Bharat Heavy Electricals Limited HIGH PRESSURE BOILER PLANT, TIRUCHIRAPPALLI 620 014.

CONTROLS AND INSTRUMENTATION / FB

3530-101-01-TR-PVI-T-006 / REV 00 PAGE 01 OF 06

WRITE-UP ON AUX. PRDS CONTROL SYSTEM

NATIONAL THERMAL POWER CORPORATION LIMITED

SIMHADRI THERMAL POWER PROJECT STAGE II - 2X500MW UNITS 3&4.

CUSTOMER No. 0645 & 0646

Reference documents :- 1. P&ID of aux. Steam system - 3530-101-01-PC-PVM-L-002 2. APRDS control scheme 3530-101-01-TR-PVI-T-004 3. List of input and output signals for APRDS control system 3530-101-01-TR-PVI-T-005

NTPC Drawing number - 3530-101-01-TR-PVI-T-006

Rev. No. DATE DESCRIPTION PREPARED CHECKED APPROVED

00 30/07/2008 Initial Release

SIMHADRI PRDS WRITEUP.DOC

NTPCDummyComment

9825Approved

9825Text BoxCAT-II

3530-101-01-TR-PVI-T-006 / Rev 00

SHEET 02 OF 06 INTRODUCTION: The purpose of the control system is to maintain the pressure and temperature of the PRDS header at the values set by the set point stations, which is located in the control room console. The control system uses microprocessor hardware design based on Max-DNA system, manufactured by Electronics Division / BHEL / Bangalore. The control system comprises of basically eleven control loops, namely:- 1) High temperature (HT) PRDS header pressure control loop (Main steam) 2) High temperature PRDS header temperature control loops:-

a) Spray water pressure control loop (Normal line) b) Spray water temperature control loop (Normal line) c) Spray water pressure control loop (Bypass) d) Spray water temperature control loop (Bypass)

3) Low temperature (LT) PRDS header pressure control loop (CRH steam) 4) Low temperature PRDS header temperature control loops:

a) Spray water pressure control loop (Normal line) b) Spray water temperature control loop (Normal line) c) Spray water pressure control loop (Bypass) d) Spray water temperature control loop (Bypass)

5) Pressure control loop to plant station A/C equipment.

1) H.T PRDS HEADER PRESSURE CONTROL (Main steam/ CRH steam): The PRDS header pressure is maintained at a pre-set level receiving steam from either main steam line or CRH steam line. The pressure controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback signals (4 to 20mA) received from the two pressure transmitters located at the HT PRDS header. The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. There are two independent pressure controllers, one for taking steam from main steam and the other for taking steam from cold reheat line. Only one pressure controller will be in operation at a time depending on the availability of steam in MS or in CRH header. 2) H.T PRDS HEADER TEMPERATURE CONTROL: The temperature controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback mill volts signals received from the temperature elements (redundant duplex NiCr-NiAl thermocouples) located at the H.T PRDS header. The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. The temperature controller receives the (maximum of) feed forward signal from the MS pressure control valves (AS-22 (LBG10AA101)) position transmitter or from CRH PRDS pressure control valve AS-26 (LBG21AA101) position transmitter. 2.1) H.T PRDS SPRAY WATER PRESSURE CONTROL: The spray water pressure controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback signals received from the pressure transmitters (redundant) located at the upstream of temperature control valve FD-31 (LAF20AA103). The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. Spray water pressure controller FD30 (LAF20AA102) receives a feed forward signal from HT temperature control valve FD-31 (LAF20AA103). Bypass temperature control valve FD28 (LAF21AA102) & spray water pressure control valve FD44 (LAF21AA101) is given in parallel to the above lines. Selection of by pass line will be decided by the operator only upon failure of main line control valves and provisions shall be available in the MMI for selection of main / bypass lines. Only one set of lines (i.e. FD28&FD44 or FD30&FD31) shall be opened at a time. Once bypass line is opened by operator manually, main line shall be given a force close signal. Other operation and interlocks shall be similar to that of main lines.

