROPORTIONAL TO

SQUARE OF SPEED. THE PR. OIL IS SENT TO HYDRAULIC

GOVERNOR WHICH GENERATES A HYD. SIGNAL PROPORTIONAL

TO VALVE OPENING /CLOSURE REQUIRED.

THE HYD. SIGNAL IS SUITABLY AMPLIFIED THROUGH A HYD.

AMPLIFIER BEFORE APPLYING IT TO CONTROL VALVES

SERVOMOTORS.

HYDRAULIC GOVERNING IN STEAM TURBINES

HYD. AMPLIFIERwith F.U. Piston

HPCV SERVOMOTOR IPCV SERVOMOTOR

HPCV IPCV

HYD. SPEED GOVERNOR

HYD. SPEED TRANSMITTER

HYD. PRESS.

HYD. SIGNAL(ERROR)

HP SECONDARY OIL IP SECONDARY OIL

MECHANICAL COUPLING MECHANICAL COUPLING

ELECTRO-HYDRAULIC

IN ELECTRO-HYDRAULIC GOVERNING SYSTEM, SPEED TRANSMITTER IS

ELECTRICAL SPEED MEASURING DEVICE WHICH SENDS AN

ELECTRICAL SIGNAL TO ELECTRONIC CONTROLLER. AN

ELECTROHYDRAULIC CONVERTER CONNECTS THE CONTROLLER TO A

HYDRAULIC SYSTEM. THE HYD. SIGNAL IS AMPLIFIED SO THAT

CONTROL VALVES SERVOMOTORS CAN BE ACTUATED.

THE ELECTRO-HYDRAULIC GOVERNING OFFERS MULTIPLE

ADVANTAGES OVER OTHER THREE TYPES OF GOVERNING SYSTEMS

AND IS VERY POPULAR FOR UNIT RATINGS LARGER THAN 200 MW.

ELECTRO-HYDRAULIC GOVERNING IN STEAM TURBINES

EHCwith F.U.Piston

HPCV SERVOMOTOR IPCV SERVOMOTOR

HPCV IPCV

ELEC. SPEED TRANSMITTER

HP SEC. OIL IP SEC OIL

MECHANICAL COUPLING MECHANICAL COUPLING

ELECTRONIC CONTROLLER

ELECTRICAL SIGNAL

ELECTRICAL SIGNAL(ERROR)

COMPARISON – LOW vs HIGH PRESSURE

Characteristic Low pr.Gov. High pr. Gov.

Operating pr. 8/32 bar 160 bar

Size of actuator Bulky Small & compact

Command toActuator Hydraulic Electronic

No. Of pipe connection In Actuator 6 2

Gov. & ProtectionElements Hydraulic Electronic

Cf piping layout

Voluminous &Congested

Simple & compact

CHARACTERISTICS OF

GOVERNING SYSTEM

STEADY STATE SPEED REGULATION

Steady state speed regulation (also called proportional band, speed

droop etc.) Is defined as the ratio of % speed droop from no load to

full load to the nominal speed.

In the steam turbines equipped with electrohydraulic governing

system, the speed regulation can be adjusted between 2.5 to 8% in

very fine steps.

TRANSIENT SPEED RISE

In the event of full load rejection, speed of the t-g set shoots up

temporarily before settling down to steady state value.

This temporary speed increase is called transient speed rise and is

expressed as percentage of rated speed.

Value of transient speed rise is restricted to 8% of rated speed.

DEAD BAND

Due to inherent inertia of the components of governing elements and

friction present in governing system, certain motion of governing

system is lost before corrective signal can actuate control valves. The

lost motion is called dead band of the system and is expressed as

percentage of the rated speed.

The order of dead band in Siemens turbines is 0.06%.

BLOCK DIAGRAM FOR LOW PRESSURE GOVERNING SYSTEM

S.No Name of Oil Source & pressure Purpose Colour Code

1. Control Oil MOP/AOP ( 7-8.5 Kg/cm2) To establish Trip Oil, Aux Trip Oil, Trip Oil, Start-up Oil, Aux Start-up Oil, Aux Secondary Oil, Secondary

Oil, Primary Oil, Etc2.

Trip OilMain Trip Valve (4.8-6

Kg/cm2)To reset , Open & Trip stop valve

3.Aux Trip Oil

Main Trip Valve (4.8-6 Kg/cm2)

To trip Hydraulic Trip Device

4.Start-up Oil

Starting Device ( 5.5 Kg/cm2)To assist Resetting & Opening Stop Valves

5. Aux Start-up Oil

Starting Device ( 5.5 Kg/cm2) To Reset Main Trip Valve & Hydraulic Trip Device

6. Aux Secondary Oil

Speeder Gear ( 2.3-4.8 Kg/cm2)

Input Signal to Hydraulic Converter

7. Secondary Oil (HP/IP)

Hyd/Electro-Hyd Converter ( 0-6 Kg/cm2)

To govern HP/IP Control Valve opening

8.Primary Oil

Primary Oil Pump ( 0-2.3 Kg/cm2)

Feedback speed signal for HG

9.Test Oil

Over Speed Test Device ( 2.0 Kg/cm2)

To test Over Speed Test Device

10. Drains

Diff. oils, their color codes, sources & purpose in KWU Governing System

CONTROL OIL OIL TAKEN FROM AOP/MOP DISCHARGE HEADER FOR THE GOVERNING SYSTEM

TRIP/AUX TRIP OILTHESE TWO OIL CIRCUITS ARE ESTABLISHED THROUGH MAIN TRIP VALVES UNDER RESET CONDITION.

START-UP/AUX START-UP OIL THESE TWO OIL CIRCUITS ARE ESTABLISHED TO ITS PEAK PRESSURE THROUGH STARTING DEVICE AT 0% POSITION AND PRESSURE STARTS REDUCING WHEN POSITION OF THE STARTING DEVICE IS RAISED.

AUX SECONDARY OIL THIS OIL CIRCUIT IS NOTHING BUT OUTPUT OF HYDRAULIC GOVERNOR AND INPUT TO HYDRAULIC CONVERTER.

SECONDARY OIL OUTPUT OF HYDRAULIC/ELECTRO-HYDRAULIC CONVERTER.

TEST OILTHIS OIL CIRCUIT IS ESTABLISHED TO TEST OVER SPEED TRIP DEVICE WHEN TURBINE IS RUNNING AT RATED SPEED.

REMOTE TRIP SOLENOID VALVESTHESE ARE TWO IN Nos. AND WHEN ANY TURBINE TRIP GETS INITIATED, THE SOLENOIDS GETS ENERGISED AND CONTROL OIL GETS DRAINED THROUGH THE VALVES AND TRIPS THE MTV.

MAIN TRIP VALVESTHESE ARE TWO IN Nos. AND CALLED THE MAIN TRIP GEAR. ALL THE TURBINE TRIPPINGS TAKE PLACE THROUGH THIS DEVICE. UNDER NON-TRIP CONDITION ESTABLISHES TRIP & AUX TRIP OIL AND USED FOR RESETTING/OPENING THE STOP VALVES AND PRODUCING HP/IP SECONDARY OIL.

STARTING DEVICE AS THE NAME OF THIS DEVICE SUGGESTS, THIS DEVICE IS USED FOR RESETTING AND OPENING THE STOP VALVES AND , MAIN TRIP VALVES & HYDRAULIC TRIP DEVICES. THIS DEVICE EITHER BE OPERATED MANUALLY BY HAND WHEEL OR FROM REMOTE (MCR) THROUGH MOTORISED ACTUATOR.

SPEEDER GEAR THIS DEVICE IN COMBINATION WITH STARTING DEVICE FORMS HYDRAULIC GOVERNOR AND PROVIDES INPUT HYDRALIC SIGNAL TO HYDRAULIC CONVERTER. THIS DEVICE EITHER BE OPERATED MANUALLY BY HAND WHEEL OR FROM REMOTE (MCR) THROUGH MOTORISED ACTUATOR.

HYDRAULIC/ELECTRO-HYDRAULIC-CONVERTERAMPLIFIES&CONVERTS HYDRAULIC /ELECTRICAL SIGNALS TO HYDRAULIC SIGNALS ( HP/IP SECONDARY OIL) FOR THE OPERATION OF HP/IP CONTROL VALVE.

