flare pilot design

8/13/2019 Flare Pilot Design

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If a flare cannot be ignited reliably and a stable flame maintainedat all times to burn the waste gas, we do not have a flare . What wehave is a vent pipe - where unburned heavy hydrocarbons or othercombustible gases can descend to grade level and become ignitedto produce a flashback, a fire, and possibly a catastrophic explosion.

Flare Pilot esign

Vicente A Mendoza, P.E.Vadim G Smolensky, PhD.John F Straitz III, P.E.

NAO Inc.

Enforcement of more stringent environmental regulations for vapor collectionand control has increased the need for ultra-safe and extremely reliable controlof waste gases and offgases from energy exploration and production, petroleumrefining and chemical/petrochemical, pharmaceutical, pulp and paper, and otherprocess industries.

Due to downsizings, deferred maintenance and other corporate cost-cuttingmeasures, major corporations worldwide are insisting on more reliable products,systems and services, including state-of-art designs for flare pilots and flaresystems that combine long, dependable service lives with minimal maintenance.

Flare technology and manufacturing practices of the 1940s, 50s and 60s aremaintenance-prone, obsolete and unsafe. To protect the global environment,plant personnel and surrounding communities, flares and their essential components must be designed, manufactured, tested, installed and serviced to ISO-9001 quality standards.

The four most critical components of a flare are:

flare burner tipo flare sealo flare pilot burner(s)

flare ignition system

A wide range of considerations must be addressed to design a safe flare system. Dependable ignition, flame stability and complete combustion are the keysto safe flare operation. If a flare cannot be ignited reliably and a stable flame


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m a i n t ~ i n e dat all times to burn the waste gas, we do not have a flare. What wehave IS a vent pipe - where unburned heavy hydrocarbons or other combustible g a s e ~can descend to grade level and become ignited to produce a flashback, a fire, and possibly a catastrophic explosion.

~ f o r ewe d.iscuss design calculations, testing, installation and service of inspiratlng flare pilot burners, which differ from regular furnace or boiler inspiratingpilot burners mainly by mixture-tube length and by severe open environmentapplication conditions versus the controlled environments of furnaces and boilers, let's consider state-of-art designs for flare burner tips and seals.

Flare Burner Tips and Seals

In the 1940s and '50s, density-differential seals were developed for elevatedflares. Positioned below refractory-lined flare burners, these massive seals(also called labyrinth or molecular seals) consume vast amounts of expensivepurge gas; but they are only partially effective. Because these seals do notprevent air intrusion, the flare burner tips employed with this obsolete technology must be refractory-lined to extend tip life and delay the inevitable and verydangerous burn-back and burn-thru conditions.

Burning inside a refractory-lined flare tip causes quick failure of the refractory,which is also subjected to temperature/expansion cycling. Refractory failure

then leaves the upper portion of the flare burner unprotected from searingflames that further accelerate damage to the flare tip.

Pieces of refractory accumulate in the base of the massive seal, where theycan interfere with waste gas/offgas flow, building up pressures that are sufficient to burp refractory from an elevated stack. Spalled refractory will alsoplug the drain in the base of a mole seal, accelerating corrosion and the verydangerous burn-thru conditions. Burn thru n a flare tip nd down inside a moleseal allows explosive gas/air mixtures to penetrate flare system - with catastrophic results.

Frequent refractory maintenance, frequent flare-.tip replacements, and c u t ~ i n gout corroded sections of a mole seal, then welding on patches create senoussafety problems. Explosive gases may surround a flare stack in a refinery, gasplant, offshore platform or chemical/petrochemical processing facility.

Dangerous Trade Ofts for Short Term Profits

Maintenance expenses average nine p ercent in refineries and large chemical/petrochemical plants. To improve profit margins, some plant managers are


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deferring maintenance, downsizing their maintenance staffs, and outsourcingmaintenance, repair and retrofit responsibilities.

Refineries and chemical plants are inherently dangerous places to work. Hundred of miles of piping transport volatile, toxic and carcinogenic liquids andgases under intense heat and pressure. Pumps and valves fail. Pipes corrode.And refractory-lined flare tips require frequent repair and replacement.

