API 421 Oil-Water Separator Sizing & Stokes Settling Calculator

Determine horizontal rectangular separator surface area, width-to-depth ratios, residence time, and Stokes rise velocity per API Publication 421 standards.

Design Flow & Wastewater Properties

Water Flow Rate Q (m³/h):

Target Oil Droplet Cut Size d_o (µm): API 421 standard design = 150 µm

Wastewater Temperature (°C):

Oil Specific Gravity (SG):

Water Depth d (m): Recommended: 1.0 m to 2.5 m

Number of Parallel Channels:

Basin Dimensions & Hydraulic Check

Stokes Droplet Rise Velocity V_t:

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Required Total Surface Area A_h:

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Channel Width B (each):

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Channel Length L:

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Horizontal Flow Velocity V_h:

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Retention Time:

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Frequently Asked Questions

What droplet cut size is standardized by API 421?

API 421 specifies standard design for recovering free oil droplets of 150 µm (0.015 cm) diameter and larger. For smaller dispersed droplets (10 to 50 µm) or chemically stabilized emulsions, downstream coalescers, DGF, or membranes are required.

Why does API 421 mandate an L/B ratio of at least 4:1?

A length-to-width ratio of at least 4:1 promotes plug-flow hydraulics, suppresses transverse eddies, and minimizes dead zones and short-circuiting across the settling chamber.