Error Analysis of Flowmeter Measurements

It is commonly recommended that a straight section of pipe, 8–10 pipe diameters long, be installed immediately upstream from a flowmeter to reduce errors in flow rate measurements due to excessive turbulence. However, this recommendation frequently is not followed for irrigation pumping plants. This project investigated the error in flow rate measurements under nonoptimal conditions caused by an elbow, a check valve, and a partially opened butterfly valve. Flowmeters used for this project included two propeller meters, a Collins pitot tube meter, a Hall pitot tube meter, two types of paddle-wheel meters, and a doppler meter. Measurements were made at 2, 5, 10, and 15 pipe diameters downstream from the turbulence source. In addition, the effect of straightening vanes was also investigated. Results showed that the propeller flowmeters, the Collins meter, and the Hall tube were the least affected by excessive turbulence. Meters that make point measurements have the potential of being severely affected by excessive turbulence. All meters were strongly affected by the partially open butterfly value.