Training Services
Flare Systems
Table of Contents
Flare Systems
– Purpose
– Flare selection
– Support structures
– Combustion theory
– Radiation theory
– Regulatory compliance
– Equipment design
– Flare design requirements
– Incineration
Disposal System
Disposal of vapors and
liquids discharged
Types of Systems
Open System
– Discharge directly to
atmosphere
Closed System
– Discharge to a collection
header
– Dispose to a flare
Relief Header Sizing – Equations to Use
V = M * SV / A / 60
SV = 379.5/MW * (14.7/(Pout+14.7)*((460+T)/520)))
SonicV = 60 * (32.17*1.1*1546*(460+T)/MW)^0.5
MACH = V/SonicV
V = velocity in ft/min
M = flowrate in lb/hr
SV = specific volume in ft3/lb
A= pipe area in ft2
MW = molecular weight
Pout = outlet pressure in psig
T = temperature in F
SonicV = Sonic Velocity in ft/min
Relief Header Sizing – 7 Steps
Step 1: Start at flare tip and calculate the pressure drop
across the flare tip at 0.5 MACH
Step 2: Determine equivalent lengths for all segments
Step 3: Limit relief header velocity to less than 0.7 MACH
Step 4: Establish properties of the gases
T=Σ(wi*Ti)/Σwi where T=temp and w is flowrate lb/hr
MW = Σwi/Σ(wi/MWi) where MW is Molecular Weight
Step 5: Calculate pressure drop
Step 6: Review allowable pressure against actual pressure
for each segment
Step 7: Review velocity is below 0.7 MACH in each segment
Purpose of Flare
Define Loadings to be Handled
– Calculate loadings for all
contingencies
– Geographic location of each
source
– Calculate maximum load
(power failure,fire case)
• Fire case limited to a
ground area of 230 -
460 square meters
• Calculate maximum
back pressure
Major Factors Influencing Flare Design
Gas Composition
Flow Rate
Gas Pressure Available
Initial Investment
Operating Costs
Gas Temperature
Energy Availability
Environmental
Requirements
Safety Requirements
Social Requirements
Elevated Flare System
Flare Tip
Steam Ring
Dry
Seal
Flare
Stack
Flare Knockout Drum Knockout Drum
Pumpout Pump
PI
TI LGR LIAH
Instrument Switch
Gas To
PI
Air TAH Pilot
Grade
Vent Solenoid
Valve M
(With Manual
Reset) Pilot Ignition
Systems Locate
Normal At Flare Knockout Drum
Emergency RO RO Gas Purge
Gas Purge PI
Steam PC
Purge Gas Slop To Fuel
Pressure Relief Plant
From Process Units Slop Tank Gas Air
Elevated Flare System
(Knockout Pot in Stack)
Elevated Flare System
(Water Seal and Knockout Pot in Stack) Title Guide
Steam Ring
Flare Tip
Flare
Stack
Flare Knockout Drum
Knockout Drum And Water Seal
Pumpout Pump
Switch
PI LIAH
Instrument
Air TAH
M
LGR Gas To
Water Pilot
Vent
Solenoid Seal Grade
Valve
(With Manual
Reset)
Pilot Ignition
Emergency Systems Locate At
Gas Purge RO RO Slop to Flare Knockout Drum
Slop Tank PI
PC
Normal
Gas Purge Steam Water
Purge Pressure Relief
Gas From Process Units Fuel Plant
Gas Air
Ground Flare
System
Knockout Drum
Flare Knockout Drum Pumpout Pump
PI
TI LGR LIAH Main
Switch Header PC Ground Flare
Instrument PI Retention Dike
Air Burners Grade
Solenoid
Vent Valve M
(With Manual
Reset) Pilot Ignition
Systems Locate At
Stage Flare Knockout Drum
Emergency PO PO PI
Header Gas To
Gas Purge
Pilot PC
Normal
Gas Purge
