How to use Horizontal Three-Phase Oil-Gas-Water Separator Sizing Calculator
- Enter Associated Gas Flow Rate (Nm³/d) and adjust the options as needed.
- Run the tool and inspect its results, evidence and limitations.
- Copy or download the result, then verify it against the actual website, source data or business records.
Try a concrete example
For an onshore oilfield central processing facility treating associated gas of 45,000 Nm³/d, crude oil rate of 120.0 m³/d, and produced water rate of 180.0 m³/d at 15.0 bar(g) and 45.0°C with an oil density of 850.0 kg/m³, requiring 8.0 min water settling and 5.0 min oil dehydration retention times: Based on API Spec 12J, gas operating density is 11.23 kg/m³ and Souders-Brown critical settling velocity is 0.547 m/s. Sizing at 75% derating (0.410 m/s) requires a gas flow area of 0.089 m². Liquid surge volume requires 1.00 m³ water phase and 0.42 m³ oil phase (total 1.42 m³). Balancing gas vapor disengagement, liquid holdup, and anti-foaming margins yields a recommended vessel ID of DN1000 (1000 mm) with a tangent-to-tangent length of 3800 mm (L/D = 3.80, ideally within the 3.0 to 4.5 hydrodynamic sweet spot). Total vessel volume of 2.98 m³ provides generous disengagement and 25% sludge surge margins. An oil weir height of 550 mm and demister mist extractor area of 0.40 m² deliver strict water-in-oil and oil-in-water separation compliance.
How to interpret results
Horizontal three-phase oil-gas-water separators are core pressure vessel equipment in wellhead platforms, central gathering stations, and refinery desalting feed sections for initial gas-liquid and liquid-liquid phase separation. Sizing and hydrodynamic retention calculations adhere strictly to API Spec 12J and GPSA Engineering Data Book standards. The incoming multiphase stream strikes an inlet momentum diverter to dissipate kinetic energy. Gas disengages via gravity settling, with maximum superficial velocity governed by the Souders-Brown equation (v_crit = K · sqrt((ρ_liq - ρ_gas) / ρ_gas)). Oil and water separate according to Stokes' law based on density differentials; oil droplets cream upward into the oil pad while water droplets sediment toward the bottom. Properly sizing retention time, vessel slenderness ratio (L/D), and weir height prevents gas liquid carryover, crude BS&W non-compliance, and excessive oil-in-water discharge.
Frequently asked questions
What are the API Spec 12J retention time guidelines for crude oil and produced water?
Separation kinetics depend heavily on crude viscosity and density differential (API gravity): First, light crude (API > 35°): low viscosity and high density contrast allow 3 to 5 minutes retention for both oil and free water. Second, medium and heavy crude (API 15° to 30°): elevated viscosity impedes microscopic droplet coalescence, demanding 10 to 20+ minutes for oil dehydration and 8 to 15 minutes for water clarifying. Third, insufficient retention leads to high water carryover in oil (tripping refinery desalter transformers) and severe hydrocarbon carryunder in water (plugging reinjection disposal wells and incurring environmental penalties).
How should the Souders-Brown K-factor be derated for foaming or high-pressure crude?
The Souders-Brown K-factor governs gas velocity to prevent liquid mist entrainment. Clean, non-foaming services with wire-mesh demisters typically size for K = 0.065 to 0.080 m/s: First, foaming crude derating: surfactants, asphaltenes, or high-salinity brines generate dense foam blankets that swallow the vapor space, requiring a 30% to 50% derating (K = 0.035 to 0.045 m/s) to prevent liquid carryover and catastrophic compressor liquid slugging. Second, high-pressure derating: above 50 bar, gas density surges and buoyancy differentials shrink, requiring progressive K-value reduction per GPSA curves.
How do the internal oil weir and interface level transmitter control three-phase separation?
An internal vertical weir plate partitions the vessel into a settling compartment and an oil bucket: First, oil control: stratified oil skims freely over the weir lip into the dry oil bucket, where a standard level transmitter regulates the crude dump valve. Second, water control: produced water drains from a bottom boot or nozzle beneath the settling section, controlled by an interface level transmitter. The displacer float is weighted with a specific gravity tuned between water (1.0) and oil (0.85), floating precisely on the emulsion boundary to discharge water continuously without dumping oil.
What are the critical maintenance considerations for demister mist extractor pads?
The mesh demister pad (100 to 150 mm thick woven stainless wire/polypropylene) captures 5 μm to 10 μm aerosol droplets beneath the gas nozzle: First, wax and scale foulings: paraffinic crudes undergoing pressure drops deposit wax and silt within mesh voids, requiring steam/chemical wash spray headers. Second, differential pressure monitoring: high-accuracy DP transmitters monitor clean drops (100 to 250 Pa); exceeding 600 Pa signals severe plugging that causes high-velocity channeling, re-entraining liquid and demanding immediate bypass cleaning.