1. Overview & Applications
Filter separators combine mechanical filtration with coalescence to remove liquid droplets and solid particles from gas streams. They are essential for protecting downstream equipment such as compressors, dehydration units, amine systems, and custody transfer meters.
Primary function
Liquid Removal
Remove entrained liquids (water, hydrocarbon condensate, compressor oil) down to sub-micron sizes.
Secondary function
Solid Filtration
Capture pipeline scale, rust, sand, and other particulates that damage equipment.
Gas conditioning
Dehydration Protection
Ideal upstream of TEG contactors to prevent glycol contamination and foaming.
Metering protection
Custody Transfer
Ensure clean, dry gas for accurate orifice or ultrasonic meter measurement.
Filter Separator vs. Standard Separator
| Parameter | Standard Separator | Filter Separator |
|---|---|---|
| Minimum droplet removal | 100-500 microns | 0.3-3 microns |
| Solid particle removal | Limited (gravity only) | Excellent (filter elements) |
| Pressure drop | 0.5-2 psi | 2-10 psi (increases with loading) |
| Maintenance | Low (mist pad replacement) | Higher (element replacement) |
| Capital cost | Lower | Higher (elements, larger vessel) |
| Best application | Bulk separation, high liquid loads | Fine mist removal, equipment protection |
Common Applications
- Compressor suction: Protect reciprocating and centrifugal compressors from liquid slugs and particulates
- Dehydration inlet: Remove free water and hydrocarbons upstream of TEG or molecular sieve units
- Amine system inlet: Prevent hydrocarbon contamination of amine solution
- Fuel gas systems: Condition fuel gas for gas turbines and engines
- Custody transfer: Ensure accurate metering with clean, dry gas
- Instrument gas: Provide clean, dry gas for pneumatic instruments
Selection criteria: Use filter separators when droplet sizes are below 10-20 microns, when solid particles must be removed, or when downstream equipment requires high-purity gas. For bulk liquid removal with droplets > 100 microns, a standard separator is more economical.
2. Coalescence Theory
Coalescence is the uniting of two or more liquid particles due to physical contact, forming a larger single particle. This is the fundamental mechanism that enables filter separators to capture and drain sub-micron droplets that would otherwise pass through conventional separators.
The Coalescence Mechanism
Coalescence Process:
Step 1: CAPTURE - Small droplets (< 10 microns) impact filter media fibers
Step 2: COLLECTION - Droplets adhere to fiber surfaces via surface tension
Step 3: COALESCENCE - Adjacent droplets merge into larger drops
Step 4: DRAINAGE - Coalesced drops (> 100 microns) drain by gravity
Key principle: Small droplets that cannot settle by gravity are
captured on fiber surfaces where they grow into drainable drops.
Surface tension forces dominate at small scales:
- Droplets < 1 micron: Strong adhesion to fibers
- Droplets 1-10 micron: Moderate capture efficiency
- Droplets > 100 micron: Drain under gravity
Fiber Media Characteristics
Coalescing efficiency depends on fiber diameter, packing density, and media thickness:
| Media Type | Fiber Diameter | Target Droplet Size | Application |
|---|---|---|---|
| Coarse fiberglass | 10-25 microns | > 5 microns | Pre-filter, high liquid loads |
| Fine fiberglass | 3-10 microns | 1-5 microns | Standard coalescing |
| Microfiber | 0.5-3 microns | 0.3-1 micron | High-efficiency polishing |
| Composite/graded | Multiple layers | 0.3-10 microns | Extended life, high efficiency |
Efficiency vs. Droplet Size
Typical Coalescing Element Efficiency:
Droplet Size Removal Efficiency
-----------------------------------------
> 8 microns 100% (guaranteed)
3-8 microns 99.5-100%
1-3 microns 95-99%
0.5-1 micron 80-95%
< 0.5 micron 50-80% (varies with velocity)
Note: As pressure drop increases with element loading,
efficiency actually INCREASES due to tighter flow paths.
