Buckling and eccentricity effects on orifice metering accuracy

Flowmetering errors arising from elastic deflection of a plate are quantified and equations to choose correct thicknesses of plates to minimize these errors and avoid plastic buckling are derived. Differences between expressions for metering error are explained. The limit on plate flatness in ISO 5167 and AGA 3 is shown to be inadequate and to lead to systematic measurement errors of up to 0.2%. An experimental investigation into the effect of plate eccentricity on the accuracy of measurement is reported. Limits in ISO 5167 are extremely difficult to meet except in specialist laboratories. Air flow tests on 5 plates ranging from 0.75 to 0.2 beta ratio in a 150 mm line show that these concentricity limits can be substantially relaxed if an increase in uncertainty of 0.1% can be tolerated.