Document Lg5vYRZEb4OdDeQvjp4Z1x625

CHAPTER 4 1946- Guide In steam measurement, a constant head of water is maintained, over each, leg of the manometer by means of condensing chambers, in order to keep the heat of the steam away from the meter. ' As the mercury level fluctuates, the difference in head as recorded on the chart, therefore, is not that due to mercury alone, but to mercury minus an equivalent head of water. To correct for this, the equation ; , -- t 1132J.555577 /(c5m9) . is applied to Equation 58. The denominator in Equation 59 is the specific gravity of mercury; and the numerator is the difference in specific gravity between mercury and water. Equation 58, hence, becomes Qt = 345.155 KDf (60) Fluid Flow 91. METERING LIQUIDS Orifices are also used for metering'liquids, and Equation 48 serves again as a starting point for, developing the working -formula. Again, too, it is necessary to convert ht to /law, the actual head in inches of water, by substituting Equation 57. It is a|so necessary to correct for the weight of the fluid above the manometer, and since this may be other than water, it is better to use an equation of more general form than Equation .59: 13.557 - -p- /law -- Z*w 13.557 where Pf = the density of the fluid over the mercury in the manometer. ,PW -- the density of water at 60 F. (64) Fic. 15. Variation of Orifice Plate Expansion Factor, P, with Temperature and Material Steam is commonly measured in terms of weight, and since W = fQ( , ' (61) where W = the rate of flow in pounds per hour. W = 345.65 KDSVh^p (62) , The expansion factor Y, and the factor P, correcting for the expansion of the orifice plate with the temperature must then be applied, so that W = 345.65 K YPDi VfcTp (63) which is the final form of the equation for the flow of steam through orifices. Values of K may be obtained from Figs. 7, 8, 9, and'10, according to the pressure taps used. Y may be computed from Equations 55 or 56. Fig. 15 gives values of the correction factor P, according to the temperature and the material of the orifice plate. Values of the density, p, may he' . obtained from steam tables, such as Keenanand Keyes4, which are widely used. Fig. 16.' Shape of ASME Long Radius Nozzle When Ratio of Throat to Pipe Diameter is 0.53 or Less . Fig. 17.' Shape of ASME Long Radius Nozzle When Ratio of Throat to Pipe Diameter is 0.4 to 0.7 Substituting Equations 57 and 64 in Equation 48 gives . ^. Qi = 44.764 K.D' - 0.00118). ; (65) Then, since liquids are generally measured in gallons instead of cubic feet, and since there are 7.4805 gal in. l cu ft, ' ' Qw = 334.86 KD,' (-L - 0.00118). - ~ . (66) in which Qv is the discharge or rate of flow, in gallons per hour. .'Since liquids,,for practical purposes, are incompressible, no expansion factor is .necessary. If circumstances demand, the 'factor P for the expansion of . the orifice may be applied. Also; if it is necessary to correct ', the volumetric discharge to a base temperature,-application'of the known1 ' expansion.characteristics pf the liquid will enable the conversion. Values-