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SAFETY IN PLANT DESIGN DESIGN PRACTICESFLARES Section

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1 of 60EXXONENGINEERING PROPRIETARY INFORMATION - For Authorized Company Use Only

DateDecember, 1999

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

CONTENTSSection Page

SCOPE ................................................................................................................................................................4

REFERENCES ....................................................................................................................................................4DESIGN PRACTICES.................................................................................................................................4INTERNATIONAL PRACTICES..................................................................................................................4OTHER LITERATURE ................................................................................................................................4

BACKGROUND ..................................................................................................................................................4

FLARE TYPES AND APPLICATION ..................................................................................................................4ELEVATED FLARES ..................................................................................................................................4GROUND FLARES .....................................................................................................................................5BURNING-PIT FLARES..............................................................................................................................5

BASIC DESIGN CONSIDERATIONS .................................................................................................................5FLARE SPACING, LOCATION AND HEIGHT ............................................................................................5FLARE CAPACITY AND SIZING ................................................................................................................5FLASHBACK SEALS ..................................................................................................................................6

DESIGN PROCEDURE .......................................................................................................................................6ELEVATED FLARES ..................................................................................................................................6MULTIJET FLARES ....................................................................................................................................7STACK DESIGN AND DIMENSIONS FOR MULTIJET FLARES (SEE FIGURE 6)....................................8BURNING-PIT FLARES............................................................................................................................11FLARE PILOTS AND IGNITERS - ALL FLARES......................................................................................13FLASHBACK PROTECTION FOR FLARE SYSTEMS .............................................................................14REDUCING FLARE PULSING AND NOISE .............................................................................................16FLARING OF H2S STREAMS...................................................................................................................17SPARE FLARE CAPACITY CONSIDERATIONS .....................................................................................18FLARE GAS METERING ..........................................................................................................................18PROTECTION AGAINST LOW AMBIENT OR FLARE GAS TEMPERATURES ......................................18

APPENDIX A - RADIATION AND SPACING FROM FLARES AND IGNITED VENTS....................................42COMPUTER CALCULATIONS .................................................................................................................42SIMPLIFIED CALCULATION OF FLAME SHAPE AND LENGTH ............................................................42

Calculation Procedure............................................................................................................................42DETAILED CALCULATION OF FLAME SHAPE AND LENGTH...............................................................43CALCULATION OF LOWER FLAMMABLE LIMIT CL ...............................................................................44

Changes shown by ➧

DESIGN PRACTICES SAFETY IN PLANT DESIGN

SectionXV-E

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FLARES

DateDecember, 1999 PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXONENGINEERING


CONTENTS (Cont)Section Page

IGNITED PRESSURE RELIEF VALVE RELEASES.................................................................................45Equal Diameter Vents ............................................................................................................................45Unequal Diameter Vents........................................................................................................................45Definition of Terms.................................................................................................................................45

CALCULATION OF RADIANT HEAT FLUX K ..........................................................................................45Information Required .............................................................................................................................45Calculation Procedure............................................................................................................................46

PERSONNEL EXPOSURE LIMITS TO HEAT RADIATION .....................................................................47Heat Radiation From Flares...................................................................................................................47Heat Radiation From Ignition of Pressure Relief Valve Releases and Start-up Vents ...........................47

EQUIPMENT EXPOSURE TO HEAT RADIATION...................................................................................48

APPENDIX B - LIMITS OF FLAMMABILITY OF GAS MIXTURES..................................................................54PRESSURE AND TEMPERATURE EFFECTS ON FLAMMABLE LIMITS...............................................54

Increased Pressure Will Widen Flammability Limits...............................................................................54Increased Temperature Will Also Widen the Flammability Limits...........................................................54

CALCULATION OF FLAMMABILITY LIMITS ...........................................................................................54

TABLESTable 1 Comparison of Flare Types.............................................................................................19Table 2 Elevated Flare Tips .........................................................................................................20Table B-1 Limits of Flammability of Gases and Vapors, Percent in Air............................................56Table B-2 The Calculation of Flammable Limits ..............................................................................57

FIGURESFigure 1A Smokeless Elevated Flare Tip (Steam Ring Type)..........................................................22Figure 1B Smokeless Elevated Flare Tip (Steam Ring and Inspirated Air Type).............................23Figure 1C Smokeless Elevated Flare Tip (Steam Inspirated Air Type)............................................24Figure 1D Smokeless Elevated Flare Tip (Steam Inspirated Air Type)............................................25Figure 2A (Customary Units) Pressure Drop in John Zink Flare Tips ..............................................26Figure 2B (Metric Units) Pressure Drop in John Zink Flare Tips......................................................27Figure 3 Nominal Steam Requirement for Typical Proprietary Flare Tips (John Zink Stf-S) .........28Figure 4 Flare Performance Chart for Propane .............................................................................29Figure 5 Multijet Flare Seal Drum Arrangement For Use on Multijet, Ground, or

Staged Elevated Flare.....................................................................................................30Figure 6A Details of Typical Multijet Flare .......................................................................................31Figure 6B Details of Typical Multijet Flare .......................................................................................32Figure 7 Typical Burning-Pit Flare.................................................................................................33Figure 8 Typical Elevated Flare Pilot and Igniter...........................................................................34Figure 9A (Customary Units) Typical Flare Seal Drum....................................................................35Figure 9B (Metric Units) Typical Flare Seal Drum ...........................................................................36Figure 10 Molecular Dry Seal (John Zink).......................................................................................37Figure 11 Fluidic Seal (National Airoil) ............................................................................................38Figure 12 Typical Non-Pulsing Sparger ..........................................................................................39Figure 13 NH4HS Deposition Temperatures ...................................................................................40Figure 14 NH4HCO3 Deposition Temperatures...............................................................................41


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DateDecember, 1999


CONTENTS (Cont)Section Page

Figure A-1A (Customary Units) Flame Center for Flares and Ignited Vents Zc- Vertical Distance, ft ..49Figure A-1B (Metric Units) Flame Center for Flares and Ignited Vents Zc- Vertical Distance, m.........50Figure A-2A (Customary Units) Flame Center for Flares and Ignited Vents Xc- Horizontal

Distance, ft ......................................................................................................................51Figure A-2B (Metric Units) Flame Center for Flares and Ignited Vents Xc- Horizontal Distance, m.....52Figure A-3 Dimensionless Flame Coordinates..................................................................................53Figure B-1 Flammable Limits for Hydrogen, Carbon Monoxide, Methane, With Nitrogen,

Carbon Dioxide and Water Vapor....................................................................................58Figure B-2 Flammable Limits for Paraffin Hydrocarbons With Nitrogen and Carbon Dioxide ...........59Figure B-3 Flammable Limits for Methane, Ethylene, Benzene With Carbon Dioxide, Nitrogen

and Water Vapor .............................................................................................................60

Revision Memo

12/99 Highlights of this revision are:

Page 14 Eq. (9), corrected formula to include denominator inside the square rootsign.

Page 54 Clarified how the flammability limits expand at higher pressures. Alsoclarified how to apply the multiplier in the table to estimate the UFL.

Page 57 Corrected typographical error of UFL of H2/N2 mixture in Calculation 8 andin Summary.

Page 59 Corrected formula for propane (C2H6) in the propane/CO2 mixture curve.


SectionXV-E

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EXXONENGINEERING


SCOPEThis section presents the criteria for selecting, designing and spacing elevated, burning-pit and multijet flares. The design ofsafety valve and flare headers is covered in Section XV-C, and associated blowdown drums, water disengaging drums etc.,are covered in Section XV-D.

REFERENCES

DESIGN PRACTICESSection VIII Fired HeatersSection XIV Fluid FlowSection XVI Noise Control

INTERNATIONAL PRACTICESIP 2-1-1, Plant Noise Design CriteriaIP 3-2-1, Sewer SystemsIP 3-9-1, Winterizing and Protection Against Ambient TemperaturesIP 4-2-1, Auxiliary Structures for Operation and Maintenance

OTHER LITERATURE1. API RP 521, Guide for Pressure Relief and Depressuring Systems, (3rd Edition, 1990).2. Predicting Radiant Heating from Flares, ER&E Report No. EE.15ER.71.3. New Seal Drum Sparger Can Eliminate Flare Pulsing, ER&E Report No. EE.57E.73.4. A Survey of Flaring Experience in Exxon Refineries and Chemical Plants, ER&E Report No. EE.45E.77.5. Safe Flaring of Concentrated Hydrogen Sulphide Streams, ER&E Report No. EE.49E.84.6. Flare Pilot Ignition and Verification, ER&E Report No. EE.47E.88.7. Use of Dynamic Simulation for Estimating Flare Loads From Process Equipment, ER&E Report No. EE.71E.97.

BACKGROUNDThe flare is a key component of the closed emergency release system in a refinery or chemical plant.Emergency releases originating from pressure relief valves, vapor blowdowns, process stream diversion, etc., and equipmentdrainage, which cannot be discharged directly to the atmosphere for reasons of safety or pollution control, are routed throughclosed systems to a blowdown drum where liquids and vapors are separated. Other emergency vapor streams are vented fromdisengaging drums handling contaminated effluent cooling water, process water drawoffs, etc.The flare provides a means of safe disposal of the vapor streams from these facilities, by burning them under controlledconditions such that adjacent equipment or personnel are not exposed to hazard, and at the same time meeting pollutioncontrol and public relations requirements.

FLARE TYPES AND APPLICATIONThree types of flare are available: the elevated flare, the ground flare, and the burning-pit flare. Although the three basicdesigns differ considerably in required capital and operating costs, selection is based primarily on pollution/public relationsconsiderations; i.e., smoke, luminosity, air pollution, noise and spacing factors. Table 1 summarizes the advantages anddisadvantages of the various types of flare.

ELEVATED FLARESThe elevated flare is by far the most commonly used type of flare used in refineries and chemical plants. The elevated flare, bythe use of steam injection and effective tip design, can be made smokeless and of reasonably low luminosity up to about 20%of maximum flaring load. Steam injection introduces a source of noise, and a compromise between smoke elimination andnoise is usually necessary. If adequately elevated, this type of flare has the best dispersion characteristics for malodorous andtoxic combustion products, but visual and noise pollution can present public relations problems. Capital costs are relativelyhigh, and an appreciable plant area may be rendered unavailable for plant equipment, because of radiant heat considerations.


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DateDecember, 1999


FLARE TYPES AND APPLICATION (Cont)Despite some of its disadvantages, the elevated flare is the general choice either for total flare loads, or for handling over-capacity releases in conjunction with a multijet ground flare. For most applications, the elevated type is the only acceptablemeans of flaring "dirty gases," i.e., gases high in unsaturates or hydrogen sulfide, or which have highly toxic combustionproducts.Three types of stack for elevated flares are used:Guyed Stack - This type is usually the least expensive to build but in some cases the guy wires result in restrictions on the useof adjacent land, in addition to normal spacing restrictions.Derrick Type Stack - This type of unit is well-suited for tall structures subject to strong winds. However, derrick type stacksare the most expensive to erect and maintain.Self-Supporting Stack - This type of unit is designed so that the flare riser pipe has no lateral structural support. For shortflares, this type is the least expensive to erect and maintain.

GROUND FLARESVarious designs of ground flare are available. The type, which has been used almost exclusively, is the multijet flare.Smokeless operation can generally be achieved, with essentially no noise or luminosity problems, provided that the design gasrate to the flare is not exceeded. However, since the flame is near ground level, dispersion of stack releases is poor and thismay result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out. Capital andoperating cost and maintenance requirements are high.The multijet flare is suitable for "clean burning" gases (i.e., where toxic or malodorous concentrations are unlikely to bereleased through incomplete combustion or as combustion products) when noise and visual pollution factors are critical. Itshould not be used in locations upwind of adjacent residential areas. Generally, it is not practical to install multijet flares largeenough to burn the maximum release load, and the usual arrangement is a combination with an elevated over-capacity flare.Other proprietary designs of ground flare suitable for refinery application are available. In some of these cases, noise isappreciable, in comparison with the multijet type, but their compact size, low space requirement, simplicity, and hence low cost,may give an overall advantage.

BURNING-PIT FLARESThe burning pit is of simple construction, with low capital and operating costs, and it can handle liquid as well as vaporhydrocarbons. Its use is usually limited by spacing requirements and smoke formation, and it is applied only in remotelocations where there are essentially no pollution restrictions.

BASIC DESIGN CONSIDERATIONS

FLARE SPACING, LOCATION AND HEIGHTSpacing, location and height of flares are determined by consideration of the following factors:Radiant Heat - Acceptable levels of radiant heat density for equipment and in areas where personnel may be present.Burning Liquid Fall-out - The possibility of burning liquid fall-out from an elevated flare, if liquid hydrocarbons should beentrained into it.Pollution Limitations (i.e., smoke formation, malodorous or toxic combustion products, noise) - May be based on statutoryand/or public relations requirements.

FLARE CAPACITY AND SIZINGFlare systems are designed to handle the largest vapor release from pressure relief valves, vapor blowdowns and otheremergency systems, which results from the design contingency. Normally the largest single contingency basis is used, asdiscussed in Sections XV-C and XV-D.Flare sizing is covered in detail under Design Procedure in this section, and sizing of the overall flare system, includingpressure relief valve and flare headers, blowdown drums and seal drums, is covered in Section XV-C.


SectionXV-E

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BASIC DESIGN CONSIDERATIONS (Cont)

FLASHBACK SEALSAs described in Section XV-B, flare systems are subject to potential flashback and internal explosion since flammable vapor/airmixtures may be formed in the stack or inlet piping by the entry of air, and the pilot constitutes a continuous ignition source.Flares are therefore always provided with flashback protection, which prevents a flame front from traveling back to theupstream piping and equipment. Design details are described later in this section.

DESIGN PROCEDURE

S ELEVATED FLARESSizing of Elevated Flares - Sizing of flare systems is a function of maximum allowable back pressure on pressure relief valvesand other sources of release into the emergency systems.Flare stack sizing and pressure drop is included with considerations of pressure drop through the pressure relief valve headers,blowdown drums, flare headers, seal drum, etc., as described in Section XV-C. Elevated flare tips incorporating various steaminjection nozzle configurations (see Elevated Flare Tip Types and Steam Injection below) are normally sized for a velocity of400 ft/s (120 m/s) at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which willensure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included ifadded through jets external to the main tip. Flared gas streams containing hydrogen can generally be flared at higher exitvelocities. In these cases flare tip manufacturers should be consulted on higher velocity limitations.A summary of suitable types of elevated flare tips is provided by Table 2, along with details shown by Figures 1A-1D.Pressure drops for proprietary tips are obtainable from manufacturers' charts such as Figure 2. Available pressure drop may insome cases dictate acceptance of a lower maximum velocity, but at least 250 ft/s (75 m/s) is recommended to insure gooddispersion. Flare tips consisting of a simple open-ended pipe with a single pilot are subject to flame lift-off and noise problemsat high velocities, and should therefore be designed for a maximum velocity of 160 ft/s (50 m/s).Elevated Flare Location, Spacing and Height - Location, spacing and height of elevated flares are a function of permissibleradiant heat densities, possible burning liquid fall-out, and pollution considerations. Design requirements are as follows:1. Flares should be at least as high as any platform or building within 500 ft (150 m) horizontally, and in no case less than 50

ft (15 m) high.2. Any source of ignitable hydrocarbons, such as separators or floating roof tanks, should be at least 200 ft (60 m) from the

base of the flare stack, assuming the potential for liquid fall-out from the flare is minimal. Burning liquid fall-out from a flarecan present extremely hazardous situations should ignitable hydrocarbons lie within the fall-out area. New facilities shouldbe designed per this section and Section XV-D to minimize the potential for burning liquid fall-out.For existing facilities that have a history of burning liquid fall-out or that are known to have design deficiencies (e.g.,undersized blowdown drum, low points in the flare header where liquid can accumulate, long distances between theblowdown or seal drum to the flare, etc.) which will increase the probability of liquid entrainment to the flare, modificationsshould be made to reduce the potential for and/or reduce the impact of burning liquid fall-out. Until such modifications canbe made, no new sources of ignitable hydrocarbons should be located close to the flare. In such situations, the 200 ft (60m) spacing mentioned above may be inadequate. Drift distances of burning liquid droplets from an inadequately designedflare system can be considerably greater than 200 ft (60 m).

3. Flares should be located to limit the maximum ground level heat density to 500 Btu/hr/ft2 (1.6 kW/m2) at any property line.The minimum distance from the base of the flare stack to the property line should be 200 ft (60 m).

4. Flare elevation and spacing must be such that permissible radiant heat densities for personnel at grade and on elevatedstructural platforms are not exceeded under conditions of maximum heat release. The appropriate calculation proceduresand personnel exposure criteria are described in Appendix A. In some special cases, flare elevation and spacing may begoverned by radiant heat exposure of certain vulnerable items of equipment, rather than personnel.

