Document Q05MOg1Q3QNBZ2kkRxyQN8z4

82 CHAPTER 4 Hence, by Equation 38 for a small value of Mi, r.-.y.W*). \k-i) (pi/ft) -1 1 -/Kpi/pi)" From/Equations 37 and 56,if Mzlis not too large, irX-~ F1 ')**1 - + Hi - 2(4 - 2)^]M^ : kMf\p J so that, approximately, . 'i V3O*3jl 4% ' IP! (69) M' Vc 5^ Fluid Flow 83 practice, with reference to fittings and valves, is shown by- Fig. Ip. If these conditions cannot be met, some flexibility is possible by introducing straightening vanes, as described more fully in References 2 and 5. The static pressure difference across the head meter is measured by suitably located static pressure taps. For orifices, the different sets of pressure taps are called flange taps, radius taps, vena contracta 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 m m where to the same degree of approximation from Equation 36 Pi - V* Mf = 2 kpi M ' -*JF' (60ll Values of are also given by Figs.. 13 and 14. . While <f> may be used for smooth nozzles and Venturi tubes, it is necessaryff in the case of orifices, where the departure from ideal flow is significant, p to replace <t> by an empirical factor Y obtained by the equation ;JV Y = 1 -- (0.41 + 0.35(5') /kj (6!> When this correction is used, the flow will be given by the equation to = KAiYyJigmijh -- Pj) Location of Head Meters and Pressure Taps >.c o)%; (< ,3$: .{ji In the installation of Venturis, orifices, and nozzles, care must be takeng in regard to upstream and downstream flow conditions. Recommended Eig. 15. Minimum Conditions to be Observed When Installing Orifices and Nozzles Between Fittings and Valves coefficients to measurements obtained 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 2 pipe diameters upstream 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 had 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 corner tap, is used in European practice. Pressures are taken from recesses in the flange connected to annular slits in the comers formed by the pipe wall and the orifice plate.