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FIG. 5-16: Fire Sizing Environmental Factors
DESIGN OF RELIEF SYSTEM TO FLARE
Back Pressure Consideration
Grouping of Systems
The next step in the analysis involves setting a preliminary
maximum back pressure for the system at various locations in
the flare system, and choosing between conventional, pilot operated, or balanced pressure relief valves for the various relief
stations. A pressure relief device inventory should be prepared,
summarizing set pressure, estimated relieving temperature,
and approximate capacity, if available. The flare style should
be considered, as well as the maximum pressure expected at
the flare base.
The first step in designing a flare system for a facility is to
determine the number of segregated vent and flare headers, if
more than one, which are required. Depending on plot plan, the
range of equipment design pressures, desirability of isolating
certain streams, temperature of the relief streams, possibility of
liquid carryover, heating value of the streams, and quantities of
the relief streams, it may prove desirable to provide two or more
segregated headers to the flare K.O. drum, or even to use totally
independent flare systems. Separation of high pressure and low
pressure headers, or low-temperature and wet headers, is not
uncommon. Some large integrated gas treating facilities have a
high pressure, low pressure, and a cryogenic flare.
Pressure relief valves that can tolerate higher back pressure
(e.g., balanced or pilot operated pressure relief valves) may be
selected if the back pressure is too high for conventional pressure relief valves. Excessive built-up back pressure will affect
the operation of conventional pressure relief valves; high superimposed back pressure will affect the set point of these valves.
Flare Header Sizing Methods
The first step in determining controlling loads for a relief
header and flare system is to identify the credible major flaring scenarios. These scenarios may be associated with pressure
relief, emergency depressuring, or transitory operating (e.g.,
startup, shutdown, etc.) events. A case may be controlling because of the back-pressure it will generate in the relief header,
the heat release at the flare stack, or the nature of the fluid to
be flared (i.e. low heating value, composition of the fluid, low
temperature, high liquid flow rate, etc.). This analysis may include dividing the plant into fire zones (fire zone size is discussed in ISO 23251 (API Std 521), identifying large individual
process relief loads, identifying common mode process failure
loads, identifying common mode local or plant wide utility failure, identifying which process valves that discharge to the flare
may already be open when an upset occurs (e.g., during startup
or shutdown), identifying maximum depressurization rates,
and identifying possible common events of pressure relief and
venting or depressurization.
Line sizing for flare headers and relief lines requires the
use of compressible flow equations. Computer programs are
normally used to size flare headers and to calculate the back
pressure at the relief devices. The header sizes are checked for
the major relief scenarios and then fixed. Based on these header
sizes, each pressure relief device is checked for proper style,
backpressure, and the effect of other devices on the set pressure and operation of the valve. API RP 520-II requires that the
pressure relief valve inlet and outlet piping be sized for the rated relief device capacity for all devices except modulating pilot
operated relief valves, while header systems may be sized using
the required capacity of the controlling scenario(s). A manual
sizing method is outlined below:
1.Start at the flare tip, where the outlet pressure is atmospheric, use design flows and work toward the individual
relief valves (pressure drop across the tip will vary with
the style of the flare and available system pressure drop
– check with the tip manufacturer).
Some favorable instrument response may be included in the
design of flare systems. ISO 23251 (API Std 521), Fifth Edition
states, “Although favorable response of conventional instrumentation should not be assumed when sizing individual process
equipment pressure relief, in the design of some components
of a relieving system, such as the blow-down header, flare, and
flare tip, favorable response of some instruments can be assumed.” In practice, the relief system design basis should be
thoroughly analyzed using appropriate methodology (i.e. layers
of protection analysis, SIL review, quantitative method), before
credit is taken. The basis of the flare design load determination
should be part of the plant formal hazard review.
2.Establish equivalent pipe lengths between points in the
system and establish losses through fittings, expansion,
and contraction losses.
3.Many users limit the maximum allowed velocity at any
part of the flare system to Mach 0.7. This limit is intended to minimize the possible effects of acoustically or flow
induced vibration on the piping in the flare system. More
detailed methods to evaluate these effects are presented
in references 8 and 9.
4.Estimate properties of gases in the headers from the following mixture relationships (i indicates the ith component).
For gas plants, another key decision is whether to design
the flare system for the maximum inlet flow of the production
header or inlet pipeline, or instead rely on a shutdown system
at the plant inlet, and/or an automatic or manual well shut-in.
