Fired Heater Operation1

Natural Draft

• Hot flue gases –lighter

• Ambient air outside –denser

• Draft

o Density difference

• Negative Pressure created inside the firebox.

2

Draft Availability vs. Losses

• Flow resistance

o Convection section

• 0.5 Vel. head per row of tubes

o Stack entrance/ friction/ exit

• 0.5 Vel. head at Entrance and 1.0 Vel head at exit

o Damper, Off take

• 1.5 Vel. Head

• Heater load vs. draft losses6/22/2012

3

Draft Profile

Arch has highest pressure

CONVECTION

SECTION

STACK

________

BURNERS

0.05"- 0.1"

W.G. DRAFT

DRAFT AT RADIANT

SECTION OUTLET,

R0

RA

DIA

NT

SE

CT

ION

Pc

(SE)a STACK

EFFECT

IN STACK

(SE)c

NEGATIVE PRESSURE

0.05"- 0.1" W.G. AT

TOP OF RADIANT

SECTION

Pb

(SE)r

Pa

NEGATIVE

PRESSURE

POSITIVE

PRESSURE0

STACK

EXIT LOSS

4

Draft

• Measured at the arch

o The pressure here is the highest (-2.54 mm WC)

• Draft units are mm of water

• The draft is measured using a water manometer or draft gauge

Stack

Convection

Section

Radiant

Section

DG

DG

DG

DG

PTPI

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5

Typical Draft Values

• Typical value at arch (-) 2.54 to –(-)3.8 mmWC

• Typical value at heater floor (-)7.6 to (-)17.7 mmWC

6

Draft

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7

• The amount of draft needed is determined for each furnace by what the draft must do.

o Provide enough air

o Provide enough energy

Draft calculations

• Draft avail.=0.53 HP ( 1/Tair- 1/Tgas)

o H= Height of stack, ft.

o P= Atmospheric pressure, psia

o Tair= Absolute temperature of air, °R

o Tgas= Absolute temperature of flue gas, °R

• Summer design temperature

• Site elevation

6/22/2012

8

Draft thumb rules

• Change in atmospheric pressure-

o 12 cm /100 m of elevation

• Draft at 760 °C- 0.8 mm /1 m

• Draft at 200 °C- 5 mm / 10 m

• Draft at 370 °C- 1.2 mm /10 m

6/22/2012

9

Control Draft

• Excess Oxygen and draft control are interrelated.

• Every time excess O2 is adjusted in the fired heater, the draft will be affected as the flue gas flow rate changes.

6/22/2012

10

O2 and Draft Control11

PT

PIC

AT

AIC

Process

Fluid Outlet

Stack

Fuel gas

Control Valve

Process

Fluid Inlet

OPEN ALL

PEEPHOLES

CHECK FLAME PATTERN/

IMPINGEMENT

INCREASE AIR FLOW

SHUTDOWN BURNER

CLEAN BURNER

LIGHTUP

RECHECK FLAME

PATTERN/ IMPINGEMENT

GOOD FLAME

CLOSE ALL

PEEPHOLES

IS FLAME

O.K.?

YE

S

NO

BAD FLAMES ?

HAZY FIREBOX ?

YE

S

NOCHECK ARCH DRAFT

(SEE STEP-2)

CHECK EXCESS O2

(SEE STEP-3)

Fig. 1. Flow Chart for Checking Flame Characteristics

6/22/2012

12

Fig. 2. Flow Chart for Checking Arch Draft

DECREASE STACK

DAMPER CLOSING

ARCH DRAFT = 0.1" WC

READ ARCH

DRAFT

IS DRAFT

O.K.?

HIGH

LOWO.K.

INCREASE STACK

DAMPER OPENING

CHECK FLAME

CHARACTERISTICS

(SEE FIGURE-1)

CHECK EXCESS O2

(SEE FIGURE-3)

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13

EXCESS O2 = 3 %

READ EXCESS O2

IS

EXCESS

O2 O.K.?

HIGH

LOWO.K.

CHECK FLAME

CHARACTERISTICS

(SEE STEP-1)

CHECK ARCH

DRAFT

(SEE STEP-2)

Heater Tuned

DECREASE BURNER AIR

REGISTER CLOSING

INCREASE BURNER AIR

REGISTER OPENING

Fig. 3. Flow Chart for Checking Excess Air

6/22/2012

14

Stack Damper / Burner Register Adjustment

Oxygen level

Draft value

Adjustment required

High High Close Close stack damper

Low Low Open stack damper

High Low Close burner registers

Low High Open burner registers

6/22/2012

15

Air Leakage

• Heater is not a pressure tight structure

• This air does not mix with fuel

• This air does not help the combustion

• It absorbs the heat that should be

transferred to the heater tubes

• Air should be entering through burners

6/22/2012

16

All Burners Must Be Operating !!

1. Air register position

- Equally open

2. Only 3 out of 5

burners are fired

Air

Fuel gas Fuel gas Fuel gas Fuel gas Fuel gas

O2

Air leakage

Air

register

Draft @ arch

-0.1"WC

Analyzer

Stack

Damper

6/22/201217

All Burners Light On

Top view of heater floor with all the 14 burners fired in each cell

Top view at half the height of radiant section with all the 14 burners fired in each cell

18

70% of Burners Light On

Top view of heater floor with only 10 burners fired in each cell

Top view at half the height of radiant section with only 10 of the burners fired in each cell

19

Combustion Controls

• Major parameters need control

o Fuel gas / Fuel oil pressure

o Excess air control

o Draft in the furnace

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20

Uniform Firing

• Fire all burners

• Adjust air and fuel correctly

• Provide required draft

• Excess air control

6/22/2012

21

Good Combustion

• Requires three steps

o Fuel and air in correct quantity

o Thorough mixing of fuel and air

o Sustained ignition of mixture

6/22/2012

22

Good Combustion

• Indications

o Firebox is clear

o No smoky appearance

o Burner flames are steady and well formed

6/22/2012

23

Incomplete Combustion

• One of the goals in combustion is to prevent CO formation

6/22/2012

24

2C + O2 2CO + 4,347 Btu/lb

Incomplete Combustion

C+O2-----------> CO + 4,347 Btu/lb

CO+ 1/2O2 -----> CO2 + 9,657 Btu/lb.

