Investigating the additional uncertainty of flowrate measurement caused by symmetrically disturbed flow

Abstract

When flowmeters are used in technological conditions significant additional errors in flow measurement are often caused by distortion of the flow velocity profile upstream the primary device. The distortion of the flow velocity profile can be caused, par- ticularly, by concentric protrusions in the pipeline. The purpose of this paper is to investigate the effect of concentric protrusions, which form a symmetric flow distortion upstream the standard orifice plate, on the uncertainty of flow measurement result. The additional component of uncertainty was studied experimentally. For this purpose a setup was developed consisting of two series-installed differential pressure flowmeters, a reference and a test one, with an inner diameter D=0.1 m. The effect of the protrusions was investigated by installing concentric rings that imitate the protrusion height h=0.025D, 0.05D, 0.1D. Experimental studies were performed for standard orifice plate with relative diameter β=0.2; 0.4; 0.5; 0.67 provided that there is a protrusion at a distance L=2D, 5D, 10D upstream the orifice plate. According to the results of experimental studies the authors calculated the values of the relative deviation of the orifice plate discharge coefficient δCp caused by the concentric protrusion upstream the orifice plate. It is confirmed that the dependence of the relative deviation δCp versus the Reynolds number is close to linear for the developed turbulent flow, so the effect of the protrusion height for each fixed value of h and L can be reproduced by the mean relative deviation. Analytical dependencies are developed to calculate the mean relative deviation from the design characteristics of flowmeter and the additional expanded uncertainty of measured flowrate is proposed to be evaluated by these dependencies. Applying the results of the paper makes it possible to evaluate the uncertainty of flow measurement in technological conditions and, respectively, to determine the possibility of using flowmeters with available additional components of uncertainty.