Flare Systems

7/17/2019 Flare Systems

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EDS 2004/Flare Systems-1

Flare Systems

Training Services

Flaring is a volatile organic compound (VOC) combustion control process in which

the VOCs are piped to a remote, usually elevated, location and burned in the open

air using a specially designed burner tip, auxiliary fuel, and steam or air to promote

mixing for nearly complete (>98 percent) VOC destruction. Completeness of

combustion in a flare is governed by flame temperature, residence time in the

combustion zone, turbulent mixing of the components to complete the oxidation

reaction, and available oxygen for free radical formation. Combustion is complete

if all VOCs are converted to carbon dioxide and water. Incomplete combustion

results in some of the VOC being unaltered or converted to other organic

compounds such as aldehydes or acids.

Flares are generally categorized in two ways: (1) by the height of the flare tip (i.e.

ground or elevated); and (2) by the method of enhancing mixing at the flare tip (i.e.

steam-assisted, air-assisted, pressure-assisted, or non-assisted). Elevating the flarecan prevent potentially dangerous conditions at ground level where the open flame

(i.e. an ignition source) is located near a process unit. Further, the products of

combustion can be dispersed above working areas to reduce the effects of noise,

heat, smoke, and objectionable odors.

The primary function of a flare is to dispose of toxic, corrosive, or flammable vapor

safely, under relief conditions, by converting them to less objectionable products by

combustion.




Table of Contents

Flare Systems

– Purpose

– Flare selection

– Support structures

– Combustion theory

– Radiation theory

– Regulatory compliance

– Equipment design – Flare design requirements

– Incineration

Flare operation and design have become more complicated than simply burning

waste gas. The major components of any flare facility are the knockout drum, flare

stack, continuous purge gas, flare tip and ignition system. Normally the flare

system will use some additional utilities such as steam or air to make the flare

smokeless. In designing a flare, considerable effort should be devoted to meeting

the prime objective, which is the safe disposal of waste gas. The overall

characteristic of the flame is a function of many variables, such as exit velocity of

the gas, air-to-gas ration, wind speed, gas composition and flame temperature.

Almost 35% of the flare design depends on the heat radiated by the flame. The less

heat radiated, the more desirable the flare. The major sources of thermal radiation

are free carbon, water vapor and carbon dioxide, but the principal source of these

three are fee carbon. Free carbon is formed from the pyrolysis of the hydrocarbon

gas. Noise is another type of environmental pollution. There are two types of noise

emitted by the flare. Normal noise amounts mainly form the gas combustion, thesecond type is shock noise generated by the gas velocity gets. In any flare system,

waste gas shall be sent to a knockout drum to knockout any hydrocarbon

condensate.

As shown in the table of contents, we will be covering a wide range of topics of

flare systems.




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

Line sizing for relief and flare headers requires the use of compressible flow

equations. UOP has developed a computer program for this purpose. The

calculation method usually starts at the flare itself where the outlet pressure is

atmospheric. The program uses design flows through the system (typically the

general electrical power failure case). Flare tip velocities are designed up to 0.5

MACH and, therefore, a pressure drop can be determined. The pressure drop

through the water seal, molecular seal, and knockout drum must also be included.

The next step is to determine the equivalent lengths of the piping this requires one

to determine the strength length of pipe, elbows, and other fittings. Try to limit the

velocity throughout the system to 0.7 MACH. Never exceed MACH since this will

cause a discontinuity in the flow pressure drop and a lot of noise. Next, the

properties will best estimated for MW = W/(MW/W), for temperature T = WT/W.

Next, the inlet pressure for each section is calculated by knowing the pressure drop

for that section.

One must continue calculations back to the relief valve. Next, one needs to check to

see if the allowable pressure is less than the actual pressure if not then one will have

to increase the relief header size to reduce pressure drop in the system. The

equations used assume isothermal flows in the relief header. This is adequate and

conservative for most applications. There are only a number of relief systems that

are allowed to atmosphere directly.




Typical Relief System

Discharge pipe size for direct spring operated valves is critical. If it is improperly

sized, it may cause valve failure. Pressure losses may occur in discharge headers

causing excessive back pressure and excessive back pressure may cause the relief

valve to close. When the valve closes, the back pressure in the discharge header

decreases, and the valve reopens. After determining the maximum load on the

system, it is necessary to decide on the location of the flares and the size of the

headers and flare lines. Location and height of the flares must consider radiation

heat and emissions. This requires fixing the maximum back pressure for the system

and choosing between conventional, pilot operated, or balanced pressure relief

valves of the various relief stations.

UOP typically divides the type of relief valve based on set pressure. Set pressures

less than 250 psig are conventional type and set pressures greater than 250 psig are

balanced type relief valves. Depending on the plot plan, the range of equipmentdesign pressures, desirability of isolating certain streams, etc., it may be desirable to

have tow or more flare systems. The maximum loads for each system needs to take

into account general electrical power failure, cooling water failure, and fire cases.




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 is a very calculation intensive. The two main criteria used in

sizing relief headers is to keep the velocity below 0.7 MACH in any segment and to

limit the velocity at the flare tip to 0.5 MACH. The second main requirement is to

maintain the pressure in the header below the maximum allowable pressure for the

relief valves.




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

Relief Header Sizing - 7 Steps

The above slide describes the seven steps to relief header sizing. When the

maximum vapor relieving requirement of the flare system has been established and

the maximum allowable back pressure has been defined, line sizing reduces to

standard flow calculations.

Since vapors in the flare headers are relieved from a high pressure system to almost

atmospheric pressure, there is an appreciable kinetic energy change throughout the

pipeline. The flow condition is that of compressible flow. The nature of

compressible flow in the case of flare headers may be assumed to be isothermal

since flare lines are normally long and not fully insulated.




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

Selection of a disposal method is subjected to many factors such as the geographic

location of each source, maximum loading for each case (i.e. General Electrical

Power Failure, Cooling Water Failure Case, etc.). The purpose of the flare and

relieving system is to conduct the relieved fluids to a location where it can be safety

discharged and burned.

The disposal system of flares consist of the relief valves, piping, drums and some

type of combustion system. In the past, many relief valves were discharge directly

to the atmosphere. This is still acceptable if environmental regulations permit such

discharges. Therefore, most steam relief valves or ones containing air are relieved

directly to the atmosphere. The relieving vapors from different relief valves and

control valves are normally collected in individual relief subheaders located near

each process area. These headers are interconnected and lead to localized knockout

drums.




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

Flaring is a volatile organic compound (VOC) combustion control process in which

the VOCs are piped to a remote, usually elevated, location and burned in an open

flame in the open air using a specially designed burner tip, auxiliary fuel, and steam

or air to promote mixing for nearly completed (>98 percent) VOC destruction.

Completeness of combustion in a flare is governed by flame temperature, residence

time in the combustion zone, turbulent mixing of the components to complete the

oxidation reaction, and available oxygen for free radical formation. Combustion is

complete if all VOCs are converted to carbon dioxide and water. Incomplete

combustion results in some of the VOC being unaltered or converted to other

organic compounds such as aldehydes or acids. The flaring process can produce

some undesirable by-products including noise, smoke, heat radiation, light, Sox,

NOx, CO, and an additional source of ignition where not desired. However, by

proper design, these can be minimized.

Major factors influencing flare design include gas composition, flow rate, gas

pressure available, initial cost, operating costs, gas temperature, energy availability,

environmental regulations, safety requirements and social requirements. The single

largest factor depends on the site. In the United States, environmental requirements

drive what, if any, type of flare can be installed.




Elevated Flare System

TI LGR

PI

LIAH

PI

ROROEmergencyGas Purge

NormalGas Purge

Purge GasPressure Relief

From Process Units

Steam

Slop To

Slop Tank

PIPC

FuelGas

Gas ToPilot

Grade

Flare Tip

Dry

Seal

FlareStack

Steam Ring

PlantAir

Knockout DrumPumpout Pump

Flare Knockout Drum

Pilot IgnitionSystems Locate

At Flare Knockout Drum

SolenoidValve

(With ManualReset)

Vent

Instrument

Air

Switch

TAH

M

The above drawing is the typical elevated flare system one finds in a refinery. The

relief valves are collected from the process units and are sent to a flare knockout

drum. In the knockout drum, the liquid is removed and the gases are sent to the

flare. The flare is elevated in this case to keep all the burning occurring above the

ground level where radiation and toxic compounds may collect.

At the top of the flare is the flare tip where all of the burning occurs. The steam

ring is shown for smokeless operation, otherwise there would be a lot of smoking

occurring. The dry stack is provide to reduce the purge gas required. Purge gas are

required to maintain a positive pressure in the flare system. If there was not a

positive pressure in the flare system, then a vacuum condition might occur and air

could get into the system. Air is not wanted in the system because an explosive

condition might occur in the header.

In the flare knockout drum, the liquids are separated and collected. Since there is

some value to the liquid, the liquid is normally sent to either the slop oil tank or to

the crude oil tanks for reprocessing. The pilot ignition system is used to keep fuel

gas supply to the pilots and the flame front generator is used to light the pilots.




Elevated Flare System(Knockout Pot in Stack)

FS-R00-02

The above drawing shows and elevated flare system with the knockout drum in the

flare stack. The advantage of this scheme is that it takes up less plot space. In

addition, the knockout drum and the flare stack/tip can be all produced by one

vendor. The disadvantage of the knockout drum in the stack is that the flowrate of

the flare gas is limited because of the limited knockout drum cross-sectional area.

The stack is also limited to a self-supporting type or a derrick type.

The emergency gas purge allows for addition fuel gas to be injected into the relief

header during an upset condition. The emergency gas purge is required if a very hot

release happens. This is because after the release the volume in the knockout drum

and the relief header will decrease because of the cooling occurring in the lines.

Therefore, the emergency gas purge needs to be designed to handle the cool down

of the gases say from 500°F to 100°F in a one hour time period.




Elevated Flare System(Water Seal and Knockout Pot in Stack)

Title Guide

EmergencyGas Purge

NormalGas Purge

PurgeGas

Pressure Relief From Process Units

Steam

Gas ToPilot Grade

FlareStack

LGR

LIAHPI

RORO

PI

PC

FuelGas

Knockout DrumPumpout Pump

Flare Tip

PlantAir

Flare Knockout DrumAnd Water Seal

Water Seal

Water

Pilot IgnitionSystems Locate At

Flare Knockout DrumSlop toSlop Tank

MSolenoid

Valve

(With Manual

Reset)

Vent

Instrument

Air

Switch

TAH

Steam Ring

The above drawing in an elevated flare system with the knockout drum and the

water seal within the flare stack. The water seal is a critical part of the flare system

because it provides a flash back protection in the system. In addition, since the inlet

gas pipe discharges into the water seal at approximately 4 inches below the top of

the water, a back pressure is produced in the relief header and therefore a positive

pressure will be produced in the flare system.

The seal drum can also be used to set up a simple staging system for burners. The

water holdup should be sufficient to provide a 10-foot slug of water in the inlet line

to the drum if an explosion occurs downstream of the drum.

