Boiler Interlock

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NTPC/ DADRI STPP- STAGE II - 2X490 MW, BOILER AUXILIARIES INTERLOCKS & PROTECTION

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WRITE-UP ON BOILER AUXILIARIES INTERLOCKS AND PROTECTION

GENERAL: The steam generators supplied today are larger in size and sophisticated in nature. Failure of

any equipment calls for expensive replacement and results in costlier down time. This

emphasises careful planning on the correct procedure for;

1. Safe sequence of start-up of equipments in the power plant.

2. Continuous trouble free & efficient operation.

3. Safe sequence of shutdown of the equipment when needed.

This also leads to provision of adequate & reliable protection to safeguard the various plant

equipments under abnormal and dangerous conditions. The operation of the protections shall

be accompanied by visual and audible annunciation, which provide definite indication of the

primary cause or causes of operation of the protection. Restarting of the equipment, which has

once been tripped by protection either by remote, automatic or manual control shall be possible

only after the elimination of the cause of tripping.

PROTECTIONS ARE CLASSIFIED UNDER THREE GROUPS NAMELY: a) Protection causing complete shut down of the unit.

b) Protection causing load reduction of the unit.

c) Protection causing annunciation only.

1. PROTECTIONS CAUSING COMPLETE SHUT DOWN OF THE UNIT

a) Failure of all feed pumps (i.e.) reserve feed pump if any, fails to start on tripping of running

pumps even after a preset time delay.

b) Boiler shutdown due to failure of both FD fans, both ID fans and other conditions.

c) Reheater protection, which is to ensure continuous steam flow through reheater tubes at,

specified conditions.

2. PROTECTION CAUSING LOAD REDUCTION

a) ID fans:

Two ID fans are required for boiler MCR. If any one of the two running ID fans trips, the boiler

load shall be run down to 60% MCR.

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b) FD fans:

If any one of the two running FD fans trip, the boiler load shall be run down to 60% MCR.

c) Mills, PA fans & Coal Feeders:

This reduces the boiler load correspondingly due to limitations in fuel firing capabilities.

d) Boiler Circulation Water Pump: As described in clause 13.0

3. PROTECTIONS CAUSING ANNUNCIATION: The causes that lead to visual/audible annunciation are covered under various sub systems

like Furnace Safeguard Supervisory System(FSSS), Secondary Air Damper Control(SADC)

system, Soot blower control system, Auxiliary pressure reducing and De-superheating Control

System, Fuel oil system, Air & Flue gas system, Steam & water system, HP Bypass system

etc. These interlocks & protection systems are divided into two portions based on the area

they cover as briefly described below.

a) Boiler auxiliaries interlock & protection

This system takes care of sequence of starting, protection and interlock of boiler Auxiliaries like

FD fans, ID fans, Air-heaters, Dampers, Valves, etc. covered in this write up.

b) Furnace safeguard supervisory system (FSSS)

This system takes care of interlocks required for starting, supervising the operating and safe

shut down of the equipments connected with fuel firing system.

1.0 ID FAN INTERLOCKS 1.1 Conditions existing when the unit is shut down and prior to starting of ID fans.

a) ID fan 'A' & ID Fan 'B' off

b) Regulating dampers of fans A & B fully open

c) Inlet gates of fans A & B open

d) Outlet gates of fans A & B open

e) The regulating dampers & VFD control will be on manual control during shutdown, and impulse from furnace draft control to regulating dampers of ID fans disconnected.

f) Lube oil pumps of ID fans off

1.2 Starting ID Fan 'A' (ID fan 'B' is off) (a) ID fan 'A' shall be prohibited from starting till the following conditions are satisfied:

i. Regulating dampers of Fan-A in minimum position.

ii. Outlet gate of Fan-A closed.

iii. Inlet gate of Fan-A closed


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iv. Fan/Fan motor bearing temperature not high.

v. Motor winding temperature is not high

vi. One of secondary air heater is running with its inlet and outlet dampers kept open.

vii. Lube oil system permissive satisfied (Refer Clause 7.0)

viii. VFD system 'Auxiliary supplies healthy' permissive satisfied

ix. VFD system 'Channel 1 ready' or 'channel 2 ready’ signal available (one contact for each channel)

x. VFD system 'Channel 1 start permissive' or 'Channel 2 start permissive’ available from VFD system (one contact for each channel)

(b) The starting of ID fan 'A' shall be done in following sequence

i. VFD system channel 1 start command starts fan 'A' and runs up fan A to minimum speed.

ii. When fan 'A' reaches minimum speed, impulse for VFD channel 2 start is given by operator. When ID fan 'A' is started (ID Fan B is off), impulses shall be given for:

iii. Opening the inlet gate and outlet gate of fan A

iv. Connecting the regulating dampers/VFD control of Fan A to auto control.

v. Closing the inlet gate and the outlet gate of ID fan B and bringing the regulating dampers to minimum position.

vi. Permission to start either FD Fan A or FD Fan B.