5. Flare location and height must be such as to meet all applicable regulatory standards of noise level (refer to EnvironmentalControl, Section IV) and atmospheric pollution by combustion products.

Elevated Flare Tip Types and Steam Injection - A simple open tip with no steam injection is used when there are nolimitations on smoke emission and luminosity.


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DESIGN PROCEDURE (Cont)Flare tips incorporating steam injection are used where smoke emission and luminosity must be minimized. Generally, tipselection and steam injection capacity are designed to give smokeless operation at flare gas rates up to about 20% of design,since this will cover a large proportion of releases in a typical plant. The economics of providing continuously available reservesteam capacity for higher loads is prohibitive unless special pollution regulations apply. The required steam to hydrocarbonratio for smokeless operation is a complex function of gas composition, hydrogen to carbon ratio, degree of saturation, tippressure drop, tip diameter steam pressure, and flare exit velocity.Nominal steam rates at 200 ft/s (60 m/s) exit velocity for a typical proprietary flare tip are shown in Figure 3 for guidance.Significantly higher or lower steam ratios may be required for different tip designs and gas composition. Therefore, definitivesteam ratios should be obtained on a case by case basis from flare tip vendors. In some cases steam control consists of a flowratio controller with adjustable ratio set point, related to flare gas flow. The ratio adjustment, located in the control house,provides for the higher steam ratios necessary at low flaring rates. More recently successful applications of infrared smokecontrol have been used - see under REDUCING FLARE PULSING AND NOISE later in this section. Manual smoke control isalso possible using TV monitors in the operating control room.Flare gas measurement is covered below under FLARE GAS METERING. If necessary, continuously vented surplus low-pressure steam may be used for smoke control at low flaring rates, with high-pressure steam cutting in through the flow ratiocontroller for larger releases.Injection of steam introduces an additional source of noise, and an effective flare tip is one which achieves a good balance ofsmoke and luminosity reduction without exceeding acceptable noise levels. Low-frequency noise is encountered at relativelyhigh steam to hydrocarbon ratios.A flare performance chart, if available for the hydrocarbon being flared, may provide additional guidelines for flare tip design.Figure 4 shows a provisional performance chart for propane, based on experimental data, which defines the design envelopeof exit velocities and steam ratios necessary to avoid smoke formation, excessive noise, flame boilover (flame-lick) and flamelift-off.Various designs of flare tip incorporating features such as central steam injection, an annular ring of steam nozzles, internal air-inspirating steam nozzles, windshields, etc., are available from a number of vendors. Details of suitable types from whichselection may be made are given in Table 2, along with typical design details provided by Figures 1A-1D.Other flare tip details:1. Incoloy 800 and Type 310 Stainless Steel are the recommended materials for flare tips. It should be noted that Alloy 800

should be used rather than Alloy 800H. The former grade has superior high temperature ductility properties.2. Length of flare tip is normally 10 ft (3 m).3. A maintenance platform should be provided at the bottom of the flare tip (at the flange), with access by means of a caged

ladder with intermediate platforms, in accordance with IP 4-2-1. Access onto the ladder should be permitted only when theflare is out of service.

Elevated Flare Pilots and Igniters - Proprietary flare tips are normally provided with the manufacturer's recommended igniterand pilot system. Typically, one to four pilots are used depending on flare tip type and diameter. The forced air supply type ofigniter system described below under FLARE PILOTS AND IGNITERS is preferred. It is recommended that controls belocated at a distance from the base of the flare such that pilot ignition can be readily observed, subject to a minimum of 25 ft(7.5 m). Since pilot nozzles are small, strainers are needed to prevent plugging.

S MULTIJET FLARESCapacity of Multijet Flares - In most cases, the economics of providing a multijet flare sized to handle the entire release fromthe largest design contingency are prohibitive, because of the low frequency of such major releases. Instead, it is designed tohandle a proportion of the maximum flow, typically 20%, so that releases up to this level will be relatively smokeless and non-luminous. To further reduce smoking, steam injection at rates of 0.3 lb steam/lb gas (0.3 kg steam/kg gas) may be provided.This will cover a large proportion of releases in a typical plant, but variations on this sizing basis may be dictated byconsiderations of the number and type of upstream process units, type and probability of major release contingencies,atmospheric pollution restrictions, and cost of the flare facilities.An over-capacity line to an elevated flare is provided to handle the excess flow when the flaring rate exceeds the capacity ofthe multijet flare. The over-capacity flare is normally not provided with steam injection, and smoke formation is accepted duringthe infrequent occasions when it discharges. The over-capacity line and flare is normally designed to handle the entiremaximum flow so that it can spare the multijet flare when the latter is shut down for maintenance.


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DESIGN PROCEDURE (Cont)Spacing of Multijet Flares - Recommended minimum spacings for heat releases between 300 x 106 and 1 x 109 Btu/hr (90-300 MW) are given below. For heat releases outside this range, a special study must be made.1. Clearance from property lines: 200 ft (60 m).2. Clearance from structures higher than the flare stack: 200 ft (60 m). In addition, no structure where personnel access may

be required while the multijet flare is in operation shall exceed in height a projected diagonal line from the base of the flarestack to the top of the stack wall diametrically opposite.

3. Clearance from structures lower than the flare stack: 150 ft (45 m).4. Clearance from refinery roads and pipe bands: 75 ft (22.5 m).5. Clearance from the elevated over-capacity flare must comply with radiant heat spacing requirements for elevated flares,

considering personnel exposure when maintenance work is being performed on the multijet flare and the over-capacityflare is taking full design load. Clearance should be sufficient to allow personnel to evacuate promptly to a safe location.This clearance should not, however, be less than 100 ft (30 m).

S STACK DESIGN AND DIMENSIONS FOR MULTIJET FLARES (SEE FIGURE 6)Dimensions - The inside diameter of the stack is based on the rate of heat release at design capacity, in accordance with thefollowing equation:

D = 0.83 Q (Customary) Eq. (1)

D = 0.47 Q (Metric) Eq. (1)M

where: D = Stack inside diameter, ft (m)Q = Heat release at design flaring rate, millions Btu/hr (MW) (Based on lower heating value)

For stack diameters up to 25 ft (7.5 m) the stack height is normally 50 ft (15 m) above grade. The bottom of the stack iselevated to allow air for combustion to enter. The minimum clearance between the bottom of the stack and grade is either6 ft (1.8 m) or 0.3 D, whichever is greater.

Insulation - The steel shell of the stack is lined for its entire length with monolithic type castable refractory.Windbreaker - A windbreaker is necessary to prevent the wind from extinguishing the flames. It serves also to hide the flames.Since a solid wall produces undesirable eddies, a louvered type is used.The windbreaker is octagonal and is placed 8 ft (2.4 m) from the stack. The height should be at least 2 ft (0.6 m) more than thestack clearance. The slats should be at least 9 in. (225 mm) wide and overlap by at least 2 in. (50 mm). They should slope 45°to direct the flow of air downward on the inside of the enclosure. Four access doors are provided in the windbreaker, equallyspaced around the periphery.The area below the stack and windbreaker is paved with concrete, surrounded by a 8 in. (200 mm) curb, and graded to acentral drain point from which a drain line is routed to a manhole in a vented section of the oily water sewer. The water inletshould be sealed and the manhole should be located at least 50 ft (15 m) from the windbreaker.Burner Design and Back Pressure for Multijet Flares - The "burner" consists of a large number of small burner jets,arranged in a grid pattern inside the stack, near the bottom. To increase the rangeability of the burners, a two-stage pipingsystem is provided, such that one set of burners handles low flows and the second set cuts in as the gas flow rate increases.1. Staging Design - A single-stage multijet burner has a turndown ratio (maximum smokeless flaring rate/minimum flaring

rate) of about 10:1. A two-stage burner therefore has a maximum turndown ratio of 100:1. In an actual two-stage design,the first stage is designed to handle about 20% of the design load of the multijet flare. Although this reduces the overallturndown ratio to about 50:1, it permits a large overlap between the maximum burning rate of the first stage and theminimum burning rate of the second. The overlap is desirable for smooth operation. The first stage burner grid shouldextend full width with the tube ends near the pilot to more readily promote ignition at very low flaring rates.A simple sequential water seal system with two seal drums is used to control the distribution of flare gas to the two stagesand to provide flashback protection, as illustrated in Figure 5. The seal in the over-capacity line is designed to startreleasing when the pressure in the second stage burner header reaches a value corresponding to maximum design flow tothe multijet flare. Design of the seal drums, loop seals, disposal of effluent seal water, etc., follows the procedure describedbelow for elevated flares under FLASHBACK PROTECTION FOR FLARE SYSTEMS, with the following exceptions:a. Vapor inlet dipleg submergence is selected to control progressive operation of the stages. Typical values are

indicated in Figure 5.


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DateDecember, 1999


DESIGN PROCEDURE (Cont)b. The second stage seal drums should be vertical, to minimize its size, provided that the required slope up to the

burners can be achieved.A butterfly valve in the line to the first stage seal drum limits the maximum flow to the first stage burner. The valve isset by observing the burners while flaring at design capacity. Once adjusted, the valve should be locked in position.Pressure indicators are specified to aid in making adjustments and in evaluating burner performance. Watermanometers may be used in this service, but must be shut-off when not in use.

2. Piping to Burners - First and second stage piping and headers, as well as the burner lines themselves, are sized tominimize pressure drop and velocity effects. Thus, maldistribution of flow to the burners will be minimized. The burnerlines are fabricated from standard 4 in. (100 mm) pipe, and are arranged in a split grid layout with distribution headers andsplit feed lines on opposite sides, for both first and second stage burners.First and second stage headers must be sloped so that any condensate will drain back to the seal drums. However, theburner lines must be accurately installed in a horizontal plane.

3. Size, Spacing and Number of Burner Jets - The jet nozzles are 15 in. (380 mm) lengths of 1 in. (25 mm) stainless steelpipe. They discharge vertically from the horizontal burner lines, which run across the bottom of the stack. The jet nozzlesare not insulated.The number of jets is based on gas velocity. For 1 in. (25 mm) standard pipe, the recommended maximum velocitypermits a flow rate of 2550 SCFH. (72.2 Sm3/h) of gas per jet. Expressed as an equation:

N = 16.4 V (SCF/day) (Customary) Eq. (2)

N = 0.0139 V (Sm3/h) (Metric) Eq. (2)M

where: N = Number of jets (N should be rounded off upward to a whole number)

V = Flare design capacity, millions SCF/day (Sm3/h)

The jets are laid out on an approximately square pitch, with spacings in both directions varying between roughly 18 and24 in. (450 and 600 mm)*. A first approximation of the required spacing is obtained from Eq. (3):

ND10PP 21 == (Customary) Eq. (3)

ND833PP 21 == (Metric) Eq. (3)M

where: P1 = Center-to-center spacing of adjacent burner lines, in. (mm)

P2 = Center-to-center spacing of jets along a burner line, in. (mm)

D = Inside diameter of stack [from Eq. (1)], ft (m)

This equation is based on a negligible wall effect. The design values of P1 and P2 must be determined from a scaledrawing, which is made to allow the required number of jets to be installed in the available area. This area is restricted bythe limitation that no jet should be placed closer than 12 in. (300 mm) from the inside wall of the stack. The spacing is alsoaffected by air flow considerations (see below), which may require the layout to be modified.

4. Air Flow - The capacity of a multijet flare to induce air flow must be calculated, to make sure that it is adequate to meet themaximum air flow requirement for smokeless combustion. [Wa of Eq. (4) must ≥ Wr of Eq. (5).]a. Air Flow Capacity - The primary air flow rate which will be induced around each jet may be estimated from Eq. (4):

2

S

o2

B

o

S1j

a

AA1138.0

AA9.101

T5201HA7.5

W

−+

+

−

= (Customary) Eq. (4)

*In the case of a low heating value flare gas (less than about 1000 Btu/SCF (37 MJ/m3) HHV @ std. condition) whichrequires relatively little air, this spacing may be as low as a minimum of 15 in. (380 mm).


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DESIGN PROCEDURE (Cont)or

2

S

o2

B

o

S1j

6

a

AA1138.0

AA9.101

T2891HA10x02.2

W

−+

+

−

=

−

(Metric) Eq. (4)M

where: Wa = Available primary air flow per jet, lb/hr (kg/s)

Aj = Net free flow area per jet, in.2 (mm2) = P2 (P1 - d)

d = Outside diameter of burner line (including insulation), in. (mm)

H1 = Net stack height, ft (m)

TS = Average stack gas temperature, °R (°K). This temperature depends on the heating valueof the flare gas and the percent excess air. Assume that TS = 2800°F (1538°C) =3260°R (1811°K). This should be close enough, since Eq. (4) is relatively insensitive tochanges in TS.

Ao = Total free flow area for primary air, ft2 = 144

AN j

= 6

j2

10

ANm

AB = Peripheral area between bottom of stack and grade, ft2 (m2)

AS = Total inside cross-sectional area of stack, ft2 (m2)

Note: Eq. (4) has a partially theoretical origin, based on the pressure drops through the three successive majorrestrictions to air flow through the flare, i.e., the cylindrical area under the stack, the burner grid, and thestack itself. However, the constants have been determined from actual performance data on two flares, of 6ft (1.8 m) and 30 ft (9.2 m) diameter.

b. Air Flow Requirement - The primary air flow rate per jet necessary for smokeless combustion depends on themolecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percentunsaturates, from a minimum of 20% excess air for a flare gas containing 0% unsaturates to 35% excess air for a gascontaining 67 mol% unsaturates. Based on this relationship and a design gas flow rate of 2550 SCFH (72.2 m3/h) perjet, the required primary air flow rate can be calculated directly from the gas composition or approximatedconservatively from Eq. (5):

Wr = 120 M + 56 yu + 22 M yu + 300 (Customary) Eq. (5)

or

Wr = 1.26 x 10-4 [120 M + 56 yu + 22 M yu + 300] (Metric) Eq. (5)M

where: Wr = Required primary air flow rate, lb/hr (kg/s) per jet, for a gas flow rate of 2550 SCFH(72.2 m3/h) per jet

M = Molecular weight of flared gas

yu = Mole fraction unsaturates in flared gas

If Wr > Wa, it will be necessary to modify the burner layout in order to provide more air flow area per jet. If necessary,the stack diameter must be increased. (Another possibility is reducing the piping diameter, if pressure drop is notlimiting.)The value of Wr calculated from the gas composition or from Eq. (5) should be considered to be a minimumrequirement. A multijet flare should be designed with a calculated air capacity Wa as high as possible, as limited bypractical considerations of geometry and economics.


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DESIGN PROCEDURE (Cont)c. Stack Gas Temperature - The average stack gas temperature, TS, which is required in Eq. (4) for calculation of Wa

has to be estimated. [An assumed value of 2800°F (1538°C) is recommended.] It can be checked as follows:i. Calculate the percent excess air which corresponds to the value of Wa calculated per Eq. (4).ii. Find the chart (Figure 7, 8, 9 or 10 of Section VIII-M, Combustion Design Data) which most nearly corresponds

to the heating value of the flare gas.iii. The intersection of the curve for the percent excess air in question with the abscissa (heat available = 0) is the

stack temperature, TS.iv. If this value of TS differs from the assumed value by 300°F (149°C) or less, the change in Wa will be less than 1%.

5. Flameholder - Flameholders are necessary to prevent the flame from "riding" up to the top of the stack. They provide asurface at which burning can take place and also promote better mixing of air and gas by the additional turbulence whichthey cause above the jets. Construction is simply a solid, 1 in. (25 mm) diameter rod of refractory material (silicon carbide)supported horizontally above each burner line. The bottom of the rod should be 1/2 in. (13 mm) above the tips of the jets.Typical flameholder support details are shown in Figure 6A.

6. Critical Burner Dimensions - The position of the flameholders and burner lines relative to the bottom of the stack iscritical for efficient operation. For example, the multijet flare has a turndown ratio of 10:1 when the flameholder centerlineis 5 in. (125 mm) below the bottom of the stack but only 2:1 when it is 6 in. (150 mm) above the bottom of the stack.Critical dimensions are shown in Figure 6A.

7. Burner Insulation - The burner lines should be insulated with a 1 in. (25 mm) layer of castable insulation.8. Pilots and Igniters - Duplicate continuous gas pilots are required at each side of the flare, corresponding to the split

burner grid layout. Selection of pilot and igniter systems follows the guidelines described below under FLARE PILOTSAND IGNITERS, and the controls should be located 50 to 100 ft (15 to 30 m) from the windbreaker.Because of the potential hazard of release of unignited hydrocarbons at ground level, a flame detection system, e.g.,scanner, flame rod, TI / TLA, with alarm in the control house is included for each pilot.If an ultra violet detector is used, the flame scanner must be located so that interference of ultra violet rays from the mainflame or other sources do not cause false readings. Mount ultra violet detectors looking straight down through the pilotstoward the ground. Provide strainers in each gas or oil line to pilots.

