Flare Systems: Types, Design & API 521/537 Selection Guide

Quick Answer

A flare system is the safe disposal route for gases released by pressure relief, depressuring, and process upsets — burning them to convert hydrocarbons into carbon dioxide and water rather than releasing them to atmosphere. The system is a chain, and every element matters: the flare header collecting relief streams, a knockout drum removing liquid (burning liquid droplets produces flaming rain, so this is a safety-critical item), a liquid seal drum preventing flashback and stabilising header pressure, a molecular or velocity seal in the stack preventing air ingress, and the flare tip with its pilots and ignition system. Two main configurations exist: elevated flares, where the tip is raised on a stack, derrick, or guyed tower so that thermal radiation at grade stays within acceptable limits (commonly designed to around 4.73 kW/m² for areas where personnel may be present for limited periods), and ground flares, where burners are enclosed within a refractory-lined windshield, hiding the flame and greatly reducing radiation and noise — favoured near populated areas at the cost of a much larger footprint. Design is governed by API 521 (relief loads and disposal systems) and API 537 (flare details and performance). Smokeless operation is achieved by adding steam, air, or high-pressure gas assist to improve mixing, and every flare requires continuous purge gas to keep the system positive and prevent the air ingress that causes internal detonation.

Every pressurised process facility needs somewhere for gas to go when something goes wrong. Relief valves lift, compressors trip and depressure, vessels are blown down, a power failure forces an emergency shutdown — and in each case a large volume of hydrocarbon has to leave the process in seconds. The flare is where it goes, and it has to work every time, without warning, having sat idle for months.

That combination — critical duty, rare operation, no opportunity to test at full load — makes flare systems unusual to design and easy to get wrong. And the failure modes are severe. Liquid carried to the tip produces burning droplets falling to grade. Air drawn back into the header can detonate inside the system. Inadequate radiation calculations expose personnel and equipment. An unlit flare vents raw hydrocarbon to atmosphere at height.

The other reality shaping flare design today is environmental. Routine flaring is under sustained regulatory and commercial pressure, with gas recovery systems increasingly installed to capture continuous streams and leave the flare for genuine relief events only. Several of Kasko's markets — Algeria's flare-recovery programme and Kazakhstan's gas capture commitments among them — are actively investing in exactly this.

For process safety engineers, facilities engineers, and procurement teams — this guide covers flare systems: configurations, the components in the chain, design principles under API 521 and 537, smokeless operation, and specification issues.

For the devices that feed the flare, see Pressure Relief & Safety Valves.

What the System Has to Do

1. Collect relief and blowdown streams from across the facility
2. Remove liquids before they reach the burner
3. Prevent flashback and air ingress into the header
4. Burn the gas completely, with acceptable smoke and combustion efficiency
5. Limit thermal radiation at grade and on adjacent equipment
6. Disperse combustion products safely
7. Ignite reliably, every time, in any weather

Flare Configurations

Elevated Flares

The flare tip is raised so distance attenuates radiation at grade. The dominant configuration in refineries, gas plants, and petrochemical facilities.

Support types:

• Self-supporting stack — economical for lower heights
• Guyed stack — taller heights, smaller foundation, but a large land area for the guy wires
• Derrick-supported — the tallest and heaviest duty, allowing multiple risers and tip replacement without full shutdown

Advantages:

• Effective radiation control through height
• Large capacity
• Well-established, straightforward maintenance access on derrick types

Disadvantages:

• Visible flame and light pollution — a genuine community issue near populated areas
• Noise
• Higher radiation footprint at grade than enclosed designs
• Tall structures require significant foundations and wind/seismic design

Ground Flares (Enclosed and Multi-Point)

Enclosed ground flare — multiple burners inside a refractory-lined windshield, hiding the flame entirely.

Multi-point (staged) ground flare — arrays of burners in a fenced area, brought into service in stages as flow increases, giving excellent combustion efficiency across a wide turndown.

Advantages:

• Flame is hidden — no visible flame, dramatically lower light pollution
• Much lower radiation and noise at the boundary
• High combustion efficiency, particularly staged designs
• Better community acceptance

Disadvantages:

• Large plot area
• Higher capital cost
• Limited capacity per unit compared with a large elevated flare
• Refractory maintenance

Use for: facilities near populated areas, environmentally sensitive locations, and continuous low-rate streams. Frequently combined with an elevated flare handling emergency loads while a ground flare handles routine flow.

Offshore Flares

Boom-mounted or tower flares on platforms, where radiation limits and available space are far more constrained, and where the flare boom's orientation relative to prevailing wind and the helideck is a critical layout issue.