A seal dam is normally provided in each drum to prevent the burner back pressure

from varying the seal level. The seal loops from the drum to the sewer should

provide a seal depth of at least 10 feet. Each drum is provided with a seal water rateof approximately 25 gpm.

The sumerged ends of the inlet lines to the drums are notched to prevent cyclic

release of the gas which accumulates from normal leakage.




Ground Flare System

FS-R00-05

EmergencyGas Purge

NormalGas Purge

Purge Gas Pressure Relief

From Process UnitsSlop To

Slop Tank

Grade

TI LGR

PI

LIAH

PI

M

POPO

MainHeader

PI

PC

PlantAir

Gas ToPilot

StageHeader

FuelGas

PC

Burners

Ground FlareRetention Dike

Knockout DrumPumpout PumpFlare Knockout Drum


Flare Knockout Drum

SolenoidValve

(With ManualReset)

Vent

InstrumentAir

Switch

The above drawing shows a ground flare installed in Mexico. As one can see from

the photograph, the ground flare is installed in a dike enclosure. The person shown

is on top of the dike and is feeling the effects of the radiation. The ground flare in

the photo is at peak rate as can one can see from the total number of burners in

operation.

The ground flare system is different from the elevated flare system in a number of

important areas. One naturally is that the ground flare is located at grade and the

elevated flare is located some distance above grade. The second difference is that

the ground flare requires more pressure at the burner tip. While the elevated flare

system requires only about 1 psi at the flare tip for pressure drop reasons, the

ground flare requires 7 to 10 psi at the burner tip. The high pressure required is that

a ground flare does not require steam to make it smokeless. The ground flare is

smokeless because of the higher pressure at the burner tip causes a lot of turbulenceat the burner tip. This turbulence causes the air and fuel to mix extremely well to

produce a clean burning flame.




Two Stage Flare System(Elevated/Ground)

FS-R00-06

EmergencyGas Purge

NormalGas Purge

Purge Gas Pressure Relief From Process Units

Slop ToSlop Tank

Grade

Water

Seal

PlantAir

Gas ToPilot

Fuel

Gas

Knockout Drum

Pumpout PumpFlare Knockout Drum

TI LGR

PI

LIAH

PI

M

RORO

Water

PI

PC

Seal

Flare

Stack

FlareTip

EnclosedGroundPlane


Flare Knockout Drum

SolenoidValve

(With ManualReset)

Vent

InstrumentAir

Switch

The above photograph shows a two stage flare system. The first stage is the

enclosed ground flare and the second stage is the elevated flare system. The

advantage of this two stage flare system is that the main flaring can take place in the

enclosed ground flare. In this manner, neighbors and people within the refinery are

usually not aware that the refinery is burning any material. This is highly

advantageous for social reasons, environmental reasons, and safety reasons.

The two stage system works by the principle of using two water seals. The water

seal for then enclosed ground flare is typically set at about 4 inches of water column

and the water seal for the elevated flare is set at about 10 inches of water column.

Therefore, the normal flowrate of flare gases is to the enclosed ground flare because

of the lower pressure drop in that system. When the flow rate becomes large, the

pressure drop of 10 inches is exceeded and the flow goes both to the enclosed

ground flare and to the elevated flare.




Two Stage Flare System

FS-R00-04

The above drawing shows a two stage flare system which utilizes an air assisted

flare for the first stage and the elevated emergency flare for the second stage. The

advantage of this system is cost compared to the two stage system which utilizes the

enclosed flare. The operation is similar to the two stage flare system which utilizes

the enclosed ground flare.

The main flare flow gases will go to the elevated air-assisted flare by means of the

water seal. The water seal will be utilized only for the elevated emergency gas

flare. The air-assisted flare will be not utilizing the water seal and the flow of flare

gas will go directly to that flare. Once the water seal differential is exceeded in

pressure drop, the emergency gas flare will open to allow both flare to be in

operation during peak flow rates.

The advantage of this system is that the main burning of flare gases occurs in theair-assisted flare and not the emergency flare. Large diameter flares (such as the

emergency gas flare above) have a difficult time burning low flows. The low flow

results in burning inside the flare tip which can result in premature failure of the tip.

By allowing the main burning to occur in the small diameter air-assisted flare,

premature failure of the tip normally does not occur.




Flare Stack

Structure

– Self Supporting

– Guy Supported

– Derrick Type

For safety reasons, a stack is used to elevate the flare. The flare must be located so

that it does not present a hazard to surrounding personnel and facilities. Elevated

flares can be self-supported (free-standing), guyed, or structurally supported by a

derrick. Self-supporting flares are generally used for lower flare heights but can be

designed up to 250 feet.

Free-standing flares provide ideal structural support. However, for very high units

the costs increase rapidly. In addition, the foundation required and nature of the soil

must be considered.

Derrick-supported flares can be built as high as required since the system load is

spread over the derrick structure. This design provides for differential expansion

between the stack, piping, and derrick. Derrick-supported flare are the most

expensive.

Guyed wire supported flares is the simplest of all the support methods. However, a

considerable amount of land is required since the guy wires are widely spread apart.




Support Structure Self-Supported

Stack less than

100’ (30M)

Tight plot area

Self-

Supported

The self-supporting structure is usually the best construction for under 35 meters. It

requires little plot area, is relatively simple to erect, and has a reasonable capital

cost. For higher stacks, the self-supporting has increased material thickness and

will greatly increase foundation requirements and normally raise the cost above the

alternatives.

Self-supporting stack should be used as follows:

• Stacks less than 100 feet

• Tight plot space

• Height less than 350 feet

• When liquid carryover is likely

• When an integral knockout drum is specified.




Self-Supportive Structures

Description

Self-supported flare

stack is utilized for

structures from 20

to 350 feet tall (6 to

100 meters).

Usually this design

has the lowest

installed cost and

requires the smallest

plot area.

When to Use

• Stacks less than

100 feet (30

meters)

• Tight plot area

• When liquid

carry-over is

likely

• When integral

drum is specified

When Not to Use

• Stacks taller

than 350 feet

(100 meters)

• Cost sensitive

applications

greater than

100 feet (30

meters

The self-supported flare stack is utilized for structures from 20 to 350 feet tall.

Usually this design has the lowest installed cost and requires the smallest land area.

Self-supporting stack should be used as follows:


• Tight plot space

• Height less than 350 feet

• When liquid carryover is likely

• When an integral knockout drum is specified.

Self-supporting stacks should not be used for:

• Stacks taller than 350 feet• Cost sensitive applications greater than 100 feet




Support-Structure Derrick

Derrick

Stacks over 250' (75M)

Tight plot area

Gas temperature over

450 F(232 C)

When plot space is a concern, a derrick structure is normally used. In addition,

when thermal expansion is a problem, derrick structures are also used. Derricks

typically require increased capital investment and foundation requirements

compared to a guyed stack. The derrick structure will typically be double the price

of guyed wire support. They also require more erection time because the derrick

must be built on-site.

Derrick supported flare stacks are used for structures from 150 to 550 feet tall.

They are used in the following application:

• When plot space is tight

• On stacks over 250 feet tall

• When gas temperatures are over 450°F

• When the customer strongly prefers the derrick structure• With offshore systems




Derrick Structures

Description

Used for

structures from

150 to 550 feet

tall (45 to 266

meters).

Relatively easy

to erect and has

superior

strength when

assembled.

When Not to Use


250 feet (75

meters)

• Cost sensitive

applications

When to Use

• Plot space is

tight

• Stacks over 250

feet (75 meters)

• Gas

temperatures

are over 450

F

(232

C)

• With Offshore

systems

Derrick supported flare stacks are used for structures from 150 to 550 feet tall.

They are used in the following application:

• When plot space is tight

• On stacks over 250 feet tall

• When gas temperatures are over 450°F

• When the customer strongly prefers the derrick structure

• With offshore systems

They should not be used for the following conditions:

• Stacks less than 250 feet tall

• Cost sensitive applications.

They are usually modular in design for ease of erection. One vendor uses 39 foot

sections which can be assembled at grade then lifted for erection. The three leg

design reduces the number of pieces which reduces the erection costs.




Support-Structure(Guyed)

Guyed

Stacks over 250'(75M)

Low capital cost

The simplest and generally most economical design is the guyed support. Capital

cost and foundation requirements are minimal. However, the guy wires can cause

some problems. A significant plot area is often required to accommodate the wires.

The radius is generally equal to two-thirds the height for most guyed flares but can

approach 80% of the height for very tall flares (on the order of 150 meters).

Thermal expansion of the stack is another problem to consider. A high design

temperature such as 300°C will require some method to accommodate thermal

expansion. Some vendors can provide this through innovative stack and guy wire

design. However, a more practical solution is to enclose the flare pipe in a structure

that can support the horizontal load yet allow freedom to move vertically.




Guyed Structures

Description

Guy wire support

flare stacks are

typically the

lowest material

cost system, but

they require the

largest plot area.

Used in systems

from 100 to 700

feet tall (30 to 213meters).

When to Use

• Stacks over 100

feet (30 meters)

• Radius equal to

stack height

available for guy

wires

• Low capital cost

is required

• Liquid carry over

is unlikely

When Not to Use


100 feet (30

meters)

• Tight plot area

• Liquid carry

over likely

Guyed wire supported flare stacks are typically the lowest material cost system, but

they require the largest plot area. Used in systems from 100 to 700 feet tall.

Guyed supports are normally used as follows:

• Stack height greater than 100 feet

• Radius equal to stack height available for guy wires

• Low cost important

• Liquid carryover is unlikely

Guyed supports should not be used in the following application:


• Tight plot spaces• Liquid carryover is likely




Radiation Effects

Solar Radiation on Earth ~ 300 Btu/hr-ft 2

Most of us learned about radiation from the effects of the sun. We found that by

staying in the sun we feel relatively warm and by going into shade we will get

cooler. Some have even learned the hard way about getting a sun burn with is

impart due to the solar radiation of the sun.

A flare stack’s height reduces the intensity of radiant heat at grade level, to protect

personnel and equipment. This intensity is often calculated by assuming that heat

radiates uniformly and spherically from the center of the flame.

Emissivity is defined as the fraction of the available chemical energy (calculated as

the mass flow times the lower heating value of the flared gas) that is radiated as

heat. As the flame, about 0.4 of this energy actually radiates as heat. However, as

this energy radiates spherically, it decays inversely with the square of the distance it

travels, and some of it is absorbed by the atmosphere.

After the heat has radiated 100 meters, the apparent emissivity is more like 0.15,

assuming spherical radiation and neglecting all other factors. The wide range of

measured radiation efficiencies (from 0.05 to 0.35) and diverse statements from

vendors reflect the complex dependence of emissivity.




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

The radiation source is generally though of as being located halfway up the flame.

A common approach to the prediction of the flare flame radiation to a point on or

near garde is simplify the geometric problem by assuming the flame has a single

radiant epicenter and to use a simple factor to cover a number of radiative heat-

transfer variables.

