TURBINE OPERATION AND CONTROL

TURBINE OPERATION AND CONTROLThe turbine can be divided into- Turbine and its governing system Turbine auxiliariesThe turbine and its auxiliaries are controlled by 2 different control systems. The turbine system which consists of the DEH system, steam governing system, turbo-supervisory system and the turbine trip system are operated using a separate control system called DEH/ETS control system. The turbine auxiliaries are operated through the plant distributed control system. Turbine and its governing system-The turbine is governed by a separate control system called the DEH/ETS control system. High pressure steam admission into the turbine is controlled by two high pressure main stop valves and four high pressure regulating valves. Intermediate pressure steam admission is controlled by two intermediate pressure main stop valves and two intermediate pressure regulating valves, which are combined. The DEH system acquires parameters like speed, power in MW, required load to be achieved and uses them to control the admission of steam into the turbine. There is an electro hydraulic servo valve which makes the high pressure regulating valves and intermediate pressure regulating valves operate depending upon load requirement through oil servo motor. The working pressure of oil used is 14.5 MPa. The ETS is a control system used for emergency turbine trip. It continuously monitors parameters of the turbine which are essential for unit safety and if any of the parameters go out of limit, it trips the turbine in a safe manner without affecting its parts. DEH system description- There is a main oil tank which is used to store the fire resistant oil. There are two oil pumps, out of which one is working and one is standby. The working pressure is 14.5 MPa and if it goes below 11.2 MPa the standby pump. The tank is provided with level switch to detect if level in tank goes low. To ensure that there is enough working oil when the servo motor acts quickly there is an oil accumulator. The oil regeneration pump is used to remove moisture and impurities from the returning oil. The oil temperature has to be maintained at 38-40 degree Celsius, and a heater and cooler are provided for this function, along with high and low temperature alarms. At low level in the EH oil tank the pumps trip. There are two actuators or servo motors in the system, one for the main stop valves and one for the regulating valve. The actuator consists of oil cylinder, unloading valve and test solenoid valve. When the turbine trips and the automatic shut down emergency trip oil manifold loses pressure the unloading pressure quickly opens under the action of pressure oil to quickly remove the pressure oil in the lower chamber of the high pressure main stop valve oil servo motor piston rod and the main stop valve quickly closes under the action of spring force. When the solenoid valve is energized the oil servomotor speed is relatively slow so that the valve moving test can be conducted through the solenoid valve. The oil servo motor is installed with position switch so that full open and full close condition can be detected. The high and intermediate pressure regulating valve oil servo motor is a continuous actuator. The respective oil operated servo motors open or close the regulating valve depending upon the control signal received from the DEH. There is an LVDT to give the position feedback signal. The servo motor actuator consists of oil cylinder, servo valve, hydraulic integrated block, LVDT, quick release valve and solenoid valve.

High pressure trip system consists of OPC and AST solenoid valves. They are activated when the EH oil pressure rises above 18 MPa, hence tripping the turbine. There are two OPC valves( Over speed protection) valves arranged in parallel, two check valves, one control block, for over speed control. In normal operation the two overspeed protection valves are closed without power and enclose the overspeed manifold oil leakage passage so that oil pressure builds up in the lower chamber of the regulating valve actuator piston rod, and when the turbine operation speed exceeds 103% rated speed the DEH control system OPC controller sends out actuation signal and OPC solenoid valves act and the oil is discharged through the oil return piping to EH oil tank, making the unloading valve on the servo actuator act quickly making all the regulating valves close. The AST solenoid valves act during emergency shutdown, when emergency shutdown signal is given by the ETS to trip the turbine. There are four ATS solenoid valves. Two pairs are arranged in parallel and the two pairs are then arranged in series. This gives a redundancy to the system because the system will work reliably even if one valve in each parallel pair is activated. In normal operation the four AST solenoid valves are closed with power and enclose the AST oil leakage passage so that oil pressure is built up in the lower chamber of the main stop valve actuator and the regulating valve actuator piston rod. When emergency trip command is given the manifold oil is discharged to the EH oil tank and the AST solenoid valves are activated. Thus the unloading valve on the main stop valve actuator and regulating valve actuator quickly open to make all steam valves quick close. The AST and OPC oil are separated by the two check valves on the AST solenoid valve assemblies. The check valves are so designed that when the OPC solenoid valve acts the AST manifold oil pressure is not affected, but when AST solenoid valve acts, OPC oil pressure is lost. Main functions of DEH control system- The following are the main functions of the DEH control system and the philosophy of operation- Automatic turbine trip-The DEH system facilitates tripping of the system, so when the turbine trip signal is received from ETS, it closes all steam admission valves(Main stop valves and regulating valves), and the emergency governor and tripping solenoid valve act to establish low pressure safety oil and allow the oil to go to the upper diaphragm chamber of the diaphragm valve. The AST oil discharge is closed and the solenoid valve of the AST is reset to establish AST oil pressure again. The pressure switches close when pressure is resumed.

