Trip and Interlock System:

Trip Values for 11-F-01

Trip Unit Value

FD fan discharge pressure (low) mm Aq. +15 ID fan suction pressure (High) mm Aq. -45 Furnace pressure (high) mm Aq. +2.95

Fuel oil pressure (low) Kg/cm²g. +2.77

Fuel gas pressure (low) Kg/cm²g. 0.20

Furnace pass flow (low) m³/h 42

The following is the description of the consequences arising from the above conditions given that all the trips are in auto interlock mode.

FD fan discharge pressure (low): The activation of this trip alarm (Annunciator alarm) may be due to the tripping of FD fan or due to the faulty indication of the combustion air discharge pressure. This results in the following

a) DOD’s gets opened

b) Stack Damper gets opened and ID fan trips

ID fan suction pressure (high): The activation of this trip alarm (Annunciator alarm) may be due to the tripping of ID fan or due to the faulty indication of the flue gas suction pressure. This results in the following

a) Stack Damper gets opened

Furnace pressure (high): The activation of this trip alarm (Annunciator alarm) results in the following

a) Stack Damper gets opened and ID fan trips b) DOD’s get opened and FD fan trips

If this alarm does not get reset within 30 seconds of its activation, the fuels to the furnace get cut off (SDV’s get closed)

Fuel oil pressure (low): The activation of this trip alarm (Annunciator alarm) results in the following

a) Fuel oil SDV’s (supply & return) close if the pressure low alarm does not get reset within 9 seconds.

Fuel gas pressure (low): The activation of this trip alarm (Annunciator alarm) results in the following

a) Fuel gas SDV closes immediately.

Furnace pass flow (low): The activation of this trip alarm (Annunciator alarm) results in the following

a) Fuels getting cut-off to the furnace (SDV’s get closed) immediately b) Stack Damper opens, ID fan trips, FD fan also trips and DOD’s open

The Operator Interface in the DCS room is provided with a set of switches, push buttons and indications to enable the smooth and safe operation of the plant. The Annunciator panel has a provision of alarms which alert the operator when unsafe operating conditions arise.

There is a provision to operate (open / close) the DOD’s and Stack Damper from the Annunciator panel. There is also a provision of lamp indications to indicate the open /close conditions of the DOD’s and Stack Damper (their activation results only when the limit switch corresponding to the open / close position is engaged as the case may be). There is a software alarm provision to indicate the limit switch (open / close) activation on the DCS panel. There is also a provision of Emergency Shut-Down push buttons to ensure the safety of the personnel and also the equipment.

The following trip switches are provided on 11-F-01

DOD – SD Check Trip Switch: This trip, when in auto mode, opens stack damper under the following conditions:

a) Furnace pressure high (Annunciator alarm)

b) FD fan trips / FD fan discharge pressure low (Annunciator alarm) c) ID fan trips

d) Both fuel oil and fuel gas pressure low (Annunciator alarm) e) Pass flow low-low

f) Activation of the DOD-SD ESD

FD Fan Trip Switch: This trip, when in auto mode, opens the DOD’s under the following conditions

a) Furnace pressure high (Annunciator alarm)

b) FD fan trips / FD fan discharge pressure low (Annunciator alarm) c) FD fan motor amps low (Annunciator alarm)

d) Any of the DOD’s is not closed completely (close limit switch deactivation) e) Pass flow low-low

f) Activation of the DOD-SD ESD

ID Fan Trip Switch: This trip, when in auto mode, opens the SD under the following conditions

a) Furnace pressure high (Annunciator alarm)

b) FD fan trips / FD fan discharge pressure low (Annunciator alarm)

c) ID fan motor amps low / ID fan suction pressure high (Annunciator alarm) d) Both fuel oil and fuel gas pressure low (Annunciator alarm)

e) Stack Damper is not closed completely (close limit switch deactivation) f) Pass flow low-low

g) Activation of the DOD-SD ESD

Fuel Oil Trip Switch: This trip, when in auto mode, closes the fuel oil supply and return SDV’s under the following conditions

a) Fuel oil pressure low (Annunciator alarm)

b) Signal from any of the three timers (details of the timers are given below) c) Activation of the Fuel-ESD

d) Pass flow low-low

Fuel Gas Trip Switch: This trip, when in auto mode, closes the fuel gas SDV under the following conditions

a) Fuel gas pressure low (Annunciator alarm)

b) Signal from any of the three timers (details of the timers are given below) c) Activation of the Fuel-ESD

d) Pass flow low-low

Timers: Three timers are provided in the furnace trip circuit, which will start counting when their respective alarms are activated. If the alarm is not brought back to normal value within 30 seconds (as counted by the timer), the timer relay will activate the fuel oil and fuel gas trips. If the alarm is brought back to normal value within 30 seconds (as counted by the timer), the timer will get reset i.e., the timer indication will become zero.

