Fired Heater Operation 1
Oct 24, 2014
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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• 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
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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
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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.
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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
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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
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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
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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
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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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Uniform Firing
• Fire all burners
• Adjust air and fuel correctly
• Provide required draft
• Excess air control
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Good Combustion
• Requires three steps
o Fuel and air in correct quantity
o Thorough mixing of fuel and air
o Sustained ignition of mixture
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Good Combustion
• Indications
o Firebox is clear
o No smoky appearance
o Burner flames are steady and well formed
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23
Incomplete Combustion
• One of the goals in combustion is to prevent CO formation
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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
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26
CO/O2 Analyzer
Flue Gas Temperature
Check:
• Firebox temperature
• Convection section temperature
• Stack flue gas temperature
o Avoid dew point corrosion
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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
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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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Monitoring Process Variables
• Tube skin temperatures
• Pass flowrates ( min./normal/ maximum)
• Pass temperatures ( min./normal/ maximum)
• Pass pressures ( min./normal/ maximum)
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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
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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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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
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Process Fluid Flow
• Equal flow distribution in all passes
• Equal heating of all passes
• No coke formation
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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.
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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
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37
Firebox Monitoring
• Look in the firebox
• Damper/ register control
• Potential problemso Tip plugging , flame impingement, hot
spotso Fallen refractory
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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