Provisions also may be needed to allow venting some or all of
the produced gas to the flare on facility start-up, pipeline depressurization, or during an emergency in one process unit.
MW= ∑ Wi / ∑ (W / MW)i
= ∑ Wi Ti / ∑ Wi
xi µi (MW)i0.5
5.Calculate the inlet pressure for each section of the line
by adding the calculated pressure drop for that section to
the known outlet pressure.
After the load is determined, it is necessary to decide on the
location of the flares, and size of the headers and flare lines. Location and height of the flares must consider flare stack height,
thermal radiation, emissions during flaring, ground level concentrations in case of a flame-out, consequences of liquid carryover, and noise. Frequently, the controlling criterion for flare
location is the minimum distance to continuously operating
equipment, which may require maintenance.
6.Calculate sections of pipe individually using the inlet
pressure of a calculated section as the outlet pressure for
the new section.
7.Continue calculations, working towards the relief valve
or other flow source.
8.Check calculated maximum superimposed backpressure,
built-up backpressure, and total back pressure at the relief valve against piping design pressure and the maximum allowable back pressure (MABP) of the flow source.
See “Discharge Piping and Backpressure,” in this section
for a definition of these terms, and API Std 520-I for maximum allowable values.
9.Adjust line size of headers until the calculated back pressure is less than both the MABP for each valve in the
system and the design pressure of the associated piping.
The method outlined above employs sizing equations which
assume isothermal flow in the flare header. This is adequate for
most uses; however, if the actual flow condition differs greatly
from isothermal, the use of more complex equations and methods is required to predict pressure and more accurately and
temperature profiles for the headers.
The choice of piping material other than carbon steel may
be dictated by temperatures and pressures in some parts of the
flare system. Flare systems relieving fluids that produce cryogenic temperatures may require special metallurgy.
Flare Knockout Drums
Gas streams from reliefs are frequently at or near their dew
point, where condensation may occur, and some systems may
relieve liquids or two-phase fluids in an overpressure event.
A knockout drum is usually provided near the flare base, and
serves to recover liquid hydrocarbons or water, prevent liquid
slugs, and remove large (300–600 micron diameter and larger)
liquid particles. The knockout drum reduces hazards caused by
burning liquid that could escape from the flare stack. All flare
lines should be sloped toward the knockout drum to permit condensed liquid to drain into the drum for removal. Liquid traps
in flare lines should be avoided. If liquid traps are unavoidable,
a method for liquid removal should be provided. The location of
the flare knockout drum also needs to take into account radiation effect from the burning flare. Typically these drums are
located between the flare and the process area, where the maximum flare radiation exposure may be higher than allowable for
continuously operating equipment, but reasonable enough to
allow properly trained personnel appropriate time to leave in
a major flaring event.
A molecular purge reduction seal is a seal device, installed in
a flare stack, which uses the difference in relative molecular
masses of purge gas and infiltrating air to reduce the rate at
which air will enter the stack. A velocity seal is a purge reduction seal which operates on the principle that air infiltrating
the stack counter to the purge flow hugs the inner wall of the
flare tip. The seal looks like one or more orifices located below the flare tip, which forces the air to the center of the stack
where it is swept up by the purge gas.
To be effective, purge reduction seals require a purge gas,
typically natural gas or nitrogen. These seals do not stop flashback, but rather minimize the chances that the air concentration below the flare tip becomes high enough to support flashback. These devices reduce the flow rate of purge gas which
otherwise would be required to accomplish this. The minimum
seal purge gas rate will be specified by flare supplier.
Purge gas is normally supplied at the end of all major flare
headers and sub-headers, to ensure that the flare headers are
free of air. Changes in ambient temperature, or cooling of the
flare header after a hot relief could cause a partial vacuum in
the flare header if no purge is provided. In most cases, the sum
of the purge rates needed for the flare headers is greater than
the purge needed for the flare seal.
Flare systems are commonly designed for a mechanical design pressure of at least 335 kPa (ga), to minimize the chances
of equipment damage due to a flashback.
Types of Flares
A number of different types of flares are used in natural gas
processing facilities. The most common can be classified as:
1.Elevated Pipe Flares — This style consists of an elevated flare riser with typically a flame stability device
constructed of stainless steel at the tip. The degree of
smokeless operation is dependent on the gas composition and discharge velocity (natural gas lean in NGL may
burn relatively smokelessly)
2.Elevated Assisted Smokeless Flare — A general classification of several different styles of elevated flares, designed to minimize smoke formation. The mechanism is
improved combustion due to the turbulence caused by the
assist gas. Assist gas mixing can be external at the flare
tip exit, internal to the flare tip, or both. These flares can
operate from below 0.5 Mach to sonic. The decision depends on the acceptable back-pressure for the flare header, the availability of utility streams, and the particular
design of the flare tip. The required quantity of assist gas
depends on the type.
