Orifice/Venturi meter

The flow through the orifice/Venturi meter also depends on the thermal expansion of the flow meter and pipe (FJ, and the expansion factor (Y). With those corrections the final equation will be ... 

Head meters with density compensation. Head meters such as orifices, venturis, or nozzles can be used with one of a variety of densitometers [e.g., based on (a) buoyant force on a float, (b) hydrauhc couphug, (c) voltage output from a piezoelectric ciystal, or (d) radiation absolution]. The signal from the head meter, which is proportional to pV" (where p = fluid density aud V = fluid velocity), is multiphed by p given by the densitometer. The square root of the produc t is proportional to the mass flow rate. 

An equation for use with venturi meters was given by Chisholm [Br Chem. Eng., 12, 454—457 (1967)]. A procedure for determining steam quahty via pressure-drop measurement with upflow through either venturi meters or sharp-edged orifice plates was given By Colhus and Gacesa [J. Basic Eng., 93, 11-21 (1971)]. 

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. 

ASMEflow nozzle. These nozzles provide for accurate measurements. Their use is limited because they are not easily placed in a process plant however, they are excellent for shop tests. Venturi meters and nozzles can handle about 60% more flow than orifice plates with varied pressure losses. 

Figure 6.14. (a) Orifice meter (b) Venturi meter (c) Nozzle... 

Figure 6.19. Pressure distribution using orifice plate, venturi meter, and Dali tube. Pressure falls by 10% from upper pressure tapping to throat in each case...

Figure 10-11 Expansion factor for orifice, nozzle, and venturi meter, (a) k = 1.3 (b) k = 1.4. (From Crane Co., 1978.)...

A 6 in. sch 40 pipeline is designed to carry SAE 30 lube oil at 80°F (SG = 0.87) at a maximum velocity of 10 ft/s. You must install an orifice meter in the line to measure the oil flow rate. If the maximum pressure drop to be permitted across the orifice is 40 in. H20, what size orifice should be used If a venturi meter is used instead of an orifice, everything else being the same, how large should the throat be ... 

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. 

Figure 8.5 shows a Venturi meter. The theory is the same as for the orifice meter but a much higher proportion of the pressure drop is recoverable than is the case with orifice meters. The gradual approach to and the gradual exit from the orifice substantially eliminates boundary layer separation. Thus, form drag and eddy formation are reduced to a minimum. 

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. 

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... 

International Organization for Standards Report DIS 5167, Geneva, 1976). Similar equations are given for other kinds of orifice taps and for nozzles and Venturi meters. 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

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. 

The coefficients for venturi meters, flow nozzles, and orifice meters vary with Reynolds number as shown in Figs. 10.7 to 10.10. The curve for an orifice meter shown in Fig. 10.10 covers an unusually wide range of both viscosity and Reynolds number. The fluids used were water and a series of oils up to a very viscous road oil, and for each fluid a number of different velocities were used, so that the curve represents points for many combinations of velocity and viscosity. Although the orifice plate may not be a standard beveled form, the value of C for high Reynolds numbers agrees closely with the value of C in Fig. 10.10 for a diameter ratio of 0.75. 

Head-type flowmeters include orifice plates, venturi tubes, weirs, flumes, and many others. They change the velocity or direction of the flow, creating a measurable differential pressure, or "pressure head," in the fluid. Head metering is one of the most ancient of flow detection techniques. There is evidence that the Egyptians used weirs for measurement of irrigation water flows in the days of the Pharaohs and that the Romans used orifices to meter water to households in Caesar s time. In the 18th century, Bernoulli established the basic relationship between the pressure head and velocity head, and Venturi published on the flow tube bearing his name. 

The Continuity and Bernoulli Equations may be used to derive equations relating flow rate to measured pressure difference for the Venturi meter (and the orifice plate meter discussed below). 

There are many other flow measurement devices including Onlicc/Venturi meters, turbine meters, and more sophisticated instruments using ultrasonic, magnetic, and Coriolis effect techniques. Orifice/Venturi type meters have a restriction causing a pressure drop related to the flow rate of liquid. Such meters are popular because of their low cost however, their accuracy can be compromised by upstream elbows and valves. Turbine meters are designed so that rotation speed varies linearly with the... 

Basic equations for the design and operation of orifice meters, venturi meters, and rotameters can be derived from the total energy balances presented at the beginning of this chapter. The following equations apply when the flowing fluid is a liquid, and they also give accurate results for the flow of gases if the pressure drop caused by the constriction is less than 5 percent of the upstream pressure ... 

The rate of flow of a liquid mixture is to be measured continuously. The flow rate will be approximately 40 gpm, and rates as low as 30 gpm or as high as 50 gpm can be expected. An orifice meter, a rotameter, and a venturi meter are available. On the basis of the following additional information, would you recommend installation of the orifice meter, the venturi meter, or the rotameter Give reasons for your choice. 

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