Problems with sophisticated electronics can, for example, put an entire plant at-risk. Recently, a pair of computers in a Texas refinery stopped communicatingwith each other for 15 seconds. That brief lapse in communications and controlcaused more than 66,000 gallons of naphtha to spew from several damagedpipe connections.

A giant vapor cloud of naphtha, a highly volatile petroleum distillate, floatingabove a refinery is very dangerous. Naphtha is so explosive it can be ignitedby sparks from the distributor of a passing car.) Skilled refinery employeesacted quickly, shutting down and bypassing damaged pipelines. The vaporcloud dissipated. The dangerous situation was over within 15 minutes.

If the naphtha had been ignited, the explosion would have leveled the refineryunit and ruptured nearby acid-gas lines containing hydrogen sulfide, H S which

can kill many people in extremely small doses.

Knowledgeable refinery managers, who may be forced by corporate decisionsto defer maintenance, realize they cannot cut any corners in the maintenance,repair and replacement of flare pilots and burner tips. They are also acutelyaware of the four essential requirements for elevated flares:

1. Safe, reliable operation2. Dependable ignition3. No air penetration into flare tips,

seals and stacks/headers4. No shortcuts in design and construction

to ISO-9001 quality standards

Refractory Free luidic lare ips

State-of-art elevated flares utilize a lightweight luidic SeaI a patented mUlti-cone device with no moving parts, that is built into the refractory-free alloy tip ofa luidic lare burner to provide a straight-thru, wall-less venturi flow path forwaste gases in only one direction. This ensures maximum exit velocities forbetter control of flare flame patterns.


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m ~ l o y i n gthe principle of the Tesla diode and the boundary-layer effect, them u l t l ~ a f f l ereverse flow seal uses the kinetic energy of air attempting to enter a

flare tiP to turn that intruding air back upon itself. (This technology has alsobeen used to develop effective Fluidic Windshields for flare pilots to maintaincontrolled flame patterns despite winds up to 250 mph.)

Wind . v e l ~ c i t yand wind direction are less significant than the kinetic energy ofthe wind Itself. The greater the turbulence and downward pressure of the wind,the greater the effectiveness of the patented kinetic seal. The effectiveness ofthis unique, maintenance-free seal is not influenced by wide variations in wastegas flow.

There is no need for any troublesome refractory lining inside a Fluidic Flare tip.Other advantages include: No bottom-mounted molecular seal with significantstructural and windloads on elevated stacks; and no need for frequent maintenance and replacement of refractory-lined flare burner tips. The working life ofa Fluidic Flare tip is typically five times longer than a troublesome refractorylined flare tip. (Many Fluidic Flares in continuous service onshore and offshoresince the 1970s have never been replaced.)

Safety is a very important advantage of the Fluidic Flare tip. In addition topreventing air intrusion, the compact, lightweight kinetic seal, proven in morethan 2700 worldwide installations, eliminates all serious safety problems in

volved in cutting out corroded sections of molecular seals, removing broken re-fractory, then welding on metal patches, high above a plant where volatilefumes may be present.

Since the late 1970s, the Fluidic Seal has been positioned at the very top of therefractory-free flare tip. Patents cover the unique design configuration and thepositioning of this multi-cone seal. (U.S. patents 3,730,670 4,092,908 . Alsoprotected by foreign patents. Other patents pending.)

Flame Stability in Crosswinds

A flare is only as good as its ability to maintain a stable flame, despite wind andweather conditions. n effective flame retainer/flame holder on the top of aflare burner is the key element in maintaining a stable flame. Without a flameretainer, the flame will lift off; and it may be snuffed out by crosswinds. Themaximum exit velocity for many flares is approximately Mach 0.2, or 220 ftls67 m/s for natural gas. With other types of flame holders such as a bluff

body, the exit velocity can increase to Mach 0.5.

Unfortunately, most flame retention designs are not very effective.