1.3 Starting ID fan B (ID Fan A is off)

Interlocks similar to those at clause 1.2 shall hold good for this condition.

1.4 Tripping of ID Fan A (ID Fan B is off)

ID fan 'A' trip shall mean that 'stop' impulses shall be given to VFD channel 1 and channel 2

simultaneously.

a) ID fan 'A' shall trip automatically under following conditions.

i. ID Fan A bearing temperature very high (prior to this, ID fan A bearing temperature high

shall be annunciated in UCB) contacts shall be generated from RTD.

ii. ID fan A motor bearing / winding temperature very high (prior to this, ID fan 'A' motor

bearing/winding temperature high shall be annunciated in UCB)

iii. Post purge fan trip from FSSS (one contact will be provided to trip all the ID and FD fans)

iv. Vibration level of Fan/motor very high (high to be alarmed). For fan, trip shall be initiated

with a delay of 10 minutes during starting and running of fan.

v. ID Fan-A VFD channel 1/channel 2 input breaker trip (6.6 KV). Here only the respective

channels shall be issued stop impulse.


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vi. Fault from VFD channel 1/channel 2. Here only the respective channel shall be issued stop

impulse.

vii. Suction or discharge damper fail to open after fan is started. (Damper operating time to be

considered.)

b) When ID fan A trips (ID fan B is off) impulses shall be given for:

i. Opening the outlet gate of ID fan B

ii. Opening the inlet gate of ID fan B.

iii. Disconnecting the impulse from auto control system (output from A/M station) to regulating

damper and VFD control.

iv. Opening the regulating damper of ID fan A to wide-open position.

v. Opening the regulating damper of ID fan B to wide-open position.

vi. Tripping the working FD fans.

vii. Boiler tripping - through FSSS

viii. To keep the inlet gate and outlet gate of ID fan A open

1.5 Tripping of ID fan B (ID fan A is off)

Interlocks similar to those at clause 1.4 shall hold good for this condition

1.6 Starting of ID fan B (ID fan A is on) a) ID fan B shall be prohibited from starting unless the following conditions are satisfied:

i. Outlet gate of fan B is closed

ii. Inlet gate of fan B is closed

iii. Regulating damper of fan B is in minimum position

iv. Fan/Fan motor bearing temperature not high

v. Motor winding temperature not high

vi. Lube oil system permissive satisfied (Refer Clause 7.0)

vii. VFD system 'auxiliary supplies healthy' permissive satisfied

viii. VFD system 'channel 1 ready' or 'channel 2 ready' (one contact for each channel).

ix. VFD system 'channel 1 start permissive' or 'channel 2 start permissive' available (one contact for each channel).

b) The starting of ID fan 'B' shall be done in following sequence.

i. VFD system channel 1 command starts fan 'B' and runs up fan 'B' to minimum speed.

ii. When fan 'B' reaches minimum speed, impulse for VFD channel 2 start is given by

operator.

iii. When ID fan B is started, impulses shall be given for:


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a. Opening the inlet gate and outlet gate of fan B after the motor reaches the prescribed

speed.

b. Connecting the regulating damper and VFD control fan 'B' to auto control (release to

auto).

c. Permission to start other FD fan.

1.7 Starting of ID fan A (ID Fan B is on) Interlocks similar to those at clause 1.6 shall hold

1.8 Tripping of ID fan B (ID fan A is on) ID fan B trip shall mean that 'stop impulses' shall be given to VFD channel 1 and channel 2

simultaneously.

a) ID fan B shall trip automatically under the following conditions:

i. Fan bearing temperature very high (prior to this fan bearing temperature high shall be

annunciated in UCB) contacts generated from RTD.

ii. Fan motor bearing/winding temperature very high (prior to this fan motor bearing/winding

temperature high shall be annunciated in UCB)

iii. Vibration level of Fan/motor very high (high to be alarmed). For fan, trip shall be initiated

with a delay of 10 minutes.

iv. 'Post purge Fan trip' from FSSS

v. ID Fan-B motor VFD channel 1/channel 2 input breaker (6.6 KV) trip. Here only the

respective channels shall be issued stop impulse.

vi. Fault from VFD channel 1/channel 2. Here only the respective channel shall be issued stop

impulse.

b) When ID Fan B trips (ID fan A is on & vice versa) impulses shall be given for: i. Disconnecting the regulator from acting on the regulating damper and VFD control

ii. Bringing the regulating damper of Fan B to the minimum position and scoop position

iii. Closing the inlet gate of Fan B

iv. Closing the outlet gate of fan B

v. Tripping the FD fan B, provided FD fan A is on

vi. Reduce the boiler load to 60% MCR.

1.9 Tripping of ID fan A (ID fan B is on) Interlocks similar to those at clause 1.8 shall hold good for this condition:

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1.10 General a) Variable frequency drive

i. Individual start/stop push buttons shall be provided in UCB/CRT for each VFD channel.

ii. Source breakers of VFD system have to be closed from plant C system&I only if 'Source

breaker close permissive’ is available from VFD system.

iii. The signal interface drawing of VFD system supplier shall be referred to for preparation of

interlock schemes.

b) Fan outlet gate

− If one ID Fan started when the other ID Fan is off, the outlet gate of starting fan shall be

opened with starting of the fan to reduce the load of electrically operated gate.