The radiant heat intensity to a unit area aligned for maximum reception can be

expressed in the above formula. Most of the published radiant intensity prediction

methods use the lower heating value (LHV) of the gas in calculating the heat release

rate, Q. In order to determine the line-of-sight distance form the radiant center to

the point of interest, you need to locate the center. This requires an understanding

of flame length and the path of the flame follows as it leaves the flare burner.

API 521 “Guide for Pressure-Relieving and Depressing Systems” presents a methodof calculating fraction of heat radiated. Flame length is determined as a function of

heat release. Flame lean is determined by a momentum relationship between the jet

velocity and the wind velocity. The radiant center is given a somewhat unrealistic

location at the center of a straight line drawn between the end of the flame and the

flare tip. An intensity of 4.73 kW/m2 (1500 Btu/h-ft2) is recommended for areas

where emergency actions lasting several minutes may be required.




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

The above formula determines the heat release from a flare. Factors to consider

regarding thermal radiation levels is that clothing provides shielding, allowing only

a small part of the body to be exposed to the full intensity. In the case of radiation

emanating from an elevated flare, standard personnel protection, such as hard hats

can reduce thermal exposure. A level of 5000 Btu/hr-ft2 (15.77 kW/m2) is the heat

intensity on structures and in areas where operators are not likely to be performing

duties and where shelter from radiant heat is available such as behind radiation

shields.

The conservative design approach used mostly ignores wind effects and calculates

the distance assuming the center of radiation is the base of the flame (at the flare

tip), not in the center.

The total emissivity of the flame is the sum of the emmissivities of the gases andsolids present in the flame. Combustion of hydrocarbon gas results in soot and

smoke. If soot is burned completely within the flame, it becomes Carbon dioxide.

If it does not burn completely, it leaves the flame envelope as smoke.

To determine gas emissivity of a flame is difficult. For gases within an enclosure

such as a fired heater, one can make a fairly accurate estimate of the gas

composition, thickness and temperature. For a flare it is more difficult




Radiation Theory

Exposure Times Necessaryto Reach the Pain Threshold

Times to Pain Threshold

(Seconds)

60

40

30

16

9

6

4

2

Radiation Intensity

Btu/hr-ft2 Kilowatts per M2

550

740

920

1500

2200

3000

3700

6300

1.74

2.33

2.90

4.73

6.94

9.46

11.67

19.87

Threshold of Pain

Safe Limit440 Btu/(hr) (ft)2

7

6

5

4

3

2

1

010 20 30 40 50 60

Exposure Time, Sec.

What level is acceptable for radiation. An acceptable level under one set of

conditions may not be acceptable under other conditions. Frequently, one radiant

intensity level is allowed for people, whereas another higher level is permitted for

equipment. When discussing the allowable level for people, it is common to tie the

level to some time period of exposure. Different levels are usually specified for

plant personnel and the public.

A heat intensity of 4.73 kW/m2 (1500 Btu/h-ft2) for several seconds (16 seconds).

This 16 seconds allows for situations where a worker is infrequently in the flare area

or must go into the flare area briefly to take some action. This level is also

acceptable when the exposure is more general but the maximum flaring event itself

is of limited duration.

The chart above shows the exposure times necessary to reach the pain threshold.The point where there is a safe zone is at 440 Btu/h-ft2. A related issue is how much

importance should be given to solar radiation in establishing the design radiant heat

intensity levels. To be additive, the worker must be aligned with the flare and the

sun in such a manner that the two exposures are truly additive. The above chart

does not take into account solar radiation.




Contours of Radiant Heat Intensity

Protection

Required for

Equipment

Boundary for Radiant Heat Intensity

(3000 Btu Hr/Sq.Ft.)

Protection

Required

for Personnel

Boundary

for Radiant

Heat Intensity

(1500 Btu/Hr/Sq.Ft.)

- Normally Fenced in with

Warning Signal

Safe Boundary

(440 Btu/Hr/Sq.Ft.)

It is important that the emissivity of the flame not be confused with the fraction of

heat radiated. The emissivity of a flame is a function of the combined emissivities

of the gases and solids in the flame at a certain temperature. The fraction of heat

radiated, on the other hand, is an overall characteristic of a flame that accounts for

gas composition, flame type,l state of fuel/air mixing, soot and smoke formation,

quantity being burned, flame temperature and flare burner design. The fraction of

heat radiated is determined empirically and must be used in the same manner that it

was determined.




FlareTip

Environmentally

acceptable

combustion

Tips normally

proprietary in

design

Flame Stability

Ignition

Reliability

Exit Velocity 1 to

600ft/s (.3 to 183

m/s)

Exit velocity at

50% of sonic

velocity

Multiple Pilot

Burners

Surrounding

Windshield

The flare tip assembly shall be complete with pilots, pilot flame sensors, noise

muffler, steam manifolds and stem jets. The flare tip shall also include all required

pilot gas manifolds and steam distribution headers, rings, manifolds and runners for

complete smokeless operation. The upper 10 feet of the flare tip shall be 310

stainless steel minimum.

The lower half shall be 304 stainless steel minimum. The flame stabilizers shall be

310 stainless steel. The noise muffler shall be refractory lined carbon steel. If

refractory lined tips are furnished, the detail of the lining and method of attachment

to the flare tip shall be shown.

Flare stack diameter is generally sized on a velocity basis, although pressure drop

should be checked. A maximum velocity of up to 0.5 Mach for peak, short-term,

infrequent flow, with 0.2 Mach maintained for the more normal and possibly morefrequent conditions for low-pressure flares.

Pressure drops of 2 psig (14 kilopascals) have been satisfactorily used at the flare

tip. Too low a tip velocity can cause heat and corrosion damage. The burning of

the gases becomes quite slow, and the flame is greatly influenced by the wind.




Flare Tip DesignConsiderations

– Design for maximum

flow rates

– Design for maximum

temperatures

– Design for wind

conditions

– Design for minimum

flow rates

Flare Tip Design

The above flare tip is one that was placed in the UK with very high wind loads.

This specific flare tip only lasted 3 years before it had to be replaced. In general,

flare tips last up to 10 years before they need to be replaced. With respect to life of

the flare tip, a failure mode which have been observed involves cracking of the shell

of the flare burner. This cracking invariably propagates from the end of a fillet weld

on brackets and attachments welded to the shell of the flare tip. During bucking of

the flare tip which results from the uneven temperature differentials of flame

impingement, the stresses concentrate at the stress riser cause by the weld and

propagates a crack through the shell. To avoid this type of problem, plug-welded

brackets on the flare tip are used. This complete circle plug-welded approach

eliminates any stress riser with its potential for cracking.




High and Too Low Relief Flow RatesCan Cause Flame Instability

a. Flame Dip b. Flame Blowoff c. Analysis Of Flame Dip

Air Aspiration

Pipe

Methane

V

D

Air

Air

Intrusion

The exit velocity, heat radiation and sonic velocity for flares is critical.

Sonic Velocity = 233(Heat radiated,(Btu/hr-ft2)*Gas temp(R)/MW)^0.5

Exit velocity f/s = (MACH)*(Sonic Velocity)

Flame stability is extremely important in flare tip design. A flameout does not

appear to be a real problem at either low or high venting flowrates. As log flow rate

velocities air intrudes into the top of the stack. If the flow rate is sufficient to

produce a flame visible from the ground, air intrusion usually is not significant. At

low velocities, combustion can take place inside the flare. When the slow upward

flow of a lighter-than-air gas permits air to flow downward along the stack wall, a

flame dip occurs. A diffusion flame propagates into this region and is quenched at

the wall, then air flows downward again, causing another flame dip, and the cycle

repeats it self. If the velocity of the relief gas rises until it exceeds the flamevelocity at every point, the flame will be lifted to another stable position above the

flare tip. This is called blowoff with flame extinguishment is called blowout.

Turbulence promoting flame holders and pilots effectively stabilize flames at high

gas velocities.




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 waterdroplets

The use of the term “flare efficiency” can be somewhat misleading because of the

wide variety of definitions that have been used in the past. The most rigorous and

universal definition of efficiency is the “Combustion Efficiency” which for a

hydrocarbon flare, is simply the mass of carbon in the form of carbon dioxide which

is produced by the flare divided by the mass of carbon in the form of fuel entering

the flare.

The “Hydrocarbon Destruction Efficiency” can also be useful in characterizing flare

performance. It is defined in a somewhat different manner as amount of

hydrocarbons entering the flare in the flare gas minus the amount of unburned

hydrocarbons leaving the flare all divided by the amount of hydrocarbons entering

the flare.

This is the fraction of hydrocarbons destroyed by the flare. It should be noted thatthis definition does not consider the form of the chemicals into which the

hydrocarbons are converted via combustion.




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

Even the best designed pilots will occasionally go out, some method should be

available for remote lighting of the pilots. In the past, methods such as a flaming

arrow, a burning rag wound around a rock, and flame guns were used with moderate

success. Modern techniques include flame front generators, electrical “spark plug”

type igniters, and aspirated air igniters. Continuous pilot burners on the flare tip

ensure ignition. The pilot burners themselves are commonly ignited by a flame

front generator system which is activated manually from a remote location. Fuel

and air are mixed upon entering the igniter tube. A spark ignites the mixture and a

flame front rushes up the tube to light the pilot. To ensure ignition regardless of

wind direction, three continuous pilots are usually spaced equally around the flare

tip. Weather shields on the pilot nozzles prevent blowout by strong wind.

Because it is difficult to see whether the pilot burners are lit in the daytime,

thermocouples on the pilot activate an alarm system to warn of a pilot flame failure.The fuel gas supply to the pilots and igniters must be clean and reliable. Low

pressure alarms are often installed to warn operators of the loss of fuel to the pilots.

Prevention of hydrate formation in the long small-diameter fuel piping needs to be

prevented. A knockout pot is normally installed after that last pressure reducer.




Pilot Burners

Automatic systemsmay be activated by:

– Thermocouples

– Infrared Sensor

– Ultraviolet Sensor

(ground flare

application)

EPA regulations require the presence of a continuous flame. Reliable ignition is

obtained by continuous pilot burners designed for stability and positioned around

the outer perimeter of the flare tip.

The pilot burners are ignited by an ignition source system, which can be designed

for either manual or automatic actuation. Automatic systems are generally activated

by a flame detection device using either a thermocouple, an infra-red sensor or,

more rarely, (for ground flare application) an ultra-violet sensor.




Installation of Thermocouples

Correct Installation Incorrect Installation

The above picture shows a correct and incorrect installation of a thermocouple. The

correct installation has the thermocouple at the interior of the pilot where as the

incorrect installation is on the outside. By placing the thermocouple on the outside,

there is a chance of an incorrect reading.

The pilot flame shall always be present at the burner tip. Each pilot shall have a

thermocouple attached to determine the flame temperature. Pilot flame failure can

be sensed by local or remote monitoring. Thermocouple failure can cause a

problem if the flare is unavailable for maintenance over long periods of time.

Sometimes two thermocouples are mounted on each pilot with separate leads

running down the stack. This allows for switching of the active thermocouple by

simply swapping the leads.