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An effective flame retaining device will allow both a higher exit velocity and asmaller flare tip. The flare flame will then be erect, stable and vertical, regardless of crosswinds. This results in lower thermal radiation at grade level and amuch cooler, longer-lasting flare tip because flame lick along the side of theflare burner is reduced. An effective flame retainer is also critical for maintaining high combustion efficiency at velocities up to Mach O B

State-of-art flame retainers employ Vor u wirt vanes to swirl approximately300/0 of the flare gas in a vortex configuration. The other 70 goes straight up.The swirling ball of flame enhances recirculation at the base of the main flareflame, thus assuring a stable flame and preventing dangerous flame lift-ofts andflame-outs, regardless of variations in waste gas flow or weather conditions -from dead-still to hurricane- or typhoon-force winds.

esign Pilots for Maximum Effectiveness

All flares require dependable pilots, reliable ignition systems, and effective pilotwindshields, which must protect pilot flames to ensure proper ignition of wastegas streams.

Pilot windshields nozzles), although often overlooked, are extremely important.Without them, the risk of an inoperable flare is real danger. Even if the flame is

not extinguished by high winds, it may be directed away from the flare and rendered useless. The number of pilots used on a flare should be increased according to the diameter of the flare to prevent this possibility.

Large flares require several pilots to assure ignition, regardless of wind direction. The size and number of pilots is determined by the size, deSign, andfunction of a flare, and the heat level of the waste gas.

t is important to nc >te that in our discussion of pilot windshields, we are referring to full windshields, which protect flare pilot flames in all directions. Partialwindshields, supplied by some manufacturers, protect pilot flames from the windin only one direction. Many ineffective windshields trap rain, snow and condensate.)

Dependable pilots incorporate stainless steel windshields, effective flame retention nozzles and pilot-air inspirating venturis with gas filters to assure a reliablesource of primary air for stable, reliable pilot flames. Fluidic Windshields, previously mentioned, assure complete protection of pilot flames, regardless of windspeed, wind direction, and hurricane- or typhoon-force rains, which can extinguish poorly designed flare pilots, while filling ignitor lines with water and thuspreventing re-ignition.

{


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If a flare ,pilot. is c o ~ s t r ~ c t e dwith only a simple retention-type nozzle, it is likelyto ~ e extinguished In high crosswinds. An effective windshield, the length of

which should be 1 3 r dthe total length of the pilot flame, will prevent this probI ~ m ~ r o p e ~ l yd e ~ l g n e dand manufactured to 180-9001 quality standards, apilot windshield will protect a pilot flame and keep it lit, despite the worst onshore or offshore conditions.

Why Constant Burning Pilots?

constant burning flare pilots are needed to guarantee effective ignition of wastegas streams. Typically, flare pilots have flames that are approximately 18(450mm) long. To maintain optimum flame stability and reliability, an inspiratingor venturi-type gas burner is used in a flare pilot. A small gas jet, installed inthe venturi tube at the base of the pilot, will introduce a gas/air mixture for astable, fast burning flame and deliver that flamefront to the pilot tip.

Pilot flames become unstable or tend to blowout if they are plugged by foreignmaterial or dirt in the pilot gas stream. To insure proper pilot operation, it isnecessary to keep pilot lines clean*.

Retractable pilots, conveniently accessible from grade level, will simplify periodic inspection, cleaning and any readjustments.

Because pilot flames are not visible to the eye in daylight, a thermocoupleshould be used to monitor pilot operation. A well-designed and thoroughlyproven thermocouple is a reliable, convenient method of detecting a pilot flame.However, a thermocouple mounted on a pilot without an effective windshield willreact erratical ly in high winds. Wind shear will tend to blow the pilot flame awayfrom the thermocouple, resulting in an inaccurate readings, thus causing falsealarms and necessitat ing unnecessary EPA paperwork. To prevent these problems, while minimizing wind or rain intrusion, the thermocouple should be located inside a Fluidic Windshield.

