Design Criteria to Select Centrifugal Gas/Liquid Separators

This article provides the essential design criteria used to size and select centrifugal gas-liquid separators for steam, compressed air, natural gas and other industrial vapor applications.

Required Design Data

To properly size a centrifugal separator the following information is required:

1. Molecular weight of the gas (or average molecular weight)
2. Design (maximum) temperature
3. Lowest operating pressure for which performance is required
4. Volumetric or mass flow rate of the gas
5. Approximate amount of liquid / particulate to be removed
6. Droplet size separation threshold required

Droplet Size Separation Threshold

99% efficiency for separation of entrained droplets and particles having a diameter as fine as:

• >10 microns: All single-stage separators satisfy this requirement and are the most common design. Includes a design variation specifically for high particle and sludge-like separation.
• >5 microns: Require a dual-stage separator design referred to as a coalescing separator where the 1st stage coalesces droplets >5 and <10 microns into droplets >10 microns to be separated with 99% efficiency in the 2nd stage.
• >0.3 microns: Requires a highly specialized dual-stage separator design where the 1st stage separates droplets larger than 10 microns and the 2nd stage removes finer droplets using specialized coalescing filter cartridges.

Once the applicable separator design is determined, the model is selected based primarily upon preferred nozzle orientation (in-line or offset) and installation footprint.

Summary of Sizing Requirements

The required separator size has three main components. It needs to:

1. Handle the design flow rate of the application
2. Satisfy the maximum acceptable differential pressure across the separator
3. Have a separation capacity ≥ the design separation volume for the application

Automation of Calculations

The online separator size calculator supports quick separator sizing and “what-if” comparisons. Separation capacity is calculated in mass and volumetric units for all models. Differential pressure is also calculated so you can quickly toggle separator size until the pressure-drop requirement is satisfied. The calculator includes unit-conversion cells and uses the underlying polynomial formulas for greater precision than printed charts.

Instructions for the Web-Based Calculator

There are two calculators: one for steam applications and one for all other gases.

Steam: Required inputs are minimum operating pressure (PSIG) and maximum mass flow rate (lbs/hr). If steam quality is known, enter it; otherwise the default value of 95% is used.

Two sizes are calculated: Standard Separator Size and Receiver Separator Size. Receiver separators are capable of separating up to 90% of the separator’s maximum flow-rate capacity and are used for heavy liquid loads or when footprint must be minimized. Most applications use a Standard design.

The “chosen size” is the next larger standard pipe size equal to or greater than the calculated requirement. Increase the chosen size if a lower differential pressure is required, then verify that the selected model’s separation capacity meets or exceeds the design liquid load.

Air and Other Vapors – required inputs:

1. Molecular weight (or average molecular weight)
2. Design (maximum) temperature
3. Minimum operating pressure
4. Volumetric flow rate (SCFM)
5. Optional: density of the liquid to be separated (water is the default)

NOTE: While design (maximum) pressure is required to determine the pressure class of the vessel, it is not required for separator sizing. Using the maximum possible temperature with the minimum possible pressure produces the “worst-case” gas volume for calculation purposes.

If both volumetric and mass flow values are available, convert mass flow to volumetric units and use the higher SCFM value. Mass flow is usually more accurate because it includes entrained liquid.

Separator Models

The >0.3 micron threshold corresponds to a single specialized model (TF). The >5 micron threshold is offered in three coalescing designs. All other designs satisfy the common >10 micron requirement.

When comparing models, pay attention to separation capacity and ability to handle liquid slugs. Each separator datasheet describes design, acceptable installation orientations, maximum separation capacity and slug-handling capability.

Separator Sizing Customizations

All fabricated designs can be customized. Common options include:

• Extra inlet/outlet/drain/instrument/inspection ports
• Oversized drains
• Resized nozzles (drain ports should not be reduced)
• Connection types: threaded, flanged, butt-weld, tri-clamp
• Materials: carbon steel, 304L SS, 316L SS, Alloy 20, Duplex SS, Hastelloy, etc.
• Pressure classes 150/300/600/900 and higher
• Support lugs, lifting lugs and support legs
• Nozzle orientation adjustments on the same plane
• Flanged bodies for clean-out or inspection
• Corrosion allowance
• Non-standard NDE/NDT (radiography, liquid dye penetrant, ultrasonic, PMI, coating tests)
• External coatings and insulation clips

Documentation

Standard documentation meets ASME Code Section VIII Division 1 requirements for code-stamped vessels (hydrostatic test confirmation and outline drawing). Additional documentation available when specified on the purchase order includes:

• ASME code calculations
• NDE/NDT procedures and reports
• Coating procedures and reports
• WPS/PQR/WPQ
• ITP (customizable for non-standard NDE/NDT)
• Weld map
• Material test reports
• Certificate of conformance
• Certificate of origin

Most of the above documentation is provided for record only and carries an applicable cost. Many documents cannot be generated after fabrication, testing and code stamping are completed.

ASME Code Section VIII Division 1

ASME Code Section VIII Division 1 provides the rules for design, fabrication, inspection, testing and certification of pressure vessels. Any non-standard NDE/NDT or documentation requirements must be communicated during the proposal stage so that we can confirm what can be provided and adjust pricing and lead time accordingly.

Use our website calculators to obtain a preliminary size, then contact us with your complete design criteria so we can prepare a detailed proposal.