Separator Internals Design Fundamentals

1. Separation Principles

Gas-liquid separation in production and processing vessels relies on three sequential mechanisms, each requiring specific internal components to function effectively. Understanding these mechanisms is essential for proper internals selection.

Primary separation

Inlet device

Bulk liquid removal at the inlet. Reduces velocity, changes flow direction, and separates large droplets from the gas stream.

Secondary separation

Gravity settling

Intermediate droplets settle by gravity in the open vessel section. Gas velocity must be low enough for settling to occur.

Final separation

Mist elimination

Fine mist droplets captured by impaction, interception, or coalescence in wire mesh pads, vane packs, or coalescing elements.

Droplet Separation Physics

The Souders-Brown equation governs the maximum allowable gas velocity in a separator. Droplets larger than the target removal size must have sufficient residence time to settle against the upward gas flow:

Souders-Brown Equation: V_max = K_SB × SQRT[(ρ_L - ρ_G) / ρ_G] Where: V_max = Maximum allowable gas velocity (ft/s) K_SB = Souders-Brown coefficient (ft/s) ρ_L = Liquid density (lb/ft³) ρ_G = Gas density (lb/ft³) K_SB depends on the internal device type and droplet size target.

K_SB Values by Device Type

Device	K_SB (ft/s)	Droplet Removal (μm)	Application
Open vessel (no internals)	0.12–0.18	300–500	Bulk separation only
Wire mesh demister	0.18–0.35	10–20	Standard mist elimination
Vane pack (horizontal flow)	0.15–0.25	15–40	High liquid load, fouling service
Coalescing elements	0.04–0.10	0.1–3	Fine mist, compressor protection
Multi-cyclone bundle	0.20–0.40	5–15	High-pressure, compact vessels

Design rule: Higher K_SB values allow higher gas velocities and smaller vessels, but at the cost of larger minimum droplet removal size. Select the K_SB value based on the downstream process requirements, not simply to minimize vessel size.

2. Inlet Devices

The inlet device is the first internal component encountered by the incoming gas-liquid mixture. Its purpose is to reduce the inlet velocity, distribute the flow, and perform initial bulk liquid separation. Inlet device selection is primarily driven by the inlet momentum parameter.

Inlet Momentum Parameter

Inlet Momentum: ρ_mV² = ρ_mixture × V_inlet² Where: ρ_m = Mixture density at inlet nozzle (lb/ft³) V = Gas velocity at inlet nozzle (ft/s) Units: lb/(ft·s²) -- commonly expressed in non-dimensional form Low momentum: ρV² < 1,000 Moderate momentum: 1,000 < ρV² < 4,000 High momentum: ρV² > 4,000

Inlet Device Selection

Device Type	ρV² Range	Separation Efficiency	Pressure Drop
No device (bare pipe)	< 500	Poor	Negligible
Diverter plate (deflector baffle)	< 1,500	Fair	Low
Half-pipe (half-open pipe)	< 2,000	Fair to Good	Low
Inlet vane (curved vane)	< 6,000	Good	Moderate
Inlet cyclone	< 8,000	Very Good	Higher
Schoepentoeter	< 8,000	Very Good	Moderate

Diverter Plate (Deflector Baffle)

The simplest inlet device. A flat or curved plate positioned opposite the inlet nozzle to redirect the gas flow downward and separate bulk liquids by impaction:

Plate positioned 6–12 inches from the inlet nozzle
Plate diameter: 1.5–2.0 times the inlet nozzle diameter
Suitable for low liquid loading and low inlet momentum
Poor distribution across the vessel cross-section

Inlet Vane Distributor

A set of curved vanes that gradually turn the inlet flow from horizontal to downward, distributing it across a larger area. This is the most commonly specified inlet device for midstream separators:

Handles moderate to high inlet momentum (ρV² up to 6,000)
Good flow distribution across vessel cross-section
Effective liquid separation by centrifugal force on curved vanes
Self-draining design prevents liquid re-entrainment
Available from multiple manufacturers (e.g., Sulzer, Koch-Glitsch, Peerless)

Inlet Cyclone

A cyclonic inlet device uses centrifugal force to separate liquid droplets from the gas stream at the inlet. Best for high-momentum, high-liquid-loading applications:

Highest separation efficiency of all inlet devices
Handles slugging and high liquid volume fractions
Higher pressure drop than vane distributors (0.5–2.0 psi)
Requires dedicated liquid drain to the vessel sump

Selection rule: For most midstream gas processing separators, an inlet vane distributor is the standard choice. It provides good separation efficiency, excellent flow distribution, and moderate pressure drop. Use an inlet cyclone only when inlet momentum is very high (ρV² > 6,000) or when slug flow is expected.

