Variable area meters

2 Variable Area Orifices. Variable area orifices consist of an orifice and a concentric obstruction. Fluid flows through the orifice around the concentric obstruction. As the flow rate increases, the area between the orifice and the obstruction changes, either by displacement of a spring loaded obstruction relative to a fixed orifice or by displacement of a spring loaded orifice relative to a fixed obstruction. The displacement is related to the flow rate. 

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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. 

Variable-Area Meters Variable-area meters, which are also called rotameters, offer popular and inexpensive flow measurement devices. These meters employ a float inside a tube that has an internal cross-sectional area that increases with distance upward in the flow path through the tube. As the flow rate increases, the float rises in the tube to provide a larger area for the flowing fluid to pass. 

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. 

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. 

Variable-area meters are well suited for the local indication of low flow rates, but are not limited to those applications. They are available in both glass and metal tube constructions (Figure 3.99). In the glass tube design, the position of the float can be visually observed as an indication of flow rate. Their advantages include their low cost, low pressure loss, direct flow indication, and the ability to detect very low flow rates of both gases and liquids, including viscous fluids. 

The last flow meter that we will address is the rotameter. This meter is relatively inexpensive and its method of measurement is based on the variation of the area through which the liquid flows. The area is varied by means of a float mounted inside the cylinder of the meter. The bore of this cylinder is tapered. With the unit mounted upright, the smaller portion of the bore is at the bottom and the larger is at the top. When there is no flow through the unit, the float is at the bottom. As liquid is admitted to the unit through the bottom, the float is forced upward and, because the bore is tapered in increasing cross section toward the top, the area through which the liquid flows is increased as the flow rate is increased. The calibration in rates of flow is etched directly on the side of the cylinder. Because the method of measurement is based on the variation of the area, this meter is called a variable-area meter. In addition, because the float obstructs the flow of the liquid, the meter is an intrusive meter. 

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. 

RP16.4 Nomenclature and Terminology for Extension Type Variable Area Meters (Rotameters). 

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. 

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