SAFE FLARE SYSTEM DESIGN.doc

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SAFE FLARE SYSTEM DESIGN

INTRODUCTION

In the safe, satisfactory operation of a process plant, the flare system is thesingle most important element for operational or emergency relief of flammablesubstances in the liquid or gaseous phases.Most people see flares as simply a fire on top of a pipe that burns gases. To thepublic, flares often appear as a source of odor, smoke, noise, fallout and light.

This paper discusses flare-related factors including location, equipment involved,and design. It also explores the most common factors involved in flaringaccidents.

TYPICAL FLARE SYSTEM

The typical flaring system consists of equipment that safely combusts ventedhydrocarbons at a pressure drop which doesn't compromise plant relief systems.

The ideal operating condition would be to eliminate the need for flares, as theywaste hydrocarbons which could become products and thereby improve profits.However, facilities to recover large amounts of released hydrocarbons underemergency conditions are currently not economically justified. the equipment torecover daily hydrocarbon leakage rates is justifiable, and is in use in manyplants.

The typical flaring system handles the discharge of all reliefs inside a designatedunit or number of units. A flaring system generally consists of the following majorcomponents and subsystems:• Collection piping within a unit (including a mix of pressure reliefs and vents)• A flare line to the site• A knockout drum to recover liquid hydrocarbon from the gas stream

• A liquid seal to provide positive header pressure without surging and to protectagainst flashbacks(optional)• A flare stack with flare tip• An assist system to maintain smokeless burning (optional)• A fuel gas system for pilots together with ignitors• Controls and instrumentation

DESIGN GUIDES

Flare Location

Economics dictate the amount of ground space allocated to a flare stack. Notonly must the location comply with all governmental regulations affecting height,noise, smoke suppression, and allowable toxic concentrations, other importantfactors must also be considered. These include the relation of the flare to thefollowing:• areas in the plant where people work• outside residential areas• commercial areas• roads• other elevated structures• guy wire location with regard to possible interference Wind Effect Wind

direction and velocity affect the head radiation produced by the flare byinfluencing the length and angle of the flame. Normal design practices assume



the average wind speed for the area and take any possible wind direction intoaccount. Temperature Inversions

Temperature inversions and other meteorological conditions affect atmosphericstability and reduce the dispersion of odors, toxic concentrations, and smoke.

These conditions should be considered on the basis of the frequency of temperature inversion occurrences and the expected effect on people in thearea.

Type of Waste Gas to be Flared

The type of gas to be flared affects the design and size of flare tip used. It is nolonger acceptable to design on only a molecular weight. Some gases burnwithout support gas; others, such as low BTU content gases, must besupplemented. Flare manufacturers can help determine the combustion

characteristics of each gas stream.Some gases require assistance to burn smokelessly, and the assist methodselection is important to flare safety. Questions such as the following must beanswered: What is the reliability of the assist medium?Will it be available in the quantities needed during an emergency? Which assistmethod is most economical?

Typically, in reviewing gas stream composition, the primary concern is assuringdestruction efficiency. A good destruction efficiency is assured if stable flame canbe established. Accomplishing this requires different techniques for various gasproperties.Another safety concern is with inappropriate materials being routed to the flare.

The watery gas oil relieved at 500°F into 1,000 feet of cold relief header may be

a waxy solid at the other end of the line. The steam line attached to the relief linemay generate an ice block in winter weather.Relief valve location is another factor to consider for safe flare design. If the flareis blocked, relief valve protection may be lost. Flare blockage may occur becauseof the waxy gas-oil mentioned above or from a poorly sized system, which putsexcessive back-pressure on the relief valve. Layout and selection of relief valvesand the operating instructions for the relief system need always consider theseproblems for safe flare system design. Heat Radiation

Radiation from the flare flame generally determines flare stack height. Duringnormal operations, design intent is to limit heat intensity at grade to levels whichare low enough for both humans to safely withstand and to protect surroundingequipment from heat-related damage. Heat intensity is measured in terms of BTU/hr/sq. ft. The recommended design level for plant areas is 1,500 BTU/hr/sq.ft. This intensity level permits a properly-dressed person to perform normalfunctions in the area of an operating flare. Equipment can withstand up to 3,000BTU/hr/sq. ft. Often overlooked, above-grade maintenance work performed whilethe flare is in service must also be considered. Without special precautions,maintenance personnel can receive more heat than recommended if workingabove-grade.Usually the largest release of hydrocarbon (heat release) occurs during instances

of major upset, such as a power loss, loss of cooling water, or a fire in the unit.Any hydrocarbon discharge under these conditions is a true emergency situation.