3. Gravity Settling Section

The gravity settling section is the open region between the inlet device and the mist eliminator. In this zone, intermediate-sized droplets (100–500 microns) settle by gravity against the upward gas flow. Proper sizing of this section is critical for overall separation performance.

Residence Time Requirements

Service	Gas Residence Time (s)	Notes
Gas scrubber (clean gas)	3–5	Minimal liquid; focus on mist elimination
Production separator (two-phase)	5–10	Standard gas-liquid separation
Production separator (three-phase)	5–10	Gas section; liquid section sized separately
Compressor suction scrubber	5–8	Clean gas delivery to compressor critical
Slug catcher	10–30	Must handle slug volume plus separation

Internal Baffles

Perforated baffles or flow straighteners may be installed in the gravity section to improve flow distribution and prevent short-circuiting:

Perforated plates: 40–60% open area. Straighten gas flow and prevent channeling. Install at 1/3 and 2/3 of the gravity section length.
Flow straighteners: Honeycomb or tube bundle type. Reduce turbulence and promote laminar settling. Used in high-efficiency separators.
Anti-foam baffles: Inclined plates in the liquid section that break foam and prevent foam carry-over into the gas outlet.

Liquid Collection

Liquid separated in the gravity section must drain to the liquid sump without re-entrainment:

Downcomers or drain pipes from the inlet device and mist eliminator to the liquid section
Liquid drain pipes should be sized for 1–2 ft/s liquid velocity maximum
Drain pipes must extend below the minimum liquid level (LLL) to prevent gas bypassing
Vortex breakers on drain pipe outlets to prevent gas pull-through

Velocity check: The actual gas velocity in the gravity section should not exceed 75% of the Souders-Brown maximum velocity. This provides margin for flow maldistribution, foaming, and operating variations.

4. Mist Eliminators

Mist eliminators capture fine liquid droplets (typically 3–50 microns) that are too small to settle by gravity. They are the final separation stage before gas exits the vessel. Three main types are used in midstream service.

Wire Mesh Demister Pads

The most common mist eliminator in midstream separators. Consists of knitted wire mesh (typically 4–6 inches thick) that captures droplets by inertial impaction and direct interception:

Parameter	Standard	High-Efficiency	High-Capacity
Wire diameter	0.011 in.	0.006 in.	0.011 in.
Density	9 lb/ft³	12 lb/ft³	5 lb/ft³
Pad thickness	6 in.	6–12 in.	4–6 in.
Surface area	100 ft²/ft³	150 ft²/ft³	60 ft²/ft³
K_SB	0.24–0.28	0.18–0.22	0.30–0.35
Min. droplet removed	10 μm	3–5 μm	15–20 μm

Wire Mesh Installation

Mount horizontally in vertical vessels, vertically in horizontal vessels
Support grid must provide structural support without excessive blanking (<10% blockage)
Minimum clearance: 12 inches between pad and gas outlet nozzle
Minimum clearance: 12 inches between pad bottom and liquid level
Pad must cover at least 80% of the vessel cross-section

Vane Pack Mist Eliminators

Vane packs consist of closely-spaced corrugated metal plates that force the gas through a sinuous path. Droplets are collected on the vane surfaces by inertial impaction and drain through liquid collection pockets:

Feature	Wire Mesh	Vane Pack
Droplet removal	10–20 μm	15–40 μm
Liquid handling capacity	Low to moderate	High
Fouling resistance	Poor (plugs easily)	Good (self-cleaning)
Pressure drop	0.5–2.0 in. WC	1.0–4.0 in. WC
Re-entrainment resistance	Poor above K_SB	Good (drainage pockets)
Best for	Clean gas, low liquid	Dirty gas, high liquid, foaming

Vane Pack Configuration

Horizontal flow: Gas flows horizontally through vertical vane plates. Most common in horizontal separators. Liquid drains downward by gravity.
Vertical flow: Gas flows upward through horizontal vane plates. Used in vertical vessels. Requires drainage channels to prevent flooding.
Single pocket: Simple drainage pocket on each vane bend. Adequate for low liquid loads.
Double pocket: Two drainage pockets per bend. Better liquid handling for high-liquid-load applications.

