SPM-1220 Advanced Fired Heater

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Process Description

The SPM-1220 Advanced Fired Heater simulator includes the following process features:

  • Gas-fired process heater with 4 parallel process passes and 8 burners
  • Forced draft fan
  • Induced draft fan
  • Rotating element air preheater
  • 2 process feed pumps with heater pass flow control system and minimum flow control system

The Advanced Fired Heater normally operates in balanced draft mode where the Forced Draft Fan and the Induced Draft Fan are running. The heater can also be run in partial natural draft mode at a reduced capacity in case either fan is out of service and in full natural draft mode in case both fans are out of service. It can be started up and shut down in any mode.

A selectable fuel gas control scheme is provided for managing fuel gas flow to meet process fluid outlet temperature requirements. It also includes a fuel gas override system to automatically reduce fuel gas flow in case of low oxygen concentration or high carbon monoxide concentration in the flue gas.

The Advanced Fired Heater is also outfitted with a protective interlock to prevent unsafe operation of the unit.

A full range of operations can be learned and practiced on the SPM-1220 Advanced Fired Heater simulator. These include normal, startup, shutdown, and emergency shutdown procedures. A rich set of faults is provided to permit training for response to a wide variety of operational problems.

The rest of this manual describes the Advanced Fired Heater simulator process and controls.

Feed Pumps P-101A/B take hydrocarbon feed from storage and pump it through four parallel tube passes within the Fired Heater F-101. The feed passes through horizontal tubes in the convection section of F-101 and then into vertical tubes along the walls of the radiant section of F-101. The heated process fluid is combined and sent to the downstream unit at battery limits.

Fuel gas from battery limits is distributed to eight burners equally situated on the floor of the radiant section of the Fired Heater. Combustion air is supplied from a plenum attached to the bottom of the heater floor and is distributed equally to the eight burners. A pilot system ensures ignition of the fuel/air mixture from the burners.

Normally, warm combustion air comes from the Combustion Air Preheater E-101 when the Forced Draft Fan G-101 is running. When G-101 is not running, doors on both sides of the plenum are opened to supply ambient air directly to the plenum.

Air Intake Filter FL-101 screens out debris from ambient air supplied to the Forced Draft Fan G-101. Air flows from G-101 to the Combustion Air Preheater which uses a rotating heating element to capture heat from the hot flue has from F-101 to preheat the combustion air from G-101. The heated air from E-101 is routed to the center of the combustion air plenum under the floor of F-101.

Hot flue gas leaving the radiant section of F-101 passes through the convection section and is then normally routed from the base of the heater stack to the Combustion Air Preheater E-101 and then to the Induced Draft Fan G-102. The flue gas damper in the base of the stack of F-101 is normally closed to divert the flue gas to E-101. The cooled flue gas from G-102 is returned to the stack of F-101 above the flue gas damper for discharge to atmosphere. When the Forced Draft Fan is not running, the flue gas damper is opened to discharge flue gas from the convection section directly up the stack.

Feed to the Fired Heater F-101 is supplied from tankage at battery limits and is fed to Feed Pumps P-101A/B which are motor-driven centrifugal pumps. Only one pump is normally in operation. The design flow is 20 MBPD.

Each pump is outfitted with a check valve on the discharge and a discharge valve (continuously adjustable opening). The normal feed to the heater is heavy gas oil (HGO), which is a mixture of hydrocarbons in the normal boiling point range starting at 227 DEG F to 765 DEG F. The feedstock can contain a mixtures of gasoline, naphtha, kerosene, HGO, atmospheric residue and dissolved light gas.

All the flow from the Feed Pumps is normally sent to the Pass Flow Control System of the Fired Heater. The Feed Pumps are protected from operating at low flow by automatically recirculating feed back to the tank at the battery limits.

