Gravity Oil/Water Separator Surface Area (API 421)
The minimum horizontal (plan) area of a gravity oil/water separator per API 421.
Example
You enter
- Design flow (gpm) 50
- Oil specific gravity 0.85
- Design droplet (micron) 150
- Water viscosity at design temp (cP) 1.1
You get
- Droplet rise velocity 0.329 ft/min
- Minimum horizontal area 30.3 ft2
Checked against published worked examples from: Applied Mechanical Technology (AMT), Water / Wastewater Division.
Details, formula, and sources
Scopean 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 = F x Q / Vt with F = Ft x 1.2, the API 421 turbulence factor Ft read at vH / Vt times a 1.2 short-circuit factor. A 150 micron droplet of 0.85-SG oil in 60 F water rises about 0.33 ft/min (F about 1.49), so 50 gpm needs about 30 ft^2; 100 gpm needs 61. 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 or tablerise_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, F = Ft x Fs with Fs = 1.2 (short-circuiting) and Ft the API 421 turbulence factor at vH / Vt (vH = 15 Vt, max 3 ft/min; Ft 1.07-1.45), Q in ft3/min.
Source and editionAPI Publication 421 (Design and Operation of Oil-Water Separators) gravity-separator method with Stokes' law, by name; the manufacturer and the engineer / AHJ govern.
Free accessStokes' law is public physics; the API 421 design factor F = Ft x 1.2 (turbulence Ft 1.07-1.45 by vH/Vt, short-circuiting 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.
What governsEstimate. Operator of record and primacy agency govern.
API field namesflow_gpm, oil_sg, droplet_micron, water_viscosity_cp, rise_velocity_ftmin, horizontal_area_ft2
- Design temperature enter the water and oil specific gravities and the water viscosity all at the design temperature (water SG 0.986 at 130 F)API 421; Applied Mechanical Technology API OWS sizing sheet
- Stokes rise Vt = g(rho_w - rho_o)d^2/(18 mu) for a 150 micron droplet; area = F x Q / Vt with F = Ft x 1.2API 421
- Free oil only emulsified / dissolved oil does not gravity-separate; needs coalescing / DAF / downstream treatmentAPI 421 / practice