Gravity Oil/Water Separator Surface Area (API 421)

an oil droplet rises at the Stokes velocity Vt = g(rho_w - rho_o)d^2/(18 mu), and the separator must let the design droplet reach the surface before the flow carries it out, so area = 1.2 x Q / Vt. A 150 micron droplet of 0.85-SG oil in 60 F water rises about 0.33 ft/min, so 50 gpm needs about 24 ft^2; 100 gpm needs 49. Colder water and smaller droplets slow the rise and demand more area, and an emulsified or dissolved oil fraction will NOT gravity-separate (it needs coalescing, DAF, or downstream treatment). A SCREEN, not a design; API 421, the manufacturer, and the engineer / AHJ govern the separator and the discharge permit.

rise_velocity Vt = g x (rho_w - rho_o) x d^2 / (18 mu) (Stokes, computed in SI, converted to ft/min); horizontal_area_ft2 = F x Q / Vt, with F ~ 1.2 (turbulence factor) and Q in ft3/min.

API Publication 421 (Design and Operation of Oil-Water Separators) gravity-separator method with Stokes' law, by name; the manufacturer and the engineer / AHJ govern.

Stokes' law is public physics; the API 421 turbulence factor (~1.2), the 150 micron design droplet, and the horizontal-velocity limits are the API 421 method, and the flow, oil SG, and viscosity come from the waste stream.

Estimate. Operator of record and primacy agency govern.

Field names used by the API: flow_gpm, oil_sg, droplet_micron, water_viscosity_cp, rise_velocity_ftmin, horizontal_area_ft2

• Stokes rise Vt = g(rho_w - rho_o)d^2/(18 mu) for a 150 micron droplet; area = F x Q / Vt with F ~ 1.2API 421
• Free oil only emulsified / dissolved oil does not gravity-separate; needs coalescing / DAF / downstream treatmentAPI 421 / practice