Flare System Design

Flare Seal Drum — Engineering Fundamentals

API 521 / 537 liquid-seal sizing for flashback prevention and stack-pulsation isolation.

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Standard

API 537 §H.9

Liquid-seal drum sizing (with API 521 droplet disengagement).

Diameter set by

3 criteria

Governing of disengagement, seal-liquid volume, and anti-pulsing.

Seal break pressure

P = ρ·h / 144

Submergence head (Eq H.21); 6 in water ≈ 0.22 psi.

Contents

Use this guide when you need to:

  • Size the drum by the governing of disengagement, seal-volume, and anti-pulsing.
  • Set seal depth and the resulting header break pressure (Eq H.21).
  • Plan freeze protection and seal-water make-up.

1. Role of the seal drum

A liquid seal drum sits at the base of every refinery / midstream flare stack with two functions:

  1. Flashback prevention — when the flare extinguishes, atmospheric air can settle down the stack and form a flammable mixture with residual flare gas. A flame propagating downward would reach the process header and ignite it. The water seal physically blocks this path.
  2. Pulsation damping — wind gusts at the stack tip create pulsation that propagates back to the header. The seal water absorbs and dissipates these pressure waves.

Distinct from the flare KO drum (which removes entrained liquid before the stack); a refinery network has one KO drum + one seal drum.

2. Sizing the drum — three governing criteria

API 537 §H.9.7 is explicit that a seal-drum diameter "has to consider liquid volume required for the seal, pulsing, and liquid vapor separation." There is no single "maximum velocity" rule — instead the drum diameter is the largest of three independent criteria. (A fixed-velocity shortcut such as "70 ft/s" has no basis in API 537 and badly under-sizes the drum, inviting seal-water carryover — "burning rain" — on the flame.)

2.1 Disengagement / re-entrainment (API 521 Eq 37)

The rising vapor must not shear the seal liquid off the surface and carry it to the tip. API 537 §A.8.1.4 defers this to the API 521 knockout-drum droplet-settling method (separate droplets in the 300–600 µm range):

uc = 1.15·√[ g·Dp·(ρl − ρv) / (ρv·C) ]    C·Re² = 0.95×10⁸·ρv·Dp³·(ρl − ρv) / μ²

The drag coefficient C comes from API 521 Figure 16 (here via the Turton–Levenspiel correlation). The drum cross-section then follows from the vapor volumetric flow: Ddis = √(4·Qv / (π·uc)). For typical flare vapor this gives uc on the order of 5–10 ft/s, not 70.

2.2 Seal-liquid volume for the design vacuum (API 537 Eq H.18)

After a hot release, cooling/condensation can pull a vacuum on the header that draws seal liquid up the inlet leg. The drum must hold enough liquid (no make-up credit) that the seal is not broken. For a vertical drum:

Dvol = d·√(H/h + 1)

where d = inlet-pipe ID, h = dip-leg submergence, and H = the seal-fluid height the design vacuum can lift (H = 144·pvac/ρ, Eq H.21). API 537 §H.9.2 cites ≈ 3 m (10 ft) of water (≈ 5 psi vacuum) as effective for typical refining service. A horizontal drum uses Eq H.19: L·w = (π/4)·d²·(H/h).

2.3 Anti-pulsing (API 537 §H.9.8)

To keep gas flow through the seal from surging, the gas area above the liquid must be at least three times the inlet-pipe area — a geometric rule, not a velocity:

Agas ≥ 3·Ainlet  ⟹  Dpulse = √3·d

This rarely governs; §H.9.8 notes the disengagement and seal-volume criteria usually drive a larger drum. Pulsing is further controlled with internal wave attenuators / anti-slosh baffles (§H.9.3–H.9.4).

3. Seal break pressure

The seal "breaks" (gas bubbles through) when the header gauge pressure equals the submergence head — there is no empirical margin. From API 537 Eq H.21, p = ρ·h / 144 (USC):

Pbreak = ρl · h / 144   (water: ≈ 0.0361 psi per inch of seal depth)

API 537 §H.9.2 puts a 150 mm (6 in) seal common for general service (≈ 0.22 psi), flare-gas-recovery seals at 500–750 mm (20–30 in) for compressor suction, and staging seals at 2.5 m (100 in) or more — the practical range is 50 mm to 3050 mm (2–120 in). The controlling limit is simply that the resulting break pressure stays within the flare header's maximum allowable backpressure (API 521).

4. Operation & freeze protection

  • Continuous water make-up via float-level control.
  • Steam tracing or heat tracing for cold climates — frozen seal = no protection.
  • Anti-freeze (50% glycol) for very cold sites + insulated jacket.
  • Periodic blowdown to remove accumulated heavies (hydrocarbon condensate floats on water; remove via skim weir).

5. References

  • API Std 537 — Flare Details for General Refinery and Petrochemical Service; §H.9 liquid-seal drum (Eq H.18/H.19 seal volume, Eq H.20/H.21 submergence, §H.9.8 anti-pulsing, §4.13.2.2 riser ≤ 0.5 Mach).
  • API Std 521 — Pressure-Relieving and Depressuring Systems; §7 knockout-drum droplet disengagement (Eq 37/39, Fig 16) and maximum allowable backpressure.
  • NFPA 30 — companion fire-code context.

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Frequently Asked Questions

What does a flare seal drum do?

It serves two roles: a liquid water seal blocks flashback (downward flame propagation from air settling into the stack after the flare extinguishes), and the seal water damps wind-driven stack pulsation before it reaches the process header.

What sizes the seal drum diameter?

Per API 537 §H.9, the diameter is the largest of three criteria: vapor/seal-liquid disengagement (the API 521 droplet-settling velocity, Eq 37 — typically only 5–10 ft/s, which keeps seal water from being carried to the flame), the seal-liquid volume needed to hold the seal under the design header vacuum (Eq H.18), and an anti-pulsing geometry rule that the gas area exceed three times the inlet-pipe area (§H.9.8). A single fixed "70 ft/s" velocity is not an API 537 criterion and substantially under-sizes the drum.

What is the seal break pressure?

Gas bubbles through when the header gauge pressure equals the submergence head — API 537 Eq H.21, P = ρ·h/144, or about 0.036 psi per inch of water seal (no empirical margin). A 6 in general-service seal is ≈ 0.22 psi; flare-gas-recovery seals run 20–30 in and staging seals 100 in or more. The only upper limit is that the break pressure stays within the flare header's maximum allowable backpressure (API 521).