The Pass Flow Control System splits the feed from the Feed Pumps P-101A/B into four streams that will pass into the convection section of the Fired Heater F-101. Note that each of the four flow control valves FV-112A/B/C/D are outfitted with minimum stops to prevent the control valves from being fully closed. This helps to protect the tube passes from excessive overheating in case the control valves are commanded to fully close

The feed mixture from P-101A/B flows through four parallel tube passes in the Fired Heater F-101. Initial heating is accomplished in the convection section of the Fired Heater where the tubes are arranged horizontally and then in the radiant section where the tubes are arranged vertically just inside the wall of the radiant section. Process fluid flows downward through the radiant tubes. Hot hydrocarbons from all four tube passes leave the radiant section and are collected in a header and flow to a downstream unit at battery limits. The normal outlet temperature of the Fired Heater is 600 DEG F.

Fuel gas from battery limits consists of hydrogen, nitrogen and hydrocarbons ranging from methane (C1) to butane (C4). The fuel gas flows through isolation valve XV-130, through control valve FV-130 and is distributed from a header through the eight burner hand valves HV-131A/B/C/D/E/F/G/H. The burners are equally spaced within the center of the floor of the radiant section of F-101. The normal specific gravity of the fuel gas is 0.747. The normal fuel gas flow rate to the Fired Heater 62.4 MSCF/H.

Combustion air is provided to the burners via a plenum at the bottom of the Fired Heater (see Schematic #6). Each burner is also outfitted with a pilot to ensure combustion of the fuel/air mixture within the burners. Most of the heat released from the burners’ flames transfers through the tube walls and into the hydrocarbon mixture within the tubes of the radiant section of the heater. The warm flue gases flowing upward from the radiant section enter the convection section to transfer additional heat to the heater feed. The flue gas leaving the convection section flows to the arch of the heater for release to the atmosphere.

The Forced Draft Fan G-101 is electric motor driven and provides combustion air to the Combustion Air Plenum below the floor of the Fired Heater F-101. Air Intake Filter FL-101 screens out debris from ambient air supplied to the Forced Draft Fan. The normal flow rate of air through the Forced Draft Fan is 899 MSCF/H.

When the Forced Draft Fan is out of service, combustion air is provided by direct flow of ambient air through adjustable doors located at each end of the plenum. The normal oxygen content of the flue gas from the Fired Heater is 2.5 VOL %.

Air flows through a duct from the Forced Draft Fan and through the Combustion Air Preheater E-101 to heat the combustion air using hot flue gas before it enters the Combustion Air Plenum. The Combustion Air Preheater is a regenerative type heater which uses rotating metallic elements which pass through the hot flue gas to warm the elements which cools down the flue gas. The hot elements then rotate into the combustion air side of E-101 to preheat the air.

The flue gas and combustion air sides are separated by a sealing system that allows the rotating elements to move through the seal. A bypass duct and damper around E-101 permits adjustment of the heat recovery rate from the flue gas to avoid excessively low flue gas exit temperatures, especially during cold weather or high excess air operation. The normal outlet temperature of combustion air from E-101 is 565 DEG F.

Within the Combustion Air Plenum, the air distributes equally to the eight burners along the floor of the Fired Heater.

Flue gas produced by the burners flows up through the radiant and convection sections of the Fired Heater to the arch of the Fired Heater and into the base of the stack. The stack damper HV-161 is situated near the bottom of the stack and is normally closed and routes most of the hot flue gas through a duct to the Combustion Air Preheater E-101. A small hole is cut into the stack damper to permit some flue gas to pass through to the stack in case the stack damper locks closed.

Cooled flue gas from E-101 is routed through a duct to the Induced Draft Fan G-102 which is motor driven. The normal temperature of flue gas leaving E-101 is 293 DEG F. The Induced Draft Fan returns the flue gas through a duct back up to the stack at a point just above the stack damper. Flue gas flows up the stack, which is very tall, and discharges into the atmosphere. In case the Induced Draft Fan is out of service, the stack damper is opened to allow flue gas to flow up the stack by natural draft.

