Two-phase separator sizing (Souders–Brown)
Diameter of a vertical or horizontal gas–liquid separator or knock-out drum from the Souders–Brown velocity, with GPSA K factors and liquid retention time.
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How this calculator works
A gas–liquid separator is sized first for the gas: droplets settle out only if the gas moves slower than the Souders–Brown limit Vmax = K·√((ρL − ρG)/ρG). The vessel's gas flow area follows from the actual gas flow and Vmax, and the diameter is rounded up to the next 150 mm (6 in). The liquid section then holds the liquid for the retention time you choose — as a height in a vertical vessel, or as length in a horizontal vessel taken as half full of liquid (with your minimum L/D).
K factor: for a wire-mesh mist eliminator the GPSA Engineering Data Book (Section 7, Fig. 7-9) gives K = 0.35 ft/s (0.107 m/s) up to 100 psig, reduced by 0.01 ft/s for every 100 psi above that (0.33 at 300 psig, 0.30 at 600, 0.27 at 900, 0.21 at 1,500 psig) — the calculator reproduces these to 0.02 %. Horizontal vessels with a vertical pad take 0.40 ft/s with the same pressure correction (GPSA range 0.40–0.50). With no mist eliminator, half the mesh-pad value is used as a cautious screening figure.
Example: 100,000 Nm³/h of gas (M = 20, Z = 0.92) at 30 bar g and 40 °C — 25.9 kg/m³, 3,446 actual m³/h — with 10 m³/h of 800 kg/m³ liquid and 3 minutes' retention, in a vertical vessel with a mesh pad: K = 0.0965 m/s, Vmax = 0.527 m/s, minimum diameter 1.52 m, so 1,650 mm; the liquid hold-up of 0.5 m³ needs 0.23 m of height. Limits: a screening size for studies and checks — inlet devices, nozzle sizes, level settings, slug volumes and the mist-eliminator rating come from your vendor and company standards (API 12J for oil and gas separators). References: GPSA Engineering Data Book, 13th ed., Section 7; Souders and Brown, Ind. Eng. Chem. 26 (1934) 98; API Specification 12J.