Other Pilot Design Parameters

For an inspirating flare pilot burner, the mixture tube length may be severalhundred feet. By contrast, mixture tube lengths for the regular inspiratingpilots of process burners are usualty in the order of 2 to 6 ft The influence of

Our experience shows that when a new ,flare or pilot is p ~ a c e din service , ~ lines mU ,st be ~ I o w nout before commissioning. Use of a strainer on fuel gas lines to keep the lines clean S deSirable ,as is a liquid trap or knockout pot to r e ~ o v eliquid from the g a ~stream , A strainer s ,hould ot beplaced in a flamefront line because It Will prevent the propagation of a flame to the pilots ,


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the extra-long tube lengths on the inspiration process must be taken into account in designing effective flare pilot burners.

Regular inspirating pilot burners can be designed or analyzed on the basis ofthe Euler momentum equation, as applied to inspirators. This equation can bepresented as:

Eq.1

Note: Nomenclature appears at end of this paper.

Taking into account the influence of a long mixture tube on the inspiration process can be done two ways. The first consists of entering the friction coefficient K into equation 1. The second consists of applying a correlation for consideration of the influence of friction pressure upon excess air mixture .

To evaluate the suitability of the aforementioned calculation concepts, extensiveflare pilot tests have been conducted. One experimental setup represented aninspirating flare pilot of nominal 1-1/4 diameter with a mixture tube that couldbe varied in length from 9 to 1 00 ft Natural gas/air flow rates, exit mixturevelocities and static pressure drop in the mixture tube were measured. Theexcess air factor for the gas/air mixture was determined by gas/air flow rates

and by analysis of gas/air concentrations in the mixture tube.

Test results are presented in figures 1 and 2 . Lengthening the pilot leads to anincrease in the mixture pressure drop (fig. 1 and an appreciable decrease inthe excess air factor (fig. 2).

On the basis of obtained experimental data and equation 1, the value of frictionfactor f in the mixture tube was determined. It is obvious from fig . 3 ( FrictionFactor vs. Pilot Length ), the value of friction factor f varies , essentially depending upon the mixture tube length. This fact contradicts the physical sense offriction factor, which for a constant Reynolds Number for a definite fluid , pipediameter and pipe roughness .

Thus, it is obvious: The introduction of the friction resistance coefficient K intoequation 1 cannot be used for a design procedure for long inspirat ing pilots.This phenomenon can be explained by the fact that equation 1 was developedprimarily for flow potential conditions. Such conditions do not take into accounta complicated vortex process in a mixing tube with the presence of s ignificanthydraulic resistance.

l


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Th.e appropriate design procedure can be done in the following way. Without~ o l n . gt ~ the root of the matter, we can consider the interaction between the~ , n s p l r a t l o np'rocess and flow conditions with significant hydraulic resistance as ablack b o ~ , ~ d a very long flare pilot can be considered as involving two parts:

a regular InspIrator and a long mixture tube.

In.spiration c ~ nbe calculated by equation 1, used for regular inspirating burnerswIth short mIxture tubes. Then, the excess air factor's empirical dependenceon mixture tube pressure drop, n = ~ L l . p ) ,can be used to enter a correction forthe value n, obtai ned from equation 1.

The function n = ~ L l . p )can be presented as dimensionless. See fig. 4 ( Excess

Air Factor vs. Pressure Drop ). This allows the use of the design procedure formixture tubes of different lengths and diameters in English or metric measurements.

Inspiration calculations carried out on the basis of this developed design procedure demonstrate good accuracy. The average design error for flare pilots inthe range of lengths up to 100 t was 5 percent. Fig. 5, with equations 2 thru 5,depicts the appropriate design procedure steps.

ssuring Reliable Ignition

A constant burning pilot flame is essential since any gas vented from a flaremust be ignited. Pilot ignition systems have come a long way in recent years.It wasn't so long ago that major companies were using flaming arrows, burningrags on pulleys and other primitive ignition methods.

The most common ignition system is a flamefront generator with remote ignitionpanel, developed in the 1940s by a major oil company. While there are certainmisconceptions about the operation and reliability of these panels, they usuallycan be attributed to a limited working knowledge and/or an improper installation.