Instrumentation

This section describes the controls and instruments of the Advanced Fired Heater.

Feed Pumps

PI-101 indicates the supply pressure of the hydrocarbon feed from storage. TI-101 indicates the temperature of the hydrocarbon feed from storage.

Switches HS-101A/B operate the motors of Feed Pumps P-101A/B, respectively. Normally only P-101A is in operation.

HV-101A/B are hand controllers to adjust the position of the discharge valves of pumps P-101A/B, respectively. Normally HV-101A is open 100% and HV-101B is fully closed.

FIC-101 is a minimum flow controller to protect P-101A/B from being operated at too low a flow rate. FIC-101 adjusts the position of control valve FV-101 to circulate hydrocarbon feed back into storage. FIC-101’s setpoint is 10 MBPD. Normally, FV-103 is fully closed.

Pass Flow Control System

The Pass Flow Control System is designed to smoothly balance pass flows in case of heat transfer differences between the four passes in the Fired Heater while keeping the total feed flow through the Fired Heater controlled to the desired value.

The total feed flow through the Fired Heater is controlled by feed flow controller FIC-102. The output of FIC-102 is indicated on FIC102OP. The output of FIC-102 is routed to a summer calculation for each pass flow (FX-112A/B/C/D). The summers add the outputs of the pass bias hand controllers HIC-112A/B/C/D to FIC-102’s output. The sums are then routed to the setpoint of the pass flow controllers FIC-112A/B/C/D.

The pass flow controllers adjust their corresponding control valves FV-112A/B/C/D. Note that these control valves are outfitted with minimum stop devices to prevent the valves from physically being closed less than 5%. This keeps a minimum flow through the tubes of the corresponding pass in case the pass flow controller commands the valve to be completely closed.

The Pass Flow Control System automatically initializes the outputs of the pass bias hand controllers HIC-112A/B/C/D whenever the corresponding pass flow controller FIC-112A/B/C/D is not in cascade mode. This ensures bumpless control whenever a pass flow controller is placed into cascade mode. Whenever a pass bias hand controller’s output is being initialized it is also locked in manual mode.

Note that the PV range of the pass bias hand controllers HIC-112A/B/C/D is -50% to +50%. An output of 50%, therefore, represents a setpoint bias of 0% in summers FX-112A/B/C/D.

The easiest way to change the setpoint bias is to place the pass bias hand controller into automatic mode and then change the setpoint to the desired bias. This can only be done when the corresponding pass flow controller is in cascade mode. The pass bias hand controller will then smoothly ramp its output to the correct value without the operator having to account for the scaling of the output in the corresponding summer if the output is changed directly in manual mode. This approach also allows FIC-102 to take action to keep the total feed flow to the Fired Heater fairly constant while the flow rates of each of the passes are gradually changed according to the pass bias changes.

FAL-112A/B/C/D are independent pass flow indications used by the Fuel Gas Interlock I-110 to trip the fuel gas in case of a low pass flow. Refer to the “Interlocks” section below for details of the trip settings of FAL-112A/B/C/D.

PI-113A/B/C/D indicate the pressure of the passes downstream of the respective pass flow control valves.

Fuel Gas System

Fired Heater Outlet Process Line

TIC-110 controls the outlet temperature of the Fired Heater. It can be cascaded to the fuel gas flow controller FIC-130, the fuel gas control valve position controller XIC-130, or the fuel gas header pressure controller PIC-131. See “Outlet Temperature Control Configuration” below for how to select the desired control configuration for TIC-110.

PI-110 indicates the fluid outlet pressure of the Fired Heater F-101.