Purge Gas Pressure Relief Slop To Fuel Plant
From Process Units Slop Tank Gas Air
Two Stage Flare System
(Elevated/Ground)
Flare
Tip
Seal
Flare
Stack
Knockout Drum
Flare Knockout Drum Pumpout Pump
PI
TI LGR LIAH Water
Switch Seal Enclosed
PI Ground
Instrument Gas To
Air
Pilot Plane
Solenoid
Grade
Vent Valve M
(With Manual
Reset) Pilot Ignition
Systems Locate At
Emergency Flare Knockout Drum
Gas Purge RO RO
PI
Normal PC
Gas Purge
Purge Gas Pressure Relief Slop To Water
From Process Units Fuel Plant
Slop Tank Air
Gas
Two Stage Flare System
Flare Stack
Structure
– Self Supporting
– Guy Supported
– Derrick Type
Support Structure
Self-Supported
Stack less than
100’ (30M)
Tight plot area Self-
Supported
Self-Supportive Structures
Description When to Use When Not to Use
Self-supported flare • Stacks less than • Stacks taller
stack is utilized for 100 feet (30 than 350 feet
structures from 20 meters) (100 meters)
to 350 feet tall (6 to • Tight plot area • Cost sensitive
100 meters). • When liquid applications
Usually this design carry-over is greater than
has the lowest likely 100 feet (30
installed cost and • When integral meters
requires the smallest drum is specified
plot area.
Support-Structure
Derrick
Stacks over 250' (75M)
Tight plot area
Gas temperature over
450°F(232°C) Derrick
Derrick Structures
Description When to Use When Not to Use
Used for • Plot space is • Stacks less than
structures from tight 250 feet (75
150 to 550 feet • Stacks over 250 meters)
tall (45 to 266 feet (75 meters) • Cost sensitive
meters). • Gas applications
Relatively easy temperatures
to erect and has are over 450°F
superior (232°C)
strength when • With Offshore
assembled. systems
Support-Structure
(Guyed)
Stacks over 250'
(75M)
Low capital cost
Guyed
Guyed Structures
Description When to Use When Not to Use
Guy wire support • Stacks over 100 • Stacks less than
flare stacks are feet (30 meters) 100 feet (30
typically the • Radius equal to meters)
lowest material stack height • Tight plot area
cost system, but available for guy • Liquid carry
they require the wires over likely
largest plot area. • Low capital cost
Used in systems is required
from 100 to 700 • Liquid carry over
feet tall (30 to 213 is unlikely
meters).
Radiation Effects
Solar Radiation on Earth ~ 300 Btu/hr-ft2
Radiation Theory
L2 (ft2) = (t)*(f)*(R)/(4*PI*K)
Where: t = fraction of heat intensity transmitted
f = fraction of heat radiated
R = net heat release (Btu/hr)
K = allowable radiation (500 Btu/hr-ft2)
PI = 3.14159
L = minimum distance from flare tip
Heat Release from a Flare
R = Heat Release (Btu/hr)
W = Flare Gas Flow Rate (lb/hr)
B = Net heating value (Btu/lb)
R = W*B
7
Radiation Theory 6
4
Exposure Times Necessary
3 Threshold of Pain
to Reach the Pain Threshold
2 Safe Limit
440 Btu/(hr) (ft)2
1
0
10 20 30 40 50 60
Exposure Time, Sec.
Radiation Intensity
Times to Pain Threshold
Btu/hr-ft2 Kilowatts per M2 (Seconds)
550 1.74 60
740 2.33 40
920 2.90 30
1500 4.73 16
2200 6.94 9
3000 9.46 6
3700 11.67 4
6300 19.87 2
Contours of Radiant Heat Intensity
Safe Boundary
(440 Btu/Hr/Sq.Ft.)
Boundary
for Radiant
Heat Intensity
(1500 Btu/Hr/Sq.Ft.)
