Magnetic flowmeters, flow measurement

The usual measuring device for feed flow is a magnetic flowmeter, which is a volumetric device whose output F must be multiplied by density p to produce mass flowMo- For most aqueous solutions which are fed to evaporators, the product of density and the function of solid content appearing above is linear with density ... 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

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. 

Flowmeters These are used to measure flocculant addition, underflow, and feed flow rates. For automatic control, the more commonly used devices are magnetic flowmeters and Doppler effect flowmeters. 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

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. 

Their major limitation is that they can be used only on electrically conductive fluids. This requirement eliminates their use on all gases and on most hydrocarbon fluids. If the liquid conductivity is 20 ps/cm or greater (Table 3.68), most of the conventional magnetic flowmeters can be used. Special designs are available to measure the flow of liquids with threshold conductivities as low as 0.1 ps/cm. 

One of the earliest methods of mass flow determination was to install two separate sensors one to measure the volumetric flow, and the other to detect the density of the flowing stream. On the basis of these two inputs, a microprocessor-based transmitter can measure mass flow. A further improvement occurred when the density and volumetric flow sensors were combined in a single package (Figure 3.75). These units are composed of either a Doppler ultrasonic flowmeter or a magnetic flowmeter and a gamma radiation-... 

Electromagnetic flowmeters can be applied only to solid/liquid slurries that contain electrical conducting liquids. A common problem of EM flowmeters is their temperature dependence for magnet stability. Thermal flowmeters, which measure the heating power that is required to maintain a constant temperature difference between two points along the stream, have not been successful in multiphase streams because varying composition changes the thermal conductivity of the flow rate. 

Magnetic flowmeters have been used to measure bulk velocity in vertical slurry flow, where the flow is homogeneous. In horizontal slurry flow of highly settling solids, the flow is heterogeneous. Even in this case, Shook and Roco (25) indicated that magnetic flowmeters mounted on horizontal slurry pipelines produced accurate velocity measurements. 

The experimental set-up (Fig. 1) was arranged in order to measure vortex shedding frequencies from very thin circular cylinders, perpendicular to the flow. A 500 liter constant-head tank was used to supply a steady flow of water or of. polymer solution. The flow was controlled by a valve at the extreme downstream end of the conduit and measured by an accurate magnetic flowmeter. 

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