Orifice Flange

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Orifice flanges, abbreviated as ORFF, are for metering the volumetric flow rate of liquids and gasses through a pipe.  Two orifice flanges are called an orifice flange union.  Each flange comes with two pipe taps for measuring the pressure drop of the flow through an orifice plate.  Orifice plates do not come with the flanges and are sized based on the requirements of the process.  Two jack screws are used to spread the flanges apart in order to change the orifice plate.  This flange is normally available in weld neck, slip-on, and threaded flanges.  These flanges have a raised face.

Orifice Flange Index

Orifice flanges are installed in a straight run of pipe in order to avoid turbulence at the plate.  As a rule of thumb, an orifice flange has ten diameters of straight pipe upstream and five diameters of pipe downstream.  There are guidelines set out in (American Gas Association) AGA Report #3 which is the Orifice Metering of Natural gas.

When installing the orifice plate flange the pressure taps need to be at the same elevation to each other.  The assumption when making the calculation for the orifice plate is that static head of the tubes is the same on both sides of the orifice plate.  Pressure taps should not be installed facing down as the taps could become plugged with debris in the line.

Weldneck orifice flanges are butt-welded into the pipeline.  The inside diameter (bore diameter) of the pipe should be specified when ordering.  Weldneck orifice flanges are available in ANSI classes 300, 600, 900, 1500 and 2500.  They are not available in ANSI 150 sizes because the thickness of ANSI 150 flanges are not enough to allow for drilling pressure taps.

Orifice Flange UnionOrifice Flange UnionPipe TapPipe TapJack ScrewJack ScrewPipe TapsPipe Taps








Orifice Flange Advantages and Disadvantages

  • Orifice flanges are primarily used for precise measurement of fluid flow rates.  The size of the orifice can be adjusted to control the flow, making them valuable in applications where accurate flow measurement is critical.
  • Orifice flanges are cost effective compared to some other flow control devices.  They provide a simple and efficient means of controlling fluid flow within a pipeline.
  • Orifice flanges have a relatively simple design, which makes them easy to install and maintain.  Their simplicity is an advantage for straightforward applications.
  • The orifice plate is a widely accepted and standardized method for flow measurement in various industries.  This makes orifice flanges a common choice for engineers and designers.
  • Orifice flanges can be used for a wide range of fluids, including gases and liquids, making them versatile in different industrial processes.
  • The presence of an orifice in the flow path causes a pressure drop.  This pressure loss should be carefully considered, especially in systems where maintaining a specific pressure is critical.
  • Orifice flow measurement is sensitive to upstream conditions.  Changes in fluid properties, such as density and viscosity, can affect the accuracy of the flow measurement.
  • Orifice flanges have a limited turndown ratio, which means they may not be suitable for applications where a wide range of flow rates needs to be accommodated without significant loss of accuracy.
  • The high velocity flow through the orifice can lead to erosion of the orifice plate over time.  This erosion may affect the accuracy of flow measurements and require periodic maintenance.
  • The accuracy of flow measurement with orifice flanges is highly dependent on precise installation, including proper alignment and positioning of the orifice plate.  Improper installation can lead to inaccuracies.
  • Orifice flanges are generally designed for one-way flow, and reversing the flow direction may affect the accuracy of the measurement.
  • Accessing and maintaining orifice flanges can be challenging, especially in systems with high temperatures or corrosive fluids.  Shutting down the system for maintenance may be necessary.


Orifice Flange Standards

  • ASME Standards
    • ASME B16.36 - Orifice Flanges
      • Slip-on Flange: Class 300, Class 400, Class 600, Class 900, Class 1500
      • Threaded Flange: Class 300, Class 400, Class 600, Class 900, Class 1500
      • Weld Neck Flange: Class 300, Class 400, Class 600, Class 900, Class 1500, Class 2500


Orifice Flange and Plate Datasheets

Class Orifice Flange Raised Face Class Orifice Plate Raised or Flat Face
 ANSI 300  Orifice WNF, ANSI Class 300 (in)  ANSI 125 - 2500  Orifice Plate - ANSI Class 125-2500 (in)
 ANSI 400  Orifice WNF, ANSI Class 400 (in)    
 ANSI 600  Orifice WNF, ANSI Class 600 (in)    
 ANSI 900 Orifice WNF, ANSI Class 900 (in)    
 ANSI 1500 Orifice WNF, ANSI Class 1500 (in)    
 ANSI 2500 Orifice WNF, ANSI Class 2500 (in)    


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Tags: Orifice and Nozzle Pipe Flange