Influence of upstream bends on the discharge coefficients of classical venturi tubes and orifice plates

From an earlier edition of DIN 1952 the identical International Standard ISO 5167 ‘Measurement of fluid flow by means of orifice plates, nozzles and venturi tubes’ was established by the ISO committee TC 30/SC 2. Newly incorporated was the classical venturi tube, whereby undisturbed straight upstream and outlet pipe sections were specified. The source for the dimensions stated is not clear. In the opinion of the VDI/VDE-GMA Subcommittee 2.5.1, ‘Measurement by means of differential pressure devices’, these dimensions are much too short in comparison with the ones required for orifices. In two papers the GMA SC 2.5.1 has made this public. Now, with new test results achieved from measurements with classical venturi tubes, the VDI/VDE Subcommittee on flow measurement is publishing these new results in the first part of this paper and putting them up for discussion. All undisturbed straight lenghts required according to ISO 5167-1 and DIN 1952 for installation between various fittings and primary devices are referenced for pipe bends of 90°. Up to now, it was assumed that the influence of disturbances would decrease with decreasing bend angles. In an additional set of tests with orifice plates it was determined that this assumption is not very reliable. Further tests with orifices show the influence of the angle between the location of the pressure tap and the plane of the pipe bend. Both these problems are described in the second part of this paper.