Pilot Windshield

Allows pilot tooperate at wind

speeds greater

than 100 mph

Should always be

specified

Prevents

misreading of the

thermocouples

Pilots shall be able to withstand the effects of rain, snow, and wind speeds greater

than 100 miles per hour. They must also stand up to the high temperature caused by

flaring.

Pilot windshield design, although often overlooked, is extremely important.

Without an effective design the risk of an inoperable flare is a real danger. Even if

the flame is not extinguished by high winds, it may be directed away from the flare

and rendered useless.

To prevent this hazardous condition from arising, the number of pilots used on a

flare should be increased as the size of the flare is increased. Large flares require

the use of several pilots to assure ignition in any wind direction. The size and

number of pilots is determined by the size, design, and function of the flare, and the

heat level of the waste gas.




Flame Front Generator Ignition System

E

DAir

Gas To Pilot #3

Gas To Pilots

B A

C

H

J

F

To Pilot #2

To Pilot #1

The above drawing show a flame front generator ignition system. They work by

introducing a flammable mixture of air and gas into a 1 inch pipe leading up to the

pilot. Once this mixture is distributed throughout the length of the pipe, it is ignited

by an electric spark plug. A flame front will then travel through the pipe up to the

flare tip, lighting the pilot. Many flame front generators are designed to light a flare

as far as one mile away.

The following is a typical operating instruction for using flame front generator

ignition system:

• Open the air valve and set air pressure at 10 psig

• Open the gas valve and the gas pressure at 10 psig.

• Purge lines for two to three minutes, depending on distance from flame front

generator to flare pilot tip.• Spark to light the mixture.

• If nothing happens, purge and spark again.

• If nothing happens, reduce the air pressure, and repeat purging and sparking.

• If still nothing happens, lower the air pressure further in several steps and

purge and spark again.

• If still nothing happens, try higher air pressures.

Pressure settings depend on the gravity of the fuel gas.




Flare Control Panel

Flare ControlPanel includes

the following:

– Pilot Gas

– Steam Control

– Pilot Ignition

System

The above photograph shows a flare control panel which contains the pilot gas

control the steam control and the pilot ignition system for the flare system.

Flare system control can be completely automated or completely manual.

Components of a flare system which can be controlled automatically include the

auxiliary gas, steam injection, and the ignitions system. Fuel gas consumption can

be minimized by continuously measuring the vent gas flow rate and heat content

(Btu/scf) and automatically adjusting the amount of auxiliary fuel to maintain the

required 300 Btu/scf for steam-assisted flares.

Steam consumption can likewise be minimized by controlling flow based on vent

gas flow rate. Steam flow can also be controlled using visual smoke monitors.

Automatic ignition panels sense the presence of a flame with either visual or

thermal sensors and reignite the pilots when flameout occurs.




Pilot Gas Requirement

Flare Tip Diameter (IN) Number of Pilot Burners (N)

1-10

12-24

30-60

>60

1

2

3

4

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:

Pilot gas is critical for flare application. The average pilot gas consumption based

on new energy efficient model is approximately 70 scf/hr. Therefore, as shown in

the above table, the annual pilot gas consumption in Mscf/yr is based on 613 times

the number of pilot burners.

The table shows that under a ten inch flare tip only one pilot burner is required.

From 12 to 24 inches, two pilot burners are required. Over 24 inches to 60 inches,

three pilot burners are required. Over 60 inches, at least four pilot burners are

required.




Multiple Pilots

Multiple pilots allow

one pilot to fail

Most flares have two

to four pilots

Equally spaced

around the flare

The above drawing shows a three pilot arrangement. This allows one pilot to fail

and have the other two in operation. The pilots in this case would be equally spaced

around the flare. Therefore, they would be space at 360 degrees divided by 3 or at

120 degrees from one another.

EPA regulations require the presence of a continuous flame. Reliable ignition is

obtained by continuous pilot burners designed for stability and positioned around

the outer perimeter of the flare tip. The pilot burners are ignited by an ignition

source system, which can be designed for either manual or automatic actuation.

Automatic systems are generally activated by a flame detection device using either

a thermocouple, an infra-red sensor or, more rarely, (for ground flare application) an

ultra-violet sensor.




Safety Aspect

Two of the three elements for explosion are

always present in a flare system

Fuel

Oxygen Ignition

Stacks shall be purged with fuel gas to ensure safe flare operation against explosion

and detonation. A question asked is how much purge gas is required? The answer

is important because of the cost to purge a flare which may run a refinery $50,000

per year.

Combustion can be defined as the rapid chemical combination of oxygen with the

combustible elements of fuel. There are just three combustible chemical elements

of significance - carbon, hydrogen, and sulfur. Sulfur is usually of minor

significance as a source of heat but it can be major significance in corrosion and

pollution problems.

The objective of good combustion is to release all of this heat while minimizing

losses from combustion imperfections and superfluous air. The combination of the

combustible elements and compounds of a fuel with all the oxygen requirestemperature high enough to ignite the constitutes, mixing or turbulence, and

sufficient time for complete combustions.




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)

Air or oxygen in a flare system is extremely dangerous. For this reason, flare

systems should be purged. Dry seals are used to reduce the rate of purge gases

required to keep air out of the system. The total volumetric flow to the flame must

be carefully controlled to prevent low flow flashback problems and to avoid flame

instability. Purge gas, typically natural gas, N2 or CO2, is used to maintain a

minimum required positive flow through the system. If there is a possibility of air

in the flare manifold, N2 or another inert gas must be used to prevent the formation

of an explosive mixture in the flare system. To ensure a positive flow through all

flare components, purge gas injection should be at the farthest upstream point in the

flare transport piping.

The minimum continuous purge gas required is determined by the design of the

stack seals, which are usually proprietary devices. Modern labyrinth and internal

gas seals are stated to require a gas velocity of 0.001 to 0.04 ft/s (at standardconditions). Using the conservative value of 0.04 ft/s and knowing the flare

diameter (in), the annual purge gas volume, F pu, can be calculated:

F pu (Mscf/yr)

= (0.04 ft/s)(πD^2/4/144 ft2)(3,600 sec/hr)(8.760 hr/yr)= 6.88D^2 (Mscf/yr)




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

There is another minimum flare tip velocity for operation without flashback or

instability. This minimum velocity is dependent on both gas composition and

diameter and can range from insignificant amounts on small flares to 0.5 ft/s on

greater than 60-inch diameter units.

Purge gas is also required to clear the system of air before startup, and to prevent a

vacuum from pulling air back into the system after a hot gas discharge is flared.

(The cooling of gases within the flare system can create a vacuum.) The purge gas

consumption from these uses is assumed to be minor.




Dry Seals

Molecular Seals

Double Seals

Fluidic Seals

Airrestors

Air in the flare stack is a serious problem. There are two common types of flare dry

seals:

• Diffusion type - (Molecular Seal, Double Seal)

• Velocity Seal - (Fluidic Seal, Airrestor)

These are located just below the flare tip and are used to reduce the amount of purge

gas required to prevent air from getting back into the stack. The molecular seal and

double seal uses the difference in molecular weights of the purge gas and the air to

form a gravity seal which prevents the air from entering into the stack. An inverted

bucket forces the incoming air through two 180 degree bends before it enters the

flare tip. If the purge gas is lighter than air, the purge gas will accumulate in the top

of the seal and prevent the air from infiltrating the system.

The velocity seal, which includes the fluidic seal and the airrestor, works under the

premise that infiltrating air enters through the flare tip and hugs the inner wall of the

flare tip.




Liquid

Drain

Molecular Seal

Flare

Assembly

Prevents explosions

Prevents entry of air

Reduces purge gas

Performs silently with

small pressure drop

Molecular Seal

The molecular seal has a baffled cylinder arrangement which forces the incoming

air through two 180 degree bends one bend up and one bend down before it can

enter into the flare stack. If the purge gas is lighter than air, the purge gas will

accumulate in the top of the seal and prevent the air from getting into the system. If

the purge gas is heavier than air, the purge gas will accumulate in the bottom of the

seal and prevent air from getting into the system.

The molecular seal reduces the purge gas velocity required through the tip to 0.04

feet per second. Some purge gas composition the rate will limit oxygen levels to

below 0.1%. These low purge rates do not prevent burnback inside the flare tip,

which will result in short tip life. This effect will deteriorate the metal wall of the

flare tip of the molecular seal and is hidden until the flame burns through the tip or

seal. This will result in the tip or seal to be replaced and cause shutdown for

immediate maintenance.

Molecular seals are purged at 0.4 feet per second to keep the flame out of the flare

tip and insure proper flare life.




Outlet To Flare Burner

Clean-Out

Double Seal

When air enters a stack, convection (not diffusion, which is much too slow) is

almost certainly the controlling process. How much purge will prevent the entry of

air depends on the type of dry seal installed.

The above picture is that of a double seal. The double seal is similar in design

principle as that of the molecular seal. Both designs work on the basis of density

differences.

The flare gases are required to go up the double seal and then make a 180 degree

reversal then go down a distance and then make another 180 degree reversal before

being sent to the flare tip.

These 180 degrees reversals prevents the air from getting back into the flare

headers. If air does get into the double seal, most of the air will be stuck at the pointnear the clean-out zone.




Flare

TipAir Air

Flow PathOf Flare

Gas

Fluidic Seal

The velocity seal or fluidic seal works under the premise that incoming air enters

through the flare tip and hugs the inner wall of the flare tip. The velocity seal is a

cone shaped obstruction with single or multiple baffles as shown in the above

picture.

These baffles force the air away from the wall where it encounters the focused

purge gas flow rate and is swept out of the tip. This seal reduces the purge gas

velocity through the tip to 0.04 feet per second. This rate keeps oxygen

concentrations below 4 to 8% oxygen. Without the velocity seal, the velocity in the

stack would have to be increased.




Smoking

Smokeless Operation

Smokeless Flare Operation

Smoke is generated from fuel that has a carbon to hydrogen ratio greater than 35%

by weight. The principle of the steam-assist flare is to reduce the formation of

smoke by reducing the carbon to hydrogen ration. This can be achieved by using a

cented steam feed and steam nozzles at the flare tip. The center feed is used to

provide a wide range of flaring rates while the steam nozzles agitate the gas

turbulently to achieve a good mix with air. The minimum allowable heating value

for all types of assist flare is 300 Btu/scf. The majority of today’s flares are steam-

assist, because of their significant advantages. First, they have been proved to be

successful in many years of operation. Next, when plants undergoes expansion, the

system can be expanded as well without having difficulty to maintain smokeless

emissions. Steam-assisted flare are extremely useful in variable flow rates of

flaring. Some disadvantages are that there is a cost of steam supply as well as the

capital cost involved.

Air assisted flares employs the same principles as the steam assisted flare but is

normally more costly. The design of the air assist flare is complicated because the

design features as stack inside a stack. Waste gas goes inside the outer stack, while

air is force through the inner stack. The amount of operational purge gas required

for air assist flares is typically 50% higher than the steam assisted flare.




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

Steam Requirements and Smoke Suppression Methods

Smoke free operation of flares can be achieved by various methods, including steam

injection, injection of high-pressure waste gas, forced draft air or distribution of the

flow through many small burners. The most common type involves steam injection.

