Design Criteria to Select Centrifugal Gas/Liquid Separators

Our website provides the necessary charts and formulas to properly size industrial grade centrifugal gas/liquid separators; it is a tedious procedure which is simplified using our web-based sizing calculator. This article describes the base performance charts and procedural overview for properly sizing a centrifugal (vortex) gas/liquid separator.

 

The sizing charts used for determining the minimize separator body size required for a given application were established decades ago.

 

Determining Performance

Wright-Austin established separator sizing guidelines via physical testing with steam and wet air at atmospheric pressure and 60F. They modified a 3” size separator to enable adjustments of its internal structure, eventually leading to establishment of vessel lengths and diameters required for the internal geometry to separate entrained droplets and particles larger than 10 microns in diameter with 99% efficiency. The “separator size” is an indirect reference to the vessel length and diameter and the “size” is actually the largest ID inlet nozzle which can be attached to a given vessels’ circumference. Example, a 6” size separator may have 6” or smaller nozzles used for the inlet/outlet. The same sizing charts and formulas are used for coalescing style separators for separation of entrained droplets and particles larger than 5 microns in diameter with 99% efficiency.

 

The maximum flow rate, for both water and steam were established for several datapoints and the performance extrapolated for other sizes based upon the physical test results. This is possible because separation efficiency is not reduced at lower flow rates and internal proportions are maintained at scale.

 

Sizing: Air and Steam

If your application is for compressed air at 60F and atmospheric pressure or saturated steam, you can simply refer to the established sizing charts. Air at another temperature requires calculating Qc to account for the difference in temperature (Ft) from the 60F at which the performance chart is based before you can refer to the chart. The “separator size required” is the size indicated to the right of the intersection of the flow rate Qc and operating pressure (PSIA).

 

Sizing: Other Gasses

Sizing centrifugal separators for other gasses requires converting the gas to the equivalent SCFM of air at 60F at atmospheric pressure (Qc). Making this conversion requires multiplying (Qsg) by (Fg) and (Ft). The (Fg) and (Ft) correction factors are provided in table form, for an approximation. Once you calculate Qc you can refer to the aforementioned chart for Air.

 

 

Steps for Sizing a Centrifugal Gas/Liquid Separator

1.  Calculate Qc

2. Refer to the Air Flow Chart for air and other gasses or the Steam Flow Chart as applicable. The “separator size required” is per the line to the right of the intersection of the operating pressure (PSIA) and flow rate (Qc). IF a reciprocating style compressor is delivering the flowrate, then double the Qc value for sizing purposes. When you refer to the various separator models, this “size” is the indirect reference to the vessel body length and diameter for that specific separator model. If your intended pipeline is smaller, then we can attach smaller connections to match. If your intended pipeline size is larger, then the separator size needs to increase to match your intended pipeline size.
   
   

   
   Use the appropriate variables to complete the sizing calculations:
   

3. Determine the pressure drop related to the data point in Step 2. It the pressure drop exceeds your design criteria, increase the separator size until the pressure drop is ≤ your design criteria. In such a scenario, the gas flow is not determining the separator size required, rather the required pressure drop is the determining factor. This is very common in low pressure and vacuum systems.

4. Determine the separators maximum flow rate (Qc) where is the intersection of the x-axis with the datapoint of the separator size line and operating pressure (PSIA).

5. Convert to the maximum flow rate capacity (Qc) in volumetric units (SCFM) to mass flow rate units (lbs/hr).
   The maximum liquid separation capacity for a given separator design is expressed as a percentage of the separators maximum mass flow rate, thus this figure will be used to determine whether the separator size is not only large enough to handle the vapor flow, but also capable of separating a sufficient volume of entrained droplets and particles.

6. Determine the droplet size separation threshold required as this will determine which subset of separator designs are required. 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
     i. 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 in diameter and the 2nd stage removes finer droplets using specialized coalescing filter cartridges.