3530-101-01-TR-PVI-T-006 / Rev 00

SHEET 03 OF 06 3) L.T PRDS HEADER TEMPERATURE CONTROL: The temperature controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback mill volts signals received from the temperature elements (redundant duplex NiCr-NiAl thermocouples) located at the L.T.PRDS header. The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. 3.1) L.T PRDS SPRAY WATER PRESSURE CONTROL: The spray water pressure controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback signals received from the pressure transmitters (redundant) located at the upstream of temperature control valve FD-46 (LAF31AA102). The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. Spray water pressure controller FD35 (LAF31AA101) receives a feed forward signal from LT temperature control valve FD-46 (LAF31AA102) Bypass temperature control valve FD38 (LAF32AA101) & spray water pressure control valve FD47 (LAF32AA102) is given in parallel to the above lines. Selection of by pass line will be decided by the operator only upon failure of any of main line control valves. Provisions shall be available in the MMI for selection of main / bypass lines. Only one set of lines (i.e. FD38&FD47 or FD46&FD35) shall be opened at a time. Once bypass line is opened by operator manually, main line shall be given a force close signal. Other operation and interlocks shall be similar to that of main lines. 4) PRESSURE CONTROL LOOP TO PLANT STATION A/C EQUIPMENT: The PRDS header pressure to station a/c equipment is maintained at a pre-set level receiving steam from LT station aux. steam header. The pressure controller receives the set point signal from the set point station located at the SG-MMI and compares it with the process feedback signals (4 to 20mA) received from the two pressure transmitters located at the A/C PRDS header. The deviation between the set point and the process feedback is processed through a PI controller and the demand signal is fed to the control actuator through drive control interface module and an electro-pneumatic converter. There is one bypass line with motorised inching valve for pressure controller. Only one pressure controller, i.e., either pneumatic pressure controller / bypass line shall be opened at a time. Selection of either of this lines shall be done manually from MMI. This control is implemented in Unit-4 of PRDS control panel only. Status feedbacks will be given in unit-5. 5.0) SEQUENTIAL OPERATION OF PRDS CONTROL VALVES: 5.1) INITIAL SET-UP: a) The main steam isolation valve AS-21 (LBG10AA201) shall be kept open manually, all the time when the

H.T / L.T PRDS lines is charged from main steam line. This valve can be opened or closed from the SG-MMI.

b) The cold reheat steam isolation valve AS-25 (LBG21AA201) shall be kept open manually, all the time when the H.T / L.T PRDS lines is charged from cold reheat steam line. This valve can be opened or closed from the SG-MMI.

c) The isolation valve FD-43 (LAF10AA101) for spray water from BFD shall be kept open, all the time when the H.T / L.T PRDS line is charged through AS-22 (LBG10AA101), AS-26 (LBG21AA201) or AS-29 (LBG22AA202). This valve can be opened or closed from the SG-MMI. FD-43 (LAF20AA101) shall be opened when AS-22/ AS-26/ AS-29 opened more than 2% and closed when AS-22/ AS-26/ AS-29 closed less that 2% with a time delay.

d) The isolation valves AS-21 (LBG10AA201), AS-25 (LBG21AA201), AS-28 (LBG22AA201), FD-42 (LAF21AA201), FD-33 (LAF31AA201), FD-37 (LAF32AA201), FD-29 (LBG20AA201) shall be blocked from manual closing when any of the associated control valves is open.

e) All the control loops set to manual mode, keeping the manual isolation valves and motorised isolation valves which are located at the upstream/ downstream of the control valves which are to be brought into service, in open condition.

f) All the control valves, manual isolation valves and the motorised isolation valves in the bypass lines are normally closed.

g) When CRH-PRDS is not in service and pressure is not adequate, start up can be done by MS-PRDS, provided MS pressure is adequate and AS-26 / AS-29 is in closed position.