HYDRAULIC TRIP DEVICESTHESE DEVICES PROVIDE MANDATORY PROTECTIONS OF THE TURBINE BY DRAINING AUX TRIP OIL. THESE DEVICES ARE RESET BY AUX START-UP OIL.

GOVERNING SYSTEM COMPONENTSGoverning Rack Consisting of following.

i) Emergency trip valve

ii) Tripping Device

iii) Change over valve

iv) Filters in control oil

v) Accumulator.

vi) Starting Device

vii) Hydraulic Amplifier with set of follow up pistons (HP&IP)

viii)Load shedding relay

ix) Trimming Device

x) Electro Hydraulic converter with set of follow up pistons (HP & IP)

xi) Extraction Relay

GOVERNING SYSTEM COMPONENTS

xii) Oil injection Test Device

xiii) Vacuum protection Device.

xiv)Set of solenoids for ATT operation.

HP Front Pedestal consisting of following. i) Impeller for primary oilii) Over speed trip governor HP Rear Pedestal Consisting of following. i) Thrust protection Device.ii) For brief description of above items, refer to O&M manualiii) HP and IP governing valves.

BOILER

Condenser

LP TURBINEIP TURBINEHP TURBINE

MS

MS

CRH NRV

HRH

HPSV

HPSV

IPSV

IPSV GENERATOR

HPCV

HPCV

IPCV

IPCV

CRH NRV

TG SET

AOP

MOP

AOP Discharge header

Filtered Control Oil

Remote Trip Solenoid Valves

Main Trip Valves

Hyd. Trip Device

TEST V/V STOP V/VSERVO MOTOR

STOP & CONTROL VALVE

CONTROL VALVE SERVO MOTOR

Hyd Converter

Electro-Hyd Converter

Starting Device

Secondary Oil

Trip

Oil

Aux Start-up Oil

Start-up Oil

Aux

Trip

Oil

Filtered Controlled Oil

Filter

BLOCK DIAGRAM OF GOVERNING SYSTEM

MAIN TRIP VALVE/TRIP GEAR• To open the trip oil circuit to drain, thereby closing all steam input to

turbine in case of abnormalities• At start up auxiliary start up oil keeps the MTV in uppermost position• Pressure oil enters and the outlet oil, Trip oil builds up and flows to

governing elements• The outlet oil through a hole fills auxiliary trip oil circuit.• The auxiliary trip oil keeps the MTV in upper position in normal condition.

(Aux. Start up oil will become ‘0’.• Any drop of aux. Trip oil pressure below certain limit will drop the MTV

and drains trip oil circuit.• Manually pressing the trip lever will also drain the trip oil circuit.• Limit switches provide electrical trip out signals

EMERGENCY TRIP SOLENOID VALVE

• It is installed in control oil line before main trip vales.• Remote operated from control room/protection circuit.• It has three connections pressure oil inlet, oil to MTV and drain• The two valve discs seals valve seats alternately• Solenoid not energized will make oil through to MTV• Solenoid energize will isolate pressure oil and drain the oil from

governing system

REMOTE TRIP SOLENOID OPERATED VALVES

TRIP OIL TO HOLD STOP V/V’S & AND TO PRODUCE SECONDARY OIL

AUX TRIP OIL TO HYDRAULIC TRIPING DEVICES

CONTROL OIL FROM AOP/MOP

AUX START-UP OIL TO RESET MTV & HTD

FilterDRAIN

AOP Dishargeheader

AOP/ MOP

RESETTING OF MTV




TRIPPING OF MTV THROUGH RTSV (ELECTRICAL)


AUX START-UP OIL TO RESET MTV

FilterDRAIN

TRIPPING THROUGH RTSV

AUX TRIP OIL TO THE HYDRAULIC TRIPING DEVICES


HYDRAULIC TRIP DEVICE




TRIPPING OF MTV THROUGH HYDRAULIC TRIP DEVICE (MECHANICAL)