     
     Once you determine the applicable separator design, the separator model needs to be determined which is typically based upon the preferred the nozzle orientation. The separator inlet and outlet nozzles can be “in-line” (180 degrees apart) and for horizontal or vertical sections of pipeline OR they can have off-set nozzles in support of a horizontal or vertical inlet relative to the outlet nozzle. The design that is most compatible for your preferred installation footprint will determine the applicable separator model.  When the physical footprint of the separator is one of the critical design criteria, compare models to find the one that is the best physical fit for your application.

7. Refer to the applicable separator base design and multiply its maximum separation capacity percentage by STEP 5 (maximum mass flow rate for the separator size required). This is the maximum amount of liquid which that size separator and base design can separate based upon the design criteria.
   
   If the estimated amount of entrained liquid and particles to separate is less than its maximum separation capacity, then you have found a separator size and model that satisfies your design criteria. If the amount of entrained liquid is higher than what that size and model separator can handle, you can either increase the separator size until it is large enough OR consider other separator models which have a higher percentage separation rate.

    

   Summary

   The required separator size has three main components, it needs:
   

   1. A. to be able to handle the design flow rate of the application

   2. B. satisfy the maximum acceptable differential pressure across the separator

   3. C. have a separation capacity ³ the design separation volume for the application

    

   Automation Of Calculations

   The online separator size calculator supports quick separator sizing and is supportive of “what if” comparisons. The separation capacity is calculated in mass and volumetric units for all separation models for easy comparison of not only size but separator model. Likewise, the differential pressure is calculated and you can quickly toggle the separator size to determine the separator size required to satisfy the differential pressure required. The calculator also has unit conversion cells in support of using just the one calculator no matter the units used for the input data. The online calculator utilizes the underlying polynomial formula for the correcting and sizing factors for a more precise calculation compared to the printed charts.

    

   Instructions for the Web-Based Calculator
   There are two calculators, one for sizing separators for steam applications and one for all other applications.

    

   Steam: The required inputs are the minimum operating pressure (PSIG) and maximum mass flow rate (lbs/hr). If you know the steam quality, input it, otherwise use the default value (95%).

    

   There are two “sizes” calculated: Standard Separator Size and Receiver Separator Size.

    

   Receiver Separators refer to a specific separator design capable of separating up to 90% of the separators maximum flow rate capacity; thus, they are for applications which have heavy liquid loads or for applications where minimizing the footprint is a priority.

    

   Standard Separators encompass all of the other designs. The reason the required Receiver Separator size is larger than the other separator designs is due to the complex internal geometry required to separate up to 90% entrained liquid loads.

    

   Thus, most applications will use a “Standard” design.

    

   The “chosen size” is the rounded-up standard pipe size that is equal to or larger than the size required.

    

   For 50,000 lbs/hr of saturated steam at 150 PSIG an 8.62” size standard separator is required, thus the chosen size is 10” (the next larger standard pipe size). If you require a receiver style separator then you would adjust the chosen size to 12”.

    

   If you need to lower the differential pressure, increase the chosen separator size until it is satisfactory.

    

   Next refer to the separation capacity and find the percentage of separation required for the application; the separator model chosen needs to separate ³ the design separation rate. Compare different models and sizes to determine the most cost effective and physically acceptable separator model.

    

   This calculator example for steam matches the Steam Chart Example.

    

   Air and Other Vapors

   The input criteria for proper sizing include:
   

   1. Molecular weight (or average molecular weight)

   2. Design (maximum) temperature

   3. Minimum operating pressure

   4. Volumetric flow rate (SCFM)

   5. Optional: if you know the density of the liquid to be separated, enter it for an accurate centripetal force ratio (water is the default value).

    

   *** input and result cells have hover-comments for detailed explanation

    

   

    

    

   The calculator will determine the standard and receiver separator sizes required. From this point the selection process is the same as previously described for steam applications.

    

   NOTE: While the design pressure (maximum) is required for determining the pressure class of the separator, it is not necessary for separator sizing. When using the maximum possible temperature with the minimum possible pressure (even if those variables are mutually exclusive), the “worst case scenario” for the volume of gas is used for the calculation.

