furnace operation and trouble-shooting

8/11/2019 Furnace Operation and Trouble-shooting

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Furnace Operation and

Trouble-shooting

By: Vineet Singh


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FURNACEFurnace, also called heaters are vital equipment in petroleum refineries.

Basic function of Furnace:Furnace is basically a heat-exchanging equipment

in which fuel is burnt and the heat generated by combustion is used to heat up

fluids flowing through tubes housed inside the furnace shell.

In process unit furnace, the fluid inside tube is Hydrocarbon.

Furnace is termed as boiler if the fluid inside tube is boiler feed water.

Different names of furnace:

Process heater, Fired heater, Process furnace, Direct fired heater and heater.

Process objective of furnace in AVUs:

For main fractionating column heat requirement and for column reboilers.


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FURNACEDifferent types of furnaces are broadly classified as based on:

SIZE :Big or small ( less than 10 MM kcal/hr heat duty)

SHAPE:Rectangular box, cylindrical

TUBES:Horizontal, vertical or whether located near or at the center of furnace

wall

BURNERS:Fuel gas, Fuel oil, or dual type and side burner, bottom or top fired.

CONVECTION SECTION:arrangement of flue gas path, studded or non studded.

STACK:Number and height

DRAFT:Natural , Induced , Forced or Balanced draft furnace.


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FURNACECOMPONNET AND ACCESSORIES OF FURNACE AND THEIR FUNCTIONS

MAJOR COMPONENT

A) Fire box

B) Burners

C) Tubes

D) Stack

ACCESSORIES:

Induced draft fan, Forced daft fan, combustion air and flue gas duct

Air preheater

Fuel system ( fuel oil, Fuel gas, pilot gas)


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FIRE-BOX:The main function of fire box is to house the tubes and burners.

BURNERS:There are four major type of burners.

a) GAS BURNERS

b) OIL BURNERS

c) PILOT BURNERS

d) COMBINATION BURNERS

Gas burners and brief on primary and secondary air

Gas burners are exclusively for burning fuel gas.

Types of gas burners:

Non-premix type:Mixing of air and gas in the furnace

Premix type:Partial or complete mixing of air with fuel before combustion.

Air that is premixed in the burn er knows as Primary air and the remaining air

adm it ted external ly is cal led Secon dary air.


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OIL BURNERS:

In addition to mixing of fuel and air, oil burners are capable to atomize the fuel

oil in small droplets.

Atomizing media: MP steam

Atomized fuel behaves like fuel gas and readily burns.

The main parameter for proper atomization is fuel oil viscosity. The fuel oil

viscosity should be preferably low. Fuel oil must be heated in fuel oil heater

prior to atomization.

PILOT BURNERS:

These are small gas burner (pre-mix type) used as pilot for safety.

These pilot burners are lit first before main gas and oil burners. In case of

emergency failure of main gas burner, pilot burners prevents the gasaccumulation and consequent explosion.

COMBINATION BURNERS:

Most widely used burner in which fuel gas, fuel oil and pilot gas are fitted

together in one assembly.


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STACK:It is a large diameter pipe to dispose off flue gas. The stack damper

is located at the base of stack. The stack damper is butterfly type valve to

regulate the draft ( pressure) in furnace.

FORCED DRAFT FAN:It supplies air to the burners for complete combustion

of fuel fired in heater.

Type:Centrifugal fan equipped with inlet guide vanes or variable frequency

drive for controlling air flow rate. Normally two fans are provided, in case offailure of any one other one takes full load.

INDUCED DRAFT FAN: It sucks the hot flue gas from down stream of

convection zone via air pre-heater and discharges it to atmosphere throughstack. It helps to maintain balanced draft in the box.

Type:Centrifugal fan equipped with inlet guide vanes or variable frequency

drive for controlling draft. In case of its failure, the stack damper gets full

open to discharge flue gas from the box.


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AIR-PREHEATER ( APH) :

The air pre-heater is installed to improve the efficiency of heater. The heat of

outgoing hot flue gas is utilized to preheat the combustion air from FD fans.

