Pitot-venturi

The pitot-venturi flow element is capable of developing a pressure differential 5 to 10 times that of a standard pitot tube. This is accomplished by employing a pair of concentric venturi elements in place of the pitot probe. The low-pressure tap is connected to the throat of the inner venturi, which in turn discharges into the throat of the outer venturi. For a discussion of performance and application of this flow element, see Stoll, Trans. Am. Soc. Mech. Eng., 73, 963-969 (1951). 

Pitot ube or Pitot Venturi Tube Turbine or Propeller Type Element Rotameter... 

Once these traverse points have been determined, velocity measurements are made to determine gas flow. The stack-gas velocity is usually determined by means of a pitot tube and differential-pressure gauge. When velocities are very low (less than 3 m/s [10 ft/s]) and when great accuracy is not required, an anemometer may be used. For gases moving in small pipes at relatively high velocities or pressures, orifice-disk meters or venturi meters may be used. These are valuable as continuous or permanent measuring devices. 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

In this chapter we will illustrate and analyze some of the more common methods for measuring flow rate in conduits, including the pitot tube, venturi, nozzle, and orifice meters. This is by no means intended to be a comprehensive or exhaustive treatment, however, as there are a great many other devices in use for measuring flow rate, such as turbine, vane, Coriolis, ultrasonic, and magnetic flow meters, just to name a few. The examples considered here demonstrate the application of the fundamental conservation principles to the analysis of several of the most common devices. We also consider control valves in this chapter, because they are frequently employed in conjunction with the measurement of flow rate to provide a means of controlling flow. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Orifice meters, Venturi meters and flow nozzles measure volumetric flow rate Q or mean velocity u. In contrast the Pitot tube shown in a horizontal pipe in Figure 8.7 measures a point velocity v. Thus Pitot tubes can be used to obtain velocity profiles in either open or closed conduits. At point 2 in Figure 8.7 a small amount of fluid is brought to a standstill. Thus the combined head at point 2 is the pressure head P/( pg) plus the velocity head v2/(2g) if the potential head z at the centre of the horizontal pipe is arbitrarily taken to be zero. Since at point 3 fluid is not brought to a standstill, the head at point 3 is the pressure head only if points 2 and 3 are sufficiently close for them to be considered to have the same potential head... 

Differential Pressure Meters Differential pressure meters or head meters measure the change in pressure across a special flow element. The differential pressure increases with increasing flow rate. The pitot tubes described previously work on this principle. Other examples include orifices [see also Eqs. (6-111) and (8-102), and Fig. 10-14], nozzles (Fig. 10-19), targets, venturis (see also Sec. 8 and Fig. 10-17), and elbow meters. Averaging pitot tubes produce a pressure differential that is based on multiple measuring points across the flow path. 

Accuracy is improved if the flowing gas stream is directed at the probe by a venturi nozzle, or by placing the sensor in the throat of the venturi (Figure 3.74). The venturi ensures a smooth velocity profile and eliminates boundary layer effects while concentrating the flow onto the sensor. These units are available for both liquid and gas services. Other designs are of the insertion probe type. Their flow ranges are a function only of the size of the pipe into which they are inserted, and their performance is a function of the correctness of the insertion depth (as are all Pitot tubes). 

Both gas flow rates and liquid flow rates can be measured by a wide variety of devices such as bellow meters, Venturi nozzles, nutating disk meters, orifice meters, rotameters, weirs (for liquids), Pitot tubes, and magnetic meters among others. Some devices measure volumetric flow directly as with meters in which the space between rotating paddles incorporates small volumetric displacements of fluid. Other device measure the flows indirectly by measuring the pressure drop caused by an orifice between two different sites in the pipe, or the change in voltage of a heated wire. 

Fig. 1 (A) Venturi meter (B) orifice meter (C) Pitot tube and (D) rotameter.

Meters that measure differential pressures over the flowmeter and such pressure changes that can be interpreted as flowrates. Such flowmeters with a large number of designs include orifices, venturi tubes, pitot tubes, elbow taps, etc. Fluids that result in changes of the cross-sectional area due to erosion, corrosion, or deposition of solids obviously change the calibrations. These meters tend to be relatively cheap but are often not very accurate. 

Gas-law constant, 8.314 N-m/gmol-K or 1545 ft-lbj-/lb mol- R Radius, m or ft r, of impeller at suction r2, of impeller at discharge Cross-sectional area, m or ft Sf, of venturi throat S , of orifice Absolute temperature, K or °R T, at compressor inlet 7, at compressor dicharge also torque, J or ft-lbf Local fluid velocity, m/s or ft/s maximum velocity in pipe Ug, at orifice Uq, at impact point of pitot tube Uj, peripheral velocity at inlet of pump impeller Hj, at impeller discharge Waj, Ho2) tangential velocity components at stations 1 and 2 Resultant velocity, absolute, in pump impeller, m/s or ft/s Vr2,... 

The pitot-static tube is the standard device for measuring the airspeed of airplanes and is often used for measuring the local velocity in pipes or ducts, particularly in air pollution sampling procedures. One can easily identify the pitot-static probes projecting from the front of modern commercial airplanes look next time you are at an airport. For measuring flow in enclosed ducts or channels the venturi meter and orifice meters discussed below are more convenient and more frequently used. 

Differential pressure Orifice, Venturi, Tuyfere, Pitot tube ... 

Kolupada, S. (1960). John W. Ledoux. Journal of the Hydraulics Division ASCE 86(HY1 42. P Ledoux, J.W. (1913).A mechanism for metering and recording the flow of fluids through Venturi tubes, orifices, or conduits, by integrating velocity head. Trans. ASCE 76 1148-1171. Ledoux, J.W. (1914). The Pitot tube theory. Journal AWWA 1(3) 536-537. 

Flowrate Differential pressure devices pitot tube, orifice plate, venturi meter Turbine meter Rotameter Hot wire anemometer... 

Instruments which measure the rate of flow (velocity) of liquids and gases are called flowmeters they may be broadly defined as being mechanical or electronic in operation. Examples of mechanical flowmeters are orifice plate and float meters (Fig. 5.8), venturi meters, and pitot tube meters, all of which depend on a constriction being introduced into the flow stream in order to produce a difference in pressure across the constriction. The rate of flow can then be obtained from the difference in pressure. 

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