    

   If provided both volumetric and mass flow units, convert the mass flow to volumetric units with the calculator and use the highest SCFM value for the calculation; mass flow is likely more accurate as it should include the entrained liquid whereas the volumetric flow may not, thus always important to factor-in the entrained separation requirement with the volumetric flow rate to ensure the proper size is calculated.

    

   Separator Models

   The separator design or type as selected in STEP 6 of separator sizing will have a subset of separator models.

    

   The >0.3 microns separation threshold is both the design and model – meaning there is only one model to choose from, it is referred to as the TF.

    

   The >5 microns separation threshold is offered in 3 designs referred to as coalescing separators.

    

   The most common >10-micron threshold designs are all of the other designs.

    

   

    

   ***When comparing separator models pay attention to their separation capacity and ability to separate liquid slugs. A liquid slug is characteristic of a large volume of liquid pushed though piping and not just simply entrained droplets. Each separator datasheet provides a description of its design, acceptable installation orientations, maximum separation capacity and slug handling capability.

    

   Separator Sizing Customizations

   All of the fabricated gas/liquid separator designs can be customized, examples include:
   

   • Extra inlet/outlet/drain/instrument/inspection ports

   • Oversized drains

   • Smaller inlet/outlet ports and other ports can be re-sized; drain ports should not be reduced but may be enlarged

   • Sometimes the inlet/outlet nozzles can be enlarged by 1 standard pipe size

   • Connection types for ports include threaded, flanged, butt weld and tri-clamp

   • Materials include carbon steel, 304L SS, 316L SS and alloys such as Alloy 20, Duplex SS and Hastelloy

   • Pressure classes are not limited to those shown in the datasheets; 150/300/600/900 and higher-pressure classes are quite standard – the limitation would be when material becomes too thick to obtain in pipe format and too thick to roll into a cylinder.

   • Support lugs, lifting lugs and support legs

   • Nozzle orientation can often be adjusted as long as it is on the same plane

   • Bodies can be flanged in support of clean-out or inspection requirements
     Corrosion allowance

   • Non-standard NDE/NDT: radiography, liquid dye penetrant, ultrasonic weld examination, PMI, external coating adhesion and thickness

   • External coating: typically, a sprayed rust inhibitor, we can provide finished coats to customer specifications.

   • Insulation clips

    

   Documentation

   Standard documentation available is per the requirements of ASME Code Section VIII Division 1 for code stamped vessels, which is basically just confirmation of hydrostatic testing and an outline drawing. Additional documentation that may be provided if explicitly specified in the purchase order include:

    

   • ASME code calculations

   • NDE/NDT procedures and reports

   •  Coating procedures and reports

   • WPS/PQR/WPQ

   • ITP (customizable per any non-standard NDE/NDT, for approval)

   • Weld map

   • Material test reports

   • Certificate of conformance

   • Certificate of origin

    

   With exception to the outline drawing and ITP, all the other documentation is not customizable from the manufacturers standardized procedures and established formats – all such documentation is provided for record only and each document has an applicable cost. Many documents are only available if specified with the purchase order and cannot be generated after fabrication/testing/code stamping is completed.

    

   ASME Code Section VIII Division 1

   ASME Code Section VIII Division 1 is a part of the ASME Boiler and Pressure Vessel Code (BPVC) that provides rules for the design, fabrication, inspection, testing, and certification of pressure vessels that are code stamped and used in various industries. Sometimes we receive inquiries requiring additional information as well as non-standard NDE and NDT. While this can sometimes be accommodated, the base system in-place with regards to data and its format are not customizable and thus if there are such requirements they must be communicated to us during the proposal stage so that we can advise what can be provided and what cannot, as well as ensure we adjust our pricing/lead time accordingly.

    

   This is a peak behind the curtain as to what is involved in selecting the most appropriate size and model centrifugal gas/liquid separator. Use our website to obtain a basic understanding of the size required and designs available and then contact us with your design criteria to enable us to provide a detailed proposal.
   
   Advanced technical support for your specific project is only a phone call or email away. Please contact us today!