Sonic Nozzles are also known as “Critical Flow Venturis” or “Critical Flow Nozzles”

Flow Systems designs and manufactures sonic nozzles in accordance with both ASME and ISO standards. The geometry is such that the gas is accelerated along the circular arc converging section and then is expanded in a conical diverging section, which is designed for pressure recovery. In the throat, or minimum area point of the Sonic Nozzle, the gas velocity becomes equal to the speed of sound. At this point, gas velocity is maximized, and the mass flow rate is a function of the inlet pressure, inlet temperature, and the type of gas. For more information on the design and performance of sonic nozzles manufactured by Flow Systems, refer to the sonic nozzle schematics and performance charts.

Sonic Nozzle (Critical Flow Venturis)
Figure 1 ASME / ISO Torodial Throat Critical Flow VenturiFigure 1 ASME / ISO Torodial Throat Critical Flow VenturiNote 1: ±0.0003″ for 0.0039″ <= d <= 0.016″, ±0.0005″ for 0.022″ <= d <= 0.707″, ±0.001 for d > 0.707″


Flow Systems will size Sonic Nozzles to meet the requirements of each application. Throat diameters are offered from 0.004 inches and larger. Throat diameter tolerances are; ±0.0003″ for 0.0039″ <= d <= 0.016″, ±0.0005″ for 0.022″ <= d <= 0.707″, ±0.001 for d > 0.707″.

Presently, the largest Sonic Nozzles we produced had a throat diameter of 14.6 inches. Certain standard sizes are available for immediate shipment with or without calibration. Refer to the charts showing estimated mass flow performance for standard sizes of Sonic Nozzles.

End Connections

Sonic nozzles can be configured with almost any desired connection such as “AN” flare, NPT, ANSI Flange, Swagelok, VCO, VCR, CPV, butt-weld or weld-in. Custom flanges with O-Ring seals (with or without V-Clamps) are also available upon request. Insert type meters are designed to be held between flanges and are available at a lower cost.

Material of Construction

Generally 300-Series stainless steel is recommended, however other materials such as aluminum, carbon steel and machinable plastics may be used.

Diffuser Options

The divergent section of Sonic Nozzles from Flow Systems are designed for maximum pressure recovery and economy. Our standard diffuser geometry is 7 throat diameters in length with a 4-degree half angle for throat diameters greater than 0.177 inches. A 10 throat diameter, 3-degree half angle diffuser is used for nozzles with throat diameters ranging from 0.044 to 0.177 inches and a 15- throat diameter, 3-degree half angle geometry is used for throats ranging from 0.016 inches to 0.031 inches. Small nozzles, throat diameters ranging from 0.0039 to 0.016 inches, have a diffuser geometry of 1 throat diameter in length with a 3 degree half angle. Custom geometry may specified by the end user.

Sonic Nozzle with a 14.6 Inch Throat Diameter

Sonic Nozzle with a 14.6 Inch Throat Diameter

Sonic Nozzle
Meter Tubes

Inlet and exit sections are available to provide users with a complete meter run. Inlet sections, designed to meet the proper upstream straight piping requirements, also contain taps for pressure and temperature measurements. Multiple or additional taps, pressure-averaging piezometer rings, and flow conditioners are also offered. Standard Inlet and Exit Sections, in stock for immediate shipment, are compatible with Flow Systems’ standard Sonic Nozzles.

Sonic Nozzle (Critical Flow Venturi) in a Meter Tube    Sonic Nozzles (Critical Flow Venturis) in Meter Tubes    Sonic Nozzle (Critical Flow Venturi) in a Meter Tube

30 inch Diameter Flange with 21 - 1 inch Sonic Nozzles     30 inch Diameter Flange with 21 - 1 inch Sonic Nozzles30″ Diameter Flange with 21 – 1″ Sonic Nozzles