Fuel Gas Line

PI-130 indicates the supply pressure of fuel gas to the Fired Heater. AI-130 indicates the specific gravity of the fuel gas. Switch XV-130 opens and closes the fuel gas emergency block valve XV-130. It is locked in the CLSD state when Fuel Gas Interlock I-110 is active. FIC-130 controls the flow rate of fuel gas to the Fired Heater. XIC-130 controls the valve position of control valve FV-130. PIC-131 controls the pressure of the fuel gas header supplying the eight burners of the Fired Heater. PAL-131 is an independent indication of the fuel gas header pressure and is used to activate I-110 in the event of low header pressure. See the “Interlocks” section below for the trip setting.

Burner Instruments

Hand controllers HV-131A/B/C/D/E/F/G/H adjust the opening of the fuel gas valves to the eight floor-mounted burners of the Fired Heater. Switches HS-131A/B/C/D/E/F/G/H turn the pilot system for each burner on and off. These instruments are accessible in the box on the lower right corner of the Fuel Gas System schematic.

Outlet Temperature Control Configuration (XY-130)

The combined outlet temperature of the process fluid from the Fired Heater is controlled by TIC-110. TIC-110 can be set up to control the fuel gas flow through control FV-130 in the following ways:

  • By direct adjustment of FV-130’s position
  • By adjusting the setpoint of fuel gas flow controller FIC-130
  • By adjusting the setpoint of the fuel gas pressure controller PIC-131

Soft switches HS-130T, HS-130F and HS-130P are used by the fuel firing control logic XY-130 to determine which configuration of the fuel gas firing control strategy is in effect. The resultant control signal to FV-130 is indicated on instrument XIC-130 which controls the valve position of FV-130.

Normally, the selected firing configuration is the cascade of TIC-110 to FIC-130 (HS-130F = ON). When changing to another control configuration, set the appropriate soft switch for that configuration to the ON position. This will change the firing controls to that configuration and set the other 2 soft switches to the OFF position. Initialization is as follows:

  • On any change of controller configuration, all three controllers (TIC-110, FIC-130 and PIC-131) are placed into manual mode (one-shot). XIC-130 always remains locked in cascade mode except in case of a fuel gas trip via Fuel Gas Interlock I-110.
  • If TIC-110 is selected (HS-130T = ON), TIC-110’s output is initialized at the current output of XIC-130 (one-shot). The two other controllers (FIC-130 and PIC-131) are locked in manual mode and their outputs track XIC-130’s output.
  • If FIC-130 is selected (HS-130F = ON) is selected, FIC-130’s output is initialized at the current output of XIC-130 (one-shot). TIC-110’s output will be initialized at FIC-130’s setpoint value until FIC-130 is placed into cascade mode. PIC-131 is locked in manual and its output tracks XIC-130’s output.
  • If PIC-131 is selected (HS-130F = ON) is selected, PIC-131’s output is initialized at the current output of XIC-130 (one-shot). TIC-110’s output will be initialized at PIC-131’s setpoint value until PIC-131 is placed into cascade mode. FIC-130 is locked in manual and its output tracks XIC-130’s output.

After any firing control configuration change, the appropriate controllers for the selected configuration must be placed into automatic or cascade mode to start regulation of the outlet temperature of the Fired Heater. Note that XIC-130 is locked in cascade mode except during I-110 activation.

Interlock I-110 Activation (XY-130)

On activation of Fuel Gas Interlock I-110, all three controllers will be locked in manual mode with an output of 0%. All three firing control soft switches (HS-130T, HS-130F, and HS-130P) are unaffected. XIC-130 is also locked in manual mode with an output of 0% whenever I-110 is in the TRIP state.

Note that I-110’s manual trip switch ES-110 is available on the Fuel Gas System schematic in case of an emergency.

Fuel Gas Override Mode (XY-130)

XY-130 will automatically reduce the position of the fuel gas control valve FV-130 in the event of low oxygen concentration or high carbon monoxide concentration in the flue gas of Fired Heater F-101. The soft switch HS-130O is used to select which concentration drives the fuel gas override logic. In the LO-O2 position, the oxygen instrument AX-180 will be used. In the HI-CO position, the carbon monoxide instrument AI-182 will be used.

