OIL & GAS EQUIPMENT | Updated May 2026 | 7 min read
What You’ll Learn in This Guide
- How to define the design basis for a sonic flare using inlet pressure and waste-gas composition
- How to calculate sonic velocity and Mach number at the tip exit
- How to size sonic flare tip diameter from peak mass flow
- How to calculate smokeless capacity for sonic flares without auxiliary air
- How to set stack height for sonic flares using API Standard 521 radiation criteria
- Why turndown ratio drives staged or multi-tip sonic flare configurations
- Common sonic flare sizing mistakes and how to avoid them
Sonic flares deliver smokeless, high-efficiency combustion without blowers, motors, or auxiliary air — but that simplicity disappears the moment the flare is undersized. A sonic flare tip that runs below choked-flow conditions stops behaving as a sonic flare, loses its self-entrainment of combustion air, and starts smoking under exactly the relief scenarios it was specified to handle. Oversize the tip and you sacrifice turndown performance; under-rate the stack and you violate API Standard 521 thermal radiation limits at occupied areas. Getting the sizing right depends on four locked-in inputs: inlet pressure, waste-gas composition, peak mass flow, and required turndown ratio.
Hero Process Solutions, founded in 2011 and headquartered in Kellyville, Oklahoma with operations in Midland, Texas, manufactures sonic flares with Coanda-effect tips for upstream production, midstream gas processing, refining, and petrochemical applications. This guide walks through the engineering decisions every sonic flare specification must answer and explains why the sizing math is fundamentally different from air-assisted or steam-assisted flare sizing.
DIRECT ANSWER: Sonic flares are sized using four parameters: minimum continuous inlet pressure (typically 15 to 30 psig or higher), peak waste-gas mass flow rate, gas molecular weight and composition, and required turndown ratio. The tip diameter is set so that gas exits at sonic velocity (Mach 1.0 at the choke point) at design flow, which produces the turbulent mixing that makes sonic flares smokeless without auxiliary air. A sonic flare needs sustained inlet pressure to function as a sonic flare — under turndown or low-pressure scenarios, output velocity drops below sonic and the smokeless performance is lost unless multi-tip staging is built in.
1. How to Define the Design Basis for a Sonic Flare
Every sonic flare sizing exercise starts with three inputs that distinguish sonic flares from every other flare configuration. The first is the minimum continuous inlet pressure at the tip — not the peak pressure during emergency relief, but the routine operating pressure that the flare must handle without losing sonic exit conditions. The second is the peak waste-gas mass flow rate in pounds per hour, which sets the required tip throat area. The third is the gas molecular weight and ratio of specific heats (the k value), which determine sonic velocity at the tip.
Sonic velocity for an ideal gas is calculated as the square root of (k multiplied by R multiplied by T divided by molecular weight). For natural gas at typical flare temperatures, sonic velocity at the tip exit is approximately 1,400 to 1,500 feet per second. For heavier hydrocarbon mixtures, sonic velocity drops to 900 to 1,200 feet per second. That number sets the design exit velocity and, with mass flow, determines tip throat area.
Confirm whether the flare service is continuous, intermittent, or emergency-only — that decision drives whether the design must guarantee sonic conditions across all routine flow or only during peak relief events.
2. How to Calculate Mach Number and Tip Exit Velocity
Mach number is the ratio of actual gas velocity at the tip exit to sonic velocity in the same gas. For a sonic flare to function as a sonic flare, Mach number at the choke point must reach 1.0 — gas accelerates to sonic velocity as it expands through the converging-diverging or straight-throat tip geometry. Above the choke point, expansion continues but mass flow through the choke is fixed by upstream conditions.
The required pressure ratio to choke a tip is approximately (k+1)/2 raised to the k/(k-1) power. For natural gas with k around 1.3, the critical pressure ratio is roughly 1.83 — meaning the absolute upstream pressure must be at least 1.83 times the downstream (ambient) absolute pressure for the tip to choke. At sea level ambient (14.7 psia), that requires upstream absolute pressure of about 27 psia, or approximately 12 psig at the tip inlet. In practice, design margins push the minimum sustainable inlet pressure to 15–30 psig for reliable choked operation across composition variations.
KEY INSIGHT: A sonic flare loses its smokeless behavior the moment it stops choking. Specifying minimum inlet pressure based on average operating conditions — instead of worst-case turndown — is the single most common engineering error in sonic flare design. Always size for the lowest sustained inlet pressure the flare must handle in service.
3. How to Size Sonic Flare Tip Diameter from Peak Mass Flow
Once minimum inlet pressure and gas properties are locked in, tip throat area is calculated from the choked-flow mass equation. The mass flow through a choked nozzle is proportional to upstream absolute pressure times throat area divided by the square root of upstream absolute temperature, with a coefficient set by the gas k value.
For typical natural gas at 100°F and 30 psig upstream pressure, the choked mass flow is approximately 0.85 lb per second per square inch of throat area. A 50,000 lb/hr peak relief therefore needs a throat area of roughly 16 square inches — a tip throat diameter of about 4.5 inches if the tip is a single circular nozzle, or a corresponding annular area if the tip uses a Coanda profile.
