Magnetic flowmeter

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. 

Liquid-Solid Mixtures Liptak [Chem. Eng., 74(4), 151-158 (1967)] discusses a variety of techniques that can be used for the measurement of sohds-in-liquid suspensions or slurries. These include metering pumps, weigh tanks, magnetic flowmeter, ultrasonic flowmeter, gyroscope flowmeter, etc. 

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. 

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. 

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. 

Another disadvantage of magnetic flowmeters is their somewhat high purchase price and the cost of maintaining the magnetic field. If the location is... 

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-... 

Mass flowmeter combining a magnetic flowmeter and a radiation-type densitometer in a single unit. 

The preneutralizer slurry is pumped at a controlled rate to a rotary-drum, ammoniator-granulator there it is distributed onto a rolling bed of recycled solids. The most commonly used metering system for the preneutralized slurry is a variable-speed centrifugal pump with automatic control from a magnetic flowmeter. Some plants have had success with a magnetic flowmeter and automatic control valve of a special ball type. 

Magnetic flowmeters 304 stainless steel, 150 Ib/in 2 16 stainless steel electrodes to4 Tetlon linings... 

In most cases, the manufacturer s own calibration procedures should be acceptable as they form part of the manufacturer s own performance guarantees and/or quality certification. If special calibration is required, then companies with custom-built test facilities must be used (e.g., a magnetic flowmeter that requires a specific certification will require calibration in a flow rig that has been certified by an approved certifying authority). 

Instrumentation on the unit consists of a magnetic flowmeter to measure feed rate and a level controller to maintain proper liquid height in the flotation cells. 

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