If a flare cannot be ignited reliably and a stable flame maintained at all times to burn the waste gas, we do not have a flare. What we have is a vent pipe - where unburned heavy hydrocarbons or other combustible gases can descend to grade level and become ignited to produce a flashback, a fire, and possibly a catastrophic explosion.
Flare Pilot Design 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 collection and control has increased the need for ultra-safe and extremely reliable control of waste gases and offgases from energy exploration and production, petroleum refining and chemical/petrochemical, pharmaceutical, pulp and paper, and other process industries. Due to downsizings, deferred maintenance and other corporate cost-cutting measures, major corporations worldwide are insisting on more reliable products, systems and services, including state-of-art designs for flare pilots and flare systems that combine long, dependable service lives with minimal maintenance. Flare technology and manufacturing practices of the 1940s, '50s and '60s are maintenance-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 ISO9001 quality standards. The four most critical components of a flare are:
o o o
o
flare flare flare flare
burner tip seal pilot burner(s) 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 keys to safe flare operation. If a flare cannot be ignited reliably and a stable flame
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maint~ined at all. times to burn the waste gas, we do not have a flare. What we have IS a vent pipe - where unburned heavy hydrocarbons or other combustible gase~ can descend to grade level and become ignited to produce a flashback, a fire, and possibly a catastrophic explosion.
B~fore we d.iscuss design calculations, testing, installation and service of inspiratlng flare pilot burners, which differ from regular furnace or boiler inspirating pilot burners mainly by mixture-tube length and by severe "open environment" application 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 elevated flares. Positioned below refractory-lined flare burners, these massive "seals" (also called labyrinth or molecular seals) consume vast amounts of expensive purge gas; but they are only partially effective. Because these seals do not prevent air intrusion, the flare burner tips employed with this obsolete technology must be refractory-lined to extend tip life and delay the inevitable and very dangerous 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 searing flames that further accelerate damage to the flare tip. Pieces of refractory accumulate in the base of the massive seal, where they can interfere with waste gas/offgas flow, building up pressures that are sufficient to "burp refractory" from an elevated stack. Spalled refractory will also plug the drain in the base of a mole seal, accelerating corrosion and the very dangerous burn-thru conditions. Burn-thru in a flare tip and down inside a mole seal allows explosive gas/air mixtures to penetrate a flare system - with catastrophic results. Frequent refractory maintenance, frequent flare-.tip replacements, and cut~ing out corroded sections of a mole seal, then welding on patches create senous safety problems. Explosive gases may surround a flare stack in a refinery, gas plant, 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 outsourcing maintenance, 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 and gases 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 atrisk. Recently, a pair of computers in a Texas refinery stopped communicating with each other for 15 seconds. That brief lapse in communications and control caused more than 66,000 gallons of naphtha to spew from several damaged pipe connections. A giant vapor cloud of naphtha, a highly volatile petroleum distillate, floating above a refinery is very dangerous. (Naphtha is so explosive it can be ignited by sparks from the distributor of a passing car.) Skilled refinery employees acted quickly, shutting down and bypassing damaged pipelines. The vapor cloud dissipated. The dangerous situation was over within 15 minutes.
If the naphtha had been ignited, the explosion would have leveled the refinery unit and ruptured nearby acid-gas lines containing hydrogen sulfide, H2 S, which can kill many people in extremely small doses. Knowledgeable refinery managers, who may be forced by corporate decisions to defer maintenance, realize they cannot cut any corners in the maintenance, repair and replacement of flare pilots and burner tips. They are also acutely aware of the four essential requirements for elevated flares: 1. 2. 3. 4.
Safe, reliable operation Dependable ignition No air penetration into flare tips, seals and stacks/headers No shortcuts in design and construction to ISO-9001 quality standards
Refractory-Free Fluidic Flare™ Tips State-of-art elevated flares utilize a lightweight Fluidic SeaI™, a patented mUlticone device with no moving parts, that is built into the refractory-free alloy tip of a Fluidic Flare™ burner to provide a straight-thru, wall-less venturi flow path for waste gases in only one direction. This ensures maximum exit velocities for better control of flare flame patterns.
