Cross-Correlation Flowmeter

The cross-correlation technique measures the time of flight of an inherent flow tag passing through two sensors separated by a known distance. The technique has been used successfully to monitor single-phase fluid flows in which turbulent eddies modulate the interrogating ultrasonic beams. This type of correlation flowmeter has also been developed for solid/liquid and gas/liquid flows, in which the density fluctuation, caused by clusters of solids and by gas bubbles, is the prime inherent flow tag. 

A coal slurry flow is a practical mixed-phase flow that requires close monitoring to ensure safe and efficient transport of the coal slurry. In addition to the flow rate measurement, detection of gas bubbles and settling of solids is equally important in coal slurry lines. In this section, we will describe a cross-correlation flowmeter that can reliably measure coal slurry flow rates over wide ranges of coal concentration and flow velocity. We will also illustrate how the flowmeter can detect settling of solids and recognize the presence of gas bubbles. Both laboratory and pilot plant flow tests are included. 

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Cross-correlation flowmeters in combination with concentration detectors are available for the measurement of the mass flow of solids in pneumatic conveying systems or for volumetric flow measurements. The cross-correlation flowmeter uses a microwave (or gamma ray, ultrasonic, or photometric detectors) as the densitometer and a measurement of the time it takes for particles to travel a known distance to determine velocity. 

FIGURE 5-8 Block diagram of basic cross-correlation flowmeter. 

Determination of the volumetric flow rate of a pipe flow requires only a measurement of average flow velocity. Both Doppler and cross-correlation flowmeters provide such an averaged flow rate measurement. Designs and performance of the two types of flowmeters are described here. Results from calibration tests conducted at an Argonne National Laboratory (ANL) flow facility and prototype instrument demonstration tests at coal-conversion pilot plants are presented. 

The basic design of an ultrasonic cross-correlation flowmeter (Sheen and Raptis, 1983), as shown by Fig. 5-8, consists of two pairs of transducers positioned in parallel and separated a known distance. The transducers can be mounted by either a clamp-on arrangement or use of special windows that provide a better impedance match to the flow so that more acoustic energy can be transmitted. Pitch-catch is the typical mode of operation. The transducers on one side generate ultrasonic waves in the form of sine tone bursts and the... 

FIGURE 5-15 Parallel and crossed acoustic-beam orientations of cross-correlation flowmeter (shaded areas are intersecting crosssections)... 

FIGURE 6-9 Typical cross-correlation flowmeter. (Source Yan, 1996)... 

To illustrate density measurement by capacitive methods, the performance of the ANL capacitive mass flowmeter is described. Instrument evaluation tests were conducted at the ANL Solid/Liquid Test Facility (SLTF), shown schematically in Fig. 6.19. The SLTF was designed as a specialized instrument-testing and calibration-loop facility for various liquid and liquid/solid media. The facility can provide volumetric flow rates that range from 0 to 10 L/s and flow speeds up to 6 m/s in 2-in. Schedule 40 pipe. It is equipped with an acoustic cross-correlation flowmeter, a PNA system to... 

Beck M S, Plaskowski A, "Cross Correlation Flowmeters - their Design and Application", Adam Hilger, Bristol, 1987, ISBN 0-85274-532-X. 

Acoustic/ultrasonic techniques that have been developed into flow-monitoring instruments are Doppler, cross-correlation, and transit-time methods. An ultrasonic Doppler flowmeter has been applied to single-phase turbulent flows and mixed-phase (solid/liquid or gas/liquid) flows. The crosscorrelation technique is mainly for mixed-phase flows, whereas the transit-time method has been applied to single-phase flows, either liquid or gas, in large pipes. 

Solid/liquid flows are commonly found in industrial processes to avoid flow obstruction, nonintrusive flowmeters are generally preferred. Flowmeters based on ultrasonic techniques are ideal nonintrusive instruments because, in most applications, the ultrasonic transducers are simply clamped on the outside pipe wall. In this section, we describe two ultrasonic flowmeters based on the Doppler and cross-correlation methods. Both require an inherent flow tag thus both are directly applicable to solid/liquid flows because of the presence of solid particles. Both flowmeters measure mainly particle velocity liquid-phase velocity, if different from the particle velocity, is not determined. 

FIGURE 6-23 Typical cross-correlation function obtained by ANL capacitive flowmeter for two slurry velocities (0.26 and 0.4 m/s) and two electrode spacings (1.52 and 15.2 cm). Coal concentration was 60 wt.%. 

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