Air and fuel gas are metered through orifices in the remote ignition panel toform a combustible mixture; then a spark is introduced to generate a flamewhich is delivered through a 1 diameter pipe to the flare pilot, safely and quickly. When there are multiple flare pilots, there are either valves to direct flamefronts to each pilot or a manifold to direct a single ball of flame to all pilots atthe same time.

The ball of flame travels thru the pipe with the combined velocity of the gas/airmixture [approximately 88 tls (27m/s)]. With natural gas, the burning speed ofthe flame is 75 tls (23 m/s . The flamefront arrives at the top of the flare stack


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to ignite the pilots within a few seconds. This type of flare ignition system hasbeen used for many years.

One of the drawbacks of this system is that ignitor lines can accumulate moisture and condensate and ultimately become fouled or plugged. Undergroundlines should be avoided since these are inevitable havens for the buildup ofwater and other plugs. f an underground line must go under a road or otherobstruction, it must have a proper slope with a drain accessible for cleanout atthe low point.

Keep Ignition System ry

Moisture is the single most significant problem to consider when locating and

installing a pilot ignition system. The air which is mixed with the gas steammust be dry. f not, it will collect condensate. A condensate trap or smallknockout pot must be employed.

Wet compressed air will flood ignition lines and short-out spark plugs, preventing ignition of the pilots. Even the use of dry instrument air and/or dryers cannot guarantee the complete prevention of moisture, because the combustionprocess generates water vapor. A drain valve should be installed at the lowestpoint o the line between the ignition panel and the pilot as a method o elimi-nating condensate produced by flamefronts.

Before ignition of a flare pilot is attempted, the air valve on the ignition panelshould be opened to allow any existing moisture to escape. The spark plug canthen be checked by using the pushbutton and sightport. t is important that theproper pipe size be used, as specified by the equipment supplier, along withappropriate air and gas pressures to deliver the flamefront to the pilot with avelocity of approximately 100 ftls (30.5 m/s .

Oversized or undersized lines will not produce good results; and they will leadto problems. Oversized lines will cause slow travel of the flamefront, allowing itto cool off and go out. Undersized lines may increase the speed and turbulence of the flamefront and cause it to be unstable.

By opening the air valve to approximately 20 psig and the gas valve to about10 psig, a mixture close to the optimum requirement for a proper flamefront willbe achieved.

t is imperative that installation instructions must be adhered to. A commonproblem is the reversal of ignitor and pilot tubes from the pilot gas supply to thepilot venturi. This can be avoided easily if proper care is taken during installa-


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tion.

Electronic Ignition of Flare Pilots

For more than a decade, the trend has been toward electronic ignition. 8tateof-art, s p a r k ~ i g n i ~ e dp i l ~ t su ~ i l i z ea high-energy excitor, originally developed ford e ~ e n d b l eI ~ n l t l o ~of Jet aircraft engines. With this high-energy excitor, anymOisture or dirt bUildup on a pilot tip will literally be blown off by the spark.

The use of a direct spark at the top of the flare has been tried many times. Itwas abandoned in the late 1940s. Top-mounted ignitors will fail quickly because they are subjected to flame impingement.

Remote-spark pilots, designed and manufactured to 180-9001 standards, offerfast paybacks for retrofit installations. Thousands are in service worldwide, including hundreds of replacements for defective, unreliable, short-life pilots.

omplete ombustion of Waste Gases

Typical flare combustion efficiencies are 99+ for hydrocarbons such as naturalgas, ethane, propane and butane. Hydrocarbons such as ethylene and propylene will have similar efficiencies if smokeless operation is achieved through theuse of steam, air blower or mUlti-tip designs. The combustion efficiency will be

lower for gases having low-Btu heating value from high contents of nitrogen,carbon dioxide, water vapor or other inert media.

For EPA compliance, if the heating value is below 300 Btu/scf3

(11.2 MJ/m3),some auxiliary fuel such as natural gas or propane must be utilized to increasethe heating value. In some applications, particularly if H 8 is present, the heating value must be further increased. When a gas contains considerable quantities of hydrogen ~ d carbon monoxide (as is the case for steel plant gases)that are fast burning, the minimum heating value for complete combustion canbe as low as 80 to 90 Btu/scf 3 (3.4 MJ/m3); but those low-Btu gases must beenriched to comply with EPA regulations.

