Effects of Upstream Flow Disturbances on Elbow Meter Performance

Abstract

In order to fill gaps in research into the use of elbow flow meters and to reconcile both a lack of published standards and differing recommendations on the necessary minimum lengths of straight pipe that should be installed upstream of an elbow flow meter to ensure sufficiently accurate flow measurement, physical data were collected on 50 mm nominal (52.5 mm or 2.067 inch actual), 150 mm nominal (154.05 mm or 6.065 inch actual), and 305 mm nominal (304.8 mm or 12.00 inch actual) long-radius elbow meters to determine discharge coefficients in a straight-line pipeline configuration. The 150 mm (6-inch) long-radius elbow meter was further tested in order to determine the effects of different upstream disturbances on the accuracy of its metering performance. Three different upstream disturbances were tested at upstream distances of 25, 10, and 5 diameter-lengths, including: a single elbow in-plane “S” orientation, a single elbow in-plane “U” orientation, and a double elbows out-of-plane orientation. Discharge coefficients were calculated for each configuration at the three variable upstream distances between the upstream flow disturbance and the meter and compared to the straight-line calibration values to identify the percent difference shifts in the average discharge coefficients. Most importantly, findings from the present study conclude that the discharge coefficients for all elbow meter installations stabilize for pipe Reynolds numbers greater than 300,000. Additionally, even at upstream distances of 25 pipe diameter lengths (3.81 m or 12.5 feet) each of the three upstream flow disturbances continued to exhibit effects on the calculated discharge coefficients for the elbow meter; the observed difference in the average discharge coefficient for the two single elbow in-plane configurations “S” and “U” were within 1.00% of the straight-line values. Finally, the double elbows out-of-plane discharge coefficient values remained constant, regardless of the three tested distances between 5 and 25 diameter lengths between the elbow meter and the upstream flow disturbance, showing a more predictable shift in discharge coefficient than the two single elbow in-plane configurations.