06-H Lost Water Study - Laserfiche WebLink

Home Browse Search

Proposal Comprehensive Water Distribution System Audit I * Allawing air to enter the process flow, resulting in metering error. * Failuretoproperlygroundtheinsfruments,resulting in induced errar in the eledronic components. * Installingameterthatisover-sizedfortheflawtobe measured. * Installing the meter in a location that makes future maintenance impossible. Meter Accuracy The accuracy of flow meters is a common problem encountered in the water industry. The science that deals with accuracy in measurements is metrology, and there have been numeraus books written on the issues associated with o6taini ng measurements that are both accurote and precise. In the United States, the Notional Institute of Science and Technology (NIST), which used to be known as the National Bureau of Standards, is the organization that is charged withsupportingmetralogyatthenationalscale. Therefore, in a purest sense, all measurements should be traceabie to NIST. Error in Flow metering systems is introduced byeach element used in the flow measuring process. (onsider a venturi tube system that consists of the venturi tube, the differential pressure transmitter, and the indicator/recorder. Each of the three devices has some intemal error that cantributes to the overall error of the measured flow. It is important to realize that these errors ore multiplimtive, not additive. In Situ Meter Calibration Several techniques can be used to perform in situ meter calibration, or to check the accuracy of ineters that are currently in service. The techniques that are most applicable to the Ciry of Paris' praject are dismssed in the following paragraphs. Carter i-Burgess Volumetric Calibration The volumetric calibration method consists of timing the increase or decreose in water volume in a tank or basin of knowngeometry. Thetechniquerequiresthatthegeometry of the volumetric tank be well-known, that the chonge in water surface elevation can be determined relatively accurately, and that the time between differentelevations mn be measured with reasonable accurary. When perfarmed rorrectly, valumetric mlibrotion is among the most accurote methods for calibrating flow meters. The method is ohen referred to as the "bucket and stop wotch" procedure, which is descriptive terminalogy, but does not indicate the relative complexiry of the procedure. The pracedure involves the measurement of three separate parameters: the volume of the tonk, the change in water surface elevation, and the time. For the procedure to be accurate, all three measurements must be preformed with reasonable accurary. Tracer Dilution Method The tracer dilution method of in situ calibration consistsof odding a known amount of the tracer to the process flow and determining the concentratian in the process flow downstreamafthemetertobecalibrated. Figure4shows o typical tracer dilution set up, and the equipment requirements are minimal. A calibrated container for the tracer is required to confirm the rote atwhich the metering pumpisaddingthetracertatheprocessflow. Theremust6e iwa taps in place, one through which to add the tracer, and the ather from which to col led the downstream sample. The flow in process is calculated from the following simple equation: q x _ (O+q) x C, where q is the flow rate of the tracer, C, is the concentration of the tmcer in the tracer solution container, 0 is the process flow rate, and Cz is the concentration of the tracer in the downstream wmple. 17