System Components

Flare Header

Collects relief and blowdown streams. Sized for the governing simultaneous relief case with back-pressure kept within the limits of the relief devices feeding it — a point directly connecting flare design to relief valve selection, since excessive header back-pressure changes what type of relief valve can be used.

Headers must be sloped and free-draining toward the knockout drum, with no pockets where liquid can accumulate.

Knockout Drum — Safety Critical

Removes entrained liquid before the gas reaches the tip.

Why it matters: liquid droplets carried to the flare tip do not burn completely and fall as burning liquid — flaming rain — a serious hazard to personnel, equipment, and surroundings.

• Usually horizontal, sized to remove droplets above a target size (commonly 300–600 µm)
• Requires level instrumentation, alarms, and automatic pump-out
• Requires heat tracing and insulation in cold climates to prevent hydrate or wax blockage
• Some systems include a downstream liquid seal and, for heavier streams, a separate condensate collection system

Liquid Seal Drum

A water seal in the line to the flare stack.

Functions:

• Prevents flashback from the flare tip into the header
• Maintains a positive header pressure, preventing air ingress
• Provides a degree of flow stabilisation

The seal depth sets the pressure it holds. Freeze protection is essential in cold climates, and seal liquid level and quality require monitoring.

Molecular Seal / Velocity Seal

Installed in the stack below the tip.

Function: prevents air from entering the stack during low or zero flow. Air inside a hydrocarbon-filled stack creates a flammable mixture and the potential for internal detonation, which can destroy the tip and stack.

• Molecular seal — a labyrinth using the density difference between purge gas and air
• Velocity seal — an internal cone directing flow to prevent back-mixing, with lower purge gas consumption

Purge Gas

Continuous purge — typically fuel gas or nitrogen — maintains positive flow up the stack at all times, ensuring air can never enter. Purge rate is set by the seal type; velocity seals substantially reduce purge consumption and therefore operating cost.

This is not optional. Loss of purge is one of the recognised precursors to flare stack detonation incidents.

Flare Tip

Where combustion happens. Selection determines smoke performance, capacity, and tip life.

• Pipe flare — simplest; smoky at high rates
• Steam-assisted — steam injected to improve air entrainment and mixing, the standard smokeless method in refineries
• Air-assisted — a blower supplies combustion air; used where steam is unavailable
• High-pressure / sonic tips — use the gas's own pressure energy for mixing; very effective and no utility required, but need adequate supply pressure
• Materials are typically high-nickel alloys and heat-resistant stainless at the burning zone, since tip temperatures and thermal cycling are severe

Tips are consumable items — a service life of several years is typical, and derrick-supported flares are often specified specifically so the tip can be changed without a total plant shutdown.

Pilots and Ignition

• Continuous pilots — usually three or more for redundancy, with wind shields
• Flame detection — thermocouples, optical, or ionisation, alarmed to the control room
• Ignition systems — flame front generator (a propagating flame down a pipe) or high-energy electronic ignition
• Reliability here is critical: an unlit flare vents raw hydrocarbon at elevation

Flame Arrestors

Fitted where flashback risk requires it, particularly on low-pressure vent and tank systems. They must be selected for the specific gas group and maintained — a fouled arrestor is a flow restriction that can over-pressure the system it protects.

Design Principles

Relief Load Determination (API 521)

The flare must handle the governing simultaneous relief scenario. API 521 defines the scenarios: fire case, blocked outlet, power failure, cooling water failure, gas blowby, and others. The design load is not the sum of every relief valve — it is the largest credible combination occurring at once.

Getting this wrong in either direction is costly: undersized and the system cannot handle the event; grossly oversized and the stack, headers, and knockout drum are needlessly large and expensive.

Radiation

Radiation intensity at grade and on nearby equipment governs stack height and exclusion zones. A commonly applied design basis is around 4.73 kW/m² (1,500 Btu/hr·ft²) for areas where personnel may be present for limited periods without shielding, with lower values for continuously occupied areas and higher permitted at the flare base within a fenced exclusion zone.

Calculation accounts for flame length and tilt, wind, emissivity, and distance. Solar radiation is added to the flare contribution.

Dispersion and Smokeless Capacity

• Unburned gas dispersion must be checked for the case where the flare fails to ignite
• Smokeless capacity is typically specified as a percentage of maximum flow — the flare need not be smokeless at full emergency rate, but should be for routine and moderate releases
• Toxic components such as H₂S require dispersion analysis of SO₂ combustion products

Back-Pressure

Header and stack back-pressure must remain within the tolerance of the relief devices discharging into it — conventional relief valves tolerate only about 10% back-pressure, bellows types more, and pilot-operated most. Flare system hydraulics and relief valve selection are therefore a single coupled problem. See Pressure Relief & Safety Valves.

