État actuel de la normalisation des diaphragmes, tuyères et venturis

Ingénieur à la Société Alsthom, Grenoble.

Résumé

The various stages of international standardisation of flow measurement devices are reviewed, with remarks on possible effects of political events on international recommendations I.S.O. R.541 and R. 781 published in 1967 and 1968, i.e. over thirty years afrter the first international documents were issued by the LS.A. The philosophy of the later documents is demonstrated, with comments on the standardised equipment, attention being drawn to the main differences from earlier recommendations and standards, among which especially the abandonment of a single straight run, the standardisation of three conventional Venturi tubes, a priori rejection of the assumption that the coefficients are invariable at sufficiently high Reynolds numbers, and allowance for measurement duct roughness. As a result of this the presentation of the discharge and expansion coefficients for certain pressure drop devices and the corresponding standard errors has changed completely. As the available experimental data for each pressure drop, device do not all cover the same ranges, the presentation of these coefficients varies, but scale requirements are met to a very large extent. Work now in progress is shortly expected to lead to the standardisation of orifice plates with pressure tappings at D and D/2 in addition to the corner tappings, or those at the vena contracta or the flange (already standardised). Pulsating flow, critical discharge, viscous fluid discharge and manometer pressure connections are further headings under which new equipment may be expected to be standardised or improved calculation or installation methods developed. In the absence of a more theoretical relationship, use is made in conjunction with the standardised data of a few experimental results obtained with conventional orifice plates, orifice plates at the end of a rough pipe, and , conventional Venturi tubes, with a view to proposing the general application to all pressure drop devices of the empirical algebraic relationship expressing the effects of Reynolds number on the discharge coefficient for the orifice plate with vena contracta or flange pressure tappings.