1. Three-Phase Separator Overview
A three-phase separator simultaneously separates a well stream into three phases: gas, oil, and water. The vessel performs gas-liquid separation in the upper section and liquid-liquid (oil-water) separation in the lower section. Three-phase separators are essential at production facilities where produced water must be separated from oil before disposal or reinjection.
Gas section
Gas-liquid separation
Identical to two-phase separator gas section. Sized by Souders-Brown method with mist eliminator.
Oil section
Oil-water separation
Water droplets settle from oil phase by gravity. Oil overflows weir to oil outlet. Retention time is critical.
Water section
Water quality control
Oil droplets rise from water phase. Clean water exits at bottom. Interface level controls separation quality.
Three-Phase vs. Two-Phase + FWKO
| Feature | Three-Phase Separator | Two-Phase + FWKO |
|---|---|---|
| Equipment count | Single vessel | Two vessels |
| Plot space | Less (one vessel) | More (two vessels + piping) |
| Capital cost | Lower for moderate water cut | Lower for very high water cut |
| Separation quality | Good for free water | Better (dedicated settling vessel) |
| Emulsion handling | Limited | Better (FWKO handles bulk water first) |
| Flexibility | Moderate | Higher (can adjust independently) |
| Best for water cut | < 50% | > 50% |
Selection rule: Use a three-phase separator when water cut is below approximately 50% and the water is predominantly free (non-emulsified). Above 50% water cut, a free water knockout (FWKO) upstream of a two-phase separator or oil treater is generally more economical and provides better separation.
2. Separation Physics
Oil-Water Settling (Stokes' Law)
Water droplets settle from oil, and oil droplets rise from water, according to Stokes' Law for viscous flow:
Stokes' Law Settling Velocity:
Vt = (g × d² × Δρ) / (18 × μ)
Where:
Vt = Terminal settling velocity (ft/s)
g = Gravitational acceleration (32.174 ft/s²)
d = Droplet diameter (ft)
Δρ = Density difference between phases (lb/ft³)
μ = Viscosity of continuous phase (lb/ft·s)
For water droplets settling in oil:
Δρ = ρw - ρo
μ = Oil viscosity
For oil droplets rising in water:
Δρ = ρw - ρo
μ = Water viscosity
Key Factors Affecting Settling
| Factor | Effect on Settling | Design Impact |
|---|---|---|
| Droplet size | Vt ∝ d² (dominant factor) | Larger droplets settle much faster. Target: 200–500 μm removal. |
| Density difference | Vt ∝ Δρ | Heavy oil (< 20 API) has small Δρ; needs longer retention. |
| Viscosity | Vt ∝ 1/μ | Viscous oil (high μ) slows settling dramatically. Heat may be needed. |
| Temperature | Increases Δρ and decreases μ | Higher temperature improves separation. Heat treaters use this. |
| Chemical treatment | Breaks emulsions, promotes coalescence | Demulsifier injection reduces required retention time. |
| Turbulence | Breaks droplets into smaller sizes | Minimize turbulence in settling section. Use inlet devices. |
Retention Time Requirements
| Oil API Gravity | Oil Retention (min) | Water Retention (min) | Notes |
|---|---|---|---|
| > 40 (light condensate) | 3–5 | 3–5 | Easy separation; low viscosity |
| 30–40 (light oil) | 5–7 | 3–5 | Standard separation |
| 20–30 (medium oil) | 7–10 | 5–7 | Moderate viscosity |
| 10–20 (heavy oil) | 10–20 | 5–10 | High viscosity; consider heating |
| < 10 (extra heavy) | 20–30+ | 10–15 | Heat treatment usually required |
Viscosity dominance: Oil viscosity has the greatest impact on settling velocity. A 20 API oil at 100°F has roughly 10 times the viscosity of a 40 API condensate. This means the required retention time increases by approximately 10x, which directly translates to 10x the liquid section volume. For heavy oils, always evaluate heating options to reduce viscosity.
3. Weir & Bucket Design
The weir system controls how oil and water are separated and removed from the vessel. Two basic configurations are used in horizontal three-phase separators.
