Safety & relief · Two-phase KO drum · API 521 §5.4.2
Horizontal Flare Knockout Drum
Sizes a horizontal KO drum for two-phase relief by droplet settling (drag-iteration terminal velocity) and liquid holdup, per API 521 §5.4.2. Half-full assumption; reports vapor-settling vs liquid-limited geometry.
Vapor
kg/h
°C
kPaa
cP
Liquid & Geometry
kg/h
kg/m³
µm
min
Engineering Basis
Terminal velocity vt from the drag-coefficient method (Schiller-Naumann Cd-Re iteration).
Half-full drum: D = √(4Qv/(π·(L/D)·vt)), L = (L/D)·D; droplet falls ≤ D/2 over length L.
Liquid holdup: required = QL·thold vs available (π/8)D²L; length extended if liquid-limited.