- Normally Fenced in with
Warning Signal
Protection
Protection Required for
Required for Personnel
Equipment
Boundary for
Radiant Heat Intensity
(3000 Btu Hr/Sq.Ft.)
Environmentally
acceptable
combustion
Tips normally
proprietary in
design
Flame stability
Ignition reliability
Exit velocity 1 to
600 ft/s (.3 to 183
m/s)
Exit velocity at
50% of sonic
velocity
Multiple pilot
burners
Surrounding
windshield Flare Tip
Flare Tip Design
Flare Tip Design
Considerations
– Design for maximum
flow rates
– Design for maximum
temperatures
– Design for wind
conditions
– Design for minimum
flow rates
High and Too Low Relief Flow Rates
Can Cause Flame Instability
Air
Air Methane
Aspiration
D
Air Pipe
Intrusion
a. Flame Dip b. Flame Blowoff c. Analysis Of Flame Dip
Flare Efficiency
Efficiency of flare
depends on the
following
– Type of fuel
– Flow rate of fuel
– Wind velocity
– Ambient turbulence
– Height of the stack
– Presence of HC
droplets
– Presence of water
droplets
Pilot and Ignition Systems
Continuously burning pilots
Flame front generator
– Fuel gas and air admitted
to the ignition pipe in a
combustible ratio
– Gas is ignited by an electric
spark
– Flame travels through the
pipe
Pilot Burners
Automatic systems
may be activated by:
– Thermocouples
– Infrared Sensor
– Ultraviolet Sensor
(ground flare
application)
Installation of Thermocouples
Correct Installation Incorrect Installation
Pilot Windshield
Allows pilot to
operate at wind
speeds greater
than 100 mph
Should always be
specified
Prevents
misreading of the
thermocouples
Flame Front Generator
Ignition System
Air D B A To Pilot #1
H
J To Pilot #2
Gas E To Pilot #3
C Gas To Pilots
Flare Control Panel
Flare Control
Panel includes
the following:
– Pilot Gas
– Steam Control
– Pilot Ignition
System
Pilot Gas Requirement
The average pilot gas consumption based on an
energy-efficient model is 70 scf/hr. The annual
pilot gas consumption (Fp) is calculated by:
− Fp (Mscf/yr) = (70 scf/hr)*(N)*(8,760 hr/yr)
− Fp (Mscf/yr) = 613*N
N can be calculated from the following table:
Flare Tip Diameter (IN) Number of Pilot Burners (N)
1-10 1
12-24 2
30-60 3
>60 4
Multiple Pilots
Multiple pilots
allow one pilot to
fail
Most flares have
two to four pilots
Equally spaced
around the flare
Safety Aspect
Fuel
Oxygen Ignition
Two of the three elements for explosion are
always present in a flare system
Purging
Flare purge gas
– Any gas which cannot go to
dew point under any condition
of operation
• Fuel Gas
• Inert Gas
• Nitrogen
– Purge Rate
• Flare Stack
– Linear velocity 1FPS
to 5FPS (.3 to 1.5 m/s)
• Flare stack with
molecular seal
– 0.10 FPS to 0.20 FPS
(.03 to 0.06 m/s)
Purge Gas Requirements
Prevents flashback problems
Flare operates at positive pressure
Purge all subheaders (upstream)
.04 feet per second to 1 feet per second
(.01 meters per second to 0.33 meters per second)
F (Mscf/yr) = (0.04 ft/sec)*((PI*D^2/4)/144 ft2))*(3600 sec/hr)*(8,760 hr/yr)
F (Mscf/yr) = 6.88*D^2
Dry Seals
Molecular Seals
Double Seals
Fluidic Seals
Airrestors
Molecular Seal
Prevents explosions
Flare
Assembly Prevents entry of air
Reduces purge gas
Performs silently with
Molecular small pressure drop
Seal
Liquid
Drain
Double Seal
Outlet To Flare Burner
Clean-Out
Fluidic Seal
Flare
Air Tip Air
Flow Path
Of Flare
Gas
Smokeless Flare Operation
Smokeless Operation
Smoking
Steam Requirements and Smoke
Suppression Methods