Servo system calibration-It is necessary to maintain linearity and reliability of the servo control system. The relation between the command and the actual valve opening should be 0% command 0% valve lift and 100% command 100% valve lift. Also the feedback received should be accurate. This setting is done by doing the pull valve test, through the operator station. When the DEH receives command to do pull valve test, it fully opens and closes the oil servo motor and records the value of the LVDT in the extreme and positions, and correct zero and span.

Pre start control-Depending on the turbine temperatures the controller decides whether the start-up is hot, warm or cold. This is necessary because in order to reduce thermal stress on the turbine, according to the type of start up, different start curves will be used. The temperature(T) in the high pressure inner casing is measured. If T < 150 degrees, cold start up curve is used. If 150 75 degree ( Direct signal from RTD) Temperature of positive thrust pad > 110 degree Positive and negative thrust pad return oil temperature > 75 degree Lube oil pressure low Condenser vacuum low EH oil pressure low Condenser level high DEH power failure Loss of AST solenoid valve 1/3 and 2/4 power supplies Boiler MFT relay activated

TSI system- The TSI system, or the turbo supervisory instrument system, is a separate system which is used to measure and monitor the critical turbine parameters. It is a set of specialized instruments which terminate in a separate cabinet. There are a number of very critical parameters which need to be continuously monitored because if any of these values goes out of limits the turbine needs to be tripped, else it will get damaged. The accuracy of the sensing and measuring instruments needs to be extremely high. The following are the parameters that are monitored using the TSI system-1. Turbine speed2. Turbine radial vibration of bearings3. Turbine differential expansion4. Axial shift5. Thrust position

Turbine auxiliariesThe steam turbine auxiliaries can be sub-divided into the following systems- ATRS system Condensate system Lube oil system Auxiliary PRDS system Gland steam system Vacuum system HP-LP bypass system Regenerative feed heating system( LPH, Dearator, BFP, HPH) Extraction steam systemThese systems are operated using the main plant distributed control system. The operation and control of each of these systems is discussed in detail below. Before rolling the turbine we need to charge some of these auxiliary systems and some, like extraction system, LPH, HPH need to be charged after turbine rolling. The auxiliaries are charged before turbine rolling using the ATRS system, i.e automatic turbine run-up system which is a sequential logic to bring all the necessary auxiliaries in line. The ATRS system consists of the sequence in which the systems are charged. Each sub-system comprises the main group logic. Each main group has a sequence start permit. In order for the ATRS to start the following permissives need to be satisfied- Boiler MFT should be reset Boiler firing should be on Boiler drum pressure > 2 kg/cm2The ATRS charges all the turbine systems in the following sequence-1. Permit to start ATRS satisfied, start condensate sub group sequence2. Condensate system ready, start lube oil system sequence3. Lube oil system ready, start APRDS system4. APRDS system ready, charge gland steam header5. Gland steam header charged, start vacuum system6. Vacuum system ready, charge turbine gland sealing system7. Gland sealing system ready, start steam pipe warmup system8. Steam pipe warmup system ready, start bypass system9. All systems ready, turbine ready for rolling. Each sub-group has its own start sequence, which the ATRs executes by itself in auto mode, and in manual mode is done by the operator. A detailed explanation of the operation of all the turbine auxiliary systems as executed in the ATRS and also during unit operation is discussed below. The operation of the closed loop controls is also discussed.

