Separator Sizing: Step-by-Step Using API 12J and Souders-Brown

Most process engineers know the theory of separator design. Very few can sit down and actually size one without looking everything up. This post gives you the sizing workflow, the key equations, and the decision points — all in one place.

What API 12J Covers

API 12J is the primary standard for oilfield separator design. It specifies minimum wall thickness, materials, safety relief, testing, and — most importantly — the sizing methodology for the gas section of a separator. It does not specify every detail of liquid section sizing, which relies on residence time criteria from your project datasheet and company engineering practices.

Step 1: Establish the Input Data

Before touching any equation, collect:

• Gas flow rate (MMscfd or Nm³/hr at standard conditions)
• Operating temperature and pressure (T_op, P_op)
• Gas density at operating conditions (ρ_g)
• Liquid density (ρ_L) — oil and/or water
• Gas-to-Oil Ratio (GOR)
• Liquid flow rates (oil + water in m³/hr or BLPD)
• Design pressure and temperature (add margins per API 12J)

Step 2: Calculate Gas Velocity Using the Souders-Brown Equation

The maximum allowable gas velocity is calculated by:

V_terminal = K × √[(ρ_L − ρ_g) / ρ_g]

Where:

• V_terminal = terminal settling velocity of the largest acceptable liquid droplet (m/s or ft/s)
• K = Souders-Brown factor (empirical constant from API 12J / GPSA)
• ρ_L = liquid density (kg/m³ or lb/ft³)
• ρ_g = gas density at operating conditions (kg/m³ or lb/ft³)

For a vertical separator with a wire mesh mist eliminator, K is typically 0.107 m/s (0.35 ft/s). Without a mist eliminator, apply a 0.5 de-rating factor. For horizontal separators, use 75–85% of the vertical K value to account for the horizontal flow path.

Step 3: Calculate the Minimum Vessel Cross-Section for Gas

Once you have the allowable gas velocity, calculate the minimum cross-sectional area needed:

A_gas = Q_gas_actual / V_allowable

Where Q_gas_actual is the actual volumetric gas flow rate at operating conditions (m³/s). For a vertical separator, this area equals the full vessel cross-section (πD²/4). For a horizontal separator, it equals 50% of the cross-section (liquid occupies the other half).

Step 4: Calculate Required Vessel Diameter

From the cross-section area:

D = √(4 × A_gas / π)   [for vertical, full cross-section]
D = √(4 × A_gas / 0.5π)   [for horizontal, half cross-section]

Round up to the nearest standard vessel diameter (typically in 6-inch increments for small vessels, 12-inch for larger).

Step 5: Check Liquid Residence Time

The liquid section must provide enough retention time for oil-water separation. API 12J recommends:

Calculate the liquid volume required: V_liq = Q_liq × t_residence. Check that the vessel diameter and the allocated liquid section height (or length for horizontal) provide this volume. If not, increase vessel size.

Step 6: Check L/D Ratio

For horizontal separators, the L/D ratio must be between 3 and 5. Below 3, gas velocity at the inlet is too high. Above 5, the vessel is uneconomical. Iterate diameter and length until you satisfy both the gas velocity criterion and the L/D constraint simultaneously.

Most Common Sizing Mistakes

• Using standard-condition gas flow rates instead of actual operating-condition volumetric flow rates — this alone can cause a 20–40% error in diameter calculation.
• Applying the same K-factor for vertical and horizontal separators.
• Forgetting to account for slug volume in the liquid surge calculation.
• Not applying K-factor corrections for foaming services or amine/glycol solutions (typically 0.6× correction).

FAQ

What is the Souders-Brown K factor for a compressor suction scrubber?

API 12J recommends applying a 0.7 de-rating to the base K-factor for compressor suction separators. This is because liquid carry-over into a compressor is far more damaging than in other services — it can destroy valve plates and cause liquid slugging in the cylinder.

Do I use API 12J for all separator types?

API 12J is specific to oilfield separators. For refinery and petrochemical separators, GPSA Engineering Data Book and process licensor data sheets are more commonly referenced. The sizing methodology is similar but not identical.