9. Steam Injection - While the multijet flare will achieve a significant reduction in the smoke produced, it does not providetrue smokeless combustion over its full operating range. This is particularly true with the heavier (C4+) and unsaturatedgases. Steam injection at a rate of about 0.3 lb steam per lb of gas (0.3 kg of steam/kg of gas) will provide an additionalreduction in smoke for most gases. Steam should be injected into the flare gas either upstream or downstream of theburner nozzles, according to the burner design.

10. Materials - In selection of materials for burner grid, stack lining, and piping inside the windbreaker, temperature rise due toheat radiation from the flame should be evaluated. A flare tip refractory lining should be provided, as shown in Figure 6A.

S BURNING-PIT FLARESBurning-pit flares can handle flammable liquids or gases or mixtures of the two. A typical design is shown in Figure 7. Acircular pit is illustrated, but any convenient shape may be used.The burning pit is simply a shallow earth-or concrete-surfaced pool area enclosed by a dike, with a liquid/vapor inlet pipethrough the wall, and provided with pilots and igniters. While the design basis below is adequate for handling emergencyreleases, a more conservative approach is recommended for continuous flaring services, incorporating up to twice thecalculated pit area.Burning Pit Flare Sizing - The burning-pit area is sized to provide sufficient surface to vaporize and burn liquid at a rate equalto the maximum incoming liquid rate. The calculation procedure is as follows:1. Determine the linear regression rate of the liquid surface (i.e., the rate at which the liquid level would fall as a result of

vaporization by radiant heat from the burning vapor above it, assuming no addition of incoming liquid):

vHLHV003.0R = (Customary) Eq. (6)

vHLHV00127.0R = (Metric) Eq. (6)M


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DESIGN PROCEDURE (Cont)2. Determine the pit area necessary to vaporize and burn liquid at a rate equal to the liquid input rate:

LRm2.0A

ρ= ; (Customary) Eq. (7)

LRm1001A

ρ= (Metric) Eq. (7)M

where: LHV = Lower Heating Value of incoming vapor and vaporized liquid, Btu/lb (kW/kg).

A = Pit area required to vaporize and burn liquid, ft2 (m2).

m = Rate of vaporization and burning of liquid, lb/hr (kg/s) (selected as equal to the rate offlashed liquid entering the pit).

R = Linear regression rate of liquid surface, in./min (mm/s).

ρL = Liquid density, lb/ft3 (kg/m3).

Hv = Liquid latent heat of vaporization, Btu/lb (kJ/kg).

3. The dike wall height above the water level is selected to provide holdup capacity for the largest liquid release resulting froma single contingency during 30 minutes, plus 18 in. (450 mm) freeboard. The liquid rate is based on the actual flashedliquid entering the pit, assuming no burning or further vaporization in the pit. The height of the dike wall above the waterlevel should not, however, be less than 4 ft (1.2 m).

Spacing of Burning Pit Flares - Spacing is based upon radiant heat considerations at maximum heat release, using asimplified calculation procedure, as follows:

K4QFD

π= Eq. (8)

where: D = Distance from flame to point under consideration, ft (m).Q = Total heat released by liquid and vapor, Btu/hr (kW).K = Radiant heat density, Btu/hr ft2 (kW/m2).F = Fraction of heat radiated. Refer to Appendix A for appropriate values of F.

The center of the flame is assumed to be 1-1/2 pool diameters from the center of the pool at a 45° elevation, in the direction ofthe point where radiant heat density is being considered. This assumption is used to allow for flame deflection by wind.Permissible radiant heat densities for personnel are described in Appendix A and later in this section. In some special cases,spacing may be governed by radiant heat exposure of certain vulnerable items of equipment, rather than personnel. Radiantheat density at the property line must not exceed 500 Btu/hr ft2 (1.6 kW/m2).In addition, the following minimum spacings apply to burning pit flares:• 500 ft (150 m) from property lines, roadways, or any process or storage facilities.• 200 ft (60 m) from any source of ignitable hydrocarbons, such as separators or floating roof tanks.Valves in the inlet, seal water and pilot gas lines should be located according to permissible radiant heat densities forpersonnel, as defined in Appendix A, consistent with being able to carry out emergency actions. Piping to the burning pitshould be suitably protected against flame impingement (e.g., by installation below grade).Inlet Piping and Flashback Protection - In non-freezing climates, for services where incoming liquid or vapor hydrocarbontemperatures do not fall below 32°F (0°C), flashback protection is provided by submerging the inlet below a layer of water in thebottom of the pit. The top of the inlet distributor is 4 in. (100 mm) below the water level, and the bottom of the distributor is 6 in.(150 mm) above the bottom of the pit. A continuous flow of at least 20 gpm (1.3 dm3/s) of makeup water into the pit isprovided, with the level maintained by an overflow seal discharging to the oily water sewer. Higher water rates may be requiredon account of ground seepage or evaporation during maximum release conditions.For services where ambient or inlet temperatures may fall below 32°F (0°C), flashback protection is provided by a special sealdrum or loop seal in the inlet line. This equipment is designed specifically for the particular liquid and vapor materials beingflared. In these cases a 6 in. (150 mm) minimum water layer is included in the bottom of the pit to prevent oil seepage into theground, and the hydrocarbon inlet distributor is mounted 6 in. (150 mm) above the water surface.Details of the inlet distributor are shown in Figure 7.


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DESIGN PROCEDURE (Cont)Pilots and Igniters - Two gas-fired pilots with igniters are installed adjacent to the inlet distributor. As described below underFLARE PILOTS AND IGNITERS, the igniter assembly and pilot gas valves must be located remote from the flare for protectionof personnel and equipment. This restricts igniter selection to the forced air supply type. Location of these components shouldbe such that the calculated radiant heat density at maximum load does not exceed permissible levels for personnel exposure.Because of the potential hazard of release of unignited hydrocarbons at ground level, a flame scanner (suitably shielded andair-cooled and connected to an alarm in the control house) is provided for each pilot.

S FLARE PILOTS AND IGNITERS - ALL FLARESAll flares must be provided with continuous pilots to ensure combustion of any releases discharged to them, and to preventflame-out. Various designs of pilot burner are available, and proprietary tips for elevated flares are normally provided completewith pilots.In addition, an ignition system is required for igniting the pilots when a flare is commissioned. Various proprietary flame frontigniters are available, which function by the application of an electrical spark to a gas/air mixture in an ignition chamber, andallowing the resulting flame front to travel through an igniter tube which terminates with an open end adjacent to the pilot burnertip. In some designs, to improve the reliability of achieving ignition, the igniter tube ignites a raw gas on-off pilot burnerprovided with a special wide-angle tip, which in turn ignites the continuous pilot.Air to the ignition chamber may be inspirated through a gas-operated venturi, which is designed to provide the correct gas/airmixture, but this type does not function reliably when horizontal igniter tube runs exceed 20 to 30 ft (6 to 9 m). The ignitionchamber and igniter tube venturi must therefore be placed close to the flare, i.e., at the base of an elevated flare or adjacent tothe windbreaker of a multijet flare. Remote operation of the ignition controls may however be achieved by locating the sparkignition pushbutton and the igniter gas and pilot gas valves at any required distance from the flare. The inspirating system isnot, however, acceptable for burning-pit flares since, even with remote control, the ignition chamber and venturis, etc. would besubject to damage by overheating.An alternative design uses a compressed air supply (from the instrument air header) for the ignition chamber, the gas/airmixture being controlled by restriction orifices. The flame front passes through an igniter tube as for the inspirating type, butthere are no limitations on horizontal distance from the flare, and the igniter assembly may therefore be located remotely.The forced air supply type of igniter system is preferred for all flare applications.When pilot and igniter systems for flares are being selected available proprietary systems may be considered but the choiceshould be based upon proven satisfactory operating experience. The John Zink Flame Front Generator, which uses aninstrument air supply to the ignition chamber, is typical of the igniter systems most frequently applied during recent years. It isillustrated in Figure 8.Flares, which depend only on PHA and PLA alarms in the pilot gas supply to indicate the pilots are lit, should be Priority 1alarms which have dedicated alarms in the control room. For flares using thermocouples to verify pilot flame, such as used ona H2S flare, the use of PHA and PLA instrumentation in the pilot fuel gas supply is not required. However, the thermocouplesshould be equipped with Priority 2 alarms.Reliability of flare pilots has been a problem at some installations when using a 45 scfh fuel supply. Plugging of the in-linestrainers has occurred and it is recommended that such in-line strainers be removed prior to pilot installation. For theseproblem installations, the strainers were not accessible without a flare shut down and the strainer mesh was too fine for theservice. Filtering of the fuel should be done at grade and cartridge filters have been used to provide adequate protection.Testing of the pilots prior to installation is strongly recommended since the primary air setting may not be valid if site location isdifferent from factory location. Pilot tip temperature must be above the ignition temperature of the fuel and testing will confirmthat the desired tip temperature of 1500°F (816°C) is achieved. Use of 80 scfh pilots appears to be adequate to ensure flamestability, and is recommended.Gas pilots must be provided with a reliable source of gas, which will remain available under any single contingency such aspower or air failure. Pilots should be designed for the anticipated gas composition, which should be reasonably constant.An alternative means of igniting flare gas is to use direct electronic ignition, instead of continuous pilots. This has beenattempted over the years but was hampered by a variety of problems. These were corrosion, erosion, melt-down, and carbonbuild-up problems associated with the electrodes. Successful flare ignition is highly dependent on the position of theelectrodes. There could be flaring conditions where the arc would not ignite the discharging gas because the flare gas/airmixture at the electrodes was either over-rich or too lean. Before specifying an electronic ignitor, a thorough investigationshould be conducted to determine the reliability of such devices.


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DESIGN PROCEDURE (Cont)

S FLASHBACK PROTECTION FOR FLARE SYSTEMSAll flares must be provided with flashback protection to prevent a flame front from traveling back to the upstream piping andequipment. A number of different flashback seal designs are available, of which the seal drum is used in nearly all applications.Details are as follows:Seal Drum - A typical flare seal drum for an elevated flare stack is illustrated in Figure 9A (Figure 9B - Metric). (Sealarrangements for multijet flares and burning pits were covered earlier in the section.) With a continuous flow of water to theseal drum, loss of level downstream of the baffle is not possible. Condensed hydrocarbon may accumulate downstream of thebaffle and skimming connections should be provided. The need for a LHA and LLA in the drum is not needed because thelevel downstream of the baffle is determined by the height of the seal leg.A baffle maintains the normal water level, and the vapor inlet is submerged 3 to 4 in. (75 to 100 mm). Drum dimensions aredesigned such that a 10 ft (3 m) slug of water is pressured back into the vertical inlet piping in the event of flashback, thuspreventing the explosion from propagating further upstream.The inlet piping or diplegs should have sufficient wall thickness to withstand an internal explosion and should be good for 50psig (345 kPa), or consistent with the drum internal design pressure.The vapor space is sized to avoid water entrainment into the flare gas. The vapor residence time in the drum should allowlarger liquid droplets to fall to the liquid surface. The method used follows API 521. The vapor space velocity as measuredthrough the vertical cross-sectional area above the baffle level (see Figure 9) determines the residence time. The liquid“dropout velocity” is determined as follows:

➧C

)(Dg15.1Uv

vLc ρ

ρ−ρ= Eq. (9)

where: Uc = Dropout liquid velocity, ft/sec (m/s).ρL = Liquid density, lb/ft3 (kg/m3) at operating conditions (water).ρv = Vapor density, lb/ft3 (kg/m3) at operating conditions.D = Liquid particle diameter, ft (m).g = Acceleration due to gravity 32 ft/s2 (9.8 m/s2).C = Drag coefficient (see figure in Section XV-D for drag coefficient).

The droplet size should be selected as follows for normal contingencies:1. 600 microns for new/existing facilities.2. The vapor velocity should be limited to 25 ft/s (7.6 m/s) or less in all cases.3. The depth of the vapor space should be no less than 20% of the drum diameter or 12 in. (300 mm), whichever is greater.For remote contingencies:1. The droplet size may increase to 1000 microns for all cases.2. The vapor velocity should be limited to 25 ft/s (7.6 m/s) or less in all cases.The former seal drum designs included a serrated edge at the base of the dipleg (see Detail A, Figure 9). Intermittent bubblingthrough the seal dipleg sometimes causes problems of flame pulsation, low frequency noise and/or smoke pollution. Thealternative is to design using successive diplegs as shown in Figure 5 plus the non-pulsing sparger shown in Figure 12 anddiscussed in subsequent paragraphs.Seal water should be taken from a reliable source (firemain water may be permissible, per Section XV-I). Salt water should notbe used where flare gas components would cause deposition of solids. Winterizing should be provided if required by IP 3-9-1.At some locations, which experience extremely cold weather, e.g., Strathcona, a recirculation glycol system is used.The vertical downflow section of the water outlet line from the drum is sized for a maximum velocity of 0.4 ft/s (0.12 m/s), toallow entrained gases to disengage. The seal loop should be sized for the design water flow. 20 gpm (1.3 dm3/s) has beenused as a guideline in the past. The make-up water rate must be sufficient to compensate for drum purge and evaporation.The maximum make-up rate assumes that the drum is empty after a blow and should be high enough to reestablish the sealwithin a reasonable time after each flaring incident. However, the seal must be reestablished before the ingress of air reachesthe seal drum. Air ingress occurs as the system cools down after a pressure release. Where higher water rates are considerednecessary then facilities for manually increasing the make up should be provided. The seal depth is equivalent to 175% of themaximum drum operating pressure, subject to a minimum of 10 ft (3 m). A seal depth of 110% of the maximum operatingpressure is permitted when applying the "1.5 times design pressure" rule to remote contingencies. (See Section XV-C.)


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DESIGN PROCEDURE (Cont)The drum is normally gunite-lined for corrosion control and must be designed to meet all the following combinations of extremetemperature and pressure requirements:1. A design pressure of 50 psig (345 kPa) at the highest possible operating temperature of the entering vapors resulting from

a single contingency. For those hydrocarbons typically encountered in refinery operations, this design pressure shouldadequately protect the drum against an internal explosion. This design pressure must also be applied to the flare stackand the water outlet seal loop. However, for those locations which may flare streams containing more than 75% by volumeethylene, a design pressure of 150 psig (1035 kPa) should be used.

2. A design pressure of 50 psig (345 kPa) at the lowest possible operating temperature of the entering vapors resulting from asingle contingency.Credit may be taken for heat transfer to the atmosphere from the flare header upstream of the seal drum when determiningthe lowest and highest temperatures of the entering vapors.

A flare seal drum may also serve as a sour water disengaging drum, if economically advantageous. In such cases, specialcare is needed to insure the drum is adequately sized to simultaneously meet all design features required for both functions.Also a separate source of makeup water must still be provided to ensure continuity of the seal.Disposal of the effluent water is discussed under Disposal of Seal Water below.The drum is provided with steam injection if required for winterizing or cold releases, per Figure 9. If winterizing is called for byIP 3-9-1, then the steam should be temperature-controlled to maintain the seal water temperature at 40 to 50°F (4 to 10°C).Oil skimming connections should be provided to remove accumulated oil, carried over from the blowdown drum, to the sealdrum. Details are shown in Figure 9.The seal drum should be located a minimum practical distance from the flare.Gas Purge - Although not normally recommended as an alternative to a water seal, a continuous hydrocarbon gas purge maybe used to provide protection against internal burning or flashback explosion which might result from air backing down from theflare tip at zero or low flaring rates. Application of this method is limited to cases where a continuous low-cost gas supply isavailable, where there are no public relations or atmospheric pollution objections to continuous flaring, and where problems thatwould be associated with a water seal (e.g., freezing up by cold vapors) can thereby be avoided.The purge gas must be non-fouling and the source must be reliable and assured during any single contingency such as poweror air failure. Purge rate (controlled by restriction orifice) must be calculated in accordance with the procedure described forpurging atmospheric vent stacks in Section XV-B.Dry Seals - Various proprietary dry seals are available, such as the John Zink Molecular Dry Seal (illustrated in Figure 10) andthe National Air-Oil Fluidic Seal (shown in Figure 11). They may be used in conjunction with a gas purge system when thepurge gas is lighter than air. The molecular dry seal functions by trapping a volume of the light gas in the internal invertedcompartment, thus preventing air from displacing light gas in the flare stack by buoyancy effects. The fluidic dry seal utilizesthe kinetic properties of the gases to prevent movement of air down the flare. The continuous purge flow requirements forthese dry seals may be 30-60% of the open stack purge rates as determined from Figure 1 of Section XV-B. The vendorshould be contacted to determine their specific recommendation for each proposed installation. This reduced purgerequirement is the only incentive for installing a dry seal.In the case of heavier-than-air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus noincentive to include a dry seal.Unlike a water seal, a dry seal cannot prevent a flashback from traveling upstream if a combustible mixture has been formed bythe entry of air into the pressure relief valve or flare headers. It only protects against internal burning flashback, which mightresult from air backing down from the flare tip at zero or low flaring rates. Dry seals are therefore not normally specified for newdesigns.The location of the John Zink Molecular Seal recommended by the manufacturer is approximately 11 ft (3.6 m) below the top ofthe flare.The NAO Fluidic Seal is located as close to the top of the tip as possible to eliminate the need for internal refractory.Disposal of Seal Water - Effluent water from water seals must be routed to safe means of disposal, considering possiblehazards arising from liquid or vapor hydrocarbons or toxic materials that may be entrained or dissolved in the water. Theseparation of liquid hydrocarbons (formed by condensation in flare lines) in flare seal drums is discussed in Section XV-D.Seal water should be discharged as follows:1. If H2S is never present in the flare gas, seal water effluent should be routed through an open funnel (to permit checking of

seal water flow) to a manhole in a vented section of the oily water sewer. The water inlet to the manhole must be sealed.


