Orifice meter applications

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

This meter may thus be considered as an orifice meter with a variable aperture, and the formulae already derived are therefore applicable with only minor changes. Both in the orifice-type meter and in the rotameter the pressure drop arises from the conversion of pressure energy to kinetic energy and from frictional losses which are accounted for in the coefficient of discharge. The pressure difference over the float —AP. is given by ... 

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. 

Turbine and rotary meters have immediate application in many areas with wider rangability and to some degree less bother that the orifice meter. We can t loose sight of the fact that there are hnndreds of thousands of orifice meters in operation. The replacement of most of these is not practical from either an economic or a physical standpoint. We have no choice bnt to improve the precision of this basic method. 

Because it is applied as a volatile liquid, anhydrous ammonia must be injected 15 to 30 cm below the surface of the soil this usually is accomplished by an application knife such as those shown in Figure 10.3. Often in sandy, loose soil ammonia is applied by an ammonia chisel, also shown in Figure 10.3. Anhydrous ammonia is usually metered by a variable orifice-type meter or by a piston pump, The rate of application using the orifice meter is determined by the speed of the applicator, the swath width, and the orifice opening. Piston pumps are usually actuated by a drive-chain operated by a sprocket attached to a wheel of the applicator. Application rate is changed by changing the length of stroke of the piston the rate is independent of the applicator s speed. 

In dilute-phase regimes the principle of using a venturi and orifice meter in series (the Graczyk or BCR meter) can be employed. In this arrangement the venturi meter will measure the acceleration of the solids and the orifice meter will measure only the gas flow, since with closely placed pressure tops the particles will undergo little acceleration. This can be a relatively inexpensive solution to a gas-solid flow problem, especially if a venturi is already on hand (see Fig. 7-2). For dense-phase gas-solid flow or systems having sizable upsets, the orifice meter-venturi setup is subject to frequent plugging with solids. The unit is also applicable only for small-diameter lines. 

Figure 2-18. Flow coefficient C for square edged orifices. By permission, Crane Co. [3], Technical Paper 410 Engineering Div. (1976) and Fluid Meters, Their Theory and Application Part 1, 6th Ed., 1971, American Society of Mechanical Engineers and, Tuve, G. L. and Sprenkle, R. E., Orifice Discharge Coefficients for Viscous Liquids, Instruments Nov. 1933, p. 201.

For liquids with entrained gas or vapor, a "vent hole" should be provided, and for gases with entrained liquid, a "drain hole." Meters for liquid with entrained gas or gas with entrained liquid services should be installed vertically. Normally, the flow direction would be upward for liquids and downward for gases. The use of vent and drain holes is discouraged, if in order to keep them from plugging, the holes would need to be large and would adversely affect accuracy. On severe entrainment applications, eccentric or segmental orifice plates should be used. 

In a target flowmeter, a target or impact plate is inserted into the flowing stream, and the resulting impact force is detected. The target meters are more expensive than orifices, but because they have no pressure taps to plug or freeze, they are better suited for applications where the process fluid is "sticky" or contains suspended solids. 

Pressure-fed applicators have a pump and metering device and dehver fumigant at pressure to the nozzle openings (orifices) as with a low-pressure sprayer. 

Harris and Magnall tested the applicability of orifice plates and venturi meters as a flow-measuring device for non-Newtonian liquids [46]. 

The most frequent application of signal conditioning is linearisation. Many of the common functions may not be obvious to the control engineer since they are often built into the DCS or transmitter as standard features. For example, where cj is the discharge coefficient, d the orifice diameter, dp the pressure drop across the orifice and p the fluid density, the flow (F) through an orifice flow meter is given by... 

There have been various applications to cryogenics of these head-type meters in the form of orifice plates, Venturi, and flow nozzles (see Fig. 8.11). This type of meter is probably the oldest method of measuring flowing fluids. The distinctive feature of head meters is that a restriction is employed to cause a change in the static pressure of the flowing fluid. This pressure change is measured as the difference between the static head and the total head at one section of the channel. 

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