Flare System: Types, Segregation, Tips, Purge System and More

Table of contents: 1. Types of Flare 2. Segregation of Flares 3. Flare Knock-Out Drum 4. Flare KOD Liquid Removal 5. Flare KOD sizing depends on two aspects 6. Liquid Seal Drum 7. Purge reduction seals 8. Flare Purge system 9. Flare stack 10. Flare Structure 11. Flare Tip 12. Ringlemann Chart 13. Pilot burner 14. Pilot Ignition 15. Other AccessoriesFlaringis defined as a process of controlled burning of exhaust gases which generates heat and noise. Flaring is a common practice in oil/gas exploration, production and processing operations. A flare system consists of a flare stack and pipes that feed gas to the stack. The type and amount of gas or liquids in the flare stack governs the sizing & brightness of the flare.There are many function & reason for flaring,few reasons for flaring are:1. During well production testing after drilling is completed2. For safety and during emergencies and maintenance3. For managing gas during compression and processing4. Flaring at well sites to recover oil1. Types of Flare1. Elevated Flare2. Ground Flare2. Enclosed Flare2. Open Flare

Elevated Flare

Enclosed Flare

Open Flare

Click to enlarge figureTypical Flare System with Elevated Flare

2. Segregation of Flares1. Service1. Acid gas flare1. Cold dry flare1. Warm wet flare0. Pressure0. Atmospheric1. Low pressure2. High pressure3. Flare Knock-Out Drum1. Objective1. Separate bulk liquid from gas1. Limit liquid droplet size entrained with gas to the flare1. Provide adequate residence time for liquid1. Sizing basis2. Based on API 5212. Separation of liquid droplet size of 300-600 microns considering the design case for the flare2. 20-30 minutes of liquid hold-up time based on a relief case that results in maximum liquid2. No internals to facilitate separation2. Many orientations / options possible, horizontal KODs most preferred

Flare KO Drum elevation1. Flare Knock-Out Drum Elevation3. KO drum elevation decidespipe rackelevation based on 1:500 slope of main flare header

Flare KO Drum elevation3. KO drum elevation determined by pump NPSH requirement3. To reduce pipe rack elevation options are3. Reduce KOD elevation (option 1)4. Use vertical can pump4. Locate pump within pit4. Locate KO drum within pit1. Use intermediate KO drums (option 2)

>Flare KO Drum elevation arrangement (Option 1)

Flare KO Drum elevation arrangement (Option 2)4. Flare KOD Liquid RemovalRemove liquid from flare KOD after relief to avoid overfill during future relief event1. Options1. Draining to evaporation pond or closed drain drums1. Liquid removal by flare KOD pumps1. Heater to be installed in KOD where freezing, pour point issues exist1. Rate of liquid removal to consider frequency and amount of liquid release1. High level in flare KOD to be considered for plant shutdown1. Sizing of Flare KOD

Flare KO Drum2. LLLL shall be sufficiently high to avoid any sludge deposition impacting LT nozzle (150 mm in above figure not correct, consider 300 mm minimum for services which are not clean).2. LLLL shall be minimum 700-300 in case flare drum electrical heaters need to be installed.2. LLLL Level at which pump trips.2. LLL Level at which both pumps stops2. HLL Level at which first pump starts2. HHLL Level at which second pump starts2. HHHLL Level at which entire plant goes into pressurized trip.5. Flare KOD sizing depends on two aspects Liquid Hold up requirement during a major liquid or two phase release. Sufficient distance shall be available between inlet device bottom and HHHLL. It is possible to have manually initiated depressurization even after HHHLLL. Any possible liquid shall be accommodated above HHHLL. Distance between HLL and HHHLL shall be designed to accommodate maximum liquid release scenario(?). Some standards this distance is between HHLL and HHHLL. Residence time required for drop of liquid particles of 300-600 micron size. Liquid particles separate When the residence time of the vapor or gas is equal to or greater then the time required to travel the available vertical height at the dropout velocity of the liquid particles and When the gas velocity is sufficiently low to permit the liquid dropout to fall. This vertical height is usually taken as the distance from the maximum liquid level.

6. Liquid Seal Drum1. Objective1. Prevent flashback from flare tip back to flare headers1. Avoid air ingress into flare system during sudden temperature changes leading to condensation and maintain positive system pressure1. Use2. Used in flare gas recovery systems2. Staged flaring between enclosed flare and full size emergency flare1. Design specifications3. Water as liquid sealing fluid not recommended for extremely cold releases; water-glycol mixtures of sufficient concentration used instead

Liquid Seal Drum7. Purge reduction seals1. Objective1. Prevent air infiltration into flare system at low flow rates1. Reduce amount of continuous purge gas injection into flare stack1. Design options2. Buoyancy seal (molecular / density seal)2. Velocity seal (fluidic seal)

Purge Reduction Seal8. Flare Purge system1. Objective1. Prevent air infiltration into flare system at low flow rates1. Prevent vacuum formation in flare headers and system following steaming or large relief event1. Design specification2. Continuous purge rate with velocity in stack1. 1-5 fps : without molecular seal1. 01 : with molecular seal1. 02-0.04: with velocity seal1. Approximate purge flow rate can be calculated using section 7.3.3.3 of API 521.