A summary of the capacities of those relief valves likely to be involved in a worstcase emergency will dictate the design rate that flare must handle. Systemvolume size will usually dampen transient peaks. Note that relief valves onvessels may not all open simultaneously, or remain open.Multiple unit use of a single flare complicates the sizing due to the likelihood of multiple units relieving to the flare at the same time. If a common utilities failurecauses dumping at the same time to the same flare, the flare must be sizedaccordingly.Radiation from more than one flare in a localized area must also be considered.Not only should thecombined radiation be calculated, but maintenance work at above-gradeelevations on one flare while another nearby system is in service must beconsidered when flare spacing is set.Unit expansions normally add to the possible heat release from a flare. Whenconsidering any majoraddition, always check on the heat release and radiation effects which will resultwith such an expansion. A new flare tip and/or a higher flare stack may be

necessary to maintain safe conditions. In an expansion scenario, header pressuredrop is also an especially critical function.Ground Level Concentration Some waste gases are lethal in high concentration. If toxic gases such as hydrogen sulfide or hydrogen cyanide are in the wastestream, flare height must be calculated to assure that unburned toxics do notexceed safe dispersion/ground-level concentrations. Thermal rise and exitvelocity effects dilute stack gases. As the gases reach grade downwind of thestack, toxic materials concentrations must be reduced to tolerable levels.Although standard calculations exist to determine proper stack height to assuresafe toxics levels at grade, these methods are theoretical and each case shouldbe investigated individually.A typical grade concentration design target is 0.10 ppm or less, downwind of the

stack. This is fine formost toxic or noxious substances; however, a few substances exist for which thissmall concentration is still far too great, and a sever odor or health nuisance maybe created.According to Volume 2 of the Manual of Disposal of Refinery Wastes, WasteGases and Particulate Matter, published by the American Petroleum Institute, thefollowing substances cause odor nuisance in the concentrations shown: .

Compound ppm

Isobutyl Alcohol 0.0003

Isoamyl Alcohol 0.0026Methyl Mercaptan 0.041Ethyl Mercaptan 0.0028n-Propyl Mercaptan 0.0016n-Butyl Mercaptan 0.001Isoamyl Mercaptan 0.00043p-Thiocresol 0.0027Methyl Sulfide 0.0037Ethyl Sulfide 0.000056Propyl Sulfide 0.011n-Butyl Sulfide 0.015

Maintenance



Because maintenance questions can impact flare system design, issues such asthe following must beconsidered:

• Can the flare system be removed from service when the unit is shut down?• How many days will the flare system be permitted out of service for repairs?Steamout and unitpurging reduces time available to work on a flare.• Are there other stacks discharging in the area which would make work at highelevationsdangerous due to pollutant concentration or heat radiation? Elevated structuresare often overlooked in design evaluations.

Flashback Protection

A sure method of preventing the backflow of air into the flare stack must beprovided to protect against explosions. Flame arresters are satisfactory in certain

situations; however, because of plugging, maintenance, and expense they mustbe carefully considered. Systems with large turndown, hydrogen, or other highflame-speed components, dirty gases or large capacity may warrant alternativemeans of flashback protection.In small systems such as terminal flares, flame arresters are valuable as aendpoint during flashback. Flame tends to "live" on the flame arrester element.When using a flame arrester, a thermocouple monitor of the flame bank with hightemperature alarm is a good safety feature. This setup will immediately alert theoperator to the problem.When a need exists to stop a flame front that might develop in mixtures of airand flammable gases, a suitably designed liquid seal is the best checking device.However, a typical dipleg liquid seal is by no means satisfactory for this service