− When starting the second fan (i.e. ID Fan A started when B is On or vice versa), the

respective outlet gate shall be opened after the fan reaches full speed.

2.0 FD FAN INTERLOCKS 2.1 Conditions prior to starting FD fans:

a) FD Fan A and FD Fan B off

b) Fan impellers tilted to maximum opening

c) Outlet dampers are fully open

d) Fan impeller regulator disconnected

e) Lube oil pumps of FD fans off

2.2 Starting FD fan A (FD fan B is off): a) FD Fan A shall be prohibited from starting until the following conditions are satisfied:

i. ID Fan A or ID Fan B is on

ii. Fan A blade pitch damper in the minimum position

iii. Outlet damper of fan A in closed position

iv. Fan/motor bearing temperature not high

v. Motor winding temperature not high

vi. Lube oil system permissive satisfied (Ref. Clause 6.0)

vii. Control oil pressure ok to release blade pitch to auto.

b) When FD fan A is started (FD Fan B is off) impulses shall be given for: i. Outlet damper of fan A to open after the motor reaches rated speed

ii. The outlet damper of FD fan B to close & impeller blades of fan B to be driven to minimum position

iii. Connecting the impeller of Fan A control drive to auto control


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2.3 Starting of FD fan B (FD fan A is off):


2.4 Tripping of FD fan A (FD fan B is off) a) FD fan A shall trip automatically under the following conditions:

i. FD fan A bearing temperature very high (prior to this, FD fan A bearing temperature high

will be annunciated in UCB)

ii. FD fan A motor bearing/winding temperature veryhigh (prior to this, FD fan A motor

temperature high will be annunciated in UCB)

iii. Both ID fans trip

iv. Post purge fan trip

vii. Vibration level of Fan/motor very high (high to be alarmed). For fan, trip shall be initiated


b) When FD Fan A trips (FD fan B is off) impulses shall be given for:

i. Disconnecting the impeller control drive from auto control system (output signal from A/M

station)

ii. Bringing the impeller of FD Fan A to the maximum position

iii. Bringing the impeller of FD fan B to the maximum position

iv. Opening the outlet damper of FD fan B

v. To open the emergency scanner air damper

vi. Boiler tripping

vii. The outlet damper of FD fan A shall remain open

2.5 Tripping FD fan B (FD fan A is off): Interlocks similar to those of clause 2.4 shall hold good for this condition

2.6 Starting FD fan B (FD fan A is on): a) FD fan B (FD fan A is on) shall be prohibited from starting until the following conditions are

satisfied:

i. Two ID fans are on

ii. Fan B impeller tilt/damper in the minimum position

iii. Outlet damper of Fan B in closed position

iv. Fan and fan motor bearing temperature not high

v. Motor winding temperature not high.

vi. Lube oil system permissive satisfied (Ref. Clause 6.0)


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b) When FD fan B is started (FD fan A is on) impulses shall be given for:

i. Opening the outlet damper of FD fan B after the motor reaches rated speed

ii. Connecting the impeller control drive of FD fan B to auto control

2.7 Starting FD fan A (FD fan B is on): Interlocks similar to those at clause 2.6 shall hold good for this condition.

2.8 Tripping of FD fan B (FD fan A is on): (a) FD fan B shall trip when FD fan A is on under the following conditions:

i. ID fan B trips

ii. FD fan B motor bearing/winding temperature very high (prior to this, FD fan B motor


iii. FD fan B bearing temperature very high (prior to this, FD fan B bearing temperature high

shall be an annunciated in UCB)

viii. Vibration level of Fan/motor very high (high to be alarmed). For fan, trip shall be initiated


b) When FD fan B trips (FD fan A is on), impulses shall be given for:

i. Disconnecting regulating impulse from acting on impeller control drive of fan B

ii. Bringing impeller of fan B to the minimum position

iii. Closing the outlet damper of fan B

iv. Reduce boiler load to 60% MCR.

v. Tripping coal elevation in service more than four through FSSS

2.9 Tripping of FD fan A (FD fan B is on):


2.10 GENERAL

i. If one FD Fan is started when the other FD Fan is off, the outlet gate of starting fan shall be


ii. When starting the second fan (i.e. FD Fan A started when FD Fan B is on or viceversa), the


3.0 PA FAN INTERLOCKS : 3.1 Conditions prior to start up

a) PA fan A and PA fan B off

b) Outlet dampers of fan A and fan B closed

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c) Regulating vanes of fan A and fan B in closed position.

d) Regulating vane regulator disconnected

e) Lube oil pumps of PA Fan off

f) First PA fan start permit contact available from FSSS.