Selection guide: Use wire mesh demisters for clean gas service with low liquid loading (K_SB = 0.24–0.28). Use vane packs when the gas contains solids, wax, hydrate inhibitors, or when liquid loading exceeds 1 gal/MMSCF. In fouling services, wire mesh pads plug rapidly and require frequent cleaning.

5. Coalescing Elements

Coalescing elements remove sub-micron to 10-micron droplets and aerosols that cannot be captured by wire mesh or vane packs. They are used in applications requiring extremely clean gas, such as compressor suction protection, fuel gas systems, and instrument gas conditioning.

Coalescing Mechanism

Coalescing filters work by three mechanisms operating simultaneously:

Inertial impaction: Large droplets (> 1 μm) cannot follow the gas streamlines around fibers and impact directly on the fiber surface.
Direct interception: Medium droplets (0.3–1 μm) follow streamlines but pass close enough to a fiber to contact it.
Brownian diffusion: Sub-micron droplets (< 0.3 μm) diffuse randomly and contact fibers by chance.

Coalescer Types

Type	Removal Rating	Max ΔP (psi)	Application
Glass fiber cartridges	0.3 μm	5–10	Compressor suction, fuel gas
Cellulose cartridges	1–5 μm	3–7	General liquid/aerosol removal
Depth-type (wound fiber)	3–10 μm	2–5	Pre-filtration, coarse coalescence
Composite multi-layer	0.1 μm	5–15	Instrument gas, high-purity applications

Coalescer Vessel Design

Flow direction: Inside-out flow through cartridge elements. Gas enters through the open bottom, flows outward through the coalescing media, and exits through the clean gas annulus.
Element spacing: Minimum 1 inch between adjacent elements. Elements must not touch to prevent bypass.
Liquid drainage: Coalesced liquid must drain downward from the element outer surface. A quiet zone below the elements allows liquid to settle.
Change-out pressure drop: Replace elements when differential pressure reaches 10–15 psi or per manufacturer recommendation.

Coalescer Applications in Midstream

Application	Target Contaminant	Element Type	Removal Target
Compressor suction scrubber	Lube oil, condensate mist	Glass fiber	< 0.1 ppmw
Fuel gas conditioning	Condensate, compressor oil	Glass fiber	< 0.3 μm
Amine contactor inlet	Hydrocarbon liquids	Cellulose	< 5 ppmw
Glycol contactor inlet	Hydrocarbon liquids	Cellulose	< 5 ppmw
Molecular sieve inlet	Liquid carryover	Glass fiber	< 0.1 ppmw

Critical application: Coalescers upstream of amine and glycol contactors prevent hydrocarbon liquid contamination that causes foaming, reduced capacity, and solvent degradation. This is one of the highest-value applications of coalescing technology in gas processing.

6. Worked Example

Select and size the internals for a vertical two-phase gas scrubber upstream of a reciprocating compressor.

Given: Service: Compressor suction scrubber Gas flow: 25 MMSCFD natural gas (SG = 0.65) Operating pressure: 400 psig Operating temperature: 80°F Liquid loading: 0.5 bbl/MMSCF (light condensate) Gas density: 1.52 lb/ft³ Liquid density: 42 lb/ft³ Inlet nozzle: 8 inches (ID = 7.981 in.)

