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:
Vmax = KSB × SQRT[(ρL - ρG) / ρG]
Where:
Vmax = Maximum allowable gas velocity (ft/s)
KSB = Souders-Brown coefficient (ft/s)
ρL = Liquid density (lb/ft³)
ρG = Gas density (lb/ft³)
KSB depends on the internal device type and droplet size target.
KSB Values by Device Type
| Device | KSB (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 KSB values allow higher gas velocities and smaller vessels, but at the cost of larger minimum droplet removal size. Select the KSB value based on the downstream process requirements, not simply to minimize vessel size.
2. Overview & Selection
Separator internals enhance separation performance beyond what the vessel alone can achieve. Proper selection and sizing of internals is critical to meeting outlet specifications and protecting downstream equipment from liquid carryover.
Internal Functions
Inlet zone
Primary Separation
Inlet devices remove bulk liquid using momentum change and impingement.
Gravity zone
Settling Section
Open vessel section allows droplet settling by gravity and gas-liquid disengagement.
Outlet zone
Mist Elimination
Final polishing removes fine mist before gas exits the separator.
Liquid zone
Collection & Control
Vortex breakers, baffles, and weirs manage liquid collection and drainage.
Internal Selection Guide
| Application | Inlet Device | Mist Eliminator | Auxiliary |
|---|---|---|---|
| Production separator (low GOR) | Diverter plate | Wire mesh pad | Vortex breaker |
| Production separator (high GOR) | Half-pipe inlet | Vane pack | Wave breaker |
| Test separator | Cyclonic inlet | Mesh + vane | Coalescing plates |
| Compressor suction scrubber | Vane-type inlet | Vane pack | Drain pot |
| 3-phase separator | Spreader plate | Wire mesh | Weir, coalescing plates |
| Flare KO drum | Tangential inlet | None or mesh | Large sump |
Design Considerations
- Gas velocity: Internals must be sized for actual gas velocity at operating conditions
- Liquid loading: High liquid rates may require first-stage separation before mist eliminator
- Foaming: Foam-prone fluids need defoaming baffles and larger settling zones
- Solids: Particle-laden gas may foul mesh pads; consider vane or cyclone alternatives
- Pressure drop: Each internal adds ΔP; total separator ΔP must be acceptable
- Maintenance: Internals require inspection and replacement access
Selection principle: Match internals to the separation challenge. Simple applications (clean gas, moderate liquid) need only basic internals. Difficult separations (high velocity, fine mist, solids) require more sophisticated devices with corresponding higher cost and maintenance.
3. Mist Eliminators
Mist eliminators are the final line of defense against liquid carryover. They capture fine droplets (typically 3-100 microns) that cannot settle by gravity in the available residence time.
Wire Mesh Pad (Demister)
Wire Mesh Pad Design:
Construction:
- Random knitted wire mesh (stainless steel, Monel, or polymer)
- Wire diameter: 0.006-0.011 inches (0.15-0.28 mm)
- Pad density: 5-12 lb/ft³ (80-192 kg/m³)
- Void fraction: 97-99%
- Standard thickness: 4-6 inches (100-150 mm)
Souders-Brown velocity:
v_max = K × √[(ρ_L - ρ_g) / ρ_g]
K-factor for mesh pad:
- Standard mesh: K = 0.35 ft/s (0.107 m/s)
- High-capacity mesh: K = 0.40-0.42 ft/s
- Co-knit (wire + fiber): K = 0.30-0.35 ft/s
Pressure correction (GPSA):
K_corr = K × [1 - 0.000035 × (P - 100)]
Where P = operating pressure in psia
Apply for P > 100 psia
Pressure drop:
ΔP = 0.2-1.0 in H₂O per inch of mesh depth (clean)
ΔP increases with liquid loading and fouling
Mesh Pad Efficiency
| Droplet Size | Standard Mesh | High-Efficiency Mesh | Co-Knit Mesh |
|---|---|---|---|
| > 10 microns | 99%+ | 99.9% | 99.9% |
| 5-10 microns | 95% | 99% | 99% |
| 3-5 microns | 80% | 95% | 97% |
| 1-3 microns | 50% | 80% | 90% |
Mesh Pad Limitations
- Flooding: At high gas velocity or liquid loading, mesh becomes saturated and liquid is re-entrained
- Fouling: Solids and viscous liquids clog mesh, increasing ΔP and reducing efficiency
- Foam: Foam can plug mesh and cause severe carryover
- Turndown: Efficiency drops at low velocity (reduced impaction)
- Mechanical damage: High-velocity slugs can collapse or tear mesh
Mesh Pad Installation
Installation Guidelines:
Horizontal vessel:
- Mount on support ring at top of vessel
- Full vessel diameter coverage
- Minimum 12" clearance above liquid level
- 6-12" above pad to gas outlet nozzle
Vertical vessel:
- Mount perpendicular to gas flow
- Full cross-section coverage
- Minimum 6" below gas outlet
- Support from below with grid
Support structure:
- Lower grid: 2" × 2" mesh, 10 gauge (carries weight)
- Upper grid: 4" × 4" mesh, 10 gauge (holds pad in place)
- Both grids welded to support ring
Sealing:
- No gaps between pad and vessel wall
- Use compression seal or welded band
- Bypass = carryover
Practical tip: Wire mesh pads are the most common mist eliminator due to low cost and good performance. However, they are easily damaged by slugs and fouled by solids. For difficult services, consider vane packs or cyclones instead.