• Carbon monoxide is a combustible

o Can react in the convection section or stack

• Convection section high temp. can induce CO reactions

• Burned Fins

o Reduce the heat transfer

o Increase the stack temperature6/22/2012

25

• Arch Installation

• Combustion process efficiency measurement

Stack

Convection

Section

Radiant

Section

O2

O2

CO

6/22/2012

26

CO/O2 Analyzer

Flue Gas Temperature

Check:

• Firebox temperature

• Convection section temperature

• Stack flue gas temperature

o Avoid dew point corrosion

6/22/2012

27

APH Operation

• Check the inlet flue gas temp.

o < Design inlet temp.

• Check the outlet flue gas temp.

o > Design outlet temp. (cold end corrosion )

• Check press. drop across flue gas

o High pressure drop very common

• Limiting the ID fan or heater load

6/22/2012

28

Fuel Oil / Gas Pressure

• Very important measurement

• Can tell the fuel firing rate

• Can estimate heater firing duty

• High ring pressure- overfiring

• Low ring pressure- turndown operation

• Low pressure cutoff6/22/2012

29

Flue Gas Analysis

• Flue gas analysis is required to control excess air

• Sampling points required at o Radiant outlet o Convection outlet/ Stack

• Components analyzedo Oxygeno Carbon Monoxideo Nitrogen Oxideso Un-burned Hydrocarbons

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30

Monitoring Process Variables

• Tube skin temperatures

• Pass flowrates ( min./normal/ maximum)

• Pass temperatures ( min./normal/ maximum)

• Pass pressures ( min./normal/ maximum)

6/22/2012

31

Fluid Temperature

• Measured at several places

• Typical installations include

o Inlet to the heater

o Crossover from convection to radiant

o Outlet of the heater

6/22/2012

32

Process Outlet Temp. Control

• Indicates heat demand

• Controls the fuel supply to the burner

• Cascade control

o Outlet temperature acts on fuel controller set point

o Not acting on fuel control valve

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33

Fluid Pressure

• Fluid pressure needs to be measured at :

o Common inlet

o Each pass inlet

o Common outlet

• Fluid pressure drop indicate

o Amount of coking

o Obstruction in the coil

6/22/2012

34

Process Fluid Flow

• Equal flow distribution in all passes

• Equal heating of all passes

• No coke formation

6/22/2012

35

What Can Operator Do ?

• Maintain equal flow through all passes

o Good guideline is to keep with in ± 5%

• Watch pressure drop across each pass

o Downstream of control valve and orifice

o At heater inlet and outlets

• Watch control valves opening

• If any parameter is abnormal, investigate.

6/22/2012

36

Fluid Flowrate

• Flow measurement and control required at the inlet to the heater

• If charge is divided into multiple passeso Flow control recommended in vaporizing

services or critical services

o Flow measurement in non critical services

• Two phase flow, flow measurement is very difficult

6/22/2012

37

Firebox Monitoring

• Look in the firebox

• Damper/ register control

• Potential problemso Tip plugging , flame impingement, hot

spotso Fallen refractory

6/22/2012

38

Routine Inspection

• Inspected at regular intervals

• Using all observation doors

• Check heater tubes

• Check burner flames

Bad flames

39

Routine Inspection

• Tube hangers

• Refractory lining

• Air Leakage at convection header boxes

• Expansion joints in ducts

6/22/2012

40

Optimization Case Study -1

• Tuning Job

• Heater:

o Depentanizer reboiler

o Horizontal tube box

o Absorbed heat duty – 21.9 MMkcal/hr

o 15 up fired burners

o Heater connected to a large common stack

o Two off take ducts provided with manual dampers

41

41

Optimization Approach

• Check draft

o Adjust using off-take dampers

• Check excess O2

o Adjust burner register

• Check burners

o Light up all burners

42

42

-17.0

-15.0

-13.0

-11.0

-9.0

-7.0

-5.0

9:36 10:48 12:00 13:12 14:24 15:36 16:48

Dra

ft , m

m H

2O

Time (7/22/2009)

Draft (B1402)

43

43

0.0

1.0

2.0

3.0

4.0

5.0

6.0

9:36 10:48 12:00 13:12 14:24 15:36 16:48

Ex

ce

ss

Oxyg

en

, %

Time (7/22/2009)

Excess Oxygen (B1402)

44

44

290

300

310

320

330

340

9:36 10:48 12:00 13:12 14:24 15:36 16:48

Sta

ck

Te

mp

era

ture

, °C

Time (7/22/2009)

Stack Temp. A

Stack Temp. B

Stack Temperature (B1402)

45

78.0

79.0

80.0

81.0

82.0

83.0

84.0

9:36 10:48 12:00 13:12 14:24 15:36 16:48

Th

erm

al

Eff

icie

nc

y, %

Time (7/22/2009)

Thermal Efficiency (B1402)

46

46

18.0

19.0

20.0

21.0

22.0

23.0

24.0

9:36 10:48 12:00 13:12 14:24 15:36 16:48

To

tal

Du

ty F

ire

d, M

W

Time (7/22/2009)

Total Heat Fired (B1402)

47

47