Condensate system- The condensate system is charged. The system consists of the condenser, hot well, CEP. The CEP discharge goes through the gland steam condenser, from where it is divided mainly into three lines- minimum recirculation line which goes back to the condenser, condensate reject line which goes to the condensate storage tank and condensate line which goes to the LPH. There is a makeup line which gives hot well makeup water from the condensate storage tank. All these lines have control valves to control the flow of condensate through them. Before charging the condensate system the following permissives need to be followed at the same time- Hot well level > minimum Condenser make-up isolation valve open Condensate reject isolation valve open Condensate recirculation isolation valve open Condensate Flow to Deaerator Isolation Valve open CEP - Ready to Start Condition available CEP-B - Ready to Start Condition availableThe steps to start the condensate system are as follows- 1. Open hot well make up CV inlet valve, condenser reject CV inlet valve, condensate recirculation CV inlet valve. Force close CEP A and B outlet valves. Force CEP minimum recirculation valve to 100% open. The following control loops should be thrown to auto- deaerator LCV, Hot well makeup valve and condensate reject valve. 2. When the above valves are open and CEP minimum recirculation valve > 80% open, open deaerator CV inlet valve and LPH 1,2 3,4 bypass valves since the LPHs are not in service. 3. When all valves feedback open is received start CEP A or CEP B or both the CEPs. In order to start the CEPs the following permissives need to be satisfied, along with the following trip conditions-

START PERMISSIVESTRIP CONDITIONS

CEP switchgear available

CEP in remote

CEP trip circuit healthy

CEP switchgear no disturbance

CEP trip relay(86) not operated

CEP trip condition not present

CEP manual inlet valve open

CEP motor winding temperature not high (any 2 out of 6)

CEP motor bearing temperature not high

CEP vibration not high( x and y axes)

Hotwell level not low

CEP in auto or discharge valve closed

CEP in auto or minimum recirculation path available

CEP motor thrust bearing temperature not high

Hotwell level low low

CEP on and discharge valve not open

CEP tripped(86)

CEP motor winding temperature high high

CEP motor bearing temperature high high

CEP vibration high high ( x and y axes)

CEP suction strnr dp high

CEP stop PB operated-console

CEP thrust bearing temperature high high

4. When CEP A/ CEP B/ both CEP are running, open either one or both the outlet valves and throw the CEP minimum recirculation loop to auto. 5. When either/ both outlet valves are open, CEP discharge header pressure not low ( > 10 kg/cm2), and CEP minimum recirculation flow established , the condensate system is ready. There are three closed loop controls in the CEP system- hot well level control, deaerator level control and CEP recirculation control loop. Description of CLCS. Lube oil system-The lube oil system provides oil for proper lubrication of the turbine bearings. It consists of main oil tank, AC lube oil pump, DC lube oil pump, AC and DC jacking oil pumps, high pressure, oil coolers and oil filters. Before starting the lube oil system the following conditions need to be checked. In ATRS these conditions are checked automatically otherwise need to be checked by the operator. Any one ACW pump should be running Lube oil tank level should not be low (< 100 mmWC) AC LOP, DC LOP, AC JOP, DC JOP, HP pump ready to start Lube oil temperature > 25 degree CelsiusThe procedure to start the lube oil system is as follows-1. When all permissives are satisfied, the oil vent fans A and B or both are started. HP oil pump is started.. when HP pump is ON, HP pump protection start command is bypassed.2. After oil vent fans are on, HP oil pump is running and HP pump outlet pressure OK (1.96 MPa), start AC Lube oil pump. 3. When lube oil pump is running and LOP outlet header pressure OK (0.08 MPa), check if barring gear motor is running. If motor is not running then stop AC LOP, else LOP continues to run.4. If AC LOP not running, and lube oil header pressure low (< 0.058 MPa), start DC LOP. If lube oil header pressure becomes OK and DC LOP is running, then it is stopped.5. If AC LOP is running, lube oil header pressure OK(0.08 MPa) and JOP inlet header pressure OK ( 0.048 MPa), then AC JOP is started6. If barring gear motor not running then stop AC JOP7. If AC JOP not running, JOP header pressure low (< 4.2 MPa), and barring gear motor running, then start DC JOP. 8. If DC JOP running and JOP header pressure ok then stop DC JOP9. If DC JOP not running, JOP header pressure low and barring gear motor running then start AC JOP10. If AC LOP running, lube oil header pressure ok (0.08 MPa), turbine speed< 1.5 rpm, AC JOP running and JOP header pressure OK then throw barring gear to auto11. If turbine speed > 4.5 rpm, barring gear motor running, barring gear engaged then message flashes on HMI turbine lube oil system ready to start:.