4. Vane Separators
Vane separators use multiple parallel plates with directional changes to capture droplets by impingement. They offer higher capacity, better slug tolerance, and greater reliability than mesh pads for demanding applications.
Vane Pack Design
Vane Separator Principles:
Operating mechanism:
1. Gas enters between parallel plates (vanes)
2. Vanes have corrugations or bends causing direction changes
3. Liquid droplets cannot follow sharp turns (inertia)
4. Droplets impact vane surfaces and adhere
5. Collected liquid drains through drainage channels
Key parameters:
- Vane spacing: 0.5-1.5 inches (13-38 mm)
- Number of passes: 3-6 direction changes
- Drainage: Hooks or pockets collect liquid
- Material: 316 SS, carbon steel, or polymer
Performance:
- 100% removal of droplets > 8 microns
- Higher K-factor than mesh: K = 0.40-0.50 ft/s
- Pressure drop: 1-3 psi (higher than mesh)
- Can handle 15% liquid by weight (slug tolerance)
Vane Configurations
Horizontal flow
In-Line Vanes
Gas flows horizontally through vertical vane pack; ideal for horizontal vessels and ducts.
Vertical flow
Stacked Vanes
Gas flows upward through horizontal vane layers; replaces mesh pads in vertical vessels.
V-Bank
Single or Double V
Angled vane banks in V-pattern; compact design for limited space.
Circular bundle
Radial Vanes
Radial vane arrangement in cylindrical housing; for vertical separators.
Vane Capacity Calculation
Horizontal Vane Capacity:
Q_h = 2400 × (V_L - 6.185) × √(P / (S × T × Z))
Where:
Q_h = Gas flow capacity (SCFH)
V_L = Vane length (vertical), inches
P = Operating pressure, psia
S = Gas specific gravity (air = 1.0)
T = Operating temperature, °R
Z = Compressibility factor
Example:
V_L = 24 inches, P = 500 psia, S = 0.65, T = 560°R, Z = 0.9
Q_h = 2400 × (24 - 6.185) × √(500 / (0.65 × 560 × 0.9))
Q_h = 2400 × 17.815 × √(1.53)
Q_h = 2400 × 17.815 × 1.24
Q_h = 53,000 SCFH = 1.27 MMSCFD per foot of horizontal length
Vane vs. Mesh Comparison
| Parameter | Wire Mesh | Vane Pack |
|---|---|---|
| Minimum droplet size | 3-5 microns | 8-10 microns |
| K-factor | 0.35 ft/s | 0.45 ft/s |
| Pressure drop | 0.5-1 psi | 1-3 psi |
| Liquid slug tolerance | Poor | Good (15% by weight) |
| Solids tolerance | Poor (plugs) | Fair (self-cleaning) |
| Turndown | 3:1 | 5:1 |
| Capital cost | Lower | Higher |
| Maintenance | Replace every 2-5 years | Minimal (clean if needed) |
Industrial Vane Models
Several vane separator models are available, optimized for different applications:
| Model | Design | Best Application |
|---|---|---|
| Model 625 | Multiple chevron pattern with drainage pockets | Bulk liquid removal, high-efficiency service, high liquid loads |
| Model 626 | Enhanced design with central channel | Lower pressure drop, moderate efficiency, polishing service |
| Model 627 | Upward flow design (horizontal stacked vanes) | Mesh pad replacement in vertical vessels |
Model 627 Vertical Vane Separator
Designed specifically for upward gas flow as a mesh pad replacement:
- Design: Coalescer vanes in stacked horizontal arrangement
- Efficiency: 100% removal of liquid droplets greater than 8 microns
- Slug handling: Internal design limits vane loading to 15% by weight
- Applications: Vertical gas separators, columns, towers, steam drums, 3-phase separators, slug catchers
- Advantages: Higher capacity, better slug tolerance, no re-entrainment at high velocities
Vane Bundle Configurations
| Configuration | Description | Features |
|---|---|---|
