Flow measurement pipe diameter

Flow Nozzles. A flow nozzle is a constriction having an eUiptical or nearly eUiptical inlet section that blends into a cylindrical throat section as shown in Figure 8. Nozzle pressure differential is normally measured between taps located 1 pipe diameter upstream and 0.5 pipe diameters downstream of the nozzle inlet face. A nozzle has the approximate discharge coefficient of an equivalent venturi and the pressure drop of an equivalent orifice plate although venturi nozzles, which add a diffuser cone to proprietary nozzle shapes, are available to provide better pressure recovery. 

Magnetic flow meters are sometimes utilized in corrosive Hquid streams or slurries where a low unrecoverable pressure drop and high rangeabiHty is required. The fluid is required to be electrically conductive. Magnetic flow meters, which use Faraday s law to measure the velocity of the electrically conductive Hquid, are relatively expensive. Their use is therefore reserved for special situations where less expensive meters are not appropriate. Installation recommendations usually specify an upstream straight mn of five pipe diameters, keeping the electrodes in continuous contact with the Hquid. 

For flow measurement of steam and water mixtures with a Herschel-type venturi in 2V2-in- and 3-in-diameter pipes, see Collins and Gacesa, y. Basic Eng., 93, 11-21 (1971). 

The characteristic length L denotes the pipe diameter or the hydraulic diameter djjyj = 4A/F A is the cross-sectional area and P is the wet periphery). If the cross-section is not circular, or in the case of a plane, the length is measured in the flow direction. 

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

Like the von Karman equation, this equation is implicit in/. Equation (6-46) can be applied to any non-Newtonian fluid if the parameter n is interpreted to be the point slope of the shear stress versus shear rate plot from (laminar) viscosity measurements, at the wall shear stress (or shear rate) corresponding to the conditions of interest in turbulent flow. However, it is not a simple matter to acquire the needed data over the appropriate range or to solve the equation for / for a given flow rate and pipe diameter, in turbulent flow. 

The simplest and most common device for measuring flow rate in a pipe is the orifice meter, illustrated in Fig. 10-7. This is an obstruction meter that consists of a plate with a hole in it that is inserted into the pipe, and the pressure drop across the plate is measured. The major difference between this device and the venturi and nozzle meters is the fact that the fluid stream leaving the orifice hole contracts to an area considerably smaller than that of the orifice hole itself. This is called the vena contracta, and it occurs because the fluid has considerable inward radial momentum as it converges into the orifice hole, which causes it to continue to flow inward for a distance downstream of the orifice before it starts to expand to fill the pipe. If the pipe diameter is D, the orifice diameter is d, and the diameter of the vena contracta is d2, the contraction ratio for the vena contracta is defined as Cc = A2/A0 = (d2/d)2. For highly turbulent flow, Cc 0.6. 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

Sample ports are also a key issue. While the EPA accepts five pipe diameters before and two pipe diameters downstream of the sample port, experience has shown that the recommended eight pipe diameters before and two diameters after the port improves testing accuracy. The proper lengths are important to flow measurement, but they are also critical to obtaining representative dust samples. Turbulence in gas flow will result in mass emission test results that are not representative. The particulate matter will be maldistributed after an elbow and the heaviest particles will be biased to the outside wall. Even if appropriate gas rates are collected, the amount of dust may be biased to the outside wall but collected at too small a rate. 

For many locations, flow could be easily measured (i.e., using a bucket and stopwatch). For other locations where it was not possible to measure flow directly the sum of individual upstream flows was used as an estimate. For several of the main pipes entering the treatment plan, a velocity meter was used to estimate flows. In these cases the velocity, pipe diameter and depth of water within the pipe were used to calculate flow. There were a total of four sampling events. Initially, all samples were to be collected under dry conditions to maximize contaminant... 

Vanes may be used to improve velocity distribution and reduce frictional loss in bends, when the ratio of bend turning radius to pipe diameter is less than 1.0. For a miter bend with low-velocity flows, simple circular arcs (Fig. 6-37) can be used, and with high-velocity flows, vanes of special airfoil shapes are required. For additional details and references, see Ower and Pankhurst (The Measurement of Air Flow, Pergamon, New York, 1977, p. 102) Pankhurst and Holder (Wind-Tunnel Technique, Pitman, London, 1952, pp. 92-93) Rouse (.Engineering Hydraulics, Wiley, New York, 1950, pp. 399-401) and Jorgensen (Fan Engineering, 7th ed., Buffalo Forge Co., Buffalo, 1970, pp. Ill, 117, 118). 

Elbow taps develop relatively low differential pressures. For this reason, they cannot be used for measurement of low-velocity streams. Typically, water flowing at an average velocity of 1.5 m/s (5 ft/s) through a short-radius elbow with a centerline radius equal to the pipe diameter develops about 2.5 kPa (10 in. H20) water differential pressure. This is approximately the minimum full-scale pressure drop that is needed for reliable measurement. If the elbow is installed with 25 diameter upstream and 10 diameter downstream straight pipe runs, the measurement error will be under 10% FS over a 3 1 range. 

The value of the coefficient of discharge Cd for orifice meters depends on the properties of the flow system, the ratio of the orifice diameter to the upstream diameter, and the location of the pressure-measuring taps. Values of Cd for sharp-edged orifice meters are presented in Fig. 14-55. These values apply strictly for pipe orifices with throat taps, in which the downstream pressure tap is located one-third of one pipe diameter from the downstream side of the orifice plate and the upstream tap is located one pipe diameter from the upstream side. However, within an error of about 5 percent, the values of Cd indicated in Fig. 14-55 may be used for manometer taps located anywhere between the orifice plate and the hypothetical throat taps. 

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