1. Scrubber Overview
A gas scrubber is a vessel designed to remove entrained liquid droplets and free liquid from a gas stream. Unlike production separators, scrubbers handle primarily gas with small amounts of liquid. They are the last line of defense protecting downstream equipment from liquid damage.
Compressor protection
Suction scrubbers
Prevent liquid slugs from entering compressor cylinders. Liquid carry-over causes valve damage, rod failures, and cylinder cracking.
Process protection
Inlet scrubbers
Protect amine contactors, glycol contactors, molecular sieves, and other process equipment from liquid contamination.
Pipeline protection
Pipeline scrubbers
Remove condensate and water from pipeline gas at custody transfer points, meter stations, and city gate stations.
Scrubber vs. Separator
| Feature | Scrubber | Production Separator |
|---|---|---|
| Primary fluid | Gas with minor liquid | Multi-phase (gas + liquid) |
| Gas-to-liquid ratio | High (gas dominant) | Variable |
| Liquid retention time | 1–3 minutes | 3–10 minutes |
| Liquid section size | Small (10–30% of volume) | Large (40–70% of volume) |
| Mist eliminator | Usually required | Required for clean gas delivery |
| Orientation | Usually vertical | Usually horizontal |
| Design priority | Gas capacity | Liquid capacity and separation |
Sizing priority: For scrubbers, gas capacity (vessel diameter) is the controlling design parameter. The liquid section is sized for minimum retention time only. For production separators, liquid capacity often controls the vessel size.
2. Souders-Brown K Factor Sizing Method
The Souders-Brown equation is the industry-standard method for sizing the gas capacity of scrubbers and separators. It determines the maximum allowable superficial gas velocity based on the density difference between liquid and gas phases.
Souders-Brown Equation:
Vmax = KSB × SQRT[(ρL - ρG) / ρG]
Where:
Vmax = Maximum allowable superficial gas velocity (ft/s)
KSB = Souders-Brown K factor (ft/s)
ρL = Liquid density (lb/ft³)
ρG = Gas density at operating conditions (lb/ft³)
K Factor Selection
| Configuration | Mist Eliminator | KSB (ft/s) | Notes |
|---|---|---|---|
| Vertical scrubber | Wire mesh pad | 0.24–0.28 | Standard design for most applications |
| Vertical scrubber | Vane pack | 0.18–0.22 | Fouling or high-liquid-load service |
| Vertical scrubber | None | 0.12–0.15 | Gravity separation only |
| Horizontal scrubber | Wire mesh pad | 0.36–0.42 | Higher capacity; half-full liquid section |
| Horizontal scrubber | Vane pack | 0.30–0.35 | Fouling service, horizontal configuration |
| Filter separator | Coalescing elements | 0.08–0.12 | High-efficiency fine droplet removal |
Pressure Correction for K Factor
At elevated pressures, the density ratio (ρL - ρG) / ρG decreases, and the K factor should be adjusted downward. The GPSA correction factor:
Pressure Correction (GPSA):
For P > 250 psig:
Kcorrected = Kbase × Cp
Approximate correction factors:
P = 300 psig: Cp = 0.95
P = 600 psig: Cp = 0.90
P = 900 psig: Cp = 0.85
P = 1,200 psig: Cp = 0.80
P = 1,500 psig: Cp = 0.75
Vessel Diameter Calculation
Required Vessel Diameter:
Step 1: Calculate Vmax from Souders-Brown equation
Step 2: Apply design margin (typically 75–85% of Vmax)
Vdesign = 0.75 × Vmax
Step 3: Calculate required area
A = Qactual / Vdesign
Step 4: Calculate diameter
D = SQRT(4 × A / π)
Step 5: Round up to nearest standard vessel diameter
Standard Vessel Diameters
| Nominal OD (in.) | Common Sizes |
|---|---|
| Small bore | 12, 16, 20, 24 |
| Medium bore | 30, 36, 42, 48 |
| Large bore | 54, 60, 66, 72, 84, 96 |
Design margin: Always apply a 15–25% design margin on vessel diameter (operate at 75–85% of Vmax). This accounts for flow variations, surge conditions, and gas property uncertainties. For compressor suction scrubbers, use 75% of Vmax as a minimum.
