furnace operations

8/12/2019 Furnace Operations

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FURNACE OPERATIONS

Pakistan Refinery Limited

Operations Department

By: Azhar ShaikhShahid Raza


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Industrial Furnaces

Types of Furnaces

Basic sections and parts of Furnace

General principles of combustion Optimizing furnace operation

Design parameters of PRL Furnaces

PRL-Fuel System

Normal operational Checks

Startup and Shutdown

Operational Troubleshooting

Contents:


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Furnace:

Furnace is a device in which the chemical energy

of fuel or electric energy is converted into heat

which is then used to raise the temperature of

material, called the burden or stock.

Performance objectives:

Maximize heat delivery of the process-side

feed while minimizing fuel consumption.

Maximize heat delivery with varying fuel

quality.

Minimize heater structural wear caused byoperation.

Minimize stack emissions (heat, CO, NOx ).

Maximize safety integrity levels.

Industrial Furnaces


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Based on mode of Operation:

1. Batch type

2. Continuous

3. Direct heating

4. Indirect heating

Based on mode of heating source:

1. Electrical

2. Nuclear

3. Combustion furnaces.

Based on type of fuel:

1. Solid fuel fired furnace

2. Liquid fuel fired furnace

3. Gaseous fuel fired furnace

4. Multi fuel fired furnace

Types of Furnaces


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Combustion furnaces:

1. Fired heaters

2. converters.

Fired heaters:

-A fired heater is a piece of equipment in which heat released from the

controlled combustion of fuel at the burners is transferred to material passing

through the tubes along the wall, roof, or floor (hearth) of the heater.

-Fired heaters are furnaces that produce heat as a result of the combustion of

fuel. The heat liberated is transferred to the material to be heated directly (in

internally-heated furnaces) or indirectly (in externally-heated furnaces).

Examples of internally-heated furnaces include submerged heaters and blast

furnaces where a solid mass is heated by a blast of hot gases.Externally-heated furnaces include ovens, fire-tube boilers and tubular heaters.

Converter :

converter is a type of furnace in which heat is liberated by the oxidation of

impurities or other parts of the material to be heated.

Combustion Furnaces:


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Based on type of draught control system

Natural Draft Furnace:

This is the most common type of draft with the air drawn into the furnaceby means of the draft created by the stack. The taller the stack, the greater

the draft available.

Forced Draft Furnace:

In this type of system, the air is supplied by means of a centrifugal fan

commonly known as forced draft (FD) fan. It provides for high air velocity,better air fuel mixing, and smaller burners. The stack is still required to

create a negative draft inside the furnace.

Induced Draft Furnace:

When the height of the stack is inadequate to meet the draft

requirements, an induced draft (ID) fan is provided to draw the flue gases

out of the heater. Negative pressure inside the furnace ensures air supply

to the burners from the atmosphere.

Balanced Draft Furnace:

When both forced draft and induced draft fans are used with the heater, it

is known as a balanced draft system. Most air preheating installation is

balance draft and large combustion furnace comes under this category.


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Types of Fired Heaters:

1. Box type heatersIt is best suited for large capacities and large heat

duties.

2. Cylindrical heaters

Cylindrical heater with vertical tubes are commonly

used in hot oil services and other processes where

the duties are usually small.

Cylindrical heaters are often preferred to box-type

heaters. This is mainly due to the more uniform

heating rate in cylindrical heaters and higher

thermal efficiency.

Cylindrical heaters require smaller foundations and

construction areas and their construction cost is

less. High chimneys are not essential in cylindrical

furnaces because they normally produce sufficient

draught.


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Basic sections and parts of Furnace

Fire box/combustion

chamber:The open area

inside the heater where the

combustion of the fuel takesplace.

Flue gas ducting:The large

diameter piping b/w the

convection of the heater and

the stack.

Convection:Where the

transfer of heat through the

circulation of gases.


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Parts of Furnace

Fire BoxRadiant Tubes

Convection Tubes

Damper and Stack

Refractory Lining

Burners.

Air Registers (lets air in

by burners)

Fire box is lined with

refractory brick (usually

white/tan in color,lightweight, chalk-like,

ceramic material) lining

that can handle high

temperatures and reflects

heat back into the

furnace.


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BURNERS

Refinery furnace burners can be classified as Premix gas burner

Non-premix gas burner

Steam atomizing oil burner

Combination burners


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PREMIX GAS BURNERS

Used to obtain good mixing and toburn the gas with a short flame.

Gas under pressure is passed through

a small orifice or spud to form jet.

The jet pulls in primary air throughthe aspirator opening, and the gasand air are mixed in the mixing tubebefore being distributed through the

holes in the burner tip or spider.

As the gas-air mixture of gas, primaryair and secondary air burns with ashort blue flame.


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All the air mixes with the

fuel beyond the burner

tip.

Combustion begins at thetip with primary air and is

aided by the burner block

which gets hot and

radiates heat back to the

burning fuel.

The muffle blocks also

gets hot and aids

combustion.

NON-PREMIX GAS BURNERS


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STEAM ATOMIZING OIL BURNERS

Oil is atomized beforeflowing through the tip.

As oil leaves the burner, mixwith air and start to burn.

Flame heat vaporizes theremaining oil, and it alsoburns.

Smoke indicates that

1.Too much oil is being fed

2.Air registers are closed toofar

3.Insufficient draft. Wet steam may cause coke

to form on the tip.

Coke should be knocked offwith rod.


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COMBINATION BURNERS

Can burn oil and gas at the same time.

When oil burner is not in use, gun should

be pulled back to keep if from burning up.

Oil burns much better with the gas burner

operating.

Oil gun safety interlock prevents removal of

oil gun with fuel flowing.

Igniter port should be capped when not in

use.


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PILOT BURNERS


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General principles of combustion

Combustion (fire) in a furnace

firebox occurs when fuel

combines rapidly with oxygen

present in the air.

The three requirements for fire

are fuel, oxygen from the air

and a source of ignition.

Complete combustion verses

Absolute combustion.


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Main combustion Reaction

Stoichiometric Combustion:Under ideal conditions, fuel combines with exactly the right amount of

oxygen to allow complete combustion. There is no

unburned fuel and no excess oxygen. This is called stoichiometric combustion.

In the simple case of methane burning in air,

CH4 + 2O2 CO2 + 2H2O

Real combustion applications are more complicated because some excess

air is always needed to ensure complete combustion of the fuel.

Otherwise, significant amounts of CO are produced, reducing efficiencyand increasing pollution levels.

When combustion is complete , one pound of carbon release 14100 BTU

heat.

When CO is formed one pound of carbon release 4000 BTU heat


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Flammability Reign

Practically completecombustion is impossible

because of imperfect mixing

of fuel and air.

Therefore, refinery furnacesmust admit more than

theoretical air in order to

burn all the fuel.

Refinery furnaces arenormally designed to admit

up to 40% excess air.


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This average curvefor gas or oil fuels

can be used to

determine % excess

air from the amount

of O2 in the flue gas.

USEFUL AVERAGE CURVE

Reduction in

10% excess air

save 1% of fuel

35 F reduction in

flue gas

temperature

save 1% of fuel.


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Combustion Control Scheme. (BMS)


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WHY DRAFT IS NEEDED?

Avoids overheating refractory,anchors and structural, that wouldoccur with outward flow of hot gas.

Prevents hot gases from exiting

sight doors, burner registers andheader boxes, thus maintainingsafe conditions for personnel.

Causes air flow through naturaldraft burners to satisfy combustion

requirements.


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DRAFT MEASUREMENT

Draft is normally measured in inches of watergauge (W.G.) (1inch H2O = 0.036 PSI).

Ideally, the damper and the burner air register

should be adjusted such that the draft at the

inlet to the convection section is about2.5

mm (0.1) H2O.

The shield will protect you from a blast of hot

flue gas if there should be a positive pressure

inside the furnace.


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EXCESSIVE DRAFT

Excessive draft is to be

avoided.

Excessive draft will increase

the unwanted air in-leakage

(tramp air) and reduce heaterefficiency.

The air in-leakage can also

cause flame distortion and/or

combustion of unburned fuelin the proximity to the tubes.

TOO LITTLE DRAFT

Too little draft will cause inadequate airflow through the burners to completelycombust the fuel. The heater will oftenpuff as a symptom of too little air.

It can cause tube and tube supportdamage.

Low draft can also cause damage due tooverheating of the structures, vibrationof the setting, and burner flashback.

In extreme cases it can cause burnerflameout and possibly an explosion.


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Burner Level (Natural

Draft Heaters)-0.2 " H2O for Low BoxHeatersUp to -1.0 " H2O for tallcylindrical units

High Point of Firebox(Arch)

-0.05 (1.2mm) -0.15(3.8mm) " H2O for atypical well-balancedsystem. A higher draftmay be required for lowfireboxes or burnerelevations near the arch

due to burner draft needs.

Excess O2:

Gas Firing: 3-4%

Oil Firing: 5-6%


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Insufficient excess air may cause the following problems:

1. incomplete combustion

2. excessive fire box temperature

3. flame impingement.

Incomplete combustion wastes fuel. Money is going up the

stack. Also, the unburned fuel may ignite explosively if

there is a sudden increase in the amount of air admitted

to the furnace.

Decreasing excess air by reducing the burner air register

and partially closing the stack damper results in higherfirebox temperature. The furnace tubes may get hot

enough to cause coking.

INSUFFICIENT AIR PROBLEMS


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Reducing excess air alsolengthens flames, and they may

touch the tubes. This condition

is called flame impingement.

Flame impingement occurs

when the length and/or the

width of the flames increase

and touch the tubes.

Flames have a temperature of

about 1370 oC and will cause

internal coking if allowed to

impinge on the tubes. For all heaters, there is min

pass flow below which tube

damage can occur due to

overheating.

FLAME IMPINGEMENT


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Uneven coke laydown will

make one side of the tube

expand more than the

other, leading to bowing

and bulging of the tube.

Also, localized hotspots

develop on tubes where

partial loss of flow or flame

impingement has occurred.

Flow to the affected pass

should be increased and

adjacent firing reduced.

TUBE BOWING & BULGING


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Flames have a temp of about 1370 oC. Assume that the oil in a clean CS furnace tubehas a temp of 480 oC. Under these conditions, with flames not touching the tube, the

tube might be about 525 oC.

Now, when the 1370 oC flame strikes the tube, the temp of the tube rises rapidly. The

layer of oil next to the inside of the tube gets very hot and turn into coke.

Coke is a good insulator. Let us suppose that, a 3 mm thick layer of coke has beendeposited in the tube.

Because of the insulating effect of the 3 mm layer of coke, the tube skin temp will

now be about 635 oC. At this temp the tube is only about one-fifth as strong as it

was at 525 oC.

The weakened tube may yield and eventually rupture. Even if the tube does not rupture, the hot metal on the tube surface will continually

oxidize and get thinner.

When tube ruptured, a tremendous amount of fuel is added to the fire box and

flames spread outside the heater through peepholes and openings b/w structural

members.

TUBE RUPTURE


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COMPARISION B/W CLEAN & COKED TUBE


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Design parameters of PRL FurnacesMatrices 101-B 103-B 104-B Consequences in case of

violation

Throughput, mtd 4185

3150

1534

1150

1381

1039

Erosion, tube leakage, deltap,

heat loss, tube failure etc.

Process Temp

in/out, 0C

190

360

190

360

190

360

overloading, coke etc

T-press barg 28.3 28.3 28.3 tube may burst, asset loss,

production loss

Delta P, kg/cm2 7.29 3.97 7.0 Heat loss, erosion, effect column

press

Duty, MMBtu/hr 101.57 36 33 -

Efficiency, % 85 83 70 -

Excess Air, vol% 20/30 20/30 20/30 heat-loss, stack temp, Envior loss,

smoke, haziness

Draft, mmwc -1.22 -1.22 -1.22 back firing


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Matrices 101-B 103-B 104-B Consequences in case of

violation

DTMT, 0C 500 500 500 tube failure bowing, sagging,

rupture.

No of tubes conv/radi 52/44 52/44 24/48 -

Eff tube length, m 19.6 9.4 3.6/12.0 -

Flue gas temp leaving

radiation zone

972 967 796 -


cone zone

287 329 594 fuel loss, envior loss

Burner norm Capacity,

MMBtu/hr

4.25/4.35 2.89/2.96 7.89/8.28 -

FG/FO consum per

burner, mtd

2/2.7 2/2.5 5/4 -

FG/FO press at B-tip,

kg/cm2g

0.9/3.0 1.5/3.0 1.8/3.5 Tip damage, flame lift off, high

flame length

A-steam press at B-tip,

kg/cm2g

4.5-6.0 4.5/6.0 4.5-6.0 Poor atomizing, improper

mixing, combustion etc.


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HTU Furnaces

Matrices 202-B 201-B Consequences in case ofviolation

Throughput, mtd 2660 3050 Erosion, tube leakage,

deltap, heat loss, tube

failure etc.

Process Temp in/out, 0C 260

300

305

360


T-press barg 51.4 40 tube may burst, asset loss,

production loss

Delta P, kg/cm2 1.5 0.98 Heat loss, erosion, effect

column press

Duty, MMBtu/hr 19.6 17.9 -

Efficiency, % - 61.6 -

Excess Air, vol% - 40 heat-loss, stack temp,

Envior loss, smoke,

haziness

Draft, mmwc - -15 back firing


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Matrices 202-B 201-B Consequences in case of

violation

DTMT, 0C 394 500 tube failure, bowing, sagging,

rupture

No of tubes conv/radi 72 48 -

Eff tube length, m 5.89 12.0 -

Flue gas temp leaving conv

zone

686/288 625 fuel loss, envior loss


MMBtu/hr

- 4.84/5.8 -

FG/FO consum, mtd - 2.7/3.0 -

FG/FO press at B-tip, kg/cm2g - 1.4/3.0 Tip damage, flame lift off, highflame length

A-steam press at B-tip,

kg/cm2g

- 4.5-7.0 Poor atomizing, improper mixing,

combustion etc.


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Matrices 301-BN 302-BN 303-BN 311-BN Consequences in case ofviolation

Throughput, mtd 660 660 660 702 Erosion, tube leakage,

deltap, heat loss, tube

failure etc.

Process Temp in/out, 0C 483

543

471

543

498

543

210

249


T-press kg/cm2g 29.46 28.39 27.16 20.4 tube may burst, asset loss,

production loss

Delta P, kg/cm2 0.77 0.56 0.63 1.5 Heat loss, erosion

Duty, MMBtu/hr 7.0 8.0 5.2 5.4 -

Efficiency, % 60 55 56 60 -

Excess Air, vol% 15 15 15 15 heat-loss, stack temp,

Envior loss, smoke,

haziness

Draft, mmwc -25 -25 -25 -25 back firing

PTU Furnaces


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Matrices 301-BN 302-BN 303-BN 311-BN Consequences in case

of violation

DTMT, 0C 582 604 602 340 tube failure bowing,

sagging, rupture

No of tubes conv/radi 42 56 40 35 -

Eff tube length, m 8.35 5.7 5.5 4 -


cone zone

765 843 832 761 fuel loss, envior loss


MMBtu/hr

3.65 4.64 2.86 3.05 -

FG consum, mtd 1.9 2.41 1.5 1.6 -

FG press at B-tip,

kg/cm2g

2.2 2.2 2.2 2.2 Tip damage, flame lift off,

high flame length

Courtesy:TSD


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Fuel Oil System Refinery furnaces burn oil, gas or both at the same

time.

Fuel gas for pilot burners is supplied from a

separate system if possible to ensure a high

integrity supply.

Viscosity affects efficient burner operation of the oil

at the burner. Electrical heat tracing is used to heat and reduce

the viscosity at the burner.

To atomize the oil properly, it is often necessary to

heat it to temperatures ranging from 65 C to 230 C.

The entire fuel oil system is heat-traced andinsulated.

At each heater, all fuel passes through a remote

isolating valve, dual filters to remove any solid

materials which might block burners, and a local

isolating valve at each burner location.


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Fuel gas is collected from various

process sources in a central fuel gasmix drum.

In mix drum, liquid is knocked out

and discharged to a closed system.

If the liquid carried forward with the

gas, unignited fuel can accumulatein the firebox or flue ducting. This

can cause an explosion when

sufficient air for combustion is

available.