The amount of steam required for smokeless burning will depend on the maximum

vapor flow at which smokeless burning is to be achieved and the composition of the

mixtures. The composition involves both the percentage of unsaturated and the

molecular weight.

The higher the molecular weight of a hydrocarbon, the lower the ratio of steam to

carbon dioxide and the greater the tendency to smoke. The amount of steam that

must be injected to maintain a constant steam to carbon dioxide ratio as molecular

weight increases can be calculated in the above formula.

The steam can be injected through a single pipe nozzle located in the center of theflare or through a series of steam/air injectors in the flare through a minifold located

around the periphery of the flare tip or in any combination of those. The steam

injected into the flame zone to create turbulence and aspirate air into the flame zone

by the steam jets. This improved air distribution combined with the steam gas shift

interaction reactors more readily with the flare gases to eliminate fuel rich

conditions which results in smoke formation.




50

40

30

20

10

0.4 0.3 0.2 0.1

H/C Ratio by Weight

P e r c e n t o f C a r b o n E s c a p i n g a s B l a c k S

m o k e

Tendency to Produce Black Smoke

As can be seen from the above chart, the lower the Hydrogen to Carbon (H/C)

ration is, the higher the percent of carbon escaping as black smoke. For example, a

H/C ratio of 0.2 results in 19% of carbon escaping as black smoke. Water or steam

reduces smoke formation by the way the steam separates the hydrocarbon

molecules, thereby minimizing polymerization and forms oxygen compounds that

burn at reduced rates and temperatures that are not conductive to polymerization

and cracking. Other theories project that the water vapor reacts with the carbon

particles to form carbon monoxide and carbon dioxide and hydrogen thereby

removing the carbon before it cools and forms smoke.




Controller Control Scheme

Flux DensitySignal

Monitor

Field Of View

Steam Nozzles

Steam ControlValve

Automatic Steam Control

Automatic control is the optimum way to minimize steam consumption, supplying

only the required amount to keep the flame smokeless at a particular waste gas

flowrate and also at atmospheric conditions. Excessive steam can produce burning

back into the flare tip, besides increasing flare noise and wasting steam. Steam

injected into the combustion zone also cools the combustion temperature and at a

high excess condition can reduce combustion efficiency.

The picture above shows that steam is controlled by flame appearance. An optical

unit is calibrated to a particular frequency in the infrared spectrum, ensuring

smokeless flaring over the range of flowrates. The system provides a continuous

output signal for control for the steam valve. The sending head detects the changes

of hot carbon flux density and generates a signal, modulating the automatic steam

control valve flowrate only allowing the right amount of steam to make it

smokeless. With this fast response system, the detector is remote and not in contactwith the flare gases. The control is independent of the gas flow rate. In addition,

the system is very convenient since the components are located at grade.




Automatic Steam Control

Minimizes steam

consumption

Controlled by the flame

appearance

Calibrated to a

particular frequency in

the infrared spectrum

Manual control of steam involves remote operation of a steam valve by operating

personnel assigned to a unit from which the flare is readily visible. This method is

satisfactory if short-term smoking can be tolerated when a sudden increase in flaring

occurs. With a manual arrangement, close supervision is required. Another method

of controlling steam is using television monitoring with manual control. A feed

forward control system pressure, mass flow or velocity can be used to control steam.

By measuring the amount of flare gas flowing to the flare, the steam rate can be

automatically adjusted to compensate for rate changes. This system would not be

acceptable if there was a lot of changing in the molecular weight say from going

from butane to paraxylene. The feedback system using an infrared sensor as the one

shown above. Infrared sensors are used to detect smoke formation in the flames and

automatically adjust the steam control valve to compensate. One disadvantage of

the feedback system is that the infrared waves are absorbed by moisture, and the

resultant feedback signal is reduced in raining or foggy conditions.




Wind

Less Than 50 psig Steam

Steam provide is usually MP steam (150 psig), special designs are available for

utilizing slightly lower pressure steam. The major impact of lower steam pressure

is a reduction in steam efficiency during smokeless turndown conditions. In very

cold climites condensation may enter the flare header and collect and may freeze

therefore, drainage of any condensate which may collect is critical.

Many refineries because they have a lot of LP steam (50 psig) they believe that by

switching from MP to LP will be better. This is not the case and can cause a lot of

problems. By using LP steam, any wind that is at the flare tip will cause the steam

not to be injected in the correct location. Therefore, much more steam will be

required to make the flare smokeless. In addition, depending on the way the wind is

blowing, the steam may not even reach the flare tip. Always try to use MP steam

for smokeless operation for flare tips.

If the steam is introduced at pressures below 10 psig, the desired turbulence or air

entrainment will not be achieved due to insufficient momentum. The use of too

much steam can also cool the flame too much and extinguish it. The flame may, in

these event be reignited by hot zones and extinguishment by the steam again, at

millisecond intervals.




Flame Arrestors

Liquid Seals

Flashback Protection

Flame arrestors are passive devises with no moving parts. Liquid seals are active

devices with water as the protection. The flame arrestor prevents the propagation of

flame from the exposed side of the unit to the protected side by the use of wound

crimped metal ribbon type flame cell element. This construction produces a matrix

of uniform openings that are carefully constructed to quench the flame by absorbing

the heat of the flame. This provides an extinguishing barrier to the ignited vapor

mixture.

Under normal operating conditions, the flame arrestor permits a relatively free flow

of gas or vapor through the piping system. If the mixture is ignited and the flame

begins to travel back through the piping, the arrestor will prohibit the flame from

moving back to the gas source.

Flare arrestors have specifically designed heat transfer characteristics for slowmoving flames and low to medium pressure fronts. But flames moving at higher

velocities and carrying higher pressure fronts can pass through a standard inline

flame arrestor.




Flare vapor pipingsubmerged

approximately 4 to

12 inches below

the water level

Effective means to

stop a flame front

Liquid Seals

The most effective seal is the liquid seal. A liquid seal is simply a head of liquid

(usually water) within the gas collection header that physically prevents air from

intruding into the system. The liquid head prevents air from going into the system

by preventing a vacuum. The liquid seal does require maintenance and utility

requirements. Make-up water must be available at all times. In addition, freezing

must also be considered. Normally, an overflow is provided to minimize the effects

of hydrocarbon which settles out of the vapor.

Depending on the effluent, a surge drum and pump may be necessary at the flare

base to dispose of the seal liquid properly. The seal drum should be located between

the stack and the other header drums and as close to the flare stack as possible. A

variation of a seal drum is sometime incorporated into the base of the flare stack as

shown in the above diagram.

Continuous removal or intermittent skimming of hydrocarbons that may accumulate

should be considered. If the hydrocarbons are not removed, there is a potential that

the inlet piping may get plugged because of the hydrocarbons.




Liquid Seal

Submerged Weir

Welded On End

Of Flare Line

Water Level

To Flare

Water

SupplyFI

Flare Header

Try Cocks For Checking Hydrocarbons

Vent

To Sewer

6" (15 cm)

Baffle

Drain

4" (10 cm) 1 0 F t . ( 3 M ) M i n i m u m

S e w e

r S e a l S h o u l d B e

D e s i g n

e d f o r a M i n i m u m

o f 1

7 5 % o f D r u m ’ s

M a x i m u m O p e r a t i n g

P r e s s u r e

The above liquid seal is of the horizontal type. A minimum of 10 feet is required

from the water level to the centerline of the outlet of the flare header. The problem

of surging in seal drums can be minimized by the use of a submerged weir with V-

notches on the end of this pipe. The V-notch provides an increased flow area to the

increasing gas flow. This V-notch type principle is similar to that of the bubble cap

trays. Some design details of the liquid seal include anti-swirl or anti-vortex

baffles on the liquid outlet lines. In addition, internally extended liquid outlet

nozzles should be used because sediment will settle out in the drums and not in the

low spot in the lines. This is shown in the above diagram with a 6 inch space

between the top of the sewer outlet and the bottom of the drum.

Designing the vessel nozzles, attachments, supports, and internals one should

consider shock loading that result from thermal effects, slugs of liquid or gas

expansion. Try-cocks as shown in the above diagram are useful in liquid leveldetection instead of level gauges. Instruments should be the simplest and most

rugged available and should be easily maintained (externally mounted and valve).

The use of seals instead of valves and of valves instead of traps is preferred,

primarily because of the nature of the materials handled and the conditions under

which these components must operate.




Stop flame propagationwithin 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

Flame Arrestor

Extended lengths of pipe allow the flame to advance into more severe states of

flame propagation such as high pressure deflagrations and detonations. Bends in

piping, pipe expansions and contractions, valves or flow obstruction devices of any

kind, cause turbulent flow.

Turbulent flow enhances the mixing of the combustible gases greatly increasing the

combustion intensity. This can result in increased flame speeds, higher flame

temperatures, and higher flame front pressure than would occur in laminar flow

conditions. High pressure deflagrations and detonations can occur more easily at

higher system operating pressures than at near atmospheric levels.

Elevated pressures condense the ignitable gas giving the flame more matter and

energy to release thereby boosting flame heat intensity,A critical concern in flame

arrestor installation is the possibility of a flame stabilizing on the face of the flamecell element. A flame that continuously burns against the flame arrestor element for

a period of time can heat the element above the autoignition temperature resulting

in flame propagation through the element. The time period varies with the type of

element, mixture of air and gas, type of gas and velocity at which the gas stream is

moving.




Product Name Liquid Seal Flame Arrestor Function Liquid seal is designed to

stop flame propagationThe arrestor is designed tostop flame propagation

Product Type Active Device Passive Device

Testing Protocol Not available FM USCG API

Maintenance Switches andcleaning

Cleaning of flamecell elements

Failure Modes Switch

malfunctions, liquidfreezing

Corrosion

Liquid Seal Versus Flame Arrestor

The above chart shows the advantages of a liquid seal and the advantages of a flame

arrestor. Liquid seals are much more common in flare system then are the flame

arrestors. Liquid seals are also designed for detonation thereby the location of the

installation is not as critical as that of the flame arrestor.

In the initial state of flame propagation, the gas is ignited and the flame has

propagated a short distance in the piping system. Flame front velocity is below the

speed of sound thus defined as deflagration, and pressure piling is very low which

makes it a low pressure deflagration. The intermediate state the flame has been

allowed to propagate further down the piping system thus increasing both the flame

velocity and pressure piling. The flame front velocity is just below the speed of

sound and the pressure piling effects have increased tremendously resulting in a

high pressure deflagration. The transformation form deflagration to detonation

transition occurs when the flame front pressure and velocity reach an undefinedlimit. This explosive transformation causes the flame to accelerate to sonic,

supersonic or hypersonic.

In the advanced flame state, the flame has been allowed to propagate further down

the piping system allowing detonation to occur. The flame front is moving at

supersonic velocity.




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 micronsin diameter.