If a flare requires steam or assist gas, one should consider using a low profileenclosed flare. An enclosed ground flare, sometimes called a thermal oxidizerflare, will allow complete combustion of a gas at a lower calorific content withoutthe use of any assist media.

Burners are the key to reliability and performance of state-of-art low profileenclosed flares, which require no steam to maintain smokeless destruction ofwaste gases and waste liquids. With these flares, there are no visible flames,


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no objectionable noise, no thermal radiation problems, and no possibility ofburning rain (liquid carryover from elevated flares with improperly designed

knockout drums) splattering plant personnel and equipment.

onclusion

With dependable pilots, flares will dispose of unwanted waste gases safely andreliably, while protecting the global environment and providing trouble-free operation. Proper operation, however, entails a joint responsibility between manufacturer and user. A good working knowledge of the principles of flare designand function by the user will preclude many problems before they occur.

To deliver the most appropriate, safe flaring system, it is the responsibility ofthe flare manufacturer to know all detailed operating parameters and to refuseto quote flare systems which are inherently unsafe. To make full use of today 'sproven technologies, there must be a complete understanding between responsible buyers (including engineering contractors), ISO-certified manufacturersand end-users .

References :

1 . Eliminate Air Polluting Smoke in Petroleum Processing , by Walter R . Smith, pp . 61-67 , Oct.

1954, Chemical Engineering .2 . Predicting Radiant Heating from Flares, by T.A. Brzutowski and E.C . Sommer , Jr . 38th API

Midyear Meeting, May 17, '73 , Philadelphia , PA .3 . Flaring in the Energy Industry , Brzutowsk i Prog . Energy Combust ion SC ience , Vo l 2 , pp . 129-

41, '76, Pergamon Press , Great Britain .4 . Flare Safety and Engineering , by John F . Stra itz III, P .E . 11th Annual Loss Sympos ium ,

AIChE, March 20-24 , '77 , Houston, TX.5 . Make the Flare Protect the Environment , Straitz , pp . 76-81 , Oct. 7 , '77 , Hydrocarbon

Process ing .6 . Solving Flare Noise Problems, Straitz, May 8-10 , '78 , Inter-No ise78 Sem inar , S . Franc isco , CA .7 . Proper Flare Operation Conserves Energy In Refinery , Stra itz , pp . 37 -42 , Jan .1, '77 , O il &

Gas Journal.8 Flare Technology Safety , Stra itz pp .53-62 , July '87, Chemical Eng ineer ing Progress .9 . High-Performance Offshore Flares , Stra itz , Fourth Annua l Flare System Sympos ium , Oct. 14-

16 , ' 86, Trodheim, Norway.10 . How New Pilots Were Installed on an Operating Flare , Straitz , pp . 33 -34 , March '92 , Oil, Gas

Petrochem Equipment.11 . Improve Flare Design, Straitz , pp 86 -90 , Oct. '94 , Hydrocarbon Process ing .12 . Where Safety Is Paramount , Stra itz , pp 62 -66 , Feb. '96 , Pe troMin.13 . Use el Quemador Optima Para Cada Aplication , Straitz , pp 76 -79 , May '96 , Pet roleo Inte rnac

ional.14 . Improve Flare Safety to Meet ISO -9001 Standards , S trai tz , pp 109 -11 0 & 1 12 , 114, Ju ne '96

Hydrocarbon Processing . '


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Fig 1 Mixture Pressure Drop vs Pilot Length

. 8 r ~ ~ ~ _ .

u3 6c

Q0

01)

:Jenen1)

CL 41)

: JB

. 2 ~ ~ ~ ~ _ r ~

o L ~ ~ ~ ~o 20 00

Pilot Length Ft

Pilpap grf


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Fig 2 Excess Air Factor vs Pilot Length