3.2 Starting PA fan A (PA fan B off) a) PA fan A shall be prohibited from starting until the following conditions are satisfied:

i. Regulating vanes of PA Fan A in the minimum position

ii. Outlet damper of PA fan A in closed position

iii. Anyone FD fan is on

iv. Purge complete

v. PA fan and fan motor bearing temperature not high

vi. Motor winding temperature is not high

vii. Any one of Primary air heater running with gas and air inlet and outlet dampers opened

viii. Lube oil system permissive satisfied (Ref. Clause 8.0)

b) When PA fan A is started (PA fan B is off), impulses shall be given for:

i. Opening the outlet damper of PA Fan A

ii. Connecting the regulating vanes of PA fan A to auto control

iii. Bringing the regulating vanes of PA fan B to the minimum position

iv. Closing the outlet damper of PA fan B

v. Permission to start any five-coal elevation through FSSS.

3.3 Starting PA fan B (PA fan A is off) Interlocks similar to those at clause 3.2 shall hold good for this condition.

3.4 Tripping of PA fan A (PA fan B is off) a) PA fan A shall trip (PA fan B is off) under the following conditions:

i. PA fan A bearing temperature very high (prior to this bearing temperature high shall be

annunciated in UCB)

ii. PA fan A motor bearing/winding temperature very high (prior to this, motor


iii. Boiler trips



b) When PA fan A trips, (PA fan B is off) impulses shall be given for:

i. Disconnecting the regulating vanes of PA fan A from auto control


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ii. Bringing the regulating vanes of PA fan A to minimum position

iii. Closing the outlet damper of PA fan A

iv. Tripping all the operating mills (done through FSSS)

3.5 Tripping of PA fan B (PA fan A is off)


3.6 Starting PA fan B (PA fan A is on)

a) PA fan-B shall be prohibited from starting unless the following conditions are satisfied: i. Both FD Fans on.

ii. Regulating vanes of PA fan B in the minimum position

iii. Outlet damper of PA fan B in closed position

iv. PA fan B and fan motor bearing temperature not high

v. Motor winding temperature is not high

vi. Both primary air heaters are running with it gas and air inlet and outlet dampers are opened.

vii. Lube oil system permissive satisfied (Ref. Clause 8.0)

b) When PA fan B is started (PA fan A is on) impulses shall be given for:

i. Opening the outlet damper of PA fan B after the motor reaches rated speed

ii. Connecting the regulating vanes of PA fan B to auto control

iii. Permission to put in service remaining coal elevation through FSSS

3.7 Starting PA fan A (PA fan B is on)


3.8 Tripping of PA fan B (PA fan A is on)

a) PA fan B shall trip (PA fan A is on) under the following conditions:

i. PA fan B bearing temperature very high (prior to this, fan bearing temperature high shall

be annunciated in UCB)

ii. PA fan B motor bearing/winding temperature very high (prior to this, fan motor


iii. Boiler trips





b) When PA fan B trips (PA fan A is on), impulses shall be given for:

i. Disconnecting the regulating vanes of PA fan B from auto control

ii. Bringing the regulating vanes of PA fan B to minimum position

iii. Closing the outlet damper of PA fan B

iv. Energisation of partial load relay ( load to be limited to 60%).

3.9 Tripping of PA fan A (PA fan B is on)


3.10 Both the PA fans shall trip simultaneously when any boiler trip condition is present (impulses from FSSS).

3.11 GENERAL

iii. If one PA Fan is started when the other PA Fan is off, the outlet gate of starting fan shall be


iv. When starting the second fan (i.e. PA Fan A started when PA Fan B is on or viceversa), the


4.0 AIRHEATER INTERLOCKS: (Applicable for both Primary Air heater (PAH) and Secondary Air heater (SAH).

4.1 Conditions before start up of AHs:

a) All AHs are off (both electric motor and air motor are off)

b) The inlet dampers and the outlet dampers in the gas path, secondary air path, primary air path are in full open conditions.

4.2 Starting SAH A (SAH B is off):

One electric motor and one air motor have been provided for SAH-A drive system.

a) SAH A electric main motor is started if the sump oil temperature of the bearings measured through RTD is less than 70 Deg. C.

b) Whenever SAH A electric motor trips impulses shall be given for starting SAH A air motor by de-energising the solenoid provided the sump oil temperature of support/guide bearing of SAH A is less than 70 deg. C.

c) Impulse shall be given for closing of the inlet and outlet dampers of SAH B which is off

4.3 Tripping of SAH A (SAH B is off)



(a) Whenever the sump oil temperature of support/guide bearing is greater than 70 deg. C an alarm shall be initiated through RTD to alert the operator. The operator shall check the circulation pump operation and the coolant flow at local to bring down the sump oil temperature. In case if the temperature of sump oil is unable to be brought down SAH electric motor/air motor shall be tripped manually.

(b) Whenever rotor stoppage alarm initiates intermittent alarm through rotor stoppage alarm box, thereby alerting the operator about rotor slowing down, the operator shall physically verify and try to rectify the condition.

(c) Air motor shall trip automatically when SAH A electric motor starts.