Main oil tank- The oil is stored in the main oil tank( MOT). The main oil tank has a level transmitter to indicate the level of the tank and also a level switch which detects high level in the tank. If level switch is acted it means that the filter placed inside the MOT has been choked hence oil is not going to bottom of the tank, hence indicating high level. If switch acts, the tank needs to be cleaned. The temperature of the lube oil inside the tank needs to be controlled using the heater, at 43 degree Celsius. There is a control panel near the heater which contains a temperature controller. If temperature goes above set point the heater is turned off and if temperature goes below set point heater is turned on. The HP pump and AC and DC lube oil pumps are on top of the MOT and draw oil directly from the MOT. Oil vent fans- the oil vent fans are used to remove moisture and oil mist from the lube oil tank. Only one oil vent fan is running at a time and it needs to be selected in auto mode from HMI. If the fan goes into overload it tripsHigh pressure pump- The high pressure pump is started manually through HMI or automatically through ATRS system. It supplies oil to turbine till 2900 rpm speed is reached after which MOP starts. The following are the conditions under which protection start command is given to HPP- Turbine speed falls below 2800 rpm, detected by speed sensors MOP outlet pressure low (< 1.7 MPa) Protection start command not bypassed. Protection start command is bypassed when HP pump is started and feedback ON is received for 15 seconds. An alarm is generated when the discharge pressure of HP pump < 1.96 MPa. AC lube oil pump (LOP)-The AC LOP along with the HP pump supply lube oil to the turbine bearings. The lube oil pump is started after HP pump, and is given protection start command under the following conditions- Turbine speed > 2900 rpm, but lube oil header pressure low (< 0.069 MPa) Turbine tripped Protection start command not bypassed. Protection start command is bypassed when AC LOP started and feedback ON is received for 180 seconds.DC Lube oil pump-The DC lube oil pump is used as standby pump if AC LOP stops working. It is also required when the lube oil header pressure is low (0.048 MPa. The AC JOP is stopped when the speed of the turbine goes above 1300 rpm and is stopped in protection when the barring gear stops.DC JOP-The DC JOP is used as a standby to the AC JOP. It is automatically started when the AC JOP trips or AC JOP fails to start and turbine speed < 1200 rpm. Also if the JOP outlet header pressure goes below 4.2 MPa for 3 seconds, the DC JOP is automatically started. Auxiliary pressure reduction and de superheating system (APRDS)-In the APRDS system, a tapping is taken from the main steam line of boiler and goes to a common header called the auxiliary steam header. Before the turbine is rolled the APRDS system is used for deaerator pegging and for gland sealing of the turbine. The steam for pegging and gland sealing is taken from the auxiliary steam header. It is necessary to maintain the temperature of the auxiliary steam header at 200-220 deg Celsius and pressure at 8-10.5 kg/cm2. Before the CRH line is charged the steam is taken from the main steam line using PCV 105 and after the CRH line is charged the responsibility of the auxiliary steam header pressure control is switched to PCV 125. The temperature control is done through TCV 105 and the spray water is taken from BFP inter stage tapping. The pressure and temperature of the steam are controlled using closed loops. For header temperature control the valve TCV 105 is used. It is air fail close type. The temperature is sensed by temperature elements TE 105A and TE 105B. If the temperature of the header line increases the control valve opening also increases to let more attemperating water into the spray, thereby reducing the temperature. If the temperature of the header decreases then the opposite action takes place, i.e. the control valve closes. Thus the controller action is direct action. For header pressure control the valve PCV 105 is used in the beginning, and after CRH pressure increases above 17 kg/cm2 the responsibility of pressure control is handed to PCV 125. If the line pressure increases above setpoint, the control valve will close, thus reducing the steam going to the line and controlling the pressure. Thus the controller action is reverse action. The following permissives need to be satisfied before starting the auxiliary PRDS system- Main steam pressure > 0.12 MPa Main steam temperature > 50 degree SH Pressure at turbine inlet > 15 kg/cm2 Temperature at turbine inlet > 100 degree Celsius BFP inter stage pressure ok MSSV open Auxiliary steam header drain valve more than 25% open MS drain valves L and R open Auxiliary PRDS PCV inlet and bypass valves openThe procedure for charging the APRDS system as given in the ATRS is as follows-1. Put APRDS PCV and TCV in auto2. Force close APRDS PCV bypass valve3. Check if auxiliary header pressure and temperature OK ( 8-10 kg and 200 deg C)4. If not then wait till ok. An alarm Auxiliary PRDS system ready is given to HMI.

