Two-Phase Separator Retention and Gas Capacity
Whether a two-phase separator is big enough, which is two independent questions in one vessel.
Example
You enter
- Vessel diameter (ft) 4
- Seam-to-seam length (ft) 12
- Liquid fraction of the vessel (0 to 1) 0.5
- Liquid rate (bbl/day) 1200
- Required retention (min) 3
- Gas rate (MMSCFD) 3.5
- Operating pressure (psig) 400
- Operating temperature (degF) 100
- Compressibility factor Z 0.92
- Gas specific gravity (air = 1) 0.7
- Liquid density (lb/cu ft) 52
- Souders-Brown K factor 0.35
You get
- Liquid volume 13.43 bbl (needs 2.50)
- Retention achieved 16.1 min against 19% of the requirement
- Required liquid bbl 2.5
- Liquid pct of required 18.6178
- Gas density 1.522 lb/cu ft
- Souders-Brown limit 2.02 ft/s
- Actual gas velocity 0.226 ft/s (11% of the limit)
- Gas pct of max 11.2296
Details, formula, and sources
The liquid side is pure residence time: flow rate times the minutes needed, which sets the liquid volume below the interface. The gas side is a velocity limit: gas moving faster than the settling velocity of a droplet re-entrains liquid and carries it out of the gas line, which sets the vessel's cross-sectional area. EITHER CAN GOVERN AND THE TWO FAILURES LOOK NOTHING ALIKE. A vessel sized only on liquid retention can be far too small in diameter for its gas rate, and the symptom is liquid carryover into the gas line and eventually a damaged compressor. A vessel sized only on gas can be too short for the liquid to degas, and the symptom is gas breaking out downstream in the oil line and upsetting the tank battery. Both are computed here and the governing one is named from the numbers, because a vessel can look generous on the side that was checked and be marginal on the side that was not. RETENTION DOES NOT FIX RE-ENTRAINMENT. If the gas velocity is over the settling limit the separator carries liquid regardless of how many minutes the liquid side shows, and the fix is a larger diameter or a mist extractor rather than a longer vessel. FOAM IS THE WILD CARD AND NO DIAMETER FIXES IT: a foaming crude can need several times the nominal retention, which is why a separator that worked on one well can fail on another from the same field, and why the answer there is a defoamer, an internal or a much larger vessel. This screens an entered geometry against entered rates and properties. It does not select the K factor, the retention time, or the compressibility factor -- all three depend on the service and the internals and are entered -- predict foaming or a foam-corrected retention, design internals, inlet devices or mist extractors, address three-phase separation and the oil-water interface, size relief or level control, or determine the vessel's pressure rating. API 12J, the operator's facility standards, and the design engineer govern.
liquid retention = liquid volume / flow rate; the gas limit is Souders-Brown v = K sqrt((rho_L - rho_G)/rho_G) with gas density from PM/(ZRT), against the actual velocity across the vapour space.
The K factor, the required retention time and the compressibility factor are ENTERED: all three depend on the service and the internals.
Cylinder geometry, the real-gas density relation, and one velocity comparison.
Estimate. AHJ and licensed professional govern.
Field names used by the API: vessel_diameter_ft, seam_to_seam_ft, liquid_fraction, liquid_rate_bpd, required_retention_min, gas_rate_mmscfd, pressure_psig, temperature_f, z_factor, gas_gravity, liquid_density_lb_ft3, k_factor, liquid_volume_bbl, actual_retention_min, required_liquid_bbl, liquid_pct_of_required, gas_density_lb_ft3, max_velocity_fps, actual_velocity_fps, gas_pct_of_max
- K factor entered it depends on the internals and whether a mist extractor is fittedAPI 12J and the vessel manufacturer
- Retention time entered a foaming crude can need several times the nominalthe design engineer
- Two-phase only the oil-water interface of a three-phase vessel is a separate problemthe operator's facility standards