Knockout Drums

Knockout drums are used to remove any liquid flowing to the flare. The knockout

drum is also designed to disengage entrained liquid droplets. Normally the

recommended particle disengagement is 300-600 microns in diameter. Although

largely vapor, hydrocarbon relief streams from process plants often contain some

liquid, even with out liquid carryover due to plant upsets, because droplets form as

the vapor is cooled below the dewpoints. Knockout drums collect such liquid

before it reaches the flare not only to reclaim it but also to prevent burning liquid

from dropping around the base of the flare. Relief systems have been over loaded

due to process upsets.

When liquid upsets happen, if the knockout drum is not designed correctly then a

problem of burning rain down the flare happens. Burning droplets can be carried by

wind, starting fires in remote areas. Sizing this knockout drum is one of the most

important aspect to flare systems. The maximum liquid flow to a knockout drumthat could occur under any circumstance must be identified. If the liquid relief is

huge then one might consider a separate relief system or may have many process

unit knockout drums located in the process area before the main knockout drum at

the flare stack. Because of the size, most knockout drums are horizontal. The one

in the picture above is a vertical knockout drum and probably be designed for lower

flow rates.




Hazardsof Burning Rain

Injury to Personnel

Damage to Equipment

Source of Fire

Separation of the entrained liquid in the knockout drum is required to prevent the

burning rain. Burning rain occurs if the separation of the liquid is not provided

adequately in the knockout drum. A separation of 300 microns is required in the

flare knockout drum before the vapors are sent to the flare. If this separation does

not occur, then there is the potential for burning rain. Burning rain is extremely

dangerous because it can injury personnel, damage equipment and start localized

fires.

The flare knockout drum needs to be monitored to make sure that the level does not

rise too high. This is because if the level rises and there is a major refinery release,

say an emergency power failure then the effective separation zone is reduced.

Therefore, the knockout drum should be checked at least daily and material which

collects should be pumped out to the slop oil tank or to the crude oil tank depending

on the composition of the material.

Burning rain can also destroy the flare tip and flare stack. The design of the

knockout drum can prevent this from happening.




Design Considerations Separation of Gas & Liquid

Hydrocarbon relief streams are mainly vapors, but they may carry some liquids that

condense in the collection line. Therefore, material entering the knockout drum will

be a mixture of vapor and liquid. A particle that is 300 microns or less can be

burned in a flare with out hazard. Larger particles shall be removed in the knockout

drum. The liquid is pumped out from the bottom of the knockout drum either for

reuse or for disposal in a slop oil tank.

Knockout drums are either horizontal or vertical. They are available in a variety of

configurations and arrangements that include horizontal drum with the vapor

entering at one end of the vessel and exiting at the top of the opposite end with no

internals. The drum can also be a horizontal drum with the vapor entering at each

end on the horizontal axis and a center outlet. The drum can also be a horizontal

drum with the vapor entering in the center and exiting at the two ends on the

horizontal axis. The drum can also be vertical as the one shown above with thevapor entering with a tangential nozzle. The drum can also be vertical with the

vapor entering at the top of a certain diameter and provided with a baffle so the flow

is directed downward.




Design Considerations Liquid Holding Capacity

Selection of the knockout drum arrangements depends on economics. When large

liquid volume storage is required and the vapor flow is high, normally a horizontal

drum is more economical. A split entry or exit reduces the size of the drum for

large flows. As a rule of thumb, when the drum diameter exceeds 12 feet the split

flow arrangements is normally economical. UOP has a number of programs to help

size knockout drums. Liquid particles dropout when the vapor velocity traveling

through the drum is sufficiently low. The drum must be of sufficient diameter to

effect the desired liquid-vapor separation.

It is general practice to assume a liquid holdup time between 10 to 30 minutes.




ProgrammableController

RuptureDisk

PressureTransmitter

Flare Header

First StageAir AssistedFlare

Stage 2

Stage 3

Stage 4

Stage 5

LRGOTips

Radiation Fence

ControlValve

Illustrative LRGO Arrangement

The above drawing in an illustrative Linear Relief Gas Oxidizer (LRGO)

Arrangement. The flow rate of gases will flow to the first stage air assisted flare.

The reason is the low pressure at that flare tip (they run at atmospheric pressure).

Once the air assisted flare gets overloaded, the back pressure builds in the system up

to a point of approximately 7 psig. At this point, stage tow of the LRGO tips open.

If the flow still increases and the back pressure is still increasing, at say 8 psig, stage

3 will open. If the flow still increases more and the back pressure increases more,

say at 9 psig, stage 4 will open. If the flow is still increasing, say at 10 psig, stage 5

will open.

The above arrangement is a 5 stage LRGO system. Therefore, the maximum

capacity of this flare is up to whatever can be released when the last stage is open.

The stages are opened when the pressure controller sends a signal to the control

valve at each of the 4 LRGO stages. In case the control valve fails closed, there arerupture disks which will break allowing the flare gas to by-pass the control valve

and go to the LRGO stages. In addition, there is a radiation fence to reduce

personnel exposure to the LRGO.




LRGO - Ground Flare - Critical Location

LRGO can be installed in almost any location. The one above was built near the

gulf on a sandy surface. The one in the photo above is a four stage LRGO system.

Open ground flares, as shown in the next few slides, come in a variety of

configurations. Some have many small burners such as that shown on two slides

later than this slide and some have large burners such as the one shown above and

on the next page.

The many small burners that together can flare larger quantities of relief gas than an

enclosed flare. Their capacity is limited by the available plot area to distribute the

burners. The individual burners are fully smokeless, using the kinetic energy of the

flare gas to entrain enough air for complete stoichiometric combustion.

The drawback of the open ground flare is that the required pressure at the flare tip is

about 7 to 10 psig, which is much greater than an open elevated flare. Rows orsections of burners are than staged using control valves, so that the proper pressure

is maintained at the burners. As flow rate increases, the header pressure increases

and more burners are made available.




LRGO - Designed

for the Tundra of Alaska

The above is a picture of a ground flare or what is known as a LRGO - Linear Relief

Gas Oxidizer. The LRGO was installed in the tundra of Alaska to minimize the

effects of radiation on the tundra. For smaller plants, open pit ground flares can

sometimes be made large enough to replace an elevated flare. They can also be

built with a refractory fence to reduce radiation effects.

The ground flare (LRGO) can have the burner stages arranged in a number of

different configurations depending on the plot plan layout. The one above is shown

as a half-semicircle which allows more plot space for other requirements. Even

with an LRGO, a knockout drum is required just prior to the burner tips as shown in

the yellow shelter above.




Ground Flare Designed in Mexico

Open ground flares can have a variety of configurations. Some have many small

burners that together can flare large quantities of relief gas, such as the one shown

above. Their capacity is limited by the available plot area to distribute the burners.

The individual burners are fully smokeless, using the kinetic energy of the flare gas

to entrain enough air for complete stoichiometric combustion.

The disadvantage of ground flares is that they require pressure at the flare tip of

about 7 psig. This is much greater than the typical elevated flare. Rows of burners

are staged using control valves, so that the proper pressure is maintained at the

burners. As flow rate increases, the header pressure increases and more burners are

made available.




Ground Flare - Operating

The above photograph shows a ground flare in operation. Note that because of the

thermal stresses the burner tips actually bend. After the release they will return to

their original position.

The ground flare tip design uses gas stream pressure to promote mixing at the

burner tip of the ground flare. With sufficient gas pressure, pressure assisted or

ground flares can be used where steam or air assist was previously required.

Multiple burner heads are staged to operate based on the amount of gas being flared.

Flare gas properties determine size, design, number and group arrangements of

burner heads. Gas streams with 10 psig or greater available pressure are normally

selected.

The problem with ground flare is that most refineries have a crude unit/vacuum unit

which have relief valves set at 50 psig (3.5 kg/cm2g) and therefore there is notenough pressure available for this type of flare. The refinery could break their flare

system into a high and low pressure system where they may send all relief valves

with a set pressure greater than 250 psig to the high pressure flare system and send

all relief valves with a set pressure less than 250 psig to the low pressure flare

system.




No structural supportrequired

Erection is relatively

straightforward

Maintenance is easy

Operating costs are

negligible

Flame of flare not visible

Fairly quite system

Advantages of Ground Flare

There are many advantages to the ground flare. No structural supports are required

and therefore the cost may be less. The erection of the ground flare can take place

at grade level so erection is relatively straightforward. If maintenance is required,

the ground flare be located at grade can be fixed easily.

The operating cost of the ground flare because no steam or air is required to make

the flare smokeless is negligible. A big advantage of the ground flare is that the

flare is not visible to neighbors or others. The system is fairly quite and, therefore,

more socially acceptable.




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 mayremain high

Disadvantages of Ground Flares

The disadvantage of ground flare is that they must be isolated from the rest of the

refinery. In elevated flares, depending on the radiation profile, the flare may be

located near the process units. As can be seen from the above photograph, the

ground flare requires considerable space and long inter connecting piping resulting

in a lot of capital cost.

A big disadvantage is that the combustion takes place on the ground so therefore the

concentration of toxic gas at grade may remain high for some time after a release.

Drainage must also be considered.




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

Air-Assisted Flare

The key to successful smokeless burning of flare gases using an air assist flare is to

properly design the burner system and to integrate that system with a properly

selected blower. Of particular importance in an air-assisted flare is obtaining a zone

to promote mixing.

The burner design must be executed to provide air flow and gas flow in proportion

to each other to properly mix. There must be more gas flow near the outside of the

circular air riser than towards the inside because the bulk of the air flow are in a

circular plenum is contained on the outside of the plenum. In addition, the burner

design must provide for stability of the waste gas under greatly varying flow

conditions and compositions.

This design is more easily achieved by using a spider-type burner whose center hub

acts as a stability point for the burner insuring stable combustion through a widerange of compositions and turndowns.




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

Quiet Flare (Low Noise)

Significant disadvantages of steam usage are the increase noise and cost. Steam

aggravates the flare noise problem by producing high-frequency jet noise. The jet

noise can be reduced by the use of small multiple steam jets and, if necessary, by

acoustical shrouding. Steam injection is usually controlled manually with the

operator observing the flare (either directly or on a television monitor) and adding

steam as required to maintain smokeless operation.

To optimize steam usage, infrared sensors are available that sense flare flame

characteristics and adjust the steam flow rate automatically to maintain smokeless

operation. Automatic control, based on flare gas flow and flame radiation, gives a

faster response to the need for steam and a better adjustment of the quantity

required. If a manual system is used, steam metering should be installed to

significantly increase operator awareness and reduce steam consumption.




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 andTurbulence

Enclosed Ground Flares

Enclosed ground flares incorporate a multi-stage burner system that injects the

auxiliary fuel separately from the vapor. The burner features a refractory tile in an

aerodynamically designed configuration to maintain flame stability and promote

mixing at much higher local temperatures that can be achieved by merely dumping

fuel in a mainly nitrogen or CO2 fume stream.

A unique “flame holder” design for the flame arrestor burner tip that accomplishes

this as well as creating excellent flame stability when auxiliary fuel is used.

EPA places significant emphasis on the effect of time, temperature, and turbulence.

Enclosed ground flares incorporates a high performance fin plate burner which can

handle most unsaturated gases smokelessly, without the cost of additional utilities.

