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FIG. 5-16: Fire Sizing Environmental Factors

FIG. 5-16: Fire Sizing Environmental Factors

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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.

Load Determination

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

Eq 5-16


= ∑ Wi Ti / ∑ Wi

Eq 5-17


xi µi (MW)i0.5

Flare Location

= ∑


xi (MW)i0.5

Eq 5-18

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

out drum.

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

natural gas.

Preliminary Elevated Flare — Thermal Radiation

Calculation —

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)

I =

14.4 π (R2)

Eq 5-19

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.

FIG. 5-17

Permissible Design Flare Thermal Radiation Levels13

­Elevated Flare Allowable Thermal Radiation


design level

K (kW/m2)

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

appropriate clothinga


Maximum radiant heat intensity at any location

where personnel with appropriate clothing a can be

continuously exposed

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

Thermal Radiation

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)



Eq 5-20

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:

ρ V2

(0.102) ρ V2

∆Pw =




Eq 5-21

The coordinates of the flame center with respect to the tip


d =



3.23 • 10–5 • W ỉ Z • T ư

ỗ ữ

P2 M

ố k MWø

 1000Eq 5-22

Sonic velocity of a gas is given by:


= k R0 T


= (Lf / 3) (sin θ)

Eq 5-26


= (Lf / 3) (cos θ)

Eq 5-27

The distance from any point on the ground level to the center

of the flame is:

R = √ (X – Xc)2 + (Hs + Yc)2

Eq 5-28

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.



Eq 5-29

= (Hs + Yc)

Eq 5-30

Hs = (R – Yc)

Eq 5-31

Hs = R – [(Lf / 3) (cos θ)]

Eq 5-32

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.

Eq 5-23

FIG. 5-19

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.

 Vw 

θ = tan–1  

 Vex 

Eq 5-24

Eq 5-25




ex = 168








FIG. 5-18

Typical Fraction of Heat Radiated Values for Flared



HS + Y C


Carbon Monoxide




Hydrogen Sulfide














The maximum value of ε for any gas is 0.13.

X - XC


Courtesy American Petroleum Institute


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FIG. 5-16: Fire Sizing Environmental Factors

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