1. Reheater Protection Logic
HP Bypass System and
Reheater Protection Logic

2. Supervisory and
Monitoring
Instrumentation

3. INSTRUMENTATION PRESSURE MEASUREMENT:
TWO Nos. PRESSURE TRANSMITTERS PROVIDED IN THE UPSTREAM OF HP BYPASS PRESSURE CONTROL VALVES, IN THE MAIN STEAM LINE.
RANGE : 0 TO 200 kg/sq.cm
TEMPERATURE MEASUREMENT:
TWO Nos. K-TYPE DUPLEX THERMOCOUPLES ARE PROVIDED IN EACH LINE, LOCATED AT THE DOWNSTREAM OF EACH HP BYPASS PRESSURE CONTROL VALVES.
RANGE : 100 TO 450°C.

4. PROCESS MONITORING PRESSURE / TEMPERATURE MEASUREMENT:
The 4 to 20 mA analog signals received from the Pressure transmitters and the mV signals received from Thermocouples are digitally processed in the DCS. These signals are monitored to generate internal and external digital signals for interlocks and alarm generation.

5. ALARMS, INTERLOCKS and PROTECTION
The following are the main Alarms generated in the system:
Pressure/Temperature transmitters failure.
Main steam pressure too high.
Downstream temperature high/very high.
Tracking Pressure set point failure.
Pressure control valves opened more than 2%.
Pressure control valves closed less than 2%.
Hydraulic Oil pressure too low.
Any fast opening criteria operated.
Any closing criteria operated.

6. ALARMS, INTERLOCKS and PROTECTION
The following are the important interlocks provided in the system:
When the Pressure control valves are closed less than 2% and
in Manual mode, a forced close signal is provided to prevent
the valve from drift open.
When the Main steam pressure is too high, then the Fast
opening device is activated.
Whenever the pressure control valve is opened more than
2%, then the associated Spray control valve will switchover
to Auto mode, if it was in Manual mode before.
When the system is in auto mode and if the process signal is
failed, then the associated control loop will be switched to
manual mode.

7. ALARMS, INTERLOCKS and PROTECTION
The following are the important interlocks provided in the system:
When the down stream temperature of Pressure control valves
is too high or when the condenser vacuum is low, then the
Pressure control valves are closed on priority.
When the Turbine trips or when the Generator CB is open,
the opening device is activated to prevent Boiler trip.

8. LEGEND HP / LP BYPASS SYSTEM HRH MSL BP BOILER HPT MPT LPT CRH NRV CRH
LPC
CON
BPE
LBPE BD
BFP
LEGEND
BP : HP BYPASS VALVE BD : HP SPRAY ISOLATION VALVE CON : CONDENSOR
LBP: LP BYPASS VALVE BPE : HP SPRAY VALVE LBPE : LP SPRAY VALVE
BFP: BOILER FEED PUMP CRH : COLD REHEAT LINE LPC : LP COOLER
HPT: HIGH PRESSURE TURBINE HRH : HOT REHEAT LINE LPT : LOW PR. TURBINE
MSL: MAIN STEAM LINE MPT : MEDIUM PR. TURBINE

10. HP BYPASS CONTROL SCHEME
MAIN STEAM LINE
To TURBINE P = BD
TEMPERATURE
SET POINT
STATION
MASTER
PRESSURE
CONTROLLER
BPE
PRESSURE
SET POINT
STATION
TEMPERATURE
CONTROLLER = T
To CRH

11. Reheater Protection Logic