Orifice/Venturi

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. 

Orifice venturi A measuring device used to determine the flow rate of a fluid by means of the pressure drop across the device. 

This sonic velocity occurs in a pipe system in a restricted area (for example, valve, orifice, venturi) or at the outlet end of pipe (open-ended), as long as the upstream pressure is high enough. The physical properties in the above equations are at the point of maximum velocity. 

Figure 11 shows a block diagram of a typical differential pressure flow detection circuit. The DP transmitter operation is dependent on the pressure difference across an orifice, venturi, or flow tube. This differential pressure is used to position a mechanical device such as a bellows. The bellows acts against spring pressure to reposition the core of a differential transformer. The transformer s output voltage on each of two secondary windings varies with a change in flow. 

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... 

Figure 8.14 shows some commonly used flow meters. Dolenc [23] reviews these flow-meter types in addition to other types. The meters in Figure 8.14 are divided into two classes the variable-head meters, which are the orifice, venturi. 

To size a variable-head meter, we must calculate the orifice, venturi throat or nozzle diameter. Using Bernoulli s equation we can derive a relationship between the flow rate, the pressure drop across the meter, and the orifice diameter. 

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. 

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

The three most common obstmetion meters include the orifice, Venturi, and Tuy6re. The operating principle is based on reducing the cross-section of the pipe normal to the flow field and measuring the increase in pressure drop the velocity head (n /2) increases at the expense of the pressure head (P/p). The reduction in pressure head is measured at two points in the pipe—one immediately downstream and the other upstream— Figure 6.5. 

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 ... 

The pressure drop calculated earlier does not represent the correct value. Downstream of the orifice/Venturi, the velocity gradually decreases, and some of the velocity head recovers. The actual pressure drop is the nonrecoverable pressure drop and is less than the pressure drop calculated earlier. The nonrecoverable pressure drop is calculated as ... 

From Equation 2.12, it is clear that for a given set of conditions, the flow through the orifice/Venturi will increase for a decrease in absolute pressure ratio P2/P1, xmtil a linear velocity in the throat reaches the velocity of soimd. The value of P2/P1 for which the acoustic flow is just attained is called the critical pressure ratio r, and flow at such a condition is called critical or choked flow. Under choked flow situations, the flow through the orifice depends only on the upstream pressure. 

Critical flow through the orifice/Venturi can be expressed as [3]... 

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