Gland steam system-The gland steam system is used for sealing of turbine glands before turbine is rolled. Before turbine rolling since there is no steam being supplied to the turbine, steam is necessary to be given to the glands externally. This is done through the gland steam system. After the steam is passed through the turbine and warm-up has been done then the turbine glands become self sealing. The gland steam temperature is required to be maintained at 150 degree Celsius and pressure at 1 kg/cm2. There are two stages of gland steam PRDS, one in the HP side of turbine and the other in the LP side of turbine. Pressure control is done at HP side through PCV 404, where steam comes through the aux. steam header. Temperature control is done at both HP and LP sides through TCV 407 at HP side and TCV 413 at LP side. HP gland steam pressure control is closed loop in nature. The pressure of the inlet is measured by PT 409 and feedback is given to PCV 404. If the pressure increases above set point then the valve closes, and vice versa. Thus controller action is reverse in nature. There is also a provision for gland steam over pressure control. If pressure exceeds the high limit then gland leak off is given to the condenser through valve PCV 411. If the pressure increases the valve opening also increases. Hence controller action is direct in nature. Gland steam temperature control is done by taking temperature measurement using temperature transmitters TT 408. If the temperature increases more attemperation is required hence TCV 407 will open at HP side and TCV 413 will open at LP side. While charging the gland steam system first the gland steam header is charged , followed by charging of the vacuum system, and after that the remaining part of the gland steam system is charged. This is because the GSC fans do not work unless there is vacuum in the condenser. The following permissives need to be satisfied before the gland steam header can be charged through ATRS system- HP Gland PRDS PCV I/L Valve Open XV 434 HP Gland PRDS TCV I/L Valve Open XV 421 LP Gland Desuperheating TCV I/L Valve Open I/L Valve to Gland Leak of Valve Open XV 423 Aux Steam Header Pressure > 8 kg/cm2 PT 206A CEP O/L Header Pressure > 10 kg/cm2 GSC Exhaust Fan -A Running orGSC Exhaust Fan - B Running Any CW Pump is Running for the Unit CW Isolation Valve at Condenser I/L and O/L are Open and Pr. Healthy

One condition before the gland steam system can be started is that any one of the GSC fans need to be on. The GSC fans operate one at a time. If the vacuum in the condenser is low ( < 0.8 kg) then the GSC fans stop.

