How to Size a Centrifugal Gas-Liquid Separator

Sizing factors, chosen size, and the online calculator

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Properly sizing centrifugal separators is often difficult for non-air applications because the test data is air at atmospheric pressure and 60°F. Other gases, temperatures, and pressures must be converted to that basis. The gas separator sizing tool does the conversion in a few seconds. A separate tool is available for steam.

 

What a Centrifugal Separator Removes

Gas enters the body. Internal geometry and the ratio of critical distances force entrained droplets across the circumference at an angle to the inlet, forming a vortex. Droplets either impinge on internal structures or are propelled to the wall. Droplets and particles larger than 10 microns collect at the bottom with 99 percent efficiency. Design Capabilities of Gas/Liquid Separators covers the internal design. Related reading: centrifugal inertia and separation efficiency.

Four Inputs

The sizing process starts with four values that set internal velocity:

1. Average molecular weight of the gas
2. Design temperature (maximum)
3. Design pressure (minimum)
4. Volumetric flow (the calculator converts mass flow to volumetric flow if needed)

 

The calculator returns two sizes: standard separator size and receiver separator size. Receiver separators have a more complex design and remove a higher volume of liquid for a given body. Use standard separator size for every design that is not a receiver. See types of centrifugal separators for the styles, and required sizing data for the full data set.

 

“Size” Is Not the Nozzle on the Drawing

Separator size is the largest nozzle that can be attached to that body. It is not the nozzle that will be welded on for the line.

 

Every centrifugal separator has a cylindrical body with dished heads. Diameter, length, and internal geometry are set so that droplets larger than 10 microns are removed at 99 percent efficiency at the design velocity.

 

The formula returns the minimum nozzle ID that matches that body. That calculated size is usually not a standard pipe size. It is the theoretical minimum. Standard bodies are then built to the next standard nozzle size. Drawings list size as the maximum nozzle the body can take. The calculation lists the minimum nozzle ID needed for a proper vortex.

 

Chosen Size

Chosen size lets you compare performance of standard bodies. If the calculation requires 2.29 inches, the body is at least a 2.50 inch size. The tool also shows a larger body, such as 3.00 inches, so differential pressure and liquid capacity can be compared before the order.

When to Oversize

A 2.50 inch body is the smallest that meets a 2.29 inch calculation. If the inlet piping is 3.00 inches, use the body that takes a 3.00 inch nozzle. Do not install a separator with an inlet smaller than the pipeline, even when the calculation allows it. The line was sized for a reason. Do not restrict it.

 

The reverse is acceptable. If the pipeline is smaller than the minimum separator nozzle, smaller nozzles can be fitted on the body to avoid transitions.

 

Common sizing errors are covered in common mistakes when sizing centrifugal separators.

 

Liquid Load and Differential Pressure

If the differential pressure at the chosen size is too high, increase the size until the pressure drop is acceptable.

 

Use the liquid removal capacity chart the same way. If the design does not hold the expected liquid load, increase the chosen size until it does. Capacity by model is listed on separators grouped by liquid separation capacity.

 

Maximum Capacity

Maximum capacity is the standardized volumetric flow (Qc), also shown as weight flow. Droplet removal capacity is stated as a percentage of maximum weight flow. That percentage also shows how much surge the unit can take. The maximum capacity figure is accurate to within 2 percent for 6 inch to 32 inch sizes, and 4 percent for other sizes.

 

Got slugs?
A separator designed for condensate removal of at least 20 percent of its maximum capacity will handle most liquid slugs from upset conditions or low points in the piping.

 

Condensate Removal and the Drain Trap

Float drain traps remove the separated liquid. They are available in several body sizes and orifice sizes. The calculator gives condensate removal as both mass and volumetric flow so it can be compared to a trap capacity chart. Size the separator and the trap together. See how to size a liquid drain trap.

 

Sizing Flow Chart

The sizing tool is also useful when comparing centrifugal separators to vane and mesh-pad designs. Vane and mesh units are typically larger, cost more, and have a higher pressure drop. They also lose efficiency at low flow, such as start-up and shut-down. See vane vs centrifugal separators and sizing charts and formulas.