Horizontal flare KO drum sizing
A flare knock-out drum separates liquid droplets from the vapour stream and provides liquid holdup. This calculator applies the API 521 droplet-settling method to a horizontal drum, comparing the time available for a droplet to settle with vapour residence time.
The stated droplet basis is 300–600 micrometres, with 20–30 minutes of liquid holdup. Horizontal geometry is used to find liquid depth and available vapour-flow area; a vertical drum requires a different geometry and is not evaluated here.
Design inputs
All calculations use SI internally.
Settling check
Residence time versus dropout time
Vapour residence time exceeds dropout time.
- Dropout velocity, Uc
- 0.729 m/s
- Vapour velocity, UV
- 1.672 m/s
- Dropout time, tdrop
- 2.20 s
- Residence time, tres
- 4.49 s
- Vapour flow, QV
- 5.556 m³/s
- Required liquid volume, VL
- 11.890 m³
- Liquid depth, hL
- 0.898 m
- Liquid depth / diameter, hL/D
- 0.359 (35.9%)
Liquid volume occupies the cylindrical tangent-length section. PASS is a settling-time check, not a complete drum design approval.
How to use the calculator
- Set the flow basisSelect SI or US units. Enter vapour mass flow, vapour and liquid densities, and vapour viscosity. Switching units converts the displayed values.
- Define separation & holdupEnter the design droplet diameter, liquid inflow, holdup time and miscellaneous liquid volume.
- Check the drum geometryEnter inside diameter and tangent length. Results update as you type. Review liquid depth, vapour velocity and the settling-time ratio.
- Find a passing sizeSet the auto-size L/D and select Auto-size D & L. It finds the smallest passing diameter on a 0.05 m SI grid or a 2 in US grid, with L = (L/D) × D.
Calculation method
1. Droplet drag & terminal velocity
Cd(Re) = (24/Re)(1 + 0.15Re0.687) + 0.42/(1 + 42500Re−1.16)
Cd(Re) × Re² = X; C = Cd(Re)
Uc = 1.1547√[gDp(ρL − ρV)/(ρVC)]
g = 9.80665 m/s². Re is solved by bisection on [10⁻⁶, 10⁷]. Droplet diameter is converted from µm to m by multiplying by 10⁻⁶; viscosity is converted from cP to Pa·s by multiplying by 0.001.
2. Flow & liquid inventory
VL = QL × holdup time / 60 + miscellaneous volume
AL = VL / L
AV = πD²/4 − AL
Mass flow is in kg/h, liquid inflow is in m³/h and holdup time is in minutes. This gives QV in m³/s and VL in m³. If AL ≥ πD²/4, the drum is reported as full.
3. Liquid depth & settling check
A(h) = R² acos((R − h)/R) − (R − h)√(2Rh − h²)
A(hL) = AL
UV = QV / AV
tdrop = (D − hL) / Uc
tres = L / UV
PASS when tres ≥ tdrop
Liquid depth is solved by bisection between zero and D. The displayed ratio is tres/tdrop; a passing settling check has a ratio of at least 1.
4. SI / US conversion
kg/m³ = lb/ft³ × 16.018463
m = ft × 0.3048
m³ = US gal × 0.003785412
m³/h = US gpm × 0.2271247
Every input is converted to SI before calculation. US velocity output is m/s ÷ 0.3048, vapour volume flow is m³/s ÷ 0.3048³, and liquid volume is m³ ÷ 0.003785412.
The equivalent US expression for X is 0.95 × 10⁸ × ρV × Dp³ × (ρL − ρV) / μ², using ft, lb/ft³ and cP. The calculator uses the SI expression in both modes.
Reference case: 50,000 kg/h vapour, densities 2.5 and 500 kg/m³, viscosity 0.01 cP, droplets 300 µm, liquid inflow 20 m³/h, holdup 30 min, miscellaneous liquid 1.89 m³, D = 2.5 m and L = 7.5 m. It gives PASS, with tdrop ≈ 2.20 s and tres ≈ 4.49 s. Auto-size at L/D = 3 gives D = 1.95 m and L = 5.85 m.
Frequently asked questions
What droplet diameter should I use?
The API 521 basis given for this method is 300–600 micrometres. The calculator defaults to 300 micrometres; enter the diameter applicable to your sizing basis.
How is liquid holdup included?
Required liquid volume is liquid inflow multiplied by holdup time, plus miscellaneous liquid volume. API 521 suggests 20–30 minutes of holdup. The calculator uses this volume to determine the liquid level and the remaining vapour-flow area.
What does PASS mean?
PASS means vapour residence time is at least the calculated droplet dropout time. A drum whose required liquid volume fills its cylindrical section is reported as full and does not pass.
Can this calculator size a vertical or split-flow drum?
No. The calculation uses the circular-segment liquid area and axial vapour residence time of a horizontal drum. It does not check vertical or split-flow drums, re-entrainment, nozzle sizing or liquid-level alarms.