Positive displacement flowmeter

Volumetric Meters Volumetric meters (also called positive-displacement flowmeters) are devices that mechanically divide a fluid stream into discrete, known volumes and count the number of volumes that pass through the device. See Spitzer (2005, op. cit.). 

Fig. 7.51 Positive displacement flowmeter (courtesy Euclid Chemicals, USA). 

Types of positive displacement meters. (Adapted from Universal Flow Monitors (UFM), "Positive Displacement Flowmeters," ttazel Park, MI, http //www.flowmeters.com/ufm/index.cfm7task = positive ... 

One common positive displacement flowmeter consists of a central screw impeller that meshes with two deeply grooved sealing screws, all selectively... 

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. 

Flow Rate. The values for volumetric or mass flow rate measurement are often determined by measuring pressure difference across an orifice, nozzle, or venturi tube. Other flow measurement techniques include positive displacement meters, turbine flowmeters, and airflow-measuring hoods. 

Other flowmeters are in common use which operate on principles differing from head flowmeters. Mechanical flowmeters have primary elements which contain moving parts. These flowmeters include rotameters, positive displacement meters and velocity meters. Electromagne-... 

Flowmeters can be classified as either positive displacement or turbine type meters. Listed below is a brief description of each flowmeter type ... 

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. 

Flow, defined as volume per unit of time at specified temperature and pressure conditions, is generally measured by positive displacement or rate meters. The term positive displacement meter applies to a device in which the flow is divided into isolated measured volumes when the number of fillings of these volumes is counted in some manner. The term rate meter applies to all types of flowmeters through which the material passes without being divided into isolated quantities. Movement of the material is usually sensed by a primary measuring element that activates a secondary device. The flow rate is then inferred from the response of the secondary device by means of known physical laws or from empirical relationships. 

Positive-displacement (PD) flowmeters are used when the total quantity of the flowing process stream is of interest or when a recipe is being formulated in a batch process. These meters operate by trapping a fixed volume of fluid and transferring that volume from the inlet to the outlet side of the meter. The number of such calibrated "packages" of fluid is counted as a measure of total volumetric flow. These measuring devices are used in both gas and liquid services. 

Displacement flowmeters for both liquids and gases. In some cases, positive displacement pumps are employed for liquids. 

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. 

Resin and hardener flowrate Can vary as a result of worn pumps, viscosity changes with temperature, blockages or leaks in pipewoik and nozzles, sticking of non-retum valves, etc. Positive displacement, electromagnetic or coreolis flowmeters... 

Less/no flow of air to plant units (drying tower)—This can cause loss of production, improper combustion of sulphur in furnace. Hence, it provides an air flowmeter, pressure gauge/manometer in airline. Operate the plant with positive displacement-type air blower. 

The first two bubblers are filled with 80/20 isopropanol/ water, with glass wool plugs wetted with the solvent to aid dissolution of the SO2 and SO3. The third bubbler is filled with 3% hydrogen peroxide as a back-up absorption system. The sampling pump used is a positive displacement type, fitted with either a flowmeter or a flow counter. 

Centrifugal pumps with flowmeters can be used for the dosing of oleum, alkylbenzene, dilution water and caustic solution. Positive displacement pumps are more accurate, but also more expensive. Furthermore, when a positive displacement pump is used, there must be a pressure relief system to prevent the pressure in the discharge line becoming too high and there must also be a pressure (h per in the discharge line. 

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