Flare System Sizing

Quick pick (TL;DR)

Add up the simultaneous relief loads (often total power failure or fire is the governing case) to get the peak flow.
Size the tip diameter to a velocity limit (≈ Mach 0.5, lower for smokeless).
Compute heat release Q = W·LHV, then the radiation distance so grade level stays within allowable flux (4.73 kW/m² for personnel).
Add a knock-out drum, continuous purge, pilots and smokeless assist (steam/air).
Everything to API 521/537.

Step 1 — Type & governing case

Choice	Use for	Note
Elevated	Large loads	Common
Ground / enclosed	Hidden flame	Footprint
Governing case	Power fail / fire	Simultaneous

Step 2 — Heat release & radiation

Relief 50 kg/s, LHV 46 MJ/kg

Step 1 — Heat release Q = W · LHV Q = 50 kg/s × 46,000 kJ/kg Q = 2,300,000 kW = 2300 MW Step 2 — Radiation distance D = √(τ · Q / (4π · K_allow)) τ = 0.2 (frac radiated) , K = 4.73 kW/m² D = √(0.2 × 2.3e6 / (4π × 4.73)) D = 88 m to the 4.73 kW/m² contour Step 3 — Tip & height tip Ø from Mach ≤ 0.5 at peak flow stack height set so grade flux ≤ allowable Answer → elevated flare, ~88 m exclusion, steam-assist

Radiated fraction reaches grade — the 4.73 kW/m² contour sets height & exclusion. Tip Ø by Mach limit.

Step 3 — System components

KO drum + seal drum / flashback protection.
Continuous purge + reliable pilots.
Smokeless assist (steam/air).

Flare radiation calculator

Heat release & radiation distance

Q = W·LHV; D = √(τ·Q/(4π·K_allow)).

Relief rate (kg/s)

LHV (kJ/kg)

Radiated fraction τ

Allowable flux

Point-source screening — a full study adds wind tilt, dispersion & stack height.

Common mistakes

Wrong governing relief case.
No knock-out drum.
Ignoring wind tilt on radiation.

Requisition & datasheet

Type, case, relief rate, LHV.
Q, tip, radiation, KO drum, assist.

Frequently asked questions

What should I know about Flare System Sizing?

Add up the simultaneous relief loads (often total power failure or fire is the governing case) to get the peak flow. Size the tip diameter to a velocity limit (≈ Mach 0.5, lower for smokeless). Compute heat release Q = W·LHV, then the radiation distance so grade level stays within allowable flux (4.73 kW/m² for personnel). Add a knock-out drum, continuous purge, pilots and smokeless assist (steam/air). Everything to API 521/537.

Which standards or codes apply?

API 521 (pressure-relieving & depressuring / flare loads & radiation), API 537 (flare details), API 520 (PSV upstream), and dispersion modelling.

Standards — API 521; API 537; API 520; dispersion modelling.