Notes:

i. Trip signal of SAH is to be taken from condition electric motors tripped and air pressure for air motor low, through pressure switch located at air motor inlet.

ii. When both electric motor & air motor are tripped, an alarm shall be initiated & the isolation dampers on primary air, secondary air & flue gas paths are to be opened automatically.

iii. When SAH A trips, the boiler will trip through FSSS.

4.4 Starting of SAH B (SAH A is off):

The interlocks said above for SAH A starting (SAH B off) is applicable here.

4.5 Tripping of SAH B (SAH A is off):

The interlocks said above for SAH A tripping (SAH B off) is applicable here.

4.6 Both SAHs are working when SAH A trips & SAH B continues to run:

The gas inlet damper, the secondary air outlet damper of the tripped AH shall be automatically closed. The gas outlet damper, the secondary air inlet damper of the tripped AH need not be closed. However, if the operator wants to manually close these dampers they can be closed. Boiler load shall be reduced to 60% MCR.

4.7 Both SAHs are working when SAH B trips & SAH A continues to run:

The interlocks said above under clause 4.6 are applicable here.

4.8. Both SAHs are tripped:

If both SAHs A & B are tripped, boiler will be tripped through FSSS.

4.9 Interlocks for PAH is similar to SAH as indicated in 4.1 to 4.7 except 4.3 iii & 4.8

4.9.1 One PAH is working other PAH is off.

Under this condition if the working PAH trips, the operator shall try restarting the PAH by its duplicate air motor drive. If restarting is not successful within 5 minutes all the mills shall be tripped manually and the PAH gas inlet dampers shall be closed.

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4.9.2 Both PAH working:

Under this condition if both PAH trip, the operator may try for starting the PAHs. If any one of the PAH is not started within 5 minutes, the operator shall manually trip all the mills and the gas inlet dampers shall be closed.

4.10 Interlocks for oil Circulation Pumps of support bearing (applicable for both PAHs & SAHs):

2 x 100% oil circulation pump have been provided for the support bearing. Anyone of the two oil circulation pumps can be pre-selected as main pump and the other as standby. Following interlocks shall be provided for oil circulation pumps.

a) Whenever the sump oil temperature of support bearing of air heater is high (40 deg.C) impulse shall be given to start the main oil circulation pump

b) The standby oil circulation pump shall start when the main oil circulation pump fails to start or trips (This is applicable where 2x100% oil circulation pumps are provided)

c) The main oil circulation pump/stand-by oil circulation pump shall trip when the temperature of the support bearing falls below a preset value (40 deg. C).

d) If the flow switch does not actuate within a reasonable time after starting of main pump or stand by pump an alarm is initiated in UCB for 'No flow’. This alarm may be bypassed when the temperature of oil is lower than the set value and pumps are not running.

e) Manual operation of the lube oil pumps shall be prohibited when the ambient temperature is too low, since viscosity of the oil will be high so that the pump couldn't be started.

4.11 Interlocks for oil circulation pumps of guide bearing: (Applicable for both PAHs and SAHs)

2 x 100% oil circulation pumps have been provided for guide bearing. Anyone of the two oil circulation pumps can be pre-selected as main pump and the other as standby. The interlocks for these pumps shall be similar to those for support bearing as envisaged in clause 4.10. The set point for starting the lube oil pump of the guide bearing is 40 deg.C

5.0 BLOW DOWN VALVE:

a) When drum water level rises to 125mm above normal water level (contact generated from drum level transmitter) an annunciation 'Drum level high shall be given in UCB.

b) When drum level rises to 125mm above normal water level, blow down valves shall open and this condition shall be annunciated. (Contact shall be derived from level transmitter)

c) Blow down valves shall close automatically when the water level falls down to NWL. One interlock switch shall be provided for both the valves for manual operation.

6.0 INTERLOCKS FOR FD FAN LUBRICATING OIL SYSTEM: 6.1 FD fan A lube oil system:

a) Pump A will start automatically if pump B trips provided A/M switch is in auto position

b) Pump A will start automatically with a time delay of 0-60 seconds if lube oil pressure falls below a set value and the pump B is working provided A/M switch is in auto position.

c) Pump B will start automatically if pump A trips/has not started provided A/M switch is in auto position.



d) Pump B will start automatically with a time delay of 0-60 seconds if lube oil pressure falls below a set value and the pump A is working provided A/M switch is in auto position.

e) Pump A or pump B can be started/tripped by pressing the respective push button provided on local pushbutton box only when A/M switch is in manual position.

f) FD fan A shall be prohibited from starting until the lube oil pressure is adequate.

g) When the fan is running, both lube oil pumps A & B shall not be permitted to stop simultaneously.

6.2 FD fan B lubricating oil system:

Interlocks similar to those at 6.1 (a), (b), (c), (d), (e) & (f) and above shall hold good for FD fan B lubricating oil system.

