Inputs
Example data
| Scenario | P1 (kPa abs) | P2 (kPa abs) | T (°C) | MW | k | Z | Cd | D (mm) | Mass flow (kg/h) |
|---|---|---|---|---|---|---|---|---|---|
| Natural gas, moderate pressure | 250 | 101.325 | 50 | 18.0 | 1.30 | 0.98 | 0.90 | 200 | Varies with assumptions |
| Air-like gas, higher pressure | 500 | 101.325 | 40 | 28.97 | 1.40 | 1.00 | 0.92 | 150 | Varies with assumptions |
Formula used
This calculator treats the flare tip as an orifice/nozzle with isentropic gas flow. It selects choked or subcritical flow using the critical pressure ratio:
Choked (sonic) flow when P2/P1 ≤ critical ratio:
Subcritical flow when P2/P1 > critical ratio:
Where A is tip area, R is specific gas constant, T is absolute temperature, k is isentropic exponent, and Z is compressibility factor.
How to use this calculator
- Choose a mode: compute capacity from a known tip diameter, or size the diameter from a required flow.
- Enter absolute upstream and downstream pressures, then set gas temperature.
- Provide gas properties (MW, k, Z). Use values consistent with the relief composition.
- Set a discharge coefficient that matches your tip and assumptions.
- Press Calculate. Review regime, mass flow, and volumetric conversions.
- Download CSV or PDF for documentation and submittals.
Technical guidance
Define the design case and limiting scenario
A flare tip capacity check should start with the governing relief or vent scenario, including composition, upstream pressure at the tip, and expected temperature. Confirm whether the case is emergency, upset, or normal venting. Use absolute pressures and document any assumed backpressure. If multiple scenarios exist, evaluate the highest required mass rate and the most conservative gas properties.
Select gas properties with traceable sources
Molecular weight, isentropic exponent, and compressibility influence mass flux and the choked transition. Property values should represent the flowing mixture at the flare tip conditions rather than at battery limits. When composition varies, use a bounded approach: evaluate a light and a heavy mixture to understand sensitivity, then apply the project’s approved basis.
Interpret choked versus subcritical operation
The calculator compares the downstream-to-upstream pressure ratio against the critical ratio. When flow is choked, the mass rate becomes largely independent of downstream pressure and is controlled by upstream conditions, k, Z, and the effective area. For subcritical flow, downstream pressure still influences capacity, so confirm the correct downstream reference for your site.
Review velocity and mechanical constraints
The estimated exit velocity provides a screening check for noise, vibration, erosion risk, and flame stability considerations. Always compare against your project criteria, vendor guidance, and allowable velocities for continuous and emergency operation. If velocity is high, consider increasing diameter, adjusting the discharge coefficient basis, or revisiting upstream pressure assumptions.
Document results and support QA checks
Export the calculation as CSV or PDF and include the input set, the selected flow regime, and the converted rates (kg/s, kg/h, and Sm³/h). Perform a reasonableness check by comparing trends: show that increasing pressure, area, or Cd increases capacity; and increasing MW reduces standard volumetric rate for the same mass rate.
P1 = 350 kPa abs, P2 = 101.325 kPa abs, T = 60°C, MW = 20 kg/kmol, k = 1.28, Z = 0.98, Cd = 0.90, D = 180 mm.
Expected: P2/P1 below critical in many cases, producing choked flow with a stable mass rate trend.
FAQs
1) Should pressures be gauge or absolute?
Use absolute pressures. If you have gauge values, add atmospheric pressure first. Incorrect pressure reference can shift the regime decision and materially change the calculated flow rate.
2) What does “choked flow” mean for a flare tip?
Choked flow occurs when gas reaches sonic velocity at the controlling section. Capacity becomes primarily controlled by upstream pressure, temperature, and gas properties rather than downstream pressure.
3) How do I choose the discharge coefficient?
Use a value supported by your tip geometry, vendor data, or project basis. If uncertain, run sensitivity checks across a reasonable range and document the selected value and justification.
4) Why is compressibility factor included?
Z corrects ideal-gas density and mass flux for real-gas behavior. At elevated pressures or non-ideal mixtures, using Z improves consistency compared with assuming ideal behavior.
5) What standard conditions are used for Sm³/h?
This tool uses 101.325 kPa and 15°C. If your project uses different standard conditions, convert outside the tool or adjust reporting to match the approved basis.
6) Can I size a diameter from a required standard volumetric flow?
Yes. Select “Diameter from required flow” and enter Sm³/h. The tool converts to mass flow using molecular weight and standard conditions, then calculates the required effective area and diameter.
7) Is this a complete flare design verification?
No. It is a sizing and capacity screening model for the tip. Full flare design may require radiation, dispersion, acoustics, stability, header hydraulics, and applicable codes or standards.