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DESIGN PROCEDURE (Cont)2. If H2S in any concentration is intermittently present in the flare gas, seal water effluent should be routed through a closed

connection (incorporating a sight glass for checking flow) to a manhole in a vented section of the oily water sewer. Thewater inlet to the manhole must be sealed. In addition, the seal drum should include a baffle (Detail B in Figure 9) topreferentially route the makeup water to the sewer, while confining H2S-saturated water to the drum.However, pollution considerations may make the routing of seal water to the sewer unacceptable, in which case disposalmust follow method (3) below.

3. If H2S is continuously present in the flare gas (see definition of "continuous" under FLARING OF H2S STREAMS, below),or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to asour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normalloop seal arrangement. Two pumps are provided: one motor driven for normal use, and the other having a steam turbinedrive for high level cut-in. Seal drum level is controlled by LIC with high and low alarm lights plus an independent highlevel alarm.

4. Disposal of effluent water from multijet ground flare seal drums should comply with paragraphs (1), (2), and (3) above,except that:• In (1), closed piping with a sight glass should be used rather than an open funnel.• In (1) and (2), the manhole receiving effluent seal water should be located at least 50 ft (15 m) from the windbreaker.

Flame Arresters - Flame arresters are not permitted as a means of flashback protection in flare systems.

REDUCING FLARE PULSING AND NOISEA major cause of pulsing in flare systems is flow surging in the water seal drum. This can be eliminated by providing a gasdistributor in the seal drum.One of several reasons why it is important to eliminate pulsing is to reduce flare noise. Combustion flare noise has beenshown to increase as the steam rate increases. Since the amount of steam required to suppress smoke in a flare is set by theflaring rate, flow surges will require a higher steam rate than for a steady flow. A past test at one of the Baton Rouge flaresshowed that through the elimination of seal drum pulsing, steam rates could be reduced by one third at about 20% of the flarecapacity.At much higher steam rates than required to suppress smoke, flares occasionally exhibit another type of noise problem, a lowfrequency rumble that can be sensed in the surrounding community as vibrations. The over steaming of the flare producespockets of uncombusted gases which then ignite sporadically. By reducing the steam requirement of a flare through eliminatingflow surges, there is a greater operating margin between smokeless flaring and high steam rate associated rumble.The likelihood of over steaming a flare and causing noise problems can be reduced by providing automatic smoke control.Since neither the gas rate nor the gas composition to a flare is rarely constant, the required steam addition rate to a flare foroptimum smoke control requires frequent adjustment. With manual addition of steam there will be occasions when a flare isreceiving significantly more steam than is required for smoke suppression and low frequency rumbling noise may result. This isparticularly likely at night when visual smoke control is difficult.Infrared techniques which rely on measuring radiation being emitted from soot particles and carbon monoxide emissions havebeen used at several Exxon locations. Experience varies from extremely poor in the case of Baytown Olefins Plant to verygood at Baton Rouge Refinery. It would seem that the key to a successful automatic infrared smoke control system is to use asensor which measures radiation in the 2.0-2.5 µ wavelength range. Raytek detectors are examples of such devices whichsense radiation over this range and have been used successfully at several Exxon locations.Other advantages of eliminating pulsing are reduced steam costs, more accurate flow measurements possible with a steadyflow, and reduced incidence of blowing and seal liquid. Also, the size of the seal drum can sometimes be reduced.Flow surges in the seal drum are generated by the cyclic formation of large bubbles as the flare gas is discharged into thedrum. These pulsations can be virtually eliminated by the use of a horizontal sparger incorporating many small diameter holesarranged specifically to allow the open area to increase as flow increases. These holes must be spaced sufficiently far apart toavoid interference between bubbles.The specific design of the non-pulsing sparger distributor is shown in Figure 12. The hole arrangement was determinedexperimentally. Hole density is low at the top of the pipe and is increased lower on the pipe. The maximum open area densityof about 10% assures reasonable bubble formation. The average velocity out of the top row of holes starts at about 130 ft (40m/s) and increases as the pressure rises and total flow increases. Total areas of holes plus bottom slot should be equal to atleast two times the cross sectional area of the inlet pipe.


XV-EPage


DateDecember, 1999


DESIGN PROCEDURE (Cont)

S FLARING OF H2S STREAMSContinuous releases of concentrated H2S streams must be segregated in a separate flare system to limit the extent of foulingand plugging problems. Releases of H2S such as diversion of Amine Regeneration Unit sour gas product to flare duringshutdown or upset of a downstream sulfur recovery unit are considered to be "continuous," but safety valve releases are notincluded in this category. Concentrated H2S pressure relief valve releases should be tied into the regular flare system ratherthan a segregated flare system for continuous releases, since the segregated H2S flaring system has a higher probability offouling. Due to the nature of H2S one should plan on frequent inspection and flushing of H2S flares to remove scale andcorrosion products.Preferably, the H2S flare system should consist of a segregated header and separate line routed up the side of a conventionalelevated flare stack, sharing the same structure, but having separate pilots and igniters. However, the H2S header may be tiedinto the regular flare seal drum if there are special mechanical design problems associated with the separate stack, e.g., in thecase of a flare, which is to be dismantled for overhaul. Flare elevation must be sufficient to meet atmospheric pollution andground level concentration requirements for the sulfur dioxide produced.Incomplete or no combustion of flared hydrogen sulfide can lead to extremely dangerous situations because of the relativelylow lethal concentration of hydrogen sulfide. Concentrated streams of hydrogen sulfide can be flared safely provided thefollowing features are incorporated into the flare design (these features equally apply to those locations where a dedicated H2Sflare is not available and concentrated H2S streams are handled via the regular hydrocarbon flare):• It is absolutely essential that flare pilots be lit at all times. The flare pilots should be provided with a reliable source of fuel

gas, a reliable forced air type igniter system and a means of automatic pilot flame verification such as thermocouplessituated inside a thermowell which is located at the base of the pilot flame. Auto relight based on pilot verification is to beconsidered where manning may limit operator attention. The fuel gas used for the pilots should be clean and free of anyappreciable quantities of condensables and should be provided with a pressure controller and high and low pressurealarms. Refer to Figure 8 for a typical pilot gas / igniter system design. If appreciable quantities of condensables in thefuel gas are likely, then a knock-out drum with a high level alarm should be provided. Piping should be self-draining backto the drum. At least three pilots should be provided where possible. For very small flares [less than 8 in. (200 mm)diameter], at least two pilot and igniter assemblies should be provided. For large flares [greater than 42 in. (1067 mm)diameter], at least 4 pilots should be provided. Each pilot burner should be rated for a minimum heat release of 80000Btu/hr (23.4 kW). Individual pilot gas lines to the flare tip with isolation capability at grade should be provided to enableisolation of a burnt out pilot. A burnt out pilot may tend to preferentially take gas from the undamaged pilots, which couldultimately lead to their extinguishment.

• The combustion characteristics of the flared gas must be high enough to ensure flame stability. Typical refinery H2Sstreams containing H2S, CO2 and water vapor are flammable without assist gas if the UFL is 30 vol. % or higher. If UFL isbelow 30 vol. %, it is recommended that assist fuel gas be added until the UFL is increased to 30 vol. % or the heatingvalue increased to 500 Btu/SCF (5.2 kJ/Sm3). Hydrogen rich streams will increase the UFL and paraffin hydrocarbonstreams will increase the heating value. Supplemental fuel gas, when required, should be automatically introduced (at theupstream end of the flare header for mixing and purging) via ratio control of the fuel gas stream to the measured flareheader acid gas rate. Ratio control of fuel gas/acid gas should be a safety critical instrument.

• To avoid flame "blow off," acid gas flare exit velocity, including any supplemental fuel gas, should be limited to 150 ft/sec(45 m/s).

• Steam should not be added to acid gas while flaring because of the quenching effect it will have on the flame temperature.This can result in incomplete or no combustion. Hydrogen sulfide burns smokelessly as will any supplemental fuel gas(unless the fuel gas contains a very high level of unsaturated compounds). Thus, steam addition for smokeless flaring isnormally not required.

• The flare stack should be high enough that dangerous concentrations of uncombusted hydrogen sulfide are unlikely toaccumulate at grade should the flare become extinguished or flaring efficiency drop off. For a given set of conditions, therewill be a certain wind velocity and downwind distance from the stack that produce the highest ground level concentration,referred to as the critical ground level concentration. As a design guideline, the flare elevation should be such that thecritical ground level concentration of hydrogen sulfide does not exceed 10 vppm over a one hour averaging time assumingnone of the hydrogen sulfide entering the flare is converted to sulfur dioxide. In addition to meeting the critical ground levelconcentration criterion, the stack should be high enough that radiant heat levels at grade do not exceed 3,000 Btu/hr ft2(9.45 kW/m2) at maximum release rate.

• Mechanical reliability of the flare tip assembly should be maximized by providing features such as wind shields to protectthe outside of the flare tip against damage from flamelick and internal refractory lining to protect against burning inside theflare tip at low flaring rates.


SectionXV-E

Page18 of 60

FLARES


EXXONENGINEERING


DESIGN PROCEDURE (Cont)• A potential problem with flaring acid gas is the deposition of ammonium bisulfide (NH3 + H2S) and ammonium bicarbonate

(NH3 + CO2 + H2O) in the flare header and even at the base of the flare stack. The potential for such deposits depends onthe temperature of the gas and the partial pressures of NH3, H2S, CO2 and H2O. Refer to Figures 13 and 14 for plots ofammonium bisulfide and ammonium bicarbonate deposition temperatures versus partial pressure. Any lines, including theflare header and flare stack, which are susceptible to the formation of such deposits should be heat traced and insulated tomaintain stream temperatures at least 25°F (14°C) higher than the deposition temperature. As a minimum, the linetemperature should be maintained above 120°F (49°C). An alternative to heat tracing and insulating the flare stack wouldbe to provide water wash connections into the flare stack approximately 30 ft (9 m) above the acid gas header inlet. Ifprotection of the stack via water wash is provided, it is imperative that routine washing be carried out.

Flashback protection is required for H2S flaring systems, either by water seal or continuous gas purge, as described aboveunder FLASHBACK PROTECTION FOR FLARE SYSTEMS. If a water seal is used, special requirements apply to thedisposal of the effluent seal water. In the case of an H2S flaring system handling a flow of H2S which is uninterruptedthroughout the period that a plant is in operation, and which stops only when the producing plant is shutdown, then flashbackprotection is not required. However, steam or inert gas connections are required to permit purging the flare line during start-up,shutdown and plant upsets.

SPARE FLARE CAPACITY CONSIDERATIONSFlare capacity must always be available for emergency releases that may occur from the upstream plant facilities which are inoperation. Flaring rates corresponding to the design contingency basis of the facilities should be assumed. Thus, when a flareis taken out of service for maintenance, either spare capacity must be available in other flares connected to the same system,or the necessary upstream process units must be shutdown. The justification of spare flare capacity is therefore considered onthe basis of an economic evaluation of the debits associated with plant downtime.As a means of providing spare flare capacity without rendering additional plot area unavailable for plant equipment, a doubleelevated flare arrangement is permissible. This consists of two flare stacks in a common supporting structure, each sized forthe full design flow. Either stack can be dismantled and removed for maintenance while the other is in service. Appropriateplatforms and radiation shielding are installed to provide safe working conditions for personnel during the dismantling andreassembly stages, but the mechanical design should be such that this work can be carried out without requiring access abovean elevation of 50 ft (15 m).

FLARE GAS METERINGMetering flare gas is important for loss accounting and for control of steam injection. A special requirement for flare gas metersis low-pressure drop and the ability to continue functioning in fouling conditions. The flare gas metering methods listed belowhave been used with varying degrees of success, or are under test or development. In view of the new techniques anddevelopments in this field, reference should be made to the Control Systems Development Section of Exxon EngineeringTechnology Department for advice on selection of flow meters for flare gas service.Venturi Meter - Limited experience of this flow meter in flare gas service has been obtained. Its advantage is that it has apermanent head loss of only 5% of the measured pressure differential. This is the lowest AP of all orifice meter designs. Flowratios as high as 1:10 (e.g., 1.0 to 10 lb/s) can be measured within + 2% of actual flow. Venturi meters are available in differentmaterials and diameters up to 60 in. (1500 mm) from B.l.F. Industries, Providence, Rhode Island.Anemotherm - The Anemotherm, a hot wire anemometer, has been extensively applied to flare gas measurement and hasbeen successfully used after modifications to reduce probe fouling. A technique used to reduce fouling is the installation of asteam coil around the tip of the probe, to prevent gases from condensing on the probe. The unmodified Anemotherm isavailable from the Anemostat Corporation of America, Scranton, Pennsylvania.Acoustic Flow Meter - Several Exxon locations now use the Panametrics ultrasonic flow meter. This meter was developed byPanametrics at Waltham, Massachusetts, USA in conjunction with Baytown Refinery. These meters use the differences insound velocity to measure flow and density of the flared gases. A flow range from 0.02 to 30 ft/s (0.006 to 9 m/s) is possiblewith this meter.

S PROTECTION AGAINST LOW AMBIENT OR FLARE GAS TEMPERATURESFlare systems must be protected against any possibility of partial or complete blockage by ice, hydrates, solidification, etc. Thefollowing design requirements should be included:International Practices - The requirements of IP 3-9-1 applying to pressure relieving systems, seal legs, bottom of flare stacksand seal drums, must be met.


XV-EPage


DateDecember, 1999


DESIGN PROCEDURE (Cont)Steam Injection - Seal Drums requiring winterizing under IP 3-9-1 should be provided with temperature-controlled steaminjection to maintain the seal water temperature at 40 to 50°F (4 to 10°C), as described above under FLASHBACKPROTECTION FOR FLARE SYSTEMS. This limits the quantity of water vapor entering the flare stack.Steam Tracing - When winterizing is required by IP 3-9-1, the steam tracing and insulation should include the first 25 ft (7.5 m)of the flare stack above the vapor inlet; and also in the case of a drum seal, the vapor line from the seal drum to the flare.Where severe ambient conditions are encountered (such as at Strathcona where temperatures lower than -40°F (-40°C) havebeen encountered) then it is recommended that the entire seal drum and flare be insulated in addition to steam tracing andopen steam injection at base of flare.Cold Gas Releases - Requirements for handling cold flare gas releases, such as by steam injection to the seal drum (shown inFigure 9), or by inline heaters, are covered in Section XV-C.

TABLE 1COMPARISON OF FLARE TYPES

COMPARISON FACTORS ELEVATED FLARE MULTIJET FLARE BURNING PIT FLARE

Pollution CharacteristicsSmoke Can be made smokeless except at

high loads.Relatively smokeless Poor

Noise Noisy, due to steam used for smokereduction (compromise necessary).

Relatively quiet Relatively quiet

Luminosity High, but can be reduced withsteam.

Some Some

Air Pollution (odor) Best obtainable, if elevation isadequate.

Poor dispersion, because of lowelevation; severe problems if poorcombustion or flameout.

Poor

Other Factors • High cost if high elevation.• Visual and noise pollution.• Radiation requires wide spacing.

• High cost.• High maintenance requirement.• Odor pollution at low elevation.• Hazardous if flameout occurs.

• Low-cost and simple;but pollution is notacceptable in mostcases.

• Wide spacing required.

Application • General choice for total flareload, or as over-capacity flare inconjunction with multijet flare.

• Generally the only acceptableflare where products ofcombustion or partialcombustion are toxic ormalodorous.

• Use for base load or partialflaring rates if noise and visualpollution are critical.

• Suitable only for "clean burning"gases, i.e. where products ofcombustion are not toxic ormalodorous.

• Not suitable upwind ofresidential areas.

• Remote locationswhere no pollutionrequirements apply andspace is available.