7.0 INTERLOCKS FOR ID FAN LUBRICATION OIL SYSTEM 7.1 ID fan A lube oil system:


b) Pump A will start automatically with a time delay of 0-60 seconds if oil pressure falls below a set value and the pump B is working provided A/M switch is in auto position.


d) Pump B will start automatically with a time delay of 0-60 seconds if lube oil pressure falls below a set value and the pump A is working provided A/M switch in auto position.

e) Pump A or pump B can be started/tripped by pressing the respective push button provided on Local pushbutton box when A/M switch is in manual position.

f) ID fan A shall be prohibited from starting until the lube oil pressure is adequate.

7.2 ID fan B Lube oil system:

Interlocks similar to those at 7.1(a), (b), (c), (d), (e) & (f) above shall hold good for ID fan B lubricating oil system also.

8.0 INTERLOCKS FOR PA FAN LUBE OIL SYSTEM

8.1 PA fan A lube oil system:


b) Pump A will start automatically with a time delay of 0-60 seconds if oil pressure falls below a set value and the pump B is working provided A/M switch is in auto position.


d) Pump B will start automatically with a time delay of 0-60 seconds if lube oil pressure falls below a set value and the pump A is working provided A/M switch in auto position.

e) Pump A or pump B can be started/tripped by pressing the respective push button provided on Local pushbutton box when A/M switch is in manual position.



f) PA fan A shall be prohibited from starting until the lube oil pressure is adequate.

8.2 PA fan B Lube oil system:

Interlocks similar to those at 8.1(a), (b), (c) (d), (e), & (f) above shall hold good for PA fan B lubricating oil system also. Note: Wherever AM stations are not envisaged the same can be implemented through DDCMIS.

9.0 FURNACE TEMPERATURE PROBE The Furnace temperature probe controls are included as a part of MISC. SG Controls.

10.0 MISCELLANEOUS INTERLOCKS:

10.1 Interlock and protection write-up for DDI gate/Biplane dampers with motor operated blowers/pneumatically operated knife edge isolation gate:

10.1.1 Guillotine Gate (ID Fan outlet & AH gas inlet) with 2 x 100% motor operated blower with pneumatically operated knife-edge isolation gate:

a) Gate will close and open according to BHA interlock and protection.

b) One of the Blowers shall be continuously working irrespective of whether the gate is in close or open position, when boiler is working. The pneumatically operated knife-edge gate shall be commanded to open and shall remain open so long as the blower is working.

c) One of the blower motors for ID fan outlet gate shall be started simultaneously with ID fan.

d) Blower shall be continuously working irrespective of whether one of the ID fan is in operation or not.

e) When the operating blower trips, the standby blower shall be started. An A/M switch shall be provided to enable auto start of standby blower when the A/M switch is in auto position. When A/M switch is in manual position, the blower start/stop operation can be done by push button commands.

f) If the blower trips due to overload, it shall be annunciated.

10.1.2 Interlocks for Guillotine Gate without blowers but with manual isolation:(EP inlet/outlet & ID Fan inlet):

Valve has to be kept open irrespective of the position of the gate.

10.1.3 Interlock for Biplane damper with 2 x 100% blower with pneumatically operated knife-

edge gate: (PAH outlet (air side) & SAH outlet (air side)):

a) Biplane damper will open/close according to BHA interlock & protection.



b) One of the blowers shall start & run when Biplane damper is commanded to close & remain in close position. The pneumatically operated knife-edge gate shall be commanded to open and shall remain open so long as the blower is working.

c) The blower shall stop and the pneumatically operated knife-edge gate shall close once Biplane damper is fully open.

d) When any one of the operating blower trips, the selected standby blower shall be started. An A/M switch shall be provided to enable auto start of standby blower when the A/M switch is in auto position. When A/M switch is in manual position, the blower start/stop operation can be done by push button commands.

e) If the blower trips due to overload, it shall be annunciated.

10.1.4 SEALING SYSTEM 10.1.4.1 Sealing System for Guillotine gate at ECO Bank - 3 Outlet ( Refer Fig-1)

Guillotine Gate at ECO Bank-3 Outlet is provided with 2 x 100% motor operated blower with

pneumatically operated knife-edge isolation gate:

a) One of the Blowers M1 or M2 shall be continuously working irrespective of whether the gate G1 is in close or open position, when boiler is working. The pneumatically operated knife-edge gate shall be commanded to open and shall remain open so long as the blower is working.

b) One of the blower motor M1 or M2 for guillotine gates at ECO-3 outlet G1 shall be started simultaneously with SAH-A or SAH-B

c) Blower shall be continuously working irrespective of whether one of the PAH A or PAH B is in operation or not.

d) When the operating blower trips, the standby blower shall be started. An A/M switch shall be provided to enable auto start of standby blower when the A/M switch is in auto position. When A/M switch is in manual position, the blower start/stop operation can be done by push button commands.

e) If the blower trips due to overload, or Blower outlet pressure dips (sensed by PS) it shall be annunciated.