Step 1: Calculate Inlet Momentum

Actual gas flow at conditions: Q_a = 25 x 10&sup6; / 1,440 x (14.7 / 414.7) x (540 / 460+80) = 620 ACFM Inlet nozzle area = π/4 x (7.981/12)² = 0.3474 ft² Inlet velocity = 620 / (60 x 0.3474) = 29.7 ft/s Inlet momentum = ρ_G x V² = 1.52 x 29.7² = 1,341 lb/(ft·s²)

Step 2: Select Inlet Device

ρV² = 1,341 (moderate momentum) Selection: Inlet vane distributor - Handles ρV² up to 6,000 - Good flow distribution - Standard for compressor suction scrubbers

Step 3: Size Mist Eliminator

For compressor suction service, use wire mesh demister: K_SB = 0.25 (standard wire mesh, 9 lb/ft³ density) V_max = 0.25 x SQRT[(42 - 1.52) / 1.52] V_max = 0.25 x SQRT[26.63] V_max = 0.25 x 5.16 = 1.29 ft/s Design velocity = 75% of V_max = 0.97 ft/s Required mesh area = Q_a / (60 x V_design) = 620 / (60 x 0.97) = 10.65 ft² Required vessel ID = SQRT[4 x 10.65 / π] = 3.68 ft = 44.2 in. Select: 48-inch ID vessel (A = 12.57 ft²) Actual velocity = 620 / (60 x 12.57) = 0.82 ft/s [OK, 64% of V_max]

Step 4: Specify Wire Mesh Pad

Wire mesh pad specifications: - Type: Standard knitted wire mesh, 9 lb/ft³ - Material: 316 SS wire, 0.011 in. diameter - Pad thickness: 6 inches - Support: Welded support ring + grid bars - Droplet removal: 10 microns and larger at design velocity

Summary of Internals

Component	Specification
Vessel ID	48 inches
Inlet device	Inlet vane distributor (curved vane type)
Mist eliminator	Wire mesh, 6 in. thick, 316 SS, 9 lb/ft³
Design gas velocity	0.82 ft/s (64% of V_max)
Inlet momentum	1,341 lb/(ft·s²)
Droplet removal target	10 μm

Design check: The 48-inch vessel with inlet vane and wire mesh demister provides good separation for compressor suction service. Operating at 64% of V_max provides adequate margin. For additional protection, consider adding coalescing cartridges downstream.

7. Operations & Maintenance

Performance Monitoring

Pressure drop: Monitor differential pressure across the mist eliminator. Rising ΔP indicates fouling, plugging, or flooding.
Liquid carry-over: Check downstream equipment for liquid accumulation. Excessive carry-over indicates mist eliminator failure or capacity exceedance.
Liquid level: Maintain liquid level below the mist eliminator drain or inlet device drain. High liquid level reduces separation efficiency and can flood the mist eliminator.
Flow rate: Operating above design capacity causes re-entrainment from wire mesh pads and bypass of vane packs.

Common Problems

Problem	Cause	Solution
Wire mesh pad flooding	Excessive gas velocity or liquid load	Reduce flow; upgrade to vane pack; increase vessel size
Wire mesh fouling/plugging	Solids, wax, scale, corrosion products	Clean or replace pad; add upstream filtration
Gas carry-under (bubbles in liquid)	Vortexing at liquid outlet, high gas velocity	Install vortex breaker; increase liquid retention time
Foaming	Surfactants, fine solids, mixing at inlet	Add anti-foam baffles; reduce inlet turbulence; inject defoamer
Inlet device damage	Slug flow, erosion, vibration	Install slug catcher upstream; upgrade materials
Re-entrainment from demister	Operating above K_SB velocity	Reduce gas rate; install larger demister area

Maintenance Schedule

Task	Frequency
Monitor pressure drop across demister	Daily / Continuous
Check liquid level controls and dump valves	Weekly
Inspect inlet device for erosion or damage	Annual turnaround
Clean or replace wire mesh pad	Annual or as indicated by ΔP
Inspect vane pack for corrosion or buildup	Annual turnaround
Replace coalescing cartridge elements	Based on ΔP (typically 6–12 months)
Internal vessel inspection	Per jurisdiction / API 510 schedule

Replacement tip: When replacing wire mesh demister pads, inspect the support grid and hold-down grid for corrosion. A corroded support grid can sag under the weight of a saturated pad, creating gaps that allow gas bypass around the demister.

Separator Internals Design

99%+ removal

ρV² < 1,500

API 12J / GPSA Ch. 7

Contents

Use this guide when you need to: