Numerical study of upstream disturbances on the performance of electromagnetic and ultrasonic flowmeters

ABSTRACT Electromagnetic and ultrasonic flowmeters are high precision devices that can measure the water flow in pressurized irrigation networks. In these networks, the inadequate installation distance of flow control valves generates upstream disturbances which will increase measurement errors and reduce the accuracy of the equipment. Therefore, estimating these errors is important to establishing the correct installation distance of the hydraulic accessories and to ensuring the metrological reliability of the flowmeters. This work proposes a numerical study using Computational Fluid Dynamics to evaluate water flow behavior in pipes and estimate measurement errors caused by gate and butterfly valves in installation configurations found at pressurized irrigation networks. The numerical simulations were made for gate (15, 50 and 75 % closed) and butterfly (open and 30º closed) valves installed three and six diameters (3D and 6D) from the flowmeters. The numerical study with three-dimensional simulation allows for evaluating the flow behavior and estimating measurement errors correctly. According to the study, the installation configurations of the gate and butterfly valves change the flow velocity profile at different positions in the pipe, generating both positive and negative measurement errors at both flowmeters. Among the tested configurations, only the butterfly valve installed 3D and 6D from the electromagnetic flowmeter ensured the measurement accuracy required by ISO 4064.

摘要：电磁流量计与超声波流量计均为高精度计量装置，可用于测量加压灌溉管网内的水流流量。在这类管网中，流量控制阀的安装间距不足会产生上游流场扰动，进而增大测量误差、降低设备的计量精度。因此，对这类误差进行估算，对于确定水力附件的合理安装间距、保障流量计的计量可靠性具有重要意义。本研究采用计算流体动力学(Computational Fluid Dynamics)方法开展数值模拟，以评估加压灌溉管网实际安装场景中管道内的水流特性，并估算由闸阀与蝶阀引发的测量误差。本次数值模拟针对安装位置距流量计3倍管径（3D）与6倍管径（6D）的闸阀（分别处于15%、50%、75%开度关闭状态）与蝶阀（分别处于全开与30°开度关闭状态）开展。本次三维数值模拟研究可准确评估管道内流场特性并估算测量误差。研究结果表明，闸阀与蝶阀的不同安装配置会改变管道内不同位置的流速分布，进而对两类流量计产生正向与负向的测量误差。在本次测试的所有配置中，仅当蝶阀安装于距电磁流量计3D与6D位置时，方可满足ISO 4064标准规定的测量精度要求。