The Venturi tube is used for the measurement of gas and fluid flow, in which there is low pressure loss primary requirement. According to type of production there are: classical Venturi tubes with cast, or machined or welded confuser.

Classical Venturi tube consists of:

• inlet cylinder with (+) differential pressure tapping
• conical confuser with total angle of 21°
• cylindrical throat with (-) differential pressure tapping, length of the throat is equal to throttle diameter d
• conical diffuser with total apex angle ranging from 7° to 15°
• annular chambers for pressure tapping
• inlet cylinder and diffuser is ended by a flange or adjustment for weld joint, so-called weld-in design, which has advantages such as material savings on the flanges and subsequently minimized leakage in the flanged joint

For lower DN there is used machining as method of production or, for higher DN it is welding from steel sheet. Classical welded Venturi tubes have cylindrical throat machined to tolerance required by the standard.

Differential pressure tapping is made in the form of individual tapping in the piping wall interconnected by annular chambers. Upon agreement with the customers outlets of differential pressure are always customized to various options of primary element attachment to measuring circuit.

ISO 5167-4 standard defines possibility to use classical Venturi tube for diameters of piping ranging from 50 mm to 1,200 mm. For the production of the Venturi nozzles there are mostly used standard carbon steel and stainless steels for the measurement of fluid and gas flow and refractory steels for the measurement of steam flow, e.g. alloyed steels 14MoV6-3/15 128, 10CrMo9-10/11CrMo910, 13CrMo4-5, 16Mo3, X10CrMoVNb9-1 and 15NiCuMoNb5. Classical Venturi nozzle is manufactured either as a working meter or, as a specified meter with metrological verification complying with the technical particulars and conditions of pattern approval.

Main advantages of classical Venturi tube:

• It requires considerably shorter straight lengths in comparison to the orifice plates and nozzles. There is a very low loss in pressure on the outlet (from 4 to 20 % of the measured differential pressure) and therefore classical Venturi tube is highly economic primary element.
• For the high velocity fluid flow, e.g. steam, classical Venturi tube has a high operational reliability.
• Classical Venturi tube is not clogged with sediments.

Classical Venturi tube is proposed and manufactured with regard to customer requirements such as e.g. both sided measurement, heating with sheating, etc. Classical Venturi tube can be incorporated both in vertical and horizontal piping.