Document k5V38gJxgnwjRgkD4qr177Ry

42 CHAPTER 4 1959 Guide From Refer*nc* 3. Utmd by pcnniwon of ASMS. Rg. 12....Row Coefficient for International Standards Association Flow Nozzle Shown in Rg. 8 as a Function of Area Ratio (Dj/DJ* and the Reynolds Number Nr* that published flow coefficients can be used for accurate de termination of flow rates. In measuring the flow of compressible fluids, the approxi mate Equations 52 and 53 must be corrected for density variations. The need for such correction is evident by com paring Equations 35 and 38 with Equation 47. Introducing a multiplicative correction factor 4>. the equation for flow with no loss becomes: W = y/2gpl{p1 - Pi) (54) By comparison with Equation 35, . A/~P~ --(l - y) Y 2 p,i (ppxtj/pd ~ 1 From Equations 34 and 35, JfiVtft* - 0Wpi)m Hence, by Equation 38 for a small value of Mi, (55) LOO 090 080 0.70 040 Ps/P. Fig. 13___ Relation of Expansion Factor, <j>, for Nozzles to Diameter Ratio and Pressure Loss for Air and Other Diatomic Gases While <t> may be used for smooth nozzles and Venturi tubes, it is necessary in the case of orifices, where the de parture from ideal flow is significant, to replace by an empirical factor Y obtained by the equation Y - 1 - (0.41 + 0.35(3*) '''/*) (61) When this correction is used, the flow will be given by the equation in = KAtY V2{fp,(p, - p,) (62) From Equations 37 and 56, if Mi is not too large, i F* -- ') -- 1 -- ** + 111 -- 2(i " so that, approximately, (5SI where to the same degree of approximation from Equation 36 M,* = 2 *Pi Values of 4> are also given by Figs. 13 and 14. (60) Rg. 14.... Relation of Expansion Factor, <t>, for Nozzles to Diameter Ratio and Pressure Loss for Steam, Carbon Dioxide and Natural Gas FluidFlow Location of Head Meters and Pressure Taps In the ingfatutiriTi of Venturis, orifices, and nozzles, care must be in regard to upstream and downstream flow conditions. Recommended practice, with reference to fittings .valves, is shown by Fig. 15. If these conditions cannot be met, some flexibility is possible by introducing straighten ing vanes, as described more fully in References 2 and 5. The static pressure difference across the head meter is 3 43 measured by suitably located static pressure taps. For orifices, the different sets of pressure taps are called flange taps, radius taps, vena contracts taps, and pipe or full-flow taps. The relative locations of the first three of these are shown in Fig: 16. The flow coefficients in Figs. 9, 10, and 11 apply only to measurements made with these pressure tap locations. The need for applying suitable coefficients to measurements ob tained by means of the different taps, is indicated by the curve of change, in pressure of the flowing fluid, shown in the lower part of Fig. 16. Pipe taps are located 2H pipe diameters up stream' and 8 pipe diameters downstream (both distances being measured from the upstream face of the orifice plate) so that pressures are measured before the orifice plate has bad any effect on the flow, and after the recovery in pressure has been completed. The use of pipe or full-flow taps has been limited to the metering of natural fuel gas in certain areas. As they are not suited to use in heating and ventilating work, no data for them are given in this chapter. Still another type of pressure tap, the comer tap, is used in European practice. Pressures are taken from recesses in the flange connected to annular slits in the comers formed by Rg. 15 .... Minimum Conditions to be Observed When Installing Orifices and Nozzles Between Fittings and Valves the pipe wall and the orifice plate. Coefficients for these taps have been adopted by the International Standards Association, but are not used commercially in America. ' It will be noted that the location of the downstream pres-, sure tap of the vena contracts arrangement is variable. Vena contracta is the terra applied to the minimum cross-section of the' jet from the orifice, where the static pressure is at a minimum. Its location, and the location of the downstream vena contracta tap, vary with the ratio of orifice to pipe di ameter, and with rate of flow, as shown in Fig. 17; the tap is generally located in accordance with the mean curve' in the'figure. Similar precautions are required when using flow nozzles and Venturi tubes. In many cases, these will be supplied