Inputs
Example data
Use this table to validate your setup and expected output ranges.
| Case | Oil (bpd) | Water (bpd) | Gas (MMSCFD) | tliq (min) | tgas (s) | fliq | L/D | Typical output |
|---|---|---|---|---|---|---|---|---|
| Sample A | 800 | 600 | 2.5 | 5 | 20 | 0.55 | 3.5 | Diameter ~ 1–2 m, Length ~ 4–8 m |
| Sample B | 1500 | 1200 | 4.0 | 7 | 25 | 0.60 | 4.0 | Larger diameter driven by gas capacity |
| Sample C | 300 | 900 | 1.2 | 6 | 15 | 0.50 | 3.0 | Length increases for residence time |
Formula used
- Liquid holdup: Vliq = Qliq × tliq
- Gas space: Vgas = Qgas × tgas
- Total volume split: Vtot ≥ max(Vliq/fliq, Vgas/fgas)
- Geometry (cylindrical shell): V = (π/4) D² L, with L = (L/D) × D
- Gas capacity check: Vs,allow = K √((ρL − ρG)/ρG)
- Gas area requirement: Agas ≥ Qgas/Vs,allow and Agas = fgas(π/4)D²
These relationships provide quick preliminary sizing for planning and construction coordination. Final design should follow project specifications and applicable standards.
How to use this calculator
- Enter expected oil, water, and gas flow rates with units.
- Provide densities that match operating conditions and composition.
- Set liquid residence and gas retention targets for your service.
- Choose a liquid holdup fraction and an L/D ratio target.
- Pick a K value consistent with internals and separation quality.
- Apply a design factor for uncertainty, surges, and margin.
- Press Calculate and review checks and notes.
- Download the CSV or PDF for submittals and documentation.
Design smarter separators with clear calculations and practical outputs.
Practical guide for three-phase separator sizing
Three-phase separators are commonly specified during facility and pipeline construction to stabilize upstream process conditions and protect downstream equipment. A well-sized vessel provides enough time and quiet volume for gas to disengage from liquids, and for oil and water to separate by gravity. This calculator supports early design, bid evaluation, and layout coordination by translating flow forecasts into preliminary diameter and length targets.
Start by confirming a consistent basis for flowrates. Many projects size on average day rates, then apply a design factor for surges, start-up transients, and future debottlenecking. For liquid holdup, the residence-time approach estimates the volume needed to keep oil and water in the vessel long enough to form distinct layers. For gas, a short retention time helps reduce carryover and prevents excessive velocity in the vapor space.
Gas handling is frequently the controlling case. The Souders–Brown relationship estimates an allowable superficial gas velocity using a selected K value and the density contrast between the mixed liquid and the gas. Higher K values may apply with better internals, mesh pads, or vane packs, while conservative values are used for foaming services or uncertain inlet conditions. If calculated gas velocity exceeds the allowance, a larger diameter is recommended.
Geometry matters for installation. A horizontal vessel often fits better on skids, supports stable liquid levels, and simplifies access to nozzles. The L/D ratio influences footprint and fabrication; higher ratios increase length and may improve level control, but they also affect transport and lifting plans. During construction planning, verify clearances for maintenance pulls, platform access, and instrument runs.
Example sizing workflow: Using Sample A (Oil 800 bpd, Water 600 bpd, Gas 2.5 MMSCFD), set a 5-minute liquid residence time, 20-second gas retention time, fliq=0.55, L/D=3.5, and design factor 1.15. The output typically falls in the 1–2 m diameter range with length roughly 4–8 m, depending on the selected K value and gas density basis.
Use the results as a screening tool, then validate with project standards and vendor design notes. Confirm operating pressure and temperature, droplet size assumptions, inlet device selection, mist eliminator type, and control philosophy. Where accurate operating densities are available, substitute them to improve the gas-capacity check and to better reflect field performance.
Finally, document your input basis and margins. When the separator is part of a construction package, clear assumptions reduce rework, support procurement, and align commissioning expectations. Good sizing is a collaboration between process requirements, mechanical constraints, and constructability realities.
FAQs
1) What is a three-phase separator used for?
It separates gas, oil, and water into distinct outlets by gravity and residence time. It improves downstream equipment reliability and enables accurate measurement and control of produced fluids.
2) Which input most often controls vessel diameter?
Gas capacity frequently controls diameter because superficial velocity must stay below the allowable limit. High gas rates or low gas density can quickly increase required cross-sectional area.
3) How should I choose the liquid residence time?
Select it from project specifications, expected emulsions, and separation quality targets. Higher water cut, foaming, or unstable inlet conditions often justify longer residence time to stabilize interfaces.
4) What does the liquid holdup fraction represent?
It is the portion of total vessel volume allocated to liquids for normal level control. Increasing it provides more liquid volume but reduces available gas space, which can affect gas velocity limits.
5) Why is the Souders–Brown K value important?
K reflects separation service and internals. A conservative K reduces allowable velocity and increases required diameter. Use values aligned with vendor guidance, mist eliminator type, and site performance history.
6) Can I use standard gas flow for final mechanical design?
Standard flow is acceptable for early screening, but final design should use operating volumetric flow based on pressure and temperature. That ensures the gas space and velocity checks reflect real conditions.
7) Does this replace vendor sizing and detailed design?
No. It is a preliminary tool for planning and comparison. Final sizing must consider internals, droplet size, foaming, slugs, controls, codes, and the full mechanical design package.