If the PV of AX-180 (firebox O2) goes below 1.0 VOL % (when HS-130O position is LO-O2) or if the PV of AI-182 goes above 20 PPM (when HS-130O position is HI-CO) and I-110 is not activated, controllers TIC-110, FIC-130 and PIC-131 will be locked in manual mode and the setpoint of XIC-130 ramped down at a pre-determined rate (normally 0.5% per second) until either AX-180 is above its 1.0 VOL% low alarm setpoint or AI-182 is below its high alarm setpoint of 20 PPM for more than 10 seconds. XA-180 will alarm to indicate that Low O2 Override Mode is active and XA-182 will alarm to indicate the High CO Override Mode is active, depending on the position of override switch HS-130O. Use Fault #130 to change the ramp rate of XIC-130. The control configuration switches remain unaffected while either Override Mode is active and controller output initialization will be made as per above. Thereafter, the appropriate controllers for the selected temperature control configuration can be placed in their normal operating modes to restart regulation of the Fired Heater’s outlet temperature via TIC-110.

Combustion Air System

Forced Draft Fan G-101

TI-164 indicates the temperature of ambient air to G-101.

Switch HS-171 operates the motor of G-101. XA-171 alarms when the motor is stopped.

Combustion air flow controller FIC-164 adjusts the position of G-101 inlet damper FV-164. FIC-164 normally receives its setpoint from flue gas O2 controller AIC-180. FIC-164OP indicates the output of FIC-164 and will alarm on low or high output values.

PI-164 indicates the discharge pressure of G-101.

Combustion Air Preheater E-101

Switch HS-172 operates the motor of E-172. XA-172 alarms when the motor is stopped.

TI-165 indicates the outlet combustion air temperature of E-101.

TI-162 indicates the inlet flue gas temperature to E-101. TI-163 indicates the outlet flue gas temperature from E-101.

Hand controller HIC-165 adjusts the position of E-101 bypass damper HV-165. This bypass keeps the flue gas temperature from E-101 (TI-163) from becoming too cold in cold weather or high excess air operation. This helps avoid corrosion of the Combustion Air Preheater E-101 due to water condensate formation of the flue gas.

Combustion Air Plenum of F-101

TI-166 indicates the temperature of the forced air to the plenum.

Hand controller HIC-166 controls the position of the direct air supply doors on both ends of the plenum. Normally, these doors are closed. When the Forced Draft Fan is stopped, HIC-166 receives its setpoint from AIC-180.

PI-166 indicates the pressure of the plenum.

O2 Control

AI-180A and AI-180B two independent flue gas oxygen analyzers located in the arch section of the Fired Heater. AX-180 is a low selector of these two analyzers, and the low signal selected by AX-180 is used by O2 controller AIC-180 and by the Low O2 Override logic of XY-130 (see Fuel Gas System above).

AIC-180 controls the oxygen concentration of the flue gas leaving the convection section of the Fired Heater by adjusting the setpoint of combustion air flow controller FIC-164 when the Forced Draft Fan G-101 is running. When the Forced Draft Fan is stopped, the oxygen concentration can be controlled by cascading AIC-180 to air plenum fresh air doors controller HIC-166. However, this cascade loop may be overly interactive to the dynamics of the draft pressure control loop. Therefore, it may be required to manually adjust HIC-166 to maintain the oxygen concentration of the Fired Heater.

