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Typical onshore gas gathering plant
The process design and operational aspects of a
flare system are wide and varied and can range
from the relatively simple to the very complex.
There are a number of basic process
requirements which are normally specified no
matter how simple or complex the flare system is.
There are also a number of operational aspects
that are common to all flare systems. These basic
building blocks in the design and operation of the
flare system are discussed below.
Flare Process
Limits on certain flare process elements such as
available back pressure, radiation and noise has a significant impact on the design of the flare
system. They all contribute to deciding the following:
Flare tip type and size
Flare riser size
Flare stack elevation
Back Pressure
This will determine whether a flare system is low pressure or high pressure, i.e., a sub-sonic flare or
a sonic type flare. A flare tip is normally specified with a back pressure of 6.0 barg or less and can
be as low as a few millibars for steel plant applications. All flow cases should be designed to vent
without exceeding the allowable flare tip or flare system back pressure.
Radiation
Once the type of flare tip has been determined a radiation level analysis needs to be performed.
This will generally determine the flare stack elevation or boom length. For an onshore flare the
sterile radius and a radiation limit at the sterile radius will normally determine the overall flare height.
For an offshore flare, radiation limits at the base of the flare tower / boom, helideck, crane cab,
weather deck or other key point locations on the platform will determine the tower height or boom
length.
Low Pressure Flares
For low pressure, sub-sonic flare tips thermal radiation is mostly influenced by the gas properties
and wind effects. The design of the flare tip is more often than not, just an open pipe that can’t
really aide gas/air mixing and exhibit low radiation unless water / steam / air / gas is used as an assist
media. The emissivity factor (or F-factor) ranges between 0.20 and 0.35 for these flares although
lower values for hydrogen can be used.
High Pressure Flares
With high pressure sonic flare tips the inherent energy and velocity of the gas stream can be used to
great effect. Thermal radiation is still influenced by the gas properties and wind effects but equally
so by the design of the flare tip itself. There are a wide range of technologies available from Argo
such as single point and multi point sonic flare tips and Coanda flares. These flare tips naturally
inspirate more air than their low pressure counterparts and exhibit lower radiation characteristics with
emissivity factors (or F-factors) ranging between 0.07 and 0.15. This results in shorter flare stacks /
booms for these flares. Being able to select from a range of high pressure sonic flares is also
Introduction & Contents
1.0 Flare System Design
1.1 Flare Design Specification Pt.1
1.2 Flare Design Specification Pt.2
1.3 Flare System Design Pt.1
1.4 Flare System Design Pt.2
2.0 Flare Types
2.1 Pipe Flares
2.2 Single Point Sonic Flare
2.3 Multi Point Sonic Pipe Flare
2.4 Coanda Flares
2.5 Vent Tips / Cold Flares
2.6 Other Flares
3.0 Pilots and Ignition Systems
3.1 Pilot Burners
3.2 Flame Front Generators
3.3 Electronic Spark Ignition
3.4 Ballistic Ignition System
3.5 ‘Very’ Pistol / Signal Gun
3.6 Flame Monitoring
4.0 Flare Components andAncillaries
4.1 Flare Process and Operation
4.2 Flare Ancillaries
4.3 Flare Emissions
5.0 Inspection and Maintenance
5.1 Close Visual Inspection (CVI)
5.2 Flyby Inspections
5.3 Schedules for Flare Tip
Inspection
5.4 Maintenance and Repair
5.5 Flare Tip Change-out
Frequently Asked Questions
Flare Photograph Gallery
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4.1 Flare Process and Operation
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booms for these flares. Being able to select from a range of high pressure sonic flares is also
particularly helpful when debottlenecking an existing flare system.
Radiation Calculations
As well as the type of flare tip, emissivity factors (or F-factors), gas composition and available
pressure, radiation levels are influence by other environmental factors such as wind speed and
direction, relative humidity, solar radiation and the calculation method employed (e.g. API,
Brzustowski and Sommer, Point, Diffuse, Mixed Point & Diffuse, Chamberlain aka Shell Thornton
method). Argo uses the commercially available FLARESIM model, from which an appropriate
calculation method can be chosen.
Radiation Plots: We normally present radiation plots with isopleths 1.58 kW/m² (500 Btu/hr.ft²), 3.15
kW/m² (1000 Btu/hr.ft²), 4.73 kW/m² (1500 Btu/hr.ft²), 6.31 kW/m² (2000 Btu/hr.ft²) and 9.46 kW/m²
(3000 Btu/hr.ft²). The different radiation levels represent permitted exposure time of personnel to
these radiation levels. Making reference to API RP 521 (Pressure-Relieving and Depressuring
Systems). 1.58 kW/m² (500 Btu/hr.ft²) is generally know as continuous full shift exposure, i.e., where
personnel with appropriate clothing may be continuously exposed. A radiation level of 4.73 kW/m²
(1500 Btu/hr.ft²) would be the limit in areas where emergency actions lasting two to three minutes
may be required by personnel without shielding but with appropriate clothing. A radiation level of
6.31 kW/m² (2000 Btu/hr.ft²) would be the limit in areas where emergency actions lasting up to 30
seconds may be required by personnel without shielding but with appropriate clothing.