FilterDRAIN

AUX START-UP OIL TO RESET HYD TRIP DEVICES

TRIPPING THROUGH HYD TRIPDEVICE

STARTING DEVICE


HYDRAULIC TRIP DEVICE



AUX START-UP OIL TO RESET HYDRAULIC TRIP DEVICES

RESETTING OF RTSV, HYD. TRIP DEVICES & MTV


FilterDRAIN

RESETTING OF RTSV, HYD. TRIP DEVICES & MTV THROUGH

AUX TRIP OIL TO THE HYDRAULIC TRIPING DEVICES

STARTING AND LOAD LIMITING DEVICE

• It can be operated either manually or by motor from control room.• At lower most position, it compresses bellow and ensures no secondary

oil generated.• At lowermost position, generates auxiliary start up oil and start up oil.• While raising up, aux, start up oil pressure first killed and subsequently

start up oil pressure reduced. This opens stop valves around 35%.• Further upward movement releases bellow and control valve starts

opening.• The release of bellow can be limited to limit the control valve opening,

thereby limiting the load.

TO TEST V/V OF STOP V/V

TO MTV & HTD V/V


STARTING DEVICE

BRINGING THE STARTING DEVICE TO “0” POSITION TO BUILD START-UP & AUX STATR-UP OIL TO RESET STOP& MAIN TRIP VALVES AND HYD. TRIP

DEVICE

Aux Start-up Oil (5.5 Kg/cm2)

Start-up Oil (5.5 Kg/cm2)

RAISING STARTING DEVICE POSITION TO DE-PRESSURISE START-UP & AUX STATR-UP OIL TO OPEN STOP VALVES AND PRODUCING AUX SECONDARY

OIL

75 100

25

50

0

Starting Device

In %

Position at MCR

Drain

Drain

Hand Wheel

Motor

Start-up Oil (0 Kg/cm2)

Aux Start-up Oil (0 Kg/cm2)

STARTING DEVICE

STOP VALVE SERVOMOTOR

• Consists of two part piston. Disc shaped lower part connected to the valve stem. Other part is bell shaped. Two spiral springs are placed between the two halves.

Opening of valve• Trip oil is admitted to space above bell shaped piston and it is moved to

seat against the piston disc, forming single unit.• Then trip fluid admitted to space below piston disc and simultaneously

space above piston is drained gradually.• This causes the two haves moving towards open as single unit.• When trip oil pressure falls below certain value, the springs separate and

leads to closing of the valve.• Back seat of piston ensures minimum leakage of oil.• While closing, just before valve disc seats the oil draining is through

restricted area leading to braking action. Valve disc seats gently.

TEST VALVE FOR STOP VALVE• Function of test valve is to open and close stop valve manually or by start up

circuits/ATT.• Test valve forced downwards against spring either manually or by start up oil

pressure.• At bottom position of test valve, trip oil admitted above piston and below

piston connected to drain.• Test valve to be moved upwards either manually and by reducing the start up

oil pressure.• At top position of test valve, trip oil is admitted below piston and the above

piston is connected to drain.• Closing and opening of the stop valve can be checked by manually operating

the test valve. • HP and IP stop valves are opened in sequence at different pressures of start

up oil.

STOP VALVE SERVO-MOTOR

SPRING

CYLINDER

SPRING DISC

PISTON

PISTON DISC

TRIP OIL ABOVE PISTON

TRIP OIL BELOW PISTON DISC

RESETTING OPENING

TRIPPING

Drain

RESETTING/OPENING/TRIPPING OF STOP VALVE

RESETTING OF RTSV(DE-ENERGIZING) & HYD. TRIP DEVICES & MTV THROUGH STARTING DEVICE AT “0” POSITION

FilterDRAIN

STOP VALVE

OPENING OF STOP VALVES BY RAISING STARTING DEVICE

START-UP OIL TO TEST V/V

TRIP OIL TO STOP V/V

STOP VALVE SERVO-MOTOR

HYD TRIP DEVICE

DRAIN

AUX TRIP OIL TO HYD TRIP DEVICEAUX TRIP OIL TO HYD TRIP DEVICE

AUX START-UP OIL TO HYD TRIP DEVICE

AUX

STAR

T-U

P O

IL T

O

MTV

MTV

S/D

RTSV’S

TEST VALVE

STOP V/V OPERATION

FOLLOW UP PISTON• Follow up pistons converts the governor movement into oil pressure

changes.• Follow up piston consists of a hollow piston which can slide in acylindrical

body.• The position of piston is controlled by a tension spring and an adjusting

screw.• At lower end of piston a sleeve with two diametrically opposite drain ports

is sliding.• Similar ports in the piston overlaps with the sleeve ports in appropriate

position.• Below top cover of the body an inlet for trip oil through an orifice.• Oil supply after orifice is called secondary oil.

