A Flow Measurement

Flows are of fundamental importance for controlling a plant and for balancing and metering liquids. A large number of measurement principles are available here. Apart from the Coriolis-force meter, which directly measures the mass flux, all of them measure the volume flow. 

As = Largest horizontal cross section of floating body 

The use of these devices is restricted to the measurement of small to medium volume flows of low molecular weight liquids free of solid components. The position of the floating body can be measured inductively, so that an electrical signal is available for 

Some important process quantities follow [Melzer 1980]  

Concentration, measured by gas chromatography or special methods such as flame-ionization detectors, IR methods including URAS for CO, CO2, NO, SO2, paramagnetism for measuring O2 concentration 

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A number of considerations should be evaluated before a flow measurement method can be selected for any appHcation. These considerations can be divided into four general classifications fluid properties ambient environment measurement requirements and economics. 

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. 

Since a spool piece in the inlet piping is required for maintenance reasons, it is recommended that this be replaced by a flow measuring section containing the required instrumentation. 

The principal requirement of a sampling system is to obtain a sample that is representative of the atmosphere at a particular place and time and that can be evaluated as a mass or volume concentration. Remote monitoring techniques are discussed in Chapter 15. The sampling system should not alter the chemical or physical characteristics of the sample in an undesirable manner. The major components of most sampling systems are an inlet manifold, an air mover, a collection medium, and a flow measurement device. 

Linford, A, Flow Measurement and Meters (Spoil, London, 1949). 

The disturbance must be detected. If we cannot measure it, we cannot use feedforward control. This is one reason why feedforward control for throughput changes is commonly used, whereas feedforward control for feed composition disturbances is only occasionally used. The former requires a flow-measurement device, which is usually available. The tatter requires composition analyzer, which may or may not be available. 

A. Flow Measurement by Displacement. A variety of flow meter designs are based on the positive displacement of a small amount of easily visualized material which does not alter the flow rate. A simple and easily constructed mass flow meter of this type, the soap-film meter, is based on timing the displacement of a soap film up a buret tube (Fig. 7.11). This type of flow meter is often placed at the exit of a gas chromatograph or small flow reactor. It is very... 

Head flow meters operate on the principle of placing a restriction in the line to cause a differential pressure head. The differential pressure, which is caused by the head, is measured and converted to a flow measurement. Industrial applications of head flow meters incorporate a pneumatic or electrical transmitting system for remote readout of flow rate. Generally, the indicating instrument extracts the square root of the differential pressure and displays the flow rate on a linear indicator. 

Pumps, such as piston pumps, can meter liquids into a reactor fairly precisely, but the chemical engineer uses a flow measurement device for greater precision. The most commonly used flowmeters are rotameters that are calibrated to translate the lifting of a float in a vertical slightly tapered tube (small diameter at the inlet of the flowmeter) into a measure of the amount of liquid delivered in a given timeframe. For greatest precision the rotameter is calibrated with the specific fluid being metered. Most modem rotameters are provided with a calibration plot that corresponds to performance. 

From the frequency measurement for different concentrations of organic substance contained in air, it is shown that the sensor can detect the concentration of different sizes of organic materials. A good recovery of the sensor to the initial condition is shown, and the ruggedness is proved from implementing the sensor in a flow measurement system. The sensor is produced from readily available materials with a simple process, and it is easily regenerated from heat treatment. 

The value of Cd depends on conditions of flow and shape of the constriction. For a well-shaped constriction (notably circular cross section), it would vary between 0.95 and 0.99 for turbulent flow. The value is much lower in laminar flow because the kinetic energy correction is larger. The return of the fluid to the original velocity by means of a diverging section forms a flow-measuring device known as a Venturi meter. 

The dominance of the flow effect suggests that flow information should be used directly to cancel this effect. This can be achieved by adding a flow measurement and controlling the ratio of the effluent and reagent flows based on the pH measurement (Section III.B.4). Despite the measurement lag and minor biases in flow measurement, this makes the concentration effect dominant. As only the two-tank configuration allows the concentration effect to be tolerated, we have two 12-m tanks with ratio control of reagent flow at the inlet to the treatment system as the starting point for the search for an effective plant. 

A flow measurement that is able to detect a flow direction can also be achieved by using two heating zones, located on the upstream and downstream side of the diaphragm. The upstream heater will need a higher voltage to maintain its temperature. The difference between the voltages to operate the upstream and downstream heater then corresponds to an air-flow signal that is dependent on the direction of the flow. 

Sampling of particulates may be directed at determining the size, size distribution, total mass and/or specific chemical identification. The sampling train usually consists of a probe or inlet, a particulate collector and pump. The pump may be calibrated for flow or a flow measuring device may be included in the train. The particulate collector may consist of one or a combination of devices such as a fitter impinger, cyclone or impactor. 

The aetive flow control unit consists of a flow measurement device, flow adjustment valves, and a feedback control unit. Flow measurement is... 

DAS) (6). The intact cells were pumped rapidly through a flow measuring cuvette. 

Harris and Magnall tested the applicability of orifice plates and venturi meters as a flow-measuring device for non-Newtonian liquids [46]. 

The experimental apparatus employed to study the catalytic performance of the samples consists of a flow measuring and control system, the reactor and an on line analytical system. The reactor is a 30 cm long pyrex tube with an expanded 2 cm long section in the middle (8 mm l.D.) in which the catalyst sample is placed. The catalyst powder is held in dace by means of quartz-wool pieces. The furnace temperature is controlled by means of a temperature contrcdler using a K-type thermocouple placed between the reactor and the walls of the furnace. The temperature inside the catalyst bed is measured by means of a K-type thermocouple (0.5 mm O.D.) placed in a 1/16 O.D. ss well which runs through the center of the cell. 

A similar argument applies to pressure compensation if the operating gauge pressure is close to zero. For example a 10% change in a pressure of 0.3 barg (around 4 psig), when converted to absolute pressure, causes a flow measurement error of about 1 %. 

If a control or other throttling v ve must be installed upstream from a flow measuring instrument, the minimum straight run requirement is as shown. If... 

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