Emissivity Factor: Also known as the F-factor. This is normally calculated by the flare vendor after
weighing up each individual case. We always provide the F-factor on our radiation plots and our
clients are always welcome to discuss how they are specified. There are various other methods of
calculating F-factors based upon tip exit velocity and molecular weight such as Tan, Kent and Cook
however these generally overestimate the figure. On occasion an operator will specify a minimum F-
factor for all vendors to work to.
Relative Humidity: RH is a factor in calculating the radiation incident at specific points. The further
away from the flare a point is, the more it will benefit from the effects of water vapour in the air.
Where permitted by a client, a Transmissivity (Tau) factor can be applied to calculated radiation
levels from the flare thereby reducing overall height or sterile radius.
Solar Radiation: Solar is a factor in calculating the overall radiation intensity levels and setting stack
heights. It is a common discussion point between flare vendor, engineering contractor and operator
where different viewpoints can be held by all three parties as to the inclusion and extent of inclusion
of solar in the calculations. Factors affecting its inclusion are; freqency of maximum flare load
occuring coincident with maximum solar radiation occuring, likelihood of personnel being exposed at
that time and common sense. Maximum and average values depend upon the geograaphical
location of the plant (latitude). The range of maximum values is 250–330 BTU/h/ft² (0.79–1.04
kW/m²). The range of average values is 140–230 BTU/h/ft² (0.44–0.73 kW/m²).
Noise
Once the type of flare tip has been determined the radiation levels and stack height / boom length
calculated a noise level check is carried out according to the customer specification. Noise levels
are advised if no limit is contained within the specification. Factors mainly affecting percieved noise
levels are combustion noise for sub-sonic low pressure flares and assist media such as steam
injection nozzles. The factor mainly affecting percieved noise levels is jet noise for sonic high
pressure flares and extended flare deck structures can be used to directly shield certain areas below
as required. Typically specified values for the maximum noise pressure levels for onshore plant are
90 dB(A) at maximum flaring and 70 to 80 dB(A) for day-to –day continuous operation. Typically
specified values for the maximum noise pressure levels for offhore plant are 120 dB(A) at maximum
emergency flaring and 70-80 dB(A) for day to day continuous operation. In order to take account off
both sonic flare jet noise and combustion noise we can also advise “C” weighted noise levels upon
request.
Flare Emissions (NOX) etc. by AB..
Mension H2S dispersion, reduced purge rates, reduced pilot gas consumption, on/off pilots,
Flare Operation
In order to keep most flares in a safe condition and functioning well there are certain minimum
operational and utility requirements. These are continuously consuming power, utility gas and
12/31/13 4.1 Flare Process and Operation - Argo Flare Services
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operational and utility requirements. These are continuously consuming power, utility gas and
nitrogen so any way the consumption levels can be safely minimised is usually welcomed subject to
excessive cost of course.
Purge & Purge Reduction
The flare header should be adequately purged with inert gas or fuel gas to reduce the oxygen level
down to that required for each individual system. For normal hydrocarbon gas a maximum oxygen
level of 6% v/v is adequate, however for flare systems containing significant quantities of hydrogen a
much lower level down to 2% v/v may be needed resulting in a larger purge rate.
Depending on the size of the flare / riser. Purge rates can vary anywhere from 0.1 Nm³/h for a small
flare system up to 500 Nm³/h for a very large flare system. Purge reduction is a high priority for most
operations as it contributes to the continuous emissions figures. For onshore low pressure flares
molecular seals have been used to good effect to dramatically reduce (by up to 10 fold) the required
purge rate, however at the expense of something else such as additional flare tip maintenance. For
both onshore and offshore low pressure flares diode type seals have been used to good effect to
reduce the purge rate (by 40-50%) when compared to for an open pipe. Starting to become more
popular are purge reduction seals for onshore and offshore high pressure flare systems.
Pilots & Pilot Gas Consumption
API 537 provides general guidance on the recommended number of pilot burners required for
various flare sizes. Some of the major oil and gas operators have their own engineering
specifications, however Argo will always make a recommendation on pilot type, duty and quantity. It is
worth noting that offshore flare tips typically operate with larger capacity pilot burners than those
onshore due to the harsher ambient conditions. Typical offshore flare pilot capacity is between
70,000 Btu/h and 250,000 Btu/h per pilot. In order to minimise pilot gas consumption where supply
may be scarse and expensive some operators use intermittant pilots which are only used if the main
flare flame becomes extinguished.
Intermittent Utility Consumption
Some other uses for utilities include plant air for flame front generators, gas for periodic flare gas
enrichment or smoke suppression, steam for smoke reduction and water for water seal top up.
Power Consumption
Power consumers include electonic pilots and ignition control systems, flame front generators, air
assist flare blowers, pilot flame detectors such as UV and IR and aircraft warning lights.
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