75 100

25

50

0

Starting Device

In %

75 100

25

50

0

Speeder Gear

In %

Starting Device Speeder GearPrimary Oil

Pump

Primary Oil (0-2.3Kg/cm

2)

Trip Oil From MTV

Aux Follow-up Valve

Link

Control Oil

Aux Secondary Oil To Hyd. Governor

Speeder Gear

Aux Follow-up Piston

Starting Device

HYDRAULIC GOVERNOR

Primary Oil

Trip oil From MTV

Aux Secondary Oil To Hyd. Governor Speeder Gear

Aux Follow-up Piston

Primary Oil Pump

Starting Device

Start-up oil

Aux Start-up oil

Control oil

75 100

25

50

0

Starting Device

In %

75 100

25

50

0

Starting Device

In %LINK

HYDRAULIC AMPLIFIER / CONVERTER

• It amplifies the signal from hydraulic speed governor.• The components are amplifier piston, pilot valve, follow up piston with

sleeve and mechanical feed back system.• Aux, sec oil flows over pilot valve. In steady state condition, pilot valve in

centre position. Pressure in aux. Sec oil is in equilibrium with spring force.• Pilot valve is kept in rotation for greater freedom of movement.• Deflection of pilot valve allows control oil above or below amplifier piston.• The motion of amplifier piston is transmitted via lever to the pilot valve as

feed back and pilot valve assumes centre position.• Each aux. Sec oil pressure corresponds to a certain position of amplifier

piston.

CONTROL VALVE SERVOMOTOR• The control valves operated by piston which is loaded one side with disc

springs and other side by hydraulic pressure.• Sec oil pressure controls the pilot valve.• During normal condition, CV at particular lift, pilot valve in equilibrium at

centre, balanced by sec oil pressure and spring force.• When sec oil pr. Raises the pilot lifts up and allows control fluid to below

piston to open the control valve.• The feed back cam mounted at the end of servomotor piston red returns

the pilot to its centre position by means of lever system and springs.• The pilot rotates continuously by means of control oil tangential jet from

disc and vibrates vertically by means of draining of sec oil at every revolution.

• Start of opening can be adjusted by adjusting pre-compression of spring with the adjusting screw provided at the fulcrum of the lever.

Secondary Oil

Drain

Drain

Drain ‘d’ Spring side

‘e’ Valve side

‘e’ Valve side

Control Oil

Pilot Valve of HPCVWhat happens when secondary oil pressure rises ?

What happens when secondary oil pressure decreases slowly ?

What happens when secondary oil pressure decreases very fast ?

CONTROL V/V SERVO-MOTOR

PROTECTIVE DEVICES IN

GOVERNING SYSTEM

1. Low vacuum trip device

2. Over speed trip device

3. Thrust bearing trip device

LOW VACUUM TRIP DEVICE• The function of the device is to trip the turbine on dropping of condenser

vacuum below a limit.• Condenser vacuum is connected to the top side of a diaphragm. The space

below diaphragm is at atmospheric pressure.• The control oil is admitted through aux pilot valve to the space below valve to

keep the valve in uppermost position against spring. The aux trip oil circuit is held blocked.

• As the vacuum deteriorates the diaphragm moves the pilot down and drains the control fluid causing the main valve to move down. The aux trip oil circuit is connected to drain causing turbine trip.

• Limit switches initiate alarm in control room.• Vacuum at which the device operates can be adjusted by the adjusting screw.• The device is isolated during start up. Aux pilot valve held upwards by spring

leading to develop aux trip oil pressure.• The primary oil pressure acting on the piston moves downwards in working

position of the device. This is to enable the protection after attaining certain speed.