3. Liquid Section Sizing
The liquid section of a scrubber must hold sufficient volume for level control, liquid dump valve cycling, and surge capacity. Because scrubbers handle primarily gas, the liquid section is much smaller than in a production separator.
Liquid Retention Time
| Application | Retention Time (min) | Notes |
|---|---|---|
| Compressor suction scrubber | 1–2 | Minimal liquid expected |
| Pipeline inlet scrubber | 2–3 | May see condensate slugs |
| Fuel gas scrubber | 1–2 | Very low liquid rate |
| Amine contactor inlet scrubber | 2–3 | Protect contactor from HCs |
| Glycol contactor inlet scrubber | 2–3 | Protect contactor from HCs |
Liquid Level Zones
Vertical Scrubber Liquid Levels:
HHH (High-High-High): Emergency shutdown level
HH (High-High): Alarm level
NLL (Normal Liquid Level): Normal operating level
LL (Low-Low): Alarm level; close liquid dump valve
LLL (Low-Low-Low): Emergency shutdown level
Minimum spacing between levels:
HH to NLL: 6 inches minimum (12 inches preferred)
NLL to LL: 6 inches minimum (12 inches preferred)
Surge volume = Volume between HH and LL levels
Retention time is calculated between NLL and LL
Liquid Volume Calculation
Required Liquid Volume:
Vliquid = QL × tret
Where:
Vliquid = Required liquid volume (bbl or gal)
QL = Liquid flow rate (bbl/min or gal/min)
tret = Retention time (minutes)
For vertical scrubbers:
Liquid height = Vliquid / Avessel
Add clearances for:
- Bottom head volume (typically not credited)
- 12-inch clearance above LLL to vessel bottom tangent
- Instrument nozzle locations
Slug capacity: For scrubbers downstream of pipelines, size the liquid section for the expected slug volume, not just steady-state liquid rate. Pipeline slugs can deliver hundreds of barrels in minutes. If slug volumes are large, consider a separate slug catcher upstream.
4. Scrubber Configurations
Vertical Scrubbers
Vertical scrubbers are the most common configuration for gas-dominant applications. Gas enters through a side inlet nozzle, flows upward through the gravity settling section and mist eliminator, and exits through a top outlet nozzle. Liquid collects in the bottom section.
| Advantage | Disadvantage |
|---|---|
| Smaller footprint | Less gas capacity per vessel weight |
| Better liquid drainage from demister | Taller structure required |
| Natural gravity separation | Higher foundation and platform costs |
| Simple liquid level control | Transportation limits on tall vessels |
| Standard wire mesh pad installation | Gas exits from top (more piping) |
Horizontal Scrubbers
Horizontal scrubbers are used when large liquid volumes must be handled or when height restrictions apply. Gas flows horizontally above the liquid level. Higher gas capacity per unit of vessel weight due to the larger cross-sectional area available for gas flow.
| Advantage | Disadvantage |
|---|---|
| Higher gas capacity (K = 0.36–0.42) | Larger footprint |
| Greater liquid surge capacity | More complex internals |
| Lower profile (transportation friendly) | Mist eliminator drainage more complex |
| Can handle slug flow | Multiple liquid level measurements needed |
L/D Ratio Guidelines
| Vessel Type | Typical L/D | Notes |
|---|---|---|
| Vertical scrubber | 2:1 to 4:1 | Height includes liquid section + gravity section + demister |
| Horizontal scrubber | 3:1 to 5:1 | Length provides gas residence time and liquid volume |
| Slug catcher | 4:1 to 8:1 | Long vessels for slug volume storage |
Selection guide: Use vertical scrubbers for most compressor suction and process inlet applications. Use horizontal scrubbers when slug flow is expected, when liquid volume is large, or when height restrictions limit vessel design. At high pressures (>1,000 psig), horizontal vessels are often more economical due to reduced wall thickness for the same volume.
