KO Drum Sizing Calculator

Method: Liquid droplet settling in gas using drag correlation (iterative Cd), then allowable superficial gas velocity = SF × V_t (safety factor). Mist eliminator face velocity check included (mesh pad). Retention time gives liquid holdup volume.

Mode & Units

KO Drum Orientation

Units System

Gas Conditions at KO Drum

Pressure, P kPa(a)

Temperature, T °C

Molecular Weight, MW kg/kmol

Compressibility, Z (≈1)

Gas Mass Flow, ṁ_g kg/h

Gas Viscosity, μ_g Pa·s

Liquid / Droplet Separation

Liquid Density, ρ_L kg/m³

Droplet Size, d µm

Safety Factor on Settling (SF) (0–1)

Cd Model

Retention Time & Liquid Flow

Liquid Mass Flow, ṁ_L kg/h

Retention Time min

Mist Eliminator (Mesh Pad)

Include Mist Eliminator?

Max Face Velocity m/s

Horizontal Options (only)

Normal Liquid Level Height (fraction of ID)

Min L/D Ratio (typical ≥3)

Gas Properties & Flows

Gas Density, ρ_g: -

Gas Vol. Flow, Q_g: -

Separation (300 µm default)

Terminal Settling Velocity, V_t: -

Allowable Superficial Gas Velocity, V_allow: -

Required Vapor Area, A_req: -

Mist Eliminator Check

Demister Required Area: -

Demister Available Area: -

Vessel Sizing (Preliminary)

Selected Internal Diameter (ID): -

Liquid Holdup Volume: -

Estimated Length/Height: -

Gas density: ρg = (P·MW)/(Z·R·T) (P in kPa, MW kg/kmol, R=8314.462618 kPa·m³/(kmol·K), T in K)
Droplet settling (iterative): Vt = sqrt( (4 g d (ρL−ρg)) / (3 Cd ρg) )
Allowable velocity: Vallow = SF · Vt
Area: Areq = Qg / Vallow (Qg in m³/s)
Liquid holdup: Vhold = (ṁL/ρL)·tret

Technical Notes

Purpose: Preliminary sizing / screening for a flare or relief KO drum based on droplet settling (default 300 µm) and optional mesh-pad face velocity check. Final design must be verified with project standards, detailed relief/flare hydraulics, and vendor internals.

Variable Definitions

P: KO drum gas pressure at separation section (absolute).
T: KO drum gas temperature at separation section.
MW, Z, μg: gas molecular weight, compressibility, viscosity.
ṁg, Qg: gas mass flow and corresponding volumetric flow at KO conditions.
ρg, ρL: gas density and liquid density at KO conditions.
d: droplet diameter to be removed (default 300 µm screening).
Vt: terminal settling velocity for the selected droplet size.
SF: safety factor applied to Vt; Vallow = SF·Vt.
Areq: required vapor cross‑sectional area from settling.
Demister: optional mesh‑pad area check using Vface,max.
ṁL, tret: liquid mass flow and retention time for holdup volume.

Formulas / Logic

Gas density (real-gas ideal form): ρg = (P·MW)/(Z·Ru·T) with P in kPa, MW in kg/kmol, Ru = 8314.462618 kPa·m³/(kmol·K), T in K
Gas volumetric flow: Qg = ṁg / ρg
Droplet settling (sphere, force balance with drag): Vt = √( (4 g d (ρL−ρg)) / (3 Cd ρg) )
Reynolds number: Re = ρg·Vt·d / μg
Drag coefficient (screening): Schiller–Naumann for Re<1000, Cd = (24/Re)(1+0.15 Re^0.687); for higher Re → Cd≈0.44
Allowable superficial velocity: Vallow = SF·Vt
Required vapor area (settling): Areq = Qg / Vallow
Demister (mesh pad) check (screening): Adem,req = Qg / Vface,max (adjusted by assumed free area)
Liquid holdup volume: Vhold = (ṁL/ρL)·tret

Assumptions / Notes (Screening)

Assumes spherical droplets and steady settling (no coalescence/foaming/entrainment effects beyond SF).
Default droplet size is 300 µm (common KO drum screening). Finer removal typically requires a mist eliminator and vendor rating.
Horizontal sizing uses the specified normal liquid level fraction to compute available vapor area at NLL.
Demister check is a simplified face-velocity screening; final mesh pad selection depends on service, liquid loading, and vendor data.
Final mechanical design (nozzles, inlet device, vane pack, internals clearances, surge volumes) must follow project specs and vendor practice.
Reference anchors: API 521 (flare/relief system design guidance, KO drum screening practice), GPSA Engineering Data Book (two‑phase separation background), and common separator design practice (drag/settling correlations).

🛢️ Knockout Drum Sizing (Relief & Flare)