The following are the steps of charging the gland steam header-

1. Open HP gland steam PRDS PCV inlet drain valve and HP gland steam header drain valve, HP GS PRDS PCV inlet valve, HP GS PRDS TCV inlet valve, LP desuperheater TCV inlet valve and gland leak off valve inlet valve.2. Once all the valves are open throw the HP gland steam PCV and TCV to auto and LP gland steam TCV to auto. Force gland steam header drain PCV (PCV 411) to 25% open. 3. Once all valves are thrown to auto and PCV 411 is 25% open, charge vacuum system4. Once vacuum system is charged come back to charging of gland steam system. 5. Once gland steam header pressure > 0.1 MPa and gland steam header temperature > 150 degree Celsius, throw gland steam header drain valve to auto.6. When drain valve is on auto start GSC fan A and B. 7. If GSC fan A or GSC fan B or both are on, monitor all feedbacks. 8. Depending on type of startup ( cold, warm, hot, very hot), monitor conditions. HP gland steam pressure should be greater than 0.1 MPa, and LP gland steam temperature should be greater than 180 degree Celsius. If cold startup condition then HP gland steam temperature should be greater than 150 degree Celsius. If warm startup condition then HP gland steam temperature should be > 150 degree Celsius. If hot startup condition then HP gland steam temperature should be greater than 250 degree Celsius. If very hot startup condition the HP gland steam temperature should be greater than 250 degree Celsius. 9. After all the above conditions have been satisfied then the HMI shows message that gland steam system is ready.

Vacuum system-

Components of vacuum system are 1) Vacuum pump2) Sealing water pump3) Vacuum breaker 4) Condensate tank5) Water cooler

The minimum pressure of vacuum to be maintained in condenser is -0.85 kg/cm2. If pressure goes below -0.9 kg/cm2 or above -0.8 kg/cm2 the system trips.

Condensate tank and water cooler- The liquid medium for the vacuum pumps is water which is taken from the CEP discharge, and filled in the condensate tank. The condensate tank has high and low level switches which indicate when level is high or low. The tank is filled through an inlet solenoid valve connecting CEP discharge line to the tank. When the level switch is high, the solenoid closes the valve so that the tank stops filling and when the level switch is low the valve is open to fill the tank. If the level in condensate tank is low for 8 seconds or more, both the vacuum pump and the sealing water pump trip. The water cooler is used to cool the water that goes into the vacuum pump. It gets cooling water from ACW pump.

Vacuum pump- The suction side of the vacuum pump has one MOV XV 536A for vacuum pump A and XV 536B for vacuum pump B, and butterfly valves SV 501A for vacuum pump A and SV 501B for vacuum pump B. The MOVs should be open before the pumps start. The butterfly valves open just after the pump starts. Thus for the vacuum pump to start the following conditions have to be satisfied- inlet butterfly valve should be closed, inlet MOV should be open, condensate tank level should not be low. The conditions under which the vacuum pumps trip are- inlet valve feedback closed (5 sec), condensate tank level low (8 sec) and vacuum pump feedback on but sealing water pump feedback off (5 seconds).

Sealing water pump- The sealing water pump is used to provide the liquid medium for the vacuum pump. Thus for the sealing water pump to be on the vacuum pump should be on and the condensate tank level should not be low. The conditions under which the sealing water pump trips are that vacuum pump is off and condensate tank level is low.