SectionXV-E

Page20 of 60

FLARES


EXXONENGINEERING


TABLE 2ELEVATED FLARE TIPS

A. SMOKELESS TYPES

GENERICNAME

BASICCONFIGURATION FEATURES

VENDOR AND TYPEDESIGNATION COMMENTS

Steam Ring See Figure 1A Basic smokeless flare.Relatively simple to build.Design of steam nozzles varies amongvendors.Center steam nozzle is sized for smokelessburning of purge gas (optional).Refractory generally disappears after severalmonths of service.Steam injection system can be sized forsteam pressures as low as 20 psig (140 kPa)at tip.External steam injection produces steamnoise.

Generally design for 1 psi (7 kPa) ∆P of flaregas at the tip, but can vary in both directions.Steam lines are usually sized for smokelessoperation at up to 20% of peak flaring rate,but can vary from this if more steam isavailable.Wind loading is relatively low, since tipdiameter is the same as the stack diameter.Cheapest type of smokeless flare.

National Airoil BurnerCompany, Inc.1284 E. Sedgley Ave.Philadelphia, Pa. 19134 NRC tipS.A.M.I.A.20129 MilanoViale L. Majno 26Italy No type designation

John Zink Company4401 South PeoriaTulsa, Okla. 74105 QS tip

Flaregas Corporation100 Airport Execute Park,Spring Valley, NY 10977 FHP tip

Airoil-FlaregasP.O. Box 92,West Drayton,Middlesex UB78BEEngland FHP tip

All five vendors arecompetitive in price.Exxon has mostexperience withZink tips.Low frequencynoise has beenexperienced withthese "Steam ring"type flare tips at lowflaring rates.

Steam Ringand SteamInspirating Air

See Figure 1B Combines steam ring with internal injection ofsteam and inspirated air.Designed to use internal steam for smokesuppression at low flaring rates, with top ringcutting in at higher rates.Acoustic muffler is used around the lowersteam manifold.

John Zink Company SA-QS tip

This was developedfrom the STF-SAStip.Low frequencynoise has beenexperienced withthis type of flare athigh flaring rates.

SteamInspirated Air

See Figure 1C Coanda effect nozzles are used to inspirateair into the mixing chamber for the FS type.In the FSX type, steam is used to inspirate airthrough steam injector units.FSX design evolved from FS tip.Both claim high efficiency use of steam andlow steam noise.Complex nozzles are a high-cost component.

Flaregas Corporation FS tipAiroil-Flaregas Ltd. FSX tip

More expensivethan “Steam Ringonly” type of tips.Quieter than“Steam Ring only”tips.Used at somelocations to reducelow frequencyrumbling noise.


XV-EPage


DateDecember, 1999


TABLE 2 (Cont)ELEVATED FLARE TIPS

A. SMOKELESS TYPES (Cont)

GENERICNAME



SteamInspirating Air

See Figure 1D Flare gas flows from the stack into the center-body and then into spokes. Air flowsbetween spokes. Basis for this design is jetengine afterburner technology.Air flow can be induced by steam injected atthroat of venturi shroud or can have forceddraft in an air stack concentric with the flarestack.Flow control plates in spokes are potentiallysubject to fouling.High aerodynamic loading. Typical shrouddiameter is 88 in. (2235 mm) and height is 96in. (2440 mm) for 24 in. (600 mm) diameterflare stack.Expensive sheet metal work.Different designs for refinery & chem. plantflaring, purging of tankers, pipeline flaring,etc.No refractory is used. Measured metaltemperatures ~ 450°F (230°C) with steamflow.

Smoke-BanManufacturing, Inc.711 E. Curtis StreetPasadena, Texas 77502 Models SVL

SFL SteamSFH InspiratorSFLH

Models AVLAFL ForcedAFH DraftAFLH

Configurations vary inspoke flow control,nozzles vs. slots, etc.

First majorinstallation in U.S.(1968) was at CitiesService, LakeCharles, La. Thistip was used fordevelopment work.Subsequentinstallations werefor Continental Oil,Denver (1970) andPonca City (1971).Two small tips weresupplied to HumblePipeline.No knowninstallations atExxon Refineries orChemical Plants.

Center Steam Sheet metal as forsteam ring tip butno steam ring ornozzles.Steam emergesfrom the straightpipe concentric withthe flare stack andterminating belowthe flare tip exitplane.

Cheapest steam-injection flare tip.Steam jet emerges at high velocity andpenetrates to the exit plane of the flarewithout mixing completely with flare gas.Results are intense steam noise (muchgreater than with steam ring for the samesteam rate) and higher steam consumptionthan the steam ring.

ER&E No longer beinginstalled.

Coanda Tulip shape to giveCoanda profile toinduce air.

Gas releases at base of elevated tulip createlow-pressure region. Gas film followsCoanda profile, mixes with air, and is ignitedby pilot.

BP Flare SystemsBP Trading LimitedChertsey RoadSunbury-on-ThamesMiddlesex, TW 16 7 LNEngland Indair Mardair

No known onshoreEsso/Exxoninstallations.Supplier statesexperience in 5 to300 million SCF/D(1.6 to 98 Sm3/s).

B. NON-SMOKELESS TYPE

GENERICNAME



Utility or FieldFlare

Sheet metal as forsteam ring tip butwithout steam ringor center steam.

Cheapest flare tip.Produces trailing smoke even with naturalgas.

John Zink Company U tipNational Airoil Burner Co. NCG tipS.A.M.I.A. No type designationFlaregas Corporation FNH tipAiroil-Flaregas Ltd. FN tip

In view of emission,quality standardsare now acceptableonly for hydrogenflaring or non-smoke producinggases.


SectionXV-E

Page22 of 60

FLARES


EXXONENGINEERING


FIGURE 1ASMOKELESS ELEVATED FLARE TIP (STEAM RING TYPE)

��

Steam Nozzles

Steam Ring

Perforated FlameRetention Ring

Alloy Tip, Typically310 SS or Incoloy 800

Center Steam Nozzle

Refractory Lining

Steam Line

Pilots and IgnitersNot Shown

DP15EF1A


XV-EPage


DateDecember, 1999


FIGURE 1BSMOKELESS ELEVATED FLARE TIP (STEAM RING AND INSPIRATED AIR TYPE)

��

��

��

Path of Inspirating Air

Acoustic Muffler

Top Steam Ring Nozzles

Top Steam Ring Manifold

Refractory

Inspirating SteamManifold

Center Steam Nozzle

Pilots and IgnitersNot Shown DP15EF1B

Internal Ducts


SectionXV-E

Page24 of 60

FLARES


EXXONENGINEERING


FIGURE 1CSMOKELESS ELEVATED FLARE TIP (STEAM INSPIRATED AIR TYPE)

��

��

Windshield

Air Inspirating Nozzles

See Detail Below

Refractory

Pilot and Igniter Lines

Main Steam Inlet

Center Steam Inlet

��

��

��

��

��

��

��

Entrained Air

Steam

FSX Type Air Inspirator(Steam Supplied Via Annulus)

��

��

��

��

��

��

Entrained Air

Steam

FS Type Inspirator(Steam Supplied Via Annular Slots

- Uses Coanda Effect Nozzle) DP15EF1C


XV-EPage


DateDecember, 1999


FIGURE 1DSMOKELESS ELEVATED FLARE TIP (STEAM INSPIRATED AIR TYPE)

Inspirated PrimaryAir Passes Between

Spokes

Steam Nozzle atVenturi Throat

Flare Gas FlowsFrom CenterbodyInto Spokes

Circular Ring Flameat Low Gas Flow Rates

Pilots BetweenSpokes

Incoloy 800

Venturi Shroud

304 SS

Steam Ring

DP15EF1D


SectionXV-E

Page26 of 60

FLARES


EXXONENGINEERING


FIGURE 2A (CUSTOMARY UNITS)(1)PRESSURE DROP IN JOHN ZINK FLARE TIPS

Det

erm

ine

Equ

ival

ent F

low

Rat

e Fr

om

Qe

= 3.

1 W

T M, o

r

Qe

= 0.

0081

Qg

TM

Whe

re Q

e =

Equ

ival

ent G

as F

low

Rat

eTh

roug

h Ti

p, S

CF/

Hr.

Qg

= A

ctua

l Gas

Flo

w R

ate,

SC

F/H

r.W

= M

ass

Flow

Rat

e, L

b/H

r.

M =

Ave

rage

Mol

ecul

ar W

eigh

t

Tip

Dia

met

er10

"16

"18

"20

"24

"30

"36

"42

"48

"

3 2

1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1 0.

10.

20.

30.

40.

50.

60.

81.

02

34

56

810

15

(Sou

rce:

J. Z

ink

Co.

)

Qe

– E

quiv

alen

t Gas

Flo

w R

ate

in m

scf

/hr.

at S

tand

ard

Con

ditio

ns

Tip Pressure Drop, psi Not

e: (

1) T

his

char

t sho

ws

tip p

ress

ure

drop

as

a fu

nctio

n of

dia

met

er a

nd e

quiv

alen

t flo

w ra

te.

DP15EF2A

T =

Tem

pera

ture

, °R

.


XV-EPage


DateDecember, 1999


FIGURE 2B (METRIC UNITS)(1)PRESSURE DROP IN JOHN ZINK FLARE TIPS

Det

erm

ine

Equi

vale

nt F

low

Rat

e Fr

om

Qe

= 26

0 W

T M, o

r

Qe

= 0.

0109

Qg

TM

Whe

re Q

e =

Equi

vale

nt G

as F

low

Rat

eTh

roug

h Ti

p, d

m3 /

sQ

g =

Actu

al G

as F

low

Rat

e, d

m3 /

sW

= M

ass

Flow

Rat

e, k

g/s

M =

Ave

rage

Mol

ecul

ar W

eigh

t

40.0

10.0 9.0

Tip Pressure Drop, kPa

30.0

20.0 8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.9

0.8

0.7

0.6

0.5

56

810

38

104

65

42

38

105

65

42

3

Qe –

Equ

ival

ent G

as F

low

Rat

e in

dm

3 / s

at S

tand

ard

Con

ditio

ns

Not

e: (1

) Thi

s ch

art s

how

s tip

pre

ssur

e dr

op a

s a

func

tion

of d

iam

eter

and

equ

ival

ent f

low

rate

.

DP15EF2B

250 mm

400 mm450 mm

500 mm

600 mm

750 mm900 mm

1050 mm

1200 mm

T =

Tem

pera

ture

, K.

Tip Diameter


SectionXV-E

Page28 of 60

FLARES


EXXONENGINEERING


FIGURE 3NOMINAL STEAM REQUIREMENT FOR TYPICAL PROPRIETARY FLARE TIPS (JOHN ZINK STF-S)

0.05200 40 60 80 100

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

> 40%Unsaturated

< 40%Unsaturated

(Source: J. Zink Co.)

Molecular Weight of Flare Gas

Stea

m C

onsu

mpt

ion

Lbs

Stea

m/L

b G

as (k

g St

eam

/ kg

Gas

)

200

50004000

150 (MJ / Sm3)

(Btu / Scf)3000

100

20001000

50

Approximate Heat Capacity of Flared Gas DP15EF3


XV-EPage


DateDecember, 1999


FIGURE 4FLARE PERFORMANCE CHART FOR PROPANE

��

��

��

��

��

Jet N

oise

Flam

e Bo

ilove

r in

Win

d

Steam Snuffs Out Flame

Low-Frequency Noise

Best Operating RegimeSmokyFlame

0 100 300200 400 5000

1.0

2.0

3.0

Wei

ght R

atio

of S

team

/Pro

pane

0 25 50 75 100 125 150

Discharge Velocity of Mixture, ft / s

Discharge Velocity of Mixture, m / s DP15EF4


SectionXV-E

Page30 of 60

FLARES


EXXONENGINEERING


FIGURE 5MULTIJET FLARE SEAL DRUM ARRANGEMENT

FOR USE ON MULTIJET, GROUND, OR STAGED ELEVATED FLARE

Flare Header

12" (300 mm)

1st StageSeal Drum

HCV(Butterfly)Lock AfterSetting

PI

20 GPM(1.25 dm3/s)

To Disposal

To Disposal

Vent

Vent

Vent

DP15EF5

Slope Down

Slope Down

PI

PI

2nd StageSeal Drum

20 GPM(1.25 dm3/s) 5 GPM

(0.32 dm3/s)

55" (1400 mm) 10' (3 m) Min.

30" (750 mm)

To 2nd Stage DistributionHeader and Burners(See Figure 6)

Overcapacity Lineto Elevated Flre

To 1st Stage DistributionHeader and Burners(See Figure 6)

Notes:1. Locate drums as close as possible to windbreaker.2. Design for 0.4 ft/sec (0.12 m/s) to allow vapor disengaging.3. Also see Figures 9 and 12 for ancilliary features.

Note 2

Seal

Water

To Sewer


XV-EPage


DateDecember, 1999


FIGURE 6ADETAILS OF TYPICAL MULTIJET FLARE

Stee

l She

llAr

ound

Ref

ract

ory

3-3"

(75

mm

) ∅ P

eeph

oles

dril

led

thru

sta

ck, a

xes

conv

erge

at c

omm

on p

oint

7'-6

" (52

50 m

m) f

rom

win

dbre

aker

and

5'-6

" (16

30 m

m) a

bove

gra

de.

Two

sets

of h

oles

on

oppo

site

sid

es o

f the

sta

ck.

26'-4

" I.D

.(8

000

mm

)

50'-0"(1500mm)

8'-0

"(2

400

mm

)

11'-0

"(3

300

mm

)

8'-0

"(2

400

mm

) 12'-0

"(3

600

mm

)5'

-6"

(165

0 m

m)

Gra

de

Win

dbre

aker

sla

tsto

be

slop

ed in

war

dto

avo

id li

ne-o

f-sig

htto

bur

ners

. See

deta

il "D

"

Seal

ed c

atch

basi

ns to

be

50' (

1500

0 m

m)

min

. fro

mw

indb

reak

er

10" (

250

mm

) ∅dr

ain

to o

il tra

p th

atfe

eds

into

dirt

yw

ater

sew

er

12"

(300

mm

)17

'-6"

(525

0 m

m)

Elev

atio

n

Plan

Not

e 8

8" &

10"

(200

& 2

50 m

m) ∅

2nd

stag

e di

strib

utio

n lin

es

4" (1

00 m

m) ∅

1st

stag

e bu

rner

line

s

4" (1

00 m

m) ∅

2nd

stag

e bu

rner

line

s

8" (2

00 m

m) c

urb

Prov

ides

4 a

cces

sdo

ors

in w

indb

reak

er

See

Det

ail "

C"

8" (2

00 m

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1st

sta

gedi

strib

utio

n an

d sp

lit fe

ed li

ne

18" (

450

mm

) ∅ m

ain

2nd

stag

e fe

ed li

ne

10" (

250

mm

) ∅ m

ain

1st s

tage

feed

line

12" (

300

mm

) ∅ s

plit

feed

to2n

d st

age

dist

ribut

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lines

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Not

e 8

See

Det

ail

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e Po

rt(N

ote

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and

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Pilo

ts

2nd

Stag

e

1st S

tage

4" (1

00 m

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f B&W

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cret

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qual

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edG

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istri

buto

rSu

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Wel

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supp

ort f

or1s

t sta

ge

Gas

and

Oil

Pilo

ts

Flam

e Ey

e Po

rt(N

ote

6)

2nd

Stag

e1s

t Sta

ge

Sect

ion

Y-Y

8" (225 m

m) Slat

Slat

2" (50

mm

)453"

(75 m

m)

Flar

eSi

de

Det

ail "

D"

Win

d B

reak

er S

lat

Cen

ter s

uppo

rtbe

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ith 3

supp

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sN

ote

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r 1st

sta

geN

ote

3

28'-4

" (ap

prox

.)(8

000

mm

)16

"(40

0 m

m)

Insi

de p

erip

hery

of s

tack

Wel

ded

supp

orts

for

1st s

tage

. Not

e 3

12" (

300

mm

) ∅ 2

ndst

age

split

feed

line

8" (2

00 m

m) ∅

1st

stag

e sp

lit fe

ed li

ne8"

(200

mm

) ∅ 1

st s

tage

dist

ribut

ion

head

ers

8" &

10"

(200

& 2

50 m

m) ∅

2nd

stag

e di

strib

utio

n he

ader

s4"

(100

mm

) ∅ 1

st &

2nd

stag

e bu

rner

line

sD

etai

l "C

"Fl

are

Dis

trib

utor

All b

urne

rs a

re p

lace

d on

16" (

400

mm

) cen

ters

alo

ng b

urne

rlin

es. B

urne

r lin

es a

re s

pace

d at

6 1/

2" (4

10 m

m) c

ente

r to

cent

er

Line

A

YY

1st

stag

e2n

dst

age

54 Jets

204

Jets��

��

4" (1

00 m

m) S

ch. 4

0pi

pe b

urne

r lin

e8"

to 1

0"(2

00 to

250

mm

)

Stag

ger a

djac

ent l

inin

g su

ppor

tst

aple

s by

45°

alo

ng b

urne

r lin

ele

ngth

and

ove

rlap

abou

t 1" t

o2"

(25

mm

to 5

0 m

m)

1/2" (40 mm)

3/4"(20 mm)

1/4"

(6 m

m) C

.S.

