Separator / KO Drum Sizing

Quick pick (TL;DR)

Vertical for low liquid / on-off; horizontal for high liquid load & better separation.
Get the max vapour velocity from Souders-Brown: Vmax = K·√((ρL−ρV)/ρV), with K ≈ 0.107 m/s for a mesh pad.
The vessel diameter comes from vapour flow ÷ Vmax; the length/level from the required liquid holdup time (typically 5–10 min).
Add a mist eliminator and a proper inlet device.

Step 1 — Vertical vs horizontal

	Vertical	Horizontal
Best for	Low liquid	High liquid / 3-phase
Separation	Good for gas	Better
Footprint	Small	Large

Step 2 — Worked diameter

Vertical 2-phase, vapour 30 t/h

Souders-Brown Vmax = K · √((ρL − ρV) / ρV) Given ρL = 600 , ρV = 15 kg/m³ , K = 0.107 m/s vapour = 30,000 kg/h Step 1 — Max velocity Vmax = 0.107 × √((600−15)/15) Vmax = 0.107 × √39 = 0.107 × 6.245 Vmax = 0.668 m/s Step 2 — Vapour flow Qv = 30,000 / 15 / 3600 = 0.556 m³/s Step 3 — Diameter A = Qv / Vmax = 0.556 / 0.668 = 0.83 m² D = √(4A/π) = √(1.058) = 1.03 m Answer → select D = 1.05 m, + 5 min liquid holdup

Vmax is the entrainment velocity — D follows from vapour flow; length from liquid holdup time.

Step 3 — Internals & holdup

Mesh pad / vane / cyclonic mist eliminator.
Proper inlet device matters a lot.
Liquid holdup ~5 min (more for slugs).

Separator diameter calculator

Souders-Brown vessel diameter

Vmax = K·√((ρL−ρV)/ρV); D from vapour flow ÷ Vmax.

K factor (m/s)

Vapour (kg/h)

ρL (kg/m³)

ρV (kg/m³)

Minimum disengagement diameter — add liquid holdup (5–10 min) for height.

Common mistakes

Diameter too small → carryover.
Poor inlet device.
Insufficient liquid holdup.

Requisition & datasheet

Type, flows, densities, P, T.
K, Vmax, D, holdup, internals.

Frequently asked questions

What should I know about Separator / KO Drum Sizing?

Vertical for low liquid / on-off; horizontal for high liquid load & better separation. Get the max vapour velocity from Souders-Brown: Vmax = K·√((ρL−ρV)/ρV), with K ≈ 0.107 m/s for a mesh pad. The vessel diameter comes from vapour flow ÷ Vmax; the length/level from the required liquid holdup time (typically 5–10 min). Add a mist eliminator and a proper inlet device.

Which standards or codes apply?

API 12J (oilfield separators), GPSA Engineering Data Book (Souders-Brown K), ASME VIII (pressure vessel), API 521 (flare KO drums).

Standards — API 12J; GPSA; ASME VIII; API 521.