Commissioning, start up, shut down, operation & Control and Decoking of furnaces
By
V Suresh Senior Manager-Core Group
Joined IOCL- 1997 Worked in different units like CRU,AU’S,MSQ,LAB etc.,
Importance of Safety in Furnaces?
Importance of Safety in Furnaces?
Importance of furnace operation? Explosions While Lighting a Furnace
Incident
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A Operator tested the atmosphere inside a furnace with a combustible gas detector. No gas was detected, so the Blind plate was removed, and two t wo minutes later, later, Ignition was done. An explosion occurred occurred.. •
Reason ?
Gas line had liquid
Importance of furnace operation? Explosions While Lighting a Furnace •
Incident
A reduction in fuel oil pressure caused the burner in an oilfired furnace to go out. and the flame failure device closed the valves in the fuel oil line . The operator closed the two handisolation valves and opened the bleed between them. Again opened when oil pressure came Explosion occurred.
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Reason ?
High oil pressure pressure
Importance of furnace operation? The burning of waste products in furnaces to save
Incident For the example, an explosion occurred in a system that collected flammable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of' the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, there are 10 such Reported incidents in literature •
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Importance of furnace operation? : Interlock not bypassed properly
Incident ? An instrument mechanic was asked to test the trip on A furnace. He put the controller on manual and then went behind the panel.. The mechanic, who had done the job many times before, took the cover off the wrong instrument , and disconnected one of theleads. The effect was the same as if the recorder had registered a high temperature. The controller closed the fuel gas valve, shutting down the furnace and the rest of the plant •
What is a Furnace ?
A furnace consists of three major components: a heating coil, the enclosure, and the combustion equipment. Heat is released from the combustion of fuel. The heating coil consists of tubes connected together in series that carry the charge being heated. Heat is transferred to the material passing through the tubes. The enclosure consists of a firebox. It is a steel structure lined with refractory material that holds the generated heat. Burners create the heat by the combustion of fuel. The furnace is fired by oil or gas. The heating coil absorbs the heat mostly by radiant heat transfer and convective heat transfer from the flue gases .
What is a Furnace?
The flue gases are vented to the atmosphere through the stack. Burners are located on the floor or on the sidewalls. Combustion air is drawn from the atmosphere. For increased heat recovery, an air preFurnace or waste heat boiler is installed downstream of the convection section. Instruments are generally provided to control the firing rate of the fuel and flow through the coils to maintain the desired operating conditions
Typical Furnace Stack Damper Arch Convection Tubes Shock Bank
Radiant Tubes Refractory Lining
Firebox Burners
Vertical Cylindrical Furnace - Side View Stack
Damper TI
Draft Gage
Sample Connection Tube Sheet Convection Section
Inlet from Process
Bridgewall Temperature Tube Pulling Door Cross Over Tube
Tube Circle Dia. I.D.
Shell Dia.
Arch Tube Guides Refractory Peep Door Snuffing Steam Cast Burner Block
Radiant Section Heating Tube Process Outlet Access Door
Burner
Peep Doors
Vertical Cylindrical Furnace Breeching Inlet from Process Header Box Outlet to Radiant Section
Header Box Drain Snuffing Steam Peep Door
End Tube Convection Section Burner Circle Diameter Inlet from Convection
Burners Access Door
Outlet to Process Radiant Section
Firing Controls
Major parameters that need to be controlled and monitored are: 1. Fuel gas/Fuel oil pressure; 2. Excess air in the form of oxygen trim 3. Draft in the furnace. 4. Burner modulation 5. Air/fuel cross-limiting
6. Total heat control in the form of pass balancing
Raw Gas Burner Burner Tile/Orifice
Gas Tip (Flame Shape)
Air Register
Cone (Flame Holder) Air
Air
Gas Pilot Gas HTR-R01-60
Staged Fuel Burner
Secondary Combustion
High Air to Fuel Ratio in Primary Zone
Secondary Fuel
Combustion Air
Primary Fuel Connection
Secondary Fuel Connection HTR-R00-64
Typical problems observed in fired Furnaces include:
High excess air operation
Fouled convection sections
High stack temperature
Over-firing
Bad flames/flame impingement
Excess air control It essentially involves answering three basic questions: a)
How much excess air is provided?
Answers : Flue gas analysis b)
How much excess air should be provided?
Answer : The oxygen concentration in the flue gas provides an indication of the excess air supplied to the combustion process c)
How efficient is the burning equipment?
Answer : The optimum excess air for a particular type of burner varies from one burner type to another and also depends on the type of fuel. Optimum excess air is the minimum excess air because it minimizes the heat loss to the flue gases, minimizes the cooling effect on the flame, and improves the heat transfer. With less than the minimum excess air, the unburned fuel will start appearing in the flue gas due to insufficient air. Minimum excess air should be specified by the burner vendor and should be verified during burner testing.