Knockout drums may be vertical external to the flare stack,
built into the bottom of a self supporting flare stack, or horizontal external to the flare stack. Internals which may break
free and block the relief path are not allowed in a flare knock
Additional material on design and sizing for flare knock out
drums, including sizing examples are provided in ISO 23252
(API Std 521).
Flare Seals and Flare System Purging
•Steam assisted flare tip: most common type of flare
used in refinery and natural gas service where sufficient steam is available. Can achieve a smokeless
operation over a wide range of flared fluids and operating conditions
A seal is provided in the flare system between the knockout
drum and the flaretip to prevent flashbacks due to air ingress.,
which can result in a sudden substantial increase in pressure in
the flare system, and potential damage. Several types of seals
can be used: 1) a water seal drum, 2) a molecular purge reduction seal (buoyancy seal), or 3) a velocity purge reduction seal.
•Low Pressure Air Assist: commonly uses air supplied
by a blower in a channel around the flare stack to
promote smokeless operation. Generally, these systems will permit smokeless operation during dayto-day operation, but not necessarily at full flaring
A water seal drum is almost always installed in refinery
flare systems, and is sometimes used in natural gas processing
plants. It separates the flare system from the flare stack and
provides a water barrier which is capable of stopping flashback.
Several non-proprietary methods for predicting thermal radiation from flares are available. One method based on flare
supplier input, which can be used for preliminary calculations
for simple flares with smokeless capacity of 10% or less, at tip
mach number of 0.5 or less, is presented below. ISO 23521 (API
Std 521)13 presents a similar method, which in general will produce more conservative results. This and other radiation models are reviewed in a paper by Schwartz and White.6
•Natural gas assisted Flare: uses high pressure natural gas to provide the discharge turbulence required
for smokeless operation.
3.High Pressure Elevated Staged Flare — Flare tips
operating at sonic velocity, which use pressure energy
to promote smokeless burning. Typically, the flare tips
are staged using valves at the flare base. This design is
most efficient when the flare stream is high pressure
Preliminary Elevated Flare — Thermal Radiation
4.Horizontal Ground Flare — A ground flare typically
consists of a flare system operated with the flame horizontally on the ground. The most common style is similar
to staged flare tips. They are often used in remote locations where emissions, noise and flame visibility are not
of significant concern.
Spherical Radiation Intensity Formula:
(Wf) (NHV) (ε)
14.4 π (R2)
This equation has been found to be accurate for distances as
close to the flame as one flame length.
5.Enclosed Ground Flare — an enclosed ground flare
consisting of a burner surrounded by a shell. The system
operates by introducing the flare gas into the unit via
a burner. Air enters the bottom of the shell via air louvers. Enclosed ground flares are normally used only for
small capacity, low pressure flaring operations (such as
tank flares) where an elevated flare is inconvenient, and
for high capacity situations where an elevated flare is not
practical due to thermal radiation or community visibility
concerns. Special flame arrestor burners are used in tank
applications to minimize the possibility of back flash.
Equation 5-19 is valid so long as the proper value of fraction
of heat radiated, ε, is inserted. Classically, ε has been considered
a fuel property alone. Brzustowski et al.2 experimentally observed
a dependence of ε on jet exit velocity. Other authors have presented models that consider the carbon particle concentration in the
flame. The fraction of heat radiated is a function of many variables
including gas composition, tip diameter, flare burner design, flowrate and velocity, flame temperature, air-fuel mixing, and steam
or air injection; therefore a flare supplier should be consulted to
determine the specific values for a given application. A list of vendor recommended fraction of heat radiated values for the most
frequently flared gases is shown in Fig. 5-18.
6.Loading and Tank Flares — Several designs of elevated flares are available that are tailored to the destruction
of vapors during truck loading and from tanks. These designs deal with the problems of low pressure, large variation in flow rate, and the potential of air ingress.