In general, the following equation can be used:
Wsteam (lb/hr) = Whc (lb/hr) * [0.68-(10.8/MW)]
Smoke Suppression Methods
– Steam injection
– High pressure gas injection
– Low pressure air
– Internal energized flare
Tendency to Produce Black Smoke
Percent of Carbon Escaping as Black Smoke
50
40
30
20
10
0.4 0.3 0.2 0.1
H/C Ratio by Weight
Automatic Steam Control
Field Of View
Steam Nozzles
Steam Control
Valve
Monitor
Flux Density
Controller Signal
Control Scheme
Automatic Steam Control
Minimizes steam
consumption
Controlled by the
flame appearance
Calibrated to a
particular
frequency in the
infrared spectrum
Less Than 50 psig Steam
Wind
Flashback Protection
Flame Arrestors
Liquid Seals
Liquid Seals
Flare vapor piping
submerged
approximately 4 to
12 inches below the
water level
Effective means to
stop a flame front
Liquid Seal
To Flare
Flare Header
10 Ft. (3M) Minimum
Try Cocks For
Checking Hydrocarbons
Vent
Water Level
4" (10 cm)
Designed for a Minimum
Sewer Seal Should Be
Maximum Operating
of 175% of Drum’s
Submerged Weir
Water Welded On End 6" (15 cm)
Pressure
Supply Of Flare Line
FI
To Sewer
Baffle
Drain
Flame Arrestor
Stop flame propagation
within a piping system
by means of breaking
the flame into very
small flames via a
crimped wound metal
grid thus quenching the
flame by means of heat
transfer and dissipation
Liquid Seal Versus Flame Arrestor
Product Name Liquid Seal Flame Arrestor
Function Liquid seal is designed to The arrestor is designed to
stop flame propagation stop flame propagation
Product Type Active Device Passive Device
Testing Protocol Not available FM USCG API
Maintenance Switches and Cleaning of flame
cleaning cell elements
Failure Modes Switch Corrosion
malfunctions, liquid
freezing
Knockout Drums
Principle Features
– Complete removal of either slugs or mists of liquid
(300 microns to 600 microns)
– Recovers valuable condensed hydrocarbons
– Ends maintenance difficulty caused by “Wet” gases
– Used as the base for the flare riser
– Ends “Wet Gas” control problems
The allowable vertical velocity
in the drum may be based on
the necessity to separate
droplets from 300-600 microns
in diameter.
Hazards
of Burning Rain
Injury to Personnel
Damage to Equipment
Source of Fire
Design Considerations
Separation of Gas & Liquid
Design Considerations
Liquid Holding Capacity
Illustrative LRGO Arrangement
First Stage
Programmable Air Assisted
Controller Flare
Rupture
Disk
LRGO
Tips Stage 2
Pressure
Transmitter
Stage 3
Flare Header
Stage 4
Control
Valve
Stage 5
Radiation Fence
LRGO – Ground Flare – Critical Location
LRGO – Designed
for the Tundra of Alaska
Ground Flare
Designed in Mexico
Ground Flare – Operating
Advantages of Ground Flare
No structural support
required
Erection is relatively
straightforward
Maintenance is easy
Operating costs are
negligible
Flame of flare not
visible
Fairly quite system
Disadvantages of Ground Flares
Must be well isolated
from the rest of the
refinery
Requires considerable
space and long
interconnecting piping
Combustion takes
place on ground
Concentration of toxic
gas at grade may
remain high
Air-Assisted Flare
Select the proper
blower requirements
Blower to provide air
flow and gas in
proportion to each
other to properly mix
Should be provided
on the outside of
circular air riser
Quiet Flare (Low Noise)
Qualitative sense decibels
(dB) describe the loudness
of sound and noise.