The control logic block AY-180 determines the initialization of AIC-180, HIC-166 (plenum fresh air doors door position) and FIC-164 (Forced Draft Fan G-101 flow controller) whenever the Forced Draft Fan is stopped or started as indicated by XA-171. The logic functions as follows:

  • On a stop of the FD Fan, ambient air is admitted into the combustion air plenum by placing HIC-166 into manual mode and setting its output to 100% (one-shot). Also, FIC-164 is placed into manual mode and its output is set to 0% (one-shot).
  • On a start of the FD Fan, the FD Fan’s inlet damper is fully opened by placing FIC-164 into manual mode with an output of 100% (one-shot).
  • On either event, AIC-180 will be placed in manual mode with an output of 100%.
  • Whenever the FD Fan is running, the fresh air doors to the Combustion Air Plenum are fully closed by locking HIC-166 in manual mode with an output of 0%.
  • When AIC-180 is in manual mode, its output tracks FIC-164’s setpoint when the FD Fan is running, and it tracks HIC-166’s setpoint when the FD Fan is off.
  • On a trip of Fuel Gas Interlock I-110, AIC-180 is placed into manual mode (one-shot) to maintain the air flow rate when the trip occurs.
  • AIC180OP indicates the output of AIC-180 and will alarm when the output is low or high.

Flue Gas System

Flue Gas Analyzers

AI-181 indicates the concentration of combustible compounds in the flue gas reaching the arch section of the Fired Heater. Its PV is used by AAH-181 which is used by Fuel Gas Interlock I-110 to stop fuel gas in case of high combustibles (see the “Interlocks” section below).

AI-182 indicates the concentration of carbon monoxide (CO) in the flue gas reaching the arch section of the Fired Heater. Its PV is used by AAH-182 which is used by Fuel Gas Interlock I-110 to stop fuel gas in case of high CO (see the “Interlocks” section below).

Stack Damper

Hand controller HIC-161 adjusts the position of the stack damper HV-161. Normally the damper is fully closed. Note that the stack damper has a small hole in it to allow flue gas flow even when the stack damper is in the fully closed position.

Flue Gas System

Induced Draft Fan G-102

Switch HS-173 operates the G-102 motor. XA-172 alarms when the motor is stopped. Hand controller HIC-173 adjusts the position of G-102 inlet damper HV-173. HIC-173 normally receives its setpoint from heater draft controller PIC-160. HIC-173OP indicates the output of HIC-173 and will alarm on low or high output values.

Draft Control

The pressure, or draft, at the top of the radiant section of the Fired Heater is controlled by PIC-160. The output of PIC-160 is indicated on PIC160OP. Normally, PIC-160 controls the draft by adjusting the setpoint of Induced Draft Fan outlet damper hand controller HIC-173. When the ID Fan is off, it controls the stack damper HV-161 hand controller HIC-161.

PAH-160 is an independent instrument measuring the draft and is used by Fuel Gas Interlock I-110 (see the “Interlocks” section below) to stop fuel gas in case of high draft.

The logic block PY-160 determines the initialization of PIC-160, HIC-161 (stack damper) and HIC-173 (Induced Draft Fan G-102 inlet damper) whenever the ID Fan is stopped or started as indicated by XA-173. The logic functions as follows:

  • On a stop of the ID Fan, PY-160 will fully open the stack damper by placing HIC-161 into manual mode with an output of 100% (one-shot). PY-160 will also close the ID fan inlet damper HV-173 by placing HIC-173 into manual mode with an output of 0 (one-shot).
  • On a start of the ID Fan, PY-160 will fully open the ID Fan damper by placing HIC-173 into manual mode with an output of 100% (one-shot). PY-160 will also fully close the stack damper by placing HIC-161 into manual mode with an output of 0% (one-shot).
  • On either event, PIC-160 will be placed in manual mode with an output of 100% (one-shot).
  • When PIC-160 is in manual mode, its output tracks HIC-173’s setpoint when the ID Fan is running and it tracks HIC-161’s setpoint when the ID Fan is off.

The operation of the draft controls is not affected by the Fuel Gas Interlock I-110.

Fired Heater Temperatures

This schematic shows the temperatures related to the Fired Heater F-101.

TI-120A/B/C/D indicate the fluid temperatures for each of the four passes at the crossover from the convection section to the radiant section of F-101.