| Removable Vane Bundles | Complete assembly removable for maintenance | Quick access, minimal vessel entry |
| Individual Removable Vanes | Each vane can be removed separately | Jacking bolts and vane removal plate provided |
| V-Bank Configuration | Single-V or Double-V arrangement | Space-efficient, includes liquid level clearance |
| Four Bank Vane Bundle | Four vane banks in cross-sectional arrangement | Maximum capacity with upward flow through center |
| Circular Vane Bundle | Radial vanes in cylindrical housing | New style filter separator design |
Vane Materials
Available materials for vane construction based on service requirements:
- Carbon Steel: Standard for non-corrosive service, lowest cost
- 304L Stainless Steel: General corrosion resistance, most common upgrade
- 316L Stainless Steel: Enhanced corrosion resistance for chloride or H₂S service
- Other Alloys: Monel, Hastelloy, Inconel for severe corrosive environments
In-Line Vane Separator Specifications
In-line vane separators are designed for high-efficiency liquid removal in pipeline and process applications where space is limited.
| Parameter | Specification |
|---|---|
| Application | Gas/Liquid |
| Efficiency | 100% of 8 microns or larger, <0.1 gal/MMSCF carryover |
| Pressure Drop | Generally less than 1 PSI |
| Liquid Loading | No slugs; maximum 15% by weight liquid content |
| Turndown Ratio | None |
| Coalescer | Available (improves efficiency to 100% at 3 microns) |
Applications: High efficiency liquid removal, dehydration, product recovery, product gathering, elimination of corrosive liquids, protection of compressors and related equipment.
Configurations with Coalescers:
- Coalescing vanes in vessel
- Coalescing vanes in nozzle
- Mesh pad on face of vanes
Horizontal Vane Separator Specifications
Horizontal vane separators provide high liquid handling capacity with integral separation features for demanding services.
| Parameter | Specification |
|---|---|
| Application | Gas/Liquid |
| Efficiency | 100% of 8 microns or larger, <0.1 gal/MMSCF carryover |
| Pressure Drop | Generally less than 1.5 PSI |
| Liquid Loading | Slugging OK; internal design limits vane loading to 15% by weight |
| Turndown Ratio | None |
| Coalescer | Available |
| Other Features | Integral liquid/liquid separation available; foam breakers for crude oil available |
Selection guidance: Use vane packs when: slug loading is expected, solids are present, high turndown is needed, or mesh pads have failed in service. Use mesh pads when: fine mist (< 8 micron) removal is critical, pressure drop must be minimized, or capital cost is constrained.
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. Inlet Devices
Inlet devices provide primary separation at the separator entrance, removing 80-90% of entrained liquid before the gravity settling zone. Effective inlet design reduces liquid load on downstream mist eliminators and improves overall separation.
Inlet Device Types
Simple deflector
Diverter Plate
Flat plate deflects inlet stream; momentum change separates bulk liquid. Low cost, moderate efficiency.
Half-open pipe
Schoepentoeter
Half-pipe forces flow downward toward liquid surface; prevents splashing and re-entrainment.
Centrifugal
Cyclonic Inlet
Tangential or vortex inlet creates swirl; centrifugal force enhances separation. High efficiency.
Vane type
Inlet Vane Device
Corrugated vanes at inlet; spreads flow, removes liquid by impingement. Best for high liquid.