– Whisper = 20 dBA
– Conversation = 65 dBA
– Food Blender = 88 dBA
– Motorcycle = 100 dBA
OSHA requires equipment
to have <85dBA over 8
hour period
Enclosed Ground Flares
Multi-stage burners
system in enclosed
ground flares injects the
auxiliary fuel separately
from the vapor
Burner features a
refractory tile to
maintain flame stability
and promote mixing
Three T’s – Time,
Temperature and
Turbulence
Typical Enclosed Flare
Sample
Connection
Refractory Temperature
Lined Control
Stack
Burner
Assembly
Flame
Arrestor
Air
Control From
Louvers Blower
Marine Vapor Control System Flare
Control of emissions of
hydrocarbon laden
vapors displaced from
ships
Combustion system must
be designed to cope with
air, nitrogen or CO2
vapors with hydrocarbon
concentrations varying
from 0 to 100%
Multiple burner tips for
large turndowns (up to 50
to 1)
Truck Loading Vapor Control Flare
Achieve high destruction
efficiencies through the
loading cycle
Systems range in size
from 100 BPH to 25,000
BPH
Enclosed burners can be
easily tested for emissions
Upper Steam Manifold Flare
High stability pilot and
flame retention system
Low noise steam
injection ring
Greater flow at lower
steam pressure drop
Multiple orifice steam
drilling ensures the
lowest possible
unshielded noise level
Designer Fuels
Stranded natural gas, so named
because no pipelines exists to
transport it, is most prevalent
(red) in Africa, Middle East,
Asia, and Europe
Ultimate energy irony: Clean,
plentiful gas fuel wasted because
it blocks access to a dirty, liquid
one
Syntroleum process, natural gas
reacts via a catalyst to oxygen in
the air to form a synthetic gas
(Syngas) then a second reaction
coverts Syngas to a liquid diesel
fuel
NH3 Plant
Pilots with Thermocouples Ignitor
Relief Tubes
Valves
Manual Gas Assisted
Vents Flare Burner
w/ Fluidic Seal Ignitor
Manifold
Pilot
Manifold
Assist Gas
FE
Flow
Relief & Vent Header Sensor
Pilot
Ammonia Gas
Storage
Tanks
Purge Compressed
Gas Air
Injection Ignitor
Gas
Flame Front
Ignitor Panel FS-R00-27
Ammonia (NH3) Plant Considerations
Auxiliary fuel required
in ammonia flares
Auxiliary fuel to
combust hydrocarbon
vapors when a clean
flare gas falls below the
necessary heating
value
Picture to the right is
one of an ammonia
plant
Offshore Platform
Ignitor Pilots
Flame Front Tubes
Ignitor Panel Ignitor Water Assisted
Manifold Smokeless
Compressed Air Pilot Flare Burner
Ignitor Gas Manifold w/Fluidic Seal
Pilot Gas
Gas
Disentrainment
Oil Drum
Boom
Water Assist
Offshore Platform
Ground Level Concentrations
Average target concentration
(downwind)
0.10ppm or less
Examples of odor nuisance:
Isoamylmercaption
0.00043 ppm
Ethylmercaptan
0.00056 ppm
Auxiliary Fuel Requirement
Amount of fuel required (F) is calculated based on
maintaining the vent gas stream net heating value at
the minimum of 300 Btu/scf required as described in
the United States Federal Register:
(Q * Bv) + (F * Bf) = (Q + F)* (300 Btu/scf)
Where:
– Q = vent stream flow rate, scfm
– Bv = Btu/scf of the vent stream
– Bf = Btu/scf of the fuel stream
Therefore,
F (scfm) = Q * (300-Bv)/(Bf-300)
