Basics of Flare Design

Sandip Lahiri

Process Design Engineering Cell

IOCL-RHO, Noida

Flare

Disposal method ofOperational

&Emergency Relief

Flammable Substanceto a safe location

Uses CombustionTo

ConvertFlammable,Toxic/CorrosiveVapors

Less Objectionable CompoundsTo

Consist of

Piping

Vessel

Stack

Environmental Control

Continuous flow of excess gas

Large surges of gas in emergency

Factors affecting Design

Physical & Chemical Properties of Fluid

Flash PointFlammable limitsIgnition TempPhase ChangeReactive Chemicals

Recovery Value

Costly Solvents

Factors affecting Design (Contd..)

Availability of Space

EconomicsInitial CostOperating Cost

Public Relations

If it is seen from residential area

How to determine Flare LoadSafety are used to protect pressure vessels.

Safety duty are calculated considering following contingencies

Blocked FlowExternal FireReflux Failure in columnsPower Failure in Units/ColumnsCooling Water Failure/Air Cooler Failure

Individual unit total flare load is computed considering the exigencies as above.The maximum of them is considered as flare load ex the unit.All units are summed up to compute the flare load.Often cooling water supply, power supply is given from different sources so that controlling flare load is not additive.

The controlling load will determine the sizing of

Flare Header

Flare Stack

Seal

K.O. Drum

Seal Drum

API RP 520 & API RP 521

Combustion Properties

FlameSmoke Radiation

Flame-Rapid self sustaining chemical reaction

9Diffusion Flame:Occurs on Ignition of Fuel jet issuing into air

9Aerated Flame:Occurs when fuel & air are premixed beforeignition

Generally Flame front is normally at the top of stack

At low gas velocity back-mixing of air occurs at the top of the stack

resulting in combustion at a flame front located part of the way

down the stack creating high stack temperature.

Flame extinguishment may also occur with subsequent formation

of an explosive mixture in the stack.

Smoke

HC Flames are luminous Carbon Particles

Released from Flames

Smoke

When the System is Fuel Rich

Steam

Reduce Smokeformation

Two School of Thoughts

Steam separates HC molecules Minimizing Polymerization

Forms O2 compounds that burns at a reduced rate &temp that are not conducive for cracking & polymerization

Water Vapor + Carbon Particles CO+CO2+H2

Removing the Carbon before it cools and forms Smoke

Suggested Steam Injection Rates

In extreme cold climates an internal steam nozzle may causeCondensate to enter flare header, collect and freeze.

In such cases high pressure air may be used.

Only disadvantage of air is that it is more expensive than steam

Air is advantageous in desert and island where water is shortage.

Steam pr-100-150 psi

Air pr-100 psi

Air requirement is approximately 20% more than steam as compressed air does not produce water-gas shift reactionthat occurs with steam.

Steam Injected smokeless Flare Tip

Radiation

Effect of Radiation Intensity on Human skin

Intensity Pain Threshold BurnBtu/hr/sqft Seconds Seconds

2000 8 207500 - 6

Intensity Pain Threshold BurnBtu/hr/sqft Seconds Seconds

250-330 - -

Solar Radiation

As allowable Radiation level is a function of length of exposure,factors involving reaction time & human mobility to be considered.

In emergency releases a reaction time of 3-5 secs may be considered.

5 secs for individual to seek cover or depart from the area.

Total exposure time of 8-10 secs.

Solar radiation to be added for considering total exposure time.

Clothing provides some shielding.

In case of radiation emanating from an elevated point,standard PPElike Hard Hat may reduce thermal exposure.

Permissible Design Total Radiation

In most cases, equipment can safely tolerate higher degrees

of heat intensity than those defined for personnel.

However, if any items vulnerable to overheating problems

are involved,such as construction of materials that have low

melting points (aluminum,plastic),heat sensitive streams,

flammable vapor space,or electrical equipment then the effect

of radiant heat on them may be evaluated.

Minimum distance from a flare to an object whose exposure to thermal radiation must be limited

Q = 2.15 x 109 BTU per hrK= 2000 Btu per hr per sq ftF=0.3T= 1.0So D= 160 ft.

Flare manufacturers have their own proprietary radiation programs based on empirical values.

The Emissivity (Fraction of heat radiated) values used in these programs are specific to equations used and are not interchangeable.

Flame Length vs. Heat release

Flame distortion due to wind velocity

Gases that have high heating value (200-300 BTU/scf) can sustain

combustion on their own without any auxiliary fuel additions.

In some cases addition of auxiliary fuel in the form of fuel gas is necessary.

Dilute NH3 and high CO2 composition gases with small amounts of

H2S are common applications.

Auxiliary Flaring Equipment

Liquid Seals-

Flashback protection- To provide positive protection against Flame propagation into plant piping.

Maintain +ve header pr- To ensure that a positive pr. is always maintained on the flare header. Any leaks in the flare header will result gas leakage to atmosphere and not air leakagein the header.