5. Compressor Suction Scrubbers
Compressor suction scrubbers are the most critical scrubber application in midstream operations. Liquid carry-over to a reciprocating compressor can cause catastrophic damage including broken valves, bent rods, cracked cylinders, and packing failures. Even small amounts of liquid can significantly shorten valve life.
Design Requirements
| Parameter | Requirement | Basis |
|---|---|---|
| KSB factor | 0.24–0.28 (with wire mesh) | GPSA Chapter 7 |
| Design velocity | ≤ 75% of Vmax | Conservative margin for compressor protection |
| Mist eliminator | Wire mesh + optional coalescer | 10 μm droplet removal minimum |
| Liquid retention | 1–2 minutes | Minimal liquid expected |
| Liquid dump valve | Snap-acting (full open/close) | Prevent gas loss through liquid system |
| High-level shutdown | Required (HHLL) | Compressor shutdown interlock |
Location and Piping
- Distance from compressor: Locate scrubber as close as practical to the compressor suction nozzle. Minimize piping length to reduce the chance of condensation between the scrubber and compressor.
- Elevation: Position the scrubber outlet nozzle at or above the compressor suction nozzle elevation. Avoid low points in suction piping where liquid can accumulate.
- Drain piping: Liquid drain line must slope continuously downward to the disposal or recovery system. Install a check valve to prevent backflow.
- Relief valve: Install a pressure relief valve on the scrubber per ASME Section VIII. Set pressure at or above compressor maximum suction pressure.
Interstage Scrubbers
For multi-stage compression, scrubbers between stages remove condensate formed by interstage cooling:
- Size for interstage conditions (higher pressure, lower temperature than suction)
- Account for condensed liquids from gas cooling
- Wire mesh demister is standard
- HHLL shutdown interlock protects next-stage compressor
Discharge Scrubbers
Scrubbers on the compressor discharge side are less common but used when:
- Compressor lube oil carryover must be removed
- Downstream equipment (dehy, amine, meter) requires clean gas
- Condensation occurs in the discharge cooler
Safety critical: The high-high liquid level (HHLL) shutdown on a compressor suction scrubber is a safety-critical function. This switch must shut down the compressor before liquid reaches the gas outlet nozzle. Test HHLL switches per the facility's preventive maintenance schedule (typically monthly).
6. Worked Example
Size a vertical compressor suction scrubber for a reciprocating compressor at a gas gathering station.
Given:
Gas flow: 15 MMSCFD natural gas (SG = 0.65)
Suction pressure: 200 psig
Suction temperature: 90°F
Expected liquid: 0.3 bbl/MMSCF condensate
Gas density at conditions: 0.82 lb/ft³
Liquid density: 45 lb/ft³ (light condensate)
Mist eliminator: Standard wire mesh pad (K = 0.26)
Step 1: Calculate Maximum Gas Velocity
Vmax = KSB × SQRT[(ρL - ρG) / ρG]
Vmax = 0.26 × SQRT[(45 - 0.82) / 0.82]
Vmax = 0.26 × SQRT[53.88]
Vmax = 0.26 × 7.34 = 1.91 ft/s
Step 2: Apply Design Margin
Vdesign = 0.75 × Vmax = 0.75 × 1.91 = 1.43 ft/s
Step 3: Calculate Actual Gas Volume
Qactual = Qstd × (Pstd / Pop) × (Top / Tstd) × (1 / Z)
Qactual = 15,000,000 / 1,440 × (14.696 / 214.696) × (550 / 520) × (1 / 0.95)
Qactual = 10,417 × 0.0684 × 1.058 × 1.053
Qactual = 793 ACFM = 13.22 ACFS
Step 4: Calculate Required Vessel Diameter
Arequired = Qactual / Vdesign = 13.22 / 1.43 = 9.25 ft²
D = SQRT(4 × 9.25 / π) = SQRT(11.77) = 3.43 ft = 41.2 in.