Type:Plate type heat exchanger having large surface area.

To avoid cold end corrosion of APH elements are provided with glass tubes.

Tube side f luid : Flue gas

Shel l s ide f luid: Air

FUEL SYSTEM:

Fuel gas is supplied from the refinery gas header. Steady FG pressure is

maintained for control system. Any liquid condensate is settled and drained in

FG KOD pot for liquid free FG supply to burners.

Fuel oilis stored in the tank and supplied by F.O pump ( gear type) to F.O header

of the unit. Separate take off to individual oil burners are provided.

The shut down valves are provided in FG and FO supply line for emergency shut-

off of firing by automatic interlock system.


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FURNACE OPERATION:

START-UP

1.Inspect furnace for readiness . Check all SDV are closed.

2. Establish flow in the furnace coils.

3. Purge the furnace with box purging steam. Minimum 30 min.

4. Start FD fan for air purging in parallel.

5. Ensure that draft is negative.

5. Light up pilot gas burners one by one with the help of ignitors.

6. Once pilot burners are lit, Open SDV and CV of FG and proceed for main FG

burner light up one by one. Adjust air to burners.

7. Open SDV of FO, Light up oil burners one by one to gradually increase the

fluid temp.


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ROUTINE MONITORING OF FURNACES

1.Check that furnace draft at arch is negative ( -2 to - 4 mmwc).

2. Furnace refractory for any damage, Hot spot.

3. Tubes, Tubes supports, Tube hammering, bending of tubes and any

vibration.

4.Check excess air , flame pattern, any flame impingement over tubes.

5.Check that Skin temp. of the tubes, arch temp. are with in range.

6.Check that stack temp. is with in permissible limit.

7.Check and ensure FG and FO pressure above tripping value.

8.Check and inspect healthiness of FD and ID fan.

9.Check for any leak in FG FO line.

10.Ensure the differential pr. of atomizing steam and FOis above 1.0 kg/cm2g.

11.Ensure daily soot blowing of the convection zone.


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FURNACE OPERATION

DOS

1. Before opening peephole ensure draft is negative.

2. Always use ignitors for pilot burner light up

3. In case of flame failure, immediately close fuel supply valve to burner.

4. Always keep the atomizing steam in open condition in idle FO burners.

5. After total flame failure or furnace trip, ensure all fuel supply is

properly isolated and box is thoroughly purged with steam or air.

6. Regular soot blowing.

7. During soot blowing ensure stack damper is fully open.

8. Always flush the oil burner after stopping the oil firing.

9. Always operate stack damper for draft control and air register for

excess air control.


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FURNACE OPERATION:

DONTS

1. While opening peephole never stand in front of peephole.

2. Never ignite the burner from other adjacent lighted burner.

3. Never allow flame impingement on tubes. Adjust flame pattern.

4. Do not bypass interlocks except during maintenance job/ start up.

5. If the oil burner is in line, never open the flushing steam of oil

burner.

6. Never light up the furnace if there is no flow in the coils.

7. Do not allow O2 % reading less than 2.0 %.

8. Do not allow + ve draft.


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FUEL GAS BURNER FLAME PATTERN

FUEL GASAIR

CORRECT GAS BURNER FLAME SHOULD BE:

CLEAR, SMOKELESS, BLUE COLOR WITH

YELLO FRINGE.

THE SHADING FROM BLUE TO YELLOW

DEPENDS UPON CARBON CONTENT IN FUEL

GAS

MORE CARBON MORE YELLOWISH FLAME



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FUEL GASAIR

INCORRECT GAS BURNER FLAME :

LONG, SMOKY, IMPINGING ON TUBES

REASON:

LOW AIR, LOW GAS PRESSURE



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FUEL OIL BURNER FLAME PATTERN

FUEL OILAIR

CORRECT FUEL OIL BURNER FLAME :

CLEAR, SMOKELESS, STEADY,

YELLOW TO YELLOWISH- ORANGE

COLOR FLAME


STEAM

O


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OIL BURNER FLAME PATTERN

FUEL OILAIR

INCORRECT FUEL OIL BURNER FLAME :

FIRE FLIES IN THE FLAME

REASON: FIRE FLY OCCURS DUE TO WET

STEAM, WATER IN THE OIL, LOW FO

TEMP.INSUFFICIENT STEAM TO BURNER.

