Gas/Liquid Separators Explained

Design and Capabilities of Centrifugal Gas/Liquid Separators

Gas/liquid separators are pressure vessels designed to remove entrained particles and droplets from gaseous processes such as steam, compressed air, and process gases.

 

Principle of Operation

Moving gas carrying condensed vapor or foreign matter is directed against a baffle, suddenly changing direction. The lighter gas flows around the baffle while heavier moisture and particles strike it, lose velocity, and fall to a coalescing area out of the gas stream.

 

A properly designed separator removes 99% of particles and droplets larger than 10 microns. The baffle and vessel proportions must collect and drain the liquid without re-entrainment. Gravity and gas velocity cause the separated liquid to cling to the vessel wall and flow downward to the drain. A vortex containment plate prevents re-entrainment.

 

The vessel must be large enough at right angles to the flow to accommodate the baffle and the change of direction without creating excessive pressure drop.

Physics of Separation

• Centrifugal Force – Internal geometry generates centrifugal force that drives droplets and particles >10 microns to the vessel wall.
• Velocity Drop – The larger cross-sectional area of the vessel reduces gas velocity so heavier material impinges on internal surfaces.
• Torturous Path – The path between inlet and outlet forces droplets and particles to contact surfaces where they coalesce and drain.

 

These separators have an infinite turndown ratio. Efficiency is maintained even when flow is drastically reduced because the drop in velocity compensates for reduced centrifugal force. This is valuable during start-up and shut-down.

 

Most units operate for decades with zero maintenance. They have no moving or replaceable parts and are considered self-cleaning due to continuous centrifugal action. An inspection port is still recommended for visual checks.

 

Drainage of Condensate

Condensate is removed without loss of process gas using a float-style drain trap. The trap contains a hollow stainless-steel float connected to a valve. As condensate rises, the float lifts the valve and system pressure pushes the liquid out. When the level drops, the valve reseats, keeping a liquid seal that prevents gas loss.

 

Manual or actuated valves with sight gauges can be used but are more complex and expensive for most applications. Float traps include a blow-down port and can be protected with a simple Y strainer. Heaters are available for freeze protection.

 

Typical Applications

The most common uses are steam systems (protecting equipment from condensate and pipe scale) and compressed-air systems. Oil refineries and petrochemical plants use them at various temperature and pressure points for product recovery. High solids loads that form a sludge can be handled with modified drain sections and rotary valves.

 

Liquid removal rates typically range from 5% to 90% of the gas weight flow, depending on separator size and design.

 

Finer Separation Options

For droplets finer than 10 microns, a coalescing mesh pad stage can be added to remove particles larger than approximately 4 microns. The fine droplets coalesce into larger ones that the main separation stage then removes. This requires a two-piece vessel for mesh access.

 

For sub-micron removal (down to 0.30 microns), a borosilicate microglass filter element stage can be added after the separation stage.

 

Nozzle Orientation & Exhaust Heads

Inlet and outlet nozzle orientations can be customized for horizontal, vertical-up, vertical-down, or combination flow paths.

 

Exhaust Heads are a special atmospheric version of a gas/liquid separator mounted at the end of a vent pipe (most often steam). One side is open to atmosphere, so they are not pressure vessels. They reduce the volume and opacity of the condensate plume and allow condensate recovery. Any noise reduction is a side benefit of the tortuous path, not a design objective.

 

Design Criteria Needed for Sizing

1. Molecular weight (MW) of the gas
2. Maximum temperature and minimum pressure (even if mutually exclusive)
3. Gas flow rate (volumetric or mass)
4. Existing or intended inlet pipe size
5. Approximate amount of condensate to remove
6. Desired flow path (horizontal, vertical, etc.)

 

The initial cost of a properly designed gas/liquid separator is essentially the final cost. These vessels operate reliably for decades with no maintenance while protecting equipment and improving process efficiency.