Governing Standards

• API 521 — Pressure-relieving and Depressuring Systems: relief scenarios, loads, and disposal system design
• API 537 — Flare Details for General Refinery and Petrochemical Service: tip performance, testing, and mechanical requirements
• API 520 — relief device sizing
• Local environmental regulation on flaring, smoke, and emissions

Common Specification Mistakes

After 15+ years supplying process equipment and pressure vessels:

Mistake 1: Undersized or Poorly Designed Knockout Drum

Drum sized without proper droplet removal criteria, or with no automatic pump-out. Liquid reaches the tip and falls as burning droplets.

Prevention: Size for the target droplet removal, provide level control with automatic pump-out and alarms, and heat trace where freezing or waxing is credible.

Mistake 2: Ignoring Back-Pressure Effects on Relief Valves

Flare header sized for cost, producing back-pressure that exceeds what the installed conventional relief valves tolerate. Set pressures shift and capacities collapse.

Prevention: Treat header hydraulics and relief valve selection as one problem. Above 10% back-pressure, specify balanced bellows or pilot-operated valves.

Mistake 3: Inadequate Purge or Seal Provision

Purge rate reduced to save fuel gas, or seals omitted. Air enters the stack and creates detonation risk.

Prevention: Maintain continuous purge at the design rate with low-flow alarm, and install appropriate molecular or velocity seals. Consider a velocity seal to reduce purge cost legitimately.

Mistake 4: Radiation Calculated Without Solar or Wind Effects

Stack height set from flare radiation alone, ignoring solar contribution and flame tilt under wind. Actual radiation at grade exceeds design limits in real conditions.

Prevention: Include solar radiation and wind-induced flame tilt in the calculation, and define exclusion zones accordingly.

Mistake 5: Summing All Relief Valves as the Design Load

Every relief device added together, producing a grossly oversized and expensive system.

Prevention: Determine the governing simultaneous relief scenario per API 521, not the arithmetic total.

Mistake 6: No Provision for Tip Replacement

Self-supporting or guyed stack specified with no means of changing the tip without a full plant shutdown.

Prevention: For facilities where shutdown cost is high, consider derrick-supported designs or demountable stacks allowing tip replacement during operation.

Mistake 7: Unreliable Pilot and Ignition System

Single pilot with marginal wind shielding and no flame detection. The flare goes unlit and vents raw hydrocarbon undetected.

Prevention: Multiple redundant pilots with wind shields, reliable flame detection alarmed to the control room, and a proven ignition system.

Mistake 8: Pocketed Flare Header

Header routed with low points where liquid collects, restricting flow and creating slug potential during relief.

Prevention: Slope headers continuously toward the knockout drum with no pockets, and provide drains.

Supply from Kasko Makine

Kasko Makine supplies flare system equipment, vessels, and components for oil and gas, refining, petrochemical, and industrial projects:

Flare equipment:

• Elevated flare stacks — self-supporting, guyed, and derrick-supported
• Ground and enclosed flares
• Multi-point staged ground flare systems
• Flare tips — pipe, steam-assisted, air-assisted, and high-pressure sonic
• Pilot and ignition systems — flame front generators and high-energy electronic ignition
• Flame detection systems
• Windshields and refractory linings

Vessels and drums:

• Flare knockout drums with internals and instrumentation
• Liquid seal drums
• Condensate collection vessels
• Blowdown drums
• Designed and fabricated to ASME Section VIII with U-stamp — see Chemical Reactors & Pressure Vessels

System components:

• Molecular seals and velocity seals
• Flame arrestors — in-line and end-of-line, selected by gas group
• Flare headers and piping — see Carbon Steel Pipe
• Purge gas systems and control
• Knockout drum pumps
• Steam injection systems and manifolds
• Structural steel, ladders, platforms, and access

Materials: carbon steel, low-temperature carbon steel, stainless, and high-nickel heat-resistant alloys for tips and burning zones; NACE-compliant materials where sour gas is present

Engineering support:

• Relief load review and governing scenario identification per API 521
• Flare type selection (elevated vs ground vs hybrid)
• Radiation calculation and stack height determination
• Knockout drum sizing for droplet removal
• Header hydraulics and back-pressure review against relief device tolerance
• Smokeless capacity and assist medium selection
• Purge rate and seal type optimisation
• Tip material and life assessment

Certification: ASME U-1 data reports for vessels, EN 10204 Type 3.1/3.2 material certificates, WPS/PQR and welder qualifications, NDE reports, hydrostatic test certificates, flare tip performance test data per API 537, third-party inspection

Logistics: Flare equipment shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Vessels and components 16-26 weeks; complete flare packages including stacks and tips 26-44 weeks.

