Variable orifice meters

To meter the two flows, a flow meter, a variable orifice meter or eddy current or sonic flow meter, for the water is generally used, and a metering pump, generally a progressive cavity pump, is used for the polymer. Unlike for solid polymer make-up. for dispersions the make-up plant is the same basic design for all sizes unless It is to be made up by hand, which is possible for small test runs. 

If dilution water is used, this is generally measured with a rotameter variable orifice meter. However if this flow has to be integrated into a control system then an electronic method, as used for the feed, will be necessary. 

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

In the meters so far described the area of the constriction or orifice is constant and the drop in pressure is dependent on the rate of flow. In the variable area meter, the drop in pressure is constant and the flowrate is a function of the area of the constriction. 

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 ... 

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. 

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. 

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. 

Because total air volume is the product of air-flow velocity (meters per second), orifice cross-sectional area (square meters), and time (seconds) any of these parameters could be changed in theory. In practice, a continuously variable orifice is difficult to build therefore, this value is fixed. The requirement for isokinetic flow will dictate the air-flow velocity. Therefore, only sample time can be independently controlled. 

Flow rates are the largest single group of process measurements used for control, and flow is the only process variable for which significant energy may be required by the measuring device. Most flows are measured by orifice meters which are heat-type devices that extract head loss from the pumping... 

Full-bore meters include variable-head meters such as venturi and orifice meters and variable-area meters such as rotameters. These will be described in some detail. Briefer descriptions are given of other full-bore measuring devices V-element, magnetic, vortex shedding, turbine and positive-displacement meters, ultrasonic meters, and mass flow devices such as Coriolis and thermal flowmeters. 

Using the orifice meters for a check was imreliable because of changing composition dne to variable enrichment which changed the density. 

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. 

The turbine-type flow meter probably has been used for flow measurement of liquefied gases more than any other type of meter. These meters have all of the disadvantages of the variable-area meters, are more expensive, have moving parts, and require electronic circuitry. There is a tendency to trust the reading of a turbine meter more than an orifice even though neither meter has been calibrated with the fluid being measured. 

For accurate metering and flow adjustment, and to ensure fast response time, adjustment of flow occurs close to the gun nozzle. To accomplish this, pressurized material flows into the gun and across a variable orifice. The variable orifice is comprised of a solid carbide needle and seat for maximum wear resistance. Downstream of the needle and seat, but before the nozzle, a pressure transducer provides pressure... 

AREA METERS ROTAMETERS. In the orifice, nozzle, or venturi, the variation of flow rate through a constant area generates a variable pressure drop, which is related to the flow rate. Another class of meters, called area meters, consists of devices in which the pressure drop is constant, or nearly so, and the area through which the fluid flows varies with flow rate. The area is related, through proper calibration, to the flow rate. 

Flow transmitters. Flow measurements are made in high-pressure lines by sensing the pressure drop across a calibrated orifice or venturi, or by the transmitting variable-area type of flowmeter. The latter meter resembles a Rotameter with float position transmitted electrically. It has the advantage of being an in-line element but is not readily applicable to large flows. 

---