Weir-Type Separator
A simple weir plate divides the vessel into a separation section (oil + water) and an oil collection section (oil bucket). Oil flows over the weir into the oil bucket, while water exits from the bottom of the separation section:
- Weir height sets the oil-water interface level
- Oil overflows the weir by gravity
- Oil bucket has its own liquid level control and oil outlet
- Water exits through a nozzle at the bottom of the main section
- Interface level is controlled by the water dump valve
Bucket-and-Weir Separator
An oil bucket is created by an adjustable or fixed weir plate. This is the most common three-phase configuration for midstream production:
| Parameter | Design Guideline |
|---|---|
| Weir height | Set above the normal oil-water interface level. Typically 50–70% of vessel ID. |
| Oil bucket length | Minimum 18 inches for level control instrumentation. Typically 24–36 inches. |
| Oil bucket width | Full vessel diameter (no restrictions). |
| Oil overflow rate | Maximum 0.5 ft/s velocity over weir to prevent re-entrainment. |
| Weir plate thickness | 1/4 to 3/8 inch carbon steel. Must withstand hydrostatic head. |
| Weir attachment | Welded to vessel shell with continuous fillet weld. Must seal to prevent bypass. |
Weir Height Calculation
Weir Height Determination:
The oil-water interface level must be below the weir height.
Only oil should overflow the weir (no water carryover).
Hweir = Hinterface + Hoil pad + Hfreeboard
Where:
Hinterface = Normal oil-water interface height from vessel bottom
Hoil pad = Oil pad thickness (depends on oil retention time and rate)
Hfreeboard = Clearance above oil level to prevent splash-over (2–4 in.)
Interface height (hydrostatic balance):
Hinterface = Hweir × (ρo / ρw)
This assumes the oil pad extends from the interface to the weir height.
Interface Level Location
The oil-water interface level in the separation section is determined by the balance of hydrostatic pressures across the weir:
Interface Level (Hydrostatic Balance):
For a vessel with oil overflowing a weir into an oil bucket:
ρw × Hw = ρo × Ho + ρw × (Hw,bucket)
Where:
Hw = Water height in separation section
Ho = Oil pad height in separation section
Hw,bucket = Water height in oil bucket (if any)
The interface level self-adjusts based on fluid densities.
Control is maintained by adjusting the water dump valve.
Weir design rule: The weir height determines the maximum oil-water interface level. If the interface rises above the weir, water will overflow into the oil bucket and contaminate the oil outlet. Proper interface level control is essential to maintain separation quality.
4. Interface Level Control
Controlling the oil-water interface level is the most challenging instrumentation task in three-phase separator operation. The interface must be maintained in the correct zone to prevent water carryover to oil and oil carryover to water.
Interface Level Instruments
| Instrument Type | Principle | Accuracy | Limitations |
|---|---|---|---|
| Displacer (torque tube) | Buoyancy change at interface | Good | Fouling, emulsion pads confuse measurement |
| Guided wave radar | Dielectric change at interface | Very good | Requires adequate dielectric difference |
| Capacitance probe | Dielectric change | Good | Affected by water salinity changes |
| Differential pressure | Hydrostatic pressure at two points | Fair | Affected by density changes; calibration sensitive |
| Nuclear (gamma ray) | Density profile measurement | Excellent | Radiation source licensing; high cost |
| Sight glass | Visual observation | Qualitative | Manual; fouling obscures view |
Control Strategy
- Water dump valve: Controls the interface level by adjusting water discharge rate. Interface level transmitter sends signal to water level control valve.
- Oil dump valve: Controls the oil level in the oil bucket. Oil level transmitter in the bucket controls the oil dump valve.
- Gas pressure control: Back-pressure regulator or control valve maintains vessel operating pressure. Independent of liquid level control.
Emulsion Pad Management
An emulsion pad (rag layer) often forms at the oil-water interface. This pad of unresolved emulsion can grow over time and reduce effective separation volume:
- Emulsion pad thickness can be monitored by interface profiling instruments (guided wave radar or nuclear)
- Chemical demulsifier injection breaks the emulsion and reduces pad thickness
- Some separators include an emulsion drain nozzle between the oil and water outlets
- Heat treatment (raising temperature by 10–30°F) can break many emulsions
- If the emulsion pad grows unchecked, it will eventually contaminate both the oil and water outlets
Interface control tip: Guided wave radar is the preferred interface level technology for new installations. It can detect both the gas-oil interface and the oil-water interface simultaneously, and can identify the thickness of an emulsion pad. Displacer-type instruments are adequate for clean services but struggle with emulsion pads.
