Flare Seal Drum — Engineering Fundamentals
API 521 / 537 liquid-seal sizing for flashback prevention and stack-pulsation isolation.
1. Role of the seal drum
A liquid seal drum sits at the base of every refinery / midstream flare stack with two functions:
- 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.
- 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. Anti-pulsation velocity
Industry sets a maximum gas velocity through the drum of 70 ft/s. Above this, gas bubbling through the water seal becomes chaotic and pulls droplets up into the stack (bad: water tower on flame tip). Cross-section sizing:
3. Seal break pressure
The seal "breaks" (gas bubbles through) when header pressure exceeds the water column + 1 inch margin. Seal depth 6–12 in is typical → break P ≈ 7–13 in WC ≈ 0.25–0.5 psi. The flare header must be designed for this static head plus any normal-operating backpressure.
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 521 — Pressure-Relieving and Depressuring Systems.
- API Std 537 — Flare Details for General Refinery and Petrochemical Service.
- NFPA 30 — companion fire-code context.