Engineering task and calculation objective
The HSA module sizes gravity separators for gas-liquid mixtures in horizontal and vertical configurations. If you want to calculate a gas-liquid separator, two tasks must be solved: the gas velocity in the vessel must lie far enough below the critical velocity for the governing limiting droplet to settle by gravity before the gas reaches the outlet — with or without a supporting demister —, and the liquid volume must provide the required holdup times between the operating levels (high, normal and low level).
The module covers the complete design: limiting droplet calculation with and without demister, inlet distributor and outlet demister, high-low levels for holdup times, the assessment of the flow pattern in the feed line (cf. module LOMA), the sizing of anti-vortex baffles at the liquid outlet, and liquid entrainment from surfaces. The basis is the relevant literature, including A. Bürkholz "Droplet Separation", the VDI Heat Atlas (flow patterns of two-phase flow), and the work of Harlemann and Patterson on vortex formation at outlet nozzles.
Typical applications in plant engineering are inlet separators upstream of compressors, flash drums, column reflux drums and slug catchers — wherever entrained droplets would endanger machinery or violate process limits.
Standard and calculation basis: Harlemann "Selective Withdrawal from a vertically Stratified Fluid", Intern. Assoc. Hydro. Research Patterson "Experimental Investigation of Critical Submergence for Vortexing in a vertical Cylindrical Tank", University of Southern California VDI-Wärmeatlas / Strömungsformen für Zweiphasen-Strömung R. Marr und F. Moser "Verfahrenstechnik 9" (1975) Nr. 8, S. 379/382 A.Bürkholz, Droplet Separation, VCH. 1989
Calculation workflow
- Record operating data and fluid properties: The inputs are the gas and liquid volume flows, the densities of both phases, the dynamic viscosities of gas and liquid, and the surface tension. From these, the total volume flow is formed and the flow pattern in the feed line is assessed.
- Determine the critical gas velocity and the limiting droplet: From the force balance between gravity, buoyancy and drag on the droplet follows the settling velocity; from this, the critical velocity of the gas and the critical limiting droplet are derived — the smallest droplet still reliably separated for the chosen geometry.
- Fix the vessel dimensions: Via the length-to-diameter ratio, the number of inlet nozzles and the distance of the low liquid level (LLL) from the vessel bottom, the vessel diameter and vessel length are chosen so that the available cross-flow area for the gas exceeds the required area and the ratio of gas velocity to critical gas velocity stays below 1.
- Check liquid levels and holdup times: The areas between the emergency level (ELL), high level (HLL) and low level (LLL) are calculated from the vessel geometry and compared with the required holdup times (e.g. from ELL to HLL and from HLL to LLL) — they give the control system and the operator the necessary reaction time.
- Size nozzles and internals: The inlet nozzle is limited via the kinetic energy or momentum of the two-phase flow (ρ·w² criterion); the liquid outlet is protected against vortex formation with an anti-vortex baffle, whose area, side length and distance from the vessel bottom are calculated. Optionally, an outlet demister is provided for fine separation.
- Verify the separation: Finally, the actually separated limiting droplet is compared with the requirement: without a demister, droplets down to a few hundred micrometers must typically be removed by gravity; with a demister, the demister takes over the droplets below the limiting droplet.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Gas flow | Vgas | m³/s |
| Liquid flow | Vfl | m³/s |
| Total flow | Vges | m³/s |
| Density of the gas | ρg | kg/m³ |
| Density of the fluid | ρfl | kg/m³ |
| ⇒ Critical gas velocity | vc | m/s |
| Dynamic viscosity of the gas | ηg | mPa·s |
| Surface tension of the fluid | σ | mN/m |
| Number of inlets (1 or 2) | n | – |
| ⇒ required flow area for the gas | ≥ | m² |
| Distance between LLL and vessel bottom | hlb | m |
| Ratio of length / diameter | L/D | – |
| Holdup-Time from ELL to HLL | ht1 | s |
| ⇒ Area between ELL and LLL | Ael | m² |
| ⇒ Area between LLL and vessel bottom | Alb | m² |
| ⇒ Available flow area for the gas | ≥ | m² |
| Holdup-Time from HLL to LLL | ht2 | s |
| Vessel diameter | D | m |
| Vessel length | L | m |
| Kinetic energy of the gas | ρg · wst2 | J |
| Inside diameter of the inlet nozzle | des | m |
| ⇒ Velocity in the inlet nozzle | wst | m/s |
| Type of inlet nozzle | 3 | – |
| Distance to impingement plate | x | m |
Frequently asked questions
When is a horizontal separator chosen, and when a vertical one?
Vertical separators are suited to high gas fractions and small liquid quantities: the droplets settle against the gas flow, and the footprint is small. Horizontal separators are advantageous when large liquid quantities must be buffered or slug flow occurs — the droplets settle perpendicular to the gas flow, and the large liquid volume provides long holdup times at a moderate diameter. For three-phase separation (gas/oil/water), the horizontal configuration is almost always the right choice because of the large phase interface.
What is the limiting droplet and what does it depend on?
The limiting droplet is the smallest droplet diameter that is just completely separated in the vessel: its settling velocity must be sufficient to traverse the gas space height within the residence time of the gas. It depends on the density difference between liquid and gas, the gas viscosity, the gas velocity and the flow geometry. Pure gravity separators achieve limiting droplets of a few hundred micrometers in practice; finer droplets require a demister, which works via inertial separation down to the range of a few micrometers.
What is the anti-vortex baffle at the liquid outlet for?
If the liquid level above the outlet nozzle falls below a critical submergence, an intake vortex forms that draws gas into the liquid line — causing cavitation and delivery problems at downstream pumps. A vortex breaker (e.g. a cross or plate baffle) above the nozzle suppresses the swirling flow. Following the investigations of Harlemann and Patterson, the module calculates the required baffle area, side length and distance from the vessel bottom as a function of nozzle diameter and outlet velocity.
Why must the flow pattern in the feed line be considered?
The two-phase flow in the feed line — stratified, wavy, slug or annular flow according to the flow pattern maps of the VDI Heat Atlas — determines the droplet spectrum and the momentum at the inlet. Slug flow delivers large liquid quantities in surges and demands buffer volume and a robust inlet distributor; annular flow with high gas momentum generates fine droplets and makes separation more difficult. In addition, the inlet momentum (ρ·w²) is limited to avoid atomization and re-entrainment in the vessel.