Discharge coefficient venturi meter

Value of the discharge coefficient C for a Herschel-type venturi meter depends upon the Reynolds number and to a minor extent upon the size of the venturi, increasing with diameter. A plot of C versus pipe Reynolds number is given in ASME PTC, op. cit., p. 19. A value of 0.984 can be used for pipe Reynolds numbers larger than 200,000. 

Discharge coefficients for critical flow nozzles are, in general, the same as those for subsonic nozzles. See Grace and Lapple, Trans. Am. Soc. Mech. Fug., 73, 639-647 (1951) and Szaniszlo, ]. Eug. Power, 97, 521-526 (1975). Arnberg, Britton, and Seidl [J. Fluids Eug., 96, 111-123 (1974)] present discharge-coefficient correlations for circular-arc venturi meters at critical flow. For the calciilation of the flow of natural gas through nozzles under critical-flow conditions, see Johnson,/. Ba.sic Eng., 92, 580-589 (1970). 

A venturi meter with a 50 mm throat is used to measure a flow of slightly salty water in a pipe of inside diameter 100 mm. The meter is checked by adding 20 cm3/s of normal sodium chloride solution above the meter and analysing a sample of water downstream from the meter. Before addition of the salt, 1000 cm- of water requires 10 cm3 of 0.1 M silver nitrate solution in a titration. 1000 cm3 of the downstream sample required 23.5 cm3 of 0.1 M silver nitrate. If a mercury-under-water manometer connected to the meter gives a reading of 20S mm, what is the discharge coefficient of the meter Assume that the density of the liquid is not appreciably affected by the salt. 

A series of tap connections in an annular pressure ring gives a mean value for the pressure at point 1 in the approach section and also at point 2 in the throat. Although Venturi meters are relatively expensive and tend to be bulky, they can meter up to 60 per cent more flow than orifice plates for the same inside pipe diameter and differential pressure [Foust et al. (1964)]. The coefficient of discharge Cd for a Venturi meter is in the region of 0.98. Venturies are more suitable than orifice plates for metering liquids containing solids. 

Water flows upwards at a speed of 2 m/s in a vertical pipe. A Venturi meter having a throat diameter equal to half the pipe diameter is fitted in the pipe and has pressure taps connected to a mercury manometer. The distance between the pressure taps is 50 mm. If the discharge coefficient of the Venturi is 0.98, what will... 

The difference between an orifice meter and a venturi meter or flow nozzle is that for both of the latter there is no contraction, so that A2 is also the area of the throat and is fixed, while for the orifice, A2 is the area of the jet and is a variable and is, in general, less than the area of the orifice A0. For the venturi tube or flow nozzle the discharge coefficient is practically a velocity coefficient, while for the orifice the value of C or K is much more affected by Cc than it is by Cv. 

Adding Cd, a discharge coefficient, to account for frictional losses (assumed zero in Bernoulli s Equation) and other non-idealities, we get Equation (36), the operating equation for the Venturi meter. 

Venturi meters usually have a tapered entrance with an interior total angle of 25 to 30 and a tapered exit with an interior angle of 7 . Under these conditions, the value of the coefficient of discharge may be assumed to be 0.98 if... 

FIGURE 3.5 Venturi meter system (a) flushing system (b) Venturi meter (c) coefficient of discharge. (From ASME (1959). Fluid Meters—Their Theory and Application, Fairfield, NJ Johansen, F. C. (1930). Proc. R. Soc. London, Series A, 125. With permission.) (d) Piezometer taps for lighter indicator fluid. 

The preceding discussion of fluid meters was concerned only with the flow of fluids of constant density. When fluids are compressible, similar equations and discharge coefficients for the various meters may be used. Equation (8.35) for venturi meters is modified to the form... 

The equation for the mass flow rate through a Venturi meter is the same as for the orifice meter. Equation 4.27. The discharge coefficient for a Venturi meter is larger and typically varies from 0.95 to 0.975 for the orifice meter the discharge coefficient typically varies from 0.6 to 0.7. For more information on the design and discharge coefficients of a Venturi meter see ASME MFC-3M-1989... 

Discharge coefficients for venturi meters. Here velocities and diameters and D, respectively, are measured at point 1 in Fig. 5.8. The solid line represents the best average of the available data the dotted lines represent the range of scatter in the experimental data. (From Fluid Meters, Their Theory and Practice, 5th ed., ASME, New York, 1959, Reproduced with permission of the publisher.)... 

As in the case of the venturi meter, experiments indicate that if we introduce a discharge coefficient and thus form Eq. 5.31, then that coefficient is a fairly simple function of the ratio of the diameter of the orifice hole to the diameter of the pipe, j, / Dp, and the Reynolds number. The relation is... 

The venturi meter in Fig. 5,30 has air flowing through it. The manometer, as shown, contains both mercury and water. The cross-sectional areas at the upstream location and at the throat-are 10 and Ift respectively. What is the volumetric flow rate of the air The discharge coefficient C equals 1.0. 

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