Venturi tubes consist of an inlet cylinder, an inlet cone to which the cylindrical neck section (throttle opening) and the outlet cone are connected. They can be designed as either venturi tubes for welding in or venturi tubes for flange mounting. According to the ISO 5167 calculation standard, a distinction is made between three venturi tube designs depending on how their inner surfaces are manufactured:
These Venturi tubes are especially relevant for smaller nominal sizes (DN 50 to DN 250). The higher roughness requirements necessitate mechanical machining of the inner surface of the inlet cone. Depending on the application, the measurement uncertainty is in the range of 1% – 3% of the flow coefficient.
Venturi tubes with a rough inlet cone welded from sheet steel are used for larger nominal diameters (DN 200 to DN 1200). Since the influence of the inner pipe surface is less with larger nominal diameters, a more cost-effective sheet metal construction can be used here. Depending on the application, the measurement uncertainty is in the range of 1.5% – 3% of the flow coefficient.
Venturi tubes with cast-grey inner profile offer the lowest measurement uncertainties and are suitable for a medium nominal size range (DN 100 to DN 800). Due to the more complex manufacturing process, these are rather recommended for particularly high demands on the measurement. Depending on the application, the measurement uncertainty is in the range of 0.7% – 2.5% of the flow coefficient.
Typical versions of the pressure nozzles are:
Venturi tubes can be equipped with shut-off valves and/or valve blocks. Condensate vessels are recommended for steam applications.