Gravity Oil/Water Separator Surface Area (API 421)
The minimum horizontal (plan) area of a gravity oil/water separator per 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.
Formula and source
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.
Audience
This tile is built for water operators and the adjacent professions in the Water and Wastewater Operations group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.