Vacuum breaker- Brings turbine speed to zero as it breaks the vacuum suddenly. By breaking vacuum you reduce the energy to drive the system, causing it to come to a stop. The vacuum breaker is an MOV XV 535. The conditions under which the vacuum breaker opens are- Lube oil pressure low ( +12 or < - 16) HP turbine inlet steam temperature falls 50 deg C in 10 min HRH temperature falls 50 deg C in 10 minutes

The following are the permissive to be satisfied in order to start the vacuum system- Condenser airline valve L and R ( XV 503 and 504) open CEP outlet pressure normal Any CEP on Any vacuum pump group( consisting of vacuum pump, sealing water pump) ready to startThe startup sequence of the vacuum system is as follows- 1. Close vacuum breaker valve, open vacuum pump A airline isolation valve, and vacuum pump B airline isolation valve. 2. After vacuum breaker closed and valves opened, start vacuum pump A and B and sealing water pumps A and B. The following are the start permissives and trip conditions for vacuum pump and sealing water pump are given below-Vacuum pump A/B start permissiveVacuum pump A/B trip condition

1Vacuum pump feeder in service

2Vacuum pump fan in remote

3Vacuum pump airline isolation valve feedback open

4Vacuum pump inlet butterfly valve closed

5Condensate tank level not low

6Sealing water pump feeder ready

7Vacuum pump inlet valve not open

Vacuum pump overload

Vacuum pump local stop PB

Vacuum pump on feedback fail

Vacuum pump control supply failed

Vacuum pump outlet condensate tank level lo (8 sec)

Vacuum pump inlet valve not open ( 5 sec)

Vacuum pump on and sealing water pump not on (5 s)

Sealing water pump A/B start permissiveSealing water pump A/B trip condition

1Sealing water pump feeder in service

2Sealing water pump in remote

3Vacuum pump on

Sealing water pump overload

Sealing water pump local stop pb

Sealing water pump on feedback fail

Sealing water pump control supply failed

Process trip 1

Vacuum pump off

Condensate tank level low

3. After vacuum pump A or B or both and their respective sealing water pumps are on, the butterfly inlet valves of the pump which is running is open and condenser vacuum ok, then the vacuum system is ready. The ATRS sequence then goes back to charging the gland steam system as described above.

HP-LP bypass system and steam pipe warm up system-The HP-LP bypass system is one of the very important systems in the turbine. It is required to bypass the steam from the boiler when the turbine is not running. Also while changing the load, or load reduction we need to open the bypass valves so that the reject steam bypasses the turbine. this bypassed steam is sent to other paths where it is reused. HP bypass steam is sent to the CRH line and LP bypass steam is sent to the condenser. Each bypass has a temperature controlling spray. The HP bypass temperature control is done through the BFP interstage tapping and the LP bypass temperature control is done through the CEP outlet. The steam pipe warm up system is used to raise the temperature of the pipes through which high pressure steam is going to be passed, so as to not raise the temperature suddenly. The permissives to start the system are as follows- Permissive from CW system Any CEP running CEP outlet pressure normal Condenser vacuum pressure normal Gland steam header pressure and temperature normalThe following are the steps in which the ATRS charges the steam pipe warm up and bypass system-1. Open HPBP and LPBP PCV warm up valves and HRH and CRH drain valves2. Monitor all feedbacks of above valves being opened3. Check if main steam and HRH pipe temperatures > 50 degrees, and then steam pipe warm up system ready.4. Open HP bypass and LP bypass spray valve inlet MOV5. When valves are opened, operator has to manually reset HP and LP bypass system6. When HP-LP bypass reset, then put HP bypass and LP bypass PCV and TCV in auto7. When valves in auto, monitor HP bypass and LP bypass inlet spray water pressure. Open HP bypass PCV 2% open. When HP BP PCV > 2%open, close HPBP PCV warmup valve8. Check both bypass spray water inlet pressure, and open LPBP PCV 2%. When LPBP PCV > 2% open, close LPBP PCV warmup valve9. When HPBP warmup valve and LPBP warmup valve are closed, open all drain valves of HP and IP turbine10. When all drain valves are open, open the drain valves of turbine extractions to LPH and HPH11. When all extraction drain valves are opened, open turbine interlayer header drain valve12. Adjust the interlayer drain valve manually13. Warm up and bypass system ready, await permit for turbine rolling.Turbine rolling permissive-After the ATRS finishes charging all the systems one by one, it waits for a signal that all the systems have been charged and that turbine is ready for rolling. The following are the permissives for turbine ready for rolling permit- HP oil pump running CEP outlet pressure > 10 kg/cm2 Any CEP on Condenser pressure < 0.85 kg/cm2 HP gland steam pressure normal (> 1 kg/cm2) HP gland steam temperature normal (> 150 degree Celsius) Barring gear on Turbine speed > 4.5 rpm HP differential expansion within limit ( +6 mm to -3mm) LP differential expansion within limit ( +7.5 mm to -3mm) Bearing vibration within limit ( +127mm to -127 mm) Axial shift within limit ( +0.8mm to -1.2mm) Turbine inlet pressure OK for rolling ( for cold start up- 40 kg/cm2 ; for warm startup- 45 kg/cm2; for hot startup- 55 kg/cm2; for very hot startup- 60 kg/cm2) Turbine inlet temperature OK for rolling rolling ( for cold start up- 320 degrees ; for warm startup- 400 degrees; for hot startup- 460 degrees; for very hot startup- 525 degrees)When all the above conditions are satisfied, a message is displayed on HMI Turbine ready for rolling.After the turbine is rolled, the regenerative feed heating system and extraction system is taken into service.