Tips on excess air control
In natural draft furnaces, the excess air is controlled by adjusting both stack damper and the Air registers.
Control schemes are installed in the balanced draft systems to control the excess air and draft more accurately.
Problems on excess air control
Measurement of the fuel and air flowrate accurately because of the fuel, the fuel gas quality (composition) keeps on changing in the refinery.
For liquid fuels, the fuel viscosity is so high and temperature dependent that a reliable flow measurement over a period of time is very difficult to obtain.
Combustion air flowrate is also difficult to measure reliably, as straight run-lengths for the installation of instruments are not available except when a venturimeter is installed in the suction stack of the FD fan.
Purpose of oxygen & combustible analyzer in flue gas in Arch
The excess air should be adjusted in such a way that the oxygen level in the flue gas is close to the minimum or optimum excess air level.
Combustibles should read close to zero during normal operation.
The combustible analyzer should not be used to make excess air adjustments.
The presence of combustibles is an indication of poor combustion.
Combustion air should not be controlled using CO or combustibles as a guide.
The presence of CO or combustibles indicates that either the air is deficient or the combustion equipment is not clean, which is generally the case.
The dirty burners or poor atomization of oil can easily lead to CO formation
Burner Troubleshooting Problem
Cause
Burners go out
Gas mixture too lean
Reduce air
Too much draft
Adjust stack damper
Low gas pressure
Raise fuel gas pressure
High hydrogen in fuel
Reduce primary air
Insufficient heat release
Low gas flow
Increase gas pressure
Burner tip plugged
Clean burner tips
Pulsating fire
Lack of oxygen
Reduce gas flow rate
Lack of draft
Lack of draft Open stack damper
Flame flashback
Solution
Open burner air registers
Erratic flames
Gas flame too long
Lack of combustion air
Adjust air registers/damper
Incorrect burner tip location
Check burner tip location
Damaged burner tile
Replace burner tile
Excessive firing
Reduce firing rate
Poor air fuel mixing
Improve burner design
Tips for proper burner operation & their solution in case of any problem
Sr
Description Indicators of correct combustion in the firebox
Significance & its remedial procedures 1. The firebox is clear 2. There is no smoky appearance 3. The burner flames are steady and well formed Check burners regularly for any signs of blockage or unusual flame conditions
Burner flames are long and lazy
It is a sign of poor mixing
Increasing the airflow to the burner can reduce flame length
With natural draft burners, increase the primary air and minimize the secondary air to the burners. Primary air mixes with the fuel and creates a short compact flame. Excess of primary air can sometimes lift off the flame and make it unstable
For oil firing, flame lift off can be corrected by increasing the atomizing steam.
Recommended Excess Air Levels
Fuel/Draft
Natural Draft
Forced Draft
Fuel gas
15 – 20%
10 – 15%
Light fuel oil
20 – 25%
15 – 20%
Heavy fuel oil
25 – 30%
20 – 25%
Fired Furnace Troubleshooting Guide Problem High or uneven tube skin temperature
Positive pressure at arch
High flue gas temperature
Cause
Solution
Flame impingement
Modify burners
Over-firing
Reduce firing
Unbalanced pass flow
Equalize flow in all passes
Coke build up
Decoke tubes
Bad thermocouple
Replace thermocouple
Damper not open enough
Open damper
Firing rate high
Reduce firing rate
Convection section fouled
Clean convection section
Convection Section fouled
Clean convection section
Fins burnt off
Replace convection tubes
After-burning in convection
Modify burners
Over-firing
Reduce firing
Fired Furnace Troubleshooting Guide Problem
Cause
Solution
High fuel gas pressure
Burners are plugged
Clean burners
Variation in pass outlet temperatures
Unequal pass flow-rates
Equalize flow in all passes
Uneven firing
Equalize firing in all burners
High pressure drop through tubes
Coke build up
Decoke tubes
High rate of vaporization
Reduce flowrate
High excess air operation
High furnace draft
Reduce furnaces draft
Poor air fuel mixing
Modify burners
Air leakage in the furnace
Plug air leakage
Commissioning
INSTRUMENTATION- Commissioning
Process flow control –
Pass flows are controlled by flow indicating/ recording control systems consisting of FE, FT, FIC, FV (on process line)
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Pass flows should be equal
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Minimum pass flow must be maintained
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Velocity steam/ water injection
INSTRUMENTATION- Commissioning
Process temperature control –
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Temperature control system consists of TE, TIC, TV (on FO/ FG line) For FG, a TIC-PIC cascade is generally used For FO, in addition to the TIC-PIC cascade, a DPIC with atomizing steam is also used For dual firing furnaces, a selector switch is provided for switching between FG/ FO modes
PROCESS HEATERS – OPERATIONS & PRACTICES TYPICAL CONTROLS
TRC
SS
PIC
Fuel Oil Atomiz ing Steam
PIC DPIC
Select or Switc h Fuel Gas
PROCESS HEATERS – OPERATIONS & PRACTICES INSTRUMENTAION
Skin Points –
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Box Temperature –
