Trip and Interlock System Trip values of 12-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/cmg. 0.20

Furnace pass flow (low) m³/h 14

The following trips are provided on 12-F-01

DOD – SD Check Trip: This trip 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

FD Fan Trip: This trip 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 e) Pass flow low-low

f) ESD is activated.

ID Fan Trip: This trip 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 (Annunciator alarm)

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

f) ESD is activated

g) Stack damper opens when in Balanced Draft

Fuel Oil Trip: This trip 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) ESD is activated

d) Pass flow low-low

Fuel Gas Trip: This trip 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) ESD is activated 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

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.

b. FD Fan Trip Timer: It gets activated when the FD fan is not running, and gets reset when FD fan is running or the FD fan trip is bypassed.

c. ID Fan Trip Timer: It gets activated when the ID fan is not running, and gets reset when ID fan is running or the ID fan trip is bypassed.

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 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.

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 is 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. As get reset i.e., the timer indication will become zero.

In case the FD fan is tripped, all the DOD’s will open automatically.

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

a. FD fan trips

b. Arch pressure high gets activated Interlock 2: SD Operation

a) 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.

b) 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.

c) 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

d) ID fan gets tripped

e) Arch pressure high gets activated

f) FD fan trips /any DOD leaves fully closed position

Interlock 3: FD Fan Operation

a) When the furnace is in natural draft mode, the FD cannot run as long as the DOD’s are in open position.

b) 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.

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

Interlock 4: ID Fan Operation

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

b) When the furnace is in forced draft mode, the ID is in stopped condition. No interlock.

c) When the furnace is in the balanced draft mode, the ID fan gets tripped if d) SD is not fully closed

e) any DOD leaves fully closed position f) FD fan is tripped

g) 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 SDV’s

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

b) When the furnace is in forced draft mode, fuel is cut off if any DOD leaves fully open position / FD fan trips

c) 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 for 12-F- 01 is 14 m³/h.

6.4.1.5 Steam Air Decoking

Steam air decoking (SAD) of vacuum heater tubes is done to remove coke deposit from inside heater tubes with the help of steam and air. Removal of coke results in clean heater tube internals and improves heater performance by better heat transfer to process fluid.

SAD† also achieves low pressure drop through heater tubes and reduces chances of hot spot on heater tubes. Need for steam air coking of heater tubes is indicated by increased pressure drop and harder firing.

This dedicated SAD arrangement comprises decoking pot (11-V-05) (which is common for 11-F-01 and 12-F-01), piping and instrumentation for plant air, service water and MP Steam. MP steam connection to each pass flow is provided to dislodge carbon deposit from inside the heater tube. Local and DCS room mounted flow indications (12-FE-111 to 113) are provided on each steam connection plant air is required to ignite the remaining coke film deposit cleaning inner wall of tubes and achieve final cleaning for each 4” size plant air connections. 12-FE-110 indicates the total plant air consumption.

Service water quench provision is given on 11-V-05 as well as on decoking line to quench the contents before letting out to atmosphere. To carry out SAD, pass flow inlet and outlet of the furnace 12-F-01 are isolated from process network and connected by means of swing elbows with the decoking network.

Heater pass flow outlets are connected to the decoking pot (11-V-05). While MP steam is introduced in the tubes it is fired from outside, thermal shock caused by the flame cracks the coke scales inside tube and flowing steam dislodges them. These coke particles are carried to 11-V-05 after being quenched in the line as well as in the decoking pot by service water. This operation is called spalling. When no more coke is removable by spalling as indicated by relatively clear colour of effluent, air along with steam is introduced into pass flow to burn out the coke inside tube while firing is on in the heater, oxygen burns coke at high temperature, burning of the smoke is indicated by increased tube metal temperature and same should not be allowed to go beyond 625 °C Number of passes selected for spalling and coke burning is largely dependent on limitations posed by steam availability and piping network. Sudden release of coke during spalling may result in choking of the piping handling effluent or may cause hot spots of the tube during burning. SAD of only one pass at a time should be done if limitations in steam and SAD piping are experienced.