Document evvJRmBZ8D6YK3O7ey7YybqJG

86 CHAPTER 4 1948 Guide or in other words, 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 is used in European practice,-- corner taps. Pressures are taken from recesses in the flange connected to annular slits in the corners formed by 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 pressure tap of the vena contracts arrangement is variable. Vena contracts is the term Fig. 10. Relative Location of Flange, Radius and Vena Conteacta Taps 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 contracts tap, vary with the ratio of orifice to pipe diameter, and with rate of flow, as shown in Fig. 11; the tap is generally located in accordance with the mean curve in the figure. Expansion Factor for Gases The expansion factor, Y, for gases (for liquids, Y = 1) is found from the empirical formula Y = 1 - (0.41 + 0.35(5*) (** ~ ^/kj (55) This is applicable to flange* radius, and vena contracts taps. Values of Y for air, computed from these equations, are given in Fig. 12. Fluid Flow 87 Fig. 11. Location of Vena Conteacta in Relation to Ratio of Orifice to Pipe Di ameter and to Rate of Flow Computing Orifice Discharge With this information it is possible to compute the discharge from an orifice if the Reynolds number is known. Here an odd complication is encountered--when the value of Nj^e is computed, the rate of flow, which is the unknown quantity, must be used in the computation. How ever, it will be noted in Figs. 7, 8, and 9 that the orifice coefficient does not change greatly as NRe changes., If, then, an estimate is made of the velocity, using this in computing jVRe, and if the corresponding coef ficient is used in Equation 54, a value for the rate of flow will be found. Using this velocity to compute a corrected value of and repeating the process, a more nearly correct value of (Jf is found. This cut-and-try method may be continued for several more cycles, but generally the first or second correction will be found sufficient. Another method would be to use the Value of K corresponding to NRe = cd , modifying this with a factor involving the rate of flow, deter mined from the temperature, and the differential and static pressures. This method is used by the American Gas Association3. STEAM FLOW MEASUREMENT While steam may be considered as a gas, its measurement differs from that of the usual gases because of a number of factors. Equation 48 serves as the starting point. Since it is usual to measure the differential pressure in inches of water, it is necessary to convert hi, the head in feet Fig. 12. Expansion Factor for Air and Other Diatomic Gases Applicable to Flange, Radius and Vena Con- tracta Taps