Installation effects upon orifice flowmeters

Abstract

An experimental study has been undertaken to quantify the effect of the inlet velocity distribution upon the coefficient of discharge, C d. A two inch (50.8 mm) diameter orifice run was operated at a Reynolds number of 91 000 with a beta ratio, β, of 0.75. The upstream pipe section was replaced with a one inch pipe mounted concentrically inside the two inch pipe. The mass flowrate was held constant by an array of sonic nozzles upstream of the concentric pipes and was split between the two. By varying the ratio of the flow split, various inlet velocity profiles were generated. The results show that the change in coefficient of discharge is related to first-, second- and third-order moments of momentum: ∫ 0 R ∫ 0 2π i=1,2,3, Analysis of data presented by Morrow et al. (Flow Measurement and Instrumentation 2(1) (1991) 14–20) shows the same relationship. This paper proposes the use of this correlation to develop criteria for correcting the discharge coefficient given the variation of the inlet velocity profile from ‘fully developed’ flow. The velocity profile can be measured at the upstream flange tap location with the orifice plate removed, and that profile can be used to generate the moment of momentum to be used to correct the coefficient of discharge.