Diverter Plate Design
Baffle/Diverter Plate:
Simple flat plate positioned to deflect inlet stream:
- Angle: 45-60° from horizontal
- Size: 1.5-2× inlet nozzle diameter
- Location: 6-12 inches from inlet nozzle
- Material: Same as vessel or wear-resistant
Design velocity:
v_inlet < 60 ft/s for gas
v_inlet < 20 ft/s for two-phase
Momentum change at plate causes:
- Bulk liquid to fall out
- Gas to redirect toward settling zone
- Some liquid to drain down plate surface
Efficiency: 60-80% of droplets > 500 microns
Simple, low cost, but limited performance
Half-Pipe (Schoepentoeter) Inlet
Half-Pipe Inlet Design:
Construction:
- Pipe cut longitudinally (half-cylinder)
- Open side faces downward
- Extends into vessel to distribute flow
Sizing:
- Diameter: Same as inlet nozzle
- Length: 0.5-0.75 × vessel diameter
- Slot area: ≥ 2× inlet nozzle area
Function:
1. Incoming stream enters closed top of half-pipe
2. Flow is forced out slots on bottom
3. Momentum is directed downward toward liquid surface
4. Prevents liquid spray from reaching mist eliminator
Advantages:
- Reduces inlet velocity
- Spreads flow across vessel width
- Directs liquid downward
- Prevents foam generation
Efficiency: 70-85% of droplets > 300 microns
Cyclonic Inlet Devices
Cyclone Inlet Separator:
Types:
1. Tangential inlet (flow enters vessel tangentially)
2. Vortex tube (external cyclone feeds vessel)
3. Internal cyclone (small cyclone inside vessel)
Tangential inlet design:
- Inlet nozzle positioned tangent to vessel wall
- Creates spinning flow pattern
- Centrifugal force throws liquid to wall
- Liquid drains down, gas spirals to center
Sizing:
v_tangential = 60-100 ft/s
Create 2-5 rotations before gas exits settling zone
Vortex tube:
- External tube with tangential inlet
- Liquid removed before entering vessel
- Gas enters vessel from tube center
- Highest efficiency inlet device
Efficiency: 90-98% of droplets > 50 microns
Higher ΔP: 2-5 psi additional
Inlet Vane Distributor
Vane-Type Inlet Device:
Construction:
- Multiple corrugated vanes at inlet
- Similar to mist eliminator vanes
- Mounted in inlet nozzle or internal housing
Function:
1. High-velocity inlet stream hits vanes
2. Direction changes cause liquid impingement
3. Liquid collected in drainage pockets
4. Gas distributed evenly across vessel
Design parameters:
- Vane spacing: 0.75-1.5 inches
- Number of passes: 2-4
- Drainage: Collected liquid falls to sump
- Material: Stainless steel or alloy
Performance:
- Efficiency: 85-95% of droplets > 20 microns
- ΔP: 1-2 psi
- Handles high liquid loading
- Good flow distribution
Best for: High GOR, slugging service, foam control
Inlet Device Selection
| Device | Efficiency | ΔP | Cost | Best Application |
|---|---|---|---|---|
| Diverter plate | 60-80% | < 0.5 psi | Low | Clean gas, low liquid |
| Half-pipe | 70-85% | < 0.5 psi | Low | Moderate liquid, foam control |
| Vane inlet | 85-95% | 1-2 psi | Medium | High liquid, slugging |
| Cyclonic | 90-98% | 2-5 psi | High | Maximum efficiency needed |
Design tip: The inlet device should remove as much liquid as possible before the settling zone. This protects the mist eliminator from flooding and extends its life. For high liquid loading or slugging service, invest in a good inlet device rather than over-sizing the mist eliminator.
7. 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.
8. Auxiliary Internals
Auxiliary internals support separation performance, control liquid behavior, and protect equipment. These include vortex breakers, wave breakers, coalescing plates, and outlet devices.