Efficiency relationship:
Higher ΔP → Tighter media → Better capture → Higher efficiency
Factors Affecting Coalescence
- Gas velocity: Higher velocity improves impaction but can cause re-entrainment; optimal range exists
- Liquid loading: Excessive liquid can flood elements, reducing efficiency and increasing ΔP
- Surface tension: Low surface tension liquids (condensate) are harder to coalesce than water
- Temperature: Higher temperature reduces liquid viscosity, improving drainage
- Pressure: Higher pressure increases gas density, affecting droplet dynamics
- Contaminants: Surfactants and chemicals can reduce surface tension and impair coalescence
Design insight: Coalescence works best when elements are sized for moderate face velocities (typically 5-15 ft/min). Too slow reduces impaction efficiency; too fast causes liquid re-entrainment from element surfaces.
3. Impingement Separation
Impingement is the collision of liquid particles with solid surfaces, causing droplets to adhere rather than follow gas streamlines. This mechanism is used in both the inlet section and vane-type mist eliminators of filter separators.
Impingement Mechanism
Impingement Principle:
When gas changes direction around an obstacle:
- Gas molecules follow the curved streamline
- Liquid droplets (higher inertia) cannot follow
- Droplets impact the obstacle surface and collect
Impingement efficiency depends on:
- Droplet size (larger = more inertia = better capture)
- Gas velocity (higher = more inertia = better capture)
- Obstacle geometry (sharper turns = better capture)
- Droplet/gas density ratio
Stokes number (St) characterizes impingement:
St = (ρ_L × d² × v) / (18 × μ × D_obstacle)
Where:
ρ_L = Liquid density
d = Droplet diameter
v = Gas velocity
μ = Gas viscosity
D_obstacle = Characteristic obstacle dimension
Impingement occurs when St > 0.1-0.2
Vane-Type Impingement Separators
Vane packs use multiple parallel plates with directional changes to capture droplets by impingement:
Configuration
V-Bank Design
Single-V or double-V arrangements provide multiple impingement surfaces with drainage channels.
Efficiency
100% at 8 microns
Vane packs achieve 100% removal of droplets 8 microns and larger at design conditions.
Pressure drop
1-3 psi typical
Multiple direction changes create moderate pressure drop but high efficiency.
Liquid capacity
Handles slugs
Internal design limits vane loading to 15% by weight; handles intermittent slugging.
Vane Separator Capacity
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
This equation sizes the vane pack for given flow conditions.
Vane length is the vertical dimension for horizontal gas flow.
Typical vane lengths: 12-48 inches
Typical vane spacing: 0.5-1.0 inch between plates
Vane Separator K_S Factor with Surface Tension
For precise vane separator sizing, the shell re-entrainment coefficient K_S accounts for liquid loading and surface tension effects:
Vane Type Shell Velocity:
V_S = K_S × √[(ρ_L - ρ_G) / ρ_G]
Where:
V_S = Maximum shell velocity (ft/s)
K_S = Shell re-entrainment coefficient
ρ_L = Liquid density (lb/ft³)
ρ_G = Gas density (lb/ft³)
K_S Correlation:
K_S = 28 × (Φ)^(-0.1856) × (σ)^0.2
Where:
Φ = Liquid to gas weight ratio (lb liquid / lb gas)
σ = Liquid surface tension (dynes/cm)
Surface Tension Effects:
┌────────────────────┬─────────────────┬──────────────────┐
│ Liquid Type │ σ (dynes/cm) │ Effect on K_S │
├────────────────────┼─────────────────┼──────────────────┤
│ Water │ 72 │ Higher K_S │
│ Light condensate │ 20-25 │ Lower K_S │
│ Heavy oil │ 25-35 │ Moderate K_S │
│ Glycol │ 45-50 │ Higher K_S │
└────────────────────┴─────────────────┴──────────────────┘
Design Note:
Lower surface tension liquids (condensate) are more prone
to re-entrainment and require lower velocities. The K_S
correlation automatically accounts for this effect.
Higher liquid loading (Φ) reduces K_S, requiring larger
vane area or lower gas velocity to prevent carryover.
