Variable-Area Flowmeters Rotameters

Variable-area flowmeters in liquid service can measure flow rates up to 15 m3/min (4,000 gal/min) and in gas service up to 40 m3/min (1,200 scfm). The measurement error of laboratory units is 0.5% AF and the errors of industrial ones are 1 to 2% FS over a 10 1 range. In special designs their operating temperatures and pressures are up to 540°C (1,000°F) and 400 bar (6,000 psig), respectively. (Glass tubes are limited to 20 bar, or 300 psig.) 

Post-Oil Energy Technology After the Age of Fossil Fuels 

In a variable-area flowmeter the area open to flow is changed by the flow itself. Either gravity or spring action can be used to return the float or vane as flow drops. 

In some variable-area flowmeters, gravity has been replaced by spring loading. In these units, an increase in flow results in a compression or deflection of a spring, and this motion is used as an indication of flow. These units can be mounted in any position, including horizontally, as flow-through pipeline devices. 

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Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

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 in-line flowmeters (Fig. 18.15), or rotameters, are sometimes referred to as sight gauges because they provide a visible indication of flow rate. These devices, when fitted with proximity sensors (such as capacitive pickups), which sense the presence of the float, can be used in on-off control applications. 

FIGURE 18.15 Variable-area in-line flowmeter (rotameter). 

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. 

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