5. Sizing Methodology
Three-phase separator sizing requires satisfying four independent criteria simultaneously. The vessel must be large enough to meet all four:
Four Sizing Criteria
| Criterion | Controls | Method |
|---|---|---|
| 1. Gas capacity | Vessel diameter | Souders-Brown K factor |
| 2. Oil retention time | Oil section volume | V = Qoil × tretention |
| 3. Water retention time | Water section volume | V = Qwater × tretention |
| 4. Oil-water settling | Settling section length or height | Stokes' Law droplet settling |
Gas Section Sizing
Gas Capacity (same as two-phase):
Vmax = KSB × SQRT[(ρL - ρG) / ρG]
Gas area = Atotal - Aliquid
Agas = Qa / Vdesign
For horizontal three-phase:
Gas area = Vessel cross-section above the liquid level
Typically 25–50% of the vessel cross-section
Liquid Section Sizing
Oil Volume:
Voil = Qoil (bbl/min) × toil (min)
Water Volume:
Vwater = Qwater (bbl/min) × twater (min)
Total Liquid Volume:
Vliquid = Voil + Vwater
Convert to vessel dimensions:
Vliquid = Aliquid × Leff
Where:
Aliquid = Cross-sectional area occupied by liquid
Leff = Effective settling length (T-T minus inlet/outlet zones)
Water Droplet Settling in Oil
Water Droplet Settling Check:
Calculate the settling velocity for a 200 μm water droplet in oil:
Vt = g × d² × (ρw - ρo) / (18 × μo)
Required settling distance = Oil pad height
Required settling time = Oil pad height / Vt
This settling time must be less than the oil retention time.
If not, increase retention time or reduce oil pad height.
Vessel Sizing Summary
| Step | Calculation | Determines |
|---|---|---|
| 1 | Gas capacity (K factor) | Minimum vessel diameter |
| 2 | Oil + water retention volume | Minimum liquid volume |
| 3 | Water settling in oil (Stokes') | Minimum oil pad height or retention time |
| 4 | Oil rising in water (Stokes') | Minimum water section height |
| 5 | Weir height and bucket sizing | Interface location and oil outlet |
| 6 | L/D optimization (3:1 to 5:1) | Final vessel dimensions |
Controlling criterion: In most three-phase separator designs, the liquid retention time (either oil or water) controls the vessel size, not the gas capacity. This is the opposite of scrubbers, where gas capacity controls. Always check all four criteria and let the most demanding one set the vessel size.
6. Worked Example
Size a horizontal three-phase production separator for a gas-condensate-water production facility.
Given:
Gas flow: 10 MMSCFD (SG = 0.70)
Oil rate: 1,500 bbl/day (40 API condensate, ρo = 51.5 lb/ft³)
Water rate: 500 bbl/day (ρw = 65.0 lb/ft³)
Water cut: 25%
Operating pressure: 300 psig
Operating temperature: 100°F
Oil viscosity: 1.5 cP = 0.00101 lb/ft·s
Gas density: 1.10 lb/ft³
Oil retention: 5 minutes
Water retention: 5 minutes
Step 1: Gas Capacity
KSB = 0.40 (horizontal, wire mesh)
Vmax = 0.40 × SQRT[(51.5 - 1.10) / 1.10]
Vmax = 0.40 × SQRT[45.8] = 0.40 × 6.77 = 2.71 ft/s
Vdesign = 0.75 × 2.71 = 2.03 ft/s
Qa = 10 × 10&sup6; / 1440 × (14.7/314.7) × (560/520) / 0.95
Qa = 6,944 × 0.0467 × 1.077 × 1.053 = 367.7 ACFM = 6.13 ACFS
Gas area required = 6.13 / 2.03 = 3.02 ft²
Step 2: Liquid Volumes
Oil volume = 1,500 / 1,440 × 5 = 5.21 bbl = 29.2 ft³
Water volume = 500 / 1,440 × 5 = 1.74 bbl = 9.73 ft³
Total liquid volume = 38.9 ft³
Step 3: Select Vessel Diameter
Try 42-inch ID vessel (3.50 ft):
Total area = π/4 × 3.50² = 9.62 ft²
Liquid at 60% fill: Aliq = 0.60 × 9.62 = 5.77 ft²
Gas area = 0.40 × 9.62 = 3.85 ft² > 3.02 ft² [OK]
Required effective length for liquid:
Leff = Vliquid / Aliq = 38.9 / 5.77 = 6.74 ft