Centrifix Vane Design
The Centrifix design (Models 625 and 626) uses a more detailed correlation for the re-entrainment coefficient:
Centrifix Vane Velocity:
V = K_R × √[(ρ_L - ρ_g) / ρ_g]
K_R Correlation:
K_R = 58 × (D)^0.1104 × (Φ)^(-0.1478) × (μ_L)^(-0.215) × (σ)^0.114
Where:
D = Droplet size (microns)
Φ = Liquid to gas weight ratio
μ_L = Liquid viscosity (cP)
σ = Liquid surface tension (dynes/cm)
Vane Models:
- Model 625: Multiple chevron pattern with drainage pockets
- Higher efficiency, handles higher liquid loads
- Used for bulk liquid removal and high-efficiency service
- Model 626: Simpler wave pattern
- Lower pressure drop, moderate efficiency
- Used for polishing and low-liquid service
The K_R correlation accounts for droplet size effects,
providing more accurate sizing for specific applications.
Vane Pack Configurations
| Configuration | Description | Best Application |
|---|---|---|
| Single-V bank | Angled plates in V-pattern | Moderate efficiency, lower ΔP |
| Double-V bank | Two V-banks in series | Higher efficiency, moderate ΔP |
| Circular vane bundle | Radial vanes in cylindrical housing | Compact vertical vessels |
| Stacked coalescer vanes | Horizontal vanes with drainage | Replacement for mesh pads |
Model 627 Vane Separator
The Model 627 is designed for upward gas flow as a replacement for mesh pads in vertical vessels:
- Application: Vertical gas separators, columns, towers, steam drums, 3-phase separators, slug catchers
- Efficiency: 100% removal of liquid droplets greater than 8 microns in diameter
- Slug handling: Internal design limits vane loading to 15% by weight
- Advantages over mesh pads: Higher capacity, better slug tolerance, no re-entrainment at high velocities
Practical note: Vane separators excel where mesh pads fail - high liquid loads, slugging service, and velocities that would cause mesh pad flooding. However, they have higher pressure drop and are less effective for sub-micron droplets.
4. Element Design & Sizing
Coalescing filter elements are the heart of filter separator performance. Proper sizing ensures adequate surface area for both filtration and coalescence while maintaining acceptable pressure drop and element life.
Element Types
Cartridge elements
Cylindrical Design
Standard cartridge filters with pleated or wrapped media; 6-40 inch lengths typical.
Sock/bag elements
Tubular Bags
Fabric filter bags for higher solids loading; easier changeout but lower efficiency.
Depth elements
Graded Density
Multiple fiber layers from coarse to fine; high dirt capacity and efficiency.
Composite elements
Multi-Layer
Combined pre-filter, coalescer, and separator in single element.
Element Sizing Parameters
Element Sizing Fundamentals:
Face Velocity:
v_f = Q_actual / A_element
Where:
v_f = Face velocity (ft/min)
Q_actual = Actual gas flow rate (acfm)
A_element = Total element surface area (ft²)
Recommended face velocities:
- Liquid coalescing: 5-15 ft/min
- Dry gas filtration: 10-30 ft/min
- High-efficiency coalescing: 3-8 ft/min
Number of elements:
N = Q_actual / (v_f × A_single)
Where:
A_single = Surface area of single element (ft²)
Standard element dimensions:
- Diameter: 2.5", 4.5", 6", 8"
- Length: 10", 20", 30", 40"
- Surface area: 1-15 ft² per element (pleated)
Pressure Drop Calculation
Element Pressure Drop:
Clean element ΔP:
ΔP_clean = K × μ × v_f / (A_media × ρ)
Loaded element ΔP:
ΔP_loaded = ΔP_clean × (1 + C × m_solids / A_element)
Where:
K = Media permeability constant
μ = Gas viscosity
v_f = Face velocity
ρ = Gas density
C = Cake resistance coefficient
m_solids = Accumulated solids mass
Typical pressure drop values:
- New/clean elements: 1-2 psid
- Change-out ΔP: 8-15 psid (manufacturer spec)
- Maximum ΔP: 15-25 psid (element integrity limit)
Element life depends on:
- Contaminant loading (liquid + solids)
- Operating pressure and temperature
- Element quality and construction
Element Selection Guide
| Application | Element Type | Micron Rating | Notes |
|---|---|---|---|
| Compressor suction | Depth coalescer | 0.3-1 micron | Protect valves and rings |
| Dehydration inlet | Composite coalescer | 0.5-3 micron | Remove free water/HC |
| Fuel gas | Fine coalescer | 0.3-0.5 micron | Turbine/engine protection |
| Instrument gas | High-efficiency | 0.01-0.1 micron | Ultra-clean for instruments |
| General pipeline | Standard coalescer | 1-5 micron | Bulk liquid/solid removal |
Element Changeout Criteria
- Differential pressure: Change when ΔP reaches 10-15 psid (or manufacturer limit)
- Time-based: Annual or semi-annual replacement regardless of ΔP
- Performance: Visible liquid carryover downstream indicates element failure
- Inspection: Physical damage, collapsed elements, or bypass leakage
- Process upset: Replace after slug events or contamination incidents
Element economics: Element cost is typically 5-15% of total filter separator cost, but proper element selection and timely replacement prevents far more expensive downstream equipment damage. Track ΔP trends to optimize replacement intervals.
