Sizing

Flow Systems will size each subsonic meter to meet the requirements of each application. Throat or bore diameters are offered from 0.02 inches and up, with a standard resolution of ±0.001 inches. An optional resolution of -0.000 +0.001 inches is available for sizes less than 0.75 inches upon request. Presently, the largest Venturi meter we have produced had a throat diameter of 32 inches and was installed in a 54-inch diameter duct. Diagrams and charts showing estimated flow performance for standard sizes of Venturis and ASME Flow Nozzles are also available.

When sizing subsonic meters, the ASME and ISO standards limit the metering differential pressure (Dp) to less than or equal to 25% of the absolute inlet pressure (P1). Good practice suggests, Dp/P1 should not exceed 0.1 to avoid increased uncertainty and decreased repeatability.

The turndown, in terms of flow, for a subsonic meter is proportional to the square root of the metering differential pressure, assuming a constant density.

If a 3 to 1 turndown in flow is desired, then the metering differential pressure must be reduced by a factor of 9. When determining the turndown, two issues related to metering differential pressure must be considered. Noise or disturbance in the piping network may bias the differential pressure. Small differentials are more likely to be affected by piping noise and a minimum Dp/P1 of 0.005 is recommended for good repeatability. Piping noise least affects Venturis; while Orifice Meters are the most affected, and ASME Flow Nozzles fall in between. The second issue concerns increased uncertainty in flow measurement due to differential pressure turndown. The technical note on “Uncertainty When Using SPEs” details the effect of differential turndown on flow measurement uncertainty. However, assuming constant density, and a variation Dp/P1 from 0.1 to 0.005 (20:1), the uncertainty in flow at the minimum differential pressure will be 10 times the uncertainty at the maximum differential pressure.

Bi-Directional Venturis

For applications requiring measurement in both directions, Flow Systems has expertise in designing, manufacturing and testing of bi-directional Venturis that can be used to accurately measure the flow of both liquids and gases.

Cavitating Venturis

Flow Systems has expertise in designing, manufacturing and testing cavitating Venturis used to limit the flow of liquids that protect system components while providing low pressure loss at normal flows.

Line Size

Line sizes vary from ½” diameter tubing to the largest pipes or ducts.

End Connections

Subsonic meters can be configured with almost any desired connections 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 offer lower initial 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. For economy, users can specify different materials for the element, body and piping. For Venturis with lines sizes greater than 6 inches, it may be more economical to combine a stainless steel throat with a carbon steel body and piping. ASME Flow Nozzles and orifice meters generally combine a stainless steel plate or element with carbon steel flanges and piping.

Venturi Divergent Options

The divergent section of a Venturi may be designed for maximum pressure recovery and economy. Our standard divergent section is a conic with a 15-degree total angle and is commonly referred to as a Short Form Venturi. Optionally, a Long Form Venturi utilizes a 7-degree total angle, conic divergent section. Please see the “SPE Overall Pressure Loss” Chart to determine the estimated loss for both Long and Short form Venturis.

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 available.