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Em~loying the principle of the Tesla diode and the boundary-layer effect, the multl-~affle reverse 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 also been used to develop effective Fluidic Windshields™ for flare pilots to maintain controlled flame patterns despite winds up to 250 mph.) Wind .vel~city and wind direction are less significant than the kinetic energy of the wind Itself. The greater the turbulence and downward pressure of the wind, the greater the effectiveness of the patented kinetic seal. The effectiveness of this unique, maintenance-free seal is not influenced by wide variations in waste gas flow. There is no need for any troublesome refractory lining inside a Fluidic Flare tip. Other advantages include: No bottom-mounted molecular seal with significant structural and windloads on elevated stacks; and no need for frequent maintenance and replacement of refractory-lined flare burner tips. The working life of a Fluidic Flare tip is typically five times longer than a troublesome refractorylined flare tip. (Many Fluidic Flares in continuous service onshore and offshore since the 1970s have never been replaced.) Safety is a very important advantage of the Fluidic Flare tip. In addition to preventing air intrusion, the compact, lightweight kinetic seal, proven in more than 2700 worldwide installations, eliminates all serious safety problems involved in cutting out corroded sections of molecular seals, removing broken refractory, then welding on metal patches, high above a plant where volatile fumes may be present. Since the late 1970s, the Fluidic Seal has been positioned at the very top of the refractory-free flare tip. Patents cover the unique design configuration and the positioning of this multi-cone seal. (U.S. patents 3,730,670 & 4,092,908. Also protected 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 and weather conditions. An effective flame retainer/flame holder on the top of a flare burner is the key element in maintaining a stable flame. Without a flame retainer, the flame will lift off; and it may be snuffed out by crosswinds. The maximum exit velocity for many flares is approximately Mach 0.2, or 220 ftls (67 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 a smaller 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 a much cooler, longer-lasting flare tip because flame lick along the side of the flare 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 VorTuSwirtM vanes to swirl approximately 300/0 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 flare flame, thus assuring a stable flame and preventing dangerous flame lift-ofts and flame-outs, regardless of variations in waste gas flow or weather conditions from dead-still to hurricane- or typhoon-force winds.
Design Pilots for Maximum Effectiveness All flares require dependable pilots, reliable ignition systems, and effective pilot windshields, which must protect pilot flames to ensure proper ignition of waste gas 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, and function of a flare, and the heat level of the waste gas.
It 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. Partial windshields, supplied by some manufacturers, protect pilot flames from the wind in 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 reliable source of primary air for stable, reliable pilot flames. Fluidic Windshields, previously mentioned, assure complete protection of pilot flames, regardless of wind speed, wind direction, and hurricane- or typhoon-force rains, which can extinguish poorly designed flare pilots, while filling ignitor lines with water and thus preventing re-ignition.
{
-6If a flare ,pilot. is co~str~cted with only a simple retention-type nozzle, it is likely to ~e extinguished In high crosswinds. An effective windshield, the length of which should be 1~3rd the total length of the pilot flame, will prevent this probI~m. ~rope~ly de~lgned and manufactured to 180-9001 quality standards, a pilot windshield will protect a pilot flame and keep it lit, despite the worst on shore or offshore conditions.