5. Two-Stage Configurations
Two-stage filter separators provide superior performance by dividing separation into distinct stages: bulk liquid removal followed by fine mist coalescing. This protects coalescing elements from flooding and extends element life.
Two-Stage Design Philosophy
Two-Stage Separation Concept:
FIRST STAGE: Bulk Liquid Removal
- Removes droplets > 50-100 microns
- Handles slug loads and high liquid rates
- Uses gravity settling, inlet devices, or vanes
- Drains liquid to separate sump
SECOND STAGE: Fine Mist Coalescing
- Removes droplets < 50 microns down to sub-micron
- Coalescing filter elements
- Protected from bulk liquid flooding
- Lower liquid load = longer element life
Benefits of two-stage design:
- Extended element life (3-5× vs. single stage)
- Higher overall efficiency
- Better slug handling capability
- Lower total pressure drop (elements stay clean)
First Stage Options
| Device | Efficiency | ΔP | Best For |
|---|---|---|---|
| Inlet diverter | > 500 micron | < 0.5 psi | Low liquid loads |
| Centrifugal inlet | > 50 micron | 1-3 psi | Moderate liquid |
| Vane separator | > 8-20 micron | 1-3 psi | High liquid, slugs |
| Mesh pad | > 10 micron | 0.5-1 psi | Steady flow, low liquid |
| Cyclone tubes | > 10 micron | 2-5 psi | High efficiency needed |
Horizontal Two-Stage Filter Separator
Horizontal Configuration:
Inlet → [First Stage] → [Second Stage] → Outlet
↓ ↓
Liquid Liquid
Drain 1 Drain 2
First Stage Section:
- Inlet diverter or vane pack
- Gravity settling zone
- Primary liquid collection sump
- Level control and dump valve
Second Stage Section:
- Coalescing filter elements (vertical or horizontal)
- Element support structure
- Secondary liquid collection
- Gas outlet with optional final vane
Typical vessel dimensions:
- L/D ratio: 3-5 (similar to standard separator)
- Diameter: Sized for element velocity
- Length: Accommodate both stages + nozzles
Vertical Two-Stage Filter Separator
Vertical Configuration:
Gas flow: Bottom inlet → Up through elements → Top outlet
[Gas Outlet]
↑
[Final Vane/Mesh] ← Optional polishing stage
↑
[Coalescing Elements] ← Second stage
↑
[Inlet Vane/Cyclone] ← First stage
↑
[Gas Inlet]
↓
[Liquid Sump] → Drain
Advantages:
- Smaller footprint
- Natural liquid drainage (gravity)
- Good for high-pressure service
Disadvantages:
- Height limitations
- Element access more difficult
- Liquid re-entrainment risk if flooded
Design Considerations
- Stage sizing: First stage handles 80-90% of liquid; second stage handles remainder plus mist
- Velocity profile: Gas must redistribute between stages; allow 12-18 inches minimum
- Liquid drainage: Separate drains for each stage prevent cross-contamination
- Element orientation: Horizontal elements drain better; vertical elements have smaller footprint
- Access: Provide manways/handholes for element replacement
- Instrumentation: ΔP across elements, levels in both sumps, temperature
Two-stage advantage: A properly designed two-stage filter separator can handle 5-10× the liquid loading of a single-stage unit while maintaining the same outlet gas quality. The first stage acts as a "sacrificial" bulk separator, protecting expensive coalescing elements.