Purge Reduction Seal

Flare P&ID9. Flare stack1. Objective1. Combustion of relief gases at elevation to minimize radiation exposure to personnel/ equipment/ structure1. Ensure adequate dispersion of un-burnt hydrocarbons and toxic components

Flare protection1. Design considerations2. Radiation: Limit radiation, either continuous and peak, on off-site properties and persons, equipment, buildings and personnel on the installation. Applicable to impacted area, restricted area and equipment lay-out.2. Flammable gas: Avoid ignition of a flammable gas cloud released from a cold vent or in case of flare flame out.2. Toxic hazards: (Mainly for H2S and SO2, but not limited to) limit the risk of a toxic gas cloud to reach off-site population, provide means of alarm and adequate protection to personnel present in the restricted area.2. Noise: Limit both continuous and peak noise2. Stack height is determined by HSE group based on permissible radiation level as per project philosophy or API 521.2. Taller stack will result in smaller sterile zone.2. Locate process plant upwind of flare.10. Flare Structure Self supported flare stack Guy wired supported flare stack Derrick supported flare stack More than one flare may be supported on the same structure

Flare Stack Support11. Flare Tip Produce desired destruction/combustion efficiency of maximum specified relief gas Establish and maintain proper ignition Pilot gas /Pilot burners/ Ignition system Ensure stable combustion Windshield Retention rings Result in smokeless operation at normal continuous flows or at100% flows Steam Air (high pressure or low pressure) High pressure water No external medium, maintain high pressure at tip by staging

Flare Tip

Flare Tip

Flare Tip: Velocity Seal (top view) Based on velocity of gas exit from tip, flare tips are considered as sonic and subsonic (pipe flare) type. This is the term used by process designer for high pressure flares and low pressure flares. General stack pressure drops are as given below. Sonic flare 2 to 4 bar Subsonic flare 0.2-0.5 bar Open Pipe flare tips: These are used for combustion of gases that do not produce smoke, gases with a low heating value, or for installations where smokeless combustion of heavy hydrocarbons is not required. These flare tips are one of the lower capital cost options for safe disposal of waste gases. In general these kind of flares have tips with very low pressure drop. Open pipe flare tips with steam injection: Steam injection is provided reduce the smoke formation. Open pipe flare tip with high pressure gas injection: This will increase the turbulence at flare tip and reduce the smoke formation. Fuel gas can be generally used as assist gas. Fuel gas injection can be either continuous or initiated manually based on monitoring of flare tip. Air assisted flare tip: When smokeless flaring is desired and neither steam nor assist gas is available, blowers can be used to inject combustion air directly into the waste gas stream as it exits the flare tip. Combustion efficiency of flared gas is increased by installing air blower which will reduce smoke formation. Multiple nozzle type flares: They are used where high flare gas pressures are available (1 barg and up) and where it is preferred to have some smokeless burning capability and also lower radiation levels. These kinds of flares are used for HP flare application. They have good combustion efficiency and less chances of smoke formation. Coanda flare tip: The Coanda effect is a gas-adhesion principle that dramatically enhances the combustion process, resulting in maximum destruction of waste gases. Coanda Effect occurs when gas is passed over and adheres to a carefully profiled, curved surface, creating a near vacuum that pulls in substantial amounts of air. The air turbulently mixes with the gas flow, resulting in high-efficiency combustion.

Open Pipe Flare

Multi Nozzle Flare

Coanda Flare12. Ringlemann Chart A series of charts, numbered 0 to 5, that simulate various smoke densities by presenting different percentages of black. Ringelmann No. 0 is clear smoke Ringelmann No. 5 is 100 percent black. Ringelmann No. 1 is equivalent to 20 percent black

Ringleman Chart13. Pilot Burner1. Objective1. Provide flame for reliable ignition of main flare gas at all times1. Design specifications2. Pilot system to comply with API 537Minimum number of pilotsFlare burner outer diameter, DN

1 (2 for toxic gas)Up to 200

2>200 to 600

3>600 to 1050

4>1050 to 1500

To be agreed with purchaser>1500

1. 3. Pilots designed to remain lit and capable of being relit at wind speeds up to 160 km/h under dry conditions

Pilot gas line14. Pilot Ignition1. High Energy Ignition (HEI)1. Electrode capable of high energy or high voltage discharge near pilot tip1. Does not require propagation of a flame front as in FFG system1. Does not require compressed air, self aspirating pilots1. Simple and easy to use and automate, require little training or maintenance. Re-ignition takes few seconds1. Shutdown of flare system required for maintenance1. Back up FFG ignition (when using HEI) may be considered for6. Very tall flares that are difficult to access6. Flare systems that can be off line only once in more than 3-5 years6. Offshore platformsin corrosive and salt environments

Electrical Ignition Panel1. Flame Front Generator (FFG)1. Ignition line from panel to flare pilot filled with flammable fuel gas- air mixture and spark introduced. Mixtures ignited and flame front travels through piping to ignite pilot at flare tip1. FFG panel located at grade1. Panel operated manually or automated to reignite of pilot flame out detection. Re-ignition can take several minutes1. Moisture accumulation can lead to corrosion, flame extinguishment Ignition lines to be heat traced

FFG Panel

FFG System15. Other Accessories Flow measurement Monitoring relief devices leaks during normal operation Assess flaring of gases due to pressure control operations Note relief flows for assessing flare system adequacy checks and potential for flare gas recovery Non-intrusive ultrasonic flow meters with wide range and no pressure drop is preferred Smokeless flaring : medium control Proper steam or air control is required By measuring gas being flared and adjusting steam rate / blower capacity Detection smoke using infra-red analyzers Aircraft Warning Lights Required when flare heights exceed 61m or when site is close to airport Type and number based on regulations