Hero’s sonic flare tips use Coanda-profile geometry to optimize the turbulent mixing zone downstream of the choke, which is what gives the tip its smokeless behavior at a given exit velocity. Tip sizing is therefore a system optimization, not a single-nozzle calculation — the throat area, Coanda profile, and downstream expansion all interact.
4. How to Calculate Smokeless Capacity for Sonic Flares
Smokeless capacity for a sonic flare is the maximum waste-gas mass flow the flare can burn without visible smoke. Unlike an air-assisted flare, the sonic flare achieves smokeless combustion through high exit velocity rather than externally supplied air, so smokeless capacity is tied directly to the choked-flow throat area and the gas composition.
For waste gas dominated by saturated light hydrocarbons (methane, ethane), smokeless capacity at the choke point is high. For olefin-rich streams (ethylene, propylene), the same throat area handles significantly less smokeless flow because the combustion chemistry produces more soot precursors. Always confirm smokeless capacity against the specific gas envelope the flare will see in service.
EPA 40 CFR 60.18 limits flares to no more than five minutes of visible emissions during any consecutive two-hour period. EPA OOOOb tightens that further for affected oil and gas sources and adds the 98% Destruction and Removal Efficiency requirement. Sonic flares typically pass OOOOb without auxiliary air because the high-velocity, high-turbulence exit jet sustains 98% DRE at choked flow.
5. How to Determine Sonic Flare Stack Height
Stack height for a sonic flare is set by the more conservative of two criteria, the same as for any flare configuration. The first is thermal radiation: API Standard 521 (Pressure-Relieving and Depressuring Systems) limits ground-level thermal radiation at occupied areas to 1,500 Btu per hour per square foot for emergency releases, with lower thresholds for personnel exposure zones. The second is dispersion: ground-level concentrations of unburned hydrocarbons, NOx, and CO at the property line must remain below applicable air-quality standards.
Sonic flares have one advantage in the stack-height calculation: the short, intensely turbulent flame envelope of a sonic tip radiates less heat at long distances than the longer, more diffuse flame of a conventional flare at the same heat release. That can translate into a shorter stack — but only after running the radiation calculation with the actual flame geometry. Never assume a height reduction without doing the math.
6. How to Specify Turndown Ratio for a Sonic Flare
Turndown ratio is the central design challenge for sonic flares. A single-tip sonic flare maintains choked-flow conditions only above the minimum mass flow that produces the required upstream pressure. Below that minimum, the tip un-chokes, exit velocity falls below sonic, and the smokeless behavior is lost.
Two design strategies preserve turndown. The first is multi-tip staging: install several sonic tips of different sizes on a manifold, with sequencing valves that bring tips on line as flow rises. The smallest tip handles routine low flow, intermediate tips handle moderate releases, and the full tip array handles peak emergency relief. The second strategy is a sonic-plus-pilot design: a continuous high-stability pilot ignition system holds a flame during sub-choke conditions while the main sonic tip waits for inlet pressure to recover.
CRITICAL RULE: Never design a sonic flare for a single peak relief scenario without verifying turndown behavior. A sonic flare optimized for 50,000 lb/hr peak that sees 500 lb/hr routine flow will smoke during routine operation unless multi-tip staging or a sonic-plus-pilot architecture is built in.
7. Sonic Flare vs Other Flare Types: When Sonic Wins
| Flare Type | Best When | Capacity Range | Key Advantage |
|---|---|---|---|
| Sonic Flare (Coanda tip) | Inlet pressure consistently 15 psig or higher; high turbulence required for smokeless flow | Single tip to multi-tip arrays in the millions of lb/hr range | No blower OPEX, no rotating equipment, simplest mechanical design |
| Air-Assisted Flare | Inlet pressure too low to choke; smokeless required at low pressure | Up to 250,000+ lb/hr smokeless via blower air | Active VFD blower control across full turndown |
| Gas-Assist Flare | Heavy hydrocarbons with available fuel gas at the site | Mid-range continuous flow | Lower CAPEX, simpler controls than air-assist |
| Low Flow Flare | Storage tank vent gas, well site applications | Low continuous flow | Simple, low CAPEX, well-suited to upstream |
| Portable Flare | Temporary commissioning, pipeline blowdown | Variable | Trailer-mounted, rapid deploy |
Sonic flares are the right choice whenever inlet pressure is sustained at design level. They lose their advantage the moment pressure falls below the choking threshold, which is why industrial flare systems selection should always start with a pressure profile analysis.
8. Sonic Flare Sizing and EPA OOOOb Compliance
For sonic flares on affected oil and natural gas sources, EPA OOOOb compliance requires 98% Destruction and Removal Efficiency on initial and annual performance tests, continuous monitoring of pilot and combustion zone presence, vent-gas flow measurement, five-year recordkeeping, and CEDRI reporting. Sonic flares typically pass the 98% DRE requirement at choked flow because the high exit velocity produces near-complete combustion in the turbulent mixing zone.