For Monitoring Tube Wall Temperatures, Thermocouples Are Provided on Heater Tubes Maximum Permissible Skin Temperatures Must Be Adhered to Facilitates the Operator to Regulate the Furnace Firing and to Maintain Even Heat Distribution
Draft Gauges –
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Draft Profile of the Furnace Is Indicated by Draft Gauges Positive Pressure Must Be Avoided
PROCESS HEATERS – OPERATIONS & PRACTICES INSTRUMENTATION
Convection Bank Temperature Thermocouples U/s and D/s of Convection Bank Indicate the Amount of Heat Transfer Stack Temperature Higher Than Normal Stack Temperatures Indicate Low Efficiency in Furnace Operation Process Fluid I/l and O/l Pressure Pressure Drop Across Furnace Indicates Coking/ Plugging in the Furnace Tubes Oxygen Analyzers Direct Indication of Excess Air in the Furnace CO, NOx & SOx Analyzers Emission Monitoring –
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PROCESS HEATERS – OPERATIONS & PRACTICES INSTRUMENTATION
Combustion Control System –
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Draft Control by Adjusting ID Fan RPM or Its’ Suction Vanes Combustion Air Control by Adjusting FD Fan RPM or Its’ Suction Vanes
Excess Air Control, Based on the On-line Measurement of O2 & CO in Flue Gas Process Fluid O/l Temperature Control by Adjusting FO & FG Pressure
PROCESS HEATERS – OPERATIONS & PRACTICES TYPICAL INTERLOCKS Low Feed Flow Thru’ Tubes
Fuel Oil & Fuel Gas to Furnace to Be Cut-off
Low FO Pressure
Fuel Oil to Furnace to Be Cut-off
Low FG Pressure
Fuel Gas to Furnace to Be Cut-off
Low Combustion Air Pressure
FO & FG to Furnace to Be Cut-off
ID Fan Failure
Stack Damper to Open Fully; If Stack Damper Doesn’t Open Within a Stipulated Time-FO & FG To Furnace Cut-Off
PROCESS HEATERS – OPERATIONS & PRACTICES TYPICAL INTERLOCKS FD Fans Failure
Fuel Oil and Fuel Gas to Furnace to Be Cut-off
High Arch Pressure
Stack Damper to Open Full If Stack Damper Doesn’t Open Within a Stipulated Time; Fuel Oil & Fuel Gas to Furnace to Be Cut- off
Fuel Cut-off to Furnace
Stack Damper to Open Fully
PROCESS HEATERS Fired Process Heaters Safety Features Firefighting Water Monitor ?Curtain :
The firefighting nozzles are installed almost 15 meter away of heater. In the case of the fire the water will be used through this equipment extinguish the fire. Explosion Doors:
These doors are installed in heater walls. In the case of explosion the door will be raptured and will cause the combustion and explosion gases to be exited and avoid from destruction of heaters walls and other different parts.
PROCESS HEATERS Fired Process Heaters Safety Features
Emergency shutdown systems
Where are they located ?
PROCESS HEATERS – OPERATIONS & PRACTICES SAFE START-UP
Preparation –
Inspect Furnace for Readiness
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Check Heater Isolation (Specially FO and FG)
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Check Dampers/ Air Registers Operation
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Check Igniters
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Check Instruments
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Check Fire Fighting Equipment
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Ensure No Loose Ends in FO & FG Circuit Check Burner Gaskets
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Ensure Required Flow Thru’ the Tubes
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Leave Header Box Doors Open
PROCESS HEATERS – OPERATIONS & PRACTICES SAFE LIGHTING PROCEDURE
Furnace Box Purge –
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It Is an Important Step Because It Safe-guards Against Formation of Explosive Mixture Due to Presence of Inflammable Gases in the Box Open Stack Damper and Air Registers Purge the Fire Box With Steam (Generally)
Fuel Lines Purge –
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N2 Purge Thru’ FO / FG/ Pilot Lines of Each Burner Pressurize Each Fuel System With N2 and Check for Leaks Pilot Gas Line Is to Be De-blinded After the Box Purging Has Commenced
PROCESS HEATERS – OPERATIONS & PRACTICES SAFE LIGHTING PROCEDURE
Lighting Pilot Burners Open Pilot Gas Main Valve Place the Igniter Tip at the Burner Tip and Open Pilot Gas Valve to Burner and Press the Igniter Button Adjust Air to Prevent Pilots From Blowing Out(pilot Burner Valve Should Be Shut-off If It Fails to Ignite Within 15 Seconds) Lighting Gas Burners After the Pilot Burners Are Lit. Deblind the FG Line and Light-up the Gas Burners From Pilots Lighting Oil Burners Open Atomizing Steam and Bypass Between Oil and Purging Steam and Heat up the Burner and Tip Close the Bypass and Open the FO –
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PROCESS HEATERS – OPERATIONS & PRACTICES ROUTINE MONITORING Check Furnace Draft at Arch Level Check Refractory: Should Not Be Damaged Check Tube Hangers and Lock-rods: Should Be Firmly Fixed and Should Not Be Red Hot Check Tubes: Hammering, Vibrations, Hot Spots, Bending, Sagging, Bowing Check the Flame Pattern and Try to Correlate With O2 Analyzer Check That Skin and Box Temperatures Are With Permissible Limits
PROCESS HEATERS – OPERATIONS & PRACTICES ROUTINE MONITORING
Ensure FO and FG Pressure Are Above Tripping Values
Ensure Differential Pressure Between FO and Atomizing Steam Is Being Maintained
Check for Heat Tracing on Instrument Pulse Line/ Seal Pots on Flow/ Pressure Taping of Process Fluid, Fuel Oil Etc.