TI-121A/B/C/D indicate the tube metal temperatures of the radiant section tubes for each of the four passes in the upper section of the radiant section.

TI-122A/B/C/D indicate the tube metal temperatures of the radiant section tubes for each of the four passes in the middle section of the radiant section.

TI-123A/B/C/D indicate the tube metal temperatures of the radiant section tubes for each of the four passes in the lower section of the radiant section.

TI-110A/B/C/D indicate the fluid outlet temperatures for each of the four passes leaving F-101.

TIC-110 indicates the combined fluid outlet temperature from F-101. TAH-110 is an independent instrument that also indicates the combined fluid outlet temperature from F-101 and is used by Fuel Gas Interlock I-110 to protect the heater from damage (see Interlock section below for trip setting). Note the heater fuel gas shutdown switch ES-110 is provided on this schematic in case immediate stoppage of fuel gas is required.

Interlocks

Fuel Gas Interlock I-110

The only interlock in the Advance Fired Heater simulator is Fuel Gas Interlock I-110. This protects the Fired Heater F-101 from damage due to conditions that can result in overheated tubes or from potentially explosive conditions within the Fired Heater.

Trip Inputs

All the process trip inputs to I-110 are shown on Schematic #8 as follows:

  • AAH-181 Flue gas high combustibles (setpoint = 500 PPM)
  • AAH-182 Flue gas high CO (setpoint = 100 PPM)
  • FAL-121A Pass ‘A’ low flow (setpoint = 1.5 MBPD)
  • FAL-121B Pass ‘B’ low flow (setpoint = 1.5 MBPD)
  • FAL-121B Pass ‘C’ low flow (setpoint = 1.5 MBPD)
  • FAL-121B Pass ‘D’ low flow (setpoint = 1.5 MBPD)
  • PAH-160 F-101 radiant section high draft (setpoint = 0.2 IN H2O)
  • PAL-131 Fuel gas header low pressure (setpoint = 4.0 PSIG)
  • TAH-110 F-101 outlet fluid high temperature (setpoint = 700 DEG F

If any one of these instruments enters the alarm condition and continuously stays in that condition for 10 seconds, I-110 will be tripped. Additionally, the operator can manually trip I-110 by changing trip/bypass switch ES-110 to the TRIP position. Note that ES-110 is accessible from Schematics #5, #8 and #10.

Interlock Actions

When I-110 changes from OK to TRIP position, the following actions will be taken:

  • XA-110 (I-110 trip) alarms.
  • Switch ES-110 is placed in the TRIP position and locked until all trip sensors are not in the trip condition or they are bypassed, and the firebox purged permissive PERM-110 is in the YES position (see Firebox Purging below).
  • Fuel gas isolation valve XV-130 is closed and its switch XV-130 is locked in the CLSD position.
  • Fuel gas control valve FV-130 is locked in the closed position (0%).
  • Fuel gas control valve position controller XIC-130 is locked in manual mode with an output of 0%.
  • Fuel gas flow controller FIC-130 is locked in manual mode at 0% output.
  • Fuel gas header pressure controller PIC-131 is locked in manual mode with an output of 0%.
  • F-101 outlet temperature controller is locked in manual mode.
  • Flue gas oxygen controller AIC-180 is placed in manual mode (one-shot).
  • The Forced Draft Fan G-101 is stopped (one-shot).
  • The Induced Draft Fan G-102 is stopped (one-shot).