because, as the flow quantity through the seal increases, there is uninterruptedgas flow through the water. Experience with hydrogen/air mixtures clearly showsthat a flame front will move countercurrent to velocities well in excess of 200ft./sec.A liquid seal protects equipment from explosions resulting from the ignition of ahydrocarbon/air mixture which backflows into the flare stack. The sealaccomplishes this protection with two functions. First, it provides a fail-safebackpressure control which maintains a positive pressure on the flare header,preventing air ingress. Second, it acts as a positive separation between the flareand process units. Normally located in the base of the flare stack, the gas streambubbles through the liquid seal or displaces the water to release the gas. A small

stream of water is added to the seal, and the liquid level is maintained with acontinuous overflow or level switch.An undesirable operating characteristic of poorly-designed liquid seals is theirtendency to surge. This occurs when relatively violent interruptions in gas streamflow create rhythmic pressure surges both upstream and downstream of thewater seal. This characteristic is evident in flare system operation in whichviolent burning is followed by complete flame extinction at the flare tip, then thegas is reignited and the burning/extinction cycle occurs again. Each time burningbegins again upon re-ignition, an annoying booming sound can be heard. Whilenot necessarily unsafe, this is an environmental concern. Good liquid seal internaldesign should prevent this surging problem.

Air Ingress Prevention



Gas purging issued to protect flare systems from explosions which would resultfrom ignition of a hydrocarbon mixture with air which backflows into the stack.Most hydrocarbons are considered safe and nonflammable with 6% or lessoxygen in the mixture. However, when large amounts of hydrogen are present, alower oxygen level is required.Any gas or mixture of gases which does not contain oxygen and cannot reachdew point at any condition of ambient temperature normal to the job site can beused as a purge gas for flare systems. This gas may also be referred to as"sweep" gas. Steam as a purge gas is not recommended for two reasons. First,steam is at an elevated temperature, and the steam content of the flare shrinksat the steam cools and condenses. Second, as the steam condenses, water is leftin the flare system. This presents a freezing hazard and encourages acceleratedcorrosion.For a corrosive waste gas, the purge gas should enter the flare systemimmediately downstream of the relief valve so that the entire system will besweeped. Most systems introduce the purge in a location that providesuninterrupted flow at design volumes. For a system with a liquid seal, it should be

downstream of the seal or designed to continually flow through the seal at a lowpressure.It is recommended that a pressure switch be installed immediately upstream of the orifice which regulates purge volume so that an alarm sounds if the purgegas pressure upstream of the limiting orifice falls below a set point. It is alsorecommended that the purge gas pass through a strainer in which the meshopenings are not more than one-quarter the diameter of the limiting orifice forpurge gas regulation.A purge gas volume which creates an upward velocity in the flare riser at .O1ft/sec. is normally recommended when the John Zink Molecular Seal is used onthe flare. If a John Zink Airrestor is used, the purge gas velocity would be 0.04ft/sec. If a Molecular Seal or Airrestor is not used, the purge gas volume must

provide an upward velocity at .25 to 50 ft/sec. to be considered adequate forsafety, depending on gas gravity. These velocities are based on full-scale testdata (30 in. outside stack diameter) and may be conservative for smaller-sizedstacks.Velocity-type purge conservation devices such as the John Zink Airrestor consistof a fixed baffle mounted in a flare tip. This arrangement presents little pressuredrop to the out flowing gas. An Airrestor helps reduce purge gas requirementsbut does not eliminate their need. For example, in a 20 in. flare stack, a purgerate of 2,500 scfh is reduced to 300 scfh with an Airrestor (oxygen level 20 ft.below tip measured at 6%). All velocity devices are rated at six to eight molepercent oxygen levels.

Molecular or diffusion seals are used to further reduce the amount of purge gasrequired. The seal is installed below the flare tip assembly and operates on theprinciple of gas diffusion. Molecular seals are large because of the complexity of their internal passages.