6. Specialized Filter Types
Several specialized filter and filter separator designs are commonly used in gas processing applications. These designs offer specific performance characteristics for different service requirements.
Dry Gas Filter
The Dry Gas Filter is designed for solid particle removal from gas streams where liquid contamination is minimal or absent.
| Parameter | Specification |
|---|---|
| Application | Gas/Solid separation (dry gas service) |
| Efficiency | 100% removal at 3 microns |
| Pressure Drop (new/clean) | 1-2 psig |
| Efficiency vs. Loading | As pressure drop increases, efficiency increases |
Efficiency note: Unlike many separation devices where performance degrades with loading, dry gas filter efficiency actually increases as elements load with particulate matter. The accumulated solids create tighter flow paths, capturing progressively smaller particles. This is why changeout is based on pressure drop limits rather than efficiency loss.
Standard Filter/Separator
The Filter/Separator is used for the highest degree of separation of liquids and solids in natural gas processing, transmission, storage, and metering.
Old Style Design
Horizontal Layout
Traditional horizontal design with filter elements in first stage followed by vane section for final separation.
New Style Design
Circular Vane Bundle
Compact design with circular vane bundle. Filter elements followed by coalescing section for improved efficiency.
Fuel Gas Filter/Separator
A specialized version of the vertical filter/separator designed specifically for fuel gas conditioning for gas turbines, engines, and other fuel gas consumers.
| Parameter | Specification |
|---|---|
| Application | Removal of liquids and/or solids from fuel gas |
| Pressure Drop (new/clean) | Typically 1-2 psi |
| Solid Loading | No slugs; typically 10% by weight (higher with shorter elements) |
| Turndown Ratio | None |
| Coalescer | Built-in |
| Special Features | Centrifugal tuyere in place of vanes; smaller filter elements |
Ultrasep Separator
The Ultrasep is a high-efficiency separator designed for applications requiring the ultimate in separation performance, typically used at compressor discharge service for removing lube oil carryover.
- Application: Compressor discharge, high-purity gas requirements
- Features: Multi-stage separation for maximum efficiency
- Position in compressor station: Recommended at discharge along with Filter/Separator
Filter Separator Configurations
| Configuration | Description | Best Application |
|---|---|---|
| Coalescing vanes in vessel | Coalescing elements followed by vane section inside vessel | Standard two-stage service |
| Coalescing vanes in nozzle | Compact design with vanes integrated into outlet nozzle | Space-constrained installations |
| Mesh pad on face of vanes | Wire mesh added to vane inlet face for pre-coalescence | Enhanced efficiency (100% at 3 microns) |
With coalescer enhancement: Adding coalescing elements or mesh to vane separators improves efficiency from 100% at 8 microns to 100% at 3 microns. This enhancement is available for in-line vane, vertical gas, and horizontal separator configurations.
7. Design Procedure
Filter separator design requires balancing efficiency, pressure drop, element life, and cost. The following procedure ensures a properly sized unit for the application.
Step-by-Step Design Procedure
Filter Separator Design Steps:
STEP 1: Define Process Conditions
- Gas flow rate (MMSCFD and ACFM at conditions)
- Operating pressure and temperature
- Gas composition (MW, SG, Z-factor)
- Contaminant loading (liquid bbl/MMSCF, solids ppm)
- Droplet size distribution (if known)
- Required outlet quality specification
STEP 2: Select Configuration
- Single stage vs. two-stage
- Horizontal vs. vertical orientation
- Element type and micron rating
- First stage device (vane, cyclone, etc.)