Purge of O2 free gas-

Prepurge removes any O2 from stack and continuous purge ensures no air enters stack during low flow conditions.To reduce amount of purge gas Diffusion type seal and velocity

Seal are used.

Diffusion type seal- This type of seal uses the difference in molecular weights of the purge gas and infiltrating air to form a gravity seal which prevents the air from entering into the stack. A baffled cylinder arrangement forces the incoming air through two 180 Deg 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 air from infiltrating the system.If the purge gas is heavier than air the purge gas will accumulate in thebottom of the seal and prevent air from infiltrating the system.

Velocity Seal- This seal works under the premise that infiltrating air enters through the flare tip and hugs the inner wall of the flare tip.The velocity seal is cone shaped obstruction with single or multiple baffles, which forces the air away from the wall where it encounters the focused purge gas flow and is swept out of the tip.

Purge reduction seals are not flame arrestors ;that is they will

not stop flashback. They are energy conservation device to

reduce purge gas flows required to prevent flashback by

reducing air infiltration in the stack.

Flare Type Elevated Flare Ground Flare

Selection of Flare type is based on

Available Space Characteristics of the flare gas (composition, quantity and pressure) Economics(investment and operating costs) Public Relations & Regulations.

Elevated Flare (Most commonly used in Refinery Operation)

Have larger capacities than ground flares. If adequately elevated, this type of flare has the best dispersion

characteristics for malodorous and toxic combustion products.

Due to steam injection / air injection , it introduces a source of noise and cause noise pollution.

Capital costs are relatively high. Appreciable plant area may be rendered unavailable for plant equipment,

because of radiant heat considerations.

Advantage

Disadvantage

Elevated Flare

Ground Flare

A ground flare is where the combustion takes place at ground level.

Self Guyed DerrickSupported Supported Supported

Types of Flare Support

Self-supported stacks are more expensive than other design, because it need greater material to ensure structural integrity over the anticipated condition. Normally the stack height for the self-supported design ranges from 200 to 300ft.

Guy-wire supported is less expensive but it need the largest land due to the

guy wire radius requirements. The radius of guy-wire is equal to 1.5 of the

overall stack height.Guyed stacks with heights from 600 to 800 feet have

been used.

Derrick supported only used when the stack is large (self-supported not

practical) and available land area is limited (guy-wire supported not suitable).

Quench is also used at times to condense the less volatile

components and reduce the release of hot condensable vapors

to the atmosphere.

Drag Coefficient

Flare gas recovery

Flare gas is treated and routed to FG system

Environmental

Economic

Safety Considerations

Path to flare

Used for both normal and emergency release.

Emergency release should always have path to the flare

Design of flare recovery system should not compromise this path

Necessary for

Safety Considerations (Contd..)

Back Flow

FGRS uses compressors which take suction directly from the flare

header.

The potential for back flow of air from the flare into the compressors

at low flare gas loads must be considered.

Oxygen content of flare gas stream should be measured and

provisions must be made to shut down the flare gas compressor if

potentially dangerous condition exist

Safety Considerations (Contd..)

Flare gas characteristicsWidely varying compositionsPotential for materials which are not compatible to treating system must be determined.

Streams containing acid gases are routed directly to flare bypassing recovery system

Design Considerations

SizingSeldom sized for emergency flare loads.Flare loads vary widely over time and the normal rate may be some average flare load or a frequently encountered maximum load.

Actual loads on these systems will vary widely and they must be designed to operate over a wide range of dynamically changing loads.

Design Considerations (Contd..)

Location

Typically FGRS are located downstream of all unit header tie-ins

and at a point where header pressure does not vary substantially with

load.

Locations upstream of process unit tie-ins should be carefully

considered because of the potential for back-flow and high-oxygen

concentration.

Design Considerations (Contd..)

Flare Tie-inA major consideration in FGRS design is preservation of a path to flare for emergency releases.

FGRS must be designed as a side stream from the flare header.Main flare flow should not be through a compressor knock out or suction piping.

The tie-in to the FGRS should come off the top of the flare line to minimize possibility of liquid ingress.

Most positive and preferred way to prevent air ingress is the installation of a water seal vessel between flare knock out drum and

flare itself

Alternate method is to use a fail open control valve

Typical Flare Gas Recovery System

Typical Flare Gas Recovery Inlet Pressure

Diagram for Stack Height Calculation

Typical Seal Drum

Typical Quench Drum

Typical Flare Installation

Sizing a Flare Stack:

Problem:

Flow rate of HC vapors = 100000 lbs per hr.

