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Oil & Gas · Interactive Engineering Model
Separator Sizing Calculator
Diameter · Height · Vapour velocity
Change the gas side and the liquid side separately and see which dimension of the vessel each one controls — Souders-Brown for the diameter the gas needs, retention time for the height the liquid needs, and the next fabricated size up.
Vapour rate at standard conditions. The vessel sees the much smaller actual volume at operating pressure.
Total liquid rate to be held for retention. This sets the height, not the diameter.
Absolute pressure in the vessel. Higher pressure means denser gas, less actual volume and a markedly smaller diameter.
How long the liquid must stay in the vessel. Two to five minutes is typical; foaming or viscous crude needs far longer.
Start from an example: 25 MMSCFD with 300 m³/day of condensate at 60 bar a →
Advanced parameters5
- —Vessel diameter
- —Minimum diameter
- —Vessel height
- —Height to diameter
- —Maximum vapour velocity
- —Suggested K at this pressure
- —Actual vapour velocity
- —Margin below the limit
- —Liquid height
- —Retention volume
- —Gas density
- —Actual gas volume
Move a parameter and the vessel answers: the diameter the gas needs, the height the liquid needs, and the size that would actually be fabricated.
The link carries the figures you entered, in the part of the address that is never sent to a server.
Change one assumption at a time. The model updates from the same calculation used for the reported result.
Try this
First increase gas flow while keeping the liquid side fixed. Then reset it and increase liquid flow or retention time. Watch which dimension of the vessel responds.
How the model works
- The gasVapour has to slow enough for droplets to fall out of it. That sets the area, and the area sets the diameter — nothing about the liquid changes it.
- The liquidBelow the inlet the vessel holds the minutes of liquid you asked for. More flow or more retention raises the level; the shell grows taller, not wider.
- The freeboardAbove the level there has to be room for the inlet zone, the disengagement space and the mist extractor. That space is added to the liquid height.
- The sizeVessels are made in steps, so the calculated minimum is rounded up to the next fabricated diameter. Push the gas a little further and the whole shell jumps to the next size.
Need sizing against a real feed characterisation?
We can work from your actual fluid data, slug and surge behaviour and internals selection, and build it into your facility engineering workflow.
Calculation basis 6 steps · view the method →
Gas density = P·MW ÷ (Z·R·T)—Actual gas volume = mass flow ÷ gas density—Maximum velocity = K × √((ρ liquid − ρ gas) ÷ ρ gas)—Minimum diameter from the area that velocity requires—Liquid height = liquid flow × retention time ÷ vessel area—Height = liquid height + inlet zone, disengagement space and mist extractor—
Assumptions & limitations Screening estimate for preliminary vessel sizing · 7 assumptions →
This is a preliminary sizing aid, not a vessel design. It covers a vertical two-phase separator in steady service and excludes slug and surge behaviour, foaming and emulsions, three-phase water separation, inlet device selection, nozzle sizing and momentum limits, level instrument spans, and all mechanical design — thickness, code, materials and relief. Mechanical design must follow the applicable pressure vessel code with a qualified engineer, and the process duty should be confirmed against a real feed characterisation.
- Vertical two-phase vessel with a mesh mist extractor, in steady service.
- Souders-Brown with the K factor entered. K is not a constant: the suggested value follows the usual screening correlation, which walks K down from 0.35 ft/s as pressure rises above 100 psig. It takes no account of foaming, surface tension or the specific internals.
- Slug and surge volume is not included — a slug catcher or surge allowance is a separate calculation.
- The height allowance above the liquid is a screening stack: 0.6 m inlet zone, disengagement of at least 0.9 m or half the diameter, and 0.3 m for the mist extractor.
- Diameters are rounded up to 6-inch increments; fabricators may offer a different set.
- Three-phase separation, water dropout and emulsion behaviour are out of scope.
- Nothing mechanical is calculated: thickness, nozzles, supports, relief and code compliance are all separate work.
Worked examples
- 25 MMSCFD with 300 m³/day of condensate at 60 bar a
Open with these figures → - The same duty separated at 15 bar a instead
Open with these figures →
What this model reveals
A separator does two different jobs, and they control different dimensions. The gas side determines how much cross-sectional area is needed for droplets to disengage. The liquid side determines how much volume is required for retention. That is why increasing gas flow tends to make the vessel wider, while increasing liquid flow or retention time tends to make it taller.
Go deeper
Oil & Gas Operations challenge
Separation, custody transfer, allocation and volume correction.
Digital Twins & Asset Management challenge
Keeping the model of a facility honest about the plant it describes.