scope:

This part of IS0 5167 specifies the geometry and method of use (installation and operating conditions) of orifice plates, nozzles and Venturi tubes when they are inserted in a conduit running full to determine the flow-rate of the fluid flowing in the conduit. It also gives necessary information for calculating the flow-rate and its associated uncertainty.

It applies only to pressure differential devices in which the flow remains subsonic throughout the measuring section and is steady or varies only slowly with time and where the fluid can be considered as single-phase. In addition, each of these devices can only be used within specified limits of pipe size and Reynolds number. Thus this pari of IS0 5167 cannot be used for pipe sizes less than 50 mm or more than 1200 mm or for pipe Reynolds numbers below  4000.

It deals with devices for which direct calibration experiments have been made, sufficient in number, spread and quality to enable coherent systems of application to be based on their results and coefficients to be given with certain predictable limits of uncertainty.

The devices introduced into the pipe are called "primary devices". The term primary device also includes the pressure tappings. All other instruments or devices required for the measurement are known as "secondary devices". This part of IS0 5167 covers primary devices; secondary devices¹⁾ will be mentioned only occasionally.

¹⁾ See IS0 2186:1973, Fluid flow in closed conduits - Conneclions for pressure signal transmissions between primary and secondary elements.

The different primary devices dealt with in this part of IS0 5167 are as íollows:

a) orifice plates, which can be used with corner pressure tappings, D and D/2 pressure tappings²⁾ and flange pressure tappings;

b) ISA 1932 nozzles³⁾, and long radius nozzles, which differ in shape and in the position of the pressure tappings;

c) classical Venturi tubes⁴⁾, and Venturi nozzles, which differ in shape and in the position of the pressure tappings.

²⁾ Orifice plates with vena contracts pressure tappings are not considered in this part of IS0 5167.

³⁾ ISA is thE abbreviation for the International Federation of the National Standardizing Associations, which was succeeded by IS0 in 1946.

⁴⁾ ln the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.