Ultrasonic Flow Measurement for Pipe Installations with Nonideal Conditions

In the arid Western United States, water is among the most valuable resources. Typically, to successfully and accurately measure a flow rate using most types of flow meters, it is recommended that a straight section of pipe be installed immediately upstream from the flow meter to avoid distorted flow patterns and extreme turbulence at the metering location. In many field piping situations, however, such flow conditions are impossible to achieve. The portable ultrasonic flow meter (USFM) is a commonly used flow meter for field flow measurements in closed conduits and emits two ultrasonic signals across the cross section of pipe. One signal travels with the direction of the flow, and the other travels against the flow. The difference in signal travel time is then used along with the known geometry of the pipe to calculate the average flow velocity of the fluid. The performance of an USFM in nonideal piping scenarios was studied using laboratory experiments and numerical computational fluid dynamics (CFD) models. A transit-time USFM was used downstream of a single 12-in.-long radius elbow, and the error in flow measurement resulting from the flow disturbance was measured. Physical model tests were performed at four locations downstream of the elbow, at three orientations on the circumference of the pipe, and for three flow rates with Reynolds numbers ranging from 250,000 to 750,000. Numerical models were also utilized and the resulting velocity profiles were used to integrate the velocity of the flow across the ultrasonic signal path. The velocity profiles were compared to similar profiles for fully developed flow to determine the error in flow measurement. The USFM measurement errors downstream of the elbow were always negative and were found to be as great as - 16%. A correction curve was applied to the results, which provided a method to correct the ultrasonic flow measurement. By applying the correction equation to the USFM measurements downstream of the elbow, the measurement error was reduced by nearly 90%. It appears that CFD is a viable tool for making appropriate corrections to ultrasonic flow measurements in pipes with upstream flow disturbances. DOI: 10.1061/(ASCE)IR.1943-4774.0000486. (C) 2012 American Society of Civil Engineers.