Vortex Breaker
Vortex Breaker Design:
Purpose: Prevent vortex formation at liquid outlet nozzle
- Vortex would entrain gas into liquid stream
- Causes pump cavitation, metering errors
- Critical for vertical separators
Construction:
- Cross-shaped or circular flat plate
- Mounted above liquid outlet nozzle
- Prevents organized rotation at drain
Sizing:
- Plate diameter: 2× outlet nozzle diameter
- Height above nozzle: 1× nozzle diameter
- Thickness: 1/4" minimum (structural)
Alternative designs:
- Cruciform (cross): Simple, effective
- Horizontal disk: Minimal ΔP
- Cage/cylinder: Maximum effectiveness
When required:
- All vertical separators
- Horizontal separators with high liquid flow
- Low liquid levels (h/D < 0.3)
- Large outlet nozzles (> 4")
Wave Breaker (Defoaming Plate)
Wave Breaker Design:
Purpose: Reduce surface turbulence and foam
- Dampens waves from inlet momentum
- Prevents foam from reaching mist eliminator
- Stabilizes gas-liquid interface
Construction:
- Horizontal perforated plates
- Positioned at or above liquid level
- Multiple plates for severe foaming
Design parameters:
- Number of plates: 2-4
- Spacing: 6-12 inches vertical
- Perforation: 40-60% open area
- Hole size: 0.5-1 inch diameter
- Material: Same as vessel
Sizing:
- Cover full vessel cross-section
- Extend 12" beyond inlet zone
- Support on vessel shell or internal clips
Effective for:
- Foaming crudes
- High inlet velocity
- Slug catching service
- 3-phase interface stability
Coalescing Plates
Parallel Plate Coalescer:
Purpose: Enhance oil-water separation in 3-phase separators
- Provide surfaces for droplet coalescence
- Shorten effective settling distance
- Increase separation capacity 5-10×
Construction:
- Inclined parallel plates (corrugated or flat)
- Stacked with uniform spacing
- Mounted in liquid zone
Design parameters:
- Plate spacing: 0.5-2 inches (13-50 mm)
- Inclination: 45-60° from horizontal
- Plate length: 3-6 feet
- Material: Stainless, polypropylene, or oleophilic coating
Enhancement factor:
E = (L / S) × sin(θ)
Where:
L = Plate length
S = Plate spacing
θ = Inclination angle
For L = 4 ft, S = 1", θ = 60°:
E = (4 × 12 / 1) × sin(60°) = 48 × 0.866 = 42×
Settling enhanced 42× compared to open settling
Other Auxiliary Devices
| Device | Purpose | Location | Key Design Point |
|---|---|---|---|
| Spreader plate | Distribute inlet flow | Below inlet | 50% open area, full width |
| Weir plate | Control oil-water interface | 3-phase liquid zone | Adjustable or fixed height |
| Oil skimmer | Collect floating oil | Water side of weir | Slot or overflow type |
| Sand jets | Flush accumulated solids | Vessel bottom | High-pressure water nozzles |
| Boot | Collect small water volumes | Vessel bottom, external | Sized for surge + level control |
| Stilling well | Stable level measurement | External or internal pipe | Small holes, isolated from turbulence |
Auxiliary importance: Auxiliary internals are often overlooked but critical to reliable operation. A missing vortex breaker can cause pump failure. A missing wave breaker allows foam to flood the mist eliminator. Include all necessary auxiliaries in the design.
9. Sizing & Installation
Proper sizing ensures internals operate within their design envelope. Proper installation ensures they function as intended. Both are essential for meeting separation performance requirements.
Mist Eliminator Sizing
Mist Eliminator Sizing Procedure:
STEP 1: Calculate actual gas flow
Q_act = Q_std × (P_std/P_op) × (T_op/T_std) × (Z_op/Z_std)
STEP 2: Select K-factor
- Wire mesh: K = 0.35 ft/s (apply pressure correction if P > 100 psia)
- Vane pack: K = 0.45 ft/s
STEP 3: Calculate maximum velocity
v_max = K × √[(ρ_L - ρ_g) / ρ_g]
STEP 4: Calculate required area
A_required = Q_act / v_max
STEP 5: Select geometry
For horizontal vessel: A = L × W (rectangular pad)
For vertical vessel: A = π × D² / 4 (circular pad)
STEP 6: Verify fit
Check pad fits vessel diameter with sealing ring
Allow 2-3" annular gap for seal
STEP 7: Check pressure drop
ΔP_mesh ≈ 0.4 × (v / v_max)² × ρ_g / ρ_water × thickness/6
ΔP_vane = K × ρ_g × v² / 2 (from manufacturer)
Sizing Example
Example: Mesh Pad for Vertical Separator
Given:
Vessel ID: 48 inches
Gas flow: 20 MMSCFD