The Molecular Seal has been shown to be the most effective purge gas reductiondevice available. For example, in the 20 in. diameter flare stack, a purge rate of 2,500 scfh is reduced to 75 scfh with a Molecular seal (oxygen level 20 ft. belowtip measured at 0%). The diffusion seal is normally rated at less than 0.1 %oxygen. The difference in the design bases between the two types of purgereduction devices can be very important for some flare gas compositions.

There is a danger of explosion in the flare system if the flare pilots are ignitedbefore the total flare system has been purged. To assure low or zero oxygen

levels, a volume of non-condensable gas equal to ten or more times the volumeof the flare system is used. The flare system includes all piping from the relief



valves to the stack and rising to the elevation of the flare at the burning point. The pilots should be ignited only after the system has been purged andpreferably while purge gas is still being admitted.

Liquid Handling

"Flaming rain" is a very real hazard for flares with liquid relief potential. Knockoutdrums are used to prevent hazards associated with flaring gas that containsliquid droplets. The drum just be large enough to effect the desired liquid/vaporseparation and have the holding capacity to contain any anticipated slug of liquid.Most flares can handle a liquid mist up to the point where the oil droplet settlesto grade faster than it is consumed by the fire surrounding it. Generally, this isconsidered to represent 600 micron particles.However, different flare tips can handle different liquid rates. The kinetic energyflare tips can handle higher liquid loads than open pipe flares because they takea significant pressure drop at the tip.

Ignition Systems Ignition of the pilots is accomplished with a flame frontgenerator. This device mixes air and fuel and ignites it. The major safety problemassociated with ignition systems is the loss of ignition through improper control of pilot fuel. The pilot is the major safety control for the flare. Any reduction in thepilot's effectiveness is a significant safety hazard. The trend to pilot monitoringand automatic relight of pilots is positive because it emphasizes pilotmaintenance.

SUMMARY

The five most frequent causes of flare accidents are:• Internal explosion

• Liquid carryover• System obstructions• Faulty maintenance procedures• Ignition lossA safe, functional flare system takes all of the design factors into consideration.Of the three primary ingredients of a disaster, two are always present in a flaresystem: fuel and fire. If proper control is maintained, oxygen will not be present.

BIBLIOGRAPHY

1. APR RP 521. "Guide for Pressure Relief and Depressuring Systems," First

Edition, (New York:American Petroleum Institute, September 1969).2. APR RP 520. "Recommended Practice for the Design and Installation of Pressure Relieving Systems in Refineries," Fourth Edition, (New York: AmericanPetroleum Institute, 1976).3. John Zink Company. "Advances in Offshore Flaring Technology," Presented at aseminar on FlareSystems arranged by The Norwegian Society of Chartered Engineers, (November1980).4. John Zink Company. "RACT for VOC - A Burning Issue," Pollution EngineeringMagazine, (July1983).



5. John Zink Company. "Flaring in Hostile Environments," Presented at a seminaron Flare Systems arranged by The Norwegian Society of Chartered Engineers,Geillo, Norway, (September 1982).6. Reed, R. D. "Design and Operation of Flare Systems," Chemical EngineeringProgress, vol. 64, no. 6, pg. 53-57.7. Bluhm, W. C. "Safe Operation of Refinery Flare Systems," presented at aseminar on OperatingPractices, 26th Midyear Meeting: Petroleum Institute's Division of Refining,Houston, Texas, (May 1961).8. Powell, D. P and Schwartz, Robert E. "Operating Experience with a Low Level-

Type Flare," APIReprint no. 59-72, 37th Midyear Meeting: Petroleum Institute's Division of Refining (1973).9. Schmidt, T. R. "A Smokeless Flare System," Ecolibrium, Shell Oil Company, vol.C, no. 1 (1977).10. John Zink Company. "Designing Flares for Enhanced Service Life," Presentedat a seminar arranged by the Canadian Gas Processors Association (June 1983).

11. John Zink Company. "Total System Approach to Flare System Design,"Presented at APISeminar: Fire Protection and Safety (October 1983).12. John Zink Company. "Flare System Design," Presented at API seminar:Fundamental Applicationsof Loss Prevention, Houston, Texas (October 1986). International HeadquartersP.O. Box 21220 • Tulsa, Oklahoma 74121-1220 U.S.A. 11920 East Apache •

Tulsa, Oklahoma 74116-1300 U.S.A. Telephone (918) 234-1800 Fax (918) 234-2700Other offices are located in major cities around the world.© Copyrighted Company 1993

Technical Paper 54908



Flare Stack Support Structure

•

Self-Supported Stack • Derrick Stack

• Guyed Wire Stack

Flare Tips

• MRW Air-Assist Flare Tips

• MRW Steam-Assist Flare Tips

• MRW Gas-Assist Flare Tips

Pilot and Ignition System

MRW pilots and ignition systems are proven to be reliable, fast, and easy to operate and maintain.

Our pilots are designed for continuous and intermittent use in adverse weather conditions and

extreme wind speeds. MRW pilots increase the reliability and performance of the flare system by

providing flame stability in any operating conditions. We are confident that our pilots are the

absolute best in the world!

MRW designs extremely energy efficient pilots using as little as 50SCFH of fuel gas. The pilots

are generally fabricated out of 309/310 stainless steel to extend the life of the pilot burner

assembly. An air/fuel gas mixer attached to pilot burner assembly allows for a combustible mixture

of pilot fuel gas at the pilot burner tip. Our local control panel powers the ignition transformers and

automatically ignites/re-ignites the pilots.

Ignition of the pilot can be done via:

1. Manual Ignition - the pilot is manually ignited via a push-button activation.

2. Auto Ignition - a spark igniting device and ignition transformer ignites the flame front.3. Auto Re-Ignition - the flare tip pilot temperature switches automatically trigger the ignition

transformers to re-ignite the flare tip pilots upon loss of flame.

4. Remote Ignition - the flare system to be capable of automatic startup and shutdown based on

either local or remote initiation.

Ignition of the pilots is accomplished using one of three methods:

1. Flame Front Generator (FFG)

2. Self-Inspirated Flame Front Generator (SI-FFG)

3. Electric Spark Ignition (EFG)

Pilot Monitoring Device



• Thermocouple - MRW uses a proprietary thermocouple design and location to safely and

accurately monitor the pilot status while also extending the service life of the thermocouples

• Flame Scanner

• Flame Rod

• Video Camera

Control and Ignition

System

MRW can design our control

systems to meet any electrical

classification. Our control

systems utilize state of the art

design techniques and

technology to obtain optimum

service potential. If you have a preference as to what

type/brand of control system,

we are happy to accommodate

your needs.

Purge Reduction Seal (Optional)

• Molecular Seal

• Liquid Seal

•Velocity Seal

Flame or Detonation Arrestor

(Optional)

In some situations, a flame or

detonation arrestor is preferred or

necessary. If a liquid seal cannot be

used, gases are in the explosive range,

and/or there are extended pipe runs or

bends in pipe, a detonation arrestor

may be used. MRW uses detonationarrestors are approved by the US Coast

Guard.



Knockout Drum (Optional)

Knockout drums are provided in situations where liquid

separation is likely in the waste stream. The knockout drum

will collect any liquids that are present in the waste stream

prior to entering the flare system. A knockout drum is

especially important if substantial cooling of heavy liquids

is expected. If the liquid is corrosive, MRW uses non-

corrosive materials of construction. A level gauge and

drain connections are built into the knockout drum.

Ladders and Platforms (Optional)

Access to the flare tip may be provided upon request from grade with OSHA approved ladders and

rest platform(s) and one 360° access platform located just below the flare tip for easy of access and

maintenance the flare tip.

Air Blower (Optional)

If assist air is required, an optional air blower will be included. The combustion airflow shall be

automatically controlled based on waste gas flow rate to the flare. A flanged inlet and outlet as well

as access door and housing drain is standard. Air duct is usually supplied from the fan outlet to the

inlet of the flare stack.

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SAFE FLARE SYSTEM DESIGN.doc