Extraction steam system-Steam is extracted from various stages of the turbine to provide steam to the HP heaters, LP heaters and for dearator pegging. The extraction steam is charged when HP heaters and LP heaters are taken into service and when deareator pegging is taken into service from extraction. The steam is provided through MOVs and there are NRVs in each extraction line to avoid back flow of the steam. LPH extraction lines-The LPH extraction lines are charged when LP heaters are taken into service at 40-50 MW. The QCNRV is opened first followed by the LPH extraction steam valve. If the selected LPH level goes high then the QCNRV and the extraction steam MOV close. There is a drain valve of the extraction line which is an NRV. It opens when load goes below 42 MW, and when the extraction steam MOV closes. When load goes above 45 MW, the drain closes automatically. When the turbine is tripped the drain valves open. HPH extraction lines-The HPH is charged when the load reaches about 80 MW. Before charging the system the drain valves are open, and when the QCNRV in the extraction line opens the drain valve closes. The main extraction steam valve opens only after the QCNRV opens. The drain valve opens when load goes below 12 MW and closes when load goes above 15 MW. If HPH level goes high or if turbine trips then the QCNRV closes and the steam MOV also closes. Dearator extraction lines-The deaerator extraction line consists of 2 NRVs, QC type, 1 extraction steam MOV, one dome valve and one isolation valve. There are 3 drains to flash tanks. The drains open when the load goes below 27 MW or if turbine trips or if extraction steam valve and QCNRV closes. The drains close when load goes above 30 MW and QCNRV, isolation valve, dome valve and extraction steam MOV open. The extraction steam MOV opens when both QCNRVs open. It closes in protection if deareator level goes high or if turbine trips. The quick closing NRVs open in protection when turbine is reset and close in protection when turbine trips, or deaerator level goes high. The isolation valve and dome valve open when deaerator temperature goes above 90 degree Celsius.

Regenerative feed heating system The regenerative feed heating system consists of 4 low pressure heaters, 3 boiler feed pumps, deaerator and 2 high pressure heaters. This system pre heats the condensed water in the condenser and pumps it back to the boiler. Low pressure heaters-From the condenser the condensate is pumped to the low pressure heaters through the condensate extraction pump. If the low pressure heaters are not in service, then the bypass valves are opened and the feedwater bypasses the LPHs and goes directly to the deaerator. There is one inlet MOV and one outlet MOV for condensate. If the bypass valve is open and both inlet and outlet MOVs are closed then the LPH is said to be out of service and if the bypass valve is closed and both inlet and outlet MOVs are open the LPH is in service.