A Guide to Gas-Liquid Separator Sizing

1. Introduction and Principles

Gas-liquid separators are fundamental to process industries, used to separate mixed-phase fluid streams into individual gas and liquid phases (including immiscible liquids like oil and water). Their primary purpose is to prepare fluids for reliable transport and to protect downstream equipment (pumps, compressors) from damage caused by multi-phase flow.

Separation is achieved through three main principles: momentum, gravity settling, and coalescing. A well-designed separator must balance these principles against practical constraints like vessel size and cost.

2. The Separator Design Workflow

A robust design follows a logical sequence, ensuring each part of the vessel can handle the process load before passing it to the next stage.

1. Define Design Basis: Establish operating conditions, compositions, and performance targets (e.g., 99% removal of droplets > 10 µm).
2. Select Vessel Orientation: Decide between vertical (good for small footprints, solids) or horizontal (better for large liquid volumes, three-phase separation).
3. Size Inlet: Design the inlet nozzle and device to manage incoming momentum.
4. Size Gas Section: Ensure sufficient space for gravity settling of liquid droplets from the gas.
5. Size Liquid Section: Provide adequate volume for liquid residence time (degassing, level control) and surge capacity.
6. Optimize Dimensions: Adjust diameter (D) and length (L) to find a cost-effective L/D ratio.
7. Size Outlets: Ensure nozzles are sized to prevent vortexing or gas entrainment.

3. Inlet Sizing and Design

3.1 The Criticality of Inlet Momentum

The first stage of separation occurs at the inlet. High-velocity feed entering a vessel can shatter liquid droplets into a fine mist that is difficult to separate, or it can churn the liquid pool, causing re-entrainment. The key design parameter is inlet momentum (or dynamic pressure), defined as ρv² (density × velocity squared).

3.2 Inlet Piping Configuration

The piping upstream of the separator is just as critical as the nozzle itself. A poor piping layout (e.g., bends close to the inlet) can create highly irregular flow regimes (slug flow) that overwhelm the separator, regardless of its internal design.

Best Practice: Provide a straight run of horizontal pipe for at least 10 pipe diameters (10D) immediately upstream of the inlet nozzle. This allows the flow regime to stabilize (ideally into stratified flow) before entering the vessel.

4. Gas Handling Capacity

4.1 Vertical Separators (Gravity Settling)

In a vertical vessel, gas flows upward. For a liquid droplet to settle out, its terminal settling velocity (Vt) must be greater than the upward gas velocity (Vg).

The maximum allowable gas velocity is typically calculated using the Souders-Brown equation:

• Vg,max = Maximum superficial gas velocity (m/s)
• K = Empirical "K-factor" (m/s), depending on the vessel type and internals.
• ρL, ρg = Liquid and Gas densities (kg/m³)

4.2 Horizontal Separators

In a horizontal vessel, gas flows horizontally while droplets fall vertically. Separation requires that the time it takes for the gas to travel the length of the vessel is *longer* than the time it takes for a droplet to fall from the top of the vessel to the liquid surface.

Horizontal separators generally have higher gas handling capacities than vertical ones of the same diameter because the gas flow path is perpendicular to gravity.

5. Liquid Handling Capacity

5.1 Residence Time (Degassing & Surge)

The liquid section must be large enough to provide sufficient residence time for two purposes:

1. Degassing: Allowing entrained gas bubbles to rise out of the liquid.
2. Surge/Control: Providing a buffer volume to allow instruments and operators to react to process upsets (e.g., slug arrivals, pump trips).

5.2 Three-Phase (Liquid-Liquid) Separation

For separating two immiscible liquids (e.g., oil and water), residence time is critical to allow droplets of one phase to settle out of the other. This is governed by Stokes' Law.

A typical design uses a "boot" or a weir to separate the phases. The required residence time can range from 5 minutes (light, easy-to-separate fluids) to over 30 minutes (heavy, viscous emulsions). Coalescing internals (plate packs, mesh) are often used to reduce the required vessel size by promoting faster droplet growth and settling.

6. Vessel Geometry and Layout

6.1 L/D Ratio

The Length-to-Diameter (L/D) ratio is a key economic factor. A vessel that is too long and skinny may be cheaper to build but harder to support and control. A vessel that is too short and fat requires thicker walls and larger heads.

• Typical L/D Range: 2.5 to 4.0 is a common economic sweet spot.
• High Pressure (>50 bar): Tends toward longer, narrower vessels (higher L/D, e.g., 4.0 - 6.0) to minimize wall thickness.

6.2 Nozzles and Connections

• Feed Inlet: Located to allow maximum separation space; should not impinge directly on liquid level or other internals.
• Gas Outlet: Located far from the inlet to prevent "short-circuiting" of gas flow.
• Liquid Outlets: Must be equipped with vortex breakers to prevent gas from being sucked into the outlet line (entrainment) when the liquid level is low.

7. Internals Selection Guide

Selecting the right internals is a trade-off between performance, cost, and fouling tolerance.