Lecture 13 Flow Measurement in Pipes （13在管道流量测量讲座）.pdf

Lecture 13 Flow Measurement in Pipes I. Introduction • There are a wide variety of methods for measuring discharge and velocity in pipes, or closed conduits • Many of these methods can provide very accurate measurements • Others give only rough estimates • But, in general, it is easier to obtain a given measurement accuracy in pipes when compared to measurement in open channels • Some of the devices used are very expensive and are more suited to industrial and municipal systems than for agricultural irrigation systems II. Pitot Tubes • The pitot tube is named after Henri Pitot who used a bent glass tube to measure velocities in a river in France in the 1700s • The pitot tube can be used not only for measuring flow velocity in open channels (such as canals and rivers), but in closed conduits as well • There are several variations of pitot tubes for measuring flow velocity, and many of these are commercially available • Pitot tubes can be very simple devices with no moving parts • More sophisticated versions can provide greater accuracy (e.g. differential head meters that separate the static pressure head from the velocity head) • The pitot static tube, shown in the figure below, is one variation of the device which allows the static head (P/γ) and dynamic (total) head (P/γ + V2/2g) to be separately measured BIE 5300/6300 Lectures 141 Gary P. Merkley • The static head equals the depth if open-channel flow • Calibrations are required because the velocity profile can change with the flow rate, and because measurement(s) are only a sampling of the velocities in the pipe • The measurement from a pitot tube can be accurate to ±1% of the true velocity, even if the submerged end of the tube is up to ±15% out of alignment from the flow direction