Fluid meters

The practical working equation for weight rate of discharge, adoptea by the ASME Research Committee on Fluid Meters for use with either gases or liquids, is... 

A simple example of an area meter is a gate valve of the rising-stem type provided with static-pressure taps before and after the gate and a means for measuring the stem position. In most common types of area meters, the variation of the opening is automatically brought about by the motion of a weighted piston or float supported by the fluid. Two different cyhnder- and piston-type area meters are described in the ASME Research Committee on Fluid Meters Report, op. cit., pp. 82-83. 

Figure 2-17. Flow coefficient C for nozzles. C based on the internal diameter of the upstream pipe. By permission, Crane Co. [3]. Crane reference [9] is to Fluid Meters, American Society of Mechanical Engineers, Part 1-6th Ed., 1971. Data used to construct charts. Chart not copied from A.S.M.E. reference.

Figure 2-38B. Net expansion factor, Y, for compressible flow through nozzles and orifices. By permission, Crane Co., Technical Paper 410, Engineering Div., 1957. Also see 1976 edition and Fluid Meters, Their Theory and Application, Part 1, 5th Ed., 1959 and R. G. Cunningham, Paper 50-A-45, American Society of Mechanical Engineers.

According to Head [Trans. Am. Soc. Mech. Eng., 78, 1471-1479 (1956)], a pulsation-intensity limit of F = 0.1 is recommended as a practical pulsation threshold below which the performance of all types of flowmeters will differ negligibly from steady-flow performance (an error of less than 1 percent in flow due to pulsation). F is the peak-to-trough flow variation expressed as a fraction of the average flow rate. According to the ASME Research Committee on Fluid Meters Report (op. cit., pp. 34—35), the fractional metering error E for liquid flow i he "... 

Bean, H. S., Buckingham, E., and Murphy, P. S., Discharge Coefficients of Square-Edged Orifices for Measuring the Flow of Air, U.S. National Bureau of Standards Research Paper 49, ASME Fluid Meters Report, 1931. 

Steam is metered with an orifice meter in a 10-in boiler lead having an internal diameter of dp = 9.760 in. Determine the maximum rate of steam flow that can be measured with a steel orifice plate having a diameter of d0 = 5.855 in at 70°F (294 K). The upstream pressure tap is ID ahead of the orifice, and the downstream tap is 0.5D past the orifice. Steam pressure at the orifice inlet pp = 250 psig (1825 kPa) temperature is 640°F (611 K). A differential gage fitted across the orifice has a maximum range of 120 in of water. What is the steam flow rate when the observed differential pressure is 40 in of water Use the ASME Research Committee on Fluid Meters method in analyzing the meter. Atmospheric pressure is 14.696 psia. 

Determine the steam viscosity and meter flow coefficient. From the ASME publication Fluid Meters—Their Theory and Application, the steam viscosity gu for a steam system operating at 640°F is gui = 0.0000141 in lb/(°F)(s)(ft2). 

Determine the expansion factor and the meter area factor. Since steam is a compressible fluid, the expansion factor Y must be determined. For superheated steam, the ratio of the specific heat at constant pressure cp to the specific heat at constant volume cv is k = cp/cv = 1.3. Also, the ratio of the differential maximum pressure reading hw, in in of water, to the maximum pressure in the pipe, in psia, equals 120/246.7 = 0.454. Using the expansion-factor curve in the ASME Fluid Meters, Y = 0.994 for ft = 0.5999, and the pressure ratio = 0.454. And, from the same reference, the meter area factor Fa = 1.0084 for a steel meter operating at 640°F. 

Adjust the flow coefficient for the actual Reynolds number. In step 2, Re = 107 was assumed and. fiT = 0.6486. For Re = 1,750,000, K= 0.6489, from ASME Fluid Meters, by interpolation. Then, the actual flow rate Wh = (computed flow rate) (ratio of flow coefficients based on assumed and actual Reynolds numbers) = (15.75)(0.6489/0.6486)(3600) = 56,700 lb/h, closely, where the value 3600 is a conversion factor for changing lb/s to lb/h. 

Related Calculations. Use these steps and the ASME Fluid Meters or comprehensive meter engineering tables giving similar data to select or check an orifice meter used in any type of steam pipe—main, auxiliary, process, industrial, marine, heating, or commercial—conveying wet, saturated, or superheated steam. 

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