Permissible Design Flare Thermal Radiation Levels13
Elevated Flare Allowable Thermal Radiation
Thermal radiation is a prime concern in flare design and
location. Thermal radiation calculations must be performed to
avoid dangerous exposure to personnel, equipment, and the
surrounding area (trees, grass). Thermal radiation exposure
limits, and the effects on personnel, equipment and instrumentation on shown in Fig. 5-17 from ISO-23251 (API Std 521).13
Maximum radiant heat intensity at any location
where urgent emergency action by personnel is
required. When personnel enter or work in an area
with the potential for radiant heat intensity greater
than 6.31 kW/m2 (2000 Btu/h·ft2), then radiation
shielding and/or special protective apparel (e.g. a
fire approach suit) should be considered. SAFETY
PRECAUTION — It is important to recognize that
personnel with appropriate clothing a cannot tolerate thermal radiation at 6.31 kW/m2 (2000 Btu/h·ft2)
for more than a few seconds.
Maximum radiant heat intensity in areas where
emergency actions lasting up to 30 s can be required
by personnel without shielding but with appropriate
Maximum radiant heat intensity in areas where
emergency actions lasting 2 min to 3 min can be
required by personnel without shielding but with
Maximum radiant heat intensity at any location
where personnel with appropriate clothing a can be
Equipment protection should be evaluated on a case by case
basis, as various pieces of equipment have different protection
Solar radiation may add to the calculated flame radiation
and is dependent upon specific atmospheric conditions and site
location. A typical design range for a temperate climate is 0.79
to 1.04 kW/m2, but depends on the location. The decision to include solar radiation, is dependent on design critieria, and is
dependent and the site and the intent of the evaluation.
Determining Elevated Flare
Flare suppliers have developed proprietary radiation modeling programs based on equations and empirical values, and
these are commonly used to assess the effects of flare radiation,
and set the flare height. The F* factor (fraction of heat radiated) values used in these programs are specific to the equations
used, and are generally not interchangeable with the F* factor
values used in other methods. These programs have not been
subject to review and verification in the open literature, and are
specific to a particular flare design and exit velocity.
ppropriate clothing consists of hard hat, long-sleeved shirts with cuffs butA
toned, work gloves, long-legged pants and work shoes. Appropriate clothing
minimizes direct skin exposure to thermal radiation.
To calculate the intensity of radiation at different locations,
it is necessary to determine the length of the flame and its angle
in relation to the stack (see Fig. 5-19). A convenient expression
to estimate length of flame, Lf, is shown below, based on information from equipment suppliers.
Lf = (0.12) (d)
For conventional (open pipe) subsonic flares, an estimate of total flare pressure drop is 1.5 velocity heads based on nominal
flare tip diameter. The pressure drop equivalent to 1 velocity
head is given by:
(0.102) ρ V2
The coordinates of the flame center with respect to the tip
3.23 • 10–5 • W ỉ Z • T ư
ố k MWø
Sonic velocity of a gas is given by:
= k R0 T
= (Lf / 3) (sin θ)
= (Lf / 3) (cos θ)
The distance from any point on the ground level to the center
of the flame is:
R = √ (X – Xc)2 + (Hs + Yc)2
Equations 5-19 and 5-28 allow radiation to be calculated at any
The stack height results from considering the worst position
vertically below the center of the flame for a given condition of
gas flow and wind velocities (see Fig. 5-19).
R2 = (Hs + Yc)2
∆Pw is the pressure drop at the tip in mm of water. After determining tip diameter, d, using Equation 5-22, and the maximum
required relieving capacity, flame length for conditions other
than maximum flow can be calculated using Equation 5-20.
The flare radiation method applies to flare tip Mach number
of 0.50 or less in Equation 5-22.
= (Hs + Yc)
Hs = (R – Yc)
Hs = R – [(Lf / 3) (cos θ)]
This method assumes that for different wind velocities the
length of the flame remains constant. In reality this is not true.
When the wind blows at more than 25 m/s, the flame tends to
shorten. For practical design, this effect is neglected.
API Preliminary Elevated Flare — Thermal Radiation Method — ISO 23251 (API Std 521) presents a similar
methodology for calculation of flare radiation. The API method
is generally more conservative to that shown above. The following are the major differences.
Dimensional References for Sizing a Flare Stack
The center of the flame is assumed to be located at a distance
equal to 1/3 the length of the flame from the tip.
The angle of the flame results from the vectorial addition of
the velocity of the wind and the gas exit velocity.
θ = tan–1
ex = 168
Typical Fraction of Heat Radiated Values for Flared
HS + Y C
The maximum value of ε for any gas is 0.13.
X - XC
Courtesy American Petroleum Institute