3530-101-01-TR-PVI-T-006 / Rev 00

SHEET 04 OF 06 5.2) H.T PRDS OPERATION: a) The set point stations for H.T PRDS header pressure and temperature and the H.T spray water pressure

are set at the desired values, at the SG-MMI. b) When the control valve (AS-22 (LBG10AA101)) opened > 2% the following actions are initiated :-

- Upstream motorised isolation valve, AS21 will open automatically. - The associated temperature controller in HT header (FD-31 (LAF20AA103)) and spray water pressure

controller (FD-30 (LAF20AA102)) will change over to auto mode. Also, temperature controller in LT line (FD-46) and spray water pressure controller FD35 will change over to auto mode.

- The motor operated isolating valves AS181, FD33 & FD29 will open automatically. - Block valve in spray line FD43 will open automatically.

c) When the control valve (AS-22 (LBG10AA101)) is fully closed (opened less than < 2%), after a delay the following actions are initiated :- - Upstream motorised isolation valve, AS21 will close automatically. - The associated temperature controller in HT header (FD-31 (LAF20AA103)) and spray water pressure

controller (FD-30 (LAF20AA102)) will be closed automatically. - The motor operated isolating valve FD29 will close automatically.

5.3) L.T PRDS OPERATION: a) The set point stations for L.T spray water pressure and temperature are set at the desired values, at the

SG-MMI. b) When the pressure control valve (AS-26 (LBG21AA101))opened > 2% the following actions are initiated :

- Upstream motorized isolation valve, AS25 will open automatically. - Temperature controller FD-46 (LAF31AA102) and spray water pressure controller FD-35

(LAF31AA101) should be changed over to auto mode. - The motorized valve in LT spray water header, FD33 & AS181 will open automatically. - Block valve in spray line FD43 will open automatically.

c) When the pressure control valve (AS-26 (LBG21AA101)) is fully closed (opened less than > 2%), then the following actions are initiated after a time delay :- - Upstream motorized isolation valve, AS25 will close automatically.

6) CHANGING OVER OF HC/LC PRDS: a) Auto change over from LC-PRV to HC PRV

- During normal running LC PRDS is in operation. HC-PRDS also comes into operation automatically when LC-PRV (AS26) lift goes above 90% & LC PRDS remains in operation.

- HC PRV (AS22) shall be on forced auto mode, automatically when :- When LC - PRV (AS26) lift goes above 90%, or when Aux. Steam header pressure comes down to 15kg/cm (adjustable) and continues to be at

that values for 0-10 seconds (adjustable) b) Auto change over from HC-PRV to LC PRV

- LC PRV (AS26) shall be on forced auto mode, automatically when :- When HC - PRV (AS26) lift goes below 10% due to decrease in aux.steam requirement and

CRH line pressure is greater than 18kg/cm. LC PRDS cuts in automatically and controls are transferred to LC-PRV. HC-PRV will be given

a force close command with a time delay of 0-100s (adjustable) 7) OPERATION OF CONTROL AND ISOLATION VALVES: 7.1) The following block valve and motorised isolation valves are operable from SG-MMI which are applicable for APRDS unit headers:

a) AS-21 (LBG10AA201) b) AS-25 LBG21AA201) c) AS-28 (LBG22AA201) d) FD-29 (LAF20AA201) e) FD-33 (LAF31AA201) f) FD-37 (LAF32AA201) g) FD-43 (LAF10AA101) h) FD-42 (LAF21AA201) i) AS-46 (LBG10AA202) j) AS-181 (LBG40AA201) k) AS-10 (LBG50AA202) l) AS-21 (LBG10AA211) m) AS-13 (LBG90AA201) n) AS-18 (LBG90AA202) o) AS-38 p) AS-39

3530-101-01-TR-PVI-T-006 / Rev 00

SHEET 05 OF 06 7.2) The following block valve and motorised isolation valves are operable from SG-MMI which are applicable for APRDS station headers (Controls for which are implemented unit -5 of SG-DDCMIS only and status feedbacks will be given to unit-6 of SG-DDCMIS):

a) AS-12 (LBG50AA202) b) AS-49 (LBG33AA201) c) AS-03 (LBG91AA202) d) AS-02 (LBG91AA201)

