Flare Header Sizing Calculator

Calculator

Enter gas conditions, flow, and design constraints. Use the hydraulic check to screen velocity, noise, and backpressure risk.

Units

Flow input type

Mach limit (header)

Common screening: 0.2 normal, 0.5 peak.

Pressure (kPa abs)

Temperature (°C)

Molecular weight (g/mol)

k ratio (Cp/Cv)

Typical natural gas ~1.27–1.32.

Compressibility Z

Gas viscosity (cP)

Mass flow (kg/h)

Pipe length (m)

Minor loss sum K (fittings, tees, valves)

Pipe roughness

Custom roughness (mm)

Reset

Formula used

This tool uses a velocity/Mach screening plus a pressure-drop estimate.

a = √(k·R·T / MW) (speed of sound, MW in kg/mol).
v_allow = Mach_limit · a (allowable header velocity).
ρ = P·MW / (Z·R·T) (gas density at flowing conditions).
Q = ṁ / ρ and A = Q / v_allow, D = √(4A/π).
Pressure drop (screening): ΔP = ( f·L/D + ΣK ) · (ρ·v²/2), using Swamee–Jain for f.

How to use this calculator

Select units and your flow input type.
Enter header pressure and temperature (absolute pressure).
Enter gas properties: MW, k ratio, Z, and viscosity.
Pick a Mach limit to control noise and vibration risk.
Optionally enter length, K losses, and roughness for ΔP screening.
Click Calculate and review the suggested pipe and ΔP.

Article

Professional reference notes to support preliminary flare header sizing.

Design objective for flare headers

Flare headers collect relief and blowdown gases and route them to a safe combustion point. The sizing goal is to move the required flow without excessive backpressure at protected equipment, while keeping velocity and noise within acceptable limits. This calculator provides a fast screening diameter based on a chosen Mach limit and then estimates pressure drop for a selected check diameter.

Key inputs and data quality

Accurate flowing pressure and temperature are essential because gas density changes strongly with these conditions. Enter absolute pressure, not gauge. Molecular weight, k ratio, and compressibility Z affect both density and the speed of sound. When the stream composition varies, use a conservative case or run multiple cases for normal, peak, and emergency scenarios.

Velocity and Mach screening

The calculator computes speed of sound using a = √(k·R·T/MW). Allowable velocity is Mach_limit × a. Using actual volumetric flow Q, the required area is A = Q/v_allow and the required internal diameter is D = √(4A/π). Lower Mach limits reduce vibration and acoustic risk but increase pipe size and cost.

Pressure drop and backpressure checks

For a practical hydraulic check, the tool evaluates Reynolds number, friction factor, and an estimated ΔP using Darcy–Weisbach: ΔP = (f·L/D + ΣK)·(ρv²/2). Roughness selection influences f in turbulent flow. Include realistic lengths and minor-loss K values for tees, valves, and reducers. Compare ΔP against available backpressure margin to avoid capacity reduction.

Using results for specification

Use the required diameter to shortlist standard pipe sizes, then confirm the suggested size with a full flare network study that includes multiple sources, elevation, liquid dropout, and sonic constraints at restriction points. Document inputs, assumptions, and export outputs for design reviews. Treat results as preliminary sizing for early engineering and bid packages. If your jurisdiction applies specific noise or velocity criteria, align Mach limits with those standards. When results are close to limits, select the next larger line to provide operational flexibility and future tie-ins during turnaround events.

FAQs

1. What is a good Mach limit for a flare header?

Many projects screen normal flow near Mach 0.2–0.3 and peak or emergency near 0.5–0.6. Use your company standard and acoustic requirements. Lower limits reduce noise but increase diameter.

2. Why does the calculator ask for compressibility Z?

Z adjusts the real-gas relationship between pressure, temperature, and density. At higher pressures or heavier mixtures, Z may deviate from 1.0. Using an appropriate Z improves density, flow conversion, and velocity estimates.

3. Can I enter standard flow instead of mass flow?

Yes. Choose Standard volumetric flow and enter Nm³/h or scfh. The tool converts standard flow to mass flow at reference conditions, then converts to actual volumetric flow at your entered pressure and temperature.

4. How accurate is the pressure-drop result?

It is a screening estimate using Darcy–Weisbach with a friction-factor correlation and a lumped minor-loss K. It does not model multiple simultaneous relief sources, elevation, two-phase effects, or fittings details. Use a network simulator for final design.

5. What roughness should I pick for carbon steel?

A typical new carbon-steel roughness is about 0.045 mm. Older or scaled lines may be rougher. If you have inspection data or a project specification, select Custom and enter the specified value.

6. What should I do if the suggested pipe is larger than my layout allows?

Reduce velocity by selecting a lower Mach limit is not possible if space is fixed. Instead, reassess allowable backpressure, shorten routes, reduce K losses, or split flow with parallel headers. Confirm changes with a full flare study.

Example data table

Case	Flow	P abs	T	MW	Mach	Required ID (mm)	Suggested pipe
Normal	2,500 kg/h	180 kPa	35 °C	18	0.20	~210	NPS 8 (Sch 40)
Peak	8,000 kg/h	220 kPa	45 °C	20	0.50	~255	NPS 10 (Sch 40)
Emergency	14,000 kg/h	300 kPa	60 °C	22	0.60	~295	NPS 12 (Sch 40)

Values are illustrative. Always run a full network hydraulic study for final design.