ADJUST ABOVE PARAMETER TO NORMAL

TO RECTIFY THE FLAME


STEAM


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FUEL OILAIR


FLAME LIFT-OFF

REASON: HIGH STEAM PREESURE



STEAM



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FUEL OILAIR


VERY BRIGHT FLAME

REASON:TOO MUCH EXCESS AIR




STEAM

OIL BURNR FLAME PATTERN


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OIL BURNR FLAME PATTERN

FUEL OILAIR


LONG, SMOKY, IMPINGING ON TUBE.

REMEDY:

Increase air, Increase steam pressure,

Increase FO temperture, Check alignment

of gun, check for any damage in the tip.


STEAM

TROUBLE SHOOTING


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SL TROUBLE CAUSE SOLUTION.

1. Flame back-fire Low Furnace draft or +vepressure

Increase damper opening.

2. Burners go out Fuel-air mixture too leanor burner tip is choke.

Reduce air, clean oil

burner tip.

3. Erratic flamepattern

A) Lack of air

B) Excess firing

C) Burner tip plugged

A)Adjust air register

B)Reduce firing rate

C)Clean the burner

4. Pulsating or

breathing

A) Lack of draft

B) Fuel rate low

A) Check stack damper

B) Increase fuel rate

5. Flame too long A) Inadequate air

B) Low atomizingsteam for oil burner

flame

A)Increase airB) Increase atom. steam

6. Thin oil burner

flame

Too much atomizing

steam

Reduce steam or increase

oil pressure.

7. Dazzling whiteflame in oil burner

Too much excess air oratomizing steam

Adjust air or steam

TROUBLE-SHOOTING

TROUBLE SHOOTING


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SL TROUBLE CAUSE SOLUTION.8. Flame ignites &

goes outalternatively

Lack of draft Reduce firing rate and

check stack damper

9. Hazy and yellow

flame

Inadequate air Increase air

10 Reddish flame with

smoke around in oilflame

Inadequate air Increase air

11 Sparkles in oil

burner flame

A) High viscosity of FO

B) Dirty burner tip

C) Water in FO or steam

A) Increase FO temp.

B) Clean the burner

C) Check steam trap

12 Uneven flame at tip Dirty tip Burner tip cleaning

13 High stack temp. A) High excess air/draft

B) Fouling in conv. Zone

C) Stack damper

open/passing

A) Reduce excess

air/draft

B) Soot blowing

C) Close stack damper

TROUBLE-SHOOTING

TROUBLE SHOOTING


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SL TROUBLE CAUSE SOLUTION14 Excess smoke

from stack

A) Lack of atomizing

steamB) Increased firing

without increasing air

C) Tube failure

A) Check oil flame

B) Adjust and increase airC) Shut down the unit

15 High skin temp. A) Coking in tubeB) Flame impingement

A) Adjust feed ratedistribution and firing

B) Adjust flame pattern

16 Uneven color of

bricks or tube

hangers

Poor flame distribution

causing uneven

heating

Readjust flame pattern light

up all burners for equal heat

distribution17 Arch

temperature too

high

Inadequate excess air Open the damper to increase

draft or adjust excess air

TROUBLE-SHOOTING

FURNACE INTERLOCKS


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FURNACE INTERLOCKS

(A) LOW FUEL PRESSURE:Low fuel pressure ( FO or FG) may cause flame

failure. This interlocks actuates on sensing low pressure in FG or FO and led

to automatic shut down of SDV to avoid flame failure.

(B) LOW FLOW IN COILS:Low flow in any of the pass may cause

overheating, coking and damage of tubes

This interlock actuates automatic SD of FG and FO firing in case any pass

flow is less than minimum.