Vessel T-T length with inlet/outlet zones:
LTT = Leff + 2 × D = 6.74 + 2 × 3.50 = 13.74 ft
L/D = 13.74 / 3.50 = 3.93 [OK, within 3:1 to 5:1]
Select: 42-inch ID × 14 ft T-T
Step 4: Verify Water Droplet Settling
Target: 200 μm water droplet settling in oil
d = 200 μm = 200 × 10-6 m = 6.56 × 10-4 ft
Vt = 32.174 × (6.56 × 10-4)² × (65.0 - 51.5) / (18 × 0.00101)
Vt = 32.174 × 4.30 × 10-7 × 13.5 / 0.01818
Vt = 1.868 × 10-4 / 0.01818 = 0.01028 ft/s
Oil pad height (at 60% fill, oil occupies roughly top 20% = 0.70 ft)
Settling time = 0.70 / 0.01028 = 68 seconds = 1.1 minutes
Oil retention time = 5 minutes > 1.1 minutes [OK - adequate settling]
Summary
| Parameter | Value |
|---|---|
| Vessel size | 42 in. ID × 14 ft T-T horizontal |
| L/D ratio | 3.93 |
| Liquid fill | 60% (oil + water) |
| Gas design velocity | 1.59 ft/s (59% of Vmax) |
| Oil retention | 5 minutes |
| Water retention | 5 minutes |
| Settling time (200 μm in oil) | 1.1 minutes |
Design check: The 42-inch x 14-foot vessel provides adequate gas capacity, liquid retention, and droplet settling. The L/D ratio of 3.93 is within the recommended range. The 200-micron water droplet settles in 1.1 minutes, well within the 5-minute oil retention time, confirming adequate oil-water separation.
7. Operations & Troubleshooting
Startup Procedure
- Fill the vessel with clean water to the normal water level before introducing production fluid
- Commission interface level instruments with water and oil (or diesel as surrogate)
- Set water dump valve to maintain interface at design level
- Start demulsifier injection before introducing production
- Bring production on slowly, allowing the separator to establish stable interface
Common Problems
| Problem | Cause | Solution |
|---|---|---|
| Water in oil outlet (high BS&W) | Interface too high, emulsion pad, insufficient retention | Lower interface; increase demulsifier; reduce flow rate |
| Oil in water outlet | Interface too low, emulsion pad growth | Raise interface; drain emulsion pad; check demulsifier |
| Erratic interface level | Slugging, emulsion pad, instrument fouling | Install slug catcher upstream; clean instruments |
| Growing emulsion pad | Inadequate demulsifier, incompatible chemicals | Optimize demulsifier type and dosage; drain rag layer |
| Gas carry-under | Vortexing at oil or water outlet | Install vortex breakers; reduce outlet velocity |
| Foaming | Gas breakout at interface, surfactants | Anti-foam baffles; defoamer injection |
Performance Optimization
- Demulsifier optimization: Conduct bottle tests with different demulsifier types to find the most effective chemical for your crude. Dosage: typically 10–50 ppm.
- Temperature: Increasing separator temperature by 10–20°F can dramatically improve separation, especially for medium to heavy oils.
- Inlet device: A proper inlet device (vane or cyclone) improves separation by distributing flow and performing initial gas-liquid separation, reducing turbulence in the settling section.
- Coalescence plates: Parallel plate packs or corrugated plate interceptors (CPI) in the water section can improve oil removal from water by reducing effective settling distance.
Operations priority: Interface level control is the single most important operating parameter for three-phase separators. An operator who maintains stable interface level will achieve good separation quality regardless of other factors. Invest in reliable interface level instruments and proper operator training.
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