1/4"

(6 m

m) c

asta

ble

cove

rre

quire

d ov

er a

ll st

aple

sex

cept

1/2

" (12

mm

) ove

r top

row

on

burn

er li

ne. N

ote

4

1"(2

5 m

m)

1"(2

5 m

m)

2"(5

0 m

m)

1" (25

mm

)D

etai

l "B

"B

urne

r Pip

e In

sula

tion

Supp

ort

DP1

5EF6

a

��

1" (2

5 m

m) ∅

std

.pi

pe ty

pe 3

04SS

Tack

Wel

d1/

4" (1

8 m

m) N

ut

Lock

was

her

3/4"

(18

mm

)2 1/

4" (5

5 m

m)

21"(525 mm)

15"(375 mm)

Botto

m o

f Sta

ckSk

irt

1" (2

5 m

m) t

hird

coup

ling

3000

lbca

rbon

ste

el

Insu

latio

nBu

rner

Line

Sect

ion

X-X

��

��

��

��

��

Burn

erPi

peC L

U-B

olt

3/4"

x 3

/4" x

2 1

/4"

(18

x 18

x 5

5 m

m) Y

oke

1/4"

x 3

/4" (

6 x

18 m

m)

Bar S

tock

Win

gs1/

4" x

1" (

6 x

25 m

m) C

olla

r

5 1/

2"(1

40 m

m)

Bi-M

etal

lic S

eal W

eld

Use

25-

20 R

od

Wel

ds

1" (2

5 m

m) ∅

std

. pip

eje

t (30

4SS)

Spac

ed 1

6" (4

50 m

m)

cent

er to

cen

ter a

long

pip

e

1" (2

5 m

m) ∅

Silic

onC

arbi

de R

od (f

lam

eho

lder

)

1/16

" x 1

/4" x

3/8

"(2

x 6

x 9

mm

) Col

lar

Stra

p

XX

Det

ail "

A"

Flam

e H

olde

r and

Sup

port

Not

e 7

See

Det

ail "

E"

8" (2

00 m

m) ∅

1st

sta

gedi

strib

utio

n he

ader

Tran

site

sla

ts in

met

al fr

ame

Onl

y 1s

t sta

gedi

strib

utor

s ex

tend

edfu

lly a

cros

s su

ppor

ts

1/2"

(12

mm

)

1" (2

5 m

m)

1" (2

5 m

m)


SectionXV-E

Page32 of 60

FLARES


EXXONENGINEERING


FIGURE 6BDETAILS OF TYPICAL MULTIJET FLARE (Cont)

��

��

3-3"

(75

mm

)Pe

ep H

oles

Dril

led

Thru

Stac

k As

Show

n

Botto

m o

fSt

ack

Skirt

10"

(250

mm

)

See

Det

ail

"A &

B"

See

Det

ail "

F" F

orPi

lot D

raw

ings

Fire

proo

f All

Supp

ort

Mem

bers

and

Arra

nge

Legs

to A

void

Bur

ner H

eade

rs,

Use

Cab

le G

uys

if R

equi

red

See

Det

ail "

H" &

"J"

See

Det

ail "

G"

26'-4

" (80

00 m

m) I

.D.

(Insi

de C

asta

ble

Line

r)

��

��

1" (2

5 m

m)

Fish

tail

Burn

er(O

n-O

ff Pi

lot)

Botto

m o

f Sta

ck S

kirt

3" (7

5 m

m) I

gnito

r Tub

e M

ust B

ea

Stra

ight

Run

Fro

m S

tack

toPo

int o

f Ign

ition

.

Line

A o

f the

4" (1

00 m

m) ∅

1st

Stag

e Bu

rner

Lin

es

Flam

e H

olde

r

Min

12 3

/4"

(320

mm

)

Flam

e Ey

ePo

rts N

ote

B

Det

ail "

E"Fl

are

Stac

k

6" (2

00 m

m)

2" (5

0 m

m)

6" (1

50 m

m)

8" (2

00 m

m)

��

��

��

Shel

l Cas

tabl

eLi

ning

C.S

. She

ll

Flam

e Ey

e Po

rtsN

ote

6

Ref

ract

ory

Line

r

Muf

fleBl

ock

Muf

fleBl

ock

Alig

n Pi

lots

So

That

Fla

me

Dire

cted

Ove

r Clo

sest

Jet

of

Line

A o

n th

e 1s

t Sta

geBu

rner

Dis

tribu

tor

110

12" (

300

mm

)

Det

ail "

F"O

il an

d G

as P

ilot

Not

e 2

��

Car

bon

Stee

l She

ll

1" +

1/2"

(25

+ 12

mm

)-

0-

0C

asta

ble

Cov

er O

ver S

tuds

1/2"

(12

mm

) ∅ W

eldi

ng S

tud

or E

quiv

. App

rox.

4" (

100

mm

)Lo

ng W

ith a

5 1

/2" (

90 m

m)

Slot

(Typ

ical

) Spa

cing

Per

Det

ail "

I" Sp

lit R

od is

Wel

ded

on S

ide

of S

tack

Tha

n Sp

read

to "Y

" Sha

de.

4" (1

00 m

m)

45

1/2"

(12

mm

)

5"(125 mm)

Stud

Sho

wn

in S

prea

d Po

sitio

n

Det

ail "

G"

Stac

k To

p

��

Det

ail "

H"

Stac

k B

otto

m

Car

bon

Stee

l She

llSt

ack

Base

P(C

arbo

n St

eel)

L

El. 1

0' (3

000m

m)

1/4"(6 mm)

4"(1

00 m

m)

1/4"

(6 m

m)

Inst

all S

tuds

With

Ran

dom

Orie

ntat

ion

Do

Not

Alig

n

10"

(250

mm

) 10"

(250

mm

)5"

(125

mm

)D

etai

l "I"

Lini

ng S

tud

Loca

tion

3/4"

(18

mm

) ∅R

od x

10"

,(2

50m

m)3" x

3" x

1/4

" x 3

"(7

5 x

75 x

6 x

75

mm

)

Adju

stab

leR

ing

12"

(300

mm

)

Stac

kSu

ppor

t -SG

Stac

kSu

ppor

tL&

G

Tack

Wel

d N

ut to

Stud

and

to A

ngle

Stac

k W

all Bo

ttom

Edg

e of

Stac

kCen

ter o

fSp

an12

" Min

.(3

00 m

m)11"

(275 mm)1"

(25

mm

)

Fron

t Vie

wD

etai

l "J"

Stac

k A

djus

tmen

t

Stac

k W

all

3" (7

5 m

m)

2" (5

0 m

m)

3" (75 mm)

7"(175 mm)

1 3/

4"(4

0 m

m)

3" (7

5 m

m)

Adju

stab

le 1

/8" x

6"

(3 x

150

mm

) Car

bon

Stee

lR

ing.

Has

a 6

" (15

0 m

m)

Tota

l Dra

ft Ad

just

men

t

Botto

m E

dge

of S

tack

6" (150

mm

)1" (2

5 m

m)

Side

Vie

w

Notes

:Al

l bur

ner l

ines s

hall b

e lev

el an

d in t

he sa

me pl

ane.

The 1

st an

d2n

d stag

e hea

der l

ines s

hall b

e slop

ed su

ch th

at the

burn

ers

drain

to th

eir se

al dr

ums.

1.

Gas-o

il pilo

ts ar

e loc

ated a

t orig

in of

each

1st s

tage b

urne

r (lin

e A).

See d

etail"F

". Pr

ovide

stra

iners

in fue

l sup

ply lin

es.

2.

The f

eed e

nds o

f the

1st s

tage h

eade

rs ar

e weld

ed di

rectl

y to t

he2n

d stag

e fee

d hea

ders.

The

1st s

tage b

alanc

e line

is su

ppor

tedon

a cro

ss tie

weld

ed on

the u

nder

side

of th

e two

adjac

ent b

urne

rpip

es of

the 2

nd st

age.

Ther

e are

a tot

al of

8 sup

port

point

s for

the

2nd s

tage d

istrib

utor.

3.

Casta

ble lin

er m

ateria

l to be

equiv

alent

to A.

P. G

reen

Co.

Kast-

O-Lit

e 30,

appli

ed pe

r man

ufactu

rer's

instr

uctio

ns.

4.

3 firs

t stag

e bur

ner l

ines t

o hav

e 9 je

ts on

each

half s

ectio

n. Se

cond

stage

burn

er lin

es to

have

9-8-

8-8-

7-6-

4 jets

on ea

ch ha

lf sec

tion.

Cente

r-line

burn

ers e

ach h

alf se

ction

, to b

e spa

ced 1

6",(4

00mm

)ce

nter l

ine to

cente

r line

.

5.

Prov

ide se

lf che

cking

flame

eyes

of th

e ultr

aviol

et typ

e for

each

grou

nd fla

re pi

lot. T

hey s

hall b

e pro

vided

in ac

cord

ance

with

the

inten

t of I

P 7-

2-1 P

arag

raph

6.46

as m

odifie

d to f

it this

insta

llatio

n.On

flame

-out

they s

hall s

ound

alar

m in

main

contr

ol ho

use.

6.

Flame

holde

r sup

port,

to be

fabr

icated

of T

ype 3

04 S

.S. m

ateria

ls.7.

Lowe

r half

of pl

an vi

ew sh

ows o

ne ha

lf of t

he 2n

d stag

e fee

d and

distri

butio

n line

s. Lin

es to

othe

r half

of 2n

d stag

e (no

t sho

wn) t

o be

identi

cal. A

ll pipi

ng to

each

side

of 2n

d stag

e bur

ners

shall

besy

mmetr

ical. S

imila

rly up

per h

alf of

plan

view

show

s 1st

stage

split

feed a

nd di

stribu

tion l

ines.

1st s

tage m

anifo

lding

to be

unde

r 2nd

stage

insta

llatio

n (se

e Sec

tionY

-Y).

8.

Cente

r sup

port

to be

two n

arro

w be

ams s

pace

d to a

ccom

moda

tebu

rner

line e

xpan

sion a

nd as

narro

w as

poss

ible t

o pre

clude

restr

iction

of ai

r flow

to bu

rner

s. Pr

ovide

guide

s for

1st s

tage p

ipes

which

trav

erse

full d

iamete

r.

9.

Contr

actor

shall

chec

k bur

ner l

ine an

d dist

ributi

on he

ader

layo

ut to

insur

e suff

icien

t allo

wanc

e has

been

mad

e for

line e

xpan

sion.

10.

DP

15E

F6b

Cas

tabl

e Li

nes

Suita

ble

for

at L

east

500

0 °F

(276

0 °C

)H

ot F

ace

Tem

p. N

ote

4


XV-EPage


DateDecember, 1999


FIGURE 7TYPICAL BURNING-PIT FLARE

Liquid and Vapor

Inlet

Pilot and IgniterLines (See Text)

Distributor andConcrete/Refractory Pad.Pad Width = 5 TimesDistributor Diameter(2 ft (0.6 m) min)(See Detail A)

Water OutletSee Detail B

Dike Wall~30° Slope to

Horizontal

See Detail B

Seal Water Make UpSupply 20 gpm (1.25 dm3/s)

R.O.

��

��

��

Normal Water Level

Maximum Oil Level

18"(450 mm)Freeboard

Rows of 1-1/2" (38 mm) Dia. Holeson 3" (75 mm) Triangular Pitch

12" (300 mm) Min.4" (100 mm)

12" (300 mm)

6" (150 mm)

Bottom of Pit

1-1/2" (38 mm)Drain Holes

RefractoryConcrete

Base distributor diameter on pressure dropalong its length of 0.025 psi (0.17 kPa) atmaximum flow. Base distributor length and numberof holes on pressure drop of 0.25 psi (1.7 kPa)across the holes at maximum flow.

��

Min. Min.

Normal WaterLevel

Seal depth sized to preventoil discharge when dike fullof liquid hydrocarbon

Overflow elevationat required normalliquid level

ToSewer

(Below Grade)

Seal WaterSupply

Detail ALiquid and Vapor Distributor

Detail BSeal Water Inlet and Overflow Seal DP15EF7


SectionXV-E

Page34 of 60

FLARES


EXXONENGINEERING


FIGURE 8TYPICAL ELEVATED FLARE PILOT AND IGNITER

Pilo

t Tip

(One

of T

hree

)

Flar

e Ti

p

Igni

ter T

ip a

ndW

inds

hiel

d

Igni

ter A

ssem

bly

Loca

ted

at G

rade

.Pi

lot I

gniti

on R

eadi

lyVi

sibl

e Fr

om th

e Ig

nite

r.

Pilo

t Gas

/Air

Vent

uri

(Loc

ated

at F

lare

Tip

Plat

form

)

Not

es:

1. L

ocat

e ig

nite

r ass

embl

y an

d in

divi

dual

pilo

t and

igni

ter v

alve

s to

geth

er (m

ay b

e re

mot

e fro

m

fla

re if

requ

ired)

.2.

Thr

ee w

ay v

alve

s ar

e re

quire

d in

the

flam

e fro

nt g

ener

ator

line

s do

wns

tream

from

the

spar

k pl

ug

ign

ition

to e

nsur

e th

ere

is a

lway

s an

ope

n pa

th fr

om th

e fla

me

front

gen

erat

or to

a p

ilot.

To N

o. 2

Pilo

t

To N

o. 3

Pilo

t

To N

o. 3

Igni

ter

To N

o. 2

Igni

ter

Low

Poi

ntD

rain

Isol

atio

n Va

lves

(at G

rade

)

Pilo

t Gas

Pres

sure

Con

trol

Stat

ion

(at G

rade

)

PI

PCPHA

PLA

PI

PI

PdI

FO(H

)

Pilo

t Gas

Stra

iner

s(a

t Gra

de)

PI

Pilo

t Gas

Supp

ly

RO

RO

PI

Inst

rum

ent

Air S

train

er(a

t Gra

de)

Inst

rum

ent

Air S

uppl

y

Elec

trica

lPo

wer

Supp

ly

Spar

kIg

nite

r

Tran

sfor

mer

and

Igni

ter P

ushb

utto

n

Sigh

tG

lass

Not

e 1

Not

e 2

DP1

5EF8

Ther

moc

oupl

e


XV-EPage


DateDecember, 1999


FIGURE 9A (CUSTOMARY UNITS)TYPICAL FLARE SEAL DRUM

BlankedFlush WithInlet

1" Drain

Clean Out Cover

Flare Stack

Slope Down

Winterized(See Text)

Siphon Breaker Vent(Locate as for manholevent per IP 3-2-1)

Normal WaterFlow RatePressure Drop

Min.10' Min.

Min.

Slope DownVapor fromBlowdown Drum

TLCI

For ColdReleases(LocateTLCIupstream toprovideadequateresponsetime).

10' Min. or175% of Max.Drum OperatingPressure (Note 2)

To Disposal

3" Drain

Min.Min.

Max. WaterVelocity =0.4'/Secin verticaldownpipe

Make-upWater viaStrainer andR.O.20 Gpm

Steam

SteamSparger 3" Drain

to Sewer

ForWinterizing

R.O.Sour Water(If Included)

TIC

Baffle

Detail "A"(Note 1)

Normal LiquidLevel

3 - 4"

d/4 Min.

60o

60o

2"

d

Gunite Lining

DETAIL A DETAIL B

BAFFLE ARRANGEMENTFOR FLARING H 2S

2"2"

4"ToDisposalMake Up

Water

H2S LadenWater 6"

6"

DP15EF9a

Notes:(1) See Figure 12 for Typical Non-Pulsing Sparger.(2) A 110% seal depth is permitted for remote contingencies.(3) Three connections, 6 in. apart for skimmed oil.

See Note (3)Skimmed Oilto VacuumTruck


SectionXV-E

Page36 of 60

FLARES


EXXONENGINEERING


FIGURE 9B (METRIC UNITS)TYPICAL FLARE SEAL DRUM

50 mm

BlankedFlush WithInlet

25 mm Drain

1 Diam. or300 mm Max.Clean Out Cover

Flare Stack

Slope Down

Winterized(See Text)

Siphon Breaker Vent(Locate as for manholevent per IP 3-2-1)

Normal WaterFlow RatePressure Drop

Min.3 m Min.

Min.

Slope DownVapor fromBlowdown Drum

TLCI

For ColdReleases(LocateTLCIupstream toprovideadequateresponsetime).

3 m Min. or175% of Max.Drum OperatingPressure (Note 2)

To Disposal

75 mm Drain

Min.Min.