Select: 42-inch ID vessel (next standard size)
Aactual = π/4 × (42/12)² = 9.62 ft²
Vactual = 13.22 / 9.62 = 1.37 ft/s (72% of Vmax) [OK]
Step 5: Size Liquid Section
Liquid rate = 15 × 0.3 = 4.5 bbl/day = 0.003125 bbl/min
Retention time = 2 minutes
Vliquid = 0.003125 × 2 = 0.00625 bbl = 0.263 gal = 0.035 ft³
This is negligible. Minimum liquid section height is governed by:
- Instrument nozzle spacing (12 in. HH to LL)
- Bottom clearance (12 in. below LLL to tangent line)
- Minimum 24-inch liquid section height (practical minimum)
Step 6: Determine Overall Vessel Height
Vessel height breakdown (tangent to tangent):
Bottom clearance: 12 in.
Liquid section (LLL to HH): 24 in.
Inlet nozzle zone: 18 in.
Gravity settling section: 36 in.
Wire mesh pad: 6 in.
Outlet clearance: 12 in.
------
Total T-T height: 108 in. = 9 ft
L/D = 108 / 42 = 2.6 [OK, within 2:1 to 4:1 range]
Summary
| Parameter | Value |
|---|---|
| Vessel ID | 42 inches |
| Vessel T-T height | 9 ft |
| L/D ratio | 2.6 |
| Design gas velocity | 1.37 ft/s (72% of Vmax) |
| Mist eliminator | Wire mesh, 6 in., 316 SS |
| Inlet device | Diverter plate (low ρV²) |
Design check: The 42-inch scrubber operates at 72% of Vmax, providing good margin. The liquid section is controlled by minimum mechanical dimensions rather than liquid volume. L/D ratio of 2.6 is within the recommended range.
7. Operations & Troubleshooting
Startup Checklist
- Verify all internals (inlet device, demister pad, vortex breaker) are properly installed
- Confirm liquid level instruments are calibrated and functioning
- Test high-high liquid level (HHLL) shutdown interlock with compressor
- Verify liquid dump valve operates correctly (open/close/fail position)
- Check that drain piping slopes continuously to disposal without low spots
- Confirm relief valve is installed and set correctly
Common Operating Problems
| Problem | Cause | Solution |
|---|---|---|
| Liquid carry-over to compressor | Flooding, high velocity, plugged demister | Reduce flow, clean demister, check liquid level |
| Frequent HHLL trips | Slug flow from upstream, stuck dump valve | Install slug catcher; repair dump valve |
| No liquid dumping | Stuck dump valve, plugged drain line | Stroke dump valve; clear drain line |
| Pressure drop increasing | Fouled or plugged demister pad | Clean or replace wire mesh pad |
| Erratic level indication | Foam or emulsion in liquid section | Install anti-foam baffles; inject defoamer |
Performance Indicators
- Compressor valve life: Decreasing valve life indicates liquid carry-over from the scrubber
- Downstream equipment fouling: Fouling of amine trays or glycol packing indicates inadequate scrubbing
- Pressure drop trend: Monitor ΔP across the mist eliminator; rising trend indicates fouling
- Liquid dump frequency: Changes in dump frequency indicate changes in upstream conditions
Maintenance tip: During every compressor overhaul, inspect the suction scrubber internals. Check the wire mesh pad for fouling, damage, or collapse. Check the inlet device for erosion. Verify the vortex breaker on the liquid outlet is intact. These inspections take minimal time and prevent costly compressor damage.
Ready to use the calculator?
→ Launch Scrubber Sizing Calculator