Flare System Design

Flare Stack Height: Engineering Design Fundamentals

Set the minimum stack height so the flame's radiant heat intensity at a ground-level receiver stays within the API 521 Table 9 limit — the primary stack-height criterion per API 537 §4.13.3.1.

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Radiation limit

API 521 Table 9

500 continuous → 3000 urgent-action Btu/hr·ft² personnel limits.

Flame length

0.00604·Q0.4776

API 521 Fig 8; Annex C example: 2.15×10⁹ Btu/hr → 170 ft.

Wind distortion

API 521 Fig 9

u∞/uj → ΣΔx/L, ΣΔy/L from the actual tip velocity (Eq C.2).

Contents

Use this guide when you need to:

  • Set stack height from the API 521 Table 9 radiation limit.
  • Compute flame length, tip velocity and wind distortion (Fig 8/9).
  • Locate the flame centre and apply the point-source model (Eq 24).

1. Radiation governs the height

API 537 §4.13.3.1 is explicit: "the primary criterion used to determine the flare stack height shall be flare flame radiant heat intensity at grade." The stack is raised until the heat flux at the worst-case ground-level receiver falls to the allowable limit. Toxic/CO dispersion is a concern only for an unignited release (the flame consumes the combustibles) and is handled as a separate dispersion study with the appropriate stability class and source term — it is not part of this radiation-based height.

2. Radiation limits (API 521 Table 9)

API 521 Table 9 gives the permissible design radiation for personnel — all four rows are personnel limits (the 9.46 kW/m² row is "urgent emergency action," not an equipment-only zone):

Exposure scenarioBtu/hr·ft²kW/m²
Continuous exposure (appropriate clothing)5001.58
Emergency action, 2–3 min15004.73
Emergency action, ≤ 30 s20006.31
Urgent emergency action (shielding/fire suit)30009.46

Solar radiation (0.79–1.04 kW/m², i.e. 250–330 Btu/hr·ft²) may be deducted from the limit when sizing for continuous exposure (API 521 §6.4.2.3). Heat flux from a point-source flame, evaluated at the flame centre:

q = τ · F · Q / (4π · R²)

τ = atmospheric transmissivity (≈0.85 clear, lower in humid/long path), F = radiant heat fraction (≈0.2 natural gas, higher for unsaturates/H₂S), Q = ṁ·ΔH_c, R = distance from the flame centre to the receiver. Rearranged, the required flame-centre→receiver distance is D = √(τ·F·Q/(4π·K)) (API 521 Eq 24).

3. Flame length & tip velocity

Flame length comes from the API 521 Fig 8 heat-release curve (the same value is used regardless of tip Mach):

L = 0.00604 · Q0.4776 ; Q in Btu/hr, L in ft

API 521 Annex C worked example: Q = 100,000 lb/hr × 21,500 Btu/lb = 2.15×10⁹ Btu/hr → L ≈ 170 ft. The flare-tip exit velocity is then the actual vapour volumetric flow divided by the tip area (API 521 Eq C.2) — not an assumed value:

uj = qvap / (π·d²/4) , qvap = (ṁ/3600)·(379.5/MW)·(TR/519.67)

For the Annex C example (MW 46.1, 300 °F, 1.53 ft tip) this gives uj ≈ 181 ft/s. The riser velocity should stay ≤ 0.5 Mach (API 537 §4.13.2.2).

4. Flame distortion & sizing

Lateral wind bends the flame. API 521 Figure 9 gives the horizontal and vertical projections of the flame tip as a function of the wind-to-jet velocity ratio:

u∞ / uj → ΣΔx/L (horizontal) , ΣΔy/L (vertical)

At u∞/uj = 0.16 the flame is already strongly bent (ΣΔx/L ≈ 0.85, ΣΔy/L ≈ 0.36); at low wind it stands nearly vertical (ΣΔx/L → 0, ΣΔy/L → 1). A simple θ = atan(u∞/uj) badly under-predicts this bend — and using a fixed jet velocity hides it entirely, which is why the actual uj matters. The flame centre is taken at half the distortion, ( ΣΔx/2 , H + ΣΔy/2 ), and placed toward the (downwind) receiver as the worst case. The stack height H is then raised until the flame-centre→receiver distance reaches D from §2.

5. References

  • API Std 521 §6.4.2 + Annex C — flame length (Fig 8), tip velocity (Eq C.2), flame distortion (Fig 9), point-source radiation (Eq 24), Table 9 personnel limits.
  • API Std 537 §4.13.3 — radiant heat intensity at grade as the primary stack-height criterion; §4.13.2.2 riser ≤ 0.5 Mach.

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Frequently Asked Questions

What sets flare stack height?

The flame radiant heat intensity at grade — the primary criterion per API 537 §4.13.3.1. The stack is raised until the heat flux at the worst-case ground-level receiver falls to the allowable API 521 Table 9 limit. Flame length, the actual tip exit velocity and the wind-driven flame distortion all feed the geometry.

What radiation limits does API 521 give?

API 521 Table 9 lists four personnel limits (Btu/hr·ft²): 500 for continuous exposure, 1500 for emergency action lasting 2–3 min, 2000 for emergency action up to 30 s, and 3000 for urgent emergency action with shielding/fire-approach suit. Solar flux (250–330 Btu/hr·ft²) may be deducted from the limit when designing for continuous exposure (§6.4.2.3).

Why does the actual tip velocity matter for the height?

Wind bends the flame as a function of the wind-to-jet velocity ratio u∞/uj (API 521 Figure 9). The jet velocity must be the real value from the flow and tip diameter (Eq C.2) — assuming a high fixed velocity hides the bend, which under-predicts the radiation on a downwind receiver and can make the stack come out too short. Toxic/CO dispersion for an unignited release is a separate study, not part of this radiation-based height.