Why Constant-Burning Pilots? constant-burning flare pilots are needed to guarantee effective ignition of waste gas streams. Typically, flare pilots have flames that are approximately 18" (450mm) long. To maintain optimum flame stability and reliability, an inspirating or venturi-type gas burner is used in a flare pilot. A small gas jet, installed in the venturi tube at the base of the pilot, will introduce a gas/air mixture for a stable, fast burning flame and deliver that flamefront to the pilot tip. Pilot flames become unstable or tend to blowout if they are plugged by foreign material or dirt in the pilot gas stream. To insure proper pilot operation, it is necessary 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 thermocouple should be used to monitor pilot operation. A well-designed and thoroughly proven thermocouple is a reliable, convenient method of detecting a pilot flame. However, a thermocouple mounted on a pilot without an effective windshield will react erratically in high winds. Wind shear will tend to blow the pilot flame away from the thermocouple, resulting in an inaccurate "readings," thus causing false alarms and necessitating 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 several hundred feet. By contrast, mixture tube lengths for the "regular" inspirating pilots 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~aced in service, ~" lines mU,st be ~Iown out before commissioning. Use of a strainer on fuel gas lines to keep the lines clean IS deSirable, as is a liquid trap or knockout pot to, re~ove liquid from the ga~ stream , A strainer s,hould not be placed 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 of the Euler momentum equation, as applied to inspirators. This equation can be presented 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, extensive flare pilot tests have been conducted. One experimental setup represented an inspirating flare pilot of nominal 1-1/4" diameter with a mixture tube that could be varied in length from 9 to 100 ft. Natural gas/air flow rates, exit mixture velocities and static pressure drop in the mixture tube were measured. The excess 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 an increase in the mixture pressure drop (fig. 1) and an appreciable decrease in the excess air factor (fig. 2). On the basis of obtained experimental data and equation 1, the value of friction factor f in the mixture tube was determined. It is obvious from fig . 3 ("Friction Factor vs. Pilot Length"), the value of friction factor f varies , essentially depending upon the mixture tube length. This fact contradicts the physical sense of friction factor, which for a constant Reynolds Number for a definite fluid , pipe diameter and pipe roughness. Thus, it is obvious: The introduction of the friction resistance coefficient K, into equation 1 cannot be used for a design procedure for long inspirating pilots. This phenomenon can be explained by the fact that equation 1 was developed primarily for flow potential conditions. Such conditions do not take into account a complicated vortex process in a mixing tube with the presence of significant hydraulic resistance.
l
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Th.e appropriate design procedure can be done in the following way. Without ~oln.g t~ the root of the matter, we can consider the interaction between the ~,nsplratlon p'rocess and flow conditions with significant hydraulic resistance as a black bo~, a~d a very long flare pilot can be considered as involving two parts: a regular InspIrator and a long mixture tube. In.spiration c~n be calculated by equation 1, used for regular inspirating burners wIth short mIxture tubes. Then, the excess air factor's empirical dependence on mixture tube pressure drop, n = ~Ll.p(), can be used to enter a correction for the value n, obtai ned from equation 1. The function n = ~Ll.p() can be presented as dimensionless. See fig. 4 ("Excess Air Factor vs. Pressure Drop"). This allows the use of the design procedure for mixture 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 in the range of lengths up to 100 ft was 5 percent. Fig. 5, with equations 2 thru 5, depicts the appropriate design procedure steps.
Assuring Reliable Ignition A constant burning pilot flame is essential since any gas vented from a flare must 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, burning rags on pulleys and other primitive ignition methods. The most common ignition system is a flamefront generator with remote ignition panel, developed in the 1940s by a major oil company. While there are certain misconceptions about the operation and reliability of these panels, they usually can 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 to form a combustible mixture; then a spark is introduced to generate a flame which 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 at the same time. The ball of flame travels thru the pipe with the combined velocity of the gas/air mixture [approximately 88 ftls (27m/s)]. With natural gas, the burning speed of the flame is 75 ftls (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 has been 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. Underground lines should be avoided since these are inevitable havens for the buildup of water and other "plugs." If an underground line must go under a road or other obstruction, it must have a proper slope with a drain accessible for cleanout at the low point.
Keep Ignition System Dry 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 steam must be dry. If not, it will collect condensate. A condensate trap or small knockout 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 combustion process generates water vapor. A drain valve should be installed at the lowest point of the line between the ignition panel and the pilot as a method of eliminating condensate produced by flamefronts. Before ignition of a flare pilot is attempted, the air valve on the ignition panel should be opened to allow any existing moisture to escape. The spark plug can then be checked by using the pushbutton and sightport. It is important that the proper pipe size be used, as specified by the equipment supplier, along with appropriate air and gas pressures to deliver the flamefront to the pilot with a velocity of approximately 100 ftls (30.5 m/s). Oversized or undersized lines will not produce good results; and they will lead to problems. Oversized lines will cause slow travel of the flamefront, allowing it to 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 about 10 psig, a mixture close to the optimum requirement for a proper flamefront will be achieved. It is imperative that installation instructions must be adhered to. A common problem is the reversal of ignitor and pilot tubes from the pilot gas supply to the pilot venturi. This can be avoided easily if proper care is taken during installa-
-10tion.