PAL-131 Startup Bypass

Low fuel gas header pressure PAL-131’s trip signal must be temporarily bypassed to permit I-110 to be reset to the OK state so that the fuel gas valves XV-130 and FV-130 can be opened to establish a high enough pressure so that PAL-131 is no longer in the low pressure alarm condition. The bypass of PAL-131 is initiated by placing switch BP-131 in the BYPS position. This causes the bypass alarm BPA-131 to go from the ARMED to the BYPS position. An audible alarm is also generated. The bypass countdown BPT-131 will change from 0 seconds to the maximum bypass time, normally set at 120 seconds. Use Fault #228 to change the bypass time. During this time, the operator will need to:

  • Change trip/reset switch ES-110 to the OK position
  • Open fuel gas isolation valve XV-130
  • Manually open FV-130 to establish a high enough fuel gas header pressure so that PAL-131 is no longer in the low alarm condition. FV-130 is opened using the appropriate controller’s output (FIC-130, PIC-131 or TIC-110), depending on which control configuration is selected (see XY-130 above).

If BPT-130 counts down to 0 seconds and PAL-131 is still in the low alarm condition, the bypass of PAL-131 will be placed back into the ARMED state and I-110 will trip.

If PAL-131 is continuously above the low alarm setpoint for 10 seconds, the bypass of PAL-131 will automatically be placed back in the ARMED state and BPT-131 will be set to 0 seconds.

I-110 Trip Instruments Bypass

The bypass of I-110’s trip instruments will completely bypass the process trip inputs to I-110 for a preset time. This bypass is not to be used except when permission is granted by the instructor. The bypass of I-110 trip instruments is initiated by placing switch BP-110 in the BYPS position. This causes the bypass alarm BPA-110 to go from the ARMED to the BYPS position. An audible alarm is also generated. The bypass countdown BPT-110 will change from 0 seconds to the maximum bypass time, normally set at 300 seconds. Use Fault #229 to change the bypass time. During this time, any trip instrument that is in its alarm state will not cause I-110 to trip. If BPT-110 counts down to 0 seconds, the bypass of I-110 trip instruments will be placed back into the ARMED state. If any trip instrument is in an alarm condition, I-110 will trip.

Firebox Purging

Schematic #9 allows the operator to initiate, control and monitor the purging of the firebox of Fired Heater F-101.

Purging of the firebox of the Fired Heater F-101 is required after any trip of the fuel gas interlock I-110, including a manual trip. Purging of the firebox should be completed before attempting to bypass the fuel gas header pressure PAL-131 as described above.

Any time the fuel gas has been tripped by interlock I-110, a timer will calculate how long the firebox has been purged. Purging is considered to be occurring under any one of these four scenarios:

  • Both outputs of HIC-166 (ambient air to plenum) and HIC-161 (stack damper) exceed 50%.
  • The output of HIC-166 exceeds 50% and the Induced Draft Fan G-102 is running and the output of HIC-173 (ID Fan inlet damper) exceeds 50%.
  • The Forced Draft Fan G-101 is running, and the output of FIC-164 exceeds 50% and HIC-161 (stack damper) exceeds 50%.
  • The Forced Draft Fan G-101 is running and FIC-164’s PV exceeds 50% of instrument range and the Induced Draft Fan G-102 is running and the output of HIC-173 (ID Fan inlet damper) exceeds 50%.

Logic block FY-164 determines if firebox purging is occurring based on the above conditions. These conditions must be held continuously to advance the countdown timer. The time required to purge the firebox is normally 120 seconds. Use Fault #227 to change the required purge time. The countdown timer FYT-164 indicates the time remaining to complete purging before fuel gas can be admitted to the Fired Heater F-101. When this timer reaches 0 seconds, the firebox purging is complete permissive PERM-110 will change from NO to the YES position.

To restart fuel gas to the Fired Heater after a trip the following conditions must exist:

  • All process trip inputs to I-110 are clear, except for PAL-131
  • The firebox purging is complete as indicated on PERM-110
  • Fuel gas header pressure PAL-131 is bypassed (see I-110 above)

Once these conditions exist, Fuel Gas Interlock I-110 can be reset by placing trip/reset switch ES-110 in the OK position. Refer to I-110 above on establishing fuel gas header pressure before the bypass timer for PAL-131 expires.