Operating P: 600 psia, T: 100°F
Gas MW: 20, Z = 0.90
Liquid: Water (ρ = 62.4 lb/ft³)
STEP 1: Actual flow
ρ_g = (600 × 20) / (10.73 × 560 × 0.90) = 2.22 lb/ft³
Q_std = 20 × 10⁶ / 1440 = 13,889 SCFM
Q_act = 13,889 × (14.7/600) × (560/520) × 0.90 = 330 ACFM
STEP 2: K-factor with pressure correction
K = 0.35 × [1 - 0.000035 × (600 - 100)] = 0.35 × 0.9825 = 0.344 ft/s
STEP 3: Maximum velocity
v_max = 0.344 × √[(62.4 - 2.22) / 2.22] = 0.344 × 5.21 = 1.79 ft/s
STEP 4: Required area
A_req = (330/60) / 1.79 = 3.07 ft²
STEP 5: Check vessel capacity
A_vessel = π × (48/12)² / 4 = 12.57 ft²
Utilization = 3.07 / 12.57 = 24.4% ✓ (< 100%, OK)
STEP 6: Specify pad
Mesh pad: 46" diameter (48" vessel - 2" seal gap)
Thickness: 6 inches standard
Type: 316 SS knitted mesh, 9 lb/ft³ density
Support: Lower grid 2"×2", upper grid 4"×4"
Actual velocity:
A_pad = π × (46/12)² / 4 = 11.54 ft²
v_actual = (330/60) / 11.54 = 0.476 ft/s
Operating at 27% of maximum velocity ✓
Worked Example: Compressor Suction Scrubber
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:
Qa = 25 x 10&sup6; / 1,440 x (14.7 / 414.7) x (540 / 520) = 639 ACFM
Inlet nozzle area = π/4 x (7.981/12)² = 0.3474 ft²
Inlet velocity = 639 / (60 x 0.3474) = 30.7 ft/s
Inlet momentum = ρG x V² = 1.52 x 30.7² = 1,432 lb/(ft·s²)
Step 2: Select Inlet Device
ρV² = 1,432 (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:
KSB = 0.25 (standard wire mesh, 9 lb/ft³ density)
Vmax = 0.25 x SQRT[(42 - 1.52) / 1.52]
Vmax = 0.25 x SQRT[26.63]
Vmax = 0.25 x 5.16 = 1.29 ft/s
Design velocity = 75% of Vmax = 0.97 ft/s
Required mesh area = Qa / (60 x Vdesign)
= 639 / (60 x 0.97) = 10.98 ft²
Required vessel ID = SQRT[4 x 10.98 / π] = 3.74 ft = 44.9 in.
Select: 48-inch ID vessel (A = 12.57 ft²)
Actual velocity = 639 / (60 x 12.57) = 0.85 ft/s [OK, 66% of Vmax]
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 | 0.85 ft/s (66% of Vmax) |
| Inlet momentum | 1,432 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 66% of Vmax provides adequate margin. For additional protection, consider adding coalescing cartridges downstream.
Installation Requirements
| Internal | Clearances | Support | Sealing |
|---|---|---|---|
| Mesh pad | 6" below outlet, 12" above liquid | Support ring + grids | Compression or welded band |
| Vane pack | 12" from nozzles | Angle frame | Gasket or welded to frame |
| Inlet device | 6-12" from nozzle | Welded brackets | Not applicable |
| Vortex breaker | 1-2× nozzle D above outlet | Welded to nozzle or floor | Welded (no bypass) |
| Wave breaker | At or above NLL | Clips to shell | None (open edges OK) |
Common Installation Problems
- Bypass gaps: Gaps around mist eliminator allow gas short-circuit; ensure complete seal
- Inverted mesh: Some mesh pads have "top" side; install per manufacturer instructions
- Insufficient support: Mesh pads can collapse under liquid loading; use adequate support grids
- Wrong orientation: Vane drainage pockets must face down; liquid cannot drain if inverted
- Inadequate clearance: Internals too close to nozzles cause flow maldistribution
- Missing vortex breaker: Easy to omit; causes gas entrainment in liquid
Inspection & Maintenance
Internal Inspection Checklist:
Mesh Pads:
□ Check for holes, tears, or collapse
□ Look for fouling (scale, wax, solids)
□ Verify seal at vessel wall
□ Check support grid integrity
□ Measure thickness (compression)
Replace if: Holes > 2", ΔP > 2× clean, thickness < 50%
Vane Packs:
□ Check for bent or missing vanes
□ Inspect drainage pockets for plugging
□ Look for corrosion or erosion
□ Verify mounting bolt torque
□ Check gasket condition
Clean if: Visible buildup; replace if damaged
Inlet Devices:
□ Check for erosion from high velocity
□ Inspect welds for cracks
□ Look for plugging or buildup
□ Verify alignment and position
Frequency: Annual or at every turnaround
Installation bottom line: The best-designed internal is worthless if installed incorrectly. Follow manufacturer instructions exactly. Ensure no bypass paths. Verify orientation. Document as-installed condition with photos for future reference.
10. 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 KSB 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.
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