It is important to remember that the

fuel used has a direct impact on thefurnaceit can change the heat

rate, the corrosion rate, the

accumulation of particles, etc.

changing the fuel specification is a

modification that should be risk

assessed formally.

Fuel Gas System


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A CDU furnace had been designed to burn low sulfur fuel oil. It was decided to burnhigh sulfur fuel oil to increase the cost efficiency. But the tube supports , which were

cast alloys of composition 25Cr-20Ni or 25Cr-12Ni, suffered rapid deterioration in a

environment of high sulfur with vanadium and sodium.

Within nine months of introducing high sulfur fuel oil, roof supports were failing in

the furnace. All 80 roof hangers had to be replaced.


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Normal operation Checks Operators should inspect fires and radiant-section tubes several times

during each shift.

Keep heat distribution as even as possible,

Keep the same amount of fuel for each burner,

Open all air registers the same amount,

Keep air registers closed on unused burners,

Allow no more than 40 o C difference b/w temp at various locations in

the firebox.


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Startup and ShutdownPRE-STARTUP CHECKS

1. Check that process pressure tests and mechanical integrityinspections are complete

2. Inspect internally and externally (no oil / material left in theheater)

3. Check blanks / blinds are installed and all isolation valves areclosed

4. Check air registers for movement

5. Check stack damper and burner for ease of movement

6. Check burners and pilots for installation and condition

7. Check purge and snuffing steam and instrument tapings foroperability

8. Brick access opening and secure doors

9. Check that fuel gas pilot gas and fuel systems are tightnoopen ends etc.


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GAS TEST

Instrument ? Explosimeter

calibrated ?

trained to use it ?

Where ? Inspection ports Convection section Flue gas ducting Air space immediately above the burner

LIGHTING PILOTS


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LIGHTING PILOTS


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TYPES OF SHUTDOWN

Normal shutdown

Heat - off

Emergency shutdown

(ESD)

Individual emergency

procedure / trips

There are a number of different types

of full and partial shutdownsassociated with heaters. These are

normally known as normal shutdown,

heat-off, emergency shutdown (ESD)

and individual emergency procedures

(eg. individual main fuel trips).

It is vital for every operator to

understand thoroughly:

What actions can occur

automatically

When to initiate such actionsmanually

The tasks necessary to resume

normal operation when the

emergency or shutdown has passed


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NORMAL (CONTROLLED) SHUTDOWN

1. Shut off all oil burners, purge and keep guns cool(fuel gas burners remain in service)

2. Reduce the feed rate

3. Reduce the heater outlet temperature

4. Stagger the shut off of individual gas burners (leavethe pilots in service)

5. Maintain fuel gas pressure to ensure stable flames

6. Adjust combustion air rate

7. When all the main burners are shut, close the main

fuel gas supply valve, and purge all lines to theindividual burners with nitrogen. Blinds must be installed

8. Shutdown the pilots, purge the system and blind off


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HEAT - OFF

Heat-off means just that. It stops heat input into theheater by isolating main fuel systems. However the pilotsremain alight.

Heat-off can be initiated automatically and / or manuallywhen an abnormal operating condition exists. Someexamples when heat-off would be initiated are:

High pressure in crude distillation tower Low process flow through the tubes

Shutdown of recycle gas compressor (cat reformer)

Individual and main isolation valves on the fuel lines toburners should be closed as soon as possible after a heat-off as an extra safeguard to prevent fuel leaking into theheater.

Pilots are kept alight again as a safety precaution so thatfuel does not accumulate and lead to an explosion.


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ESD

The ESD would be used in the event of a hazardous

situation such as fire, major gas leak, heater tube failure,

etc.

The ESD would initiate heat-off, in addition, the pilotswould be shutdown, pumps would be stopped, vessels

would be isolated.


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INDIVIDUAL EMERGENCY PROCEDURE/TRIP

Apart from ESD and heat-off, there are procedures / tripsto cater for failures of individual pieces of equipment, e.g.extra low fuel gas pressure to the burners causes isolation ofthe fuel gas system.

On forced draught heaters, extra low FD fan driver speedwill trip the fuel supplies to the heater.

These individual trips are designed to prevent theaccumulation of unburned fuel in the firebox.


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Operational Troubleshooting


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THANK YOU

furnace operations

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