Max. WaterVelocity =0.12 m/sin verticaldownpipe

Make-upWater ViaStrainer andR.O.1.25 dm3/s

Steam

SteamSparger 75 mm Drain

to Sewer

ForWinterizing

R.O.Sour Water(If Included)

TIC

Baffle

Detail "A"(Note 1)

Normal LiquidLevel

75-100 mm

d/4 Min.

60o

60o

50 mm

d

Gunite Lining

DETAIL A DETAIL B

50 mm50 mm

100 mmToDisposalMake Up

Water

H2S LadenWater 6"

6"

DP15EF9b

BAFFLE ARRANGEMENTFOR FLARING H 2S

Notes:(1) See Figure 12 for Typical Non-Pulsing Sparger.(2) A 110% seal depth is permitted for remote contingencies.(3) Three connections, 15 cm apart for skimmed oil.

See Note 3Skimmed Oilto VacuumTruck


XV-EPage


DateDecember, 1999


FIGURE 10MOLECULAR DRY SEAL (JOHN ZINK)

��

(Source: J. Zink Co.)

Drain to Grade

Flare Gas Inlet

(Into Flare Stack or Header)

Flare Stack

Continuous Purge

Flare Tip

DP15EF10


SectionXV-E

Page38 of 60

FLARES


EXXONENGINEERING


FIGURE 11FLUIDIC SEAL (NATIONAL AIROIL)

Entering Air

Velocity Gradientof Flared Gas

Baffles

Flare StackPurge Gas Flow

(Source: National Air-Oil Company)DP15EF11


XV-EPage


DateDecember, 1999


FIGURE 12TYPICAL NON-PULSING SPARGER

12

34 5

6

Level Number

Vertical Distance BetweenLevels: 1 in. (25 mm)

Hole Diameter: 1/2 in.(13 mm)

Cut Slot AlongBottom of Pipe

W

W =π4

DL / DTotal Number of Levels = 3D

(L / D) – 1 + 2

Level NumberHole Spacing (inches) Center-To-CenterHoles/Level Per Foot of PipeHole Spacing (mm) Center to CenterHoles/Level Per Metre of Pipe

1347513

2387526

32125040

42125040

5 or More1.5164050

Hole Spacing Required on Each Level

Detail "A"

DL/2 D/2+6" (D/2 + 150 mm)

Minimum RecommendedDistance to Overflow

L

D = Pipe DiameterL = Pipe Length

Drill Holes in These Two ZonesOnly. See Detail "A"

ClosedEnd

D

Seal Depth Slope 1/8" per ft (10 mm per m)

Normal Liquid Level

Note: Minimum Recommended L / D = 7 DP15EF12


SectionXV-E

Page40 of 60

FLARES


EXXONENGINEERING


FIGURE 13NH4HS DEPOSITION TEMPERATURES

105

2252001751501251007550

Temperature, °F DP15EF13

NH

4HS

Dis

soci

atio

n C

onst

ant,

Kp, (

psia

)2

104

103

102

10

1

Reference:U.S. Bureau of Mines Bulletin 406,p. 56 (1937)

NH3(g) = H2S(g) NH4HS(s)Kp = (PH2S)(PNH3)

In this area, deposition ofsolid NH4HS can occur.


XV-EPage


DateDecember, 1999


FIGURE 14NH4HCO3 DEPOSITION TEMPERATURES

103

140130120110100908070

Temperature, °F DP15EF14

NH

4HC

O3

Dis

soci

atio

n C

onst

ant,

Kp, (

psia

)3

102

10

1

10–1

10–2

In this area, deposition ofsolid NH4HCO3 can occur.

NH3(g) + CO2(g) + H2O(g) NH4HCO3(s)Kp = (PNH3)(PCO2)(PH2O)


SectionXV-E

Page42 of 60

FLARES


EXXONENGINEERING


APPENDIX A -RADIATION AND SPACING FROM FLARES AND IGNITED VENTS

The following procedures for determining flame shape, flame center, and radiant heat density at any adjacent location areapplicable to flares and to vents (such as atmospheric pressure relief valve discharges), which may inadvertently becomeignited. This material is abstracted from Report No. EE.15ER.71, Predicting Radiant Heating from Flares, to which referenceshould be made for further background information, if required.

COMPUTER CALCULATIONSA computer program (FLARAD, Program No. T-3576) is available for performing the radiant heat density calculations describedin this Appendix. The input information required is:Atmospheric Conditions - Average wind speed at grade, ambient air temperature, air density, relative humidity, FLARADautomatically converts normal average wind speed [measured 33 ft (10 m) above grade] to wind speed at the flare tip elevationabove grade.Flare Gas - Molecular weight, lower flammable limit, density at flare tip, fraction, F, of heat release radiated by the flame.Flare Parameters - Heat release rate, tip diameter, gas discharge velocity, stack height.

SIMPLIFIED CALCULATION OF FLAME SHAPE AND LENGTHA simplified graphical procedure has been developed for the determination of Xc (horizontal) and Zc (vertical), the flame mid-point coordinates for flares and ignited vents. A more detailed procedure for calculating the flame center is described in thenext section.The attached graphs, shown in Figures A-1A and A-2A, one for Xc and one for Zc, are based on two independent variables:

LC and a modified form of djR. The latter variable has been modified to include gas and air temperatures and molecularweights instead of densities. The ideal gas law assumed. The graph curves have been smoothed over the LC range of 0.5 to1.5 to remove discontinuities and curve fit inaccuracies existing in the design practice procedure. No significant difference isintroduced with the data smoothing.

Information RequiredCL = Lower flammable limit concentration of flare gas in air, volume fraction (see Page 45).dj = Inside diameter of flare tip, ft (m)Mj = Molecular weight of flare gasM∞ = Molecular weight of air = 28.97Tj = Temperature of flare gas, °R (°K)T∞ = Temperature of air, °R (°K)U = Normal average wind speed at 33 ft (10 m) elevation above grade, ft/s (m/s), based on local

meteorological dataUj = Flare gas velocity at the flare tip, ft/s (m/s)ρj = Flare gas density at the flare tip (before ignition), lb/ft3 (kg/m3)ρ∞ = Ambient air density, lb/ft3. If it is not known, use .0706 lb/ft3 (1.22 kg/m3)h = Height of flare tip above grade, ft (m)

Calculation Procedure

Step 1 Calculate the average wind speed, U∞, at the flare tip elevation above grade and the concentration parameter LC forthe flare gas:

14.014.0

10hU)s/m(U;

33hU)s/ft(U

=∞

=∞

∞∞=

MM

UU

CC jjLL


XV-EPage


DateDecember, 1999


APPENDIX A (Cont)RADIATION AND SPACING FROM FLARES AND IGNITED VENTS

Step 2 Calculate the graphical parameter 5.0

j

jjj T

MTUU

d

∞∞

, then enter Figure A1A and determine Zc.

Step 3 Enter Figure A-2A and determine Xc.This locates the flame center, which is treated as the source of all radiation from the flame.

DETAILED CALCULATION OF FLAME SHAPE AND LENGTH

Step 1 Calculate the average wind speed, U∞, at the flare tip elevation above grade and the concentration parameter LC forthe flare gas:

14.014.0

10hU)s/m(U;

33hU)s/ft(U

=∞

=∞

∞∞=

MM

UU

CC jjLL Eq. (A-1)

Step 2 Calculate the value of LS from Eq. (A-2a) or (A-2c). This term is a dimensionless coordinate of the concentration LCalong the centerline of the flame. From LS , calculate the downwind coordinate XL from Eq. (A-2b) or obtain it fromFigure A-A3.

1. If LC ≤ 0.6,

( ) 03.1L

L

C

04.2S = Eq. (A-2a)

and

65.1SX LL −= Eq. (A-2b)

(See Figure A-A3).

2. If LC > 0.6,

( ) 625.0L

L

C

51.2S = Eq. (A-2c)

a. If LS > 2.35,

65.1SX LL −= Eq. (A-2b)

b. If LS ≤ 2.35, obtain LX from Figure A-A3. This figure is a plot of the equation:28.0

L2

LL X05.2X04.1S += Eq. (A-2d)

The term LX is used later [Eq. (A-5b)] to locate the tip of the flame.

Step 3 Calculate the dimensionless rise LZ of the flame tip above the flare tip:28.0

LL X05.2Z = Eq. (A-3)


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Step 4 Calculate the scaling parameter R, which accounts for the effect of flame shape of the relative thrusts of the wind andthe gas jet discharging from the flare tip:

5.0jj

UU

R

∞ρρ

∞= Eq. (A-4)

Step 5 Calculate the full-scale coordinates of the flame tip:

RdZZ jLL = Eq. (A-5a)

RdXX jLL = Eq. (A-5b)

where: ZL = Vertical distance of flame tip above flare tip, ft (m)

XL = Horizontal distance of flame tip from flare tip, ft (m)

Step 6 Calculate the location of the flame center, which is treated as the source of all radiation from the flame. Only flamesbent over by the wind are considered, since for nearly vertical flames (calm air), the effective center of flame radiationis higher off the ground and therefore not limiting for spacing purposes.

ZC = 0.82 ZL Eq. (A-6a)

XC = 0.5 XL Eq. (A-6b)

CALCULATION OF LOWER FLAMMABLE LIMIT CL

Flammable limits for pure components may be obtained from Table B-1 in Appendix B. Note that values for lower flammablelimit in the table are expressed as percentage, while CL is a volume fraction. For calculating CL of a mixture of gases, thefollowing equation is recommended:

1

Ln

n

3L

3

2L

2

1L

1L C

Y...CY

CY

CYC

−

++++= Eq. (A-7)

where: CL = Lower flammable limit concentration of the mixture in air, volume fraction.

Y1 = Mole fraction (or volume fraction) of Component 1 in the mixture

Yn = Mole fraction of the nth component

CL1 = Lower flammable limit concentration of Component 1 in air, volume fraction

CLn = Lower flammable limit of the nth component, volume fraction

Eq. (A-7) is accurate for mixtures of paraffinic gases or for mixtures of H2, CO and CH4. It is only approximate for mixtures ofH2 and C2H4, H2 and C2H2, H2S and CH4 or CH4 and C2H2Cl2. It is even less accurate for mixtures of flammable gases withsteam or inerts. Nevertheless, for the accuracy required here, this simple equation should be tried first in all cases. When theflare gas is a mixture of flammable gases and inerts, the simplest way to approximate CL is to use the above equation with CL =∞ for the inert components. This step treats the inerts as a simple diluent in an ideal mixture. It would give only a roughapproximation for mixtures such as H2 and H2O vapor, where a component, which is inert in the cold mixture, neverthelessenters into the reactions taking place in the flame. In such cases, if a rough approximation of CL is not good enough, then moreaccurate methods described in Appendix B should be used.


XV-EPage


DateDecember, 1999



IGNITED PRESSURE RELIEF VALVE RELEASESApplication of the above procedure to inadvertently ignited pressure relief valve discharges can involve a special problem.Certain combinations of pressure ratio and length of pressure relief valve riser can result in choked flow, with a pressurediscontinuity at the exit. The pressure of the jet then adjusts to atmospheric pressure in a system of shock waves or expansionwaves over a distance of a few pipe diameters. These waves can affect the local mixing of the jet with the cross-wind. Sincethe calculation procedure incorporates correlations for subsonic jets, it cannot be expected to be entirely accurate in this case.Nevertheless, since the wave system occupies a very small portion of the flow field influenced by the jet, the procedure can stillbe counted on to provide a useful approximation of the gross flame length and flame shape when the actual discharge velocityand diameter are used in the calculation.Credit for additional height of the flame center for multiple valve installations may be taken by clustering the pressure reliefvalve discharge pipes to the atmosphere. The following procedure should be used for determining equivalent diameter and exitvelocity to be used in the flame center calculation. Diameter and velocity are based on the total actual area of the clusteredvents.

Equal Diameter Vents

nd)areasventofsum(4.D 2equivj =

π=

=.v equivj actual velocity of any one vent

Unequal Diameter Vents

2n

22

21equivj d...dd)areasventofsum(4.D ++=

π=

2.equivj

equivj)D(

G273.1AreaVentTotalRateGasTotal.U ==

Definition of Terms

d, d1, d2, dn = Diameter of individual vents, ft (m)Dj equiv. = Equivalent vent diameter to be used in flame calculation, ft (m)N = Number of vents in cluster (n ≥ 3)G = Total gas rate of all vents, in ft3 (m3/s) at one atm and release temperatureUj equiv. = Equivalent vent exit velocity to be used in flame calculation, ft/s (m/s)

For further background, see Report EE.15ER.71, Predicting Radiant Heating from Flares.

CALCULATION OF RADIANT HEAT FLUX KThe following calculation procedure is used to predict the heat flux K incident on a surface normal to the direction of radiation,at any distance from the flare stack and any elevation above grade.

Information Required

Xc = Downwind distance from flare stack to flame center, ft (m)Zc = Height of flame center above flare tip, ft (m)h = Height of flare tip above grade, ft (m)F = Fraction of heat release radiated from the flame•m = Mass flaring rate, lb/hr (kg/s)H = Lower heating value of the flare gas, Btu/lb (kJ/kg)r = Relative humidity, percent


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Calculation Procedure

Step 1 Calculate the rate of heat released Q, Btu/hr (kW)

Q = Hm•

Eq. (A-8)

For pure components, the value of H may be obtained from the Exxon Blue Book. In general,H ≅ 20,000 Btu/lb (46,600 kJ/kg) for most paraffins, olefins, and diolefinsH ≅ 18,000 Btu/lb (41,900 kJ/kg) for aromatics, andH ≅ 51,600 Btu/lb (120,000 kJ/kg) for hydrogen

Step 2 Select the appropriate flame emissivity factor F, based on flare gas composition, by weight. Note that for ignitedpressure relief valve releases, lower values of F are used, as described later in this section.

FLARE GAS COMPONENT FHydrogen, methaneC2 HydrocarbonsC3 and C4 HydrocarbonsC5 and heavier

0.20.250.30.4

Step 3 Calculate the distance from the center of the flame to a position with coordinates X and Z, representing personnel orequipment exposed to radiation from the flame. (Since the limiting case is the one in which the flame is blown over inthe direction of the exposed personnel or equipment, they are in the same plane, and a two-coordinate system isadequate for describing their relative locations.) Note also that all wind directions must be considered, even inlocations, which have a prevailing wind direction.

2c

2c )hZZ()XX(D −−+−= Eq. (A-9)

where: D = Distance from the flame center to point (X, Z), ft (m)

Xc = X coordinate of the flame center, ft (m)

H + Zc = Z coordinate of the flame center, ft (m)

Step 4 Calculate the atmospheric transmissivity, τ:16/116/1

D100

r10079.0

=τ Eq. (A-10)

Note: τ is dimensionless. D (ft) and r are as defined above.

or

16/116/1

D5.30

r10079.0

=τ Eq. (A-10)

Note: τ is dimensionless. D (m) and r are as defined above.

Step 5 Calculate K from Eq. (A-11). K is incident on a surface normal to a line from point (X, Z) to the flame center (Xc, Zc).This is the maximum value of K at that location and is the value used when personnel exposure is being considered.

2)z,x(D4QFK

πτ= Eq. (A-11)

where: K = Heat radiated to the surface, Btu/hr/ft2 (kW/m2)


XV-EPage


DateDecember, 1999



S PERSONNEL EXPOSURE LIMITS TO HEAT RADIATION

Heat Radiation From Flares

In most cases, heat radiation is the controlling factor in determining the spacing of elevated or burning-pit flares, consideringpersonnel exposures at grade under maximum heat release conditions.Maximum permissible flare radiation levels for personnel exposure are:

MAXIMUM PERMISSIBLEK, Btu/hr/ft2 (kW/m2) CONDITIONS

3000 (9.45) • Maximum value of K at design flare release at any locationwhere personnel have access, e.g., at grade below the flare, oron a platform or nearby equipment. Exposure must be limitedto a few (approx. six) seconds, sufficient for escape only. Ontowers or other elevated structures, ladders must be providedon the side away from the flare, so that the tower or structurecan provide some degree of shielding.

2000 (6.3) • Areas where emergency actions lasting up to 1 minute may berequired by personnel without shielding.

1500 (4.75) • Areas where emergency actions lasting several minutes may berequired by personnel without shielding, e.g., at the batterylimits of a process unit.

500 (1.6) • Maximum value of K at design flare release for continuousexposure of personnel and maximum value of K at property line.[By comparison, solar radiation in a hot climate may be as highas 300 Btu/hr/ft2 (1 kW/m2).]