The annual auxiliary fuel requirement (Fa) is:
Fa (Msfm/yr) = (F scfm) * (60 min/hr) * (8760 hr/yr)
Fa (Mscfm/yr) = 526 * F
Flare Tip Diameter
Sizing must also comply with Federal Register (40 CFR 60.18)
for maximum velocity of steam-assisted, elevated flares:
Net Heating Value of Vent Stream Maximum Velocity
Bv (Btu/scf) Vmax (ft/sec)
300 60
300-1000 log10(Vmax) = (Bv + 1214)852
> 1000 400
It is standard practice to size the flare so that the design velocity
of flow rate Qtot, is 80 percent of Vmax:
Dmin (in) = 12*[((4/PI)(Qtot/60sec/min))/(0.8*Vmax)]^0.5
Dmin (in) = 1.95 * (Qtot/Vmax)^0.5
Where:
Qtot = Q + F (measured at stream temperature and pressure)
Dmin should be rounded up to the next largest available
commercial size
Improve Flare Burner Life
Vector Diagrams
F
F
W W W
L F
G L L
G S G
W= Wind High Exit Low Exit Low Exit Velocity
F = Flare Velocity Velocity With Secondary
LP = LP Zone Energy Source
G = Gas Jet
S = Supplementary Energy
Incinerator Design
Mechanism
– Oxidation reaction
Factors
– Time, temperature
and turbulence
Principles of Combustion
Burners
– Ignite the fuel and
organic material
Chamber
– Appropriate residence
time
for oxidation process
Three T’s of Combustion
– Temperature
– Time
– Turbulence
Coupled Effects of Temperature and
Time on Rate of Pollutant Oxidation
100
1 sec
Pollutant Destruction, %
80 1.0 sec
0.01 sec
0.001 sec
60
Increasing
Residence
40 Time
20
0
600 800 1000 1200 1400 1600 1800 2000
Increasing Temperature, °F
Residence time of gases in combustion chamber calculated from: t = V/Q
t = Residence Time (s)
v = Chamber Volume (ft3)
Q = Gas volumetric flow rate at combustion conditions (ft3/s)
Schematic of a Thermal Incinerator
Fume
Fuel Exhaust
Combustion
Air
(Fume)
Typical Marine Vessel Loading System
Product
Loading Arm
Product from
Storage Tanks
Vapor Natural Gas/ Vapor
Arm Inerting Gas Mover Hydrocarbon Vapor
Enriching to Control Device
Detonation Gas
Arrestor Analyzer
Knockout
Drum(s)
Discharged Condensate
Vapors to Tanks
Sump
Ship Pump
or Barge
Dock Facilities Shoreside Facilities
Storage Tank and Tank Truck Loading
Hydrocarbon Concentration Profile
70
60
Total Hydrocarbon Vapor
Concentration in Vessel
Gasoline Vapors
(Volume Percent)
50
40
30
20 Crude Vapors
0
0 25 50 75 100
Percent of Storage Tank or Truck Filled
Marine Vessel Loading
Hydrocarbon Concentration Profile
70
60
Total Hydrocarbon Vapor
Concentration in Vessel
50
(Volume Percent)
40
30
20 Gasoline Vapors
Crude
10 Vapors
0
0 25 50 75 100
Percent of Marine Vessel Filled
FS-R00-33
Typical Lean Oil Absorption Vapor
Recovery System
Typical Refrigeration Vapor
Recovery System
Vapor Condensing
Precooler Unit (Evaporator)
Thermostatic
Expansion
Valve
Hydrocarbon
Vapor Inlet Compressed
Refrigerant
Freon Compressor-
Blower Return Condensor
Treated
Vapor
Power
Input
Power
Input Recovered
Product
Condensate
Collection Tank
Typical Catalyst Oxidation System
Hydrocarbon Treated
Vapor Inlet Vapor Out
Thermal Burner
Zone
Heated Feed
Catalyst Air or Air or
Bed Process Process
Stream Stream
Oxidation
Zone
Preheat Optional Stack
Heat
Recovery