STEP 3: Size First Stage (if two-stage)
- Calculate K-factor velocity for vane/inlet device
- Size for 80-90% bulk liquid removal
- Include liquid holdup and drainage capacity
STEP 4: Size Coalescing Elements
- Select face velocity (5-15 ft/min typical)
- Calculate required element surface area
- Determine number and size of elements
- Check element layout fits vessel diameter
STEP 5: Size Vessel
- Diameter: Accommodate elements + clearances
- Length: First stage + elements + nozzles
- Check L/D ratio (typically 2-4)
STEP 6: Calculate Pressure Drop
- First stage ΔP (if applicable)
- Clean element ΔP
- Piping/nozzle ΔP
- Total system ΔP (clean and at changeout)
STEP 7: Verify Performance
- Check efficiency meets specification
- Confirm element life expectation
- Verify liquid handling capacity
Design Example
Example: Filter Separator for Dehydration Inlet
Given:
Gas flow: 50 MMSCFD
Operating P: 800 psia
Operating T: 100°F
Gas SG: 0.65, Z = 0.88
Liquid loading: 0.5 bbl/MMSCF (water + condensate)
Outlet spec: < 0.1 ppmw liquid carryover
STEP 1: Calculate actual flow
Q_std = 50 MMSCFD = 34,722 SCFM
ρ_std = 0.0765 × 0.65 = 0.0497 lb/ft³
Q_act = Q_std × (14.7/800) × (560/520) × (0.88) = 534 ACFM
STEP 2: Select configuration
- Two-stage horizontal (high liquid loading)
- First stage: Vane separator
- Second stage: 0.3 micron depth coalescers
STEP 3: Size first stage vane
- K = 0.35 ft/s for vane separator
- ρ_gas = 2.85 lb/ft³ (at conditions)
- ρ_liq = 50 lb/ft³
- v_max = 0.35 × √[(50-2.85)/2.85] = 1.45 ft/s
- A_vane = 534/(60×1.45) = 6.14 ft² vane area
STEP 4: Size coalescing elements
- Face velocity: 10 ft/min (coalescing service)
- A_element = 534/10 = 53.4 ft²
- Select 6" OD × 30" elements, 4.5 ft² each
- N = 53.4/4.5 = 12 elements (use 12)
STEP 5: Size vessel
- Element bundle diameter: ~24"
- Vessel ID: 36" (allows clearance + first stage)
- Length: 8 ft (first stage) + 4 ft (elements) = 12 ft S-S
- L/D = 12/3 = 4:1 ✓
STEP 6: Pressure drop
- First stage vane: 2 psi
- Clean elements: 1.5 psi
- Nozzles/internals: 0.5 psi
- Total clean: 4 psi
- At element changeout (10 psid elements): 12.5 psi
FINAL DESIGN:
Vessel: 36" ID × 12'-0" S-S, horizontal
First stage: Vane pack, 6.5 ft² area
Second stage: 12 × 0.3-micron coalescers, 6"×30"
Design pressure: 900 psig
Element changeout ΔP: 10 psid
Common Design Mistakes
- Undersizing elements: Too few elements means high velocity, poor efficiency, short life
- No first stage: Single-stage designs flood elements with bulk liquid
- Wrong element type: Using dry gas filters for wet service (immediate failure)
- Ignoring liquid loading: Underestimating slugging or upset conditions
- Insufficient drainage: Liquid backup into element zone
- Poor access: Cannot replace elements without full vessel entry
Specification Checklist
| Parameter | Typical Value | Notes |
|---|---|---|
| Removal efficiency | 99.5% @ 0.3 micron | Varies with element selection |
| Clean ΔP | 2-4 psid | Including all internals |
| Changeout ΔP | 10-15 psid | Across elements only |
| Element life | 6-24 months | Depends on contaminant loading |
| Liquid handling | 5-10 bbl/MMSCF | With two-stage design |
| Design pressure | 1.1× MAOP | Per ASME Section VIII |
Final verification: Always confirm with element manufacturer that selected elements are appropriate for the service conditions. Provide gas composition, liquid type, temperature, and pressure for proper material selection (especially seals and end caps).
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