PROCESS HEATERS – OPERATIONS & PRACTICES DOS Before Opening ‘Peep-hole’ Covers, Ensure Fire Box Is Under ‘Draft’ Always Use Pilot Burner for Lighting up Main Burner After Total Flame Failure, Ensure All Fuel Supply Is Properly Isolated and Fire Box Is Thoroughly Purged Always Steam Flush the FO Burner After Stopping Oil Firing Religiously Drain the FG KOD
PROCESS HEATERS – OPERATIONS & PRACTICES DONTS
While Opening Peep-hole Covers, Never Stand Directly in Front of Hole
Never Try to Ignite a Burner From Another Lighted Burner
Never Allow Impingement of Flame on Tubes
Never Light-up Main Burner Without Ensuring Flow Thru’ Coils
Do Not Bypass Furnace Interlocks Except During Maintenance Jobs
PROCESS HEATERS – DECOKING
STEAM / AIR DECOKING
Mechanics of steam-air decoking
Principles used to complete the task of heater coil decoking
Precautions
STEAM / AIR DECOKING Steam-air decoking is the art of removal of coke deposited inside heater tubes by spalling and/or burning, using steam and air as agents.
STEAM / AIR DECOKING The mechanics of steam-air decoking for heater tubes are: A. Contraction of the tubes due to cooling will cause the coke deposits within the tube to crack and spall (fall off). This action is enhanced by reduction in firing of the heater and the introduction of steam. Steam injection and sweeping, in addition to the manipulation of heater firing will remove (spall) loose coke from the tubes.
STEAM / AIR DECOKING The mechanics of steam-air decoking for heater tubes are: B. It is required that steam be injected into the tubes not being decoked to prevent damage to these tubes. Injection of steam into the tubes generates a chemical reaction – 3H2O + 2C
CO2 + CO + 3H2
The oxygen in the air also generates a chemical reaction with the heated coke – 3O2 + 4C 2CO2 + 2CO
STEAM / AIR DECOKING The decoking requires that operators continuously monitor the coke burning rate by observing the meta temperature of the tubes and checking the effluent water Night is the best time TO CHECK The metal content of the tubes governs the controlling temperature a which the operation shall be conducted. Coke will burn at temperatures between 565°C to 650°C The time required for completion of the decoking operation can vary from six hours to three days, dependent upon the thickness of coke deposits •
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STEAM / AIR DECOKING Schematic piping diagram
STEAM / AIR DECOKING When the pressure drop across a pass of the heater increases by around 10%, the tubes require decoking. An alternative method is to schedule decoking at regular intervals. Normally there is no requirement for reversible flow. The deposition of coke is toward the outlet end of the heater.
STEAM / AIR DECOKING OPERATING PROCEDURES Introduce steam into the tubes. Approximate steam flows various tube diameters are: Tube ID, mm
Steam Flow, Kg/hr
50.8 ( 2”)
304
63.5
476
76.2
703
101.6 ( 4”)
1270
127( 6’)
2042
STEAM / AIR DECOKING OPERATING PROCEDURES A pressure-drop of 0.68 Kg/cm2 per 30.5 meters is to be expected when the recommended steam is introduced. If pressure drop is more do reverse Turn on quench water Introduce small amount of air When it appears that coke burning is about to stop or is nearing completion, gradually increase the quantity of air being injected with the steam During decoking operations the CO2 content will be about (1-5%), and the CO:CO2 ratio will be high because of insufficient oxygen. As the decoking operation nears Completion, the CO:CO2 ratio will decrease, and in the last stages O2 will be evident in the sample.
Note: tubes are not overheated during the burning operation.