Need flare system equipment or components? Send us your relief loads and governing scenarios, gas composition including H₂S and molecular weight, required smokeless capacity, plot constraints and distance to occupied areas, available assist medium (steam, air, or high-pressure gas), and applicable standards to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll recommend the flare configuration, size the knockout drum and stack, verify radiation limits, and provide a quotation within 72 hours.

Continue Reading: Process Safety & Equipment Guides

• Pressure Relief & Safety Valves — The devices feeding the flare header
• Oil & Gas Separators — Upstream separation
• Chemical Reactors & Pressure Vessels — ASME vessel design
• Industrial Process Burners — Combustion equipment
• Carbon Steel Pipe — Header piping
• Chemical Factory Setup — Plant safety systems

Frequently Asked Questions

Q: What is a flare system and why is it needed?
A: A flare system is the safe disposal route for gases released by pressure relief, emergency depressuring, and process upsets in pressurised facilities. When relief valves lift, compressors trip, or an emergency shutdown forces blowdown, large volumes of hydrocarbon must leave the process within seconds. The flare burns them, converting hydrocarbons to carbon dioxide and water rather than releasing them to atmosphere. The system comprises a collection header, a knockout drum removing entrained liquid, a liquid seal preventing flashback, a molecular or velocity seal preventing air ingress into the stack, and the flare tip with pilots and ignition. Design is governed by API 521 for relief loads and disposal systems and API 537 for flare details and performance.

Q: What is the difference between an elevated flare and a ground flare?
A: An elevated flare raises the burner tip on a self-supporting stack, guyed tower, or derrick so that distance attenuates thermal radiation at grade. It offers large capacity and straightforward maintenance access, but produces a visible flame, light pollution, noise, and a larger radiation footprint. A ground flare places multiple burners inside a refractory-lined windshield or in a staged multi-point array within a fenced area, hiding the flame entirely and greatly reducing radiation and noise at the boundary while achieving high combustion efficiency across wide turndown. The trade-offs are a much larger plot area, higher capital cost, and lower capacity per unit. Many facilities combine both — a ground flare for routine flow and an elevated flare for emergency loads.

Q: Why is a flare knockout drum safety critical?
A: The knockout drum removes entrained liquid from the relief stream before it reaches the flare tip. This matters because liquid droplets carried to the tip do not burn completely and instead fall as burning liquid — sometimes described as flaming rain — creating a serious hazard to personnel, equipment, and the surrounding area. The drum is usually horizontal and sized to remove droplets above a target size, commonly in the range of 300–600 µm, with level instrumentation, alarms, and automatic pump-out so collected liquid is removed continuously. In cold climates heat tracing and insulation are required to prevent hydrate or wax blockage, and flare headers must be sloped free-draining toward the drum with no pockets.

Q: What is purge gas in a flare system?
A: Purge gas is a continuous flow of fuel gas or nitrogen maintained up the flare stack at all times, even when there is no relief flow, to keep the system at positive pressure so air can never enter. This is safety-critical: air inside a hydrocarbon-filled stack creates a flammable mixture and the potential for internal detonation capable of destroying the tip and stack, and loss of purge is a recognised precursor to such incidents. Purge requirements are reduced by installing a molecular seal, which uses a labyrinth exploiting the density difference between purge gas and air, or a velocity seal, which uses an internal cone to prevent back-mixing at substantially lower purge consumption and therefore lower operating cost.

Q: How is flare stack height determined?
A: Stack height is set primarily by thermal radiation limits at grade and on adjacent equipment. A commonly applied design basis is around 4.73 kW/m² (1,500 Btu/hr·ft²) for areas where personnel may be present for limited periods without shielding, with lower values for continuously occupied areas and higher levels permitted within a fenced exclusion zone at the flare base. The calculation accounts for flame length and flame tilt under wind, emissivity, and distance from the flame centre, and importantly must add solar radiation to the flare contribution rather than considering the flare alone. Dispersion of unburned gas in the event of ignition failure, and of SO₂ where H₂S is present, must also be checked.

Q: How is smokeless flaring achieved?
A: Smoke forms when there is insufficient air mixing for complete combustion, leaving unburned carbon particles. Smokeless operation is achieved by adding an assist medium that improves air entrainment and mixing at the tip. Steam-assisted tips inject steam and are the standard method in refineries where steam is readily available. Air-assisted tips use a blower to supply combustion air, suiting facilities without steam. High-pressure or sonic tips use the relief gas's own pressure energy to create mixing, requiring no utility but needing adequate supply pressure. Smokeless capacity is normally specified as a percentage of maximum flow rather than at full emergency rate — the flare should be smokeless for routine and moderate releases, with some smoke accepted during rare full-capacity emergency events.