Improved air management is achieved utilizing an advanced fluidic windfence, theresult of extensive research using wind tunnel technology. The system gives an

unrivalled level of smokeless operation, low burner and refractory temperatures,

low noise, and the flexibility of high turndown.




RefractoryLinedStack

AirControl

Louvers

BurnerAssembly

SampleConnection

TemperatureControl

FlameArrestor

From

Blower

Typical Enclosed Flare

An enclosed flare’s burner heads are inside a shell that is internally insulated. This

shell reduces noise, luminosity, and heat radiation and provides wind protection. A

high nozzle pressure drop is usually adequate to provide the mixing necessary for

smokeless operation and air or steam assist is not required. In this context, enclosed

flares can be considered a special class of pressure-assisted or non-assisted flares.

The height must be adequate for creating enough draft to supply sufficient air for

smokeless combustion and for dispersion of the thermal plume. These flares are

always at ground level.

Enclosed flares generally have less capacity than open flares and are used to

combust continuous, constant flow vent streams, although reliable and efficient

operation can be attained over a wide range of design capacity. Stable combustion

can be obtained with lower Btu content vent gases than is possible with open flare

designs (50 to 60 Btu/scf), probably due to their isolation from wind effects.Enclosed flares are typically found at landfills.




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 50to 1)

Marine Vapor Control System Flare

The marine vapor control flare is similar to the enclosed ground flare. The picture

above takes the emissions which are displaced from ships when unloading and

loading. Note the large loading arms located to the right of the vapor control system

flare. These flares generally have less capacity than open flares and are used to

combust a wide range of flows. Stable combustion can be obtained with lower Btu

content vent gases that is possible with open flare designs (50 to 60 Btu/scf),

probably due to their isolation from wind effects.




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

Truck Loading Vapor Control Flare

Truck loading terminal flares are designed to burn waste gas streams which can

range from very lean (high oxygen/nitrogen content) to very rich (high hydrocarbon

content). The design heat release turndown is often 10 to 1. The rich steams

require forced air blowers to supply part of the combustion air to produce

turbulence in the flame zone for smokeless burning.

The lean streams can be extinguished (blown out) by the forced air blowers if they

are not properly controlled. Most systems have two-speed blowers, one for each

stage’s burners, a two speed quench air blower for efficient temperature control, and

all necessary instrumentation and controls to automatically maintain stable,

smokeless combustion in the unit.




High stability pilot andflame 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

Upper Steam Manifold Flare

This design incorporates a high stability pilot and flame retention system with a

high efficiency low noise steam injection ring. The flare provides smokeless

burning with lower noise than traditional steam injection systems.

The upper steam injection is mechanically superior in design while providing

greater flow at lower steam pressure drop. Special multiple orifice steam drilling

ensures the lowest possible unshielded noise level. Flares larger than 30 inches

include internal refractory lining.




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 reactioncoverts Syngas to a liquid diesel

fuel

Designer Fuels

Stranded gas sites around the world where no pipeline is available to capture and

transport natural gas, it is vented and fired. The worldwide supply of stranded

natural gas equals all the know U.S. petroleum reserves. It a huge potential

resource, disposed of in a most wasteful way. But a new method of capturing it

means it could soon be powering the car you drive.

In the Syntroleum process, natural gas reacts via a catalyst to oxygen in the air to

form a synthetic gas (hydrogen/nitrogen/carbon monoxide), which is dubbed

Syngas. A second reaction converts Syngas to a liquid diesel fuel that would power

special high-efficiency, clean-burning diesel engines. The byproducts are water,

nitrogen, and carbon dioxide.

Much of the incentive to create new fuels stems from the limitations of today’s

catalytic converters, which have proven to be a major boon to clean air - today'scars are approximately 99 percent cleaner than their 1960s counter parts, according

to the American Automobile Association - but now stand in the way of highly fuel

efficient engines. The reason lies with how catalysts clean exhaust.



EDS 2004/Flare Systems-76FS-R00-27

IgnitorTubes

IgnitorManifold

PilotManifold

PilotGas

Compressed

AirIgnitor

Gas

Flame FrontIgnitor Panel

Pilots with Thermocouples

Gas AssistedFlare Burner

w/ Fluidic Seal

Assist Gas

FlowSensor

FERelief & Vent Header

AmmoniaStorageTanks

Purge

GasInjection

Relief Valves

ManualVents

NH 3 Plant

The above diagram is that of an ammonia plant. Ammonia is a problem with flare

systems because of its low heating value. Ammonia does not burn very effectively

and therefore requires assist gas. This is also required because of EPA requirements

with flares. As can be seen from the above diagram, there assist gas is added based

on the flow rate from the ammonia storage tanks and other relief valves.

The flow sensor will increase the assist gas flow rates proportionally with the

increase in ammonia flow rates. A Net lower heating value of approximately 300 to

1000 Btu/scf is required as defined in 40 CFR 60.18, which was discussed earlier in

the material. The flare utilized for this ammonia is a gas assisted flare. This means

that there is some high pressure gas available within the plant. Steam is not the

preferred choice for the ammonia flare. The flame front igniter panel is also

utilized in the ammonia flare.




Auxiliary fuel requiredin 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

Ammonia (NH 3 ) Plant Considerations

Ammonia plants are a unique application for a flare. The only time a flare is needed

is during a power or refrigeration failure. Normally the gas is contained in a low-

pressure refrigerated storage tank and dispensed on to railroad cars or tank trucks

usually bound for fertilizer or metallurgical process applications. In most instances,

there are not releases of any kind from an ammonia plant. However, if a power or

refrigeration failure occurs, the pressure in the tank will gradually build up.

A short-term power failure or a minor refrigeration failure will not result in the flare

operating, since the tank is well insulated and the problem can be corrected quickly.

The flare, however, is the final safeguard. The ignition system has a built-in battery

backup system that can still activate and ignite the pilot in the event of a power

failure.

Since ammonia has a very low heating value, it is necessary to administrate assistgas to make the ammonia burn effectively.




Flame FrontIgnitor Panel

Compressed Air

Ignitor Gas

Oil

IgnitorTubes

Pilots

Ignitor

ManifoldPilot

Manifold

Water Assisted

SmokelessFlare Burnerw/Fluidic Seal

Boom

Water Assist

Pilot GasGas

DisentrainmentDrum

Offshore Platform

The above drawing is an offshore platform layout and in the requirements for a flare

system. One of the first items one notices is the large boom required for the flare.

The purpose of the boom is to keep the flames and radiation produced away from

the workers on the offshore platform. The type of flare shown is a water assisted

flare. Since steam is not available and air is expensive, the most effective method

to make the flare smokeless is to use water from the sea.

When designing a flare for an offshore platform, one should consider the worst case

for gas flow. In addition, to reduce the cost of the boom, accurate radiation

calculations need to be made.




OFFSHORE PLATFORM

It is estimated that nearly one third of the world's oil comes from offshore fields, in

particular from the North Sea, the Arabian Gulf and the Gulf of Mexico, where one

of the world's first offshore platforms was built in 1947, in just seven meters of water.

Thanks to great advances in engineering, it is now possible to build platforms taller

than most of the world's skyscrapers and anchored to the seabed in more than 400

metros of water. These platforms contain thousands of tones of equipment and can

accommodate hundreds of men who work in shifts to ensure that oil is produced,

stored and pumped ashore around the clock.

In smaller fields, such huge fixed structures may not be economically viable.

Engineers have developed ingenious alternatives, such as floating production

systems. These are converted drilling vessels or tankers, which are used to treat and

store the oil, which flows through risers linking the vessels to wells on the seabed. An

example is the Anasuria Floating Production, Storage and Offloading System(FSPO’s). With subsea production systems, there is no dedicated platform. Instead,

oil is pumped from wells and manifolds on the seabed to a platform in a nearby field.

In the future, many smaller fields in areas like the North Sea might be produced as

"satellites" using such systems.




Average target concentration

(downwind)

0.10ppm or less

Examples of odor nuisance:

Isoamylmercaption

0.00043 ppm

Ethylmercaptan

0.00056 ppm

Ground Level Concentrations

After the stack height has been established from radiation intensity values, the

maximum permissible ground level concentration of toxic gases in the event of a

flame blowout should be evaluated. The above examples show odor nuisance

concentrations which can be extremely low. Estimated ground level concentrations

should be based on emergency condition of flame blowout. For a rough estimate,

the following formula can be used:

Cmax = 3697VMDz/(uH^2Dy

Xmax = (H/Dz)^(2/(2-N))

Where: Cmax = concentration at grade in ppmv

V = specific volume of toxic gas ft3/lb

M = weight discharge of pollutant component in tons per day

Dz = vertical diffusion coefficient (~0.13)u = air velocity at grade

H = stack height, ft

Dy = horizontal diffusion coefficient (~0.13)

X = distance from stack to point of maximum concentration, ft

N = environmental factor (~0.25)




Amount of fuel required (F) is calculated based on

maintaining the vent gas stream net heating value atthe 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

Auxiliary Fuel Requirement

The flare tip diameter is a function of the vent gas flow rate plus the auxiliary fuel

and purge gas flow rates. The purge gas flow rate is very small relative to the vent

gas and fuel flow rates, so it may be ignored when determining the tip diameter.

The flow rate of the auxiliary fuel, if required, is significant, and must be calculated

before the tip diameter can be computed.

Some flares are provided with auxiliary fuel to combust hydrocarbon vapors when a

clean flare gas stream falls below the flammability range or heating value necessary

to sustain a stable flame. The 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 by rules defined in the Federal Resister:

QBv + F Bf = (Q + F)(300 Btu/scf)

where: Q = vent stream flow rate, scfm

Bv and Bf are the Btu/scf of the vent stream and fuel respectively.




Sizing must also comply with Federal Register (40 CFR 60.18)

for maximum velocity of steam-assisted, elevated flares:

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

Flare Tip Diameter

Net Heating Value of Vent Stream

Bv (Btu/scf)

Maximum Velocity

Vmax (ft/sec)

300300-1000

> 1000

60log10(Vmax) = (Bv + 1214)852

400

Rearranging gives

F (scfm) = Q * (300 - Bv)/(Bf - 300)

The annual auxiliary fuel requirement, Fa, is calculated by:

Fa (Mscfm/yr) = (F scfm)(60 min/hr)(8760 hr/yr)

Fa (Mscfm/yr) = 526F

Typical natural gas has a net heating value of about 1,000 Btu/scf. Automatic

control of the auxiliary fuel is ideal for processes with large fluctuations in VOC

composition. These flares are used for the disposal of such streams as sulfur tail

gases and ammonia waste gases, as well as any low Btu vent streams.




Wind

FlareLP Zone

Gas Jet

Supplementary Energy

High ExitVelocity

Low ExitVelocity

Low Exit VelocityWith SecondaryEnergy Source

W W W

F

F

G G G

FLL LS

W =

F =LP =

G =

S =

Improve Flare Burner LifeVector Diagrams

The above diagram shows how steam (supplementary energy) injected into the flare

tip can improve the flame tilt. Pressure drops as large as 2 psig have been

satisfactorily used at the flare tip.