10.1.4.2 Sealing System for Guillotine gate at SAH Inlet ( Refer Fig-2) Guillotine Gate(SAH Inlet) is provided with 2 x 100% motor operated blower with

pneumatically operated knife edge isolation gate:

a) One of the Blowers shall be continuously working irrespective of whether the gate is in close or open position, when boiler is working. The pneumatically operated knife-edge gate shall be commanded to open and shall remain open so long as the blower is working.

b) One of the blowers motor ‘M1’ or ‘M2’ for Guillotine Gate (SAH Inlet) ‘G1’ shall be started simultaneously SAH-A or SAH-B.

c) Blower shall be continuously working irrespective of whether one of the SAH is in operation or not.



d) When the operating blower trips, the standby blower shall be started. An A/M switch shall be provided to enable auto start of standby blower when the A/M switch is in auto position. When A/M switch is in manual position, the blower start/stop operation can be done by push button commands.

f) If the blower trips due to overload, or Blower outlet pressure dips (sensed by PS) it shall be annunciated.

10.2 Interlocks for motor operated valves with Bypass valves:

a) First open the bypass valve fully

b) After main valve is fully opened, bypass valve may or may not be closed.

c) Main valve can be closed wherever required. The bypass valve, if not already closed, can be closed when main valve is commanded to close.

11.0 MOTOR OPERATED VALVE INTERLOCK REQUIREMENTS

11.1 Gate valves 100mm and above in 1500 CL and above ratings:

Valve closing:

Actuator motor tripping shall be done by limit switch. Torque switch shall be used in the control circuit as a back up. Valve opening:

Actuator motor tripping shall be done by limit switch. Torque switch shall be used in the control circuit as a back up.

11.2 Other gate and globe valves:

Valve closing:

Actuator motor tripping shall be done by torque switch. Limit switch shall not be used in the control circuit for tripping.

Valve Opening:

Actuator motor tripping shall be done by limit switch. Torque switch shall be used as a backup in the control circuit.

11.3 Thermostat shall be connected without fail in the starter circuit.

12.0 REHEATER PROTECTION LOGIC:

Reheater protection shall be provided as part of Boiler House Auxiliaries Interlock and protection system. This protection is provided to protect the Reheater against starvation of steam flow during abnormal conditions. To take care of Reheater protection, the Boiler shall be tripped after 10 seconds, when any of the following conditions occur: a) When Boiler is working and if

i) Both generator circuit breakers open and ii) HP Bypass or LP Bypass valves remain closed less than 2%

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b) When Boiler is working and if i) Turbine trips and ii) HP Bypass or LP Bypass valves remain closed less than 2%

c) Copy of Recommended Reheater Protection Logic is enclosed.

13.0 INSTRUMENTATION AND CONTROLS FOR REMOTE MANUAL OPERATION OF BOILER CIRCULATION PUMPS ON CONTROLLED CIRCULATION UNITS 13.1 Unit operation The boiler circulation system of a Controlled Circulation Unit is designed for the following unit

operating modes.

− Two or three pumps in service - no restriction in unit operation at design maximum

continuous rating.

− One pump in service - unit load is limited to not more than 60% MCR.

− No pumps in service - unit must be tripped.

It is BHEL’s strong recommendation that two (2) pumps be in operation prior to purging the

furnace and during initial firing. One (1) pump is sufficient, however. If two (2) pumps are in

operation at a boiler load greater than 60% MCR, and one (1) of the pumps trips, an alarm and

load limit signal must be generated. A manual or automatic load run back at the maximum

controlled rate must be commenced after a time delay of "5" sec. and continued to 60% load or

until the other pump has been started and proven in service. If only one (1) pump is in service,

and this pump is tripped, an automatic master fuel trip (MFT) must be generated if another

pump is not started with in five seconds.

Please refer table below for more details regarding the operation of boiler circulating water

pumps:

TABLE – A: No. Of

pumps in operation

Unit Load allowed for different DP (psi) across circulating water pump

MCR 60% MCR MFT

3 pumps a. Each>18psi b. Two>18psi and

One <18psi

a. Each>10psi b. Two>10psi and

One <10psi c. One >10psi and

Two <10psi

a. Each<10psi

2 pumps a. Each>18psi a. Each>10psi b. One >10psi and

one <10psi

a. *Each <10psi

1 pump a. >10psi a. *<10psi



NB: * 5 secs delay for starting the standby pump before resorting to automatic action for

rundown or MFT as applicable.

The above table is based on the following concept: • Unit load of 100 % MCR shall be allowed only if two BCWPs generate DP > 18 psi • When at least one BCWP pump generates DP > 10 psi, unit load of 60 % MCR shall

be permitted. • When all pumps generate DP < 10 psi, boiler shall be tripped.

Vent valves HAN 11 AA201, HAN 11 AA 202, HAN 14 AA 201 and HAN 14 AA 202 shall be

closed before starting the boiler circulating water pump.

13.2 Unit protection To prove a pump is in service, BHEL is furnishing two (2) differential pressure transmitters for

each pump to monitor the differential pressure across the pump. When only one pump is in

service, the differential pressure across that pump will be lower than the differential pressure

across the pump if more than one pump is in service. The differential pressure transmitters will

be installed so that they sense the pressure at the pump suction nozzle and the pressure at the

pump discharge nozzles.