Avg. molecular weight (MW) = 46.1

Temp (T) = 760 0R (149 0C)

Compressibility Factor (Z) = 1.0

Heat of Combustion = 21500 Btu/lb

Ratio of specific heat of gas (K) = 1.1

Pr. at Flare tip = 14.7 psia

Design Wind Velocity = 20 miles /hr

= 29.3 ft/sec

Solution:

Flare Diameter:

MACH = (1.702) x (10-5) x (W / P2 D2) x (ZT / K x MW )0.5

For, MACH = 0.2

So, 0.2 = (1.702) x (10-5) x (100000/ 14.7 x D2) x ( (1x 760) / (1.1) x (46.1) )0.5

So, D2 = 2.24

So, D = 1.5 ft (ID)

Flame Length:

Q = (100000) x (21500) = 2.15 x 109 Btu /hr.

From Graph(Flame Length Vs Heat Release).

L (Flame Length) = 170 ft.

Flame Distortion by Wind Velocity:

Vapor Flow Rate = (100000 / 3600) x (379.1 / 46.1) x (760 /520)= 334 A ft3 per sec.

Flame Distortion:

Ua = Wind Velocity Uj Flare Tip Velocity

Uj = Flow / {(3.14)xD2/ 4}= 334 / { 3.14 x (1.5)2 / 4} [ For, MACH = 0.2]=189 ft/sec.

Ua = 29.3 Uj 189

= 0.155

From GraphDy / L = 0.35Dx / L = 0.85So, Dy = (0.35) x 170

= 59.5 ft.So,Dx = (0.85) x 170

= 144.5 ft.

Calculation of Stack Height:

D = (TFQ / 4 x 3.14x K)0.5 [ F=0.3, T =1 The max. allowable radiation at 150ft from Flare Stack = 2000 (Btr /hr) / sqft]

So, D = (1x (0.3) x (2.15x 109) / 4 x (3.14 )x (2000))0.5= 160.2 ft .

Flare Stack Height:

H = H + DyR = R - DxDx = 144.5 ftDy = 59.5 ftR = R- x (144.5)

=150 x (144.5) = 78 ft.

D2 = R 2 + H 2H 2 = D 2 - R 2

= (160)2 (78)2= 19516 ft2.

So, H = 140 ft. So, H = 140 D y

= 140 (59.5) = 110 ft.

Sizing a Knock out Drum: -

Problem:

Single Contingency results in the flow of 200000 lbs/hr of a fluid

Liquid density of 31 lbs /ft3 & vapor density of 0.18 lb /ft3.

Pr = 2 psi.

Viscosity of Vapor = 0.01 Cp

Fluid Equilibrium results in 31000 lbs per hr of liquid & 169000 lbs per hr of vapor.

In addition 500 gallous of storage for miscellaneous draining from the units is desired.

The droplet size selected as allowable is 0.000984 ft in diameter.

Solution: Vapor Rate (Rv) = 169000 lbs / hr (3600 sec. /hr) x (0.18 lbs/ft3) = 261 ft3 /sec PV PL Drag Coefficient (C) from graph D PV C (RC)2 = 0.95 x 108 x (0.18 )x (0.000984)3 (31-0.18) (0.01)2 = 5021. So, C from graph = 1.3

Drop Out Velocity (VC): UC = { (gD (L - V) / V x C )0.5}x 1.15 = {((32.2) x(0.000984)x(31-0.18) / (0.18)x(1.3))0.5 } x 1.15 = 2.35 A horizontal vessel with an inside diameter Di and a cylindrical length L should be assumed, At (Cross Section Area) = /4 (Di)2 AL1 = ((500 gallous) /(7.48 gallous per ft3)) x (1/L) Considering liquid hold up of 30 minutes during emergency, A L2 = (31000 lbs per hr / 31 lbs per ft3) x (30 minutes / 60 min per hr) x (1/L) The cross sectional area for the remaining vapor flow, A V = A T - (A L1 + A L2). h L1 = Depth of slops & Drains. h L1+ h L2 = Depth of all liquid accumulator. hV = Remaining metrical space for the vapor flow. ht = Total drum diameter = hL1 + h L2 +hr (Liquid Dropout time, in seconds) = (hV / 12 inches per ft) x (1/ VL ft per sec.) UV (Vapor Velocity in ft per sec) = (260 ft3 per sec / A V ft2) DRUM LENGTH (LMIN) = (UV ft /sec) x ( secs.) LMIN should be less than or equal to the assumed drum length L. Other wise the calculation must be repeated with a newly assumed cylinder length. Assume dMIN inside dia = 8 ft Assumed dMIN length = 19 ft AT = 50.26 ft2 AL1 = 3.25 ft2 h L1 = 11.25 inches. A L2 = 26.32 ft2 h L1 + h L2 = 55 inches. AV = 20. 69 ft2 hV = 41 inches. ht = 96 inches. Liquid dropout time (sec) = 1.45. Vapor velocity UV (ft/sec) = 12.73. Require Drum Length LMIN (ft) = 18.5

Modes of Flare Failure

Flow Restriction

Failure to burn

Mechanical Failure

Modes of Flare Failure (Contd..)

Failure to burn can be caused by loss of pilot functioning.Pilot failure can often be traced to a failure of the pilot gas Supply system rather than pilot itself.

Modes of Flare Failure (Contd..)