Drain

Control Oil

Atmosphere

Vacuum

Aux Trip Oil

Drain

LOW VACUUM TRIP DEVICE

Resetting Of Low Vacuum Trip Device

Tripping Of Low Vacuum Trip Device

Low Vacuum Trip Device

OVERSPEED TRIP

• The function is to stop turbine when permissible speed is exceeded.• It is fitted in turbine rotor and consists of eccentric bolt, adjusting screw,

spring and screw plug.• By appropriate adjustment of screw, the centre of gravity of bolt is positioned

eccentrically to the turbine shaft.• Below the tripping speed the bolt is held in position by spring and bears

against the plug.• At speed higher than over speed limit, the centrifugal force overcomes the

spring force and forces the turbine rotor out of turbine. This activates the turbine trip mechanism.

OVER SPEED TRIP DEVICE

• The function of the device is to open the aux trip oil circuit to drain and shut down the turbine on over-speeding.

• The trip device situated in front bearing pedestal consists of valve body, pistons, pawls and spindle for limit switch operation.

• At over speed the eccentric bolt fly out of the shaft and strike the pawls and turning this against spring. The latches of the pawls release the piston and this piston moves towards shaft. The piston aided by aux trip oil pressure and spring loaded spindle.

• The aux trip oil pressure drops causing turbine trip.• Spindle provided with limit switches for alarm initiation.• For restarting the turbine the trip device is latched in by admitting the aux

start up oil.

OVER SPEED TRIP TEST DEVICE

• The function of the test device is to test the over speed trip device and trip bolt without actually raising the speed.

• Test device consists of three valves combined in valve block. One valve(left) is held at upper position by spring. In this position aux trip oil can pass to over speed trip device. When the knob is pressed, aux trip oil circuit to over speed trip device is blocked.

• The centre valve on moving downwards by rotating hand wheel admits test oil to the eccentric bolts causing them to protrude out of turbine shaft to strike against the pawl of trip device. The valve can be operated by motor during ATT.

• The third vale (right side) is used for resetting the over speed trip device after the test operation. When knob is pushed in against spring control oil enters aux start up oil circuit of over speed trip device and gets latched.

SEQUENCE OF OPERATION FOR TESTING OVERSPEED TEST DEVICE

1. Press the left valve knob to isolate the aux trip oil circuit to the over speed trip device.

2. Gradually rotate the hand wheel of middle valve to move downwards and observe for test oil pressure raise.

3. Observe and record the pressure at which the trip device is actuated. Raise pressure till both pins are acted.

4. Reduce the test oil pressure by moving the middle valve fully in opposite direction.

5. Press the right side valve knob to admit control oil to aux start up oil circuit of over speed trip device. This will reset the device to normal position.

6. Release the left side valve knob to normalize the aux trip oil circuit.

TURBINE OVERSPEED TRIP DEVICE

FLY BOTL

Aux Trip Oil

AUX START-UP OIL TO RESET

AUX TRIP OIL

Drain

TURBINE OVERSPEED TRIP DEVICE

THRUST BEARING TRIP DEVICE

• The function of the device is to drain the aux trip oil circuit in the event of high axial displacement of turbine shaft.

• The trip device consists of valve body, piston, pawl and limit switch.• During start up aux start up oil moves piston in latched position with pawl.• When shaft is axially displaced, the cam in the turbine shaft engages the pawl

to rotate. This moves the piston of trip device towards turbine shaft. The piston aided by aux trip oil pressure and spring force. The aux trip oil circuit is connected to drain causing turbine trip.

• Limit switch provided initiates alarm in the control room.• The healthiness of the trip device can be tested by admitting the test oil to the

piston to actuate the pawl.

TURBINE SHAFT

AUX START-UP OIL TO RESET

AUX TRIP OIL

DrainTRIPPING OF Thrust Bearing TRIP DEVICERESETTING OF Thrust Bearing TRIP DEVICETEST

OIL

THRUST BEARING TRIP DEVICE

a) Condenser 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 )

PROTECTION DEVICES CAUSE CLOSURE OF BYPASS STATION IN CASE OF FOLLOWING EVENTUALITIES

low pressure governing system

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