Variable head meters

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. 

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. 

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. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

In general positive displacement pumps have limited flow capacity but are capable of relatively high pressures. Thus these pumps operate at essentially constant flow rate, with variable head. They are appropriate for high pressure requirements, very viscous fluids, and applications that require a precisely controlled or metered flow rate. 

A second requirement of the scrubbing control system is to maintain the total solids level in the effluent below 10% w/w. The single variable-speed metering pump motor can therefore be used to drive a second head for the addition of water, thereby controlling the correct water sodium hydroxide ratio. The temperature and flow-rate of the recirculation liquor should be measured and a low-flow alarm incorporated in the liquor recirculation line. 

Ardel polyarylate resins, 10 190 Ardeparin, 4 95t Arduengo carbenes, 26 847 Area, exponents of dimensions in absolute, gravitational, and engineering systems, Cross-sectional area Head- area meters Surface area Variable-area flowmeters Area detectors, 26 431 Area per surfactant molecule, 24 136 Arechloral hydrate, anesthetic properties of, 2 69... 

In this section the notation and symbols used throughout the book are listed alphabetically under their appropriate headings. Constants are usually given in nonitalic type and variables in italic. (This is only a general rule. By convention, Boltzmann s constant k, Planck s constant h, and other physical constants are in italic.) Boldfaced type indicates vectors and matrices. Except in section L2.4.A., the cgs (centimeter-gram-second) and the mks (meter-kilogram-second) systems of notation are used in parallel. Any symbols not listed in this section are defined where they are used or in the notation section of Level 3. 

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... 

The positioning of the sensor over the slip surface must be exact from cast to cast and must not change during a casting run. This is the only way to maintain a constant surface level, since the sensor has a distinct range of operation for maximum detection efficiency. Most sensors are secured by a locking nut or other mechanism. The best way to control the slip flow to the doctor blade is with the use of a continuously variable proportional valve. This valve can meter the flow of slip to the reservoir and maintain a very constant head behind the doctor blade. 

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