7.3) The following motorised inching (control) and pneumatic control valves are operable form SG-MMI (applicable for each unit headers): A) Control valves.

a) AS-22 (LBG10AA101) b) AS-26 (LBG21AA101) c) FD-30 (LAF20AA102) d) FD-38 (LAF32AA101) e) FD-35 (LAF31AA101) f) FD-46 (LAF31AA102) g) FD-28 (LAF21AA102) h) FD-44 (LAF21AA101) i) FD-47 (LAF32AA102) j) FD-31 (LAF20AA103)

B) Inching valves. a) AS-29 (LBG22AA202)

7.4) The following motorised inching and pneumatic control valves are operable form SG-MMI (applicable for APRDS station headers - Controls for which are implemented unit-5 of SG-DDCMIS only and status feedbacks will be given to unit-6 of SG-DDCMIS): A) Control valves.

a) AS-04 (LBG91AA101) B) Inching valves.

a) AS-05 (LBG91AA203) 7.5) All the above mentioned pneumatic / motorised (inching) control valves are operated through Drive control Interface modules (DLI) located in the SG-MMI: 7.6) Opening of all the control valves are interlocked with its upstream motorised isolation valves so that when the control valves are opened > 2%, isolation valves are to be fully opened automatically; and the isolation valves will get a close command when the associated control valve is closed less that 2% with a time delay. 7.7) The valve FD-43 (LAF10AA101) shall close only when all the control valves namely AS-22 (LBG10AA101), AS-26 (LBG21AA101), AS-29 (LBG22AA202) are closed. FD-43 is a block valve, envisaged with dual coil solenoid valve, each coil is rated for 24VDC. 7.8) The motorised inching valves AS-29 (LBG22AA202) need not be interlocked as they are normally closed valves, opened only during initial start-up or after a long shutdown. 8) DETAILS OF SG-MMI: The following facilities are available in the SG-MMI: a) Auto / Manual station are available with open / close operation. b) Set point station with indicators in engg. units. c) Process value indicators with engg. units. d) Position indication for motorised inching and control valves. e) Open/ Close indication for motorised isolation valves. f) Process signal selection from redundant process signals. 9) SYSTEM INTERLOCKS AND PROTECTION: a) On MFT signal from FSSS system, HC and LC PRDS pressure control valves, temperature control valves

and spray water pressure control valves shall be closed. b) Whenever the header temperature exceeds the too high set point the pressure control valves in operation

get a force close signal. c) Whenever there is a failure of both process feedback signal which is used for control, the associated

control loop shall trip to manual mode. d) Whenever the main valve is modulating in auto mode, if its bypass valves are opened manually, the main

valve will be given an auto-block signal.

3530-101-01-TR-PVI-T-006 / Rev 00

SHEET 06 OF 06 e) When any control valves upstream and downstream isolation valves are closed, the control valve shall be

given a forced close signal, to prevent from drift opening. 10) SYSTEM SIGNAL INTERFACES: a) This PRDS control system is implemented in SG-DDCMIS b) The list of analogue and digital inputs/outputs of the system has been given in document ref. List of input

and output signals. 11) CONTROL INTERFACE FOR MOTORISED ACTUATORS: a) Integral starters are used for motorised actuators and open/close commands (24V DC) are issued from

control panel and disturbance monitoring signals and open/close limit switch feedbacks will be provided from integral starter.

b) For modulating drive the command to actuator shall be in the form of 4-20mA signal. The necessary positioning circuit shall be provided.

c) The sequence of operation for integral bypass valve AS21 (LBG10AA211) is performed in the control system itself. First, the integral bypass valve opens fully. The main valve is opened only after the bypass valve is fully opened. On close command, both main and integral bypass valves shall close simultaneously.