(C) LOW COMBUSTION AIR PRESSURE:Less combustion air to furnace

creates accumulation of unburnt fuel.This interlock actuates on low pressure

of air and led to automatic SD of FG and FO SDVs

FURNACE INTERLOCKS


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FURNACE INTERLOCKS

(D) HIGH ARCH PRESSURE:positive pressure in the furnace causes back fire.

This interlock actuates on sensing high arch pressure and led to automatic SD of

FG and FO SDVs.

(E) FD FAN FAILURE:In the event of FD fan failure Fuel supply to furnace will cut-

off followed by opening of stack damper and tripping of ID fan.

(F) ID FAN FAILURE:ID fan failure causes pressurization of furnace if the dampers

are closed. This interlock actuates automatic opening of stack damper with in set

time, failing which fuel supply to furnace will cut off and FD fan shall trip to stop

cold air entry to avoid thermal shock.

(G) HIGH ID SUCTION TEMPERATURE:High temp. at ID fan suction may damage

the IGV. This interlock actuates tripping of ID fan.

BURNER MANAGEMENT SYSTEM


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BURNER MANAGEMENT SYSTEM

Burner Management System (BMS):

The control system dedicated to boiler-furnace safety, operator

assistance in the starting and stopping of fuel preparation and burning

equipment, and prevention of damage to fuel preparation and burning

equipment.

ENERGY CONSERVATION


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ENERGY CONSERVATION

BENEFIT OF LOW EXCESS AIR:(10-15 % excess air is optimum for gas firing

and 20 % for oil firing)

Reduced stack losses

Reduced stack temperature

Reduced firebox temperature.

Reduced cold end corrosion

Reduced Nox

Reduced flue gas pressure drop

Fuel savings

Increased capacity

However, too low excess air can be dangerous as it can lead to secondary

burning, localized heating, flame impingement, high skin, and coking.

Air deficiency may lead to Furnace explosion , High CO emission.

DEFINITIONS OF THE TERMS COMMONLY USED IN HEATERS


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Atomizer:a device used to turn a liquid stream in to very fine droplets.

Blow-off:The loss of stable flame in a burner when the fuel exit velocity fromburner tip exceeds the flame velocity.

Burn-back:The internal burning with in the tip is called as burn-back.

Burner block:it is the especially formed refractory pieces that mount around the

burner opening inside the furnace and forms the part of the burner throat.

Burner throat:The opening formed by burner tile where the air flow is regulated

by air registers is the burner throat. It helps to stabilize the flame.

Burner velocity:The speed at which a flame front travels into an unburned

combustible mixture.

Detached stable flame:A flame that is not in contact with the burner itself but

burns with a stable flame front near the burner.

Diffuser:A device used in natural draft burner to create the continuous ignition

zone required for stable operation.



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Diffusion flame:The combustion state controlled by mixing phenomenon where

the air and fuel diffuse into one another until a flammable mixture is formed.

Arch pressure (draft): The pressure difference between the atmosphere and the

inside of the heater at a particular location where the draft is being measured.

Excess air:The amount of air needed by a burner that is in excess of the perfect

amount required for complete combustion of a given fuel.

Firing port:The orifice in the fuel tip where fuel is released in to the air stream for

mixing and burning.Lift-off: A condition where the fuel/air mixture velocity is higher than the flame

velocity, causing it to detach from the burner.

Mounting plate:The part of a burner assembly that attaches the burner plenum to

the heater floor/wall.

Regen tile: A block of refractory material in oil burners, to redirect the intense

flame radiation back to the root of the spray to enhance the vaporization rates and

help to stabilize the oil flame.

Tramp air:The air that infiltrates in the heater through leaks.


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SUMMARY

The efficient operation of furnace require

continuous monitoring of firebox conditions,

excess air, draft, stack temperature, burner

flame pattern etc. and skill of identifying the

problem and taking appropriate timely action.

furnace operation and trouble-shooting

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