The compliance risk for sonic flares is at sub-choke conditions, not at peak flow. The sizing specification must address what happens when inlet pressure drops — through multi-tip staging, sonic-plus-pilot architecture, or operational procedures that route low-flow vent gas to a separate low-flow flare or vapor combustor.
9. Common Sonic Flare Sizing Mistakes
| Mistake | Why It Hurts Operations / Compliance | Fix |
|---|---|---|
| Sizing minimum inlet pressure to average instead of worst-case turndown | Tip un-chokes during low-pressure scenarios, smokes, fails OOOOb DRE | Specify minimum inlet pressure at the lowest sustainable continuous flow |
| Ignoring gas composition variability | Sonic velocity changes with molecular weight, throat area sizing becomes wrong | Run sizing math for full composition envelope |
| Single-tip design for wide-turndown service | Fails to handle routine low flow without smoke or OOOOb deviation | Specify multi-tip staging or sonic-plus-pilot architecture |
| Treating sonic flare as a drop-in replacement for an air-assist flare | If inlet pressure is insufficient, performance collapses | Confirm full operating pressure profile before changing flare type |
| Skipping radiation calculation because sonic flame is shorter | Under-designed stack height violates API 521 at occupied areas | Run radiation calc with actual sonic flame geometry, do not assume reduction |
| Missing sample-port placement for OOOOb annual test | Cannot run EPA Method 1-compliant performance test | Locate ports per Method 1 during fabrication |
Article Summary
- Sonic flare sizing requires four locked-in inputs: minimum inlet pressure, peak mass flow, gas composition, and turndown ratio.
- The tip must operate at choked flow (Mach 1.0 at the throat) for smokeless combustion to occur without auxiliary air.
- Critical pressure ratio for natural gas (k≈1.3) is approximately 1.83, requiring at least 12 psig at the tip inlet, with design margin pushing minimums to 15–30 psig.
- Tip throat area is sized from choked-flow mass equation using upstream absolute pressure, temperature, and gas k value.
- Smokeless capacity depends on choke throat area plus gas composition; olefin-rich streams reduce smokeless flow relative to light saturated hydrocarbons.
- Stack height is set by API Standard 521 thermal radiation or dispersion, whichever is taller.
- Turndown ratio drives the choice between single-tip, multi-tip staged, or sonic-plus-pilot architectures.
- Hero Process Solutions designs Coanda-profile sonic flares as turnkey OOOOb-ready packages from Kellyville, Oklahoma.
Frequently Asked Questions
What is the minimum inlet pressure for a sonic flare?
For natural gas, the theoretical minimum inlet pressure to choke a sonic tip is approximately 12 psig (about 1.83 times atmospheric absolute pressure). In practice, design margins push the specified minimum to 15 to 30 psig at the tip inlet, depending on gas composition. Pressure-assisted sonic flares are the preferred choice when inlet gas pressure is consistently above 30 psig.
How is sonic flare tip diameter calculated?
Tip throat area is calculated from the choked-flow mass equation: mass flow equals a coefficient (set by gas k value) times upstream absolute pressure times throat area divided by the square root of upstream absolute temperature. For typical natural gas at 100°F and 30 psig, choked mass flow is approximately 0.85 lb per second per square inch of throat area. A 50,000 lb/hr peak relief therefore needs roughly 16 square inches of throat area.
Do sonic flares need a blower or auxiliary air?
No. Sonic flares achieve smokeless combustion through high exit velocity at the tip rather than through auxiliary air supply. The high-velocity, high-turbulence exit jet entrains ambient air into the combustion envelope from the surrounding atmosphere, producing the smokeless flame without a blower, motor, or compressed-air source.
Can a sonic flare meet EPA OOOOb 98% DRE?
Yes, at choked-flow conditions. Sonic flares produce 98% or greater Destruction and Removal Efficiency at design flow because the high exit velocity creates near-complete combustion in the turbulent mixing zone. The compliance risk is at sub-choke conditions, which is why turndown architecture (multi-tip staging or sonic-plus-pilot) is critical in the sizing specification.
What turndown ratio can a sonic flare achieve?
A single-tip sonic flare has limited turndown because it must maintain choked-flow conditions to remain smokeless. Practical turndown for single-tip designs is typically 3:1 to 5:1. Multi-tip staged designs with sequencing valves can achieve turndowns of 50:1 or higher by bringing tips of different sizes on line as flow rises.
What sizing data does Hero Process Solutions need to quote a sonic flare?
Hero typically asks for minimum and maximum continuous inlet pressure at the tip, peak relief mass flow rate (lb/hr), gas composition with heating value and molecular weight, the operating temperature envelope, required turndown ratio, site layout for radiation modeling, and OOOOb / OOOOc compliance status. With those inputs, the sonic flare can be sized including tip, stack, pilot, ignition system, monitoring, and control package.
