furnaces 2012

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 furnace operation? Explosions While Lighting a Furnace

•Incident

A Operator tested the atmosphere inside a furnacewith a combustible gas detector. No gas was detected, so the Blind plate was removed, and two minutes later, Ignition was done. An explosion occurred.

• Reason ?

Gas line had liquid

• Incident

A reduction in fuel oil pressure caused the burner in an oil-fired furnace to go out. and the flame failure device closed the valves in the fuel oil line . The operator closed the two hand-isolation valves and opened the bleed between them. Again opened when oil pressure came Explosion occurred.

• Reason ?

High oil pressure

Importance of furnace operation? Explosions While Lighting a Furnace

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

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

ConvectionTubes

ShockBank

RadiantTubes

RefractoryLining

Firebox

Burners

Vertical Cylindrical Furnace - Side ViewStack

Damper

Draft Gage

Tube SheetConvection Section

RefractoryPeep Door

Snuffing Steam

ArchTube Guides

Cross Over Tube

Cast BurnerBlock

Peep DoorsAccess Door

Process Outlet

Radiant SectionHeating Tube

Tube Pulling Door

Inlet from Process

Sample Connection

Tube CircleDia.I.D.

Shell Dia.

Burner

TI

Bridgewall Temperature

Vertical Cylindrical FurnaceBreeching

End Tube

Convection Section

Header BoxDrain

SnuffingSteam

Inlet fromConvection

Burner Circle Diameter

Burners

Peep Door

Access Door

Inlet from Process

Header BoxOutlet to

Radiant Section

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

HTR-R01-60

Air

Pilot Gas

Gas

Air

Air Register

Gas Tip (Flame Shape)Burner Tile/Orifice

Cone (Flame Holder)

Staged Fuel Burner

HTR-R00-64

High Air toFuel Ratio inPrimary Zone

Secondary FuelConnectionPrimary

Fuel Connection

CombustionAir

SecondaryCombustion

Secondary Fuel

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 Solution

Burners go out Gas mixture too lean

Too much draft

Reduce air

Adjust stack damper

Flame flashback Low gas pressure

High hydrogen in fuel

Raise fuel gas pressure

Reduce primary air

Insufficient heat release

Low gas flow

Burner tip plugged

Increase gas pressure

Clean burner tips

Pulsating fire Lack of oxygen

Lack of draft

Reduce gas flow rate

Lack of draft Open stack damper

Open burner air registers

Erratic flames Lack of combustion air Adjust air registers/damper

Incorrect burner tip location Check burner tip location

Damaged burner tile Replace burner tile

Gas flame too long 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 Significance & its remedial procedures

Indicators of correct combustion in the firebox

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 lengthWith 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 unstableFor 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


High or uneven tube skin temperature

Flame impingement

Over-firing

Unbalanced pass flow

Coke build up

Bad thermocouple

Modify burners

Reduce firing

Equalize flow in all passes

Decoke tubes

Replace thermocouple

Positive pressure at arch Damper not open enough

Firing rate high

Convection section fouled

Open damper

Reduce firing rate

Clean convection section

High flue gas temperature

Convection Section fouled

Fins burnt off

After-burning in convection

Over-firing

Clean convection section

Replace convection tubes

Modify burners

Reduce firing

Fired Furnace Troubleshooting Guide


High fuel gas pressure Burners are plugged Clean burners

Variation in pass outlet temperatures

Unequal pass flow-rates

Uneven firing

Equalize flow in all passes

Equalize firing in all burners

High pressure drop through tubes

Coke build up

High rate of vaporization

Decoke tubes

Reduce flowrate

High excess air operation High furnace draft

Poor air fuel mixing

Air leakage in the furnace

Reduce furnaces draft

Modify burners

Plug air leakage

Commissioning

Process flow control– Pass flows are controlled by flow indicating/ recording control

systems consisting of FE, FT, FIC, FV (on process line)– Pass flows should be equal– Minimum pass flow must be maintained– Velocity steam/ water injection

INSTRUMENTATION- Commissioning

Process temperature control– 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

INSTRUMENTATION- Commissioning

PROCESS HEATERS – OPERATIONS & PRACTICES

TYPICAL CONTROLS

DPIC

PICSS

PIC

TRC

Selector

Switch Fuel

GasAtomiz

ing Steam

Fuel Oil

Skin Points– For Monitoring Tube Wall Temperatures, Thermocouples Are

Provided on Heater Tubes– Maximum Permissible Skin Temperatures Must Be Adhered to

Box Temperature– Facilitates the Operator to Regulate the Furnace Firing and to

Maintain Even Heat Distribution Draft Gauges

– Draft Profile of the Furnace Is Indicated by Draft Gauges– Positive Pressure Must Be Avoided


INSTRUMENTAION

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


INSTRUMENTATION

Combustion Control System – 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


INSTRUMENTATION

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


TYPICAL INTERLOCKS

FD Fans Failure Fuel Oil and Fuel Gas to Furnace to Be Cut-off

High Arch Pressure Stack Damper to Open FullIf 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


TYPICAL INTERLOCKS

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 Fired Process Heaters Safety Features

Preparation– Inspect Furnace for Readiness– Check Heater Isolation (Specially FO and FG)– Check Dampers/ Air Registers Operation– Check Igniters– Check Instruments– Check Fire Fighting Equipment– Ensure No Loose Ends in FO & FG Circuit– Check Burner Gaskets– Ensure Required Flow Thru’ the Tubes– Leave Header Box Doors Open


SAFE START-UP

Furnace Box Purge– 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– 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


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


SAFE LIGHTING PROCEDURE

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


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.


ROUTINE MONITORING

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


DOS

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


DONTS

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 DECOKINGThe 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 DECOKINGThe 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

STEAM / AIR DECOKINGSchematic piping diagram

STEAM / AIR DECOKINGWhen 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 DECOKINGOPERATING 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 DECOKINGOPERATING 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 waterIntroduce small amount of airWhen 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 stagesO2 will be evident in the sample.

Note: tubes are not overheated during the burning operation.

Thank you

furnaces 2012

Documents

oilfired furnace

generated heat

combustion of fuel

fuel oil pressure

fuel oil line

fuel gas valve

convective heat transfer

radiant heat transfer