Too low a tip velocity can cause heat and corrosion damage. The burning of the

gases becomes quite slow, and the flame is greatly influenced by the wind. The

low-pressure area on the downwind side of the stack may cause the burning gases to

be drawn down along the stack for 10 feet (3 meters) or more. Under these

conditions, corrosive materials in the stack gases may attack the s




Mechanism

– Oxidation reaction

Factors

– Time, temperature

and turbulence

Incinerator Design

Incinerators are called thermal oxidizers, combustors and thermal reactors. The

function of an incinerator is the destruction of organic and inorganic wastes using

an oxidation reaction. The products of combustion are Carbon monoxide Nox, Sox,

acid gases, particulate and unburned hydrocarbon. The carbon monoxide are

generally a function of burner operation and the unburned hydrocarbon and SOX

and acid gases generally a function of incinerator operation.

There are many permit requirements such as tons per year that can be emitted and

ppm that can be discharged. The equipment shall be designed for the worst case

scenario if limited by an hourly average ppm level. A tons per year emission limit

may allow designing for a normal case, while merely handling an infrequent or

intermittent surge.

The waste characterization determines the type of incinerator. The burner selectionis dependent on waste type and special consideration such as low NOX. The

incinerator design for introduction of some waste must provide for retention time.

Flue gas treatment may be necessary to meet the required emissions levels if

particulate or acid gases are present in fuel gas.




Burners

– Ignite the fuel and

organic material

Chamber

– Appropriate residence

time

for oxidation process

Three T’s of Combustion

– Temperature

– Time – Turbulence

Principles of Combustion

Some process design consideration are low waste gas pressure drop of 4” w.c. and

NOX requirements because of high fuel rate. The mechanical design consideration

for natural draft and for forced draft must be considered. Natural draft is less

expense, forced draft provides better turndown, and forced draft is better for adding

heat recovery .

The layout of the incinerator must be considered. Is horizontal better than vertical?

The cost of the vertical unit is less expensive, the vertical unit requires less plot

space. The horizontal is easier to access and add downstream equipment. If the

heat release is large, then the horizontal may be required to eliminate impingement.

The refractory of brick castable or blanket must be considered. Highly abrasive

particulate requires special castable or brick. The blanket has limited velocity

capability. The blanket provides ability for on/off operation and heat up and alsocosts less.




100

80

60

40

20

0600 800 1000 1200 1400 1600 1800 2000

1 sec

1.0 sec0.01 sec

IncreasingResidence

Time

Increasing Temperature, °F

0.001 sec

P o l l u t a n t D e s t r u c t i o n ,

%

Residence time of gases in combustion chamber calculated from: t = V/Qt = Residence Time (s)v = Chamber Volume (ft3)Q = Gas volumetric flow rate at combustion conditions (ft3/s)

Coupled Effects of Temperature andTime on Rate of Pollutant Oxidation

The above chart is the coupled effects of the three T’s of temperature, time and

turbulence on the rate of pollutant oxidation. By increasing temperature only and

holding the retention time, the same increases the pollutant destruction efficiency.

By increasing the retention time only and maintaining the same temperature

increases the pollutant destruction. Depending on the percent pollutant destruction

required determines what temperature is required in the incinerator and how long of

a residence time is required.

Therefore, for the same gas volumetric flow at combustion conditions, a higher

temperature and lower retention time or a lower temperature and higher retention

time may be the more optimum design for the incinerator.




Fume

Exhaust

Combustion

Air

(Fume)

Fuel

Schematic of a Thermal Incinerator

The above drawing is a schematic of a thermal incinerator. As you can see from the

drawing, the waste gas or fume is introduced at the top left hand corner.

Combustion air and fuel are added to ignite the mixture. The time it takes for the

fume to enter and leave the incinerator as exhaust is know as the retention time.

The temperature in the incinerator is the number used in calculating the efficiency.

You can also see the turbulence when the air, fuel and fume are combined within

the incinerator.

Using low NOX burners or 2 stage furnace (reducing then oxidizing), catalytic

NOX reduction or special scrubber may be required after the exhaust to maintain an

NOX ppm limit which is set by the local law and governments such as EPA.




ProductLoading Arm

Product fromStorage Tanks

Natural Gas/Inerting Gas

EnrichingDetonation

Arrestor

Hydrocarbon Vapor to Control Device

Vapor Mover

SumpPump

Condensateto Tanks

DischargedVapors

Ship

or BargeDock Facilities Shoreside Facilities

GasAnalyzer

Vapor Arm

KnockoutDrum(s)

Typical Marine Vessel Loading System

To achieve high destruction efficiencies through the loading cycle requires that

provision be made to keep the combustor at a minimum temperature prior to

introducing loading vapors and during high vapor flow rates when the majority of

the vapor is air, nitrogen or carbon dioxide depending on the ship being loaded.

Achieving high destruction efficiencies requires that the oxygen in the combustion

air be given adequate opportunity to come in contact with hydrocarbons which can

be a very low percentage of an inert vapor stream. Mixing is critical to making this

happen. The more time the vapor stream is at temperature in turbulent contact with

combustion air, the higher the likelihood of complete combustion which means no

carbon monoxide or hydrocarbons. The less combustion air that is required (excess

air) to achieve complete combustion, the less auxiliary fuel is required to maintain

temperate. This is especially important when dealing with very low Btu content

vapors associated with inert vessels or non inert ships that use inerting techniques to

meet the Coast Guard safety criterion. The design of the combustion equipment,which accounts for approximately 20% of the total marine terminal equipment cost,

is influenced by the emission rules. Due to the cyclic nature of the use pattern of

combustors in marine service, ceramic fiber is generally the refractory system of

choice due to its low heat capacitance and resistance to thermal shock. These units

are routinely brought from ambient to 1500°F in less than 5 minutes.




Gasoline Vapors

Crude Vapors

Percent of Storage Tank or Truck Filled

70

60

50

40

30

20

00 25 50 75 100

T o t a l H y d r o c a r b o n V a p o r

C o n c e n t r a t i o n i n V e s s e l

( V o l u m e P e r c e n t )

Storage Tank and Tank Truck Loading Hydrocarbon Concentration Profile

The loading occurs at the transportation loading rack, the vapor combustion system

is in a standby mode with no pilot flame, the vapor block valve is closed, and the air

assist blower is off. Automatic startup of the vapor combustion system is initiated

by an electrical signal from the loading rack that product loading will occur shortly.

The startup sequence consists of a short air purge using the air assist blower to

purge the air plenum of the open flare or the enclosed flare of any combustibles

prior to pilot ignition. This brief air purge is followed by automatic electronic

ignition of the pilot. After pilot ignition, product loading begins at the loading rack

and an air vapor mixture begins to flow from the transports being loaded to the

vapor combustion system.

Flow through the vapor combustion system first consists of the air vapor mixture

from the lading rack bubbling through a liquid seal. As soon as sufficient flow is

available, it will be detected by the pressure monitoring controls which willautomatically open the vapor block valve allowing the air vapor mixture to flow

thorough the flame arrestor to the burner, where the combustible vapors are ignited

by the pilot and burned. The air assist blower provides partial combustion air and

mixing energy to the burner tips to assure smokeless combustion. As the loading

operation at the loading rack is completed, vapor flow to the combustion system

decreases. The pressure monitoring system closes the vapor block valve when the

vapor flow is insufficient to maintain minimum burner velocity. The pilot and air

assist blower remain on for a brief time period after loading is complete. If no

further loading occurs, the combustion unit will shut down in the standby mode toawait automatic restart.




Gasoline Vapors

CrudeVapors

Percent of Marine Vessel Filled

70

60

50

40

30

20

00 25 50 75 100

T o t a l H y d r o c a r b o n V a p o r

C o n c e n t r a t i o n i n V e s s e l

( V o l u m e P e r c e n t )

10

Marine Vessel Loading Hydrocarbon Concentration Profile

As shown in the above chart, the hydrocarbon vapors are in higher concentration

during the final portion when the vessel is being filled. The vapors immediately

upon being transferred to the loading facility, are analyzed and conditioned, if

necessary, to assure they are well outside the explosive limit.

The combustion system must be designed to cope with air, nitrogen, and carbon

dioxide vapors with hydrocarbons concentrations varying from 0 to 100 percent.

The application must also deal with flow rates which are wetted, loaded and topped

off.

The large turndown (up to 100 to 1) encountered in marine terminals, satisfying the

low end velocity criteria makes it impossible in a single burner configuration to

have stable and efficient combustion at the high flow rates.

To accommodate the wide range of vapor flows, multiple burners are used and are

brought in and out of service to assure the flame arrestor tips are always kept cool

and that exit velocities never exceed the stability limit of the burner




Typical Lean Oil Absorption Vapor Recovery System

Economic consideration as well as environmental considerations have led to vapor

recovery systems as the one shown above. Since a vapor recovery system is used

for extreme emergencies one must not compromise the integrity of the flare system.

The above drawing shows a lean oil absorption vapor recovery system.

Hydrocarbon vapor is introduced into the absorption column where lean oil is

absorbed by the vapor. The treated vapor after being “scrubbed” can be released

into the atmosphere.

The rest of the section is the absorber regeneration process. Instead of adding lean

oil continuously, most of the lean oil can be regenerated. This involves separating

the absorbed material from the lean oil used to absorbed the material.




Precooler Vapor CondensingUnit (Evaporator)

ThermostaticExpansion

Valve

CompressedRefrigerant

Compressor-Condensor

Power Input

RecoveredProduct

CondensateCollection Tank

Power

Input

TreatedVapor

Blower

HydrocarbonVapor Inlet

FreonReturn

Typical Refrigeration Vapor Recovery System

The above drawing is a refrigeration vapor recovery system. The hydrocarbon

vapors are precooled and then sent to a vapor condensing unit or what is know as an

evaporator. The hydrocarbons will condense once it fall below its dew point. The

product that condenses is stored in a condensate collection tank before being sent

back to the process unit for reprocessing.

The treated vapors from the condensate collection tank are sent to the atmosphere

by a blower. The refrigeration vapor recovery system is similar to that of a

refrigerator or an air conditioning unit which uses some type of freon depending on

the temperature the stream needs to be reduced to.

The disadvantage of the refrigeration vapor recovery system is the high cost of

electricity required for the system.




HydrocarbonVapor Inlet

TreatedVapor Out

Burner

CatalystBed

ThermalZone

OxidationZone

Preheat OptionalHeat

Recovery

Stack

FeedAir or

ProcessStream

HeatedAir or

ProcessStream

Typical Catalyst Oxidation System

The above drawing is that of a catalytic oxidation system. The hydrocarbon vapors

are fist preheated and then sent into the thermal zone of the oxidation system. After

being heated to over 1000°F, depending on the catalyst, it is sent through a catalyst

bed and the oxidation zone.

The vapors are then preheated against the hydrocarbon vapor air going into the

thermal section. The vapor are still going to be hot and then can be either sent back

to the process streams or an optional heat recovery unit can be installed.

After the heat recovery unit, the cooled treated vapor will be sent to a stack for final

disposal to the atmosphere.

Documents

Flare Systems