A monitoring system that will monitor the output of the differential pressure transmitters for all

three pumps shall be provided. The monitoring system shall develop contact outputs for each

pump which, in conjunction with a control signal that the respective pump motor breaker is

closed, will give a positive indication that the pump is running and pumping effectively.

Required motor control interlock for starting a boiler circulation pump:

− Stems in both discharge valves must be proven open by position switches.

− The interlock must prevent starting a pump with either one of the discharge valve stems not

open and trip the pump if either one leaves the open position.

13.3 Boiler Circulation Pump protection The following conditions must be verified prior to local or remote manual starting of a circulation

pump. (Start permissive interlocks)

− Low pressure cooling water flow established.

− Differential temperature between pump casing and suction manifold less than 50°C.

− Motor cavity temperature more than 2°C (cold boiler start up), but less than 63°C (abnormal

hot standby condition). (Manual check only)

After the pump starts, it must be tripped if the motor cavity temperature is greater than 66 °C. If

an automatic trip functions adopted, a five (5) second time delay is allowable to permit remote

manual starting of the other pump. Starting a hot standby pump should reduce the motor cavity



temperature immediately. An instantaneous alarm signal must be activated if the motor cavity

temperature exceeds 63°C.

13.4 Pump Instrumentation and unit operation control functions In addition to the normal motor overload protection devices, the recommended instruments and

major control circuitry items for the boiler circulation pump system are listed below.

In the following tabulation, the A series covers items for the boiler circulation pumps. Item - B

applies to the discharge valves. The items in the C series apply to the circulation pump

differential pressure transmitter. The initiating differential pressure contacts for the load limit

alarm and fuel trip under series - C shall be part of DDCMIS. These contacts, used in

conjunction with the contact proving that the respective pump motor breaker is closed,

determine whether each pump is running and delivering the expected capacity. This indication

is used within the pump control system, which counts the number of operating pumps and

develops the load limit alarm signal and fuel trip.

Item

No. Quantity Description Location

A1 3 Start-run-stop switch for pump motor and associated lights (on-off) from motor breaker

UCB

A2 3 Ammeter for pump motor current UCB / DDCMIS

A3 3 Motor cavity temp. high alarm Annunciator / DDCMIS

A4 3 Motor cavity temp. high trip Motor control cabinet / DDCMIS

A5 3 Pump casing & suction manifold temp. differential indication (thermocouples by BHEL)

UCB / DDCMIS

A6 3 Pump motor breaker closed signal to C1 and C2

Motor control cabinet / DDCMIS

A7 3 Motor cavity temperature trend recorder UCB / DDCMIS A8 i

A8 ii

3

3

Low flow alarm for cooling water to pump motor cooler

Low flow alarm for Heat Barrier

Annunciator / DDCMIS

A9 3 Motor overload/ground fault trip Motor control cabinet

A10 3 Purge filter DP high alarm Annunciator / DDCMIS

B 3 pairs Discharge valve stem position pairs lights (open contact by BHEL) UCB / DDCMIS

C1 1 Low differential alarm and load limit signal for less than two pumps operating

Annunciator / DDCMIS



C2 1 Fuel trip signal for less than one pump operating (5sec. time delay) FSSS

C3 1 Differential pressure 3-pen Local or strip chart recorder or computer trend recorder UCB / DDCMIS

C4 3 Differential pressure indicator (not required if differential recorder is at UCB) UCB / DDCMIS

C5 3 Differential pressure adequate or circulation correct FSSS

13.5 Alarm and trip settings for Boiler Circulation Pump Instrumentation

Item No. Pump Operation Control Instruments

A3 Motor cavity temperature high alarm Y =63°C*

A4 Motor cavity temperature high pump trip X = 66°C after 5 sec. time delay

A5 Pump casing/suction manifold temperature differential: +/- 50°C max.

A7 Motor cavity minimum starting temperature Z = 2.0°C

A8 i

A8 ii

Low pressure cooling water to Heat Exchanger low flow alarm 6.3 m3/hr

Low pressure cooling water to Heat Barrier- low flow alarm 1.26 m3/hr

A10 Purge filter pressure drop alarm = 25 psi Unit Operational Controls

C1 Pump differential pressure alarm and load limit signal with any two pumps less than H = 18 psi* or any two motor breakers open. Refer Table-A.

C2 Pump differential pressure fuel trip with all three pumps less than L = 10 psi or all three motor breakers open (5sec. time delay for transient pressure fluctuations and emergency bus transfer). Refer Table-A.

* These alarm circuits must always be active on idle pumps.

14.6 The two (2) pneumatic valves in this P&ID i.e. ( One from Emergency tank & the second

one in the common drain line) shall open automatically during boiler black out condition.

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Boiler Interlock

Documents

c protection

b protection

boiler load

protection general

boiler mcr

interlocks protection

c reheater protection

b boiler shutdown