a) Furnace Pressure High Timer: It gets activated when furnace pressure high alarm comes on the annunciator panel and gets reset when the alarm is normalized. If the alarm condition is not reset, the fuels to the furnace get cut-off (given the fuel trips are in auto mode)

b) FD Fan Trip Timer: It gets activated when the FD fan trips. This should result in ID fan tripping and DOD’s and Stack Damper opening with in 30 seconds or else fuels get cut-off. Gets reset when FD fan is running and all the DODs close or the FD fan trip is bypassed.

c) ID Fan Trip Timer: It gets activated when the ID fan trips. This trip should result in the opening of Stack Damper within 30 seconds or else fuels get cut-off. Gets reset when ID fan is running and the stack damper is completely closed or the ID fan trip is bypassed Note: There is a provision to set the timer to any time duration between 0-60 seconds

Interlock system: Interlocks are provided to ensure safe operation of the equipment. They ensure that corrective action is taken automatically whenever unsafe operating conditions arise due to process upsets, mal-operation of Instruments or equipment etc. But the interlocks can be made ineffective / inactive by bypassing the trip switches provided on the DCS panel.

Interlocks on furnace operation: The following interlocks ensure the safe operation of furnace. The following description is valid only when the trips described above are in auto-interlock mode.

Note: All the references to the ‘open’ and ‘close’ positions of DOD’s and Stack Damper mean that the open or close limit switches are engaged and the close indication is active on the Annunciator panel

a) Drop out doors opening or closing b) Stack damper opening or closing c) FD fan starting or stopping d) ID fan starting or stopping

e) Fuel SDV’s opening or closing f) Low Pass Flow

Interlock 1: DOD Operation

i) When the furnace is in natural draft mode, all the DOD’s have to be in fully open position. If any one DOD leaves the fully open position (either by operation or by instrumentation malfunction), then the fuel to the furnace will cut off.

ii) When the furnace is in forced draft mode, DOD’s are in fully closed position and if any of the DOD’s is not in fully closed position, FD fan is tripped. In case the FD fan is tripped, the DOD’s open automatically.

iii) When the furnace is in balanced draft mode, DOD’s are in fully closed position. They get opened automatically if the

• FD fan trips

• Arch pressure high gets activated

• ID fan trips and Stack Dampers gets opened Interlock 2: Stack Damper Operation

• When the furnace is in natural draft mode, the SD has to be in fully open position. If the SD leaves this position, the fuel to the furnace is cut off.

• When the furnace is in forced draft mode, the SD has to be in fully open position. If the SD leaves this position, the fuel to the furnace is cut off.

• When the furnace is in the balanced draft mode, the SD has to be in fully closed position. If it leaves this position, the ID fan is tripped. The SD gets opened if

• ID fan gets tripped

• Arch pressure high gets activated

• FD fan trips /any DOD leaves fully closed position

• Fuel oil / fuel gas SDV’s both get closed (command)

• Activation of DOD-SD ESD Interlock 3: FD Fan Operation

• When the furnace is in natural draft mode, the FD cannot run as long as the DODs are in open position.

• When the furnace is in forced draft mode, FD fan gets tripped if any DOD leaves fully closed position or the furnace pressure is too high.

• When the furnace is in the balanced draft mode, the FD gets tripped if any DOD leaves fully closed position.

• FD fan cannot run if furnace pressure high alarm is active

Interlock 4: ID Fan Operation

i) When the furnace is in natural draft mode, the ID cannot run if FD fan is not running.

ii) When the furnace is in forced draft mode, only FD fan runs and ID fan is stop condition.

iii) When the furnace is in the balanced draft mode, the ID fan gets tripped if

• SD is not fully closed

• any DOD leaves fully closed position (FD trips, hence SD opens)

• FD fan is tripped

• Furnace pressure is high Interlock 5: Fuel SDV’s Operation

The fuel SDV’s get closed by operating the ESD on the auxiliary panel. Individual ESD’s are also provided for fuel oil and fuel gas. The fuel SDV’s get opened only after the individual “SDV Reset” push button on the auxiliary panel is operated. The following are the interlocks on the Fuel SDVs

i) When the furnace is in natural draft mode, fuel is cut off if any DOD or stack damper leaves the fully open position.

ii) When the furnace is in forced draft mode, fuel is cut off if stack damper leaves fully open position / FD fan trips.

iii) When the furnace is in the balanced draft mode, the fuel is cut off in case any of the three timers gets activated.

Interlock 6: Low Flow in the Furnace Passes

This interlock shuts down the fuel SDV’s to the furnace whenever the furnace pass flow in any coil is low. The trip value set for the low pass flow is 42 m³/hr.