. 9 ~ - - - - - - ~ - - - - - - - - ~ - - - - - - - - ~ - - - - - - - - ~ -

. 8 ~ - - - - - - ~ - - - - - - - - ~ - - - - - - - - - 4 - - - - - - - - -

- - -u 70u..-

/)/)

l)

uX

LU

.6

Pilot Length Ft

pilpopl grf


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0

)0

u..c0-)

~

u

14

Fig 3 Friction Factor vs Pilot Length

0 2 r - - - - - - - ~ - - - - - - - - ~ - - - - - - - - ~ - - - - - - ~ -

.A

. 0 2 ~ ~ - - - - - - - - - - - - - ~ - - - - - - - - ~ - - - - - - _ 4 - -

01

. . . . ~ ~

. - 01 L - - - - - - - - - - - - - - - - ~ - - - - - - - - r _ - - - - - - ~ - - -

oo

p lpop1 grt

2 4 80 100

Pilot Length Ft


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......, / ), / )

Q

co( , / )

Q

E- 0'-

- -o

+-

oo

u -

, / ), / )

Qox

U J

-15-

Fig 4 Excess Air Foetor dimensionless) vsPressure Drop dimensionless)

, 8 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 - - - - - - - - - -

, 6 - - - - - - - - - - - - - - - ~ - - ~ - - - - - - - - - - 4 - - - - - - - - - -

,0. _ _

4 ~ - - - - - - - - - - - - - - ~ - - - - - - - - - - - - - - ~ - - - - - - - - - -o 10 20 30

Pressure Drop dimensionless)

p pap 1 grtI =>


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Fig. 5 Pilot Design Procedure Steps

Step 1 Determine the mixture excess air factor no for given valuesof gas jet. diameter dj bumer nozzle diameter d n , type of gas, andfor gas/air temperatures as for a regular short) inspirating burner.This determination can be done using a regular inspirationequation for example eq. 1).

Step 2 Determine gas volume flow rate Qg, mixture volume f lowrate Qmix and mixture velocity in a mixture tube V for given valuesof et diameter dj, burner nozzle diameter d n and gas pressure Pgas for a regular short) inspirating burner.

Step 3 Determine friction factor f by a value of Reynolds Number inthe mixture tube.

Step 4 Calculate the friction pressure drop value flp1.0 for a regularinspirating burner with a short mixture tube with a length 10:

I V2n I f X X X P .

.0 dll

2g mu Eq 2

Step 5 Calculate the friction pressure drop value /1 Pf for the realpilot mixture tube with the length I

I V2f l p l = f x d x 2 x P m a Eq 3

gStep 6 Calculate the dimensionless value of friction pressure drop

An - 1p1- j / I -

1p1.0Eq . 4

Step 7 Find on Fig.4 the dimensionless value of excess air factor by

value of I PI

Step 8 Calculate the actual excess air factor value for flare pilotwith the length I :

Eq.5


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n

AL

rorgrmix

ff

non

g

===

====

====

=

=

=======

=

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Nomenclature

cross sectional area of a burner nozzle ft2cross sectional area of a gas jet ft2actual volume of inspirating air per unit volume ofgas ft3/ft3air density Ib/ft 3gas density Ib ft3air-gas mixture density Ib/ft 3drag coefficient for air entrance and air-gas mixture

exitfriction factorfriction resistance coefficientexcess air factor for a regular inspirating burnerexcess air factor for an inspirating pilot with a longmixture tube

pressure drop for a regular inspirating burner Ib/ft

pressure drop for an inspirating pilot with a longmixture tube, Ib/ft 2

gas jet diameter, ft Iburner nozzle diameter, ftgas volume flow rate scfhgas-air mixture volume flow rate scfhmixture velocity in a mixture tube, fpsgas pressure psigmixture tube length for a regular inspirating burnerftmixture tube length for an inspirating pilot with along mixture tube, ftacceleration of gravity 32.17 ft/sec 2

if = :;Pf dimensionless friction pressure dropflp / 0

_ n-

o

= dimensionless excess air factor

flare pilot design

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