Document B8pNr5dLnmdkKVkQ1xZMgaJ6o

78 CHAPTER 4 1958 Guide FLOW MEASUREMENT The measurement of quantity of flow of fluids is generally accomplished! either by making observations on the change of state due to flow systems configuration, such as the pressure drop across a metering orifice, or by theS. displacement of some device, such as a rotating vane system. The selefi# tion of the metering system will be determined by the type of fluid, this precision of measurement desired, the cost of equipment and installations the range of flow quantity, the ease of maintenance, and the method of| observation and recording. Fluid meters may be classified as follows: (1) Head Meters (Pressure^ Sensitive), which may be of Venturi, flow nozzle, orifice plate, or Pitot tubM types; (2) Area Meters, which may be of gate, tapered tube, or tapered phiM types; (3) Force Meters, which may be of vane, propellor, or turbine types-M (4) Quantity Meters, which may be of weighing tank, reciprocating piston, orfj geared impeller types. Rate meters are generally of the first three groups, although rates arel Fig. 8. Dimensions fob Intebnational Standards Association Flow Nozzle Note: From Reference 3. Used by permission of A.S.M.E. .f obtainable from timed observations of quantity meters. Similar quantity! measurements can be obtained by suitable integration of rate meter in dications. i Head Meters |i The head meter is of sufficient flexibility so that almost any type of flow measuring problem can be handled. For this reason, standard and referj ence measurements are usually made in this way. With proper care, ex treme precision can be obtained. Also, an inexpensive installation can b|i made to give moderate precision. In general, a head meter requires fairly' competent installation and maintenance to give satisfactory service. Among the types of head meters, the Venturi has the advantage of having a low pressure loss, but requires considerable length of space. The orifice plate reduces the required length to a minimum at the cost of considerabl| pressure drop. The flow nozzle represents a compromise, but must d| calibrated individually, or made with extreme care from standard specific^ tions, to obtain a good precision of measurement. Standard configurations for representative orifice plates and flow nozzles, are shown in Figs. 7 ami 8, respectively. Additional specifications are given in References 2,.? and 4 at end of chapter. The measurement of flow in head meters is dependent upon observation