Electronic Ignition of Flare Pilots For more than a decade, the trend has been toward electronic ignition. 8tateof-art, spark~ign.i~ed pil~ts u~ilize a high-energy excitor, originally developed for de~endable I~nltlo~ of Jet aircraft engines. With this high-energy excitor, any mOisture 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. It was 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, offer fast paybacks for retrofit installations. Thousands are in service worldwide, including hundreds of replacements for defective, unreliable, short-life pilots.
Complete Combustion of Waste Gases Typical flare combustion efficiencies are 99+% for hydrocarbons such as natural gas, ethane, propane and butane. Hydrocarbons such as ethylene and propylene will have similar efficiencies if smokeless operation is achieved through the use 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. 3
For EPA compliance, if the heating value is below 300 Btu/scf (11.2 MJ/m3), some auxiliary fuel such as natural gas or propane must be utilized to increase the heating value. In some applications, particularly if H2 8 is present, the heating value must be further increased. When a gas contains considerable quantities of hydrogen a~d carbon monoxide (as is the case for steel plant gases) that are fast burning, the minimum heating value for complete combustion can be as low as 80 to 90 Btu/scf3 (3.4 MJ/m3); but those low-Btu gases must be enriched to comply with EPA regulations.
If a flare requires steam or assist gas, one should consider using a low profile enclosed flare. An enclosed "ground flare," sometimes called a thermal oxidizer flare, will allow complete combustion of a gas at a lower calorific content without the use of any assist media. Burners are the key to reliability and performance of state-of-art low profile enclosed flares, which require no steam to maintain smokeless destruction of waste 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 of "burning rain" (liquid carryover from elevated flares with improperly designed knockout drums) splattering plant personnel and equipment.
Conclusion With dependable pilots, flares will dispose of unwanted waste gases safely and reliably, 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 design and function by the user will preclude many problems before they occur. To deliver the most appropriate, safe flaring system, it is the responsibility of the flare manufacturer to know all detailed operating parameters and to refuse to quote flare systems which are inherently unsafe. To make full use of today's proven technologies, there must be a complete understanding between responsible buyers (including engineering contractors), ISO-certified manufacturers and 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," Brzutowski, Prog . Energy Combustion SCience, Vo l 2, pp. 12941, '76, Pergamon Press , Great Britain . 4. "Flare Safety and Engineering," by John F. Straitz III , P.E., 11th Annual Loss Symposium , AIChE, March 20-24 , '77 , Houston, TX. 5. "Make the Flare Protect the Environment, " Straitz , pp. 76-81 , Oct. 7, '77 , Hydrocarbon Processing . 6. "Solving Flare Noise Problems, " Straitz, May 8-10 , '78, Inter-Noise78 Sem inar, S. Francisco , CA . 7. "Proper Flare Operation Conserves Energy In Refinery," Straitz , pp . 37 -42 , Jan .1, '77 , Oil & Gas Journal. 8. "Flare Technology Safety," Straitz, pp .53-62, July '87, Chemical Eng ineering Progress . 9. "High-Performance Offshore Flares," Straitz , Fourth Annual Flare System Sympos ium , Oct. 1416, '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 Processing . 12 . "Where Safety Is Paramount," Straitz , pp 62-66 , Feb. '96 , PetroM in. 13 . "Use el Quemador Optima Para Cada Aplication," Straitz , pp 76-79 , May '96 , Petroleo Intern acional. 14 . "Improve Flare Safety to Meet ISO-9001 Standards," Straitz , pp 109 -11 0 & 112, 114, Ju ne '96 Hydrocarbon Processing . '
II
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Fig. 1 Mixture Pressure Drop vs Pilot Length
.8r-----------~----------~----------~----------_.
u
3
.6
c Q.
