API 421 Oil-Water Separator Sizing Calculator

Size gravity oil-water separators per API Publication 421: calculate droplet rise velocity, chamber cross-sectional area, required plan surface area, and chamber dimensions.

Design Wastewater Flow & Oil Properties

Design Flow Rate Q (GPM):

Water Temperature (°F):

Oil Specific Gravity (So): Crude/Fuel Oil ~ 0.85 - 0.92

Target Oil Droplet Size (µm): API 421 Standard: 150 µm

Basin Geometry Constraints

Liquid Water Depth d (ft): API 421 limits: 3 to 8 ft

Number of Parallel Channels: Min 2 for maintenance

Turbulence & Short-Circuit Factor F:

API 421 Basin Sizing & Hydraulics

Total Plan Surface Area Required

-- sq ft

-- sq ft per channel

Oil Droplet Rise Rate (Vt)

-- ft/min

-- m/h

Horizontal Velocity (Vh)

-- ft/min

API Max Limit: 3.0 ft/min

Channel Dimensions (L x W x D): -- ft x -- ft x -- ft

Channel Width-to-Depth Ratio: -- (API standard: 2.0 to 3.0)

Hydraulic Retention Time (HRT): -- min

Vh / Vt Ratio Check: -- (API max: 15.0)

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Frequently Asked Questions

What is the basis for API 421 oil-water separator design?

API Publication 421 defines standards for sizing gravity oil-water separators in refinery and petrochemical applications. It specifies Stokes Law rise rates for free oil droplets (conventionally 150 microns), limits horizontal water velocity to 3.0 ft/min (to prevent scour), and adds turbulence and short-circuiting factors F.

Why is the horizontal velocity limited to 3.0 ft/min?

When wastewater flow velocity exceeds 3.0 ft/min (0.91 m/min) or 15 times the droplet rise rate (Vh > 15 Vt), hydraulic turbulence at the oil-water interface shears rising oil droplets and re-entrains them into the effluent discharge.

Can an API 421 separator remove emulsified oil?

No. API gravity separators only remove free, non-emulsified floating oil droplets down to approximately 150 microns. Dissolved hydrocarbons and chemically or mechanically emulsified oil droplets (<20 microns) require coalescing plate media, dissolved air flotation (DAF), or chemical coagulants.