Heat Radiation From Ignition of Pressure Relief Valve Releases and Start-up Vents

The following personnel exposure limits should be used to protect personnel against exposure to high radiant heat resultingfrom inadvertent ignition of atmospheric vents such as pressure relief valve releases and start-up vents:

TYPE VENT PERSONNEL EXPOSURE LIMIT

• Pressure Relief Valve Release toAtmosphere

6,000 Btu/hr ft2 (18.9 kW/m2) maximum at grade or at frequently usedoperating platforms. Refer to Section XV-C for additional design criteria.Because of the higher exit velocities and additional air entrainment associatedwith PR valve discharges, the heat release is less than for a low velocity flare.The following factors (fraction of heat release radiated) should be used:

F for PR Valve DischargesHydrogen, MethaneC2 and Heavier Hydrocarbons

0.10.25

• Start-up Vent to Atmosphere 3,000 Btu/hr ft2 (9.45 kW/m2) maximum at grade or at frequently-usedoperating platforms. Note that the higher F factors in this section for flaresshould be used for start-up vents.


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EQUIPMENT EXPOSURE TO HEAT RADIATIONIn most cases, equipment can safely tolerate higher degrees of heat density than those defined for personnel. However, ifanything vulnerable to overheating problems is involved, such as low melting point construction materials (e.g., aluminum orplastic), heat-sensitive streams, flammable vapor spaces, or electrical equipment, then the effect of radiant heat on them mayneed to be evaluated. When this evaluation is required, the necessary heat balance is performed to determine the resultingsurface temperature, for comparison with acceptable temperatures for the equipment, e.g., up to 176°F (80°C) is permitted forelectronic instrument transmitters. For tanks, this heat balance reduces to the following equation:

( ) 3/4s

s

4s2 TT

E21.0

100T1713.0)fthr/Btu(K ∞−+

=− Eq. (A-12)

or

( ) 3/4s

s

4s2 TT

E0015.0

100T0057.0)m/kW(K ∞−+

= Eq. (A-12)

where: K = Radiant heat flux at the tank surface, Btu/hr ft2 (kW/m2)

Ts = Tank surface hot-spot temperature, °R (°K)

T∞ = Ambient air temperature, °R (°K)

Es = Tank surface emissivity

Copper, oxidized black : 0.8

Copper, dull : 0.15

Copper, polished : 0.1

Stainless steel, after repeated heating and cooling : 0.5-0.7

Stainless steel, polished : 0.1-0.2

Aluminum, heavily oxidized : 0.2-0.3

Aluminum paint : 0.27-0.67

White paint : 0.77-0.95

The above equation is based on no wind, negligible unsteady state heating of the tank shell since it is thin, and negligibletransient conduction of heat into the tank contents because large bodies of liquid hydrocarbons act like thermal insulators forshort exposures to radiation. Refer to Report No. EE.15ER.71 for calculation procedures for other surface types and for tankswhere wind cooling is taken into account.


XV-EPage


DateDecember, 1999


FIGURE A-1A (CUSTOMARY UNITS)FLAME CENTER FOR FLARES AND IGNITED VENTS Zc- VERTICAL DISTANCE, ft

200

100

50 20 1010

.05.

01.

00.

50.

10.

050.

01102050100

200

500

10000.

010.

10.

051.

00.

5

4000

3000

2000

1500

1000

700

500

400

300

200

150

100

2015

10

3040

5070

Zc, Vertical Distance FromFlare Tip To Flame Center, ft

CL

Uj

U∞

Mj

29.0

DP15EFA1a

dj

Uj

U∞( ) ( )T∞ Mj

Tj

0.5


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FIGURE A-1A (METRIC UNITS)FLAME CENTER FOR FLARES AND IGNITED VENTS Zc- VERTICAL DISTANCE, m

100

50 510

10.0

5.0

1.0

0.5

0.1

0.05

0.01

550100

1800.

010.

10.

051.

00.

5

Zc, Vertical Distance FromFlare Tip To Flame Center, m

CL

Uj

U∞

Mj

29.0

DP15EFA1b

10

180

105.

0

1200

1000

750

500

300

200

150

120

100

75

50

30

2015

107.

55.

03.

0

12

dj

Uj

U∞( ) ( )Ta Mj

Tj

0.5


XV-EPage


DateDecember, 1999


FIGURE A-2A (CUSTOMARY UNITS)FLAME CENTER FOR FLARES AND IGNITED VENTS Xc- HORIZONTAL DISTANCE, ft

200

100

50 20 1010

.05.

01.

00.

50.

10.

050.

01102050100

200

500

10000.

010.

10.

051.

00.

5

Xc, Horizontal Distance FromStack To Flame Center, ft

CL

Uj

U∞

Mj

29.0

DP15EFA2a

10.0

5.0

1000

500

4000

3000

2000

1500

1000

700

500400

300

200

150

100

70

50

4030

20

15

10

7

5

43

2

1.5

1

d j

U j

U ∞( ) ()T ∞

M j

T j

0.5


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FIGURE A-2B (METRIC UNITS)FLAME CENTER FOR FLARES AND IGNITED VENTS Xc- HORIZONTAL DISTANCE, m

100

50 510

10.0

5.0

1.0

0.5

0.1

0.05

0.01

550100

1800.

010.

10.

051.

00.

5

Xc, Horizontal Distance FromStack To Flame Center, m

CL

Uj

U∞

Mj

29.0

DP15EFA2b

10

180

105.

0

50

1200

7501000

500

300

200

150120

100

30

1015

1210

7.5

3

21.5

1.2

.751

0.5

0.3

75

5

d j

U j

U ∞( ) ()T ∞

M j

T j

0.5


XV-EPage


DateDecember, 1999


FIGURE A-3DIMENSIONLESS FLAME COORDINATES

3.02.52.01.51.00.50

SL

XL

0.001

0.002

0.004

0.006

0.01

0.008

0.02

0.04

0.06

0.08

0.1

0.2

0.4

0.6

1.0

0.8

SL XL XL= 1.04 2 + 2.05 0.28

DP15EFA3


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APPENDIX B -LIMITS OF FLAMMABILITY OF GAS MIXTURES

When small increments of a combustible gas are successively mixed with air, a concentration is finally attained in which a flamewill propagate if a source of ignition is present.This is referred to as the Lower Flammable Limit of the gas in air. As further increments of the gas are added, a higherconcentration of flammable gas in air will finally be attained in which a flame will fail to propagate. The concentration of gasand air just as this point is reached is referred to as the Upper Flammable Limit of the gas in air.Safety requires that only the most reliable experimentally determined flammable limit data be considered in purgingcalculations. This is included in Table B-1.

PRESSURE AND TEMPERATURE EFFECTS ON FLAMMABILITY LIMITS

Increased Pressure Will Widen Flammability Limits

Below atmospheric pressure there is no effect on the limits of flammability of natural gas-air mixtures and most other gas-airmixtures. Below about 1 in. (25 mm) absolute pressure, carbon monoxide-air mixtures are not flammable.

➧ From atmospheric pressure, up to 300 psig (2170 kPa), the lower limit of flammability is not affected, but the upper limit rises asthe pressure on the mixture is increased. This widens the limits of flammability as the pressure increases, as shown below.Above 300 psig (2170 kPa), the lower limit will be reduced, and the upper will approach an asymptotic limit. The multipliersgiven below cannot be prorated to higher pressures. Consult your Safety & Risk Regional contact for estimates at higherpressure.

EFFECT OF INCREASE IN PRESSURE ONRAISING THE UPPER FLAMMABILITY LIMIT (U.F.L.)

psig 0 100 200 300 350Methane and NaturalGas U.F.L.

Approx. 15 18 24 32 40

Coke Oven Gas U.F.L. Approx. 30.7 38.4 54 74.9 95.7

Multipliers* 1 1.3 1.9 2.7 3.5

Note:➧ * For estimating the upper flammability limit of other gases where the U.F.L. and L.F.L. are known at 0 psig, apply the multipliers

indicated in the table to the difference between the limits. Add this product to the lower limit to find the new upper limit.

Increased Temperature Will Also Widen the Flammability Limits

Raising the temperature also lowers the lower flammable limit, L.F.L. and raises the upper flammable limit, U.F.L.The following equations can be used to estimate the flammable limits at an elevated temperature, t°C:

);77t(000401.01F77at.L.F.L

Ftat.L.F.L −−=o

o

)25t(000721.01C25at.L.F.L

Ctat.L.F.L −−=o

o

);77t(000401.01F77at.L.F.U

Ftat.L.F.U −+=o

o

)25t(000721.01C25at.L.F.U

Ctat.L.F.U −+=o

o

CALCULATION OF FLAMMABILITY LIMITSThe calculation of the flammability limits of complex gas mixtures is carried out by the application of the mixture rule firstapplied in such estimations by Le Chatelier in 1981. Stated simply, the mixture rule is that if two limit mixtures of differentgases are added together, the resulting mixture also will be a limit mixture. The equation expressing this law is written asfollows:

∑

+++

=

Ln

n

3L

3

2L

2

1L

1L

CY

CY

CY

CY

1C


XV-EPage


DateDecember, 1999


APPENDIX B (Cont)Where Y1, Y2, Y3, etc. are the volume fractions of each combustible gas present in the original mixture, free from air and inertsso that Y1 + Y2 + Y3 + … + Yn = 1. CL1, CL2, CL3, etc. are the lower limits of flammability of the components in air expressed asvolume fractions. CL is the lower limit of flammability of the mixture. A similar procedure would be used for determining theupper limit of flammability.An example of the application of this law is indicated by a natural gas of the following composition.

GAS VOLUME FRACTIONVOLUME FRACTION GAS IN AIR

LOWER LIMITMethane 0.80 0.05Ethane 0.15 0.031Propane 0.04 0.021Butane 0.01 0.0186

Lower Limit =

0186.001.0

021.004.0

031.015.0

05.080.0

1

+++ = 0.043 Gas in Air

Any oxygen contained in a mixture may be considered as though it were a part of the air required for combustion, and theanalysis of the flammable mixture should be converted to an air-free basis before the flammable limits are calculated.When mixtures contain appreciable quantities of the inert substances, such as nitrogen and carbon dioxide, calculation of theflammability limits becomes somewhat more complicated and requires the use of an extension of the mixture rule. In thismodification, the inert gases are taken into consideration by assuming that the original mixture is composed of a number ofsubmixtures of inert gas-combustible gas, the flammability limits of which have been experimentally determined in a similarmanner as have the limits for the pure gases, as given in Table B-1. Figures B-1, B-2, and B-3 show the flammability limits ofmixtures of some of the more common gases present in fuels with CO2, N2, and H2O.As an example of the application of this extension of the mixture rule, take a producer gas of the composition shown in TableB-2. The CO2 and N2 may be apportioned with the different combustibles in any of several ways, two of which are representedby Calculations A and B in Table B-2.In these examples, the inerts CO2 and N2 are combined with the combustible H2 and CO, and the small amount of CH4 is takenalone. Next, the ratio of inert to combustible is obtained for each group as shown, and the flammable limits for each suchmixture are obtained from Figure B-1. The mixture rule formula is now applied, using the data as just obtained, and the limitsare calculated as shown in Table B-2.The summary at the bottom of Table B-2 indicates the relative agreement between the calculated data and that experimentallydetermined for this particular producer gas. It was suggested that the difference between calculated and determined data inthis case may have been due more to inaccuracies in the analysis of the producer gas (particularly for methane) than to thefault of the mixture rule formula. This points up the fact that reliable gas analyses are also a necessary part of the calculatedflammability limit data.


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TABLE B-1LIMITS OF FLAMMABILITY OF GASES AND VAPORS, PERCENT IN AIR

GAS OR VAPOR LOWER UPPERHydrogenCarbon monoxideAmmoniaHydrogen sulfideCarbon disulfide

4.0012.5015.004.001.25

75.0074.0028.0044.0050.00

MethaneEthanePropaneButaneIso-butanePentaneIso-pentaneHexaneHeptaneOctaneNonaneDecaneDodecaneTetradecane

5.003.002.101.901.801.501.401.101.051.000.830.800.600.50

15.0012.509.508.508.407.807.607.506.706.502.905.40

EthylenePropyleneButadieneButylene (1-Butene)Amylene (1-Pentene)

2.702.002.001.601.50

36.0011.1012.0010.008.70

AcetyleneAllylene (Propadiene)BenzeneTolueneStyreneo-XyleneNaphthaleneAnthracene

2.501.741.201.100.900.900.900.63

100.00

7.807.106.806.705.90

Cyclo-propaneCyclo-hexeneCyclo-hexaneMethyl cyclo-hexane

2.401.221.301.20

10.404.818.006.70

Gasoline-regularGasoline-73 octaneGasoline-92 octaneGasoline-100 octaneNaphtha

1.401.501.501.451.10

7.507.407.607.506.00

Jet fuel JP-4KeroseneGas oil

1.800.700.50

8.005.005.00

Note: Data taken from NFPA 325M, 1991 edition.


XV-EPage


DateDecember, 1999


TABLE B-2THE CALCULATION OF FLAMMABLE LIMITS

GAS ANALYSIS COMBINATIONS RATIO FLAMMABLE LIMITS (FIG. B-1)GAS COMPOSITION CHOSEN TOTAL INERT/COMBUSTIBLE LOWER UPPER

H2COCH4CO2O2N2

12.4%27.30.76.20.0

53.4

12.4 H2 + 6.2 CO227.3 CO + 53.4 N2

0.7 CH4

18.6%80.70.7

0.501.960.00

6.0%39.85.0

71.5%73.015.0

Lower Limit = %0.19

0.57.0

8.397.80

0.66.18

100 =++

CALCULATION A

Upper Limit = %8.70

0.157.0

0.737.80

5.716.18

100 =++

Gas Analysis 12.4 H2 + 53.4 N227.3 CO + 6.2 CO2

0.7 CH4

65.8%33.50.7

4.310.230.7

22.0%15.05.0

76.0%71.015.0

Lower Limit = %7.18

0.57.0

0.155.33

0.228.65

100 =++

➧ CALCULATION B

Upper Limit = %2.72

0.157.0

0.715.33

0.768.65

100 =++

Lower Limit Upper Limit

➧ Summary Determined 20.7 73.7Calculation A 19.0 Use 18.7% 70.8 Use 73.7%Calculation B 18.7 72.2


SectionXV-E

Page58 of 60

FLARES


EXXONENGINEERING


FIGURE B-1FLAMMABLE LIMITS FOR HYDROGEN, CARBON MONOXIDE, METHANE,

WITH NITROGEN, CARBON DIOXIDE AND WATER VAPOR

80 70 60 50 40 30 20 10 0

2

4

6

8

10

12

14

16

18

20

Volu

me

of In

ert (

N2,

CO

2 or

H2O

) Per

Vol

ume

of C

ombu

stib

leH2 + N

2

H2 + CO

2

H2 + H2O

CO + N2

CO + H2O

CO + CO2

CH4 + N

2

CH4 + H

2 O

CH4 + CO

2

Flammable Limit %[ (Volume of Gas + Volume Inert) Per (Volume of Gas + Volume Inert + Volume Air) ] x 100 DP15EFB1

Flammable Limits ForHydrogen

Carbon MonoxideMethane

With Nitrogen, Carbon Dioxideand Water Vapor


XV-EPage


DateDecember, 1999


➧ FIGURE B-2FLAMMABLE LIMITS FOR PARAFFIN HYDROCARBONS

WITH NITROGEN AND CARBON DIOXIDE

70 60 50 40 30 20 10 0

2

4

6

8

10

12

14

16

18

20

Volu

me

of In

ert (

N2,

or C

O2)

Per

Vol

ume

of C

ombu

stib

le

CH4 + N

2

C2 H

6 + CO2

CH4 + CO

2

DP15EFB2

Flammable Limits ForParaffin Hydrocarbons

With Nitrogen and Carbon Dioxide

C3 H

8 + CO2

C4 H

10 + CO2

C2 H

6 + N2

C3 H

8 + N2

C4 H

10 + N2

Flammable Limit %[ (Volume of Gas + Volume Inert) Per (Volume of Gas + Volume Inert + Volume Air) ] x 100


SectionXV-E

Page60 of 60

FLARES


EXXONENGINEERING


FIGURE B-3FLAMMABLE LIMITS FOR METHANE, ETHYLENE, BENZENEWITH CARBON DIOXIDE, NITROGEN AND WATER VAPOR

70 60 50 40 30 20 10 0

2

4

6

8

10

12

14

16

18

20

Volu

me

of In

ert (

N2,

CO

2 or

H2O

) Per

Vol

ume

of C

ombu

stib

le

CH4 + CO

2

C2 H

4 + CO2

CH4 + N

2

DP15EFB3

Flammable Limits ForMethaneEthyleneBenzene

With Carbon Dioxide,Nitrogen, and Water Vapor

C6 H

6 + CO2

C6 H

6 + H2 O

C6 H

8 + N2

C2 H

4 + N2

CH4 + H

2 O

Flammable Limit %[ (Volume of Gas + Volume Inert) Per (Volume of Gas + Volume Inert + Volume Air) ] x 100

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