b'LT ENGINEERINGCATALOGsignificant loss of head, and are not suitable for measuringFlumes can be more expensive than weirs. However, flumes flows with solids that may cling to the weir or accumulateresult in a lower head loss and are self-cleaning, requiring upstream from it. less maintenance than a weir.Automatic measurement of the flow rate in an open channelArea Velocityflume or weir can be performed manually by reading a singleThe Area Velocity method is an open channel water level measurement and calculating, or by the means of anmeasurement design that does not require the installation automatic flow meter. The most common open channelof a weir or flume. Instead, it can be used directly in a new or meters are the Ultrasonic, Bubbler, & Pressure Transducer.existing pipe channel. The area velocity method calculates The Ultrasonic Meter measures the time required for anflow rate by multiplying the area of the flow by its average acoustic pulse to travel from a transmitter to the liquidvelocity. This is often referred to as the continuity equation, surface (where it is reflected) and returned to a receiver. Q=AxVThe Bubbler Meter consists of a bubbler tube that is anchored in the flow stream at a fixed depth, then the tubeFor convenience, most area velocity flow meters use a supplies a constant bubble rate of pressurized air. The airsingle sensor to measure flow rate. Doppler ultrasonic pressure required to maintain the bubble rate is measured;characteristics is used to measure average flow velocity,Wastewater Systemsthis pressure is proportional to the liquid level.while an integral pressure transducer measures the level in the channel. The flow meter converts this level into the area The Pressure Transducer consists of a sealed pressureof the flow based on the size and shape of the channel.transducer submerged in the flow stream at a fixed depth. The pressure measured by the transducer is proportional The main advantage of the area velocity method is that it can to the liquid level. be used to measure flow under a wide range of conditions. Open Channel FlumesSurchargedA flume is a specially shaped open channel structure that Full Pipe temporarily modifies the flow through the channel to enable Submerged the measurement of the flow rate. Specifically, the flume Reverse Flow restricts the channel area and/or changes the channel slope, resulting in an increased velocity and a change in the levelSlope-Hydraulic Radiusof the liquid flowing through the flume structure. The flowVarious resistance equations are used to estimate flowrate through the flume can be determined by measuring therate based on measurements of the water surface slope, liquid depth at a specified point in the flume, and using thecross-sectional area, and wetted perimeter over a lengthflumes associated equation (head-flow rate relationship). of uniform channel. The most popular of these equationsis the Manning formula: There are several popular flume types; Parshall, Palmer-Bowlus, and Manhole Flume. The most common flume isQ=K_n/nR^(2/3)S^(1/2)Athe Parshall Flume. The flow rate through a Parshall flumewhere: is determined by measuring the liquid level one third of the way into the converging section. Parshall flumes areQ = flow ratedesignated by the width of the throat, which ranges fromA = cross sectional area of flow 1 inch to 50 feet. The throat width and all other dimensionsR = hydraulic radius (cross sectional area dividedmust be strictly followed so that standard discharge tablesby wetted perimeter) can be used. Also, note the drop in the floor of the flume,S =slope of the hydraulic gradient which makes it difficult to install a Parshall flume in ann = roughness coefficient based on channel materialexisting channel. and condition K = constant dependent upon units Another popular flume is the Palmer-Bowlus Flume. This flume is designed to be installed in an existing channel withThe cross-sectional area A and the hydraulic radius R are minimal effort. The flow rate through a Palmer-Bowlus flumecalculated based on the liquid depth, and the size and is determined by measuring the liquid depth at a point shape of the channel. The slope S is often estimated based one-half pipe diameter upstream from the flume throat.on installation drawings of the channel. The roughness Palmer-Bowlus flumes are designated by the size of the pipecoefficient n is selected from standard references based on into which they fit. Standard sizes range from 4 to 42 inches.the material of construction of the channel, and its condition. The dimensional configuration is not rigidly established for each flume size. However, a Palmer-Bowlus flume with aGiven the size, shape, slope and roughness of the channel, trapezoidal throat with a flat bottom has emerged as thean open channel flow meter can calculate flow rate using standard design for circular pipes. the Manning formula based on a measurement of the liquid depth. The Manning formula is not as accurate as the The Manhole Flume is a unique variation of the Parshallhydraulic structures and area velocity methods, but it can Flume. An advantage of the Manhole Flume is that itprovide sufficient accuracy in some applications. In addition, designed for easy installation in standard manholes andno weir or flume is required.existing pipe sewer lines.INFO@PARKUSA.COM|888-611-7275|WWW.PARKUSA.COM 363'