0
~
0 (1) ~
:J en en (1) ~
CL (1)
.4
~
:J
:B ~
.2~----------~~--------~----------_r----------~
oL---------~----------~----------~--------~
o
20
~
00
Pilot Length Ft. I
Pilpap.grf
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Fig. 2 Excess Air Factor vs Pilot Length
.9~------~--------~--------~--------~---------'
.8~------~--------~---------4---------+--------~
0
-+-
u 0
.7
-
u..
~
(/) (/)
(l)
u X
LU
.6
Pilot Length Ft. I
pilpopl.grf
-14-
Fig. 3 Friction Factor vs Pilot Length
.02r-------~--------~--------~------~--------~
.
A·
.02~~-----+--------~--------~------_4--------~
~~
'-
0 -+-
()
0
u..
c
.01
,
-0
()
.~
u...
-. . . .
~~.
--. -- .01 L--------+--------~--------r_------~--------~~~
o o
20
40
80
Pilot Length Ft. I
pilpop 1.grt
100
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Fig. 4 Excess Air Foetor (dimensionless) vs Pressure Drop (dimensionless)
,...... (,/) (,/)
Q)
c
,8+---------------+---------------4---------------~
o c
(,/)
Q)
E -0
'-" '--
o
+-
o o
u-
(,/) (,/)
~~ ,6+---------------~--~----------4---------------~
Q)
o x
UJ
',0. __
,4~--------------~--------------~--------------~
o
10
20
Pressure Drop (dimensionless)
pilpap 1.grt
I I'=>
30
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Fig. 5 Pilot Design Procedure Steps
Step 1. Determine the mixture excess air factor no for given values of gas jet. diameter dj , bumer nozzle diameter d n , type of gas, and for gas/air temperatures as for a regular (short) inspirating burner. This determination can be done using a regular inspiration equation, for example eq. (1). Step 2. Determine gas volume flow rate Qg, mixture volume flow rate Qmix and mixture velocity in a mixture tube V for given values of jet diameter dj, burner nozzle diameter d n and gas pressure Pg as for a regular (short) inspirating burner. Step 3. Determine friction factor f by a value of Reynolds Number in the mixture tube. Step 4. Calculate the friction pressure drop value flp 1.0 for a regular inspirating burner with a short mixture tube with a length 10: AnI
~
.0
Io V2 =fXX-XP . d 2g mu
Eq.2
ll
Step 5. Calculate the friction pressure drop value /1 Pf for the real pilot mixture tube with the length I: I V2 flpl=fxdx2xPma g
Eq.3
11
Step 6. Calculate the dimensionless value of friction pressure drop An -
'-'j/ I
-
!1p1 !1p1.
Eq . 4 0
Step 7. Find on Fig.4 the dimensionless value of excess air factor by value of I1PI Step 8. Calculate the actual excess air factor value for flare pilot with the length I : Eq.5
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Nomenclature
An
= = =
A'
LJ
= = = =
ro rg rmix K f Kf no
= = = =
n
=
pressure drop for a regular inspirating burner, Ib/ft 2
=
pressure drop for an inspirating pilot with a long mixture tube, Ib/ft2 gas jet diameter, ft burner nozzle diameter, ft gas volume flow rate, scfh gas-air mixture volume flow rate, scfh mixture velocity in a mixture tube, fps gas pressure, psig mixture tube length for a regular inspirating burner, ft mixture tube length for an inspirating pilot with a long mixture tube, ft acceleration of gravity, 32.17 ft/sec 2
= = =
= = = = =
g
=
!¥if
=
_
n
n=-
no
cross sectional area of a burner nozzle, ft2 cross sectional area of a gas jet, ft2 actual volume of inspirating air per unit volume of gas, ft3/ft3 air density, Ib/ft3 gas density, Ib /ft 3 air-gas mixture density, Ib/ft3 drag coefficient for air entrance and air-gas mixture exit friction factor friction resistance coefficient excess air factor for a regular inspirating burner excess air factor for an inspirating pilot with a long mixture tube
/:;Pf == flp /.0 =
I
dimensionless friction pressure drop
dimensionless excess air factor
